What happens when a helicopter engine fails, and why is a Robinson Helicopter different? Autorotation explained.
One of my passengers asked me the other day, “What happens if the helicopter engine stops working?”
A quick look at Google’s autocomplete shows she isn’t alone.
There’s a short answer that I commonly give to passengers, and then a longer answer, which I will explain in this post.
Short answer first: helicopters can “glide” when they don’t have sufficient engine power to sustain flight. This gliding is called autorotation. A simple illustrative example can be found by watching a falling maple seed. A helicopter with a failed engine operates roughly like a maple seed.
That explanation is enough for many. If you want to go deeper, read on.
We can begin discussing the longer answer by continuing our discussion of the autorotation of the maple seed. As the seed spins, the “wing” generates lift. That lift force significantly reduces the descent velocity. Maple trees employ this technique to increase seed dispersal.
Returning to helicopters, the most common reason for an autorotation (other than a practice autorotation) is the failure of the engine, or failure of the drive line. This is usually caused by a design defect, a manufacturing defect, or improper maintenance practices.
Helicopters are equipped with freewheeling units which disengage the engine from the rotor system in the event of an engine failure. Any time the geared engine RPM (revolutions per minute) is lower than that of the rotor RPM, the freewheeling unit disengages.
Helicopters also have controllable pitch rotor blades. The blades are collectively controlled through the pilot’s (appropriately-titled) “collective.” The collective is a lever mounted in the cockpit which hinges from the rear of the pilot’s seat, and looks like the handbrake in a car. Before the engine failure, the main rotor blades were producing lift and thrust through their AOA (angle of attack) – controlled by the collective. Immediately after engine failure, the pilot needs to quickly lower the collective. The helicopter will begin an immediate, and steep, descent, which will produce an upward flow of air through the rotor system. The descent rate is high as the helicopter enters autorotation, but can decrease to around 50-60 knots as the autorotation stabilizes. The helicopter is now flying like a maple leaf.
However, helicopters are not very crashworthy, and a descent rate of 50-60 knots is still far too high for landing. As the helicopter nears the ground, it can do one better than the maple leaf. Because the pilot can control the pitch and AOA of the rotor blades of the helicopter, the pilot has the ability to transfer the kinetic energy stored in the blades to thrust and cushion the landing.
Landing from an autorotation should be as follows: As the helicopter nears the ground, the pilot should use aft cyclic to “flare” the helicopter. This will arrest much of the forward and downward travel of the helicopter and simultaneously increase the rotor system RPM. As the helicopter exits the flare, the pilot can then level the helicopter and increase the collective. This will increase the AOA of the rotor blades and will exchange the stored rotor inertia for cushioning lift. Ideally, the helicopter will touch down softly with no forward airspeed.
Helicopter pilots practice autorotations as part of their flight training. I can speak from experience that the fist “auto” is quite the ride. However, with proficiency, auto’s become second nature.
Now – what about Robinson Helicopters, like the R-66 that crashed in the town of Colusa, California?
Remember above, I explained that the pilot must quickly lower the collective? How quickly depends on the mass in the helicopter’s rotor system (remember that increased mass means increased inertia). Generally speaking, the more mass the helicopter has in its rotor system, the more time a pilot has to get the collective down due to rotational inertia. Unfortunately, some helicopters have “low-inertia rotor systems.” For example, the Robinson R22, a popular training helicopter, and its larger sibling, the R-66 have low inertia rotor systems which require that the pilot lower the collective within just a few seconds to safely enter autorotation. Other helicopters, such as the Bell 206 and 407, the Eurocopter/Airbus AS350 “A-Star”, and the MD/Hughes 500 have higher inertia rotor systems which afford slightly more reaction delay.