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boufogre avatar image
boufogre asked boufogre answered

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.

aircraftflightmodel
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7 Answers

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FlyingRaccoon avatar image
FlyingRaccoon answered FlyingRaccoon commented

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:

1630075209522.png

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


1630075209522.png (807.5 KiB)
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boufogre avatar image
boufogre answered boufogre edited

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.

1625642986142.png


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boufogre avatar image
boufogre answered

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:

1625731089410.png


However, the Cd0 value indicated here is including the induced drag at AOA = 0°, which is wrong.


1625731089410.png (21.0 KiB)
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donstim avatar image
donstim answered

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?

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boufogre avatar image
boufogre answered boufogre commented

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.

2 comments
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Alec246 avatar image Alec246 ·
Very good reading! Congrats on the findings! Hope Asobo takes a good look at that
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boufogre avatar image boufogre Alec246 ·
Thanks, I literally spent weeks trying to work out the drag;
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Sherv avatar image
Sherv answered

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


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boufogre avatar image
boufogre answered

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.


1630920674832.png







"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... "

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.


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