Dimme: That would clearly never happen if they drove on the correct side of the bloody road! xD
Also if the CG was not too aft ;)
Pilot_DAR wrote:
Airplanes (well, GA airplanes, anyway) are designed so that as the plane stalls, if you loose elevator effectiveness, the nose will drop, to aid recovery – because the tail stalled, and stopped lifting down. To be able to stop lifting down, it had to be lifting down in the first place.
Is that really what is happening? The tail is lifting down. Increasing AOA (of the aircraft, or the main wing if you want) will decrease the AOA of the horizontal stabilizer, and at some point it will start lifting up. Increasing elevator (pulling the stick) will of course practically increase the AOA of the elevator, but if that does not make the horizontal stab stall in horizontal flight, I really don’t see why it should make it stall when the AOA of the aircraft as a whole has increased. I mean, when the wing looses lift, stalls, and the horizontal stab keeps on flying, the nose will go down.
Pilot_DAR wrote:
Airplanes (well, GA airplanes, anyway) are designed so that as the plane stalls, if you loose elevator effectiveness, the nose will drop, to aid recovery – because the tail stalled, and stopped lifting down. To be able to stop lifting down, it had to be lifting down in the first place.
Do you mean that the tail will stall before the wing!?
As far as I’m aware, you can only make the tailplane a lifting surface if you put it in front of the wing – otherwise the aircraft becomes unstable in pitch.
I’ve not studied this enough to have an educated understanding, but my mental model has been that the tail is downloaded at cruise angle of attack (AoA), to a greater or lesser extent depending on CG location. As the AoA of the aircraft increases, the wing center of pressure shifts forward until at some point it transitions forward of the CG, at which point the ‘aircraft nose up’ horizontal tail geometry is automatically lifting and creating an opposing nose down moment, countered by elevator up deflection to the extent necessary for stable pitch attitude. When at some AoA the wing stalls, the tail does not and the lifting force on the horizontal stabilizer lowers the nose to un-stall the wing.
Obviously a fixed geometrical relationship between the wing and horizontal stabilizer means that loading the plane with a rearward CG creates a situation where at high AoA less nose up elevator deflection is required to counter the nose down moment of the lifting horizontal stabilizer. Move the CG far enough aft and you have to add more nose down moment with forward stick elevator deflection to stop the nose rising on its own, the definition of static pitch instability.
Is that anywhere near correct, or is the center of pressure always behind the CG at every angle of attack? (meaning downforce would always be required on the horizontal stabilizer).
It doesn’t matter if the wings or tail stall first. A stable aircraft with forward CG location will drop the nose regardless.
Silvaire wrote:
Is that anywhere near correct, or is the center of pressure always behind the CG at every angle of attack? (meaning downforce would always be required on the horizontal stabilizer)
Imagine what would happen if you violently push the stick forward. The tail starts producing lift upwards. Of course, the nose pitches down. But in that case, neither your wings nor your tail is stalled. (unless you are too violent and get in an inverted accelerated stall)
For positive stability, and predicable stall and spin recovery, the horizontal tail should produce a downforce, (lifting down) which when no longer possible (tail stalls) allows the nose to drop. If the wing stalls first and lowers the nose, that’s fine too. but the stability comes from what the tail does. I have flown two types, where during planned slow flight and stall testing, the pitch force reversed and a push was required to fly more slowly, and power was required to speed up. These airplanes were both non compliance, and difficult to fly precisely. Certainly, in that condition, taking your hands off the controls would be dangerous.
I’ve also flight tested a highly modified C182 amphibian, where the tail no longer had sufficient effectiveness at slow speed to raise the nose as desired to flare to land (particularly important for water landings), the tail was stalling. I had to create, test and approve an aerodynamic fix for this. That airplane tended toward being very nose heavy, and ballast was required.
Another variation is the Ercoupe, which cannot be brought to the point where the nose drops in a stall, because elevator deflection and downforce are design limited. Ultimately, it is not possible to have a manual control airplane where the aft horizontal tail is lifting in flight, and the plane is stable, with the required control forces. If the tail were to be lifting, and stall before the wing, the airplane would pitch up, and be entirely unrecoverable. perhaps highly computerized planes can manage this, but GA types cannot. I have found the reasons for C of G limits during flight testing.
Pilot_DAR wrote:
it is not possible to have a manual control airplane where the aft horizontal tail is lifting in flight, and the plane is stable, with the required control forces
I think that might be true for a single surface stabilator, but it seems to me with a conventional fixed horizontal stabilizer and elevator, the fixed stabilizer is lifting at high aircraft angle of attack, counteracted to the degree necessary by nose up elevator control. The combination creates the pitching moment required to counteract the pitching moment of wing lift around the aircraft CG.
The stated advantage of a stabilator is that you don’t have the fixed horizontal tail and elevator fighting each other in creating the required force.
My question is the fore and aft location of the wing center pressure in relation to the CG at max AoA not cruise.
