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Inverted Flight and Negative G


bart
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The aircraft will happily fly inverted for a sustained period and even do an outside loop without any loss of engine power.

 

Not sure if our Mk IX could do this, if so ignore this! otherwise could be a bug..

 

EDIT

 

I think I may have found my answer.....

 

Another old problem was solved on Mk. VC: the negative G-force cutting the engine. Every time a Spitfire chased an enemy aircraft, which went into a dive by pushing forward the stick, the Spitfire had problems with engine cutouts and consequent power and speed loss. The negative G’s pressed the floats in the carburettor to top of their housing and thereby cut off the fuel and thus starving the engine and after that overdosing the engine with fuel and cause suffocation. So far the Spitfire pilots had to make a half roll before starting the dive. This slowed down the Spitfire during combat. Therefore a Bf109 could with relative ease dive away from a Spitfire, because the Daimler-Benz engine used in the Bf109 was fitted with fuel-injection. Many attempts were done to overcome this problem. The best solution to this problem was to fit a restrictor into the fuel line ensuring that the engine never got more fuel than it could “eat”. It worked quite well, but the problem was not solved until a standard SU AVT40 float carburettor was modified by removing the floats and replacing them with a diaphragm unit. This came as an unpleasant surprise for many German pilots when they discovered that their old manoeuvre did not work any more. The engines fitted with the the new type of carburettor was designated as the Merlin 50-series. A Bendix Stromberg injector-carburettor was also tested, however, it increased the fuel consumption at high altitude without being better than the AVT 40. Later Mk. V’s used an SU injector-carburettor that - depending of the altitude - increased the top speed by 5 mph (8 km/h) and up to 15 mph (25 km/h) compared to AVT 40 carburetted Spits.

 

So our Merlin 66 didn't suffer from this issue then?.


Edited by bart

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The aircraft can withstand negative G, however it can not handle sustained negative or inverted flight.

 

There is no inverted oil system, and no inverted fuel system. So the engine wont cut out immediately but it will after several seconds of sustained negative G or inverted flight.

 

If the engine doesnt cutout from fuel starvation, it will eventually seize from oil starvation.

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The aircraft can withstand negative G, however it can not handle sustained negative or inverted flight.

 

There is no inverted oil system, and no inverted fuel system. So the engine wont cut out immediately but it will after several seconds of sustained negative G or inverted flight.

 

If the engine doesnt cutout from fuel starvation, it will eventually seize from oil starvation.

I thought the ran a dry sump, p.t.o. mechanical pumps, with backup electric pumps both for pressure and scavenging !

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There were 2 issues with the Mk I (and the Mk V?), stemming from the fact that it used a float-chamber carburettor (UK_En)/carburetor (US_En).

 

1. In a negative-G environment, fuel flow through the carburetor to the engine was interrupted, because it was dependent on gravity to cause the flow.

 

2. Once a positive-G environment was reestablished, the float chamber of the carburetor would be over-full and it would flood the engine.

 

The infamous Miss Shilling's Orifice only alleviated the second issue (see the Wiki article), but it did nothing to address the first issue. The first issue was only eliminated when the Merlin engine was fitted with a pressure carburetor, which was not dependent on gravity to move the fuel.

 

In the P-51D you are only allowed to fly inverted (with negative G's) for 10 seconds, because of the lack of lubrication when inverted, as has been stated. With the Spitfire having basically the same engine, more or less, the situation will be similar, if not identical.


Edited by Captain Orso

When you hit the wrong button on take-off

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Our Spit will fly inverted continuously for about 1.5 mins, which it can't do RL. so it is something that needs looking into.

 

Oil press/temp. remains constant

Water temp. constant.

Amber fuel light comes on to stop engine. Roll upright to restart.

No damage.

Continue flight.

..


Edited by Holbeach

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I thought the ran a dry sump, p.t.o. mechanical pumps, with backup electric pumps both for pressure and scavenging !

 

Yup it did, however the external oil sump only has a pick up at one end, the low end right under the engine. The sump is directly under the engine following the cowl line up, the far end just being a cap. Because of the angle it sits on there is considerable air gap inside the sump for the oil to slosh into when inverted

 

Compare that to say a Pitts Special which has a pick up and breather on both the top and bottom of its inverted oil system.

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