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I am working on an enclosure that is to be ejected from a rocket at apogee (10,000') and fall to a target altitude at a descent rate below 20ft/s before releasing its payload. The idea is to have coaxial rotors with collective pitch control deploy and autorotate to slow the descent of the enclosure and payload.

My question is how I can start autorotation with blades that have essential zero angular momentum, and are there special considerations for the use of coaxial rotors?

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Go back to Estes rocket days. There is a whole class of competition rockets known as "heliroc" or helicopter recovery. These ascend like a conventional rocket, in a vertical(ish) path and fin stabilized; at (or near) apogee, they release a rotor by some means and the rotor spontaneously begins to autorotate, often giving a lower descent rate than a parachute.

Go even further back: look at maple seeds (aka maple keys). They autorotate so efficiently that they'll enter stable rotation and constant (low) sink rate in as little as one blade length of fall (they do have very low wing loading, however).

So, yes, it's possible for a rotor disk to spontaneously begin autorotation from a dead stopped starting condition. Mother Nature does it billions of times every autumn, and school children do it thousands of times every year.

Yes, this will work with a coaxial rotor, if you can keep the rotors from interfering with one another during deployment (for instance, by braking them until fully deployed). However, since there's no reaction torque from freewheeling, autorotating rotors, there's no reason to have coaxial rotors. It's far simpler to use an ordinary rudder for yaw control than differential collective, and you'll have half as many parts for pitch and roll.

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  • $\begingroup$ Are there any special considerations needed for coaxial rotors? $\endgroup$ – Mr. Sir Sep 26 '19 at 0:07
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    $\begingroup$ That depends on how you deploy the rotors. If they're folded along the rotation axis (i.e. flap hinge folded) and can be deployed simultaneously enough to prevent one rotor interfering with the other, it shouldn't matter. BTW, you do know there's no torque reaction during autorotation? That makes coaxial rotors and unnecessary complication. $\endgroup$ – Zeiss Ikon Sep 26 '19 at 11:04
  • $\begingroup$ The current idea is the have the blades folded "down" along the rotational axis. The reason I was considering coaxial rotors is that one of our design goals is to generate electrical power during descent. If I am not mistaken, a single rotor connected to a motor would have torque. Could I use a single rotor and vanes to keep the enclosure from rotating? $\endgroup$ – Mr. Sir Sep 27 '19 at 6:11
  • $\begingroup$ Pulling energy off the rotor for electric generation will produce a torque drag (opposite direction from torque reaction), so you'll need some means off offsetting the torque. Vanes would have to be large, since your speed is low. Might be lighter to put a rotor-driven tail rotor on it. Deployment setup is left as an exercise... ;) $\endgroup$ – Zeiss Ikon Sep 27 '19 at 11:02
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...how I can start autorotation with blades that have essential zero angular momentum...

By using the collective pitch control mentioned in OP. At release, the blades should be aligned with the free stream like a feathering propeller, then gradually be set with decreasing blade AoA as the rotor speeds up.

...are there special considerations for the use of coaxial rotors?

Yes, coaxial rotors complicate all design efforts tremendously. Both rotors would have to have blade pitch control, one blade shaft must be mounted within the other which counter-rotates, etc. Much simpler to use only one, larger rotor, since yaw compensation is not required during autorotation.

Note that at vertical autorotation, a rotor has a slightly lower drag coefficient than a flat plate of equal area, which has a lower drag coefficient than a parachute. As mentioned in this answer.

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  • $\begingroup$ Thank you for the advice! A design goal I did not mention in the OP is power generation during descent. Keeping the net torque = 0 is the reason I was looking at coaxial rotors in the first place. If I were to go with a single rotor, are there methods of providing counter-torque such as vanes (no including a tail rotor), and would 4 blades have advantages over 2 blades? Again, I appreciate your input and would appreciate sources to self educate if you know of them! $\endgroup$ – Mr. Sir Sep 27 '19 at 6:24
  • $\begingroup$ Ah OK. When the tail rotor of a single rotor helicopter fails, the engine is idled and the helicopter autorotates back to earth: autorotation causes close to zero shaft torque. Which also means that zero shaft torque can be used for power generation... Note that this sort of autorotation delivers blade lift which slows down the deceleration from gravity. If a propeller is just windmilling, it creates shaft torque and no lift, and indeed does require yaw compensation, via simple adjustable vanes for instance. $\endgroup$ – Koyovis Oct 3 '19 at 1:46
  • $\begingroup$ A good book for self education is Principles of Helicopter Aerodynamics by j. Gordon Leishman, with a chapter on windmills as well. $\endgroup$ – Koyovis Oct 3 '19 at 1:47

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