I had this argument with a friend of mine, on the subject of autorotation. We both agreed that it is a theoretical way for a helicopter to safely land after an engine failure. We differ on the specifics, though.
He says it is a way to do a rolling landing, using your helicopter as if it were an autogyro. You would use gravity and your forward momentum to keep the blades spinning, and do a controlled dive into the ground, and use the cyclical at the last minute to pull out and land like a glider.
I say forward momentum isn’t necessary. You just keep energy in your whirling blades by “zeroing” (don’t know the technical word) your angle of attack, or even making it negative. That is, you let your rotors windmill on the way down, without changing position overland. Then, at the last minute, you move the collective back to a positive angle-of-attack, and your rotors act normal again. If you did it too late, of course, you’d be a pancake, and if you did it too early, you’d run out of energy and your rotors would quit rotating. But that’s why you avoid engine failures.
His theory is dependent on doing a rolling landing in a type of craft not designed for it (skids or wheels, neither necessarily designed to land with forward motion). My theory is dependent on the collective, and the rotors, having sufficient range in their angle of attack - maybe the blades are designed to never angle downward. I mean, it would really bite if you weren’t paying attention, and you pulled your collective to “negative lift” propelled yourself into the ground. So one (or both) of us is probably wrong. Which one? Him! Him!
Now I’m going back there and read it again. The author likens it to gliding in an airplane, but he also says a helicopter can autorotate with zero or negative airspeed.
I didn’t read the links yet, but as a helicopter pilot I think I’m qualified to answer.
Generally, you’re moving in a helicopter. Entry into autorotation is likely to be with forward airspeed. When the engine quits (or in a practice autorotation), lower the collective, add right anti-torque pedal, and set up a glide (generally about 60 knots in the helicopter I fly). Keep the rotor RPM “in the green”. Too low and you stall the rotor, too fast (“overspeed”) and you can damage the rotor system. If you’re heavy, you may have to pull the collective up to “catch” the RPM. Start your flare about 50 feet AGL. At about 10 feet, level the skids and touch down skids-level, cushioning the landing by pulling up on the collective. You can land with no forward motion if there is a bit of a breeze, but generally you’re moving forward. Not to worry though. That’s why they’re called “skids”!
If you’re hovering, you need to get moving forward to get some air over the rotor system. Contrary to popular belief (or so I was trained), it’s not like a pinwheel. There is a “driving” section of the rotor that keeps it spinning aerodynamically. If your airspeed gets too slow, you may not have the energy to keep the rotor turning at an acceptable RPM.
For those who don’t know, helicopters have a “freewheeling unit” that disengages the rotor system from the engine, which allows it to keep turning.
It’s easier to do than to write it out… and rather fun!
The problem with the “windmill” technique is that helicopter
rotors are NOT the same as propellers or fan blades. A propeller has slanted planes designed to move air over the
wings, which generate lift. An airplane wing is shaped so that the top is a curve (the highest point somewhere near the middle) and the bottom is flat. Air traveling along the top has further to go than air traveling on the bottom, making the air on the bottom slightly more dense, generating lift. A helicopter rotor works on the same principle. The blades (each one is the same thing as a wing) move through the air, generating lift. The lift is dependent on the blades movingthrough the air, just like an airplane (in the case of a helicopter, they’re simply moving in a circle.) If the helicopter were to maintain a constant position relative to the earth and move down, the rotors would not be moving through the air in the correct plane, thus generating no lift.
Johnny LA, it looks like your account agrees with beatle’s autorotation link. You do use a lot of forward motion, and then cancel it at the last minute with a flare, i.e. a pull back on the cyclical.
The “hovering autorotation” link looks a little closer to my idea, but it never talks about making the blade angle of attack negative, to allow the rotors to windmill. Probably because nobody would design a collective with enough range to make the blades push down. Anyway, the author of that link says hovering autorotation isn’t really autorotation at all, it’s just using the saved kinetic energy (RPM) in your rotors at the last minute with a yank on the collective, to save yourself from a hard landing. Sounds like it’s not too easy to do. Better than leaping out the door over a neighborhood swimming pool, though.
I wouldn’t say they’re “hard”; they just take a bit of practice. There’s a lot going on. Once you enter the autorotation (collective down, right pedal (unless you’re in a Russian or French heli ;)), a little aft cyclic) the glide is pretty-much just a ride. You need to adjust it so you can make your intended landing site (which you picked out while you still had an engine. I frequently play hop-scotch: “Where would I go if my engine quit now?”) but it’s fairly low-stress. When you initiate your flare, you also have to watch the rotor RPM. This is where it gets a little busy. Aft cyclic increases RPM and you don’t want to overspeed; so you lower the collectve (saving the inertial energy that built up when you flared). Shortly after that you need to level the skids. If you don’t, you can strike the tailboom. You also need to make sure the nose is pointing in the direction of flight. A little sideways on touchdown and you can roll it. (That happened to my first instructor once.) While you’re doing all this, you also need to pull collective for a soft touchdown… but not too much. You don’t want to run out of energy before you’re down! (Actually, you can drop a Schweizer from about 5 feet without damaging it, I’m told.)
It’s funny how persistent the Bernoulli/friedo explanation for flight is, in the face of evidence to the contrary. Don’t get me wrong, I accepted that wing shape played a big role in flight, because “everyone said so”, but I also understood that angle of attack was important. It turns out that angle of attack is the main thing that provides lift to an airplane (as per explanation’s on SingleDad’s link), and wing shape is mainly an attempt to reduce turbulence.
As weird as the Bernoulli/wing shape theory is, it’s even weirder when applied to helicopters. I mean, you can see the angles of attack in stationary rotors. Any explanation of helicopter control must invoke angles of attack to describe how the cyclical and collective work. I’d like to see that explained by the Bernoullites. With fixed-wing aircraft, at least there is the excuse that angle-of-attack is less obvious, and only variable in the F-8 Crusader (as far as I know).
As I mentioned earlier, there are some interesting dynamics going on in a rotor system. I’d have to dig out my copy of the U.S. Army manual Fundamentals of Flight to explain it (although it might also be in the FAA’s basic rotorcraft manual), but a rotor in autorotation has a “driving section” and a “driven section”. That is, aerodynamic forces on the airfoil “pull” it forward, allowing the “driven section” to generate lift and to keep the whole shebang spinning. Again, I’d have to look in my reference books; but I think Bernoulli would be a good explanation as to why it works.
Oh, I wasn’t really talking about autorotation per se. And when I talk about the “Bernoulli explanation” I’m not really talking about actual contributions to aerodynamics by the dude named Bernoulli. I just mean, the idea that curved-top-flat-bottom wing shape is the primary cause of lift in rotary and/or fixed wings. This explanation is quite common, and not very good, and is handily debunked on SingleDad’s link.
Another helicopter question: is there a separate motor for their tail rotor, or does it usually use the same power source as the main rotor?
The angle of attack of the rotor blades will be the resultant vector of the blade pitch, the angular velocity of the rotor system, the forward velocity of the helicopter, and the descent rate of the helicopter. If any of those get too far out of whack you’re in trouble.
I’ve never seen or heard of a helicopter that had a separate engine for the tail. There may be some out there somewhere, but there’s a lot of problems with that idea. A failure of the tail rotor engine would mean a loss of the aircraft, for instance, whereas if the system is geared then as long as the main rotor is turning the tail rotor is turning.
There is a relatively new system for counteracting torque called “NOTAR”, which uses bleed air from the main jet engine, pumped through exhaust vents on the boom of the helicopter. The jet effect from that counteracts torque. Very simple, no tail rotor to hit people or get fouled in trees, and more aerodynamic. However, it requires a jet engine to work.
Actually, NOTAR uses a fan inside of the boom. The exhaust is angled to the left side for a little extra anti-torque. The fan inside of the boom forces air out through a long slot on the starboard side of the boom. The pressurized air, assisted by the rotor wash, as well as the shape of the slot, cause a downward flow of air over the starboard side of the boom. This “Coanda effect” is similar to the Bernoulli Principle, in that the air is flowing more rapidly on the starboard side of the boom, resulting in a lower pressure area and thus, a lift vector to the right (counter-clockwise yaw) to counteract torque. The “ash-can” on the end of the boom rotates so that turns can be made.
One comment from a non-pilot. It is my understanding it is far from theoretical, and is in fact a standard emergency procedure you are required to learn to get a license. No?
