Are there any safety features to prevent reversible-pitch props from reversing in flight?

Question

Some aircraft have propellers capable of reversing pitch, which is also known as beta angle. The example I saw was an Aviat Husky.

(Youtube demonstration)

I want to know if there are any safety features that prevent this from activating while airborne. For example, what kind of controls activate it? How long does it take to actually reverse the blades, and how much physical effort does the pilot have to exert? Would it be impossible to force the reverse when at high cruising speed?

If it is a mechanically powered reverser, then is it possible for it to be inadvertently triggered?

And if it is activated, can it quickly be set back to original blade angle?

Note, I am interested in any propeller-driven aircraft, not just commercial ones, but smaller ones as well. Piston prop, turboprop, GA, anything.

Answer 1

Beta props are pretty rare on anything smaller than Twin Otter or King Air or Porter or Turbo Beaver. Can't speak for MT's system, but most beta mode systems use mechanisms to switch control of propeller blade angle from the propeller rpm governor to the throttle. Beta mode is the mode of throttle or power lever control of blade angle, with engine torque controlled by a governor system of some kind to regulate rpm.

Beta mode is very similar to turbine helicopter collective control, where a lever, the collective, directly changes rotor pitch and a governor system regulates rotor rpm by adjusting engine torque.

Beta mode is normally entered just above flight idle (flight beta) once the propeller speed governor is no longer regulating blade angle (blades at fine pitch stops).

There is normally some kind of detent or gate that requires a deliberate movement or unlatching to move below flight idle (the Twin Otter requires you to twist the throttle grips - on others there are finger triggers).

Also, some turboprops have beta lockout systems that prevent power lever movement below flight idle until you're on the ground. But a lot of airplanes don't and can be put into ground beta in flight if the flight idle gate or latch is deliberately operated by the pilot.

During the Twin Otter program there was some controversy at DeHavilland Canada because demo pilots were using ground beta (coming into DISC - blades flat, like a disc) to make crazy steep descents (I've also heard of Porter pilots doing it). A very dangerous thing to do, because if the prop doesn't want to come out of beta mode when you move the lever(s) back up, you are dead meat.

Answer 2

The aircraft I fly has a constant speed, reversible prop with beta. It has several protective devices to prevent the blade angle from decreasing. The main ones that you're interested in are the low pitch stops and the flight idle stop.

The low pitch stop is essentially a latch inside the prop dome that prevents the blades from going into the beta range by blocking the pitch change piston. It can be overcome by an increase in oil pressure (260-310 psi on ours). Normal operating pressure is between 150-250 psi, but we don't have a gauge to verify that. The additional hydraulic pressure is not available unless a valve inside the prop control mechanism is opened, which happens when the pilot pulls the power levers into the ground range. The power levers do many other things, and one of the nice things about the P-3 is that you don't have to deal with prop levers or condition levers. But I digress. Low pitch stops are more likely to fail in the engaged position, which would prevent reversing the prop altogether. In flight, it would take a failure of several other prop protective devices to even get to the point where they are needed.

The flight idle stop is a solenoid-activated stop that makes it more difficult (though not impossible) to pull the power levers into the ground range and reverse the blade angle. It is energized and pulled out of the way when the plane touches down and when the power levers have been pulled "over the ramp". If the solenoid fails it takes an addition 12 lbs of force to overcome it. The flight idle stop additionally prevents the prop control from going into revers should the power lever linkages break. Occasionally the solenoids come apart entirely, which prevents you from going into reverse. At that point, you feather the engine to prevent forward thrust while you're trying to stop, or else you're going off the end (or the side) of the runway.

"Over the ramp" refers to the fact that the power levers need to be lifted up when they are at flight idle to get into the ground range. If you really wanted to, you could overcome everything and put the prop in reverse inflight. At that point, the negative torque (-500 to -1700 shaft horsepower) should activate the safety coupling, disconnecting it from the engine. The prop would overspeed severely, and could conceivably throw the blades right out of the hub if the RPM got high enough. You have to be going pretty fast for it to get that bad though.