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?

[Brett_Henderson]

I think you're comparing apples and oranges. Kinda like the cyclic/collective controls are "similar" to a variable-pitch prop, but are actually, completely different, in flight.

The lifting surface (wing) on a fixed-wing craft relies on aerodynamic thrust to maintain an angle AoA.

The AoA of the lifting surface (rotor blade), or for that matter,

[chornedsnorkack]

The lifting surface (wing) on a fixed-wing craft relies on aerodynamic thrust to maintain an angle AoA.

But my point is, actually it does NOT rely on thrust. The angle AoA is maintained by the horizontal stabilizer irrespective of whether there is any thrust. If the thrust vanishes, the horizontal stabilizer ensures the plane will continue to fly at unchanged angle of attack and airspeed, dropping its nose and entering into a steady descent.
[quote]

The AoA of the lifting surface (rotor blade), or for that matter,

[Brett_Henderson]

So, if and when the engine/s stop providing mechanical energy, the rotor blades will slow down, suffer increase of AoA and stall, with no stability which would allow the blades to accelerate from the energy of airflow/descent of the copter?

Sort of.. yeah.. I see your point. I'm gonna call my cousin (5,000 hour Bell206 pilot) and ask him about engine failure / auto-rotation procedure. I suppose if the collective mechanism allows for "leading edge down" attitude of the rotor blade.. that would be like nosing down to reduce AoA and gain airspeed. But even pondering that points out the difference. If you can do that we'd have to equate airplane airspeed to helicopter rotor rpm... and again.. you can't alter the attitude of a fixed wing without redirecting the mass of the whole craft.

Think about the engine failure scenario. Altitudes being equal.. airpeed is your friend in a airplane and your enemy in a helicoter. Airspeed in a helicopter comes at the expense of lift and in a airplane you have lift because of airspeed.

Edit:

[yaarpanjabi]

Hi guys,

With the helicopters, thats why to autorotate, you only have a couple of seconds before the angle of attack reaches the stalling angle at which point, you will lose stability and pretty much drop out of the sky. When an engine on the helicopter fails, the rotors keep spinning at the same AoA.

The chopper starts to descend and therefore increases AoA on the rotors. Now, if you can get the thing to autorotate, you basically twist the "wing" (rotor) so that the angle of attack is such that it can sustain rotation and therefore gives you the ability to cushion your landing.

In other words, say that the rotor is a wing of a fixed wing aircraft, you're essentially twisting the wing forward, so that you get the same effect as to if you had an elevator pushing the nose down. You want the rotors to keep cutting through the air at a very low angle of attack (it may well be negative) so that the "dropping" of the helicopter is powering the rotors themselves. When you get close to the ground, you use that momentum and convert the low AoA to a higher AoA and decrese your descent rate and then touchdown while still moving forward.

Please note, I am not a helicopter expert but the above statement is what I can logically think of. I stand to be corrected.

Cheers.

[Brett_Henderson]

...You're pretty close.. But it's mechanically impossible to get a "leading edge down" attitude for a rotor blade (realtive to the body of the helicopter).. The blades are prevented, mechanically, from pitching that far negative..That's why airspeed is indeed important so you can maintain a nose-down attitude for the chopper itself.

The way it was explained to me... Each blade goes from negative to positive AoA for every revolution. As long as you are nose down and descending.. you get the "effect" of a diving airplane for enough of that cycle to maintain enough rotor RPM to pull the collective.. making all blades go WAY positive (AoA).. just before "flaring".. using up all that energy as to not hit the ground too hard.

Now.. this all for a Bell206.. I don't know about other helicopters.

[chornedsnorkack]

...You're pretty close.. But it's mechanically impossible to get a "leading edge down" attitude for a rotor blade (realtive to the body of the helicopter).. The blades are prevented, mechanically, from pitching that far negative..That's why airspeed is indeed important so you can maintain a nose-down attitude for the chopper itself.

The way it was explained to me... Each blade goes from negative to positive AoA for every revolution. As long as you are nose down and descending.. you get the "effect" of a diving airplane for enough of that cycle to maintain enough rotor RPM to pull the collective..

But what happens if the helicopter has no airspeed to begin with? That is, the helicopter is hovering (out of ground effect... plenty of height to fall from)... all blades are facing steady airflow and have constant, positive AoA. Now, when the engine quits, what would be the next thing to do before the blades stall?

[Brett_Henderson]

[quote]But what happens if the helicopter has no airspeed to begin with? That is, the helicopter is hovering (out of ground effect... plenty of height to fall from)... all blades are facing steady airflow and have constant, positive AoA. Now, when the engine quits, what would be the next thing to do before the blades stall?

[chornedsnorkack]

So:

a fixed-wing plane is close to a gliding attitude anyway with power, so in principle, it can transition from powered cruise to glide by itself

a helicopter is unable to transition to autorotation without quick pilot reaction, and it is physically prevented from autorotating in hover/vertical descent.

What about autogyros/gyroplanes? They are autorotating in cruise all the time... are they liable to get into trouble when an engine quits?

Then again, a rotary wing is said to be inefficient, so that a glide ratio of autogyro is poor compared to a, aeroplane. What is the range of the glide ratio of an autorotating helicopter?

[Brett_Henderson]

a fixed-wing plane is close to a gliding attitude anyway with power, so in principle, it can transition from powered cruise to glide by itself

I'd say that's a good way to put it. Level flight is pretty much a glide, with enough thrust to keep the airspeed/lift equal to gravity.

a helicopter is unable to transition to autorotation without quick pilot reaction, and it is physically prevented from autorotating in hover/vertical descent.

Tthat's the way it was explained to me, by someone who flies them every day.

[quote]What about autogyros/gyroplanes? They are autorotating in cruise all the time... are they liable to get into trouble when an engine quits?

Then again, a rotary wing is said to be inefficient, so that a glide ratio of autogyro is poor compared to a, aeroplane. What is the range of the glide ratio of an autorotating helicopter?