I’ve just uncovered a major aerodynamic bug: it appears that the induced drag is wrongly influencing the basic Cd0 parameter.

The SDK states that Cd = Cd0 + k(Cl-Cl0)²

Cd0 is the zero-lift drag component, in other words the basic parasitic drag and k(Cl-Cl0)² is the induced drag component, with k = 1/(PI*AR*e) according to the scientific litterature, AR being the aspect ratio of the wing, and e the Oswald efficiency factor.

Here is the desired Cd0 of 0.0171 set in the flight_model.cfg file:

And here is the [FLIGHT_TUNING] section of the flight_model.cfg file:

However, when looking at the numbers in the sim, here is what appears:

That Cd (Cx is the French notation for Cd) is way too high.

My manually calculated Cd for the conditions would be according to the SDK formula:

Cd = 0.0171 + k x 0.812² = 0,04796 (I voluntarily chose a Cl0 = 0), with k = 1/(PI x 8.46 x 0.804) = 0.04680

The difference between the sim and the manual calculation is huge: 0.07376 - 0.04796 = 0.0258.

After of lot of time, playing with the various parameters, I’ve come to realise that if the induced drag coefficient is set to 0 in the flight_model.cfg, then the correct Cd0 is calculated by the sim:

The induced drag shouldn’t have any influence over the Cd0 in the sim, but be added to it as the SDK says it does.

I hope this post was clear, and I hope this will get fixed at the next possible opportunity.

Thank you for reading me!