Modern gliders, but not only (some STOL aircraft and even the B787) make use
of negative flap settings to adjust their L/D to higher speeds. This shifts
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. in the Flaps section of the flight-dynamics.cfg a table
can be used for flaps-position.i where the parameters are, for each flap
deployment stop: flap_position airspeed_limit drag_scalar
lift_scalar area_scalar add_camber add_aft_feet
add_incidence Are these parameters incremental or absolute (maybe a mix)?
Some seem redundant, like add_camber/add_incidence/area scalar/add_aft_feet…
OTOH wonder if using negative values, for instance in add_camber and in
lift_scalar and drag_scalar can be used to try to replicate the effect
of setting negative flaps?
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.
Ah! Now I understand. Interesting idea indeed !!! Thank you for your
suggestion and explanation.
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 tried to have a little test of the CTSL and the negative flaps do seem to
work. Using the SDK Sim Polar VhVs, the L/D ratio of Flap -12 is better than
the L/D ratio of Flap 0 at high speed (140 knots), and worse at lower speed
(50 knots). Not entirely reliable numbers as the magneto-off prop was spinning
at 385 rpm (50knots) 2800 rpm (140 knots) but that was similar for both -12
and 0 flap settings.
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.