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As a followup to the general question on length I found this webpage:
http://www.mh-aerotools.de/airfoils/jp_propeller_design.htm . Also, "Design for Flying" by Dave Thurston is a good book that covers the considerations in aircraft design- I've found it to be very helpful. He has a section on propeller design and what I get from it is that large diameter is preferable but is restricted by the need to keep the tip speed subsonic and by ground clearance. "For example, study of a recent 250-hp constant-speed installation indicated that a three-blade 76-in diameter propeller was slightly more efficient than a 76-in diameter two-blade, and much more efficient than a 74-in diameter two-blade one, particulary at climb speed. However, an 80-in diameter two-bladed design was the most efficient of all throughout the entire flight range, provided such a large diameter could be used."
It may be that Hartzell doesn't recommend a 70" blade due to an interaction with the IO-360. Shorter or longer might change the propeller's resonant frequencies so it wouldn't be excited by the engine pulses. I'm not sure that the engine counterweights would dampen the prop- I believe they're tuned for crankshaft frequencies. Since you're midway through the overhaul I don't know what your options might be. Your best sources might be Lancair and other builder/owners who could tell you what has worked for them. -Bill Wade
----- Original Message ----- From: <mmcmanus@grandecom.net>
To: <lml@lancaironline.net>
Sent: Saturday, November 15, 2008 7:36 AM
Subject: [LML] Prop Length for 360
I'm in the middle of getting my Hartzell 2 blade prop overhauled for my Lancair
360. I got a new hub at 50% discount from Hartzell to eliminate the recurring
eddy current inspection. The prop shop doing the overhaul has issues with the
blades and they will not issue the formal 8131 (I think) paperwork for the
overhaul. The problem is the blade length.
I bought the airplane 3 years ago and it had an overhauled "0" time prop when
the airplane was built in 2002. The blades are 70" long according to the prop
shop. They also say that the Hartzell recommends a 72" blade. But Hartzell
also allows a 68" blade for the Lyc 360. They do not however, recommend a 70"
blade.
The question is, since I've had no problems or vibration issues with my 70"
blades - should I be concerned? I guess Hartzell's recommendation is based on
some harmonic resonance or some other vibration related things (which are
outside my knowledge). Does my 70" blade length provide cruise or climb
capabilities that are greater or lesser than a 68" prop.
I'm planning to have the 70" blades reinstalled on the new hub, and the log
entry will just say the standard overhaul stuff (except the IAW stuff), and it
will specify "experimental A/C usage only."
Any thoughts?
Thanks,
Matt McManus
lnc2 360
408 hours total time airplane.
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