Re: Coaxial counter-rotating airplane propellers pitch

This is a fairly difficult question to answer completely, but easily answered generally. Unfortunately I have not been able to find details of the gannet propeller pitch. Below is what I have been able to gleen from the net or deduce for myself.

In general if you have a counter rotating prop system the two props will need to have different pitches.

A propeller is a wing. For maximum efficieny you need to control the angle of attack (the angle between the airflow and the wing chord). This becomes complicated for a propeller because as you move out from the hub of the prop the speed of the blade (thus angle of attack) changes, and as the propeller cuts through the air at different forward velocities the angle of attack changes.

Now for a counter rotating system the second disc is meeting already disturbed air. In general the first disc will cause a "swirl" of air behind it. The air behind the fist disc is travelling backwards faster now, but it also has a rotation applied to it. The increased backwards speed of the airflow tends to require a higher pitch on the second disc, the rotation tends to cause a requirement for a lower pitch. The second disc has different pitch requirements to the first, however just what those requirements are changes depending on the type, speed, use etc of the system.

Finally it should be noted that a pilot does not directly control propeller pitch (in aircraft where there is any involvement of pilot with propeller pitch). Variable pitch propellers almost always use a constant speed device. This device alters propeller pitch to maintain a constant rotational speed. The pilot adjusts for engine speed and the CSU adjusts propeller pitch to maintain that speed.

This article talks about pitch requirements for contra rotating helicopter blades: http://www.helosim.com/coaxiala rticle.htm

A close up of a contra roating system for a torpedo - the obvious difference is blade design for each disc: http://www.larwe.com/sub/ img/mk3_tail_hires.png from here http://www.larwe.com/sub/groton.html

The AN-70 is a modern aircraft with contra rotating propellers: http://www.aeronautics.ru/nws0 02/ai016.htm
http://www.aeronautics.ru/img002/an70-progres-d-27- drawing.jpg

Martin Smith