Propeller Challenge – Calling Jan Carlsson

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:


Well-Known Member
Lifetime Supporter
Feb 24, 2008
San Jose, CA
Jan, this is somewhat inspired by the "quiet props" thread, but also just a pragmatic question.

The HKS 700T engine is an 80 hp turbocharged engine (77 hp continuous) that produces peak power at 5300 rpm (4900 rpm continuous), and has an integrated 2.62:1 redrive, for a prop speed of ~2025 rpm (1870 rpm continuous).

The Sonex Onex is designed around the Aerovee engine, which produces its 80 hp at 3400 rpm, direct drive. It spins a 54" x 44" fixed-pitch propeller, giving a ground clearance of 8" (both taildragger and tricycle).

To stay within the 7" ground clearance requirement for a tricycle gear aircraft prescribed in Part 23, and assuming a similar thrust line between the HKS and Aerovee, the maximum prop diameter for the Onex would be 56". This leads to an HKS tip speed of 150 m/s (~0.44 Mach), which is, as expected, incredibly slow.

So, what does an HKS-driven Onex look like? Lots of blades, I assume. Noise level? Efficiency?

Jan Carlsson

Well-Known Member
Jan 11, 2009
With the same power?? it will have the same speed, so with a much lower rpm the pitch will be high, we can suspect poor take off, many blades help with take off though.

to get the effective pitch, take speed mph * 88 / rpm * 12 = inch
so some back "engineering" 44 / 12 * 3400 / 88 = 142 mph
so (the long way around the problem) 142 * 88 / 2022 * 12 = 74"

Tan-1 74/(56 *,75* 3,1416) = 29 degree at r/R .75

you will need 4 blades or more. a normal AR on 2 blade is 6, (5-7) on 3 blade AR 4.00 and 4 blade AR 3, 5 blade AR 2,4, 6 blade AR 2
That is the diameter devided with the combined max chord, example (72" /(6+6)= 6 )
I might think the take off will suffer, on the other side the turbo and geared engine will have more low(er) rpm torque.
People that don't like airplane will only complane if they see you, = little noise
Last edited: