Re: [Flightgear-devel] YASim question: propeller idle RPM

[Andy R. <an...@pl...>]

David Megginson wrote:
> Is Andy Ross still on this list?

He is, he's just caught up in another personal project (a
maybe-possibly-kinda fundable startup) and isn't doing much FlightGear
hacking these days.

[He's also sitting on a PistonEngine patch from Vivian for the turbo
 modelling -- I'll get to that RSN, I promise!]

> If so, I'd be grateful for any suggestions about changing
> Propeller.cpp to slow down idle RPM for constant-speed propellers
> (drag? friction?).

The current propeller model is based on a nominal efficiency curve; it
doesn't bother to simulate blade AoA & such, and just works up a force
model that matches the energy inputs.

This works really well in the middle of the range (where the solution
happens), but the outer areas tend to be poorly constrained.  So very
high AoA situations (windmilling or idling) don't match reality well.
Ditto for very low AoA (max RPM at takeoff), although that one was
impotant enough to have its own tunable hacked into the model.

Really, I think the right answer here is to rewrite the propeller code
to use a more typical aero-based model and use the efficiency curve
trick to fix the coefficients to known performance at solution time.
But yeah, that's a lot of work. :)

I'm really not sure what the right answer to the idling issue is.
When we were tuning fixed-pitch idle speed, the easiest fix was to
play with the idle power of the engine.

But the variable speed code will always seek to the specified RPM, and
the current model gives it enough wiggle room to push the torque as
far down as it wants, essentially.  You could try setting a
different/tunable "floor" to the pitch value (see the clamping in
Propeller::modPitch()), but getting that value right is going to take
a lot of experimentation.

Andy