Propeller behavior with engine off

[cclaudel]

Recently, when testing some airplanes on Il2 BOS, I found that some have a peculiar behavior: if you cut the engine in the air at very low speed, the propellers keep spinning into the wind, except for 2 models (that I found so far): the Spitfire Mk IX and the P-47. If you take these and do a vertical pull-up and cut the engine off, the propeller will be almost "blocked" and will very slowly spin as if it was stuck.

 

Technically, what is the reason for this? I guess it has to do with the internal mechanism of the constant speed propellers? But why do some behave this way while others behave more like regular propellers (spin at relatively high rate). Is it related to prop feathering?

For the Spitfire mk IX and the P-47 that behavior only happens if airspeed drops below around 50 mph in the air.

[Asgar]

Most single engine planes don’t have a feathered position just coarse to fine… but there are different methods of actuation. For one there are hydraulic and electrical systems and even with hydraulic systems there are differences.

1 using oil pressure for both adjustments towards fine and coarse setting

2 using oil pressure for adjusting to fine setting and using centrifugal forces to adjust the other was

3 using oil pressure to adjust to coarse setting and using centrifugal force to adjust the other way

 

now, when the engine stops the oil pressure will drop and the centrifugal force for the spinning prop will adjust either to fine or coarse setting. Thus either you get windmilling even at low speed or you don’t

 

damn I hate typing in the phone xD 

Edited December 26, 2021 by Asgar

[cclaudel]

Sounds good! So it is just that on Spitfires mk IX and P-47s when oil pressure drops the spinning prop adjusts to coarse? But this only happens if the propellers spins down to a minimal value. If you cut the engine with those and your speed remains above say 150 mph then it will keep spinning at the usual pace. So it must be related to oil pressure and propeller RPM somehow...

[Art-J]

8 hours ago, cclaudel said:

Sounds good! So it is just that on Spitfires mk IX and P-47s when oil pressure drops the spinning prop adjusts to coarse? But this only happens if the propellers spins down to a minimal value. If you cut the engine with those and your speed remains above say 150 mph then it will keep spinning at the usual pace. So it must be related to oil pressure and propeller RPM somehow...

 

Bubble top P-47 featured in Il-2GB series has a Curtiss Electric prop, so oil has nothing to do with its operation. At the same time, I believe Spitfire's Rotol prop is mechanically the same as oil-supplied Hamilton Standard props on other US aircraft in this sim, so it shouldn't behave different (nor should the Razorback P-47 with HS prop for that matter). In old Hamiltons oil is by default used to coarse the pitch, while loss of pressure (which should happen only if engine is off completely by the way) would make them go fine pitch (high RPM) automatically.

 

At the same time, in the sim we can see things like P-47's props spinning down after engine shut-off on the ground waaaay longer than they should (almost as in non-geared turboprop engines), which is clearly wrong, but honestly a cosmetic issue rather than gameplay-affecting one.

 

All and all, you just have to accept that some aspects of physics/systems modelling in this sim are simplified or somewhat incorrect in unusual, out-of-envelope operational modes. Nothing wrong about that, mind you - in the end it's a videogame (excellent at that!), not some industrial-grade CFD research software. As long as the flight model follows the relevant laws in typical flight situations it's all good. One just have to be aware of the game engine's justified limitations and not "overthink" them too much.