More drag questions

[MSFS 2020] Questions & Community Discussions Aircraft
1

I am having difficulty understanding what is being presented in the drag debug
window and how it can be used to achieve a target drag level. See the
following screenshot from a level, unaccelerated flight condition like those
normally used in airplane performance flight testing. The aircraft speed,
drag, engines, and polar clcd debug windows are open in the screenshot.

3534Ă—1873 1.32 MB
First, a very significant
difference can be seen in the drag level shown in the speed debug window vs
the total drag force shown in the drag debug window – 10,622 lbs in the speed
debug window and 60,857.6 lbs in the drag debug window. This being a stable,
level, unaccelerated flight condition with thrust of 10,633 lbs, it is obvious
that the current drag level at the time of this screenshot was 10,622 lbs, and
the drag coefficient, Cd is 0.07305. None of the drag coefficient build-ups in
the drag debug window are anywhere close to this value, so what purpose do
those build-ups serve in refining this value? A couple of other questions:
“Other drag” in the drag debug window is quite high – 0.33364 on average. It
supposedly consists mostly of “slip_stream_&prop_wash" and
"side_slip&_control_surfaces.” Why would there be drag for slip stream and
prop wash for a turbojet airplane with underwing mounted engines (A320) and
high sideslip and control surface drag for a steady-state, level,
unaccelerated flight condition with no wind? What is the “wingledcd” parameter
in the “target_cd” buildup? Is this a delta cd value for winglets? The
description of the “wing_winglets_flag” in the SDK says that this is a legacy
FSX parameter not used in the modern flight model. How is this value
determined, and can it be adjusted by the developer?

2

I suggest you screenshot the CD vs CL curve in debug and paste it as a
background in excel. It’s the best way I found for the tuning process of the
drag against desired model

3

Thank you, but that it not what I am asking.

4

Was there any clear answer to the above questions?
I am also struggling to understand how the other_drag parameter is computed in the debug window and how it can be sometimes negative (i.e. drag becomes thrust which is not possible in real life…)
It has an impact on the real-time calculation of Cd0 which is obviously important to determine CD and the aircraft polar.

Also I see that for some aircrafts (e.g. A320 Fenix) the cfg_cd0 parameter is different from the drag_coef_zero_drift set in the flight_model.cfg file so I was wondering how it is determined in that case?

5

I think the other drag variable is mostly from fuselage drag. In the original post, the drag does seem abnormally high for the lift.

One thing about this sim flight model is that you can’t enter a table for the drag coefficients even if you have that data. You set some limits and the sim seems to calculate it’s own curves. The only useable parameters to work with are the parasitic and induced drag scalars. They can “push” the curve around a bit.

I think the drag debug screen shows the best information when tuning a model. The numbers are changing so it’s not perfect, but you can get a picture of the balance. You can see the factors of each calculation so that’s nice.

6

Hello,
I agree with you, we need tables that are easier to use.
We have this in mind, and this will be improved one day.

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7

Also, regarding the initial questions in this topic:
some improvements of the aerodynamics debug windows are coming with FS2024.

3 Likes