Pitch vs.radius of shrouded propeller

[amorrison]

Most heli rotors are constant pitch while the pitch of airplane propellers varies with radius.

What are the recommendations -pitch wise- for shrouded propellers for "full size" aircraft.

 

[insideman]

Most helicopter rotors are constant angle. Propeller pitch is usually increasing toward the tip.

 

[Dan320]

Most helicopter blades do have twist.
Propeller pitch decreases towards the tip.

Just to get the basics right.

 

[rb1957]

it's all about the local velocity vector.

airplane propellers are twistied because they have two sources of velocity components ... the plane's speed (out of the plane of rotation) and the in-plane circular component (due to the spinning propeller).

helicopter blades are probably less twisted because they are (i guess) less sensitive to the velocity component due to the vehicle's motion.

 

[moon161]

I have no first hand knowledge of rotor blades.

I would expect that prop/rotor twist is so that the each slice of the blade from the hub out flies at an angle of attack that optimizes L/D, or at least that this is a consideration.

For a hovering helicopter and a prop in forward flight, your simple vector diagram will look the same.

If V_infinity is the same across the rotor/prop span, omega is the rotational speed, r the radius, I would expect to find r*omega/V_infinityto be a constant ratio across the span. I don't see how a duct would make much of a difference in this respect, except that the hub may be large relative the to the duct. In this case r_tip/r_root may be close enough to 1 that the twist is hard to notice.

 

[GregLocock]

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920074916_1992074916.pdf

may help

Cheers

Greg Locock


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