Let's make a helicopter blade

[Graviman]

"...top-down approach should also be taken toward the development of an optimal blade."

Agreed (i take this approach with most things). An important part of this is also to ensure that the design goes down avenues most likely to result in practical design. In this way you get the perfomance, but keep good reliability without introducing cost.

"IMHO, Active Blade Twist is, BY FAR, the best way."

Not disputing this at all. Just trying to say that there are easier ways of implementing it. Consider a "fixed" wing - IMHO the tip should ideally offer 0 degrees AOA to the airflow, with the rest of the blade increasing AOA elliptically towards root. This produces the weakest tip vortices (ie induced drag) for any given lift. The wing root would ideally have AOA altered for manouvering and velocity trim change, but the elliptical lift distribution is a given. On banking the elliptical lift would effectively develope a slight inflexion. Once the aircraft is established in a turn, the tip will twist to keep it's AOA 0' rel airflow, so the span pressure distribustion becomes elliptical once again. This is easist achieved by a wing that is stiff in bending, but allows torsional flex - ie the wing/blade warping you have proposed. And the easiet way to achive this? - a powerful tip tab, set to 0 degrees.

In a rotor wing the situation is complicated by the wing rotating around an axis, hence inflow velocity being proportional to radius. This means the blade must now have it's form multiplied by an inverse taper (ie 1/radius) for constant downwash velocity, but the dynamics still hold. The stiffness is chosen to allow an elliptical response to any torsion along the blade. A powerful tip trim tab ensures that the tip is always at 0 degrees relative to the airflow. The blade thus only need be controlled from the root, allowing a simple actuation method for Active Blade Twist.

Naturally this only works as long as the air is flowing in the forward direction across the tip. The reverse flow region of the root presents it's own difficulties, and is why i suggest feathering the retreating blade so as to produce no lift. In this way only the advancing blade actually produces downwash, allowing a near perfect downwash pattern in all regimes of flight. Since Prouty's example helicopter achieves a "glide" slope of 6:1, i suggest you aim for at least 10:1 with such a system.

Mart