Gimbal / spindle

[ymmv]

I've got a pretty good conceptual handle on gyro aeronautics, not the kind of mathematical understanding that some of you have, but still I get it in reasonable detail. Some of what I don't know about rotor dynamics I'm learning with the help of a book suggested by Chuck Beaty. Good.

The gaping hole in my understanding is the precise way in which a gimbaled rotor head contributes to stability. I understand how it goes together and could draw a decent picture of one, but I don't know why it matters. Why is a gyro with a gimbaled rotor head stabler than one with a straight spindle head?

Can someone give me a clue, please, or direct me to a good explanation?

[C. Beaty]

The term cyclic flapping is misleading, Bart, inasmuch as blades don’t actually flap in a physical sense.

The teeter bolt in the case of a seesaw rotor and flap/drag hinges in the case of 3 or more blades permits the rotor disc to rotate about an axis that differs from the rotorhead axis; flapping articulation amounting to a universal joint connecting rotor disc to rotorhead.

Forward flight produces the appearance of a blowback of the rotor disc relative to the rotorhead axis as a result of cyclic flapping.

Viewed along the rotorhead axis, the blades appear to flap without cyclic pitch change.

Viewed along the tip plane axis, the blades don’t flap but undergo a cyclic pitch variation.

The thrust developed by a rotor is normal to the tip plane (nearly). For all practical purpose, the rotor thrust vector is coincident with the tip plane axis.

Offsetting the gimbel head pitch axis forward of the rotor thrust vector couples a fraction of rotor thrust into the cyclic control system that is balanced by a trim spring.

An increase of rotor thrust from say, an upward gust, overpowers the trim spring and produces a nosedown force in the control system, tending to tilt the rotor in a nosedown direction and head the rotor into the relative wind. Stable.

With a spindle rotorhead, the rotor thrust vector passes forward of the pitch axis, producing a nose up force in the control system. An increase of rotor thrust from an upward gust applies an increased nose up force to the control system. Unstable.

[rehler]

Vance Breese,

At the SWRFI this morning, as we were leaving the AAI display, you asked me a question, which I tried to explain. Chuck answered it much more clearly in his above post.

[ymmv]

Viewed along the tip plane axis, the blades don’t flap but undergo a cyclic pitch variation.

Yep, got that far on my own....


The thrust developed by a rotor is normal to the tip plane (nearly). For all practical purpose, the rotor thrust vector is coincident with the tip plane axis.

Yeah....


Offsetting the gimbel head pitch axis forward of the rotor thrust vector couples a fraction of rotor thrust into the cyclic control system that is balanced by a trim spring.

Okay, fair enough....


An increase of rotor thrust from say, an upward gust, overpowers the trim spring and produces a nosedown force in the control system, tending to tilt the rotor in a nosedown direction and head the rotor into the relative wind. Stable.

Ah! Eureka. Okay, right, that makes perfect sense at last. Thanks, Chuck: this is the first time I've heard it put clearly.