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SWS-AlexVletsas avatar image
SWS-AlexVletsas asked SWS-AlexVletsas answered

Propeller advance ratio jumping at 0.3

First off, for our propeller simulation we use the legacy tables system. After SU9 we had to modify the factory tables to get proper propeller drag out of the aircraft, but we have hit a bug. When the propeller is at its ground pitch, as soon as the aircraft reaches an advance ratio of J=0.3 the thrust and advance ratio will "jitter". One instance the advance ratio is 0.29, the next it is 0.38. This also makes resultant thrust jump. At that pitch & advance ratio, the aircraft is able to maintain a stable 34kts on the ground. All this would go unnoticed if we didn't hear a "sputtering" sound from the propeller when that point is reached.

So in short:

>The problem wasn't there before we tweaked our propeller tables.

>There is no problem from an aircraft performance perspective with the new version.

>There is jittering at the point of equilibrium. The advance ratio and thrust vary greatly as propeller RPM drops.

>There is a sound problem that stems from the change in thrust, although propeller pitch is stable at 11.4 degrees.

Any suggestion on how we can fix that problem, or what it is?1654587498536.png

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SWS-AlexVletsas avatar image
SWS-AlexVletsas answered

After doing some more research into this, the problem occurs when the propeller efficiency changes from positive to negative values -I use negatives to fake propeller drag. If it happens so, that the balance point is near the change of sign, then the sim jumps back & forth. It becomes especially evident if the jump is big between entries (i.e from 0.4 efficiency to -0.6 efficiency).

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SWS-AlexVletsas answered

Some further research into the problem. The jump can be somewhat "improved" if one reduces the difference in efficiency between two values of J. The sound cutting problem, though, seems related to thrust. When the propeller produces "negative thrust", the sound cuts out.

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