Merlin negative G cutout too quick?

[whoarmongar]

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

[klem]

Originally Posted by 41Sqn_Stormcrow

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.

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

[41Sqn_Banks]

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.

[fruitbat]

Originally Posted by 41Sqn_Banks

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.

[klem]

Originally Posted by 41Sqn_Stormcrow

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

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.

[IvanK]

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.

[klem]

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.

[Viper2000]

Originally Posted by klem

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"?)

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.

Originally Posted by klem

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.

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:

Originally Posted by klem

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.

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.

[klem]

[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