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- from people who have no business flying.
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- Re: Autorotation, or “gliding” in a helicopter, as when the engine fails. (no, you don’t get far, but you can hit the ground soft enough to avoid injury)
#1. Our first question concerns the little build-it-yourself helicopter plans that some(?) company advertised in the back pages of Popular Science/Popular Mechanics (and may still, I haven’t looked lately). What this was, was some unknown type of engine that turned a rotor that was mounted on a shaft with a U-joint in it. The 1-passenger carriage “hung” from the rotor, and there was a handle not unlike that of a hang-glider, and you effected limited steering of this thing by basically shifting your weight. It doesn’t have any real rotor head, like with collective or cyclic pitch: the rotors are mounted solid. The rotors also would have to be mounted in a slight positive position, but when the engine stopped running, you wouldn’t be able to feather them, so would this thing autorotate to any useable degree, or not? Say, you’re cruising along at a thousand feet and the engine dies, and there’s a linkage that allows the rotor to spin freely of the engine. It’s definitely gonna spin, but if it will save your hide is what I’m asking. The two people I know who both fly helicopters in the military say that they have no idea; every “real” helicopter has a “real” rotor head.
#2. Also about the same helicopter plans, , we recalled that the engine they showed for these things was rather tiny, but most real helicopters have at least 50-60% of their weight in horsepower: a 4500 lb helicopter has at least a ~3000 HP engine. Say our little minicopter weighs 150 lbs itself (with fuel), and the daring pilot weighs 200 lbs, for a 350 lb total. How much power does our little “dance with death” need to get airborne? - MC
Is what you are after an autogyro? Here’s the first good link I found on the subject:
http://freespace.virgin.net/jim.montgomerie/html/the_merlin_autogyro.html
With regard to #2: The kit-built RotorWay Exec 162F seems to have only about 1 hp for every 10 lb. (1500lb with a 150 hp engine). Here are the specs, but it doesn’t give the horsepower rating. For that you have to go to another of their pages, an article reprinted from Kitplanes Magazine, which gives the 150 hp figure.
Heli pilot reporting in…
kferr, I don’t think he’s looking for an autogyro. I seem to recall seeing the described helicopter, and it wasn’t a gyrocopter. (BTW, gyrocopter and helicopter licenses are two different ratings.)
MC, I’ll have to weigh in with an “I don’t know” regarding the rotor head. Not having a proper rotor head would make it impossible to control. Why? Because of “dissymmetry of lift”. That is, when the helicopter is in forward flight, there is more “wind” flowing over the advancing blade than over the retreating blade. For example, let’s assume your blades are going round and round at 300mph. If your forward speed is 100mph, then there is 400mph of wind flowing over the advancing blade; but only 200 mph flowing over the retreating blade. (You’re probably saying to yourself that the speed of a rotating surface varies along it’s length, but the idea is the same.) If there was no compensation, the helicopter would generate more lift on the advancing blade side than on the retreating blade side and it would roll. To compensate, helicopters have a “flapping hinge” or have a “semi-rigid rotor system” with a “teeter hinge”. The “U” arrangement you describe would be the teeter hinge.
But the blades also have to feather. When you move the cyclic forward, the “stationary star” tilts to starboard (assuming right-to-left rotation as you are sitting in the cockpit). Due to gyroscopic precession, the reaction manifests itself 90º later in the plane of rotation resulting in a forward tilt of the rotor disc. The “rotating star” has linkages to the “feathering hinges”. Since the stationary star is tilted, the tilt is transmitted to the rotor blades. In forward flight, the blades are feathering and unfeathering as they trace their paths. So dissymmetry of lift is compensated for by the blades “flapping” and “feathering”. (And “leading and lagging” on the… you guessed it, “lead-lag hinge” on a rotor systen with more than two blades. Rigid rotor systems use the flexibility of the blades themselves to perform the functions of the lead/lag and flapping hinges.)
Okay, back to the PS/PM heli. I have seen ads for helicopters that use two tiny jet engines on the blade tips. And I think I remember seeing an ad with a “hang-glider” style control. Could this be it? One advantage of having the tip jets is that you don’t need an anti-torque rotot in the back. You also wouldn’t need a “free-wheeling unit” (or “Sprague clutch”) to disengage the engine. The rotor is already spinning freely.
But what about autorotation? A gyrocopter is always in autorotation when it’s flying. Due to the thrust of the engine, the lift generated by the autorotating blades is enough to keep it airborne. If there is an engine failure in a helicopter, this is what happens: The pilot lowers the collective lever to feather the blades. He sets up his glide using the collective and cyclic to maintain glide speed. When he’s close to the ground he begins to flair. This ingreases the rotor RPM and the pilot lowers the collective to compensate (if he can. A light helicopter might have the collective all the way down already.). At about 8-10 feet above the ground, the pilot levels his ship. This is when you want to use all of that energy you have saved up in your blades. The pilot pulls up on the collective to increase the blade pitch and cushion the landing. (It’s important to keep the skids pointed straight ahead, as most autos I’ve done include a little forward speed.)
It seems to me that the experimental heli must have some way to feather the blades, or else it would not be able to perform an autorotation. Of course, I’ve never seen one, much less flown one, so my guesses are based on flying FAA certified helicopters.
That’s not quite right. For example, a Robinson R-22 Beta has a maximum gross weight (pilot, fuel and baggage included) of 1,370 pounds. But the Lycoming O-320 it uses is only 160hp, and it’s “derated” (i.e., they put a red line on the manifold pressure guage) to 124hp max. continuous (131hp max for 5 minutes at take-off). A Schweizer 300CB has a max. gross weight of 1,750 pounds and uses a Lycoming HO-360-C1A engine rated at 180hp. So the power requirement is considerably less than 50%-60% of the gross weight.
[the “stationary star” tilts to starboard
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Oops. What I meant to say was that the stationary star moves the linkages that apply pressures to change the pitch on the retreating blade, upward on the port side and downward on the starboard side, such that the reaction is manifested 90º later in the direction of rotation. Upward tilt in back and downward tilt in front, resulting in forward motion of the helicopter.
I think I’ve found a photo of the helicopter in question here: http://www.rqriley.com/glu-heli.htm (click on the photo for the larger picture)
I can’t see too much detail of the rotor head, but there is a collective lever next to the pilot’s seat. This would indicate that it does have feathering blades (as I said above, it would have to).
The tip-jets themselves are described at http://www.rqriley.com/gluharef.html