My 1970s helicopter

C. Beaty

Gold Supporter
Sketch showing that if JC had used a 2x6 plank rather than a square one, aerodynamic lift force would have brought plank axis into alignment with rope axis. plank.JPG

Doug Riley

Platinum Member
Chuck, I've copied the plank-on-a-rope experiment and, of course, had the same results. Fun and cheap entertainment.

Another interesting trick to try while you have the plank and rope assembled is a demonstration of gyroscopic precession. While the plank is spinning, tap one end from above or below and watch which way the "disk" tilts.

C. Beaty

Gold Supporter
Much can be learned from simple models, Doug.

My favorites, in addition to JC’s plank, are scale model rotors made up from welding rod and aluminum strips and run by a portable electric drill. With such models, it’s easy to demonstrate delta-3 coupling, pitching moments and the effects of chordwise balance.

A rotor made from a 1” x 12” strip of aluminum flashing will flutter violently if run without noseweights.

C. Beaty

Gold Supporter
Here’s another simple training aid that will help in the understanding of autorotation; a 6” long slice from an 1.25” dia wood dowel (Lowe’s stocks 1.25” wood dowels in 6’ lengths for $8 –each member of the family can have his own windmill).

Give it a flip in either direction, hold it out the window of a moving car and it spins merrily away. Once a person understands how this can be, he’s well on his way to understanding autorotation,



Junior Member
Hey Chuck,

I just read the Bell Engineering/NTSB final report on the cause of the 525 crash that killed 2 Bell test pilots in 2016. The finding identified "excessive blade flapping" during a high-speed test, which struck the tailboom causing it to fail, followed by complete in-flight breakup.

I have to interject, flapping is not imaginary. If the rotor behaves like a rock on a string, it would not have "flapped" low enough as the blades passed the tailboom to chop it off.

C. Beaty

Gold Supporter
Flapping is a common term, used when viewing a rotor relative to its drive shaft.

A long range artillery shell when fired north-south and viewed from the Earth’s surface appears to follow a curved path as a result of the Earth’s rotation but when viewed from space, moves in a straight line just as Newton said it must.

That’s where Coriolis comes into the picture: when using the Earth reference we must apply an imaginary force to explain the imaginary curvature.

Same with rotors; when viewed from the shaft axis, we must apply an imaginary force to explain imaginary lead-lag and to avoid violating Newton.

But viewing a rotor along its real axis of rotation, the tip plane axis, there is no flapping nor lead-lag.

JC’s plank on a rope is flapping in the exact same sense as a see-saw rotor in forward flight; is it flapping? It is if you were rotating with the plank and viewing it along the rope axis.

Flap and drag hinges constitute a universal joint which permits a rotor to rotate about its own axis rather than the shaft axis. A teeter hinge is all that is required in the case of a see-saw rotor.
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Chuck, back when I started experimenting with rotors I had never seen a two blade machine up close but I had seem three bladed machines and I did know that they had a hinge that allowed them to flap or cone, I built a rotor on a fixed test stand that only had the flap hinge and would it up in a breeze and let it do it’s thing and in a few hours it failed and tore up everything, I built another one stronger and kept checking it and it was starting to crack, a helicopter pilot told me that I needed a second hinge, I didn’t understand how it helped but it did, I knew that stress can’t cross a hinge but I didn’t understand where this movement was coming from, I built a small scale model and walked around with the blade and I finally saw what was happening and why it was cracking without the drag hinge, with the blades coned up the blades were swinging the tips through an arc, the higher the cone angle the greater the swing and I now understood the purpose, it simply allowed the hub to change the pitch of the blades as they went around without swinging the tips and over stressing the attaching points. I never could find a lead leg motion even though everyone said that it was there.

C. Beaty

Gold Supporter
flap.JPG Without a drag hinge, it looks like this, Norm. Has nothing to do with speeding up and slowing down, leaving poor Mr. Coriolis out in the cold.
Angles are exaggerated for illustration.

Doug Riley

Platinum Member
Well, with only a flap hinge, the mechanism WANTS the blade to speed up and slow down.

The blade would NEED to speed up in order to arrive on time at the spot that Chuck has labelled "flap hinge only forces blade here."

This is the old familiar "snap" that occurs in U-joints when their input and output shafts aren't parallel. It's why U-joints in drive trains are used in pairs -- with the U-joint there really IS a speedup-slowdown 2/rev.

Seesaw rotors accomplish all this with a minimum of mechanical complexity -- but at a cost. The seesaw arrangement becomes aerodynamically unstable if too much of the retreating blade stalls. In effect, the mechanism adds more and more angle of attack to the retreating blade until the whole blade stalls. That leaves the rotor with lots of lift on the advancing blade and a sudden lack of lift on the (stalled) retreating blade -- like a seesaw when a kid on one side jumps off. It's also not much different, conceptually, from a spin entry in a FW plane (a pair of fixed wings is rather like a semi-rigid rotor). This runaway stall, with lots of lift on the advancing side, is what forces the retreating blade into the tail, prop, etc, whether in a Bell, Robbie or gyro.

You can call this phenomenon "flapping" (and gyro folks do) but that does confuse the issue. "Catastrophic flapping" or "divergent retreating blade stall" would be more accurate, but quite a mouthful.