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Ron Jensen: fixed a potential NaN and Segfault in JSBSim propeller code

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The property /fdm/jsbsim/propulsion/engine/prop-induced-velocity_fps
gives wrong answers, and can become NaN under certain conditions. When thrust is
negative and forward velocity is small we can take the square root of a negative
number. This could occur, for example, when using reverse thrusters on landing.
The value comes out much too high when alpha is near 180, such as taxing with a
tail wind.
This commit is contained in:
ThorstenB 2012-02-11 17:11:13 +01:00
parent a8d5ac90c6
commit 58e79013e3
1 changed files with 9 additions and 1 deletions
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10
src/FDM/JSBSim/models/propulsion/FGPropeller.cpp
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@ -228,7 +228,15 @@ double FGPropeller::Calculate(double EnginePower)
// Induced velocity in the propeller disk area. This formula is obtained
// from momentum theory - see B. W. McCormick, "Aerodynamics, Aeronautics,
// and Flight Mechanics" 1st edition, eqn. 6.15 (propeller analysis chapter).
Vinduced = 0.5 * (-Vel + sqrt(Vel*Vel + 2.0*Thrust/(rho*Area)));
// Vinduced = 0.5 * (-Vel + sqrt(Vel*Vel + 2.0*Thrust/(rho*Area)))
// Since Thrust and Vel can both be negative we need to adjust this formula
// To handle sign (direction) separately from magnitude.
double Vel2sum = Vel*abs(Vel) + 2.0*Thrust/(rho*Area);
if( Vel2sum > 0.0)
Vinduced = 0.5 * (-Vel + sqrt(Vel2sum));
else
Vinduced = 0.5 * (-Vel - sqrt(-Vel2sum));
// P-factor is simulated by a shift of the acting location of the thrust.
// The shift is a multiple of the angle between the propeller shaft axis