I offer a correction on two points raised in David Godfrey's letter (Flight International, 8-14 October).
Steel is becoming about three times "stiffer" than aluminium, with a Young's Modulus of roughly 30x106 lbs/in2 against the average aluminium alloy's 10.4x106 lbs/in2. Young's Modulus is a ratio of stress versus strain. Steel is also roughly three times heavier per unit volume. Aircraft-guide steels have higher yield strengths than aluminium. With stress proportional to axial load, it follows that it is roughly three times more resistance to axial load. Structural analysis must balance stiffness, mass and stress to achieve a design that works. Tip thrust reduces the need for anti-torque control, but does not eliminate it. Tip driven rotorcraft still have bearing friction to overcome. If a rotorcraft can maintain a forward speed, a rudder can trim this out. But if I remember correctly, this aircraft can transition to a fixed rotor configuration. How do you stop a moving rotor? With a brake. How do brakes work? Friction. What happens to the friction versus the angular momentum of the rotor? It results in torque about the rotors axis.This braking force could be reduced by reversing the tip thrust, but the rotor must be stopped at some point to go to fixed-wing mode. If this happens at lower speeds, anti-torque may be needed.
Joe Draper Seattle, USA
Source: Flight International
