Effect of touchdown on angle of attack, tailwheel vs tricycle

Question

I have a theory but I can't find any documented confirmation. Please let me know if my theory seems right:

My theory: During a regular touchdown, a tricycle gear equipped airplane, since the main wheels are behind the center of gravity, rotates around its transversal axis (nose down), thus reducing the angle of attack. This should reduce the plane tendency to rebound (unless the nosewheel touches the ground).

On a tail-wheel airplane, the effect in reversed, on touchdown the airplane rotates around in transversal axis, thus increasing the angle of attack.

Does that make sense?

Answer 1

Yes, it does make sense.

However, the angle of attack increase of the tailwheel airplane would only be possible if it touches down on the main wheels while there still is much clearance of the tail wheel or skid. Normally, the ground attitude of tailwheel airplanes should be very close to their landing attitude, so all three wheels touch down almost simultaneously.

Answer 2

Yes. With a tailwheel airplane, if you are trying to make a "wheel landing" (on the main wheels only) rather than a 3-point landing, it is critical that the sink rate be very low at the moment of touchdown, or else the plane will tend to pitch nose-up (tail-down) which will make it bounce back into the air. A plane with tricycle gear doesn't have this tendency-- it pitches the other way and tends to stay on the ground. On the other hand, in a tailwheel plane, if you hold it off the ground with aft stick input till it runs out of airspeed and settles to the ground in a three-point attitude, there should be little tendency to bounce even if the sink rate is a little high.

Answer 3

Agreed. Tail wheel aircraft generally have the required angle of attack on touchdown (for a given landing speed) very close to the angle achieved when all three wheels are in contact with the ground. Hence, if the aircraft is landed at too higher speed the tail will be high, and without care, the touchdown can cause the aircraft to rotate around the main wheels with the tail wheel descending and further increasing the angle of attack causing the aircraft to get airborne again or bounce. Even at the correct landing speed, too high a rate of descent on touchdown can have the same effect.

Answer 4

If a taildragger is configured and flown properly, actually the result is the same as for a tri-gear.

If you land 3-point, a "full stall" landing, the ideal is for the tailwheel to make contact just before the mains, so that ground contact has the result of reducing AOA (a tiny amount). If you contact mains first you are likely to skip or bounce as AOA increases, but you've screwed it up in the first place if that happens. You could contact with all 3 wheels simultaneously, occasionally, but most of the time it's one or the other and tailwheel first is preferable to mains first.

If you wheel land, on the mains, although the moments are trying to make the plane pitch up at ground contact, the result is the same because with the proper technique, a pitch over is induced by pilot input as part of the landing process. A proper wheel landing is done by leveling off with the wheels a foot or two above the surface, and then you push the stick (it takes some nerve at first until you get the hang of it). When ground contact is made, the down elevator input will normally raise the tail a bit more, reducing AOA further, and this also overpowers the pitch up tendency from the CG behind the mains and prevents bouncing.