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boufogre avatar image
boufogre asked GrimPhoenix9349 commented

Wing sweep and CP position

Hello,

I would like to add to a problem that I have mentioned before here and in the forums, and that problem is how wing sweep affects the CP position.

For the two unlocked airliners coming with the sim, the 747-8 and the A320, it can be clearly seen that for the both of them, featuring a significant amount of wing sweep, their CPs appear to be in front of their CGs, by a significant amount.

My hypothesis is that the sim considers the longitudinal position of the MAC to be the same as the position of the wing section located at the centre of the fuselage. It can be seen by using the Sim Forces overlay alongside with the Pitch overlay and the Weight window from the debug UI.


When setting the wing sweep to 0, the CG and CP positions correspond to the one set in the config file. However, as soon as a sweep angle is introduced, the fuselage centre section of the wing starts shifting forward, as seen using the Sim Forces overlay, and so does the CP in the Pitch overlay.

I think that to solve this problem, the MAC and the CP longitudinal position should become independent of the wing section at the centre of the fuselage.

Please take that into consideration as it is an important topic having big consequences for the physics of the simulator.




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boufogre avatar image
boufogre answered GrimPhoenix9349 commented

Here are some screenshots illustrating my thinking:

boeing-default-w.jpg


boeing-default-sfp.jpg


These first two screenshots show the default 747, without any modification. It can be seen that the CG is at 4.56% MAC according to the weight window, and that the CG is at -6.50 ft, while the aero centre is located at -14.00 ft. The LEMAC is at -4.83 ft and the TEMAC is at -41.51. However, and as it can be seen in the second screenshot, the wing pressure centre is located at -14.3% MAC, which means outside of the MAC, which does not make sense, while the wing aero center is located at 99% MAC, which would mean at the TE, which does not make more sense. It can also be seen that the green vector (lift?) origin from the pitch overlay seems to be the position of the wing pressure centre as it shows in front of the MAC. Now if changing only the sweep parameter from the original value of 41 to 0, here is what we obtain:


boeing-sw0-w.jpg


boeing-sw0-sfp.jpg


This time, it can be noticed that the CG and aero centre have the same positions as before (-6.50ft and -14.00 ft), the LEMAC and TEMAC as well (-4.83 ft and -41.51ft), but this time the wing pressure centre is located at 32.1% MAC and that the wing lift centre is located in the same spot (32.0% MAC). This time, the wing aerodynamic centre shows 0% MAC (99% before the modification).


I think this example shows exactly the point I am trying to make. The wing sweep has dramatic consequences on the flight model, and the current calculations are wrong.


boeing-default-w.jpg (376.6 KiB)
boeing-default-sfp.jpg (419.4 KiB)
boeing-sw0-w.jpg (379.9 KiB)
boeing-sw0-sfp.jpg (427.9 KiB)
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FlyingRaccoon avatar image FlyingRaccoon ♦♦ commented ·
Hello.


Just trying to collect some more info before we review this.
According to you, changing the wing sweep angle shouldn't have any influence on the real CP at all ?

From what I understood from our FM people (assuming I understood everything correctly :) ), with an airliner with important wing sweep and twist, you can have a situation where the outer part of the wing will have 0 AOA hence 0 lift, and the inner part of the wing will produce all the lift. The inner part being in front, the true CP will be shifted forward.

Regards,
Sylvain

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boufogre avatar image boufogre FlyingRaccoon ♦♦ commented ·

Hello @FlyingRaccoon,


Thanks for your comment.


“According to you, changing the wing sweep angle shouldn’t have any influence on the real CP at all ?”

This is not exactly what I am saying.

According to my observations, adding wing sweep in the simulator is moving the reference chord position forward, when it should not. You can test that yourself by changing the sweep angle to 0 and comparing how the reference chord had moved aft compared to the default case. Another clue to this happening is how the empty CG position changes in %MAC, without changing the empty CG position in the configuration file.

The reference chord position is set in the flight_model.cfg file by using the aero_center_lift parameter, which by positioning the aerodynamic center of the wing (25% MAC) is also setting the MAC position alongside the aircraft longitudinal axis. The problem is that this position should not be modified by the simulator when adding sweep to the wing. This position is already considering the wing sweep of the aircraft. Indeed, if you consider an aircraft with a straight wing and a taper ratio of 1 (constant chord along the span), the MAC position and the wing longitudinal positions will coincide. However, if you add wing sweep to this aircraft, the MAC position will be moved AFT compared to the first example. The MAC position is sometimes directly given in the TCDS, as it is the case for the older variants of the 747 (1258 in aft of datum).


“From what I understood from our FM people (assuming I understood everything correctly :) ), with an airliner with important wing sweep and twist, you can have a situation where the outer part of the wing will have 0 AOA hence 0 lift, and the inner part of the wing will produce all the lift. The inner part being in front, the true CP will be shifted forward.“

0 AOA does not necessarily 0 lift unless we are talking about a symmetrical airfoil. By the way, in the debug UIs, the AOA indicated does not include the angle of incidence of the wing. For example, an aircraft in level flight, with a pitch of 4° and a wing angle of incidence of 2° should have an AOA of 6° (assuming no twist). In the simulator, with this example, the debug UIs show an AOA of 4°.

In the case of the 747, with a wing angle of incidence of 2°, a wing twist of -3°, a pitch of 4° (level flight), the wing root AOA should be equal to 6°, while the wing tip AOA should be equal to 3°, hence the full wing creating positive lift. To see an AOA of 0° for the wing tip, it would require the plane to fly level with a pitch of 1° (pitch = 1° ==> root AOA = 3° ==> tip AOA = 0°). And even then, assuming same airfoil across the span, which I guess MSFS does, the tip would create upward positive lift.


That is a long answer, and hopefully I was clear. Let me know if that was not the case.

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GrimPhoenix9349 avatar image GrimPhoenix9349 boufogre commented ·

And then we come to the extreme case - Concorde (which I am currently working on and the inbuilt behaviours are potentially going to see the project shelved). There appears to be no way to actually create a delta wing, nor a wing with multiple sweep angles. However, the closest we can get is a highly-swept wing of 55 degrees. Combine this with a CG%MAC range also in the 50s (52.5% for take off, moving aft during flight) and the potential problems can already be seen.


It flies reasonably 'okay' if you dive it first to high speed / low angles of attack but if you bring it down to circuit speeds (at which you could be looking for AoAs of between 7 and 12.5 degrees) the forward movement of the CP causes backflips, absolutely uncontrollable. At those high angles of attack, most of Concorde's lift comes from the vortex created on the upper surface of the wing, which is stronger the further aft you get. Obviously, this has not been simulated in the core flight model (I accept it would be a lot of work for very little return) but when so much else which is Concorde-specific has been modelled (intake ramps and the control of Mach number of air entering the intake, for example) it is doubly frustrating to see the entire balance of the aircraft being ruined with seemingly no way for us to take control of this behaviour.


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boufogre avatar image boufogre FlyingRaccoon ♦♦ commented ·

Hello @FlyingRaccoon,


I keep investigating this issue and I have to disagree partly with the explanation of your people.

Here is a scheme showing the span-wise lift distribution for different wing shapes:

1647346463680.png


As you can see, the higher lift coefficients are generated around mid-span for the wing, and that would explain why the CP doesn't get shifted FWD, but AFT.

It looks to me that when inducing sweep, in the sim, the CP is moved FWD with the chord section located at the centre line of the fuselage. To reflect the above figure, nothing should happen to CP position.

I hope I am clear. Again, I am free and available to explain this directly via a Teams or any other way.

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1647346463680.png (42.5 KiB)
FlyingRaccoon avatar image FlyingRaccoon ♦♦ boufogre commented ·

Hello @boufogre


From what you show here and the explanations I got from the FM devs, I think the difference is the wing twist your scheme is not considering.

I am paraphrasing the FM devs:
When you add sweep to a twisted wing, there's more positive lift towards the front, and more negative lift towards the back (especially at high speed). So the CP actually moves.
In addition, on a high sweep angle wing, CP position will move a lot depending on twist and incidence.

Our method is to align wing surfaces depending on a CP position we explicitly set in the cfg. But that CP is averaged on the chord, independently from sweep and incidence .
Otherwise, every time incidence or sweep would be changed, you'd have to readjust the CP position to realign the wing geometry. It would be hard to tweak.

So instead, the real CP in the sim will match the one in the cfg only at a certain incidence, and that will change depending on twist and sweep.
Does that make sense?


New debug information will appear with SU9 regarding this:

We display the CP position in a local chord reference for several wing sections.
It should always be around 25%, let's say in a 15% to 35% bracket. This is the case in this example on all sections.
We also display the CP position in %MAC. It will show that everything happens between 0 and 100% MAC when there's no sweep, as the mean chord matches each section chord.
But if you have a high sweep angle, the local chord is not aligned at all with the mean chord so we go from -25%MAC to 135%MAC.
And if you add twist to that model, you can have negative lift at the tip of the wing (135%MAC) causing a big displacement of the CP.


I hope this helps you understanding how the current model works.
Please take advantage of the upcoming SU9 to check out those new debug values and come back to us if there's more explanations needed or you if you have more feedback.

Regards,
Sylvain


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Sherv avatar image
Sherv answered Sherv edited

Hi MSFS team, can you advise on this? Is the only way to have proper swept wing logic by using a WASM module? From what Romain has mentioned, there are some serious issues with how the swept wing behaves. In fact I am willing to bet that PMDG and other major devs are probably using WASM for the flight model for their swept wing models. If this is the only way can you please expedite some documentation on using WASM for flight models? Thanks.

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FlyingRaccoon avatar image FlyingRaccoon ♦♦ commented ·
PMDG and "other major devs" are not using WASM for their flight model.
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Sherv avatar image Sherv FlyingRaccoon ♦♦ commented ·

Well I stand corrected. That is good news to hear. Thanks Sylvain.

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Cammac avatar image
Cammac answered

I would like to add that with my current project I am also seeing the exact same wing sweep behavior shown here in this thread.

My projects wing sweep is 28 degrees and the CP position is around 1-2% MAC most of the time, with the empty weight CG being at 25%. This same aircraft inside another simulation platform showed a CP position of around 40% which matches closer to real world data.

Thanks hope this helps.


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boufogre avatar image
boufogre answered

Hello,

Did you have a chance to review this topic?

Thanks!

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boufogre answered

Hello again,

Any news on that topic?

Thanks.

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