I don't understand drag

Hello, As if it was not yet obvious from my previous questions on the topic, I
must be stupid because I cannot understand how the drag is calculated. An
example in pictures:

How can
the parasitic drag be negative? If I go back to a normal speed for this
altitude: Even at normal cruise
speed, some “negative” drag is calculated and reducing the parasitic drag by
half. Could you explain this please? I have a really hard time tuning the drag
of the aircraft, and I believe the missing drag in cruise (reflected in a too
low N1 in cruise) is due to this negative drag. Thank you.

Hello @FlyingRaccoon, Could you take a look at this by any chance? Thank you.

Hello @boufogre and my apologies for the late reply. Can you provide us with
files and repro steps to end up in this configuration? I would have to trace
the value in the code to tell where this negative drag is coming from with
accuracy. Regards, Sylvain

Hello @FlyingRaccoon, Thank you for the reply. In the screenshots, you can see
a development version of the Salty 747, as I’m working on its flight model.
However, this is not specific to this plane. Here are new screenshots with the
default 747, showing the same negative drag. To reproduce, just load any plane
and bring up the Drag debug window. The other_drag will show negative drag.

I can share with you my files
from the Salty, but as you can see, the problem is not limited to it.

Hello @FlyingRaccoon, Could you track down where this other_drag is coming
from? Thank you!

Hello again, I have just downloaded the SU11 beta, and saw the new drag debug
graphs and lines. Thank you for these additions, I will keep experimenting
with them.

Hello @FlyingRaccoon,

After more time playing with the new graphs, I can confirm that the negative
drag previously reported here is due to control deflection.

Whilst I agree that control deflection should have an impact on the drag
coefficient, the magnitude of the current effect seems to be exaggerated in my

Furthermore, if the normalization process assumes neutral control deflection,
then any deflection should increase the drag coefficient, not reduce it.

Another thing I have noted using the new curves is that it appears the Cd0 is
following the theoretical drag polar when it should be the overall Cd.

Here are two screenshots taken at different speeds (330 and 220 KCAS):

As you can see, the apex of the red curve (Cd0) is matching the theory drag
polar, when it should be the actual Cd. The red curve should match the yellow
one, no matter what the speed is, at least up to the stall AOA.

A video to further document the effects: first is elevator full deflection,
then ailerons, and finally rudder.

Hello @FlyingRaccoon, I have been thinking and I wanted to make a suggestion:
would it be possible to create scalars to fine tune the drag of each plane
component? For example:

  • elevator_drag_scalar
  • elevator_trim_drag_scalar
  • rudder_drag_scalar
  • rudder_trim_drag_scalar
  • fuselage_drag_scalar
  • aileron_trim_drag_scalar

Another suggestion is to give the drag created by the flight controls a
positive value only (absolute value of the currently calculated value should
work). That way, the parasitic drag would only ever be higher than the Cd0 set
in the file and never lower, and that would make the Cd0 set in the file a
real Cd0, in the sense of the minimum drag coefficient (drag coefficient
cannot be lower). By the way, this should not break any existing addon. Let me
know what you think. Thank you.