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Attempt at a fix for the propeller torque problems in the

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"slow/windmilling propeller" regime.  I'm happy with the foundations
of the solution, but this hasn't been complete tested yet.  The
solution behavior seems fine on the planes I tried.
This commit is contained in:
andy 2004-05-01 00:25:56 +00:00
parent b1c964030d
commit 9f2a8d4651
1 changed files with 28 additions and 36 deletions
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64
src/FDM/YASim/Propeller.cpp
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@ -58,55 +58,47 @@ void Propeller::setPropPitch(float proppitch)
void Propeller::calc(float density, float v, float omega, void Propeller::calc(float density, float v, float omega,
float* thrustOut, float* torqueOut) float* thrustOut, float* torqueOut)
{ {
if (_manual) { // For manual pitch, exponentially modulate the J0 value between
float pps = _proppitch * 0.9999f; // avoid singularity // 0.25 and 4. A prop pitch of 0.5 results in no change from the
pps = 1 + ( Math::pow(pps,-1/(pps-1)) - Math::pow(pps,-pps/(pps-1)) ); // base value.
_j0 = (4*_baseJ0) - ( ((4*_baseJ0) - (0.26f*_baseJ0)) * pps ); if (_manual)
} _j0 = _baseJ0 * Math::pow(2, 4*_proppitch - 2);
float tipspd = _r*omega; float tipspd = _r*omega;
float V2 = v*v + tipspd*tipspd; float V2 = v*v + tipspd*tipspd;
// Clamp v (forward velocity) to zero, now that we've used it to // Sanify
// calculate V (propeller "speed")
if(v < 0) v = 0; if(v < 0) v = 0;
// The model doesn't work for propellers turning backwards.
if(omega < 0.001) omega = 0.001; if(omega < 0.001) omega = 0.001;
float J = v/omega; float J = v/omega; // Advance ratio
float lambda = J/_j0; float lambda = J/_j0; // Unitless scalar advance ratio
float torque = 0; // There's an undefined point at lambda == 1.
if(lambda > 1) { if(lambda == 1.0f) lambda = 0.9999f;
lambda = 1.0f/lambda;
torque = (density*V2*_f0*_j0)/(4*_etaC*_beta*(1-_lambdaPeak));
}
// There's an undefined point at 1. Just offset by a tiny bit to float l4 = lambda*lambda; l4 = l4*l4; // lambda^4
// fix (note: the discontinuity is at EXACTLY one, this is about float gamma = (_etaC*_beta/_j0)*(1-l4); // thrust/torque ratio
// the only time in history you'll see me use == on a floating
// point number!)
if(lambda == 1.0) lambda = 0.9999f;
// Calculate lambda^4 // Compute a thrust coefficient, with clamping at very low
float l4 = lambda*lambda; l4 = l4*l4; // lambdas (fast propeller / slow aircraft).
// thrust/torque ratio
float gamma = (_etaC*_beta/_j0)*(1-l4);
// Compute a thrust, clamp to takeoff thrust to prevend huge
// numbers at slow speeds.
float tc = (1 - lambda) / (1 - _lambdaPeak); float tc = (1 - lambda) / (1 - _lambdaPeak);
if(_matchTakeoff && tc > _tc0) tc = _tc0; if(_matchTakeoff && tc > _tc0) tc = _tc0;
float thrust = 0.5f * density * V2 * _f0 * tc; float thrust = 0.5f * density * V2 * _f0 * tc;
float torque = thrust/gamma;
if(torque > 0) { if(lambda > 1) {
torque -= thrust/gamma; // This is the negative thrust / windmilling regime. Throw
thrust = -thrust; // out the efficiency graph approach and instead simply
} else { // extrapolate the existing linear thrust coefficient and a
torque = thrust/gamma; // torque coefficient that crosses the axis at a preset
// windmilling speed. The tau0 value is an analytically
// calculated (i.e. don't mess with it) value for a torque
// coefficient at lamda==1.
float tau0 = (0.25f * _j0) / (_etaC * _beta * (1 - _lambdaPeak));
float lambdaWM = 1.2f; // lambda of zero torque (windmilling)
torque = tau0 - tau0 * (lambda - 1) / (lambdaWM - 1);
torque *= 0.5f * density * V2 * _f0;
} }
*thrustOut = thrust; *thrustOut = thrust;