Merlin negative G cutout too quick?

[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.