Aha - I thought this diagram might appear! I'll answer each point in turn.
1/ Agreed, but bear in mind these downwash velocities come from Prouty's blade sweep integration rotor charts, not taking into acount the tip loss vortex. The effect i'm talking about is in addition to the effect shown, since its purpose is to minimise tip loss power.
Downwash from a circular disk is elliptical (in forward flight) assuming constant downwash velocity throught the disk. Trouble is you never get that, since you want the fewest blades to keep the profile drag to a minimum. By using an elliptical downwash velocity across radius you minimise tip losses, and still virtually provides the overall elliptical lift pattern shown.
2/ Agreed, but this does also raise the important point about how to combine the velocity from overlapping blades. Both Prouty and Stepniewski treat overlap regions as having the same downwash velocity as a single rotor. If you have differing downwash velocity, such as when a retreating blade overlaps an advancing blade, i believe that averaging the velocities is a more accurate estimate since the lower downwash blade interferes with the higher downwash blade. My belief in feathering retreating blade is to minimise this interference.
3/ True, but the higher pressure air from below would stillleak through the zero velocity circle. An advancing blade below it, with reverse velocity blade twist for feathering, stops this
Mart
Last edited by Graviman; 4th May 2005 at 19:08.