AOA stability of fixed and rotary wings

[chornedsnorkack]

Do rotary wing craft have the stability to preserve their angle of attack?

Consider what happens to a plane that flies level and steady, then loses thrust. It continues to have drag, so the airspeed decreases. Decrease in the square of airspeed at constant angle of attack causes decrease of lift, so the craft accelerates downwards. Once the craft begins to descend, the angle of attack is no longer equal to the angle of attitude - it increases and causes increase of lift.

However, the plane has a horizontal stabilizer - tailplane or canards. This reacts to changes of angle of attack by creating a torque changing the pitch angle of attitude. The craft would drop nose - and the lift would acquire a forward component until the forward component of lift equals the drag. By the operation of the horizontal stabilizers, the plane would transition from level flight to descent while the trimmed angle of attack and speed of the craft would remain unchanged - though the transition would also excite the phugoid oscillations in pitch.

Now consider a rotary wing losing thrust!

The wing would slow by drag... square of airspeed would decrease... the craft would accelerate downwards... the angle of attack would increase above the angle of attitude...

But is there any stabilizer or feedback capable of changing the angle of attitude of a rotary wing in response to changes of the angle of attack?

[Dave_Jackson]

Both the plane and the rotorcraft (should) have a center of gravity that is ahead of the center of lift. This, combined with the 'up-flow' on the horizontal stabilizers, will pitch the fuselage of both craft nose down.

When the fuselage of the plane pitches nose down, so do the airfoils (wings). When the fuselage of the rotorcraft pitches nose-down, the loosely coupled airfoils (rotor) do not pitch down at the same rate.

Two solutions to this shortcoming of rotorcraft are;
~ 1/ 'Absolutly' rigid rotors [ http://www.unicopter.com/0815.html ] so that the change of the rotor-disk's angle of attitude matches that of the fuselage's angle of attitude.
~ 2/ Rotor Govenor [ http://www.unicopter.com/Governor.html ] so that the blades' angle of attitude changes with the blade's rotational airspeed.

[chornedsnorkack]

Both the plane and the rotorcraft (should) have a center of gravity that is ahead of the center of lift. This, combined with the 'up-flow' on the horizontal stabilizers, will pitch the fuselage of both craft nose down.


In balanced cruise, the center of lift and of gravity should be exactly aligned, giving zero torque. Of course, the centre of lift should be taken over all surfaces - main wing and any stabilizers.

Helicopters often have no horizontal stabilizer, the tail rotor working only sidewards.

Do rotorcraft have any restoring stability in case of deviations of airspeed? Fixed-wing planes have such stability.

[Flingwing207]

In cruise flight, while it is convenient to think of the "rotary wing", you really have to consider the individual blades. Of course, if you don't lower the collective, the rotor will stall, rendering your discussion moot. If you do lower the collective, the nose will pitch down due to reduced flapping amplitude. This will increase airspeed and reduce RPM, so no cyclic inputs at power loss in cruise flight could also cause rotor stall.

[NickLappos]

While CG relationships to the rotor are a contributer to stability, airplane textbooks tend to tell the wrong story for helicopters. Regardless of CG relationships, the rotor itself has very strong negative angle of attack stability. A rotor in forward flight wants to "blow downstream" if it is disturbed, and this strong negative moment stability dominates any discussion of rotorcraft stabilization.
What do we mean by "blown downstream"? If a rotor is trimmed in forward flight, and we disturb it by tilting it up a bit, it will tend to flip upside down of its own accord, because the change in angle of attack of the upsweeping blade is great, and this tends to create a nose-up moment change. This nose up moment lead to yet more nose up, and so on, because the stability is negative (or the instability is positive). In fact, without a horizontal tail, all helicopter flights would involve one more take-off than landing.

There are some very good discussions of this in the several helo design texts.

[Dave_Jackson]

Chornedsnorkack,

When considering the center of lift, center of gravity and horizontal stabilizer you might find Bell's solution interesting.

The HS on (some?) of their helicopters consisted of an inverted airfoil. During cruise the HS has negative lift and this contributes to keeping the nose of the fuselage up. During autorotation the HS stalls and this results in a positive lift on the HS.

Its location in respect to the rotor disk is also relevant.

[Graviman]

But is there any stabilizer or feedback capable of changing the angle of attitude of a rotary wing in response to changes of the angle of attack?

Yup, the pilot! Control augmentation is the way to go in the heelicopterr.

In fact, without a horizontal tail, all helicopter flights would involve one more take-off than landing.

That is a scary fact. High effective hinge offset will help reduce flapback.

Mart

[NickLappos]

Actually, Mart, the backflap is still quite severe in a "rigid" rotor. Although the flapping contribution is much reduced due to the smaller blade motions, the rotor control moment is much higher, so they are probably less stable than a conventional rotor!

[Graviman]

Must admit i remember being mildly alarmed when Prouty discussed pitch/roll coupling in a rigid, Nick. I've never flown a rigid rotor, but i get the impression pilot authority is improved at the expense of higher flight workload (without control augmentation). Is this your experience?

Mart

[NickLappos]

Mart,
The basic stability of an unaugmented "rigid" rotor is generally worse, but the controls are more powerful, and the control damping is better, so they feel more pleasant, overall. There is a sharpness or quickness that is very nice.