Merlin negative G cutout too quick?

[winny]

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.

[klem]

Originally Posted by winny

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.

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.

[IvanK]

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.

[PE_Tigar]

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).

[Viper2000]

Originally Posted by PE_Tigar

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).

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

[Deadstick]

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.

+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.

[effte]

Originally Posted by Deadstick

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 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.

[winny]

Originally Posted by klem

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.

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

[whoarmongar]

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