Any thoughts on new neg G cutout post beta patch?

[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

[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.

[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

[Viper2000]

Originally Posted by Wolf_Rider

erm... where?

Try this from page 10 of the thread, complete with diagram.

The analysis linked to above only talks about mixture strength, and not the effects of that mixture strength on the engine.

During the lean cut, power initially falls away a bit slower than fuel flow because the design point was rich of stoichiometric, and therefore the temperature rise per unit fuel flow goes up. EGT would also initially rise. There might be incipient detonation, followed by misfiring as the mixture becomes too weak to support combustion (cylinder to cylinder variations in temperature and mixture strength mean that this behaviour will not affect all of the cylinders at the same time).

The lean cut will be less severe at lower power output under reduced positive g because fuel flow is less and so the fuel in the small chamber will therefore last longer; indeed at high altitude the engine might just go straight to rich cut. (Under negative g the small chamber empties much faster because fuel flow can escape from the holes in the small chamber which were provided to admit it.)

During the rich cut, power output falls because the Fuel:Air Ratio is approximately 2.4 times design point (which was already somewhat rich of stoichiometric for high power running). This lowers the combustion temperature considerably which kills the engine's BMEP.

However, combustion will continue because there is still oxygen and a source of ignition in the combustion chamber - the chemistry just changes so that it makes CO, H20, sundry cracked petroleum products, and of course some carbon (soot).

Boost shouldn't initially change during the cut unless engine rpm starts to fall, because the ABC doesn't know anything about the cut, and the supercharger pressure ratio is simply a function of its rpm; since it is geared to the engine.

Therefore, the chain of events is

1. Power reduction
2. Possible rpm reduction
   • Boost reduction

The rich cut is worse at lower engine power outputs because the fuel flow from the pumps is a fixed function of rpm, whilst the amount of fuel needed by the engine is a function of power.

At 30000' where the engine is only capable of producing about 500 bhp instead of nearly 1300 at its +12 psi FTH, the fuel flow during the cut would be 2.4*(1300/500) ~ 6.24 times that actually required by the engine. This is such a severe over-rich case that nett power output might fall to zero or even go negative (the prop will obviously windmill, so the engine won't stop; as long as it keeps turning the mags will keep going and so there will still be a spark and therefore recovery will follow).

Recovery time to float-controlled operation upon return to positive g would probably be longer at higher altitudes because the recovery depends upon the rate at which the carburettor can dispose of the excess fuel in the float chamber. This depends upon how much fuel can be carried away by the airflow through the venturi (which is mostly a function of ambient conditions & engine rpm; fuel temperature will also have a 2nd order effect).

At or below FTH, with the mixture about 2.4 times as rich as it should be, power will probably fall to about 1/3rd of the expected value, or perhaps a little less. I could do a cycle analysis if there is sufficient interest; the main unknown is the design point Fuel:Air ratio; I can't lay my hands on my copy of The Performance of a Supercharged Aero-Engine by Hooker et al at the moment...

[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.

[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.

[IvanK]

You dont call Minimum RPM a cut out ??? ... how much power and thrust do you think the prop/engine RPM combination is putting out with the RPM full scale anticlockwise ? You went from 3000RPM to something less than perhaps 1000RPM in a heartbeat !