Thorp Air Command - T18.net

why are most thorps 66 or 68in? i know a guy with a 72 and he does fine. does a longer prop decrease cruise? is a longer prop worse for aerobatics due to higher gyro forces?

Fixed or CS? Hartzel doesn't recommend less than 72" due to harmonics. Interested to know what aerobatics you are contemplating.

_________________
Bob Highley
N711SH
SN 835
KLAL

fixed pitch. nothing fancy just basic stuff.

let me rephrase: are there any downsides to a longer prop as long as tip speed and ground clearance are maintained?
i know a longer prop will make more thrust and thus better climb. will it increase drag and thus slow cruise?
or just stick with "keep it as long as possible for as long as posshble"?

I have a 68" prop, with the original "short" gear I had 6-7" ground clearance so that's reason #1, I also have an O290 140HP motor, reason #2, makes it easier for the motor to make rated power, bigger motor can swing a bigger prop. I've since replaced the gear with the "long" gear, 4" longer legs with an increased ground clearance of 10-11".

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

I have found that longer props require more horsepower to turn, by a lot! On the flip side, with the same motor, a longer prop will make your motor work harder. As far as cruise speed, it depends more on the airfoil. If you ask Catto, they will tell you shorter is faster.
Aerobatics are much easier in a CS than FP. You get more vertical penetration and it keeps you from over speeding on the down line. But they are heavier.
Cubes