R/C Whirlygig Is Terrifyingly Unstable

In the days during and immediately after World War II, aerospace research was a forefront consideration for national security. All manner of wild designs were explored as nation states attempted to gain the upper hand in the struggle for survival. The Hiller Hornet was one such craft built during this time – a helicopter which drove the rotor through tip-mounted ramjets. Unsurprisingly, this configuration had plenty of drawbacks which prevented it from ever reaching full production. The team at [FliteTest] had a soft spot for the craft, however, and used it to inspire their latest radio controlled experiment.

Initial experiments consisted of a modified foam wing from a model seaplane, with two left wings facing opposite directions, and joined in the middle. Two motors and props were fitted to the wings to provide rotational motion. After some initial vibration issues were solved, the improvised craft generated barely enough lift to get off the ground. Other problems were faced with centripetal forces tearing the propellers off the wing due to the high rotational speeds involved.

A second attempt started from scratch, with a four wing setup being used, with much higher camber, with the intention to generate more lift with a more aggressive airfoil, allowing rotational speeds to be decreased. The craft was capable of getting off the ground, but instabilities likened to the pendulum rocket fallacy prevented any major gain in altitude.

We’d love to see a redesign to solve some of the issues and allow the craft to sail higher into the air. If you think you know the solution to the whirly bird’s dynamic problems, be sure to let us know in the comments. It should be possible, as we’ve seen successful designs inspired by maple seeds before. Video after the break.

[Thanks to Baldpower for the tip!]

10 thoughts on “R/C Whirlygig Is Terrifyingly Unstable

    1. You just need a control algorithm to balance things out. There’s a few papers on monocopter / maple seed copter control algorithms. You could probably do pretty well just monitoring power output of each motor to ensure balanced current draw.

  1. Use rotors in counts of prime numbers, as it it significantly reduces the harmonic vibrational modes available. A 3 sided rotor can produce harmonics at 1x and 3x per rotation, whereas 4 sided can produce 1x, 2x and 4x. The lower numbers are generally higher amplitude so prime numbers eliminate most of those.

  2. The rotational inertia perpendicular to the spin axis is low and the center of gravity is high.
    In a traditional whirly, the dowel rod can’t be too light or it generates the same oscillations.
    If they double the rod length and pull the center of gravity down one prop diameter, the oscillations will decrease in amplitude and increase in frequency to the point they disappear. They could add mass to the bottom of the rod and decrease weight in the head.

  3. A fairly serious inventor tried this basic idea sometime around 1942: Arthur Young, who abandoned this approach and went on to invent the first few models of the Bell helicopter lineage, culminating in the Model 47, the first ever to be commercially certified and which saved many lives by evacuating wounded in the Korean War. Young’s description of the prop-driven helicopter attempt is in the opening pages of his book “The Bell Notes.” … And, concerning the vibrations you’re noticing, a large part of that problem is 2-blade props. In one orientation, the blades are generating a gyroscopic torque, due to the interaction of the prop-shaft rotation with the rotary-wing rotation.. Then 90 degrees later in prop shaft rotation, the gyroscopic torque is zero. So that torque appears and disappears, twice per prop revolution, generating huge vibrations thru the shaft and motor. In the history of large wind turbines, 2-blade turbines were abandoned in favor of 3-bladers, because when the nacelle is rotating to a new heading, the on-off-on-off fluctuations in gyroscopic torque cancel out with the symmetry of 3 or more blades and you get smooth rotation (but cyclic bending moments in the individual blades are still terrible.) So try 3-blade props, or you can make a decent 4-blade prop using two 2-blade props stacked at right angles. Then you mention the “pendulum rocket fallacy.” Here you’ve got that going on, combined with gyroscopic aspects of your 4-blade main rotor. Controlling that will be non-trivial. You took the right direction by using broad, high-lift rotor airfoils and four of them instead of two. You will find, however, that you do better with four LITTLE props and motors, one per rotary-wing blade, and move those out to the rotor tips, which means that you can get the same torque at higher prop speed but with MUCH SMALLER props and motors. Cutting prop radius and prop weight and motor weight, those all work together to lessen your problems with both centrifugal and gyroscopic forces (which both come strongly into play.) The way the weight and centrifugal and gyroscopic forces scale, you do better at or near the blade tips rather than near the blade roots, but you get major changes in the proportions that you need.

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