Number of Blades
Dividing the total blade area up into a finite number of blades is the next decision. The fewer the blades, the cheaper the tail rotor is to build and maintain; but if the
result is a very stubby blade, high tip losses may penalize performance. Most designers will select the number of blades to satisfy an aspect ratio (radius/chord) criteria of 5 to 9.
High twist is beneficial on the main rotor for improving hover performance and for delaying retreating tip stall in forward flight. High twist also helps tail rotor hover performance; but, as mentioned in the discussion of tip speed, even untwisted tail rotor blades will not have high retreating tip angles of attack. Thus part of the usual advantage of high twist does not apply. A further consideration, reported in wind tunnel tests in reference 10.12 and in flight test observations in reference 10.8, indicate that untwisted rotor blades go through the vortex ring state more gracefully than twisted blades do. This then is a consideration regarding the ability of the new design to fly steadily in sideward flight.