Gyro Accident in the UK

C. Beaty said:

Check this thread. Rotor damping is discussed in detail.

http://www.rotaryforum.com/forum/showthread.php?t=18121&highlight=rotor+damping

I have been able to analyze an RAF rotor blade section and was surprised at how good it was. The CG is almost exactly on the ¼ chord line and pitching moment coefficient is nearly zero. The only real negative was that the leading edge portion had some lack of fairness that increased the drag. I have the computer plots around here somewhere.

Nearly all gyros follow Bensen’s lead and use the NACA 8H12 airfoil section. Both Bensen and Sportcopter took some liberties and flattened out the lower surface. DW rotors are about the only serious brand which does not use the 8H12.

Click to expand...

reelmule said:

Doug, Thanks for the update on sailplanes...
Perhaps one advantage of being tail heavy is that most of these machines have provisions for jettsionable water ballast of 100-200 lbs probably located forward of the CoG so that the max L/D occurs at a higher airspeed...
Handling is probably greatly improved with the ballast.

Click to expand...

C. Beaty said:

Skyjinks, long, thin structures such as rotorblades are flexible in torsion by their very nature. An idea of flexibility can be derived from the modulus of elasticity of the material from which they’re constructed. Aluminum has a modulus of ~10 mpsi, fiberglass ~ 3 mpsi and wood ~ 1 mpsi.

Rotorblades can and often do periodically twist as they rotate. If, for example, they have a nosedown pitching moment coefficient (too much camber, insufficient reflex for neutralization), it is possible to run out of rearward stick at high speed. The advancing blade, having the greatest airspeed, twists nosedown more than the retreating blade and tips the rotor disc nose down.

The aerodynamic center of a rotor blade airfoil is at ~ 25% of chord from the leading edge. The entire aerodynamic load can be imagined to act on the aerodynamic center plus a moment load if not with zero pitching moment coefficient.

If a rotorblade is nose heavy; that is if its CG is forward of its aerodynamic center, a rapidly applied aerodynamic force will cause it to twist nose down.

An upward gust increases the angle of attack of both blades equally but the advancing blade, having the greater airspeed, twists nose down more than the retreating blade. The rotor disc responds by tilting nose down and thus keeping it headed into the relative wind, enhancing angle of attack stability.

A rotor with nose heavy blades follows control stick displacement with a greater lag, causing greater stick force.

A disturbance that tends to tilt the airframe is resisted because the nose heavy rotor lags behind a tilt of its spindle to a greater extent than a normal rotor.

Addendum: Here’s a plot of an RAF rotorblade section done on a desktop wind tunnel.

These computer programs are quite accurate for pitching moment coefficient and angle of zero lift but less so with drag, especially near and beyond stall.

Click to expand...

C. Beaty said:

Skyjinks, a simple demonstration of the effects of noseweights can be performed with a strip of aluminum flashing material, -typically 0.005” thick- and a portable electric drill; preferably a reversible one. Running in reverse will prevent the mandrel screw from unscrewing.

Without the noseweights, it will flutter violently. With 3 or 4 noseweights in a row, it will only barely follow the tilt of the drill.

The holes for the noseweights should be as near to the leading edge as possible without breaking through.

Click to expand...

C. Beaty said:

Vortech blades aren’t the only ones that are tailheavy, Giorgos. One would think that with skins as thick as extruded blades must have, twist wouldn’t be a problem but apparently not so.

The response of tail heavy rotorblades to gusts is sometimes called “ballooning” by pilots. An upward gust causes the gyro to pitch nose up and gain altitude, a symptom of angle of attack instability.

It is most noticeable on heavy machines with high blade loading.

Click to expand...