Doug Riley
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- Joined
- Jan 11, 2004
- Messages
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Brian: Sounds like you want to know what the "doomsday" situation looks like. What happens if your RRPM goes too low in flight?
Well, I'm still here, so the simple answer is I don't know. We can predict with some confidence, based on what many, many observers have seen in the seconds before a crash, as well as what the aggressive yank-and-bank people, and the curious experimenters, report.
First of all, we've all experienced divergent retreating-blade stall on the ground. You pile on too much airspeed before RRPM is up (some call it "crowding" the rotor) and so much of the retreating blade stalls that the rotor rocks violently around the teeter hinge and hits the teeter stops and/or parts of the aircraft. We call this "flapping" but specifically it's DIVERGENT flapping (as opposed to the normal kind, which happens all the time in flight). On typical light gyros, on-the-ground divergent flapping can happen at up to 200 RPM or more, if you really crowd the rotor hard.
Why "divergent"? Consider the familiar normal operation of the teeter hinge in forward flight. The retreating blade descends below the plane that's perpendicular to the rotor spindle. This descent means that the blade "sees" the air approaching it more from beneath than it otherwise would -- IOW, the blade sees a larger angle of attack (AOA). This larger AOA causes the blade to make more lift than it would if it stayed in a higher orbit. Voila! The retreating blade's relatively low airspeed is compensated for by higher AOA, so it makes the SAME lift as the fast advancing blade.
All this happens automatically. The retreating blade sails down until its AOA is high enough to balance out the lift across the disk. Trouble is, airfoils stall when their AOA gets a too high (around 12 deg.). Our mindless automatic mechanism will keep dropping the retreating blade lower and lower as long as lift isn't balanced between the two blades. In low-RPM cases. this mechanism will drop the blade so fast that, before the retreating blade can make enough lift to balance, large portions of the retreating blade will reach their stalling AOA -- and stall. That's divergent flapping.
At a steady one G, the gyro either will take off and fly (with the teeter mechanism working normally) or it'll never get off the ground. There's bound to be a rotor size and pitch that will just flap endlessly if you try to take off with it. I've never heard of that happening -- you're talking a truly humungous rotor. Chuck Beaty got close by cranking some 6 deg. of pitch into a Hughes rotor one time. His RRPM was in the low 200's.
But a more realistic possibility is that your big rotor will take off and fly -- but, if a monetary low-G** maneuver or turbulence drops the RRPM by X%, the rotor will go into divergent flapping once the load is reapplied.
I don't have a mathematical model to offer you to determine X. It will vary with airspeed, blade pitch, airfoil section, chord and rotor diameter.
It would be ideal if all rotors were tested on a motor vehicle, to discover the "point of no return" RRPM at various airspeeds and angles of attack. Short of that, we're left with a random bunch of data points (aka hangar- flying tales). From these, we know that
(1) a rotor can lose enough RPM in flight that the teeter mechanism stalls the retreating blade down into the tail or prop this seems to have happened in some PPO/porpoising accidents);
(2) divergent flapping is more likely to occur at higher airspeeds; it won't happen at all in a vertical descent.
(3) normal* rotors on Bensen-style gyros at Bensen-style speeds seem well able to tolerate a drop of 10+% of normal RRPM without any issues -- below that, when you re-load the rotor, you will likely to experience stick hammering at the very least, and maybe much worse.
A corollary to #2 is that a low-powered featherweight gyro can get away with a very lightly-loaded rotor, with less chance of divergent flapping than a more powerful gyro of the same weight that can fly faster.
__________________________
* By "normal," I mean disk loading at 1.2 or more, mu ratios at or below 0.2 - 0.25.
** The use of "G" in describing a rotor's operating conditions can be confusing. Of course, gravity always exerts a force of one G on the gyro. However, if we suddenly reduce the disk angle of attack of the rotor, the rotor will quit making an opposing lift force. The gyro will accelerate downward, just as the Vomit Comet does. From the rotor;'s viewpoint, "zero G really means "low disk AOA". A by-product of low disk AOA is a fairly rapid loss of RRPM.
Well, I'm still here, so the simple answer is I don't know. We can predict with some confidence, based on what many, many observers have seen in the seconds before a crash, as well as what the aggressive yank-and-bank people, and the curious experimenters, report.
First of all, we've all experienced divergent retreating-blade stall on the ground. You pile on too much airspeed before RRPM is up (some call it "crowding" the rotor) and so much of the retreating blade stalls that the rotor rocks violently around the teeter hinge and hits the teeter stops and/or parts of the aircraft. We call this "flapping" but specifically it's DIVERGENT flapping (as opposed to the normal kind, which happens all the time in flight). On typical light gyros, on-the-ground divergent flapping can happen at up to 200 RPM or more, if you really crowd the rotor hard.
Why "divergent"? Consider the familiar normal operation of the teeter hinge in forward flight. The retreating blade descends below the plane that's perpendicular to the rotor spindle. This descent means that the blade "sees" the air approaching it more from beneath than it otherwise would -- IOW, the blade sees a larger angle of attack (AOA). This larger AOA causes the blade to make more lift than it would if it stayed in a higher orbit. Voila! The retreating blade's relatively low airspeed is compensated for by higher AOA, so it makes the SAME lift as the fast advancing blade.
All this happens automatically. The retreating blade sails down until its AOA is high enough to balance out the lift across the disk. Trouble is, airfoils stall when their AOA gets a too high (around 12 deg.). Our mindless automatic mechanism will keep dropping the retreating blade lower and lower as long as lift isn't balanced between the two blades. In low-RPM cases. this mechanism will drop the blade so fast that, before the retreating blade can make enough lift to balance, large portions of the retreating blade will reach their stalling AOA -- and stall. That's divergent flapping.
At a steady one G, the gyro either will take off and fly (with the teeter mechanism working normally) or it'll never get off the ground. There's bound to be a rotor size and pitch that will just flap endlessly if you try to take off with it. I've never heard of that happening -- you're talking a truly humungous rotor. Chuck Beaty got close by cranking some 6 deg. of pitch into a Hughes rotor one time. His RRPM was in the low 200's.
But a more realistic possibility is that your big rotor will take off and fly -- but, if a monetary low-G** maneuver or turbulence drops the RRPM by X%, the rotor will go into divergent flapping once the load is reapplied.
I don't have a mathematical model to offer you to determine X. It will vary with airspeed, blade pitch, airfoil section, chord and rotor diameter.
It would be ideal if all rotors were tested on a motor vehicle, to discover the "point of no return" RRPM at various airspeeds and angles of attack. Short of that, we're left with a random bunch of data points (aka hangar- flying tales). From these, we know that
(1) a rotor can lose enough RPM in flight that the teeter mechanism stalls the retreating blade down into the tail or prop this seems to have happened in some PPO/porpoising accidents);
(2) divergent flapping is more likely to occur at higher airspeeds; it won't happen at all in a vertical descent.
(3) normal* rotors on Bensen-style gyros at Bensen-style speeds seem well able to tolerate a drop of 10+% of normal RRPM without any issues -- below that, when you re-load the rotor, you will likely to experience stick hammering at the very least, and maybe much worse.
A corollary to #2 is that a low-powered featherweight gyro can get away with a very lightly-loaded rotor, with less chance of divergent flapping than a more powerful gyro of the same weight that can fly faster.
__________________________
* By "normal," I mean disk loading at 1.2 or more, mu ratios at or below 0.2 - 0.25.
** The use of "G" in describing a rotor's operating conditions can be confusing. Of course, gravity always exerts a force of one G on the gyro. However, if we suddenly reduce the disk angle of attack of the rotor, the rotor will quit making an opposing lift force. The gyro will accelerate downward, just as the Vomit Comet does. From the rotor;'s viewpoint, "zero G really means "low disk AOA". A by-product of low disk AOA is a fairly rapid loss of RRPM.