PPRuNe Forums - View Single Post - prop design
Thread: prop design
View Single Post
Old 11th January 2001 | 01:51
  #2 (permalink)  
Genghis the Engineer
Guest
 
Posts: n/a
Post

I'm not going to be much help on the Saab, since apart from a vague recollection that I've read somewhere that the thrustlines are about 2½° off parallel with the longitudinal axis to counteract torque, I've never been near a whole one.

I do however do quite a lot of work matching fixed pitch props to smaller engined GA aeroplanes. It's a bit of a black art, but in general I find that if at max RPM you design for a tip speed of about 0.65M and you set pitch on the ground so that firewalling the throttle you can generate about 90-95% max.rpm in still air, the prop matching is pretty damned close, and no more than a couple of degrees of adjustment is likely to be needed.

I certainly wouldn't even contemplate flight testing an aeroplane that had tip speeds in excess of 0.85M - my career is too precious to me.

There's quite a useful document that covers microlight engine : propeller matching at http://www.bmaa.org/011.pdf which might be useful to you (bear in mind that it's written around Rotax engines which have a max.rpm of aroun 6,500 to 7,000 rpm and usually a 2.58:1 gearbox.)

G