Date: 30-Aug-00 15:40: Author: Dave Jackson: Subject: Lift vs. Drag

Hennie;

Your statement "You still have the advancing blade delayed and the retreating one speed up." raises an interesting hypothesis.

Assume that sufficient cyclic has been applied to eliminate flapping. The two blades must be producing identical lift. To do this the pitch of the retreating blade must obviously be greater that of the advancing blade. A blade's coefficient of drag is not in a direct correlation with its coefficient of lift. The blade elements on both blades have different angles of attack and are subjected to different air velocities.

The implication is that, when the two blades are producing identical lift, they are not subjected to identical drag. Therefor vibration.

This is only theoretical speculation and it would be appreciated if Chuck or some other knowledgeable individual would comment.

Date: 26-Nov-00 11:50: Author: Doug Riley: Subject: blade failure

The Cierva gyros flew before the invention of the 2-blade teetering rotor that most of us now use. Honors for that invention go, I believe, to the designer of the original Bell helicopter.

We get relief from the much of the lead-lag stress in our rotor-and-head system through the use of an underslung teeter hinge. There is still some 2-per-rev bending stress, however, that must be accommodated by a flexible mast. Bell mounted his whole engine-transmission-mast assembly on rubber to allow the flexing. On Bensen-style gyros, leaving the tubular mast relatively unbraced does the same thing. A few uninformed home-builders over the years have built rigidly triangulated masts on their gyros. Not a good move at all.

The mast on a gyro without rotor lag hinges must be free to flex (or somehow to allow a bit of movement of the rotor head in the plane of the rotor disk). A very rigid mast will allow the bending vibrations to work havoc on some other part of the gyro; possibly the blade roots or possibly something else.

Going way back, I remember one of our PRA chapter members complaining about an increase in frame vibration when he switched from the old 2x2 single-tube mast to the (then-new) "redundant" mast made of two 1x2's. The increased mast stiffness in the fore-and-aft plane probably did make a difference that an educated seat-of-pants could feel!

Date: 01-Dec-00 Author: Chuck Beaty: Subject: Drag hinges

Kevin, the drag hinges resulted from chasing 2/rev rotor shake. With a 2-blade rotor there is a 2/rev aerodynamic "hit:" Broadside to the flight path, there is more drag than endwise. It is centered by a pair of coil springs. It is nearly dead smooth and the total movement of the "slider" is less than 1/16 inch, judging from the wear pattern.

This supports using the solution below. - DBJ.