You could construct such a helicopter, yeah. It would need a special clutch, as you mention, and special blades to avoid the problem of flapping.
These systems would add complexity and weight, diminish reliability, and quite possibly compromise safety in many emergency situations. But sure, you could make it work. It would purely be an engineering exercise and likely useless for any practical purpose, though.
Ok, I didn’t get that part right. So you already have that in a helicopter, the blade pitch is the collective control, and to auto-rotate you pitch the blades down and they act like a turbine, lift from the air flowing over the rotor blades causes the rotor to turn.
Yes, that would be it. Maybe a rigid non-flapping rotor with an asymetrical foil optimized for auto-rotation. Maybe greater disk density is needed to get it up to speed more quickly. A manually controlled clutch could be made somehow, maybe a fluid torque converter of some type. It wouldn’t be the first helicopter to be designed and built to test a principle, but you’d have a hell of a time finding a test pilot for it.
yes they can. After initial training, the most important of all maneuvers, given the right altitude, the blades on the helicopter must flip and catch enough air on the way down, to then be flipped back which if done correctly staggers the chopper within 5-10 meters from the ground, limiting fatalities and causalities.
Note that the pitch can’t be negative. Negative-g is something to be avoided (in the kind of helicopters I’ve flown – might be different on more sophisticated ones), and reducing pitch to a negative angle would have the same effect as a negative-g pitch-over. I’ve never heard of a manned helicopter that could push down. I mention this because the implication of reducing pitch to negative is that the rotor blades act like a pinwheel, being blown by the wind.
Here’s the article on Autorotation, from Army Field Manual FM 1-203, Fundamentals Of Flight.
Look at the illustration of the ‘driving region’ and ‘driven region’ (and ‘stall region’). Note that the blade has positive pitch in all regions. The reason the driving region keeps the blades turning is that the sum of the force vectors produce lift behind the axis of rotation.
Now: Self-deploying rotors.
Can a helicopter-like object ‘self-deploy’ rotors and come to a soft landing? Yes. Sycamore seeds do it. I had a toy when I was a kid that was like a plastic dart, with soft plastic ‘blades’ (basically rectangular strips about an inch wide and four or so inches long). The launcher was a stick with a rubber band on it. You grasped the tips of the blade-strips, nocked the hooked nose on the rubber band, and shot it into the sky. At apogee, the heavier plastic body would point down, and the blade strips would act like sycamore seeds to bring the toy down. (I think these are still being made, but I don’t know what they’re called.) IIRC, Estes made a ‘helicopter-recovery’ model rocket with deploying rotor blades.
I haven’t actually looked into the aerodynamics of sycamore seeds or toy darts, so I could only make an educated guess as to how they work. But I won’t. I just got out of bed, and I’m already logged into my job; which I should be doing instead of posting here. But as a helicopter pilot, I can see some problems with making a ‘drop-launch’ helicopter.
The rotor system must be designed so as not to hit any part of the airframe. In the toys, the rotor blades don’t have cyclic pitch control; the body swings at the whim of the blades. The toys come straight down, with the body spinning with it. I can imagine a rotor system on a manned helicopter, where the cyclic pitch control is locked such that the airframe is always 90º to the rotor disc. This would be quite a wild ride for the occupants, and I’m not at all certain it would work.
The blades have to flap. Unless you’re coming straight down, you’re going to have dissymmetry of lift. If you’re coming straight down, you’re going to come down pretty fast. You need the forward speed to power the driving region of the blades. If the blades are rigid enough not to flex, they would probably be too heavy to work on a full-size helicopter. In a helicopter with a fully-articulated or semi-rigid rotor system, you’re going to be bashing the blade stops. Also, the blades need to ‘cone’ so that the airframe can better hang like a pendulum.
Let me think out loud here. A helicopter is stowed in the cargo bay of a large aircraft. You get in, and slide out the back of the airplane. First, the helicopter is going to tumble. Then you need to deploy the rotor blades into their flight positions. That’s going to take some strong motors to make them do that on a tumbling helicopter. Next, you need to get the airframe below the blades. It works in the toy I mentioned, but the moments on the blades would be enormous on a full-sized helicopter. I see breakage. Assuming your blades are made of unobtainium and they don’t break, you need to keep them from hitting the airframe. You also need to keep the rotor hub from breaking off of the rotor mast, as it pounds against the stops.
But let’s say that you do get your blades deployed, you’re upright, and nothing breaks or gets chopped. Can you get the blades turning in the right direction? A video I posted earlier shows a helicopter entering autorotation from a hover. But it already had its blades turning. To go from a stop? Maybe. If you remain upright and have a a few thousand feet, the sum of the force vectors on the driving region may be enough to turn the rotor system. Only without the initial rotation you have when you’re already flying, the angle of attack on the driving region is going to be much greater. They’ll be in a stall. If they are stalled, then I think the rotors will act like a pinwheel and start turning in the wrong direction.
IANA aerodynamicist. This might be an interesting experiment for Mythbusters. Get a few r/c helicopters (proper ones with cyclic and collective controls; not the toy ones with the counter-rotatating rotors) and a vertical wind tunnel, and see if it’s at least theoretically possible.
Crikey, you chaps spend a long time not saying “nah, it’s Hollywood, get over it”…
Yes, I was too simplistic. But in the case of a non-turning rotor I think you’ll need negative pitch or an unusual airfoil to get the blades turning initially. You really don’t want negative or positive pitch in autorotation because it increases drag slowing the rotor, so you’ll need to get an effective negative pitch to get the rotor turning, gradually increasing it to level pitch or the point where there is the least drag. Once up to speed you should be able to descend under control with a few degrees of positive pitch. But initally without some negative pitch I think the blades will be in a stall and won’t rotate.
It’s how we are. 
Yes, in the scenario in the hypothetical start-in-a-fall helicopter, you’d need negative pitch to get the blades turning. I am not aware of any real, manned helicopter that can reduce the pitch of its main rotor blades to a negative angle.
Once you’re in autorotation, you do want positive pitch. You need the lift vector on the driving region (behind the axis of rotation) to drive the rotor system forward. There’s drag, but the lift produced by the driving region is great enough to overcome it. The game is to balance the slowing force of drag, and the driving force of the inboard region of the airfoil, such that the rotor system maintains a constant RPM. (Note that helicopter tachometers show percent of RPM, rather than actual RPM. You keep the RPM within a narrow range.) But yeah; on the fictional helicopter, you would need the negative pitch to get the blades pinwheeling.
As I recall, the Huey has a twisted blade so when the air reverses through them, they will be driven in the correct direction. I do not know how a little Robinson is actually set up.
A Huey can auto rotate straight down or even backwards.
I know of no airfoil shape that produces lift in a FWD direction. That would be a sail pilots wet dream.
No matter what shape you make a fan blade, when the direction of air through the blades is reversed, the blade turn the opposite way. You have to get over 50% of the blade to reverse.
This all assumes you are too high for angular momentum to be useful like in a hovering auto-rotation where you do not dump the collective as in auto-rotations from higher up.
I’m not aware of any certified aircraft that do. Some experimentals will because of the simplified control system, that’s what makes them death traps. I believe some blades can be cyclically negative in pitch relative to the helicopter, but not relative to the angle of the disk. Negative collective pitch would result in inverse coning which is a recipe for disaster. Most model helicopters allow negative pitch so they can do acrobatics, but that doesn’t apply to the discussion.
Yes, I mentioned a few degrees positive. Autogyros typically have between 2 and 4 degrees fixed positive pitch, usually with a symetrical foil. For either type of device the angle of attack of the blades has to be inside the stall range based on forward speed of the blade in rotation and the rate of descent. But you have to ‘fall’ to auto-rotate, except for a short time when you can burn off the energy in the rotor.
Not sure what you mean here. The air flow doesn’t reverse in auto-rotation. When actually auto-rotating the blades are spinning and the air-flow is always from front to back over the blade, only the relative angle of attack is changing.
ISTR there’s at least one certified turboprop that has a reversible pitch propeller (or two, if it’s a twin), but I can’t remember what it is. I read something about a crash where reversing the pitch was the main factor.
Of course there are other aircraft with reversible pitch. The C-130 comes to mind. It was either an uncle or a former coworker who said they used to back P2V Neptunes (radial piston engines) into parking spaces.
But no, I can’t think of a helicopter that will do negative pitch either. Nor can I think of why one would.