Why are drag polar CD and speed debug window CD diffferent?

Another drag-related question – One can see the drag polar (CL vs CD) with
the debug polar window. You can also get the current CL and CD from the speed
debug window. However, the CD from the drag polar for the CL shown in the
speed debug window can be quite different than the CD in the speed debug
window. See the screenshot below. In the speed debug window CL is 1.77436 and
CD is 0.17123. If you go to the drag polar for that CL, the CD is well above
0.175. Digitizing that plot, you will find that CD from the drag polar is
0.18236. Why is the CD from the drag polar so much different than the CD shown
in the speed debug window?

I am interested too in knowing the answer to this one.

Hello The displayed values depend on if the engine slipstream is taken into
account or not. Once again, we improved the debug panel to make it more

Regards, Sylvain

@Nocturne FYI

What do you mean by “engine slipstream?” Are you talking about propeller
slipstream? My question was relative to the A320, which has wing-mounted jet
engines. What slipstream effect is being added? Any engine effect (with both
engines operating) should already be included in the basic lift curve and drag
polar. There should be identifiable windmill drag and yaw (control) drag in
the case of shut down engine. (At least that’s the way it is done for airplane
performance modeling IRL.)

Hello Yes I was talking about propeller slipstream. Ok that’s not the culprit
if you’re talking about the A320. In fact there are a lot of possible
explanations for this, I’ll try to give a bigger picture. The value printed in
the Debug Aircraft Speed is the final value measured on the aircraft in game
while the polar is the result of the normalization process. The normalization
process happens in a very controlled and theoretical (simplified) environment,
while the final value is the result of a more complex calculation. During the
normalization process, there’s no wind, turbulences, side slip, engine side
effects, control surfaces are in a perfectly neutral position, pressure is
homogeneous, etc… this is basically a plane gliding in perfect conditions.
The final values are computed using the final aircraft FM in its final
environment so any of these will have an influence and that’s the difference
you’re observing. For example, the dynamic pressure is homogeneous in the wind
tunnel but for the final FM, we perform dynamic pressure measures all over the
aircraft (thousands of those). Typically the top of the aircraft is in a less
dense air than the bottom. I’m not saying this is having a big influence, just
demonstrating the difference between the theoretical model and the final more
complex FM. Another more relevant example would be the the way the lift is
computed. The normalization computes the lift at AOA0 and just before the
stall and considers a straight line between the two while in reality (and in
our end FM) this is more of a parabolic. On the A320, that can account in some
cases for a 5% difference in the drag value between the normalization and the
final FM. We double checked with
the A320 and flying in the appropriate conditions, the difference is really
negligible. (We’ve added Min/Max/Avg for the final values so you have a better
idea how it changes across the aircraft)

Note: a bug has been fixed with
debug values being incoherent when too many debug panels are opened. I hope
this answers your concerns. Regards, Sylvain

Thank you. This makes sense and does provide a complete answer to the
question. I had done a number of comparisons of the drag coefficient from the
speed debug window compared to the drag polar. The difference seemed to be
configuration dependent for our FBW A320. In most cases, the difference was on
the order of a percent or two, with some cases being around 6%. The worst case
I saw was pretty high though, 13.5% in CONF 3.