Rotorcraft Rookie: Do helicopters stall?

May 8, 2014

The training to avoid, recognize, and recover from stalls is such a major foundation of fixed-wing training that I was curious if rotorcraft had an equivalent. It turns out helicopters also stall, but in much different situations.

If you think of the rotor blades like the wings they are, this makes sense. And in that regard the retreating blade stall is probably easiest to understand. As the helicopter flies forward and the blades are rotating, the advancing blade is striking the relative wind at a higher speed than the retreating blade. Remembering that in general terms angle of attack plus airspeed equals lift, an obvious asymmetrical lift situation would develop. To compensate, designers allow the blades to flap, or move up and down, to increase and decrease the angle of attack. Just as in an airplane, the wing or main rotor blade will stall if it gets slow enough and the angle of attack is exceeded. That’s the primary value in determining a helicopter’s VNE. Any faster and the retreating blade will stall.

Assuming the pilot stays below VNE, retreating blade stalls are easy to avoid. The more insidious condition is the rpm stall. Essentially the rotor rotation slows down enough that the blades can now longer support the helicopter’s weight and it simply falls out of the sky.

As the blades slow down the pilot’s natural response is obviously to ask for more lift by raising the collective and increasing the angle of attack. But this creates more drag, slowing down the rotation even more. In that way it’s very much like an airplane situation. In an airplane we’re taught to lower the nose or decrease the back pressure and add power. Lowering the nose won’t do anything in the helicopter, but lowering the collective will. In both cases you’re lowering the angle of attack.

What makes the helicopter situation so difficult is that rotor rpm is directly tied to power. Whereas in an airplane you simply avoid or break the stall with nose attitude, in a helicopter the situation is still tenuous even after lowering the angle of attack. This gets really confusing. The collective controls both power and blade angle. There’s a twist-grip throttle on the collective, but it’s controlled automatically with a governor. So lowering the collective to decrease drag on the rotor system also decreases engine power.

This also speaks to the sneaky nature of the problem. It can happen in any attitude, any configuration, and during any maneuver. Robinson uses its characteristic frank talk with this mention in a safety notice. “Rotor stall…can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky.” OK, then. Who’s ready to go flying?

Rotor stalls happen most often on takeoff or landing, when the pilot has to pull maximum power to either take off from a hover or descend in to one. It’s also common on autorotation recoveries, when students yank on the collective as they see the ground rushing up.

In all cases a warning light and horn will come on, alerting the pilot that he has dropped below the normal range. The recovery is something called “milking the collective.” Originally I was taught to pump the collective up and down until the engine was able to garner the power to increase the rotor rpm. You end up in this sort of silly washing machine ride as you bob up and down waiting for the engine to come back to life. A new technique I learned during a checkride preparation flight seems to make more sense.

Because the angle of attack of the blades has to come down to decrease drag, the first action is to lower collective. This is akin to lowering the nose or decreasing back pressure in an airplane. Then the pilot simply rolls on throttle to manually increase engine rpm. If done properly, a controlled demonstration shows that it’s possible to recover without losing any altitude.

The most interesting fact about all this training is that despite it being a prevalent problem in light helicopters, we spent virtually no time discussing how to avoid a low rotor rpm scenario. Unlike fixed-wing training that harps on stalls in every possible configuration, the helicopter practical test standards call for a quick demonstration of milking the collective and nothing more. That’s not to say the training was at all inadequate. Far from it. I think it stems from the fact that most people learn to fly helicopters expecting to do it for a living. And given that it’s less of a problem in turbine-powered aircraft normally found in that arena, it’s less of a long-term problem for the students.

Next time: The checkride.

Read all the stories in the Rotorcraft Rookie series.