Aerodynamic findings: induced drag and parasitic drag

Bug Reporting Hub Aircraft & Systems
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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/(PIARe) 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:

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

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

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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:

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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!

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I have found a temporary fix to this one, but it is really ugly and not really accurate.

The added value to what should be the Cd0 can be remove using the drag_coeff_zero_lift_tab by entering 0:X, with X being the correction value between the calculated Cd0 by the sim and your desired Cd0.

This allows to set the induced_drag_scalar to 1.0 and to have a decent amount of drag.

However, this cannot be considered the definitive solution. We need to have a clean Cd0 calculation by the sim, so that the drag can be fine tuned depending on its different components : parasitic drag, wave drag, induced drag, etc.

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I have spent the last few days investigating deeper into this issue.

I have tested all the default planes, and all appear to show the same incorrect behaviour of induced drag modifying the Cd0 (parasitic drag) in the simulation.

More of my findings can be found in MSFS DevSupport platform:
https://devsupport.flightsimulator.com/questions/644/major-aerodynamic-bug-induced-drag-and-parasitic-d.html

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So it turns out that this bug really isn’t one, but more a consequence of misleading labelling of the values in the debug windows. The value indicated as FSX and Total NEW are actually Cd(alpha = 0) and not Cd0, hence why they include the induced drag component of the Cl(alpha = 0).

However, there are still interesting things to be learnt from the experiment, for example how setting induced_drag_scalar = 0 influences how the drag_coef_gear is included or not in the Cd(alpha = 0) (cf results of the DA40NG).

I’m changing the title of this topic to reflect this.

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