I agree one must arrange for the nose to pitch down in a stall, but that merely needs the wing to stall before the elevator (regardless of how the “elevator” is arranged) loses effectiveness.
What I don’t understand is why it is necessary for the elevator to be pushing down for this to work. In fact if the elevator was pushing up then the nose-down pitch would happen even sooner.
The direction of load on the horizontal tail assembly at max AoA (just prior to stall) counters the moment of the wing lift around the CG, and thereby maintains maintain stable aircraft pitch attitude. Hence my question of the fore/aft location of the wing center of pressure relative to the CG in that condition – because it answers your question.
Pilot control forces, static trim required etc is indeed a separate question.
Peter wrote:
In fact if the elevator was pushing up then the nose-down pitch would happen even sooner.
Correct. Very soon. Actually so soon you would never be able to rotate.
Peter wrote:
What I don’t understand is why it is necessary for the elevator to be pushing down for this to work.
The HTP load points down to be able to balance the aircraft in straight and level flight. Why? Because the CG sits forward of the NP. Why? Because if it wasn’t so the aircraft wouldn’t be stable in pitch.
Just before the stalling AoA, the lift on the wing is high (up), and the load on the HTP is also high (down) for balance.
Just after passing the AoAmax (i.e., stalling), the lift on the wing goes down (it doesn’t disappear), so its pitch-down moment about the CG is reduced. The aircraft tends to pitch up and increase the AoA even further. By increasing the aircraft AoA, the down load on the HTP reduces in magnitude (it becomes slightly less of a down force, even if the stick is still full aft), this then makes the aircraft pitch down. By pitching down, the wing lift increases (because we are still at AoA greater than AoAmax), which makes the aircraft pitch down even further. But now the HTP downforce is increased, so the aircraft will pitch up again and so on. You are in a fully developped but stable stalled condition.
This should be demonstrated during PPL training, and I mean, descend 1000 ft or so in the fully developped stalled condition to demonstrate that it’s stable.
In an aircraft with conventional configuration like the TB20 you can’t stall the HTP and therefore as as soon as you unload the stick the down load on the HTP is greatly reduced (in magnitude, though it will still point down) and the aircraft will pitch down because the wing lift has a strong nose-down pitching moment associated with it, even when stalled.
Alpha_Floor wrote:
Just before the stalling AoA, the lift on the wing is high (up), and the load on the HTP is also high (down) for balance.
The lift on the wing is the same as at lower AoA, equal to the weight of the plane, and the center of pressure is closer to the CG, having moved forward. That lowers the tail moment required to maintain stable pitch attitude.
The fixed horizontal tail has meanwhile with high wing AoA assumed an attitude that decreases its download.
Alpha_Floor wrote:
Just after passing the AoAmax (i.e., stalling), the lift on the wing goes down (it doesn’t disappear), so its pitch-down moment about the CG is reduced. The aircraft tends to pitch up and increase the AoA even further. By increasing the aircraft AoA, the down load on the HTP reduces in magnitude (it becomes slightly less of a down force, even if the stick is still full aft), this then makes the aircraft pitch down. By pitching down, the wing lift increases (because we are still at AoA greater than AoAmax), which makes the aircraft pitch down even further. But now the HTP downforce is increased, so the aircraft will pitch up again and so on.
Which raises some interesting questions about what happens if the pre-stall AoA increase were to shift the center of pressure forward of the CG, with the horizontal tail transitioning by design to upload to maintain stability. In that case the loss of wing lift moment at stall drops the nose and the horizontal tail transitions again to download. That would seemingly reestablish a stable condition at lower AoA, no?
Regardless of any of the above, when wing lift is lost the plane drops vertically, creating an upload on the horizontal tail in the transient condition.
Silvaire wrote:
The load on the wing is the same as at lower AoA, equal to the weight of the plane,
Well, not exactly because it’s the TOTAL lift that remains constant, the combination of wing lift and tail lift.
Silvaire wrote:
and the center of pressure is closer to the CG
The center of pressure is not very useful in aerodynamics.
We use the Aerodynamic Centre (AC), where the pitching moment is independent of angle of attack.
Alpha_Floor wrote:
Well, not exactly because it’s the TOTAL lift that remains constant, the combination of wing lift and tail lift.
That’s a good point and as AoA increases, tail download/moment must decrease as the center of pressure of the wing moves closer to the GG. This lowers the wing lift required at 1G, lowering the stall speed.
I’m not sure this is getting any closer to answering Peter’s question, despite my best efforts 
What he wants to know is why it is necessary to maintain a wasteful download on the tail in cruise condition, which obviously requires a clear understanding of static pitch stability of conventional light aircraft to explain in simple non-mathematical terms.
A clear explanation of what the the total aircraft pitching moment versus angle of attack slope does in effect, i.e what positive and negative means other than being a mathematical criterion, would be useful. And how the individual wing and tail characteristics of a conventional light aircraft combine to produce the required negative slope for the aircraft as a whole. An aircraft designer clearly has to understand it on that level. I’m not there yet and would also enjoy seeing an explanation on that level.
I’m sure if Al Mooney and Richard Van Grunsven could master this successfully, many others can too. Neither of them had any education in aircraft aerodynamics or stability and control.
I think it’s actually easier for a canard, as explained to me: both wings lift and the CG is in the middle. Beyond that, for pitch stability you need the rate of increase of lift with aircraft angle of attack for the fore plane to be lower than the main plane, and for the fore plane to stall at a lower aircraft AoA. Hence a very ‘round top’ section wing section on the front, one that doesn’t make a lot more lift with more AoA, plus appropriate installed incidence on both wings. Not really very complicated in principle.
Silvaire wrote:
as AoA increases, tail download/moment must decrease as the center of pressure of the wing moves closer to the GG. This lowers the wing lift required at 1G, lowering the stall speed.
The AoA increases precisely because we are increasing the HTP load in magnitude (by either deflecting the elevator or the whole tailplane). To maintain overall balance, the wing lift increases as well. The 1G stall speed is what it is. You don’t “lower” it by increasing the AoA.
You are considering the point of application of wing lift as the Centre of Pressure: CP. This is impractical because this point moves around.
It’s better to apply the lift at the Aerodynamic Centre, which is fixed. To move the lift point of application from the CP to the AC, we also apply a compensating pitching moment. By definition of AC, this pitching moment is constant with angle of attack.
Understanding the aerodynamics of the whole thing is easier this way.
The Centre of Pressure moves forward with increasing AoA, yes. But what you’re not considering is that when the CP moves forward, the lift applied on the CP is increased and the moment about the AC is conserved. By virtue of the increased lift and increased pitch-down moment of the AC about the CG, the HTP has to compensate by increasing the down load.
Alpha_Floor wrote:
The AoA increases precisely because we are increasing the HTP load in magnitude (by either deflecting the elevator or the whole tailplane). To maintain overall balance, the wing lift increases as well. The 1G stall speed is what it is. You don’t “lower” it by increasing the AoA.
I don’t get why the downward tail moment is increasing as the wing center of pressure moves forward, closer to the aircraft CG, with increased AoA. The total lift on the aircraft is constant and the pitching moment between the wing lift and aircraft weight is decreasing, no? What am I missing?
The forces on the horizontal tail assembly that combine to create the required tail downforce are a separate issue: As explained by those who like stabilators, the movable elevator has to overcome an upload on the horizontal stabilizer before a net download on the aircraft tail is produced, a wasteful situation. The increased elevator deflection required to do that is what creates the backward stick force to maintain tail download and the high wing AoA.
Good stuff, anyway.
Silvaire wrote:
I don’t get why the downward tail moment is increasing as the wing center of pressure moves closer to the aircraft CG.As you reduce the airspeed and increase the AoA approaching the stall:
Silvaire wrote:
The pitching moment is decreasing, no?Which pitching moment? The overall aircraft pitching moment about the CG remains constant and zero for balance.
Why does the lift applied at the CP increase with increasing AoA?Because of the need to balance the increased HTP down load.
