Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!rpi!bu.edu!att!cbnews!cbnews!military From: veeneman@mot.com (Dan Veeneman) Newsgroups: sci.military Subject: Re: Autorotation of Helicopters (was ... AV-8B Harrier) Message-ID: <1990Dec17.043547.23971@cbnews.att.com> Date: 17 Dec 90 04:35:47 GMT Sender: military@cbnews.att.com (William B. Thacker) Organization: AT&T Bell Laboratories Lines: 105 Approved: military@att.att.com From: veeneman@mot.com (Dan Veeneman) > From: paj > > I have been following this discussion and am somewhat puzzled. > > First, a note on my expertise: I fly a hang-glider. I have never even sat > in a helicopter. I fly helicopters and airplanes. I have never even touched a hang-glider. > Gliders fly by descending such that (for constant speed) their drag:weight > ratio equals the glide slope. I was under the impression that helicopter > blades are aerofoils and hence capable of gliding (rather than being simple > flats which would be far less efficient). Yes, helicopter blades are airfoils, usually symmetrical with an increasing twist near the tip (so the tip stalls first). > Hence the principle of > autorotation is to lower the collective such that the "glide angle" of the > blades is equal to the ratio of drag:weight where drag includes engine and > pitch mechanism and weight includes the aircraft itself. Yes, for an in-flight engine failure the proper response is to immediately lower the collective pitch, which will alter the _relative wind_ on the blades and allow them to be driven by the air rushing upward through the disk (instead of driving air down through the disk when powered). Some instructors refer to this as an "inertia bank," from which you make slow, steady deposits during descent and a very large withdrawl at the end. BTW, for powered aircraft lift overcomes weight and thrust overcomes drag. The ratio drag:weight is not used. > Hence the > helicopter will descend slowly. I wouldn't call it "slowly", unless you're compairing it to freefall. In a Robinson R-22, a common piston trainer, autorotation descent rates are somewhere around 1500-2000 fpm. Starting from a practice entry point of 1500 feet AGL, this gives less than a minute to: 1. enter autorotation (lower collective, yaw correction with pedals, cyclic to reach best glide speed). 2. select a landing spot (free of trees, wires, cows, etc). 3. manuever the ship into the wind 4. keep the blades from overspeed or rundown. (overspeed = too much inertia built up in the blades -- rotor RPM exceeds redline. On most helicopters maximum rotor speed is set to avoid either transmission failure or excessive centrifugal force on the point where the blades meet the hub. Overspeed can be cured by raising the collective slightly to increase the angle of attack and convert some of the RPM to lift.) (rundown = too much inertia is leaving the blades -- rotor RPM falls below minimum. If this keeps up you won't have the energy you need when you reach the ground). 5. At about 50' AGL pull aft cyclic to flare the ship. Aft cyclic is used to bring forward airspeed from best glide (somewhere around 55 knots on most light pistons) to zero. The forward speed energy will be used to bring the descent rate from 1500 - 2000 fpm to near zero. 6. Add up collective to use the remaining rotor system intertia to cushion the eventual ground contact. NOTE: Step six would only occur in an actual emergency. In practice step six is to re-open the throttle, rejoin the needles (bring engine RPM up to match rotor RPM, which will re-engage the drive clutch and power the blades), and stabilize into a hover. At one time the US Army practiced autorotations to touchdown, but this changed after too many helicopters were suffering damage due to "excessive vertical speed at time of ground contact." > [...] > I expect that > in a helicopter stalling the blades is not necessary but the inertia > would be useful for decreasing vertical speed in the last seconds. > I would have thought that the big thing to avoid when autorotating a > helicopter is stalling the blades. This would cause the aircraft to loose > lift until the rotor can be unstalled. This would have to be done > by lowering the collective as far as possible and praying. Blade stall in a helicopter occurs for the same reason it does on an airplane -- excessive angle of attack. It is something to be avoided. Stalling can eventually occur during autorotation only by *not* lowering the collective and letting the rotor system run down. Blade stall on a helicopter is usually in reference to "retreating blade stall", which is generally the limiting factor for speed in forward flight. > Have I completely misunderstood how helicopters fly or am I about right? No, not completely. It's different mechanics, but the same physics. -- Dan veeneman@mot.com