Merlin negative G cutout too quick?
 

I am finding that the Merlin engine misfires at the slightest nudge down of the nose, nothing like a nose pushover. This also happens in level flight with some slight movement say due to turbulence or a mere touch of the stick however much I adjust the Mixture.

Can this be correct? We all know the story of negative G and the later Tilly Orifice but would our Fighter Aircraft engines ever have been so susceptible to Negative G that it is impossible to fly them straight and level without them constantly misfiring/puffing black smoke?

I was wondering the same....

I agree, it is way too touchy.

too much

http://www.youtube.com/watch?v=9BCl8...eature=related

25 seconds in you can see and hear the real behaviour. It's obviously difficult to estimate what the g loads were during the roll, but it looks more like reduced positive than actual negative g to me...

An excellent description of the problem is found on page 5 of this accident report:

http://www.aaib.gov.uk/cms_resources...pdf_501355.pdf

It is important to recognise that the negative g cut is a two stage phenomenon of lean cut possibly followed by rich cut, with the large amount of black smoke in the exhaust being symptomatic of over-fuelling (whether the engine actually cuts out or not is a function of the degree of over fuelling).

You can see this in the video; the smoke starts some time after the aeroplane passes 180º roll angle.

Yup I think that cinches it. I had forgotten that little tidbit form the movie. You can clearly see it just takes a second for it to be a problem.

This has been brought up directly with the Devs. I agree its way to sensitive at the moment.

Well, it seems more forgiving than IL2 to me, at least the engine doesn't completely turn off. Plus, the video shows it doesn't really take long to cause trouble and is consistent with a veteran's accounts in BBC's "battle of britain: the true story".

The way he describes it is just the way it appears on the BoB film intro posted above, which clearly indicates fuel starvation followed by an over-rich condition:

stage 1: lack of positive G forces the carb floater to the top of the fuel sump along with any fuel in it(is sump the correct word? let's say "feeding reservoir between fuel tank and engine" if it's not), causing fuel starvation

stage 2: as soon as positive G is restored, fuel flows around the floater and down into the engine at increased rate (since the floater, well, floats in the fuel and thus descends slower to its original position :-P), resulting in too much fuel drowning the engine out, signified by the tell-tale signs of black smoke that accompanies an incomplete burn

I actually like it quite a lot the way it's done in CoD. Between the individual aircraft quirks, the extra details and restrictions and the all-around higher level of challenge in all flyables, i feel like flying for the allied team much more often than i used to in IL2.

It's not way too much, but it is overdone indeed (also showing in the video) when he noses up and rolls over, he is forcing his nose straight for a second. In COD it would start cutting out in this stage already.

I don't think it's wrong, any ammount of negative G in early Spitfires causes the engine to cut with very little delay. That's why it was such a problem. There is no inbetween.

If anyone can prove otherwise I'd like to see it.

I also just read an account where a RAF pilot deliberately nosed down to produce a cloud of black smoke to fake being hit when he was bounced.

+1

Yes but it is happening intermittently in level flight give or take a little turbulence/undulation. I'm sure that never happened. The pilots reported the problem when they positively nosed over to dive. It's mentioned in many of their bios but no mention of problems in normal flight or even during normal descent but it is happening on CoD. I doubt it would have been cleared if that was the case.

Spot on Klem. Normal Flight inputs such as setting up a descent shouldnt result in -Ve G cuts.
A healthy push to say -0.5G okay but anything between say 0.1 and up G should be okay.

I agree with Klem. I guess the effect starts now with anything less than 1G. In my around 300 hours of flying, most of which I've spent flying atmospheric piston engine airplanes with carburetors pretty much similar to WWII design (no neg G capability), I haven't seen this happening, not in heavy turbulence, nor in powered stalls (which is a zero G maneuver on full RPM).

If it's doing it just from bumping around then you're probably right about it.

I can show you plenty of YouTube videos of people doing silly things in 172s etc at zero g where no engine misbehaviour is evident, and I can tell you that back in my younger days I played map catching games* in a Grumman AA5 at zero g on a couple of occasions without the engine missing a beat.

What I haven't done is taken the carburettor apart to investigate its design, so I can't tell you whether the lack of a zero g cut was by accident or design. I don't recall reading anything about carburettor performance in the POH for the non-aerobatic types I've flown; I suppose that it's not considered to be relevant information.

However, I have managed to find a video of some rather foolish people (no HASSLE checks**) obviously pushing into negative g in a 172, which does produce what sounds like a lean cut:
http://www.youtube.com/watch?v=HE64UwGGOZE
There are actually quite a few YouTube videos out there of people confusing zero g with negative g and getting these sort of engine cuts, which perhaps explains why most rental aeroplanes are so clapped out. Of course, this would also imply that airframe and engine safety factors are being rapidly consumed by people incapable of judging what they're doing to the aeroplane, which isn't a happy thought...

OTOH, genuine zero doesn't produce misbehaviour in this video:
http://www.youtube.com/watch?v=cwwlk...eature=related

Of course, this sort of comparison is hardly scientific because not all 172s were created equal, and even if they were, they certainly aren't Merlin powered! But perhaps it can inform the discussion by analogy if drawings of the carburettors concerned can be produced for comparison with the particular carburettors the simulator intends to model.

That, of course, is the other important detail; there were several different carburettors which might be fitted to the Merlin, so it's important that we are specific as to which one we're trying to match, because clearly an early SU carburettor will behave differently from for example an RAE anti-g carburettor.

My understanding is that all of the Merlins which saw service during the height of the Battle were made at the Nightingale road factory, and so they're probably more likely to have a consistent set of ancillary components than later engines which were built at a dazzling array of Rolls-Royce and shadow factories in both the UK and USA. So arguably our task is easier than would be the case for later engines, provided that we can find the required source material.

*Place map on instrument panel glare shield. Pitch up 20-30º, then push to zero g and catch the floating map between your teeth before getting uncomfortably close to VNE.

Obviously, very slight negative g is required to get the map off the dashboard (say -0.01 or something), but once it has floated up a couple of inches you obviously have to stay almost exactly at zero if you're going to catch it. So I never pushed deep into negative whilst playing the game; doing so would probably be unwise in a non-aerobatic aeroplane, though I'd be more worried about the lubrication system than the carburettor TBH. Of course, I've pushed into negative g in aerobatic aeroplanes, but I can't say I've ever been a fan of negative g; it always used to give me a headache...

**Yes, I know most people would say HASELL/HELL, but the alternative is another one of the "interesting" habits I've picked up over the years; I find it easier to remember HASSLE because the checks are a pain. So, before starting aerobatics:
Height - sufficient for recovery/legality/insurance, whichever is greater
Airframe - capable of safely executing the manoeuvre intended in its current condition (snags, weight & CoG etc).
Straps/Security - straps tight, no loose objects in the cockpit, especially near control runs. Positively identify strap quick release box and parachute quick release box, since mixing them up in case of emergency would be terminally embarrassing.
Situation - not over built up areas, close to danger areas, restricted airspace etc.
Lookout - clearing turns and all that jazz, making sure to check both above and below. Set lights & transponder as required.
Engine - set power required, ensure throttle friction nut tight, check instruments for abnormalities (Temperatures, Pressures, Manifold Pressure, rpm)

Then for subsequent manoeuvres:
Height
Engine
Location
Lookout

+1 on that.

The above post is an impressive demonstration of knowledge, and it is very interesting...BUT as mentioned above, the Cessna 172 is not a Spitfire.

I understand the need for explanation,and comparison, but you have to take into account at least 15-20 years difference in aero-engine technogical development between the Merlin Mk II and the Continental O-300(Early 172 engines).

From what I gather the problem of cut-out in the Merlin was not completely fixed until 1942 when pressure carburettors were introduced.

And as the CLoD manual states, pilots had to develop the tactic of half-rolling the Spitfire to chase the fuel injected 109s in negative G dives.

This seems to suggest to me that even a small amount of negative G was causing the cut out, or why else would the tactic be necessary?

..the amount of guesswork some of you guys do on this forum is astonishing sometimes..

On average a Merlin engine gulps an astonishing 3 UK gallons (almost 14 litres) per minute at take off and circa the half (roughly 6,5 litres) at 75% FEC.

It's literally like pouring petrol off a jerrycan on the ground.

A minimum negative acceleration that can occur also in turbulent air can cause a misfeed and an irregular detonation for such delicate but thirsty engines.

This was sorted with the introduction of new carburetor designs, but the early configs suffered from an instant cutout when being hit by negative Gs. Even a 0 G situation could cause trouble.

The tactic was necessary as the Bf109 pilots pushed more negative Gs than the Merlins could handle. In short, it's anecdotal evidence showing it happened, but next to nothing about the hows and whens.

Specifically, it doesn't say anything about whether problems began at 0 G, 0.2 G, -8G or 0.95 G.

Has anyone found a good description of the inner workings and design of the early Merlin carbs? I'd like to see one to form an opinion on when we should realistically expect the engine to cut.

Sure. Please provide source material that will save us the effort of guessing. ;)

Actually the Spitfire I Pilot's Notes quote 89 imperial gallons per hour all out at FTH, which is only about 1.5 gallons/minute.

You might see 3 gallons/minute from a Merlin 66 running at +25 psi, since it produced roughly double the shaft horsepower; but that's very much a horse of a different colour. Actually I seem to recall that somewhere I've got the accurate figures for the mighty RM.17 SM engine which was of course the thirstiest Merlin of all; but they're not at university with me.

Why exactly does the absolute flow rate matter? We care far more about dimensionless parameters such as FAR than we do about the absolute rate of fuel flow, which serves merely to tell us that Big Engine Is Big...

Actually, if you want to get into analysis, the Merlin should be inherently more forgiving of the failings of carburettors than a more modern naturally aspirated GA engine because the supercharger both vigorously mixes the charge and heats considerably. Therefore the fuel is considerably more likely to be fully evaporated and homogeneously mixed than would be the case for a naturally aspirated engine.

I would submit that the Merlin was not especially delicate; whilst its reliability was imperfect, especially during its early life, it was considerably better than many other engines (e.g. the Rolls-Royce Vulture, almost anything ever made by Napier, many early Bristol sleeve valve engines etc).

The early ramp or "penthouse" head engines certainly had trouble passing type tests, but of course we're not talking about them in this context, since all of the aeroplanes we're interested in are fitted with Merlin II or later engines.

How exactly do you think anything with a carburettor could be "instant"?

The carburettor is quite some physical distance from the cylinders. The flow velocity in the induction system, other than at the supercharger impeller tip, is subsonic and thus decidedly finite. It therefore obviously takes some time for any leaning of the mixture at the carburettor to impact upon the mixture at the cylinders and thus the engine shaft power output.

So even if the leaning of the mixture at the carburettor was instantaneous upon reduction in positive g, any effect upon the engine clearly could not be.

But of course, the impact of g load upon the carburettor could not be instantaneous because it is caused by physical displacements brought about by inertial loads. So obviously there is a time lag involved here as well.

Actually, inherent time lags are one of the (many) arguments raised against the carburettor, especially for automotive applications where swift throttle response is considered important.

Did you read the accident report I posted, which contains considerable information on the history of negative g cut behaviour in the Merlin?

Did you watch the video I posted which shows the actual phenomenon in flight? You can quite clearly see and hear the lags involved.

Here is some more source material:

http://www.flightglobal.com/pdfarchi...%20carburettor

http://www.flightglobal.com/pdfarchi...%20carburettor

http://www.flightglobal.com/pdfarchi...0-%202734.html

There's probably a cutaway of the SU carburettor out there somewhere on the internet, just waiting to be found... I've almost certainly also got one in my library at home, but that's several hundred miles away...

This from AP2095 Pilots Notes General. These were supplemental Generic aircraft handling notes to the usual individual aircraft notes:

http://img20.imageshack.us/img20/8356/neggcarby.jpg

Talk about "a sufficiently hard push down" and finally a reference to a G value ..."usually around 1/2G"

Now a 0.5G push to do something like set up a descent is a reasonable push.

After a bit of searching I foundthe document source you quoted above: A.P 2095, PILOT'S NOTES GENERAL, PROMULGATED BY ORDER OF THE AIR COUNCIL , 2ND Edition , April 1943. FOR OFFICIAL USE ONLY.

A.P. 2095 is not type-specific. It is about explaining the different operating requirements of a wide variaty of operational aircraft at the time - and over time - like engine management or handling props.

So where does it mention the Spitfire 1? As you said they are GENERIC notes that might not specifically relate to the model.

Spitfire fans clutching at straws again? :-P (JOKE)

I am a 109 driver by choice. As I said they are generic but its a clue as to the numbers involved.

I thought I would post this extract from the Accident Report that Viper gave us the link to. It refers to a Mosquito crash in which it was found that the later carburettor modification to overcome the negative G effect was negated by a fault in the carburettor that failed to sustain fuel flow to the carburettor chamber.

"In the event that the combined dynamics of the aircraft and float chamber fuel mass caused the floats to be forced towards their fully depressed conditions, then it is likely that the ensuing restricted fuel flow could cause a loss of engine power, as the residual fuel in the chamber would last only a few seconds."

This indicates to me that the engine cut would not be instantaneous and I simply don't believe that the aircraft would have been accepted if persistent cutouts or misfires were experienced in normal flight. It is certainly not mentioned once in the many bios I have read including those that talk about the nose-over dive cut-out problem.

Is it 0.5G, 0G, -ve G? 1 second, 2, 3? I don't know but it's not 0.25G and 0 seconds.

It needs some research but if there isn't any I'd be happy with -1G and 2 seconds to onset (rpm reduction) and a couple more to engine cut. After all, we're only interested in making normal flight and descent not give problems. When we nose over in combat we NOSE OVER because we wan't to catch to so-and-so - we ain't setting up a descent - and we can expect the engine to cut out. No point in splitting hairs over half a G and a second or two. And that should be acceptable to the Blues because they still get to cut and run. Sorry, make evasive maneouvres :)

Also I've never read of it being stated as a problem during those RL fast descents and landings made to reduce the risk of being vulched by prowling 109s.

Here's the fuel consumption of a Hurricane Mk.I for comparison;

http://img156.imageshack.us/img156/1...uelconsump.jpg

Bloody norah! nother engineer! ;)

my humble knowledge comes from a thing called experience (in this specific case experience with Merlin and other aviation engines), which is something you can't learn on a manual or at university, but for the sake of science (and in order to pass my exams) I also had my fair share of theory books..

Anyway, I meant on average going from the early marks to the late ones, but regardless of that, the comparisons that I've seen here are somewhat out of place. Comparing an air cooled boxer engine of a Cessna with a liquid cooled V12 is a bit of a silly thing, since the engine have humongous differences.

As for the Merlin not being a delicate engine, I beg to differ, and seriously too. The most delicate part of the Merlin is actually the cylinder banks, where -because of the external cooling jacket - cracks and microcracks are hard to spot unless going with internal inspection. A radial engine can still fly with a damaged piston and/or cylinder, a Merlin simply can't. Damages to cylinder banks on Merlins are normally the reason why we have engine failures still today. You have to appreciate that the planes we have today use 60+ years old banks, not to mention that some pilots today still think you need 100% throttle for takeoff, while we normally takeoff at 75% to save on engine life and play it safe (and because there's SO much power!). Another cause of cracks is not paying attention to temperatures, which can cause further thermal fatigue to the banks, especially on a cold start.

The G loads don't affect only the carburetor btw, but the inlet manifolds as well, so cutouts can be more or less abrupt. I haven't had the chance to fly the sim yet, maybe a video would help understanding better.

gosh, I would spend hours at the pub (or at the hangar with a pint) talking about this stuff mate lol

Sternjaeger what would be your opinion on how much of a push or reduction in G would be required in an early Merlin to cause it to cough ?

At present a smooth (like doing your best IF technique) lowering of the nose to say 10 degrees nose down for a descent causes the engine to cough.

IvanK it's no guesswork, you need a G-meter to determine it properly. A sustained 0G or a minimum negative G load (-0.1) are enough to interrupt the flow. Despite what our scientist friends here say, the mixture gets to the cylinder inlet almost instantaneously. As you know it's not like cylinders burn mixture all at the same time, so one cylinder not receiving enough mixture for a full detonation is enough for a cutout sometimes. Again, please guys provide me with a video and I will be able to tell you (in my humble opinion) if the effect is overmodelled or not.

Here is a small video. Starts with throttle at idle then to max Boost.
Then a slow descent initiation with max Boost set.
I would be interested in your opinion on the RPM needle Bounce as well as the cough which starts about 19seconds in:

http://www.youtube.com/watch?v=Py7lfQUxgRA

I figure (based on experience) that the descent entry is less than 0.2g decrement on 1G i.e by estimation G is never getting less than +0.8G ... pretty slow entry and still it coughs

Just to add to IvanK's vid.

Crusing speed with various moves to invoke this issue. Full realism.

http://www.youtube.com/watch?v=L7bxmmh0VzE


Dunno if it's just me, but the problem seems worse with engine heat realism off.. so that you can't control the rad. Prolly just me. *shrugs*

Hope the video helps anyway.

Sternjaeger, please don't take this the wrong way, I would really like to understand this and I know you will reply on the videos but can you tell me if you actually worked on Merlins (or just study them), did you ever experience or observe the negative G effect and do you feel that an engine that misfires in level flight would ever have made it onto the Spitfire?

The vast majority of us can only use three ways to assess this: Factual official reports (hard to find), common sense (would it ever have been fitted like that) and first hand reports in biographies which, in the dozen or so I have read including Geoffrey Quill, Alex Henshaw, Al Deer, Johnny Johnson etc, only ever refer to it when pushing over into an aggressive combat dive.

Can you also explain the part about it also affecting the inlet manifold please?

I'd be truly interested to know what you think.

It's probably worth pointing out that Sternjaeger will never have experienced the Merlin engine at combat power or indeed emergency boost because unfortunately (or perhaps very fortunately, we don't want to destroy every Merlin on the planet ;) ) no one does that anymore for practical reasons...this being the condition at which the cut-out is least severe in its onset.

Klem, it's understandable and I'm well happy to explain things, so no worries!
My relatively modest knowledge of Merlins is because of my involvement with a warbirds association, I don't feel like giving too many info about it because I have had a lot of "new friends" approaching me more for my interests and luck than for anything else, so I hope you appreciate why I prefer to keep it confidential.

Anyway, I know three qualified engineers who work on pretty much any mark of Merlin ever produced, plus a pair of pilots who fly with them regularly. As you might understand, the wealth of information I have access to is pretty much blinding. Having said this, I prefer to keep things quite simple, mainly because not all of us are literate in engineering matters but still want to try and understand how things work.

Anyway, back to the topic, the videos I have seen seem show quite a jolly response, but then again if it's on full throttle they are quite spot on.

As a simple reference, the response to the negative G should be same or even less than the throttle response time.

Regarding the inlet manifolds, the answer is pretty easy: mixture, just like air or water, is a fluid, and as such is affected by gravity and G forces.

Check out this video, especially towards the end (and behold of the divine flying skills!) and see how fluids behave in the right situation.

http://www.youtube.com/watch?v=9ZBca...eature=related


now watch what happens to another kind of fluid (a more "organic" one) when under zero then negative G-load

http://www.youtube.com/watch?v=wbOU3l864H0

flying is a fun, fun thing ;0)

I have experienced 100% throttle but not on take-off. It's highly not recommended to use full military power or 100% throttle anyway, again because of the components age concern.

A very quick but effective way to check the health of your engine is checking the lubricant oil for metal particles: they can be microscopic, but if found (often by means of special filters) they're a bad, bad sign.

There was a certain P-51 driver a couple of years ago who had the jolly habit to take off firewalling the throttle: needless to say the cylinder banks didn't want to know about it and an inflight engine failure followed, which fortunately happened close to landing.. the plane and pilot were ok, but the engine needed major (expensive) maintenance costs. Let's not forget that like any other liquid cooled engined, these monsters were meant to perform, not to last ;)

regarding the needle bounce I think it's overmodelled. RPM gauges are connected by flex cable to a reduced gearbox attached on a specific slot on the back of the engine, so they're as accurate as it gets. You can get high frequency vibrations, but oscillations like that means that either your instrument is fooked or your engine is actually doing those oscillating RPM peaks (which is weird..).

The problem in the Merlin carb was the fuel resevoir/float tank.
It's pretty much like the cistern on a toilet. With a float/valve system to refill

Fuel sit's in there and the supply to carb is taken from there. It's done purley by gravity. In negative G the fuel moves to the wrong end of the resevoir and so starves the engine.

Miss Shillings work around was a metal plate with a hole in it that slowed the liquid movement down. It didn't completely get rid of the Neg - G problem but it gave you more time before the fuel emptied from the resevoir

btw, I forgot to tell that my flying experience isn't with early Merlins, so de facto I have never had a cut out with the Merlin. I had it with other gravity fed machines (I once had a very hair raising experience with a Tiggie which left me falling down like a leaf with a dead engine..), but I asked one of the engineers this afternoon and he said that yes, power loss and cut out would be quite abrupt. As you probably know negative G or inverted flight is not recommended on a Merlin anyway because of its lubrication system configuration: you'd have oil coming up and messing up the cylinders and leaving the crankshaft dry. Another advantage that the DB engine had over the Merlin apparently.

A.

+1 ~20 hours for the Merlin III with the 100 octane fuel and +12lbs boost used. :o

Could you ask one of the engineers or the actual pilots if it is modeled good enough?

Prolonged inverted was not allowed on the DB engined 109 either, for the same reason (lubrication system not prepeared for it)

Sternjaeger you said:

"Anyway, back to the topic, the videos I have seen seem show quite a jolly response, but then again if it's on full throttle they are quite spot on. "

So let me get this right in your opinion an Engine cut with entry into a descent as shown in my video where G never gets lower than say +0.8G is enough to induce
engine cut ?

In a previous post you said: "A sustained 0G or a minimum negative G load (-0.1) are enough to interrupt the flow."
In this video we don't get any where near those values .

http://www.youtube.com/watch?v=Py7lf...layer_embedded

An engine cut at 0G or negative G value I wouldn't have an issue with. Here we are positive G throughout.

LONG POST ALERT
Those with limited tolerance for extended, somewhat generalised technical geekery should probably stop reading about now...

It's certainly not ideal, but in the absence of superior material, it's better than nothing.

Thermodynamically, the arrangement of the cylinders doesn't matter. Firing order does if you want to get into some kind of method of characteristics type modelling approach in the induction manifold in order to investigate scavenging etc., but this isn't really important to a discussion of negative g cut behaviour.

The size of the engine has in impact; bigger engines are closer to being adiabatic because the surface area:volume ratio is smaller so the heat transfer losses are less. This is important both inside the cylinders and inside the intake manifold (especially if fuel is not injected directly into the cylinders).

For our purposes, the main impact of engine size is that the Merlin's induction manifold is nice and neat, and would probably give better mixture distribution than is possible for GA boxer engines.

But really, induction manifold scale effects are going to be very much second order.

The main point of interest is the carburettor itself. The main scale effects of interest will be those associated with the float chamber.

Reynolds number scales with linear dimension, whilst one might reasonably assume that float chamber volume would scale approximately with power, and thus float chamber linear dimension would scale with power^(1/3).

So if we assume that the Merlin II and III had carburettors sized for about 1100 bhp, whilst the GA boxer engine is sized for about 110 bhp (ie Cessna 152 powerplant) then the characteristic Reynolds numbers determining the behaviour of the fluid flows within the carburettor will only differ by about a factor of 10^(1/3) which is about 2.15.

Without making reference to design drawings or going into a relatively deep analysis, it's quite difficult to say whether this Reynolds number difference will dramatically affect the flow.

However, I would observe that the chances are that the flow is turbulent in both cases, and therefore my gut feeling is that the qualitative differences in flow phenomena should be relatively small. This is supported by the fact that people have built and run much smaller piston engines without needing to radically alter their carburettor design. Indeed, there's even a very nice scale Merlin out there somewhere:

http://www.youtube.com/watch?v=0xe1LL1IC7Y

What is more likely to be significant is that the larger physical size and mass of the moving components in the Merlin's carburettor will make it easier for inertial loads to defeat stiction.

So a computer model of a Merlin assuming zero friction in carburettor moving parts would be closer to reality than one of a smaller engine.

However, the fact that the smaller carburettors work would tend to imply that the "controlling" forces still tend to dominate. Naturally, the vibration of the engine will tend to help the smaller carburettor to overcome stiction (which is of course the engineering justification for the price of an aerotow being set by the reading on the tug pilot's altimeter; but I digress).

Therefore, the implication is that the biggest difference will be that the Merlin's carburettor float will be less well damped than that of a smaller engine.

My judgement based upon this argument is that a pair of photographically scaled carburettors sized for the order of 100 bhp and the order of 1000 bhp should behave in a similar manner when subjected to reduced or negative g for sufficient time that oscillation of the float is damped. This is not to suggest that their behaviour will be identical; what I'm saying is that they should misbehave at roughly the same g loading.

This sort of simple argument from first principles is far from the ideal way to approach the problem; but I don't have any alternative available to me at present, so it will have to do.
Well yes; I don't think anybody expected Merlins to still be flying 70+ years after the first one came off the line. It's hardly surprising that really really old banks are somewhat delicate.

When I say that the Merlin is a relatively tough engine, what I mean is that you can actually go through the RRHT books and see the records of endurance running at high power, which will show you that in general, by the time a rating was signed off, the engine for which it was approved could probably run at that sort of power for a very extended period (100 hours+) continuously, without maintenance, and not break.

You can see how strong the basic engine is by looking at how hard they push them at Reno (albeit with various mods like V-1710 con rods etc) whilst still reasonably expecting to get through a race distance without failure. It's quite hard to get accurate data because naturally the teams are secretive, but they're probably at least +36 psi/3200 rpm for around 2600 bhp plus exhaust thrust. Which isn't bad for a machine originally designed for an international rating of roughly 1000 bhp. It's particularly impressive that the reduction gears stand up to it...

Of course, the early engines that we're dealing with in CoD were rather less robust than the later ones because they hadn't been subjected to such intensive development.

Cooling leaks were a continual problem, and were one of the main motivations for switching from pure glycol to 70% water 30% glycol. Of course, this is often spun in less intelligent history books as "allowing engine temperatures to be reduced by 70º C, greatly improving reliability", but what it really means is that the reliability was poor, so lowered the coolant temperature and accepted the drag penalty of bigger radiators rejecting heat at lower temperature, greatly reducing the scope for exploiting the Meredith effect (i.e. running the radiator as a low temperature ramjet; a peak cycle temperature of 393 K rejecting to about 250 K gives much less useful work potential than 463 K rejecting to 250 K).

You'd hope that the stuff going through the intake manifold was hot air plus gaseous fuel, so you've got a mixture with a reasonably low density (<2 kg/m^3 for an early engine below FTH). This is going to be far less susceptible to the effects of g loading than liquid fuel of density c.720 kg/m^3.

The g is more likely to have an impact upon the intake manifold during the rich cut if droplets of liquid fuel persist downstream of the supercharger, but since we know that the initial cut is a lean cut rather than a rich cut, it seems reasonable to relegate this to the status of a second order effect.

There might be a bigger impact for a naturally aspirated engine, because they tend to be much less good at mixture distribution, and can suffer from liquid fuel pooling around in their induction manifolds. Obviously this liquid fuel will tend to mitigate any lean cut caused by misbehaviour of the carburettor.

You're on. :grin:

Sustained zero g means that the position of the float is undefined. This means that if the system would work at small positive g, then it should keep working at zero g because a body at rest remains so until disturbed by an outside force or influence.

OTOH, small negative g redefines the system "upside down", and so misbehaviour should result as soon as that negative g becomes sufficient to overcome stiction.

However, it's not immediately obvious to me that the system should work at small positive. "Modern" GA piston engines seem happy enough for a few seconds at zero, but unhappy under small negative g, which would tend to support this model.

But OTOH, the video of the Hurricane flying in The Battle of Britain film intro I posted earlier looks more like reduced positive than genuine negative to me, and AP2095 posted by IvanK imply that there might be trouble at +0.5 g.

So I don't know. It's quite possible that even the humble GA piston engine has benefited from WWII experience and now has a zero g capability not found in the first generation Merlins. Or it could be that the Hurricane in that video I posted actually did push into negative (in which case it was a pretty bad barrel roll...).

My approach to the problem needs more information in order to proceed, specifically, design drawings or cutaway drawings of both the original Merlin SU carburettor on the one hand, and of a representative carburettor fitted to a modern GA engine on the other, so that we can then decide whether or not, based upon the logic I have outlined above, the GA carburettor's reduced and negative g behaviour might be expected to be broadly similar to that of the Merlin's SU or not.

As for the characteristic time for an engine cutout we can say several things.

The volumetric flow capacity of the Merlin flat out is roughly

27 litres * 3000 rpm/120 litres per second = 675 l/s.

For the sake of argument, let's consider a Merlin II at about 17000 feet and +6.25 psi boost.

We can assume that the supercharger has an isentropic efficiency of something like 67%. Ambient pressure is about 52.7 kPa and ambient temperature is about 254 K. We shall neglect intake ram effect because life's too short.

+6.25 psi boost = 20.95 psi absolute ~ 144.4 kPa

Supercharger pressure ratio is therefore 2.74.

Isentropic temperature ratio is therefore 2.74^(0.4/1.4) ~ 1.33

Isentropic delivery temperature is therefore 338 K. Delta T is therefore about 85 K (I just rounded up). Assuming constant Cp, which isn't unreasonable for this sort of temperature in a back of envelope calculation, this means that the real temperature rise is

85 K/0.67 ~ 125 K

The absolute delivery temperature is therefore about 379 K.

We want to know the density at compressor delivery. Since I'm not about to break out the really heavy computational firepower to do this properly, the simplest way I can think of doing this is to treat the real compression process as 2 processes, namely, isentropic compression and constant pressure heat addition.

So the isentropic delivery was 338 K and 144400 Pa.

From the ideal gas law we know that V2/V1 = (P2/P1)^(-1/gamma)

Ambient density is 0.72209 kg/m^3 at 17000' on a standard day.

Specific volume is 1/roh, therefore V1 = 1.385 m^3/kg. Thus V2 = 0.674 m^3/kg and therefore roh2 is 1.483 kg/m^3.

Now we add heat at constant pressure;

V3/V2 = T3/T2 = 338/379

Therefore

roh3/roh2 = 379/338 = 0.892

Thus roh3 = 0.892*1.483 kg/m^3 = 1.323 kg/m^3.

(So we actually haven't won a great deal compared with standard sea level density, but at least we're getting it at 17000'...)

Since we already know that the volume flow rate is 675 l/s, we now know that the mass flow rate is about 0.893 kg/s, which gives an hourly air consumption of about 3214.5 kg.

Cross-check with the known fuel flow - 89 imperial gallons is about 404.6 litres, so at specific gravity 0.72 it has a mass of about 291.3 kg.

Therefore the FAR is about 0.09, for an air:fuel ratio of 11:1, which is pretty much bang on given that we'd expect to be considerably rich of peak.

This separation of air and fuel is based upon the assumption that the fuel is very much denser than the air, and that most of the mixing takes place in the supercharger and intake manifold. It's rather quick and dirty stuff because IRL you'd get about 25 K temperature reduction from fuel evaporation, but then if you start accounting that the compression process isn't really adiabatic anymore and the whole thing becomes too much like real work...

We now have a reasonable idea of what's going on in the intake manifold.

The next stage is to consider how long it takes for a change in FAR to impact the engine.

The induction manifold is about 2 m long. Given dimensions we can work out the approximate steady flow Mach number needed to pass the calculated total mass flow, and hence the time taken for the charge to reach the last cylinder.

From these pictures we can infer that the intake manifold is about 3" internal diameter, which for the sake of argument we might as well call 7.5 cm. So the area is about 0.0044m^2.

Since the volume flow rate has already been defined as 0.675 m^3/s, the steady flow velocity down the pipe must be about 153 m/s, which is around Mach 0.4, which is within the realms of reasonable expectation.

The time to deliver charge along the manifold is about 1/75th of a second; say 1/150th of a second for the average cylinder.

The engine is turning at 3000 rpm. There are therefore (1500*12)/60 = 300 induction strokes per second.

So the time taken for the charge to travel along the induction manifold is somewhat longer than the interval between induction strokes for all but the rearmost cylinders.

Whilst 1/75th of a second may sound like a very short interval, it's worth pointing out that a lot of people will be aiming for that sort of framerate once the sim is patched & they've bought their shiny new upgrade hardware; and that's before we even consider the possibility of multiple FM frames per visual frame (which is a technique used in at least one other sim, i.e. X-Plane).

So, just as we have an animation showing individual exhaust pulses, it wouldn't be unreasonable to model this sort of time delay between the mixture starting to lean out at the carburettor and the FAR seen by the engine starting to lean.

However, it is likely that the longest lag in the system will be associated with the carburettor itself, which is not the sort of thing I'm equipped to analyse; as a thermodynamicist, I'm not not really a master of the dark arts that are the fluid mechanics of multi-phase flows.

(Ok, so quite a lot of the thermodynamics wasn't really necessary, because I guess I could have just inferred the induction manifold speed from the dimensions and the volume flow rate, but I was distracted so I hit it with an excessively large computational hammer; it seems a shame to delete it all, now, so I've just left it in for the amusement/horror of the assembled company).
They push them much harder than emergency boost at Reno...
http://www.youtube.com/watch?v=iBqpn9VXek8
You can even see how much power they pull for takeoff and the run in; but unfortunately the race power settings are censored.

However, 2800/55" Hg for 257 IAS/318 TAS should tell you that he's not exactly sparing the horses during the racing laps, given that power required is roughly proportional to v^3.

Very roughly, if 2800/55" is 1500 bhp, and that's giving 318 mph TAS then the race power to average 440 mph TAS would be about 3900 bhp. :shock:

The highest power cleared for ground running during WWII was about 2600 bhp, achieved by the RM17SM engine at 3100 rpm, +36 psi with ADI.

Of course, the Reno guys will never see a negative g cut, but that's not because they coddle their engines!
I believe that in most fighters the recommended takeoff power was about +12/3000, which will give around 1200 bhp depending upon MS gear ratio, which should be more than enough given a sensible runway and no war load.

I would have though that in general it would be safer to use less power in order to mitigate the handling issues, because most people wouldn't try to operate their extremely expensive warbird out of a length limited airfield.

That's a whole lot of horses:
http://www.wwiiaircraftperformance.o...ang-IV-ads.jpg
And even the wartime manual suggests that full chat isn't a good idea for takeoff...
http://www.wwiiaircraftperformance.o...ilotsnotes.jpg

I suppose this is another example of the sad fact that many people have more money than sense; I mean, if I was going to fly a Mustang at high power, I'd wait until it was going fast to start bending the throttle in order to maximise the benefit and minimise the cooling and handling worries.

Most engines don't like extended inverted running; generally the limit is less than about 30 seconds of sustained negative g unless the engine & fuel system are specially designed for the job (i.e. in a dedicated aerobatics machine).
+1. I can't really think how the bounce could be that bad unless something was broken, especially since you can rule out a bad CSU in the case of the Spitfire I with its 2 position prop, implying that either there's continual engine rpm variation or an awful instrumentation problem...

The other possibility is that the prop mass is out by an order of magnitude and there therefore is far too little angular momentum in the system to smooth out engine rpm variations. It just depends on how complex the underlying model really is I suppose...

[/procrastination] (though actually this little lot was mostly written whilst waiting for code to run; which of course is why it's ended up getting so massive; the real procrastination has been tidying it up at the end...)

Fascinating post thanks for the effort in writing it.

@Viper 2000, I am both amused and horrified. Thank you for the post.:)

+1 ;)

Viper, you need to get out more mate, think about all the hot birds you could have met at uni whilst the code was running :mrgreen:

on a serious note, yes, your calculations are somewhat interesting (if I can suggest something, try and keep it simpler..), but not conclusive :???:

two things though:

a) don't take Reno Racers into account, those folks really have more money than sense and 1) have custom built cylinder cases, pistons, piston rods, valves and modified superchargers (their special mixtures+the very low altitude means that the mixture gets VERY hot, and the intercooler of the two stages supercharger can struggle to keep things to the right temperature - the rule of thumb with Reno Racers is "don't believe in what they say, they play a lot on the "secrecy" thing..") 2) run on "special" fuels. It's fascinating but somehow appalling to see them do what they do with those machines..

b) not always warbirds owners fly their own planes. And when you're a pilot who's flying a £2mln warbird that doesn't actually belong to you, you might get a tad too jolly on it.. another important thing to take into account for modern warbirds is that most of the heavy stuff (old radios/batteries, central fuel tank, armour, ammunition and armament) is gone, a slimming process that makes the thing sensibly better in terms of perfomance.

Repost of post 41 for your comment Sternjager:

Sternjaeger you said:

"Anyway, back to the topic, the videos I have seen seem show quite a jolly response, but then again if it's on full throttle they are quite spot on. "

So let me get this right in your opinion an Engine cut with entry into a descent as shown in my video where G never gets lower than say +0.8G is enough to induce
engine cut ?

In a previous post you said: "A sustained 0G or a minimum negative G load (-0.1) are enough to interrupt the flow."
In this video we don't get any where near those values .

An engine cut at 0G or negative G value I wouldn't have an issue with. Here we are positive G throughout.

if you never get lower than +0.8G then no, it's not correct, but you need to be sure of the G-load value mate.

just had a word with one of the pilots, who confirmed me that as soon as you hit 0G with a float carburettor it's enough for the engine to falter or cut. He said the Harvard does it regularly (I never experienced stalls or 0G manouvres with it so I couldn't tell personally).

Ok thanks for that, then based on that 0G is significant. If we accept 0G as the point at which the coughing starts then what we have now is GROSSLY over done.


I have a pretty good idea of 0G and the pitch rates required to achieve it in high performance aeroplanes and we are getting nowhere near it in COD before the engine starts coughing.

It does seem very strange to me that the Merlin installation, lacking fuel injection as it was, though intended for an aerobatic use, is less robust to negative G than either a Prewar Matchless scrambler or the pilot's MG TA.

I can keep an MG TA for negative G for about2 to 3 seconds without it cutting out. The lube system is much more sensitive than the carbs, as merely throwing it hard into a roundabout can get the oil thrown to one side of the sump, but while I've heard bearing knock on round abouts and good hump back bridges, it's never had metering problems.

Of course it's a completely spurious comparison, but nonetheless odd that a machine designed for 2 D is more robust in this matter than a machine designed for 3D.

I cant even set up trim to fly level without it nearly cutting out :/

..erm, you got me a bit confused mate, how can you exactly put a car into negative G load? :confused:

lol

the weirdest comparissons are made in this thread to find arguments to convince the developers to tune the favorite plane.

If this happens for blue side I instantly see 10 posts calling s.o. Luftwhiner.

Please get rid of the Blue v Red comparison for once... This thread is inteded to find out if a.) the devs did it right or b.) the devs got it wrong and it needs fixing.

Just like many others in here, I am under the impression that this is meant to be a Sim.

You can read some very bad comparisons in this thread.
My take on this is the attempt to improve the FM of a favored AC, no matter if the comparison relates or not.

Anyway I just had to smile.
Just ignore my post.

I´ll listen to what Sternjaeger has to say.

In the official mind, interceptors go up, shoot a bomber or two, come back down, re-arm, refuel and repeat.

Or words to that effect.

If you just think of the interception task, and assume that the target is a cooperative bomber flying in a straight line, you really only need 1 g straight & level, plus axial acceleration/pitch changes to get the job done.

Is this silly? Of course. But if the people designing the aeroplanes have never flown them then it's unrealistic to expect them to imaginatively embellish the specifications given to them by the man from the Ministry, especially since they probably wouldn't be thanked for it anyway.

Or to worsen the FM of a hated AC, in this case the door does swing both ways..but you are quite correct.

Nothing speaks louder more than documentational fact as opposed to opinion.

yes and no... it depends on the nature of the documented fact and the opinion.. ;)

Beta patch seems to improve the cutouts. 0G<Instant G<1G = no cut out, instant G<0= cut out, as far as i can say without ingame gmeter.

Oh absolutely it does, but having documentational evidence puts the ball squarely into the other court and says "there you go fella, let's see what you can do with that."

I think the RAF\Supermarine ran negative G tests sometime pre-war at Martlesham Heath or Boscombe down. I truly believe the answers lay there in those records.

Crest of a hill...? Or, roll it.

I still believe that better than papers and talk it's the real deal that solves any doubt. Even papers, made by men, could have been "retouched" in favour of this or that person/situation etc.. (think of aerial victories reports..),whilst first hand experience does hardly lie at all..

Having said this, it'd be interesting to read these records.

Oh Gawd,

this is getting more surreal by the minute.

Common sense guys, do you really believe the MoD/RAF would have put it's most cutting edge fighter into service with an engine that persistently misfires in level flight with its normal turbulence and undulations.

Push-overs, yes, that's well reported on by the people that flew them but problems in normal flight? None that I have ever read and lets be honest no-one here has ever pushed one of those Merlins into negative G, you all only have your various written references to go by and so far I don't think anyone has found a report of the time or operation of those engines that says normal flight is a problem. Lets stay away from conjecture, guesswork and revisionism.

So, feet back on the ground (no pun), the model is too sensitive in normal flight, I leave it to the devs to determine where negative G effects should have an effect but atm its just plain daft.

Bear in mind that Hawker and Supermarine were ahead of the MoD on aircraft development, both producing what they thought was best and better than the basic MoD specs of the time. Imaginative embellishment is precisely what they did to the gratitude of the MoD. Hawkers had all their experience of the Hart/Fury line from which the Hurricane directly descended (some of those had Merlin engines, do you think they never hit negative G in their life or development?) and Mitchell had all the Se5/Se6 development behind his work on the Spitfire.

Added to that, these aircraft had to be passed fit for purpose by the appropriate authorities, from normal flight to combat.

Is the issue about believe or about knowledge?

Its about an absence of knowledge and the application of common sense.

I hope this is not going to be a sim of common sense.
Common sense is very often far off reality or truth.

Anyhow we have people like Sternjaeger and basically it should not be too hard for the developer to pick up the phone and call Tangmere and ask questions.

I have just been talking to a friend who is current on Spitfire MKVIII,XVI,P40E,P40F and P51D and the Harvard/T6, Wirraway and Hudson. His day job
is an airline pilot, his Warbird flying is with the Australian Temora collection and also flies for a number of Warbird owners. Below is a summary of the conversation.

The discussion was to determine -Ve G effects and cut outs and RPM fluctuations and how they compare to what we have in COD.

COD RPM FLUCTUATIONS
On RPM Fluctions he was quite adamant that in the Spitfires which are running Hydraulic Constant Speed units the RPM is "Rock Steady" throughout a display. Now the MKI,MKII and Hurricane both have Hydraulic CSU's that are pretty much the same as those fitted to the MKVIII and XVI. The bottom line is we shouldn't be seeing ANY of the fluctuations we see at present.

Interestingly he made the comment that the Merlin powered P40F (currently the only one flying in the world) that has an Electric CSU is not so stable. In a typical display with fixed prop lever position he says the electric CSU is slow and this results in RPM fluctuations of around +-150RPM over 2-3second period. The Allison P40E he flies has an Hydraulic CSU and like the Spits is also rock steady.


COD -Ve G CUT OUT
Now obviously the Spit VIII and XVI don't have an issue with -Ve G but the Harvard he flies does. It suffers similar issues that the early Merlins do. I asked him at what G value does the engine start to cough ? The answer was 0G. As long as the G is positive he said there were no issues. Normal flying is unaffected by the cutout phenomena. This is the best level of information we have at present. This is also backed by a separate reference from a UK Harvard pilot spoken to by sternjaeger and referenced in an earlier post. It bolsters the position that what we now experience is too sensitive.

I post this here for info.

I work at Tangmere as a volunteer, I'll go through our library again. I'll also ask around and talk to a friend there that maintained the Grace Spitfire (not a MkI/II) but I think IvanK has already covered the 'AFAIK' aspect. What I really need is a Hurricane MkI veteran to walk through the door :)

Klem that would be great! And yes, meeting a BoB veteran would be great, but be prepared for some disappointment: after many years memories are not that sharp anymore :( I met a WW2 luftwaffe pilot who had confused memories and apologised saying "people expect me to talk about those days like it happened yesterday, but it was just 5 years over 75 of my life, SO many other things happened!"

On a semi related issue (very semi!), I need to have a chat with you sometime about the museum klem.

Your "Wing's" name arose a week ago - a telephone call?
pm me.

That's a very over-simplistic attitude to take.

Firstly, people tend to have pretty imperfect memories. That's why we write things down. Very few people will remember precise details about aeroplanes that they flew 70 odd years ago.

I last flew a Bulldog about 10 years ago. I can remember the numbers which were most important to me; climb at 80 knots, loop entry at 140, stall turn entry at 120, VNE was about 200 knots, but you'd only get that going downhill very steeply (at which point, if you're 17 and will live forever, puling as hard as you can will get you about 6 g - "do what you want - it's going to the scrapyard tomorrow!").

I can remember that it used to be happier to stall turn one way than the other, but I couldn't honestly tell you which way was the easy way.

I couldn't tell you the engine limits off hand; there's a tendency to just remember that green is good and red is bad, so if it's green it's good and that's it. I know it'll handle negative g without complaint, because I did it.

The stall was well mannered, but I couldn't tell you the stall speed because I wasn't interested in stalling it; I wanted aerobatics. Anyway, there was plenty of buffet to warn you if you were close to the edge of the envelope.

I landed it quite a lot of times, but I can't even remember the speeds for that - 70 knots on approach? Probably a bit less over the fence and then you're not looking at the ASI anyway...

That's 10 years ago - ask me again in 60 and it'll be a wonder if I can remember what a Bulldog even was!

Secondly, it's not even as though you can just talk about "a Hurricane". Even in 1940 there were a heck of a lot of potential mod states knocking around.

The sim is seeking to model the performance of a couple of Hurricanes in defined mod states. Information relating to a different mark or mod state won't necessarily read across.

Of course, this observation equally reads across to current flight experience. Nobody operates Hurricane Mk I aeroplanes in their 1940 mod state anymore.

The average warbird knocking around has a civil Merlin (500 series for the single stage engines, 700 series IIRC for the two stage engines, though quite often you'll also see single stage engines retrofitted into aeroplanes that would originally have had two stage engines), or a transport command merlin (T.25) rather than an authentic fighter Merlin, because the latter offer considerably longer overhaul intervals. Most of the time they have FS gear disabled because they're only interested in low level work at airshows; hence the unfortunate tendency to retrofit single stage engines into aeroplanes which would originally have had two stage engines*.

It can actually be very difficult to work out exactly what engines and mod states current warbirds operate in, because the pilots often don't much care, the PR people haven't the slightest idea, and the engineers are generally too busy. In fairness, it's not the sort of question that they'll generally be asked...

///

In other news, I have been experimenting with the Rotol Hurricane this evening using the latest patch, and it's crazy. I get black smoke and misbehaviour at even slightly reduced positive (like 0.9ish g I guess; not much point getting out the stopwatch and calculating g from pitch rate and TAS; the rates would be too low for the results to be sensible). It's set off by turbulence, and you'd probably also get it from a gently phugoid (which would probably be the most repeatable way to test & quantify it; trim for say 200 mph IAS at 10,000 feet and then pull to x mph IAS slower than the trimmed speed and release the stick - naturally this this will generate a stick-fixed phugoid given the nature of the sim's modelling methodology - anyway, a given value of x should correspond to a fixed g load for the first cycle, and this is probably repeatable to smaller g increments than directly hand-flying a pushover).

The cut behaviour is obviously wrong, quite apart from the fact that it's on a hair trigger, because we're straight into rich cut with no preceding lean cut.

*Of course, a single stage engine will give more bhp at any given boost level because less power is consumed driving a single supercharger stage than driving two. Single stage engines are also lighter. So if you only want up to about +18 psi at low level then a single stage engine will deliver more performance, especially since single stage engines are obviously lighter, and the lack of an intercooler/aftercooler means that you've instantly gained a load of extra radiator if you're prepared to plumb the intercooler/aftercooler radiator into the main loop...

Viper,don't forget Packard Merlins on Mustangs! :-)

You'd be surprised to see how many warbirds owners and operators tend to go for an accurate engine selection: the third generation of warbirds owners do any possible effort to have their machines to wartime specs,down to wirings,radios and equipment (working gunsights are a must!).

Same goes for superchargers: the era of non working superchargers is gone,pretty much everyone is going for working ones.

It's all down to how deep the pockets are and availability of parts.

Times have obviously moved on! In that case, mine's a Dh-103 with real Merlin 130/1s please (since Eric Brown gave it such a glowing review). Or an MB.5, perhaps modded with spring tab ailerons...

I'd also love to see a Tempest V with Napier Sabre, because despite the huge reliability issues it would be such a wonderfully different sound! But my true love is and remains the English Electric Lightning... *sigh*...

As for the whole Packard thing, I rather like the original Mustang X with Merlin 65; just a pure scrapheap challenge job by the installation department, but it gave quite a good showing against the vastly more elaborate & expensive P-51B/V-1650-3 combination; AFAIK a few examples even flew on ops. But that's a really obscure machine. You know they also generated performance curves for Mustang + Merlin XX series? It would have arrived earlier and presumably served as a Hurricane replacement (since there would then have been no supply of XX series engines for the Hurricane line). But then Packard made Ford's refusal to play ball irrelevant and the idea got parked.

Did anybody ever compare the behaviour at low g of the Spit and the Hurri? My impression is that the Hurri is less sensitive though still quite sensitive.

I too think it is a bit overdone. The slightest dip will lead to a significant performance loss.

I do understand that when you carefully push the stick this will reduce the lateral g from 1g to something like let's say 0.9g or 0.8 g which will lead to reduced hydraulic pressure at the fuel outlet of the fuel tanks hence reducing the fuel flow hence leading to a leaner mixture. Cut-out only should happen when the total pressure at tank outlet (including hence the hydraulic pressure) is equal or less to the necessary pressure for combustion in the cylinder heads plus the pressure losses in the feed lines. Pressure losses are a function of fuel flow and decrease with decreasing mass flow rate as the flow velocity decreases. I though do not know at which g this could happen.

Up to now I just wonder why there was no mechanical blocking of the forward movement of the stick as even levelling out from climb is extremely tricky.

Just to get me right: If it was historically this sensitive I wish to keep the effect. Currently I have some slight doubts.

Your misunderstanding as to why the engine cuts out during negative G maneuvers.

Naw naw. I understand why the engine cuts out under NEGATIVE g manoeuvers.

The thing is I don't understand why any slight push of the stick should lead to negative g :)

You only get negative g when the total acceleration acting upward exceeds gravitational acceleration in horizontal flight. I don't see why this should happen at any slightest forward move of stick even in horizontal flight ... but perhaps you can enlighten me.

I hope to get into the library this week but please don't hold out too much hope on that as if there were a publication I'm sure someone would have popped up with it by now. The chances of bumping into a Hurricane pilot are extremely thin!

However, on this general subject, a small point for the devs:

To confirm the reversed Rich/Lean problem, if you open an info Window you can see that the Hurricane lever in the rear 'Rich' position is giving 0% Rich and in the forward 'Lean' position is giving 100% Rich. You get the same results whether you are using an Axis or Keys for your mixture control. I assume that is what is feeding into the simulation and its not just showing my Axis position.

On the Rotol prop Hurri FT-N, when you close the throttle it pushes the mixture lever to the rearward opsition, i.e. Lean not Rich (remember its backwards) which is not what you want when starting the engine, you want Rich, and I have to move the Throttle forward to around halfway up the Rich-Lean gate so that I can move the mixture lever out of fully Lean to start the engine. My point for the devs: Strangely, when I then close the throttle the info window shows the mixture doesn't change until I give my axis a slight tweak when it goes to 0%. Just another tweak needed when they can.

Rich for the Spit should be forward and Lean backwards. It just reads the wrong way in the cockpit model. Physically it is correctly implemented.

Anyone ever play the empire game "battle of Britain" ? In that the cut out was vertually identical to this game.

I'm sorry but that's wrong:

http://spitfiresite.com/2010/07/anat....html/03it_001

also some 'gathered' notes checked over by a Spitfire pilot
http://forum.keypublishing.co.uk/arc...p?t-72714.html

Also this is from the A2A Simulations "Accusim" Spitfire which seems to be a well repsected simulation of the Spitfire MKI amd MKII which particularly models engine behaviour, wear etc.:

Mixture Controls - The Mk I and II have an automatic mixture control which will weaken (lean) the mixture as height is increased, regardless of whether the mixture control handle is set rearward to RICH, or set forward to WEAK (lean). If the mixture control handle is set to WEAK an extra weak mixture will be provided with a 3% drop in R.P.M. DO NOT use the extra-weak mixture at more than +2 ¼ lbs./sq. in. Boost.

As you'll know the Spitfire has two position mixture control, unlike the Hurricane which is adjusted by hand and again the Rear position is Rich.

The actual effect on mixture is reversed.

Mh. I thought to have read in the Pilot's note of the Spit that mixture should be forward for rich. I try to find this passage.

EDIT: You're right. I must have mixed some stuff up. The pilot notes say richt is backwards and lean is forwards. So actually the lever positions in CoD are wrong.

PS: However I know how mixture ratio works for the early Merlin nonethelss :)

Has anyone tried to climb a Spitfire or Hurricane to 30 000 ft? Above 20 000 ft I suffer constantly from misfires and negative g cut outs.

yep me to.

Yes and if you get the Engine Info in an info window you'll see that: 0% is at Rear position (should be 100%), 100% is at forward position (should be 0%). And of course its a real problem on one Hurricane because the Throttle moves the mixture rearward when it closes, forcing it to Lean when it should be Rich.

Here is a response to the question on this issue passed by a warbird engineer friend to a current early model Spitfire and Hurricane pilot in the UK:

Pilot quote:
Basically, the Spit/ Hurri with the standard float carb (SU) has the cut-out issue with a definite unload. The problem does not occur in normal flight or bumpy conditions but, does happen with low +ve G.

Engineers opinion:
He has not explored the limits of the problem as, you
avoid it by rolling and pulling (no suprise!). The limit is "between
zero and +0.5G".

My Opinion:
So in general there is agreement in these comments and the info written in the AP2095 that has been quoted in this thread. To me this confirms that the current setup is too sensitive.A good solution imo would be < +0.25G for more than 0.5seconds results in the cut.

Well, I spent a few hours in the Tangmere library today but did not find any factual data or reports, just some observations from former Spit/Hurri Mk1 pilots, Alex Henshaw (Spit test pilot) and the pilots notes. It paints a picture which seems to be "negative G is quick to affect the engine but not instantaneous and it recovers in a couple of seconds" but here's what I found so you can form your own opinions:

From the cockpit: Spitfire by Wing Cdr T.F. Neill DFC AFC
"it caused the carburettor to flood after the briefest period of negative G"
"engine ceased to pull for a second or two"

Spitfire - The Biography by Jonathan Glancey
"When the Spitfire is thrust into a sudden dive the carburettors would flood causing the engine to cut out."
"the Merlin always came back on song in a matter of moments"
"All this took precious seconds"

Pilots Notes Spitfire IIa and IIb
Merlin XII
"Inverted flying. This is Normal"
"A moderately slow roll is best as the engine can be kept running normally...best if slight barrel roll... if engine shows signs of beginning to fade the stick should be brought back a little, almost imperceptibly"

"True Slow Roll* This can be done if high speed is used at the start but the engine will cut out when inverted. If the engine is throttled back** as the roll is started it will be possible to get the engine started again earlier in the final stages of the roll."

*In opening sequence of the film "Battle of Britain" (is that a "True Slow Roll"?) if you look carefully the roll is begun with an upward pitch and a slight barrel element iaw the Pilots Notes. From the moment the lift vector ceases (inverted) there is about one second before engine response and about two seconds after rolling out before it picks up again.

** Presumable reduces flooding

Sigh for a Merlin by Alex Henshaw
"I would open the engine flat out in a vertical climb and at approximately 1200 feet push the nose over forward and with the engine closed complete the half of an outside loop... usually round off to a few feet above the ground *** ... push the machine into an almost vertical climb.... then pull the control gently over to form a half loop, hoping as I did that the engine would burst into life"

***(klem:inverted)

There are frequent references to diving in pilots notes, Jeffrey Quill's and Alex Henshaw's books etc with no mention of engine problems. I expect the severity of the dive would have had some influence, perhaps a low -G or reduced G pushover to a sustained dive would allow the Carburettors to keep up? Some of these dives achieved very high speed and were quite steep.

There is a time element to onset but it's hard to quantify and almost certainly related to the severity of the pushover as the floats and fuel rose at various rates vs reducing G value. A sudden severe pushover would presumably have had the floats and fuel wanging up in the float chambers.

The recovery or catchup appears to be a matter of only a couple of seconds once more normal G values are recovered.

No it's a barrel roll.

A slow roll involves keeping the aeroplane flying straight (not quite the same as pointing straight) by using rudder and elevator as required. You therefore see -1 g when inverted.

A barrel roll is a 1 g manoeuvre if flown correctly; if your name is Bob Hoover then you can demonstrate this by pouring iced tea backhanded as the aeroplane rolls inverted. Normal human beings should not attempt this sort of thing unless they are confident that they can:

1. Still fly the aeroplane after iced tea goes everywhere and shorts out the electrics and,
2. Run faster than the person responsible for maintaining the aeroplane after landing...

To execute a barrel roll, you trim the aeroplane for 1 g at your target speed, pitch the aeroplane up to a certain angle (which is a function of your TAS), reached at target speed, and then roll with the elevator neutral. Since the aeroplane is trimmed for 1 g, you should get 1 g all the way around. The nose will drop, and if you selected the correct pitch attitude then you should roll wings level with the nose in the correct attitude for level flight. The only g the airframe needs to see is that associated with pitching up to the entry attitude.

If your aeroplane has a very slow roll rate then you'll need to either retrim during the manoeuvre or else apply some elevator to maintain 1 g as the aeroplane departs from its trimmed speed.

During the BoB movie sequence, the pilot conspicuously fails to maintain 1 g all the way around the manoeuvre. This was probably deliberate as the cut was intended to demonstrate the incompetence of his character, because flying a barrel roll properly isn't exactly rocket science. I therefore suspect that he deliberately pushes forward on the stick to induce the cutout.

Also, remember that the cut is a 2 stage phenomenon:

1. Lean cut as the fuel flows away from the uptake point and pools elsewhere in the float chamber.
2. Rich cut due to the float rising away from the fuel level.

The rich cut can happen either as part of the recovery from #1, or else almost immediately if given sufficient negative g with sufficiently rapid onset (in which case the engine doesn't notice the lean cut before the float chamber completely fills with fuel and the rich cut happens).

In simple theory, the lean cut shouldn't happen until g <0 because Newton says that a body at rest remains so unless disturbed by an outside force or influence, and therefore the g would have to be slightly negative to move the fuel away from the uptake point.

However, it has just occurred to me that in reality, the cut could happen earlier because the fuel is being sucked through the pipe into the venturi.

The pressure of the fuel at the uptake point is ambient static pressure + gz, where g is the local acceleration and z is the height of the column of fuel. above the uptake point.

As g tends to zero, the pressure at the uptake point tends to ambient static.

Depending upon the suction at the uptake point, and the vapour pressure of the fuel, it might actually start to cavitate, which would obviously greatly reduce the mass flow rate passing through the uptake pipe.

This would provide a mechanism for lean cut at 0<g<1.

Note that the "fade" mentioned in the barrel roll case is due to lean cut.

The rich cut is caused by incorrect float position, and wasn't even partially solved until the RAE restrictor was introduced. This was sized for the combat power case however, so rich cut would still happen if the engine's demand for fuel was such that it couldn't handle the full combat power fuel load. The real fix was to redesign the carburettor.

I don't know if you can really infer much from the dives to VNE mentioned by Henshaw, because if the carburettor was a problem then he would just have half-rolled for the entry.

Also, there wouldn't have been much need to pull a lot of g on entry anyway, as the Spitfire could fly high enough to provide quite a lot of space and the objective of the early dive testing was to hit the Q limit not the Mach limit.

If you look at the g history for a later transonic dive (where there was a greater need to expedite entry due to the need to get high TAS at high altitude) you'll see that it was quite possible to dive very steeply without ever seeing negative g:

http://www.spitfireperformance.com/sd2011.jpg

The catchup time is that required to pass the excess fuel through the engine and resume normal FAR, plus however long it takes for the engine to get back up to speed. It therefore depends upon float chamber volume and engine inertia. The latter was probably more important than the former due to the massive angular momentum of the supercharger.

[QUOTE=Viper2000;261610]If you look at the g history for a later transonic dive (where there was a greater need to expedite entry due to the need to get high TAS at high altitude) you'll see that it was quite possible to dive very steeply without ever seeing negative g:

QUOTE]

Thanks for that Viper.

I'm really just a layman but looking at the dive figures it seems that quite a high rate of descent can be achieved in quite a short time without even hitting 0.5G.

The average rates of descent over the 2.7 sec periods are in the order of (without splitting hairs) and against g values
1........0.82.....0.65....0.7.......0.42.....0.28. .....0.29.......0.54
0........-2667...-2000..-2000...-5333...-12000...-15778...-14444

The g values seem quite benign and I did wonder if the g figures were variations in g value (although I would have expected them to be -ve values) which would have made the actual g values
1...0.18...0.35...0.3...0.58...0.72...0.71...0.46. ..0.56
That would take us into the questionable g territory for engine cutout at the beginning of the dive, perhaps less than 0.5 as is being speculated.

Anyway, thats even more speculation :)

I don't know if this relevant or helpful but there's a section about negative G cut outs in the RAF Pilot's Notes General from 1943 (apologies if this has already been posted);

http://img685.imageshack.us/img685/4564/neggcutout1.jpg

http://img861.imageshack.us/img861/6268/neggcutout2.jpg

http://img849.imageshack.us/img849/3033/neggcutout3.jpg

Negative G - an answer
 

Guys I have been fortunate to get a reply from a current Hurricane MkI display pilot. He asks to remain anonymous but gives me the following:

"I have the privilege of flying and displaying Hurricane Mk1, [serial deleted]. It will not surprise you to know that in deference to it's age and historical importance we do not fly the aircraft as aggressively as it would have been flown during combat. Particularly, we avoid negative g so I am not well placed to answer your question specifically. However, I can give you some clues.

First, I can tell you that it does not require negative g to make the engine suffer from a shortage of fuel supply; a significant reduction of g down to, say, 0.3g can be enough to make the engine misfire. This can be experienced towards the top of a wing-over but I would estimate that the reduction in g needs to be maintained for 2 seconds or more before there are any effects. Undoubtedly, if the reduction in g was greater (to less than zero g) and particularly if the bunt was abrupt then the effect could be instantaneous. I have never, though, experienced any misfiring in turbulence; albeit, were the turbulence severe enough to produce g spikes to less than zero g, I would not rule out the possibility of the odd cough from the engine. Of interest to you I am sure is that on recovery from an episode of fuel starvation the engine recovers through a short period of over-richness shown by, I would estimate, up to a second of black, sooty exhaust before normal combustion is resumed.

Good luck with your simulation."

It's not a complete profile of the problem but it gives some useful check points for whatever 1C come up with so I hope they will use the information for that purpose.

I suppose the thing the Merlin flyers want to know is that the effect isn't overmodelled to an unrealistic and irritating degree in level flight or modest pushovers into what might be called 'normal' descents.

What the Axis flyers want to know is that an aggressive pushover in combat will have the Melin cutting out and preserving their escape maneouvre.

The truth will lie somewhere in the middle but is probably not too critical if the two situations are preserved.

I think what the report tells us is that for modest maneouvres there is little if any effect and for sustained reduced G of perhaps 0.3G there is something like a 2 second delay or more before it occurs, perhaps due to the carburettor trying to keep up on the knife-edge of the problem. Recovery seems to be in the order of a second once positive G is restored.

A question remains, for an aggressive pushover causing that level of negative G that first causes virtually instantaneous fuel cutoff at the carburettor inlet, exactly how fast would the fuel cutoff at the carburettor inlet occur and how quickly would the carburettor fuel be used up and the engine be affected? If 1C were able to decide on a G figure for a near instantaneous cutout they would I suppose still be left looking at the 'curve' for intermediate values and times.

Hope this has helped.

More information
 

Ok guys, several things here.

Firstly, the dive was only posted for the g history; there would have been no risk of cutout because the aeroplane was a PR.XI, which would have had a 2 stage engine, and all the 2 stage engines had later carburettors.

The point was just that you can get the nose down quite smartly without recourse to negative g, which is probably why people didn't immediately realise that a negative g capability was required for a military engine.

Secondly, I've gone digging through my archives and have found some relevant RRHT books.

This means that I can now hopefully shed some light both on the engine ratings and FTHs (which are slightly more conservative than given in some Pilots Notes) and also the negative g cut.

Harvey-Bailey (1995).
I have preserved the original formatting as far as possible, which means that the paragraphs are rather long. I have therefore highlighted some important sections in red so that they aren't missed.

The illustration is of interest because if you remove the modifications then you're back to the standard S.U. carburettor fitted during the Battle.

You can see that the lean cut was caused by the exhaustion of the fuel in the small chamber; whilst the rich cut was caused by the big chamber filling up with fuel and subsequently flooding this small chamber, as well as by fuel leaking through the air line which the mod protected using the ball valve.

Recovery from the rich cut requires that the engine consume the excess fuel in the big chamber. We know that this took about 1½ seconds.

This allows us to make several observations about the behaviour of the system.

1. The maximum rate of fuel pump delivery was about 2.4 times the rate at which the engine could consume fuel.
2. The amount of time taken to flood the big chamber would therefore be about 1.5/2.4 = 0.625 seconds in the worst case.
3. The small chamber is something like half the size of the normal space above the fuel level in the big chamber. We can therefore infer that full power engine demand would empty the small chamber in something like 0.75 seconds.
4. However, under negative g, fuel can also escape the small chamber through the holes that admit it under positive g. Since these holes were at least big enough to supply full engine demand, this would at least double the rate at which the small chamber could empty; therefore the lean cut would be expected within 0.3 seconds of negative g onset.

This means that we can probably safely say that when reduced or negative g is applied, nothing much will happen for at least say ¼ a second, because there is certainly enough fuel in the small chamber to supply the engine for that amount of time even if it's flowing out of the entry holes. And under reduced positive, close to zero g, it will take more like ½ a second.

Once the fuel in the small chamber is exhausted, the engine will start to suffer a lean cut.

However, we have already calculated that the big chamber is likely to be flooded in about the 0.6 seconds.

Once the big chamber is full of fuel, an unregulated supply of fuel will be forced into the small chamber under pressure.

It will take perhaps another ¼ second or so to fill the small chamber, at which point it will then proceed rapidly into the carburettor and cause the rich cut.

So the sequence of events was probably:

ZERO G Onset:

1. t = 0 s, pilot pushes to zero g
2. t = 0.6 s, small chamber empties; engine suffers weak cut
3. t = 0.75 s, fuel flow into small chamber resumes
4. t = 1 s, rich cut begins

NEGATIVE G Onset:

1. t = 0 s, pilot pushes to negative g;
2. t = 0.3 s, small chamber empties; engine suffers weak cut
3. t = 0.75 s, fuel flow into small chamber resumes
4. t = 1 s, rich cut begins

Once the rich cut has started, both chambers are full of fuel; therefore the recovery from negative g would be identical to the recovery from reduced positive g.

Recovery:

1. t = 0, pilot returns positive g
2. t = 1.5 s, excess fuel in float chamber consumed; float resumes controlling

The final piece of the jigsaw is the fact that it was felt tactically advantageous to roll the aeroplane into a dive rather than to suffer the cut.

If you look at the roll rate diagrams here you can see that the worst-case for high speed roll rate would have been about 60º/s, and therefore it would take about 3 seconds to roll inverted for dive entry.

The alternative, of pushing through the cut would result in the engine going on strike for roughly the push time plus 1¼ seconds (because the engine would keep supplying full power for roughly the first ¼ second of the manoeuvre which is therefore subtracted from the 1½ second rich cut recovery after positive g is restored).

If we assume -20 m/s^2 acceleration and constant 150 m/s TAS, since
a = v^2/r,
r = v^2/a = 1125 m
The turn circumference is therefore about 7 km, and so the time to execute a complete outside loop would be about 47 seconds; the time taken to push through 90º would therefore be something like 11.75 seconds, and so the total duration of the loss of power would be about 13 seconds.

So it's fairly obvious that in this sort of situation you'd win by rolling and pulling rather than pushing, because you'd lose a lot of distance in 13 seconds.

The critical case would be a pitch change of about 20º, because at -2 g you'd get there in about 2.5 seconds, for a total cut duration of 3.75 seconds or so, which is of the same order as the amount of time lost in the roll.

This all seems pretty reasonable to me.

The cut duration lines up with the current reports, and the calculated 0.3 second grace period explains the lack of misbehaviour in turbulence. :)

But what about the reduced positive case?
Well, that's been puzzling me for a while, because the float position is still defined by the float position, and therefore it's not immediately obvious why there would be a problem.

But if you look at the diagram, you'll see that the fuel has to flow down through the holes in the bottom of the big chamber into the small chamber in order to supply the jet. The driving force for this is the head of fuel in the big chamber, which is of course gz.

So when g reduces close to zero, the force driving the fuel through the holes into the small chamber is dramatically reduced, and therefore it follows that the flow rate reduces.

If the flow rate is less than engine demand then the small chamber will gradually empty and starve the jet. This explains the fact that it takes such a long time for reduced positive g to induce a cut. Indeed, it implies that if you waited long enough at say 0.75 g you'd probably get a lean cut eventually; it's just that in reality this never happens because people don't fly like that.

Of course, under reduced positive g the float is still controlling. However, because the flow rate through the holes into the small chamber is less than would normally be the case, whilst the pump delivery rate remains normal, the big chamber starts to over-fill. The float moves up and reduces the rate of fuel supply, but the equilibrium under reduced positive g will be a higher float position such that the progressive reduction in fuel flow into the float chamber balances the reduced rate at which fuel leaves to enter the small chamber.

This means that when 1 g flight is restored, there is going to be too much fuel in the system until equilibrium is restored.

This will happen somewhat more quickly than in the zero or negative g cases because the equilibrium point for reduced positive g is reached when the float chamber is only partially (albeit still excessively) filled rather than totally filled.

Therefore the duration of the rich cut recovery time should be expected to progressively increase towards the 1½ second maximum as g tends to zero.

Finally, what about g onset rate?
I've been thinking about this, and I suspect that it wouldn't make a lot of difference. If the sudden negative g was applied then the float would rise at the same rate as the surface of the fuel in the float chamber; this would momentarily cut off the fuel flow into the float chamber. However, as soon as the fuel hits the top of the float chamber, the float will instantly float downwards, re-opening the valve and admitting fuel at the full pump delivery rate. It won't bounce around because buoyancy would just peg it to its stop.

Therefore any misbehaviour is likely to simply be a function of g and duration.

Reference:
Harvey-Bailey, A. 1995. The Merlin in Perspective - the combat years. Derby: Rolls-Royce Heritage Trust.

Hi Viper,

well I think I followed that (amazed).

Just two points.

1. You estimate it takes 1/4 second for the small chamber to refill as it heads towards Rich cut but previously you said the entry holes were sized to permit the full power demand which you felt would contribute an inlet hole emptying under -G in 0.75 seconds (along with the emptying due to engine demand). So wouldn't it take 0.75 seconds to refill the small chamber through the inlet holes as it heads towards Rich cut?

2. I didn't understand the part under sudden -G where you said "as soon as the fuel hits the top of the float chamber, the float will instantly float downwards". Why would the float float downwards when it is being held to the top by the raised fuel surface?

I think your estimates fit in with the info I was given by the MkI Hurricane pilot at reduced G where he felt it did not cause a problem down to about 0.3G with around a 2 second delay before engine response. His rich cut recovery from that he estimates to take about 1 second which is pretty close to your 1.5.

btw for interest and on a parallel topic, the Spitfire MkIa/Ib pilots notes say that it is quicker to make a turning dive onto a target passing below you in the opposite direction than to roll inverted and pull through, presumably because of the fairly slow roll rate. Its not the tail chase pushover case we are talking about but based on that and the earlier barrel roll comments I have found it very effective to barrel or corkscrew down into the dive in a tail chase with the slightest stick-back as it maintains +ve G and I think is quicker than the pushover.

The holes must be bigger than required for max engine demand otherwise the rich cut couldn't happen. The fuel pump is providing fuel at roughly 2.4 times maximum engine demand (2 pumps each sized for 120% of the requirement). I rounded to ¼ second because the 0.75 second figure was also an estimate, so there's not much point being over-precise; in the end the diagram is somewhat schematic, and only shows one of the 2 jets. There's also obviously the float adding depth to the float chamber, so the error bars on the calculation are quite large. But having said that, I think that the results are in pretty good agreement with the available data, so the chances are that it's not a million miles away from the truth.

Once the fuel hits the top of the float chamber, the situation looks like the "Negative G conditions" part of the figure I uploaded; remember that local acceleration is pointing upwards and so buoyancy pushes the float down.

To put it another way, if the float chamber was huge and you were sat in an inflatable boat inside it when negative g was applied, your experience would be:

1. weightlessness, with the ceiling appearing to rush towards you
2. headache from hitting the ceiling!:-P
3. then you'd start floating "up" towards the surface of the liquid again; but upon reaching the surface you'd realise that the floor and ceiling had changed places...

The 1.5 second figure is from RR; it's approximate and it's predicated upon the assumption of negative g rather than reduced positive.

In case of reduced positive g the rich cut won't last as long because the equilibrium is just that the float will sit higher. So there's more fuel than the 1 g equilibrium, but it's not like the negative g case where the float chamber is literally filled to overflowing. As such, recovery would be quicker, because the rate at which the engine can suck away the excess fuel is fixed for any given rpm and OAT (since the supercharger is supersonic and therefore the non dimensional flow passing though its diffuser is fixed if you want to be technical about it).

So the figures line up quite nicely.

In the end, this sort of thing turns into a parametric study because there are quite a lot of factors involved.

But if you're unconstrained by other threats and have the necessary energy then it's probably best of all to loop and roll off the top, because you'll exit the manoeuvre with at least as much energy as you came into it with.

But in the end this is just another way of restating the energy vs angles/ lead vs lag tradeoff, and the best option will always be a function of the geometry.

The bigger the height difference, the more attractive the idea of going straight into the vertical becomes.

Got it :)

For the float I forgot the G had reversed to upside down.

its too sensitive definitely...even a very gentle push forward with the stick and engine cuts...

The documented value in RAE documentation specifically investigating this problem is cutout onset at 0.1G ...."i.e. at accelerometer readings of less than 0.1g"

http://img59.imageshack.us/img59/5658/vegcutfile.jpg

http://img593.imageshack.us/img593/7585/vegcutfile2.jpg

The evolution of the cutout and time taken for recovery is also well documented in AVIA 18/1281 Tests of RAE devices for the reduction of "Negative G" engine cutting on merlin engined fighter aircraft" Though specifically looking at various cutout reduction methods some good info on cut duration and recovery in there with various amounts of negative G application.

Both these documents are available at the UK National Archives. The devs have copies of both these documents.

Hi! what about Tilly orifice? Do you have any data about minimal G with this improving?