While we're on the topic of calculating thrust, I thought it would be nice to touch turbofans/turbojets. I've been using an empirical method via interpolation to nail down exact numbers in simulation, but it would be really nice to do it all by crunching numbers.
Here's what I have:
Assuming all input values are between data points in tables 1502-1507. Going from Throttle input => CN2 => CN1 => Thrust Factor/Ram Drag Factor appears to be a simple matter of linear interpolation. From there I've used these equations (found at avsim for major, and aerodynamics textbook for minor), but the numbers produced aren't quite matching up in simulation.
Net Thrust = Engine Thrust - Ram Drag
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Engine Thrust = (Static_Thrust*Thrust_Factor*Delta_2)*Thrust Scalar
Delta_2 = Ptotal / Psl = Standard Pressure Ratio/Total Pressure Ratio
SPR = Pamb / Psl
TPR = Pamb / Ptotal = (1+((G-1)/2)*M^2)^(-G/(G-1)) where G (gamma) is assumed constant at 1.4 for air
=> TPR = (1+0.2*M^2)^-3.5
*I have no idea how table 1524 fits into this equation. The scalar is direct at M0.0 || SL, but does not appear to correlate linearly at higher speeds/altitudes
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Ram Drag = (Inlet_Area*Ram_Drag_Factor/SQRT(Theta_2))*(TAS(ft/s)/g0(ft/s^2))
g0 = 32.174
Theta_2 = Ttotal / Tsl = Standard Temperature Ratio/ Total Temperature Ratio
STR = Tamb / Tsl
TTR = Tamb / Ttotal = (1+((G-1)/2)*M^2)^-1
=> TTR = (1+0.2*M^2)^-1
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N1 = CN1*SQRT(Theta_2)
N2 = CN2*SQRT(Theta_2)
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Results:
For two test points at M0.80 || FL250 / FL350:
Starting with Throttle percentage, CN1 was calculated with 0% error for both data points
Thrust errors were 6.7% and 0.8% respectively.
As a check, I also empirically calculated Theta_2 using the defined equations for CN1 => N1
For the same two test points, errors in Theta_2 were 3.8% and 1.3%. However, substituting the empirically determined Theta values increased the thrust error.
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From here, I'm not sure where to go....but if any of you see an error in the equations or have a deeper insight into the inner workings of FSX, I would be hugely grateful.
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