Is it possible....

[JudgeSmails218]

This is a really dumb question, but I'm not a helo guy! Is is possible to stall a helicopter somehow? Maybe if you had a problem with the tail rotor? Thanks for the edjumacashun!

 

[Kuma]

yes you can...

You can stall a helicopter in a whole bunch of ways. First, you can fly too fast. When you do, you stall the retreating blade and the aircraft rolls into the stalled blade. On American helos this is to the left.

Second, you can fly too slow. Hovering takes the most power, once you achieve "translational lift" the entire rotor system gets more efficient. If you slow below translational lift, the entire rotor stalls and you may not have the power or the altitude to fly out of it. Ground effect is very important in this regard as it takes way more power to hover out of ground effect than it does within ground effect.

Third, you can set up a descent that causes the rotor to fly through its own down-wash resulting in a stall.

Forth, you can fly through an airmass that is changing direction or speed (most commonly in the mountains, but could be anywhere like around buildings or other man-made structures).

Fifth, you can fly too fast sideways or backwards. Sideways causes the tailrotor to stall causing loss of directional control and flying backward causes a barn door effect with the aircraft fuselage that the flight controls may not be able to overcome.

There are probably more ways, but these are the quick 5 that I can think of.

Kuma

 

[JudgeSmails218]

Very interesting...thanks for the info!

 

[Hobit]

Two neat references:

An old one that started me off: (AC 61-13b FAA AC 61-13B Basic Helicopter Handbook )
http://www.hothelicopters.com/faa_ac_6113b.htm

The new, updated one: (FAA-H-8083-21 Rotorcraft Flying Handbook, PDF down-loadable)
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

Helicopters aerodynamics are quite interesting, worth the read.

 

[Kuma]

Two neat references:

An old one that started me off: (AC 61-13b FAA AC 61-13B Basic Helicopter Handbook )
http://www.hothelicopters.com/faa_ac_6113b.htm

The new, updated one: (FAA-H-8083-21 Rotorcraft Flying Handbook, PDF down-loadable)
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

Helicopters aerodynamics are quite interesting, worth the read.

Thanks Hobit...that reference is great.

Kuma

 

[rajflyboy]

You can stall a helicopter in a whole bunch of ways. First, you can fly too fast. When you do, you stall the retreating blade and the aircraft rolls into the stalled blade. On American helos this is to the left.

Second, you can fly too slow. Hovering takes the most power, once you achieve "translational lift" the entire rotor system gets more efficient. If you slow below translational lift, the entire rotor stalls and you may not have the power or the altitude to fly out of it. Ground effect is very important in this regard as it takes way more power to hover out of ground effect than it does within ground effect.

Third, you can set up a descent that causes the rotor to fly through its own down-wash resulting in a stall.

Forth, you can fly through an airmass that is changing direction or speed (most commonly in the mountains, but could be anywhere like around buildings or other man-made structures).

Fifth, you can fly too fast sideways or backwards. Sideways causes the tailrotor to stall causing loss of directional control and flying backward causes a barn door effect with the aircraft fuselage that the flight controls may not be able to overcome.

There are probably more ways, but these are the quick 5 that I can think of.

Kuma

This is it exactly. You are a smart smart smart man!

 

[Kuma]

This is it exactly. You are a smart smart smart man!

Why, thank you; you are too kind.

 

[Fox6]

If you slow below translational lift, the entire rotor stalls and you may not have the power or the altitude to fly out of it.
Kuma

Kuma, a question. If the entire rotor system is stalled, what is providing lift in the hover? ETL is characterized by moving into undisturbed air, thereby increasing the efficiency of the rotor system. Moving from ETL back to a hover puts the rotor system back into disturbed air caused by its tip vortices. My understanding is these vortices obviously disrupt the smooth flow of air over the airfoil, thus reducing rotor efficiency, but not necessarily stalling the the entire disk.
Maybe I'm missing something, could you elaborate?

 

[Kuma]

Kuma, a question. If the entire rotor system is stalled, what is providing lift in the hover? ETL is characterized by moving into undisturbed air, thereby increasing the efficiency of the rotor system. Moving from ETL back to a hover puts the rotor system back into disturbed air caused by its tip vortices. My understanding is these vortices obviously disrupt the smooth flow of air over the airfoil, thus reducing rotor efficiency, but not necessarily stalling the the entire disk.
Maybe I'm missing something, could you elaborate?

You are correct that the entire rotor is not actually aerodynamically stalled. Because of the disturbed air, different places on the rotor blade are experiencing a different angle of attack and some locations are stalled; the net result is less lift available to hold up the weight of the helicopter.

Think about a blade rotating through air. For simplicity, lets only think about the advancing blade. The speed at the tip is faster than the speed at the hub because the rotational rate (rotor rpm) is the same, but the radius is longer. Translational lift (clean smooth airflow) keeps most of the blade in a regime of positive lift. As you slow down, the part of the blade closest to the hub sees an ever increasing AOA until you build a stall. So at this point, perhaps the inside 1/3 of the blade is stalled and the outer 2/3 are producing lift. Now, as the tip vortecies swing around and come down on the middle 2/3, the AOA is increase even more and perhaps 60% of the middle blade is stalled. As a result, you only have a fraction of the rotor arc producing lift (we all think of it as simply being inefficient).

For instance, if you are at a pressure altitude and temperature where your helicopter can only produce 12000 lbs of lift while in the out-of-ground-effect hover (disturbed air) and your helicopter weighs 14000 lbs, as you slow below translational lift the aircraft will "stall". Lift is still being generated, but it is insufficient to continue flight. In this scenario, you must trade altitude for airspeed (get back into translational lift), get into ground effect (less disturbed air means more lift available), jettison weight (ordnance, sling load) or face a mishap.

My statements are from the point of view of the pilot, not the aerospace engineer. I could talk at a cocktail party level of detail, but if you want me to start talking about blade element diagrams and taking the integral of the lift equation over the entire span at each position with respect to the rotational arc...I'm out.

Kuma

 

[Fox6]

My statements are from the point of view of the pilot, not the aerospace engineer. I could talk at a cocktail party level of detail, but if you want me to start talking about blade element diagrams and taking the integral of the lift equation over the entire span at each position with respect to the rotational arc...I'm out.

Kuma

Yes, I probably read too much into what you were saying. I am a perpetual student in aviation and am always interested in another point of view, theory, understanding, concept, etc., thats the reason I asked. I agree with you about the cocktail party level, thats mine too, no engineer here. Thanks for the civil reply, and thank you for your service.

F6