Hey Dave,
Welcome back. I followed and enjoyed your blog while you were
stationed in
Iraq. We also sent you a care package of cookies and coffee but it was
returned to us. I guess the Japanese postal markings looked suspicious.
Anyway glad you are safe.
A three blade for your RV. Hmmmm.... Wouldn't a two blade C/S be
more than
adequate?
Doug Fir
BTW guys here are some of Butch's nice 3D's on the prop hub.
--
Paul Lamar ...No rotor no motor.
Thanks for the package Doug! Its the thought that counts.
Yes, a 2-blade is adequate and the most efficient. However, I have come
to note what I feel is a sort of harmonic between the engine and prop.
It works like this with a 2-blade prop and 2.17:1 gear drive:
Say there is a power stroke as a prop blade is passing 12:00 (call that
zero degrees). The next power stroke is going to occur after 165 deg.
of prop rotation. That puts the other prop blade 15 degrees behind
where the last power stroke occurred. And the next power stroke 15 deg
behind that. Give it 10 more power strokes and we will be about where
we started. But since there are 2 blades the same phenomenon was
occurring 180 deg away and doubles the frequency.
At 6000 engine RPM it all works out to a 16 Hz cycle that I swear I am
hearing and feeling in the cockpit. It is noticeable by my passengers
(and I think somewhat disconcerting), and my instrument panel seems to
vibrate to this freq.
I know that I am not the only one to notice it. If you go do "the roll"
on Tracy's website, you can clearly hear the same ~16 Hz cycle in the
background.
With a 3 blade prop everything changes. Because of the lack of opposing
prop blade the main cycle becomes 8hz and the apparent amplitude will
drop in half. There will then be two additional half amplitude nodes
causing distracting sounds 120 deg away.
I think this will have the effect of increasing the freq of the sound to
24 Hz (making it less noticeable?) and decreasing its strength by 1/2.
Of course none of this makes any sense even to me, but maybe it is
responsible for the profound improvement in smoothness reported by
rotary guys who try out 3-bladed props on their planes.
--
David Leonard
Turbo Rotary RV-6 N4VY
My websites at:
http://members.aol.com/_ht_a/rotaryroster/index.html
<http://members.aol.com/_ht_a/rotaryroster/index.html>
http://members.aol.com/_ht_a/vp4skydoc/index.html
<http://members.aol.com/_ht_a/vp4skydoc/index.html>
http://leonardiniraq.blogspot.com <http://leonardiniraq.blogspot.com/>
Dave,
Yes it does make sense. I believe I read somewhere Tracy experienced a
similar phenomena years ago with his first generation bird. I thought the
rubber donuts on the flex plate were suppose to take care of this torsional
resonance problem though. Your prop is probably different than his in weight
and pitch though. Sounds like a rubber donut hardness 'tuning' challenge
to me. Wouldn't a 2.83 gearbox be a better ratio for the HP numbers you are
generating? Of course you would still need another prop.
BTW I have to have a three blade for my bird and I think I'll go with a 3:1
gear box. That would mean the prop is rotating at the same speed as the
rotors. Good, bad, ugly? I am prepared for trouble and hope it shows up on
the test stand and not in the air.
Doug Fir
One advantage of a integer ratio for a pusher is the prop
can be clocked to miss the exhaust pulses.
Paul Lamar
Paul
Thanks for that info. I hadn't thought of that. Here is a good article
perhaps everyone should read before matching up a prop and an engine.
http://www.avweb.com/news/columns/193075-1.html
DougFir
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