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Imagine that a helicopter raises its nose up and then there is a loss in speed. Can it enter a stall? If so, how do you recover from the stall?

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  • $\begingroup$ By the way, raising the nose up and loss of speed is a typical entry to a hover from forward flight. A helicopters forward speed does not contribute to a stall like it does for fixed wing aircraft unless you are just talking about the retreating blade stall. At zero forward speed the retreating blade hits the relative wind just like the advancing blade, so it won't stall. Moving forward the retreating blade is traveling slower (relative to the air) so it can stall. $\endgroup$ – Ron Beyer Mar 4 '16 at 18:48
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    $\begingroup$ The simplest answer is yes, but not how you might think. There are no useful comparisons between stalls in fixed wings and stalls in helicopters. $\endgroup$ – Simon Mar 4 '16 at 19:01
  • $\begingroup$ The blades can stall if the AOA is high enough, this is known as a rotor stall. See the link that @Simon provided for more information Wikipedia article. $\endgroup$ – Ron Beyer Mar 4 '16 at 19:12
  • $\begingroup$ @Simon what is OP? $\endgroup$ – kepler22b Mar 4 '16 at 20:51
  • $\begingroup$ @kepler22b Sorry, "original poster" or "opening post". It means you ;) $\endgroup$ – Simon Mar 4 '16 at 20:53
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Yes and No.

The definition of a stall is that the airfoil stops generating lift, which happens when the critical angle of attack is reached.

In an airplane, this happens during normal flight by pitching up until the critical angle of the airfoil is reached.

In a helicopter, aft cyclic ("pitching up" in an airplane) will only serve to make the helicopter climb and slow down until it reaches equilibrium in the new attitude.

Helicopters, however, can suffer from what is called "retreating blade stall," when the blade is at full pitch on the retreating side of the rotor:

enter image description here

This doesn't happen under normal circumstances, but there are four factors which contribute to retreating blade stall:

  • fast -- close to Vne

  • heavy -- at max gross weight

  • hot -- above 35C

  • high -- at high altitude

Retreating blade stall is considered an emergency, and can be recovered from by reducing power.

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    $\begingroup$ Above 35F or 35C? $\endgroup$ – wbeard52 Mar 4 '16 at 23:22
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    $\begingroup$ Is it possible to stall all blades by yanking up on the collective? $\endgroup$ – Steve Mar 4 '16 at 23:44
  • $\begingroup$ no because so long as the rotor is turning at the right RPM, the relative wind is below the critical angle. if the rotor isn't turning fast enough, then you have much bigger problems. $\endgroup$ – rbp Mar 4 '16 at 23:48
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    $\begingroup$ @Steve It might be possible on some models, but it would be the helicopter equivalent to Pugachev's Cobra: stall by extreme pitch up while far away from stall speed. $\endgroup$ – Rainer P. Mar 5 '16 at 0:24
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A stall, as in slowing down and then descending too fast - yes, in the following scenario:

  • Helicopter flies with forward speed, above the hover ceiling.
  • Pilot moves cyclic to aft, and pulls up collective to maintain height.
  • When all airspeed is gone, the helicopter cannot maintain hover at this weight/altitude combination and starts to lose altitude.
  • Descending vertically, the helicopter descends in its own downwash and enters a dangerous situation called Vortex Ring State. Even if more collective travel was available, it would not create more lift in this state.

There is a discussion possible about what stall exactly is. If it is flow separation, the rotor blades are technically not stalled as @Jan Hudec points out. If it is stopping the generation of aerodynamic lift, this is exactly what happens during both fixed wing stall and rotor wing vortex ring state.

Even the way to get out of vortex ring state is the same as with fixed wing stall: move the cyclic forward so the nose tips down. Or sideways alternatively left/right and descend like a leaf from a tree.

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    $\begingroup$ This is technically not a stall, as the flow is indeed still attached. In fact, the air is moving downwards, reducing the angle of attack and that is why the rotor can't provide enough lift (and raising collective just accelerates the circulation instead of providing more lift). It is, however, just as dangerous, if not more, than a stall in a fixed-wing aircraft. $\endgroup$ – Jan Hudec Jun 13 '17 at 19:17
  • $\begingroup$ @JanHudec Have amended the answer, thx. $\endgroup$ – Koyovis Jun 14 '17 at 3:54
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    $\begingroup$ The rotor is moving the air down, at about the same velocity as the helicopter is coming down. $\endgroup$ – Koyovis Jun 14 '17 at 8:55
  • $\begingroup$ @qqjkztd, when creating lift, the air must accelerate downward (by principle of action and reaction). So below the helicopter there is air moving downward (called downwash). And vortex ring state (described in this answer) occurs when the helicopter descends into its own downwash. $\endgroup$ – Jan Hudec Jun 14 '17 at 9:11
  • $\begingroup$ It is possible to slow down, indeed come to a stop, and descend "too fast" (whatever that means) without stalling. There is only one type of stall in a helicopter which is a blade stall, either main rotor or tail rotor. I don't understand you last sentence - "descend like a leaf from a tree." Are you talking about auto-rotation? This is one way to escape VRS if height permits but the standard recovery is to fly out of the vortex in any direction. Moving the cyclic forwards is just one possible recovery. VRS is not a stall condition and in general, your answer does not address stalls. $\endgroup$ – Simon Jun 16 '17 at 9:52

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