Why tailwheel planes rise its tail?

[Alfredson007]

In a conventional airplane, i've understood, the horizontal stabilizer pushes the tail down, creating downforce, otherwise the nose would dive in flight. But in a tailwheel plane you can rise the tailwheel only with propblast, so it's creating upforce, or lift. Why is this?

[Oracle427]

Assuming the brakes are not held and the stick is positioned neutral or full back, the tail will not rise with the prop blast. The relative wind of the prop blast will generate a low angle of attack on the tail. As the prop blast is flowing along the longitudinal axis.

As the aircraft starts to roll with the tail down and the stick neutral, the relative wind will transition to coming from beneath the tail with an increasing angle of attack. At some it will generate enough lift to raise the tail. As the tail rises the angle of attack will decrease until it is fairly close to neutral or wherever it needs to be to cancel out any pitch moment up or down.

Another thing to think about is the way that the elevator control on the Cessna Skyhawk behaves. The yoke does not have centering springs, so it will extend all the way back when the engine is off out at very low power while on the ground and motionless.

The elevator will hang down as a result. The moment the power is applied the prop blast will hit the underside of the elevator and push up on it and force the elevator and yoke forward to a neutral position. The tail itself won't try to move as it is aligned into the prop blast unless you try to stop the elevator from centering. Once it is centered the AoA will be low and the control will remain steady.

BTW: Conventional landing gear = tailwheel landing gear.

[AKar]

In a conventional airplane, i've understood, the horizontal stabilizer pushes the tail down, creating downforce, otherwise the nose would dive in flight. But in a tailwheel plane you can rise the tailwheel only with propblast, so it's creating upforce, or lift. Why is this?

Exactly like wings, tailplane can create lift both ways, up or down in relation to the airplane, there would be nothing mysterious in that. Prop blast only introduces another component to the relative airflow over the tail surfaces, as Oracle already said.

-Esa

[bobsk8]

I look at the tail as just another wing. When the airflow over the wing gets to the point where the wing starts producing Lift, the tail rises.

[AKar]

Yea, basically. If OP wonders about the principal difference of lifting the tail up in a tail dragger versus pushing it down in a tricycle, there is a chance of confusion if thinking the lift as a uni-directional entity, up for the wing and down for the tail plane, as it is often quoted (though not in perfectly accurate thems: even a statically stable airplane can have its tail plane producing some upwards lift in cases).

Both direction and magnitude of lift depend on the angle of attack. When speaking of airplane's stability, in aerodynamic terms the significant thing is something called decalage, or the difference in angle of the main wing and horizontal stabilizer. But in case of a take off roll of a tail dragger, both the main wing and the tail plane are at highly positive angle of attack, therefore lifting upwards. The tail plane has a good distance to the airplane's "pivot point", so fairly quickly the tail comes up.

If the question is instead that why the tail rises when one increases the power while standing still against the brakes, the same applies, however augmented by the thrust also turning the airplane around its locked pivot point (the main wheel).

-Esa