Dear "J...."

In my previous post, I neglected to include that a symmetrical airfoil needs to have a positive angle of incidence or, if it is a wing,
a positive angle of attack. Otherwise it doesn't produce lift.

So for a symmetrical airfoil rotor blade to get into autorotation, it must have a positive angle of incidence relative to the plane of rotation.

However, a blade with positive incidence is very very very difficult to get up to speed so that it is producing enough lift to get into autorotation.

That is why the Clark-Y and the SG6042 are so well suited to autogyro rotors, They produce lift even at negative angles of incidence!

In fact a well adjusted rotor (that is one that has been fiddled with enough to start and yet go into autoroation) will easily get into autorotation by simply walking into a 5mph wind holding the rotor at about a 45 degree angle to the wind.

It is worth spending a lot of time, getting the angle of incidence of the rotor blades just right. In fact a rotor that won't spin up into autorotation can be "fixed" by adding as little as one layer of masking tape under the trailing edge of the blades. Conversely, a rotor that starts too easily, probably is simply windmilling and not really going into autorotation. Here the fix is to add a layer of masking tape at a time under the leading edge of the blades.

By the way, three blades are better than two and 4 blades are better than 3.

Bill