question

To simulate negative flap you need to use both the flaps and a spoiler. For the animation you would animate based on the positive flap or negative spoiler. So you need to have two animated parts. For the cfg side you need both flaps and spoiler.

So if there is no positive flaps just use spoilers only for the animated reverse flaps mesh parts.

Thank you @Flysimware

I didn't fully understand your suggestion, namely that a spoiler has to be used for the negative flap "reflex". I thought that the add_camber and add_incidence parameters could take negative values, that way allowing to change the wing profile for negative flap settings.

I am not worried with the graphical side of the system but rather with the aerodynamics and the tuning required to allow for the necessary shift ( to the right and down ) of the L/D curve, allowing for the same L/D at higher speeds.

The point is I am pretty sure there is no negative flaps performance options. And here is why I think we can't simulate this control surface the way you are asking.

We have to tell the sim 4 major factors for performance under [AERODYNAMICS] for lift and loss of lift.

1. lift_coef_flaps (Defines the lift coefficient that will be added to the target lift coefficient obtained with the lift_coef_aoa_table of the airplane when at maximum flap expansion.)

2. lift_coef_spoilers (This allows you to correctly tune the spoilers for ground usage where there is very strong drag and very strong loss in lift.)

3. lift_coef_air_spoilers (This allows to correctly tune the spoiler behaviour in the air where you have strong drag, and little loss in lift.) Note: You can have some lift to simulate negative flaps.

4. drag_coef_spoilers Note: You can have some some drag to simulate negative flaps.

So as you can see #1 there is no way to have flaps produce loss in lift as mentioned in SDK, but for air spoilers we do have loss in lift.

Hold on just a second here! Negative flaps was an issue significantly highlighted on the build up to the gliders development and saw effort put into the sim, actually emerging in the Flight Design CTSL "-12 flaps" with a slight hiccup at first but apparently now fixed so that flap setting is the most efficient for high-speed cruise. The idea that spoilers or increased drag are somehow related to negative flaps was an earlier misunderstanding that took a while to correct.

The original description in this thread by @jcomm is correct, and the thread has gone adrift since then: Negative flaps "shift the L/D to the right allowing to have lower induced drag / lift coefficient when travelling at higher speeds while maintaining the same L/D characteristics. "

The entire point is to IMPROVE the LIFT/DRAG ratio (which equates to the glide ratio) with negative flaps and additional drag is absolutely the last thing you're aiming for. Counter-intuitively negative flaps do this by reducing lift by a small amount and reducing total drag by a larger amount so the L/D actually improves. So I'm afraid the idea that you'd want to deploy spoilers in some kind of simulation of negative flaps is nonsense. I *would* be interested to know if the negative lift effect of spoilers actually reduces the overall 'induced drag' of the wing, which would be interesting, but I doubt that in the simulation as previously the control surfaces have been modelled separately from the wing.

So the best test of negative flaps in MSFS would be to try and test the -12 flap setting in the CTSL.

I've added negative flaps to the Jonkers JS3 sailplane we're developing and they work as advertised/expected with the small deflections we use (like -3 degrees), i.e. the glide ratio improves at high speed (compared to zero flap) and is less good at low speed. So definitely thanks Asobo!

The subtlety I'm working on how is how to tune the +ve/-ve effect i.e. how bad the low-speed L/D is vs how good the high speed is, and the speed of the cross-over with zero flap. Plenty of parameters so we'll be fine, I think.