Airborne_Again wrote:
I don’t see why a minimum flap setting which is negative would be more dangerous that a zero flap setting?
Any flap setting will change the span wise distribution of lift, and hence effect the general stall characteristics, and in particular the initial resistance to enter a spin. However as long as it checked in the certification (which it should) I can’t really see the problem. However I think many aircraft when at the edges of the envelope are either very close to the certification limits, or what designer will tolerate with regarding those margins.
Universally GA aircraft have wings that could be easily designed to be more efficient in cruise. A simple taper at the tips, or just make the wings smaller with better flaps. e.g The TBM uses longer span flaps, with small ailerons supplemented with spoilers.
Even the F-16 has wings that are too big for cruise, probably because it was designed to do more than cruise. 
UdoR wrote:
Therefore, I assume that this is the reason why you don’t find such flaps in typical GA aircraft.
I don’t think it’s a lack of safety but rather a lack of need.
The operating airspeed range of SEP is quite limited in cruise – esp. compared to high performance gliders where negative flap settings are common.
If the airfoil is optimized for efficiency at Vc, it doesn’t really matter if you could improve it with negative flaps for the rare cases where you actually fly faster than Vc.
Stall characteristics with negative flaps are comparatively irrelevant as when you use negative flaps you should be in a speed range that is far away from Stallspeed – it can only become dangerous if you forget to adjust when slowing down (e.g. for landing). But then it is more th higher stall speed than any kind of characteristics that get dangerous.
Airborne_Again wrote:
I don’t see why a minimum flap setting which is negative would be more dangerous that a zero flap setting?
Not necessary would, but certainly could. With negative flaps, the tip could stall first. This will cause a rapid roll since one tip would stall before the other.
LeSving wrote:
With negative flaps, the tip could stall first. This will cause a rapid roll since one tip would stall before the other.
One wing may drop, speed will build up quickly but it does not mean you are going into spin, it’s hard to spin with negative flaps just as much as hard to spin with GC forward or lift P forward, in any case with negative flaps, the aircraft will stall at lower geometric AoA values unlike with full positive flaps
At extreme scenario -40deg negative flaps: there not much room for AoA to start with, and the delta between wings AoA is just too small so that both wings will just stall at the same time, maybe roughly 1deg max AoA near VNE, with +40deg flaps, max AoA is near 15deg, you can have one wing at 15deg and the other at 10deg with 2.5deg from slip or skid near VS0
What you get with negative flaps is some nasty dives, high performance gliders wings tend to bite when one is thermaling on cruise flaps
I think any speed below maneuvering speed (Va) is a speed at which stall characteristics are important. Above Va (or Vo) the plane breaks before you can stall it.
That is a quite common misconception. The maneuvering speed is the speed at which full deflection of a control surface can create forces that exceed the structural limit of the airplane. It is – if at all – only very loosely connected to stall as stall is an aerodynamic condition while maneuvering speed is also significantly influenced by structural factors.
And like Silvaire said the Va protects the structure by stalling first instead of exceeding g limit.
“Very loosely connected” ?
With a slow, strong aerobatic plane you can stall it at cruise speed without issue, just apply enough G. That is for example how a snap roll is done, with rudder applied as it stalls to make it a horizontal spin. It’s when you don’t want to inadvertently do the snap roll that stall characteristics in general would be significant.
I would not want a plane that has poor stall characteristics, e.g. tip stalling induced by reflexed inboard flaps or similar, at any speed that it could be stalled without structural issues being dominant. That means at any speed below maneuvering speed, Va or Vo as applicable.
Airborne_Again wrote:
I don’t see why a minimum flap setting which is negative would be more dangerous that a zero flap setting?
OK I must admit that I don’t remember the details, nor can I puzzle it out now. This is stuff from when I was young, more than 20 years ago, when I was flying gliders with such features. I was told to not stall or go in thermals with negative flap setting, as this could end real bad. So I can’t contribute more to this, sorry.
Airborne_Again wrote:
I don’t see why a minimum flap setting which is negative would be more dangerous that a zero flap setting?
I think it’s “dangerous” in the way it will remove washout from the wings. Most light GA aircraft have washout (and if you look down some plane’s wings spanwise it’s really very visible) – the wing has a twist in it which means the inboard section is at a higher AoA than the outboard, so the inboard stalls first, leaving the ailerons at least somewhat effective at the onset of a stall. So you will get a warning of an impending stall (some stall buffet) during the onset, but all the aircraft’s controls continue to work as expected.
Now if you put in negative flaps (enough to be worthwhile), now the washout is “backwards” – the wing tips are at a higher AoA than the inboard section of the wing, and the outboard section will stall first. You also will not have much warning of this – there’s no disturbed air now flowing over the tail so no buffet. You might put some aileron in not realising the outboard section of the wing is about to stall, immediately causing the outboard wing on the downgoing aileron to stall, leading to control reversal (e.g. if you turn the wheel right, the plane will start to bank left instead, which will cause you to add even more right aileron input, causing the stall to deepen and the roll rate to the left to increase, causing you to add even more…till all the aileron is in and the aircraft rolls on its back).
So negative flaps in a certified plane are generally not worth it. To make the aircraft meet certification requirements on stalls, you would need so much washout to have a negative flap setting that doesn’t result in the tips stalling first, that you might as well have less washout and no negative flap setting.
