paj@gec-mrc.co.uk (paj) (12/13/90)
From: paj <paj@gec-mrc.co.uk> 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. 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). 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. Hence the helicopter will descend slowly. In a conventional aircraft (glider or powered), one lands by flying close to the ground and then leveling out (with power right down) to allow the aircraft to slow down until it stalled onto the ground. This ensures that the landing is at the absolute minimum speed (amongst other things). 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. There is a kind of aircraft known as an "autogyro". It consists of a helicopter rotor with no drive and a conventional aircraft propellor for thrust. The rotor is driven entirely by autorotation. A gentleman named Frank Wallis (no relation to Barnes) has been pushing these for military use with little success for some years, and was also the builder and stunt pilot of "Little Nellie" on one of the Bond films (Thunderball I think). Have I completely misunderstood how helicopters fly or am I about right? Paul.
veeneman@mot.com (Dan Veeneman) (12/17/90)
From: veeneman@mot.com (Dan Veeneman) > From: paj <paj@gec-mrc.co.uk> > > 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
Steve.Bridges@Dayton.NCR.COM (Steve Bridges) (12/20/90)
From: Steve.Bridges@Dayton.NCR.COM (Steve Bridges) [mod.note: Followups to rec.aviation, please; I think this has been covered sufficiently for our purposes in sci.military. - Bill ] In <1990Dec13.034019.20161@cbnews.att.com> paj@gec-mrc.co.uk (paj) writes: >From: paj <paj@gec-mrc.co.uk> >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. >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). 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. Hence the >helicopter will descend slowly. Gliding is a misnomer (Note: I have a fixed wing liscense, single engine and multi-engine, and have been slowing working on rotary-wing. The purpose of lowering the collective is two-fold -- Decrease the angle of attack of the main rotar system. Doing this decreases the total lift, resulting in a descent. The rotar blades in flight (e.g. rotating) are rigid only by the fact that the centripetal forces acting on the blades keeps them from folding up in flight (e.g. excessive coning angle) Since the helicopter is descending, the airflow through the rotar system keeps the system turning. The transmission clutch disconnects the main rotar system from the engine, turning into a free-wheeling system. The tail rotar (anti-torque rotar) is connected to the main rotar shaft, so it is still turning, producing torque (or anti- torque if you want to do a pedal turn). There is also one other thing that helps -- stored energy in the rotar system. A helo like a UH-1 (or Bell Jet Ranger) has a tremendous amount of stored energy in the rotar system due to the fact that the blades have tip weights. A helo like a Robinson R-22B has a very light rotar system. In the R-22, we generally practice autos from 1000 AGL. The drill is to lower the collective fully to the floor. This generates a descent rate of 1500FPM. The cyclic is used to control the forward airspeed to 50-60 KIAS. Then about 100' AGL, the collective is slowly raised to slow the rate of descent, back cyclic is fed in to slow the forward airspeed, and the rest of the collective is used to cushion the landing. All of the above happens in about 45 seconds. >In a conventional aircraft (glider or powered), one lands by flying close to >the ground and then leveling out (with power right down) to allow the aircraft >to slow down until it stalled onto the ground. This ensures that the landing >is at the absolute minimum speed (amongst other things). 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. That is not how I learned to fly airplanes! The approach is made at a comfortable speed above the stall speed for the approach (depending on the amount of flaps, temperature, density altitude, and so on). The speed (called Vref) is generally 1.3 times the stall speed of the airplane. The glideslope ideally is a 3 degree one, stabilized in both the rate of descent, power setting, and speed. Once the landing is assured, power is reduced (about 20' AGL for the airplanes I fly), the nose is raised slightly to bleed off airspeed, and in a single, the stall warning horn should just be coming on as the main mounts kiss the pavement. A full stall landing in an airplane is hard on the airplane. When the airplane stalls (note - the stall warning comes on generally 10KIAS above the actual stall speed), Mr. Newton takes over, and the airplane will accelerate downward as a speed of 32 ft/sec^2. If done at any more than 1 foot above the ground, the least it will do is embarras the pilot. At worst, you might have a broken, burning airplane and some dead people. In twins, they are flown onto the runway in a controlled descent above the minimum single engine control speed (Vmc). If you attempt a full stall landing in a twin, the FAA should come out and shoot you right away, to save an airplane. >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. If both blades stall, they will probably fold up and you will die. -- Steve Bridges | NCR - USG Product Marketing and Support OLS Steve.Bridges@Dayton.NCR.COM | Phone:(513)-445-4182 622-4182 (Voice Plus) ..!ncrlnk!usglnk!uspm650!steve | AOPA #916233 ..!uunet!ncrlnk!usglnk!uspm650!steve| PP-ASEL, AMEL (I want a P-38 type rating)