Drag calculation, round 2

Hello, After my previous question about the effect of induced drag on Cd0,
here I am again because I am unable to set the drag I desire in the sim. I
have created an Excel to help me calculating the drag, and it works as I am
able using it to calculate exactly the FSX and Total NEW value that were
discussed in my previous question on the topic. However, I find it impossible
to obtain the same drag in the sim than the one I calculate in my Excel. The
sim drag is consistently lower than the drag from Excel. Here is a link to the
file, for you to experiment with it:
https://1drv.ms/x/s!AohwTJSZ0H8DgcEdzeuwq9Zj5EraZQ?e=wblNY4 To use it,
simply load the default 747, and note the CL (Cz) and CD (Cx) from the
Tracking debug window and put them in the file. I would like to know what is
going on with the drag calculations, as it looks like I am missing something
somewhere and it is really hindering me in my progress. Thank you.

To better show the discrepancy between what is expected and what is obtained
in the sim, here is a screenshot from my Excel, showing two drag polars
(Cd(Cl)) together. The blue one is the theoretical one, calculated using
Cd=Cd0+k(Cl-Cl0)² and the orange one is read from the sim ClCd debug window.

Hello, I have realised yesterday that the drag of each plane component is
given in the Sim Curves debug window, opened by pressing Sim in the aircraft
editor. It is very useful to control your drag:
However, the Cd0 value indicated
here is including the induced drag at AOA = 0°, which is wrong.

Giving this a bump. I’ve seen these differences, too and have all but given up
on duplicating MSFS drag numbers via calculation. Also, exactly how are the
parasite and induced drag scalars applied?

After a lot of testing, I realised there is a connection between the wing
sweep applied to the aircraft and the drag curve calculated by the simulator.
More on this link: <https://forums.flightsimulator.com/t/wing-sweep-and-
drag/419385> You need to be part of the current beta to read the post.

Very good reading! Congrats on the findings! Hope Asobo takes a good look at

Thanks, I literally spent weeks trying to work out the drag;

Also bumping up. Can the Asobo team at least acknowledge if they are looking
into this? We don’t expect answers right away but hope in the long run these
issues are addressed. Thanks

Hello. @boufogre I’ve added some more informations about the drag in your
first question. I’ve also had our FM developer checking your wing sweep topic
on the forums. It’s not reflecting how our FM actually works due to some wrong
assumptions and misunderstanding from you and some missing documentation and
unclear debug tools from us. Our model doesn’t rely on the thin airfoil
theory. What makes you think this is the case? Obsolete doc maybe? Let us know
so that we can clarify this if needed. Also, the CP is not located at 25% of
the MAC. This is on us, our Debug Aircraft Weight panel showing an " Aero %
MAC: 25%
" string is misleading (this is more of a theoretical value we
wanted people to consider when placing their CG). Once again, we’ve used your
feedback to review our calculations and debug tools. We’ve added some debug
informations and improved the way we compute the aerodynamic centre Now we
show the wing/aircraft aerodynamic/pressure/lift center positions. Keep in
mind that while you might think there will be pitch because the lift center is
not aligned with the CG, this doesn’t take into account the pitch induced by
the drag, engines, etc… These positions are depending a lot on the wing
sweep and twist in our model. For example, with a liner aircraft, the outer
part of the wing will have less AOA, producing less lift compared to the inner
part of the wing, having more AOA and producing more lift, shifting the center
of lift towards the front. Here’s what the Debug Aircraft Weight panel will
look like starting from SU6 illustrated with the 747:

The lift center and aerodynamic
center are not fixed positions, the change in real time depending on a lot of
factors (AOA, ailerons positions, flaps, stall, turbulences, etc…) I hope
this helps you having a better understanding of our model. Regards, Sylvain

@Nocturne FYI

Hello @FlyingRaccoon, Thank you for the detailed answer. My assumptions were
very much wrong but they were based upon my experiments and the information
available at the time. I thought it was relying on the thin airfoil theory as
the Flight Model Physics SDK page mentions it several times and also because
the aero center position in the sim was always reported as 25% MAC in the
debug UI, no matter what was changed and notably the wing sweep.

" Keep in mind that while you
might think there will be pitch because the lift center is not aligned with
the CG, this doesn’t take into account the pitch induced by the drag, engines,
" Regarding this, it would be nice to know with more details what is
represented in the Pitch debug UI, as it showing vectors but without any
labels. It is also missing the drag representation and it would be nice if
more details could be provided on what information this overlay gives exactly.
When conducting my experiment, I tried to counter this by placing the engines
at the CG, which would cancel any pitching moment generated by them. I think
that clearer debug tools and more detailed SDK can only help the flight model
designers to produce the results they want in the sim. Looking forward to
seeing the improvements SU6 will bring to the sim and the SDK.