Good point. That's something that's also not too hard to fix.
I tried to fix this problem in JSBSim a year or two ago, and I seem to recall that no one on the flight model list could quite figure out how to code it back then. I also took a stab at YSBSim, and failed just as miserably. Neither model is set up to have the propeller driving the engine rather than the engine driving the prop.
The rule of thumb for pilots is that a windmilling propeller creates as much drag as a disc of the same size, but that's too vague for modelling (plus, it doesn't handle the partial-windmilling situation). What we need to figure out is how much drag we get from the propeller turning the crankshaft, compressing the cylinders, and spinning the accessory drives (vacuum pump, alternator, etc.).
I could not (yet) find my NACA report on the light twin, but here are some interesting numbers:
Cn_beta for some aircraft (per rad):
Navion: 0.071 (Raymer ?) C-172p (JSBSim, from Raymer): -0.349 -0.0205 0 0 0.349 0.0205 This is roughly 0.06. Cherokee (McCormick): 0.067
C-310 (JSBSim): 0.1444
This is twice as high as the other aircraft. It could be due in some measure to a larger vertical tail, but I wonder if perhaps this value is too high? When coupled with the correction of drag due to prop, then I suspect we'll be a lot closer.
Twins and taildraggers need a lot of rudder authority; tricycle-gear singles, not so much.
All the best,
David
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