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December 24, 2013
By Ian J. Twombly
I’m ashamed to admit that up until a few years ago I assumed that if the engine quit in a helicopter it basically tumbled as it fell like a rock out of the sky. Now, with a stout nine hours in my logbook, I know I was only partially wrong.
Maneuvers: Autorotations—lots of them
Assuming the pilot has his act together, a helicopter will not tumble if the engine quits. But it will fall like a rock out of the sky. The silver lining in that fact is that the aircraft’s vertical takeoff and landing capability allows it to land in something as small as a suburban backyard. So, airplane pilots, we can glide, but we will eventually have to land on an unbroken strip longer than a football field.
Coming down without power in a helicopter is called an autorotation. In other words, the main rotor blades are rotating automatically. They can do this because the engine becomes disengaged from the rotor system and air comes up from below to spin the blades. Imagine those whirlybird seeds from a maple or elm tree coming down and you pretty much get the idea. So long as those rotors are spinning, the pilot has a fighting chance of getting the machine on the ground in a controlled manner. That’s why helicopter pilots say “maintain rotor rpm” the way airplane pilots say “speed is life.”
The big secret about autorotations is that they are great fun to practice. The maneuver probably seems a bit crazy when you’re watching it from the ground, but to actually do it is a rush. And because the rotor system in a Robinson R22 loses rpm quickly, we practice them a lot.
If you remember practicing engine failures in an airplane—with all those memory items you could recite as it happened—you have a sense of how an autorotation is practiced. At this flight school we count to three and then say “engine failure.” Then it’s quickly down collective, right pedal, aft cyclic. In a few seconds the rotor rpm starts to come up as air flows through it so we “check” the rotation rate with a little bit of up collective. Then start to pitch for 70 knots. When we’re about 40 feet off the ground we flare (just like landing an airplane), and then roll on the throttle and level off.
It all sounds rather calm and collected; I suppose it is for someone who’s done it 5,000 times. But for newbies like me, the autorotation is generally a blur. The time from pattern altitude to rolling on the throttle is probably fewer than 10 seconds. To give you an idea of how fast the helicopter comes down, with no wind and 70 knots on the airspeed indicator, we are able to make it only half a mile forward when starting from pattern altitude. The vertical speed indicator says 2,000 feet per minute, but that’s only because it’s as high as the instrument goes.
Done well, autorotations can be extremely versatile. Private pilot applicants don’t practice enhanced autorotations, but more advanced pilots do. Helicopters can go straight down, backwards, sideways, and forward during an autorotation. So if the pilot finds himself directly over a nice, lush backyard in suburbia somewhere, he can autorotate to that spot. Advanced pilot applicants also practice what they call “full-down autos.” Like practicing engine failures in an airplane to a field, we never get to see if we would actually make it on an autorotation. More advanced students do. The difference is that instead of rolling on the throttle, the helicopter is leveled off just above the ground. At this point there’s obviously no speed and no air coming up through the rotor. So the only energy left is what’s in the spinning blades. That’s all dissipated as the pilot pulls the collective to soften the touchdown. It’s sort of like flaring an airplane with the engine at idle.
For more on autorotations, check out this Hover Power post.
You can read all the stories in the Rotorcraft Rookie series on AOPA Online.
Next time: Maximum performance takeoffs and landings
Flight Training Editor Ian J. Twombly joined AOPA in 2003 and is an instrument flight instructor.
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