Merlin negative G cutout too quick?

[Osprey]

Originally Posted by Crumpp

It was a positive G maneuver most likely and certainly did not have a float carboretor. AFAIK, Spitfires were not outfitted for inverted flight.

Float type carburetor's are not capable of inverted flight as fitted to the Merlin.



http://www.airspacemag.com/how-things-work/upside.html

Did you watch the video? I was referring to the video.

[41Sqn_Banks]

Originally Posted by Osprey

Did you watch the video? I was referring to the video.

Indeed. Positive G maneuver? No way ...

[Osprey]

That's what I thought. And it had power applied too otherwise there's no way it could retain height.

All I would like to know is if this was an injected engine or Ms Schillings Orifice

[klem]

Originally Posted by Osprey

That's what I thought. And it had power applied too otherwise there's no way it could retain height.

All I would like to know is if this was an injected engine or Ms Schillings Orifice

Looking at the video again the engine was not supplying power when inverted, it was windmilling with the occasional detonation. When he was inverted he was almost certainly bleeding airspeed as he went until he rolled right way up and the engine picked up - quite quickly too. Pretty sure its a carburettor not an injector.

[JtD]

The Mk.Va came with a float type carburettor. The Tilly orifice would only have an effect when flying at 12lb boost and 3000 rpm, if you did not do that, it was pretty much meaningless. If you did, however, the engine would continue to work under negative g just fine.

To me it sounds as if combustions stops during the inverted flight and the engine spools back up thereafter, which indicates the behaviour to be typical for a Merlin with a standard float type carburettor.

On the other hand the engine spools up quite nicely (though on a very rich mixture) immediately after returning to positive g's, so maybe a fuel flow restrictor (i.e. Tilly orifice) was in place. The Merlin handbook states up to 10 seconds of recovery after a neg g situation, which clearly isn't there.

[Viper2000]

Originally Posted by cheesehawk

Interesting, cold air induction does have its merits, but never would have thought it mattered what point you introduced the cold. If it made a 25C difference at the supercharger intake, why couldn't it be the same 25C at the cylinder head?

The supercharger compresses the charge adiabatically.

In the isentropic case, (T2/T1) = (P2/P1)^((gamma-1)/gamma)

Isentropic Supercharger work = W*Cp*(T2-T1)

Actual Supercharger work = (Isentropic work)/(Isentropic Efficiency)

Reducing the temperature upstream of the supercharger therefore reduces the supercharger work at fixed supercharger efficiency, and therefore increases the overall efficiency of the machine.

Furthermore, because the supercharger temperature ratio is > 1, it follows that the the temperature reduction in the induction manifold will be correspondingly greater than that due to the evaporation of the fuel alone (though in this case the 25 K figure was measured in the induction manifold, the point is that you wouldn't actually get a 25 K temperature drop from direct fuel injection at TDC).

[41Sqn_Banks]

Originally Posted by JtD

The Mk.Va came with a float type carburettor. The Tilly orifice would only have an effect when flying at 12lb boost and 3000 rpm, if you did not do that, it was pretty much meaningless. If you did, however, the engine would continue to work under negative g just fine.

To me it sounds as if combustions stops during the inverted flight and the engine spools back up thereafter, which indicates the behaviour to be typical for a Merlin with a standard float type carburettor.

On the other hand the engine spools up quite nicely (though on a very rich mixture) immediately after returning to positive g's, so maybe a fuel flow restrictor (i.e. Tilly orifice) was in place. The Merlin handbook states up to 10 seconds of recovery after a neg g situation, which clearly isn't there.

Which Merlin handbook are you referring to? Don't you mean Pilot's Notes General? Standard procedure for inverted flying was close the throttle to allow a smooth recovery.

[JtD]

Not a plane handbook / pilot notes, but a Merlin document, which I can't seem to find at the moment. Sorry.

Good point about closing the throttle, slipped my mind and it would indeed limit the problem, but the fuel pump would still flood the engine to some extend.

[41Sqn_Banks]

The throttle also had to be closed when negativ g cut out occured. IIRC Reason is that when the throttle remains opened and the fuel flow is restored the engine might overrev as power is regain so quickly that the CSP can't react quick enough.

[ZaltysZ]

Originally Posted by 41Sqn_Banks

The throttle also had to be closed when negativ g cut out occured. IIRC Reason is that when the throttle remains opened and the fuel flow is restored the engine might overrev as power is regain so quickly that the CSP can't react quick enough.

Engine starves briefly at first and then gets flooded with fuel, because things get reversed: the more fuel you have in float chamber, the more fuel you get into it. Tilly orifice simply restricted fuel flow to amount engine needs at max power setting, so even when things are reversed, float chamber gets no more fuel than engine can burn (at max power setting). Lower the fuel usage (cut the throttle or lower the RPM), and Tilly orifice won't prevent the flooding anymore. Without orifice things are the same, except engine gets flooded even at max power setting.

Running engine at max RPM and open throttle softens the flooding by some extent, but recovering with same settings can indeed result in overrev. It is probably better to cut the throttle just before recovery and not the moment cut out occurs - this seems more optimal.