Any thoughts on new neg G cutout post beta patch?

[Extreme_One]

I'm no expert, just entering the debate with what I've read and picked up from previous sims.

I believe the engine would cut out if subjected to constant negative G

The early versions of the Rolls-Royce Merlin engine came equipped with a SU carburettor. When these aeroplanes performed a negative G force manoeuvre (pitching the nose hard down), fuel was forced upwards to the top of the float chamber of the carburettor rather than into the engine, leading to loss of power. If the negative G continued, the fuel would collect in the top of the float chamber, forcing the float to the floor of the chamber. This would in turn open the needle valve to maximum, flooding the carburettor with fuel and drowning the supercharger with over-rich mixture. This would lead to a rich mixture cut-out, which would shut down the engine completely, a serious drawback in combat.

This engine cut out no longer appears to be modelled with the current beta patch.

[Wolf_Rider]

^ beat me to it




The early versions of the Rolls-Royce Merlin engine came equipped with a SU carburettor. When these aeroplanes performed a negative G force manoeuvre (pitching the nose hard down), fuel was forced upwards to the top of the float chamber of the carburettor rather than into the engine, leading to loss of power. If the negative G continued, the fuel would collect in the top of the float chamber, forcing the float to the floor of the chamber. This would in turn open the needle valve to maximum, flooding the carburettor with fuel and drowning the supercharger with over-rich mixture. This would lead to a rich mixture cut-out, which would shut down the engine completely, a serious drawback in combat.[1]

Negative G commonly occurs when manoeuvring to fire on an enemy aircraft in a dogfight. Moving the stick forward would starve the engine of fuel, producing a sudden loss of power. This would let the enemy get away, and if continued the maneuver would cause the carburettor valve to open, provide far too rich a mixture and kill the engine. During the Battles of France and Britain, the German fighter aeroplanes had fuel injected engines and therefore did not suffer from this problem as the fuel injection pumps kept the fuel at a constant pressure whatever manoeuvre were made. The German pilots could exploit this by pitching steeply forward while pushing the throttle wide open, the pursuing British aircraft being left flat footed since trying to emulate the maneuver would result in loss of power, or fuel flooding and engine shutdown. The British countermeasure, a half roll so the aircraft would only be subjected to positive G as they followed German aircraft into a dive, took enough time to let the enemy escape in most instances.

1 Price, Alfred. The Spitfire Story: Second edition. London: Arms and Armour Press Ltd., 1986. ISBN 0-85368-861-3 p78

[IvanK]

This is becoming painfull and full of genearalisations and quotes from fairly generic accounts.
For those that haven't followed this discussion in the various threads understand these words below come from someone who currently flies a Hurricane I equipped with the early style carburettor affected by the cut out out issue and was asked specifically for his opinion. We have a similar though more abbreviated comment from another pilot current on similarly equipped aircraft. So read the following closely:

""First, I can tell you that it does not require negative g to make the engine suffer from a shortage of fuel supply; a significant reduction of g down to, say, 0.3g can be enough to make the engine misfire. This can be experienced towards the top of a wing-over but I would estimate that the reduction in g needs to be maintained for 2 seconds or more before there are any effects. Undoubtedly, if the reduction in g was greater (to less than zero g) and particularly if the bunt was abrupt then the effect could be instantaneous. I have never, though, experienced any misfiring in turbulence; albeit, were the turbulence severe enough to produce g spikes to less than zero g, I would not rule out the possibility of the odd cough from the engine. Of interest to you I am sure is that on recovery from an episode of fuel starvation the engine recovers through a short period of over-richness shown by, I would estimate, up to a second of black, sooty exhaust before normal combustion is resumed."

Quote from a current Hurricane I pilot !

[Wolf_Rider]

"say, 0.3g" is uncannily close to "0.5/ 0.25" eh?


do you have a link perhaps... to see what else may be gleaned?








"The use of carburettors was calculated to give a higher specific power output, due to the lower temperature, hence greater density, of the fuel/air mixture compared to injected systems.[40] However, the Merlin's float controlled carburettor meant that both Spitfires and Hurricanes were unable to pitch nose down into a steep dive. The contemporary Bf 109E, which had direct fuel injection, could "bunt" into a high-power dive to escape attack, leaving the pursuing aircraft behind because its fuel had been forced out of the carburettor's float chamber by the effects of negative g-force (g). RAF fighter pilots soon learned to "half-roll" their aircraft before diving to pursue their opponents.[41] "Miss Shilling's orifice",[nb 7] a holed diaphragm fitted across the float chambers, went some way towards curing the fuel starvation in a dive; however, at less than maximum power a "fuel rich" mixture still resulted. Another improvement was made by moving the fuel outlet from the bottom of the S.U. carburettor to exactly halfway up the side, which allowed the fuel to flow equally well under negative or positive g.[42]



40 Hooker, Stanley Not Much of an Engineer London: Airlife, 1984. p62
41 McKinstry, Leo. Spitfire – Portrait of a Legend. London: John Murray, 2007 p205
42 Smallwood, Hugh. Spitfire in Blue. London: Osprey Aerospace, 1996 p135

[Extreme_One]

The writer doesn't mention continued negative G effects.

What we had before the patch was too sensitive.
We shouldn't have suffered misfires and reduced revs because of turbulence so in that respect what we have now is (more) correct.

Now if I attempt to completely starve the carburettor by continually pushing negative G I do suffer from reduced revs as expected but I expect the engine to cut out after pushing negative G further.

Sure my expectations may well be wrong, the accounts I've read may be wrong and previous sims may have modelled this badly in the past.

[IvanK]

Yes its currently about 5 threads below this !!!

http://forum.1cpublishing.eu/showthread.php?t=20462

Extreme One what do you mean by Cut out ? .... Prop stopped ? That wont happen if you have any forward airspeed it will windmill at very low RPM. This can easily be demonstrated in the current Beta following sustained negative G. In these circumstances there is pretty close to Zero thrust output from the prop.

[Targ]

This is what we have now with the beta.

http://www.youtube.com/watch?v=LQesJ..._order&list=UL

Does a full red out with no engine cut out seem right?

I put some pretty good negative G moves in that vid with very little effect.

This is a huge change from before.

[Extreme_One]

Originally Posted by Targ

...

Does a full red out with no engine cut out seem right?
...

That's what I'm talking about. I don't think this is right.

[Targ]

I am not a pilot nor do I know what "should" be right or wrong. Nor do most folks here.

This just seems like a huge swing in the other direction from before. I read all the threads Ivan and it seemed to me that the merlin engine neg G was off a bit and seemed to need some tweaking, what we have now is no small tweak but a 180.

[Wolf_Rider]

Originally Posted by IvanK

Yes its currently about 5 threads below this !!!

http://forum.1cpublishing.eu/showthread.php?t=20462

erm... where?



"Further improvements were introduced throughout the Merlin range: 1943 saw the introduction of a Bendix-Stromberg pressure carburettor that injected fuel at 5 pounds per square inch (34 kPa; 0.34 bar) through a nozzle directly into the supercharger, and was fitted to Merlin 66, 70, 76, 77 and 85 variants. The final development, which was fitted to the 100-series Merlins, was an S.U. injection carburettor that injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures.[43]"


43 Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003 p212