Is it necessary for an elevator / horizontal stabiliser to produce a down force?

Alpha Floor explains this better than I do. I have experienced a couple of planes with reversing pitch control forces, and a couple with inadequate pitch control, both nose up, and nose down. It makes you very willing to operate within the C of G limits, and appreciate stable planes!

Alpha_Floor wrote:

The total moment of wing/body and HTP loads about the CG remains constant and zero (otherwise there would be no balance in pitch)

Agreed!

Alpha_Floor wrote:

because of the worsening Lift/Drag ratio with increasing AoA approaching the stall, the whole system becomes more inefficient and more lift is “wasted” in balancing it out.

I don’t immediately get that. The total lift on the aircraft is unchanged but as a result of CP shift moves closer in longitudinal position to the CG, in itself bringing the plane closer to balance, not further from it. However, point taken, there is also more induced drag. However, unless the additional drag force acts on a significant moment arm relative the the propeller thrust line, it should have no effect on longitudinal balance. Also, the effect on longitudinal balance would depend on whether its a high or low wing plane, e.g. a high wing plane would typically see a nose up trim change with power added to oppose increased induced drag, not the converse.

I cannot so far see a reason in the overall moment diagram why the tail download required increases with increasing angle of attack, as opposed to the pitch control force gradient felt by the pilot through the elevator control, which is a different thing.

Pilot_DAR wrote:

I have experienced a couple of planes with reversing pitch control forces, and a couple with inadequate pitch control, both nose up, and nose down.

I’m sure that was unpleasant

As per my post above, the key issue in answering Peter’s question is an accurate description of how the ‘negative slope aircraft pitching moment versus angle of attack’ stability concept introduced by @Alpha_Floor is created by the individual wing and tail characteristics of a conventional configuration light aircraft. I don’t think anybody has done that yet

There is no need for a tail. Tailless aircraft flies just fine (one single wing). Or moving fast enough, one single fuselage (missiles). The original question by Peter is concerned with dynamic stability. For instance, what will happen if one pertubate a steady flight with a small input on the stick, or a small gust.

The stability properties are similar to those of tail wheel vs nose wheel. A small perturbation will first change the path from a straight line to a curve. The curved path will create a centrifugal force (in addition to gravity). Depending on the CG and the center of lift, the centrifugal force will try to restore the straight line, or amplify the curved path. CG aft of the center of lift will therefore be unstable, while CG fwd of the center of lift will be stable. Just like tail wheel vs nose wheel. The tail surface lifting down, is really more by coincidence to counteract the moment in straight flight. It could be moved fwd of the wing lifting up, or removed altogether. A canard is the obvious choice aerodynamically, but has some practical disadvantages.

Nobody mentioned the Piaggio Avanti yet? :-)

https://en.wikipedia.org/wiki/Three-surface_aircraft#Pitch_equilibrium

I remember it being described as having all three surface generating positive lift, but that link above seems to suggest that the tailplane might still be producing a down force, just a lot smaller than on more common types.

LeSving wrote:

The original question by Peter is concerned with dynamic stability.

That’s what may have been missing from the discussion. It’s still too early here for my brain to function properly, but comparison of static versus dynamic stability issues is something to think about. Thanks.

Re the tailwheel analogy, when I started flying in a tail wheel I went through the stability and control thing for many different scenarios and found it to be a very complex situation, with factors and situations that are never explained to pilots, and possibly not anybody else either. A lot of interesting things like the possibility to back a tailwheel plane into its parking place using inertia and differential braking.

Peter wrote:

My brain is fully stretched now and I still don’t get it.

Mine, too! I just looked at the “Stability and Control” chapter of the “Principles of Flight” part of the Oxford Aviation Academy ATPL Manual. In every figure, they have an up force from the horisontal stabiliser! I think that chapter has to be my evening reading.

Peter wrote:

I can’t see why a down force during flight is necessary for stability, stall recovery, etc.

The down force in a light GA aircraft of conventional configuration is required for BALANCE, not for stability.
Stability is achieved by having the CG forward of the aircraft’s aerodynamic centre.

For stall recovery what matters is that wing stalls before the HTP. A larger angle of attack on the HTP will increase its load in the UP direction: so if the load was pointing down, its magnitude will decrease. If it was pointing up, its magnitude will increase.

This is all considering there are no other effects like the nose-up pitching moment caused by engine thrust line placed below the CG. The moment of the thrust about the CG can revert the direction of the load on the HTP. In a normal light GA aircraft this is not the case.

Airborne_Again wrote:

In every figure, they have an up force from the horisontal stabiliser!

Sure enough the force vector in the diagrams is usually pointing in the UP direction because that’s the POSITIVE direction. What happens when you do the calculations is that the force turns out to be of negative value, that is, it’s pointing down.

This is the best “simple terms” explanation I could find as to why the tail load is negative in a conventional and stable aircraft (case 3 below):

From Roskam, Airplane Flight Dynamics. Reproduced here under fair use.