Wouldn't that put too much heat on the spar? A melted spar would be

really

bad news!

It is easy to insulate the spar but real hard to insulate the prop :-)

Al Gietzen wrote:

Many pushers have the exhaust going through the prop near the hub with no
ill effects on the prop, except for some discoloration caused by the exhaust
gases.  If the exhaust hits the blade a foot or so from the center of the
hub, that part of the blade is in the cool air more than 10 times as long as
it is in the exhaust gas stream.

I guess I'll find out if it's OK.  That's how mine is set up.

Al

Most of those pushers are direct drive Al. In that case it is possible to
index the prop so the exhaust pulses miss the blades. Can't be done
when the gear ratio is other than an integer number. Every so often the
pulse hits the blade. With Tom Jordans EZ you could hear this happen.

Perry Mick wrote:

This should be a better situation (with the gear ratio) than the direct
drive.  With two blades you can clock the direct drive so the exhaust
always goes between the blades.  I heard from people who run 3-blade
props on pushers that no matter how you clock it, one blade or another
will always get hit some.  But with the gear ratio, aren't you spreading
that stuff (heat and soot) around equally to all blades instead of
hitting
one the hardest?

I get soot on two blades, one worse than the other, with my three-blade
direct drive.  But no heat damage. I could probably clock it to improve
that, but don't because it has been dynamically balanced in that
position.
--
Perry J. Mick
Mick Duckt N7XR
http://www.ductedfan.com

Just a little background info here. A four cylinder fires twice per
rev just like a rotary. There fore it is possible to clock the
prop with a direct drive so no blade ever gets a full exhaust pulse. 

With a non integer gear box one or more blades gets the full exhaust 
pulse in a periodic manor.  If I were building a rotary pusher with 
a gear box  I would move the exhaust pipes outboard as far as possible. 
That is all I am saying and what is so hard about that? 

BTW I am glad I am building a tractor :-)

Paul Lamar
 
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