Newbie question on auto-conversions and Props?

[rnbraud]

(caution newbie)

Why do auto conversions require PSRU's. I understand that auto engines rev at a higher RPM, but, is the need to reduce the propellor speed for the sake of the prop or the engine?

 

I guess I am wondering if normal prop cruise speed is between 2500-3000 rpm why not just put a different prop on an auto-conversion which operates at the 4000-6000 rpm range?

 

Or, is it possible to add more smaller blades to a prop, i.e. 5, 6, or 8 blade props of a smaller diameter, which would either slow down the engine or would be able to operate at higher rpms.

 

Just asking.

[Jon Matcho]

Why do auto conversions require PSRU's.

Common props that you can buy operate in the 2,000 RPM range, and that's why you need the PSRU. I believe you're entirely correct that you could possibly use a different prop geometry, but as the prop diameter was reduced (with more blades added), you would begin to approach the need for and design of a ducted fan.

 

Check out Perry Mick's attempt to do exactly this on a Long-EZ powered by a Mazda rotary. www.ductedfan.com The results were not as desired, and Perry has since removed the fan in favor of a PSRU setup.

 

This is not to say that the concept is bad -- I believe it could be perfected with enough time, money, and energy (pun intended).

[rnbraud]

Yep, kinda came to the same conclusion. My main driver for this was with the PSRU, however, there still needs to be some way to attach the prop to the crank which would mean some sort of housing.

 

PSRU's are more comples than asimple connection to the crank, but if you still need a housing, and therefore bearings and such, than a PSRU isn't all that much more complex.

 

Later.