Now, it’s been waaaaaay too long since I’ve had the discretionary funds to fly a helicopter; but autorotations were always my favourite thing to do in a heli. Here’s a very good article on autorotations: Autorotative Flight (Or Yes Virginia, A Heli Can Glide!).
Many people wonder, ‘What happens if your engine quits in a helicopter? Do you fall out of the sky?’ Fortunately, you don’t fall out of the sky. Well, you do; but it’s a controlled fall. The key is to maintain rotor RPM. The Robinson R22s and Schweizer 300s (aka TH-55 for you Army types) have ‘low inertia’ rotor systems. Basically, the rotors aren’t very heavy. Without power, they slow down very quickly unless drag is reduced immediately. During our practice autorotations in training, I’d lower the collective as soon as the power was chopped. Usually there was a ‘3-2-1’ countdown, but sometimes the instructor would chop the power unexpectedly. Those were annoying, since there were mountains to climb over and it takes time to climb 2,500 feet after an autorotation in a low-powered helicopter on a hot day with two big guys in it. I had the collective down in half a second in the unexpected power chops. It becomes automatic.
Hovering autos are different. In that case, you don’t want to lower the collective immediately. If you do, you’ll gain altitude and then run out of lift when you’re five feet in the air. If your engine quits when you’re hovering, you need to wait one second before you raise the collective to cushion your landing. Telling someone you have one second to lower the collective if you have a power failure in flight before you lose rotor RPM and plummet to a nasty death sounds like a very short time. When you’re hovering three feet above the ground and have to wait a second, it seems a very long time indeed.
A mundane post, I know. But I thought I’d share the article.
So the key thing here is the low inertia rotor systems? I always had the impression that in a bigger helicopter like say… a EC135, you weren’t quite so time-crunched if the engines cut out.
I have heard that a UH-1 on the ground could chop its power, rise into a hover, make a 180º pivot, and set down softly. I don’t know if that’s true or not, but they do have some pretty big blades. Robinsons have weights on the rotor tips for a little extra inertia, but you still have to be pretty quick on the stick.
In a hovering autorotation you’re only about three feet off the ground. In that case, you don’t want to lower the collective. Instead, you wait for drag to slow the rotor and then raise the collective to cushion the landing. At the instant of power failure, you have enough inertia to climb if you pull the collective; so you need to let it bleed off and then pull the collective to use the remaining energy for touch-down.
If you’re in flight, as in you’re a few hundred feet up and you’re going someplace, you need to keep the rotor turning. (See the linked aerodynamics page in the OP.) The drag from your forward motion is going to slow the rotor down very quickly, and you have a lot more than three feet to cushion. So you lower the collective to preserve your inertia and to allow the ‘driving region’ to drive the rotor system for the glide.
Hah I say, hah to your rotating wings. My plane won’t stall if the engine quits. Not enough elevator. I could finish reading a book if the engine quits. OK it’s got to be a short story but still, no panic involved. All I have to do is pull the yoke all the way back and prang it in if there’s no place to land. OK, I’m not going to test it but I think the undercarriage would absorb the shock enough to walk away.
Aha! But fixed wings have a fatal flaw. You can’t see them stop working!
Serious question, since it’s been a couple of years since I’ve flown. In case of a power failure in a Cessna 172, you can set up ‘best glide’ by rolling the trim all the way up. You’ll come down at 500 fpm at 70 kts. That’s with flaps up, right?
Pardon my ignorance, but as a non-pilot, non-expert I suspect I’m asking a really dumb question, so I apologize in advance:
I thought ground effect came from the air pushed down by the rotors into the ground buoying up the aircraft, yes? So if there’s a power failure, and the rotors begin slowing down…doesn’t that more or less instantly weaken the ground effect? Is it just that the change is negligible for the first few seconds?
A helicopter is flown with both hands and both feet, which manipulate the cyclic control (‘cyclic stick’), collective lever (‘collective stick’), throttle, and anti-torque pedals.
The throttle is like a motorcycle throttle, only it’s ‘backwards’ compared to a motorcycle. It sits on the end of the collective lever, which is controlled with the left hand.
The rotor blades are attached to the ‘rotating star’, which goes round and round. The swash plate (aka ‘stationary star’) is below the rotating star. The swash plate is controlled by the collective lever. When the collective is raised or lowered, it causes the swash plate to move up and down. This movement is transferred to the rotating star, and linkages change the pitch of the rotor blades. The control is called the ‘collective’ because it causes the rotor blade pitch to change by the same amount at the same time; i.e., collectively.
The cyclic tilts the swash plate in any direction. Since the swash plate is tilted, the pitch of the rotor blades changes depending upon where the blades are in their circle of rotation. So the pitch is changed in a cyclic manner.
The anti-torque pedals control the tail rotor; which is like the main rotor mounted vertically, and with only collective pitch changes. The anti-torque rotor is there to counteract the torque of the engine and keep the helicopter from spinning around – like thisor this.
It depends how much kinetic energy is available. Gravity will spin the blades as you descend and you can slow down the process by changing the angle of the blade. But as you do this you give up some of the energy. So the trick is to apply all that energy just before landing. Think about a fixed wing plane. When the engine quits you can maintain altitude briefly by pulling back on the elevator. But as your airspeed decreases you eventually run out of forward energy until you stall. The advantage in a fixed wing is the plane will stall in a way that is recoverable. It will break left or right as the nose drops and you straighten that out with rudder allowing you to maintain control until enough airspeed is built up to fly normally. Helicopters that stall have no aerodynamic properties to them. Thus the need to “do something” in the very near future.
The key is to maintain rotor RPM. The Robinson R22s and Schweizer 300s (aka TH-55 for you Army types) have ‘low inertia’ rotor systems. Basically, the rotors aren’t very heavy. Without power, they slow down very quickly unless drag is reduced immediately. During our practice autorotations in training, I’d lower the collective as soon as the power was chopped. Usually there was a ‘3-2-1’ countdown, but sometimes the instructor would chop the power unexpectedly. Those were annoying, since there were mountains to climb over and it takes time to climb 2,500 feet after an autorotation in a low-powered helicopter on a hot day with two big guys in it. I had the collective down in half a second in the unexpected power chops. It becomes automatic.
