REFLECTOR: firewall dimensions

Vance Atkinson nostromo56 at tx.rr.com
Thu Jul 17 06:49:05 CDT 2008


Speaking of measurements from the firewall to the prop....  We in the 
EZ/COZY group have found that a prop placed further away from the cowl, 
the better it is.   don't know how far out you can go to still get 
benefits, but here is what we found.

The greatest benefit is a reduction in noise, a close second is better 
cooling, and third is a very slight increase in speed.   The speed thing 
comes in depending on how bad your cowl is stifling the efficiency of 
the prop arc.  Most of us are running 8 inch extensions.  I've been told 
a longer extension (more than 8 inches) is problematic in that the 
slightest crankshaft wobble will be amplified enough to damage your 
engine.  The 8 incher is about the practical limit.

Some cut back the cowl an inch or two, some just add the longer 
extension.  Now, a lot of builders are converting to "exhaust in cowls"  
I personally haven't tried this arrangement but those who have tell me 
again better cooling ( as the higher pressure  of the exiting exhaust 
creates a lower pressure area just before the end of the pipes and helps 
move more air thru the cowl.  Again, Everyone Ive talked to, got more 
speed as the high pressure disturbed air is now pretty much aimed at the 
prop blades root which is not doing any work leaving a cleaner air to 
pass over the prop where the exhaust used to be.

Down side is your cowl gets messy inside where the pipes end (about 4 to 
6 inches)  Some have made augmentation tubes to keep the mess down ( 
like the really old Cessna 310)

Enclose is a pic of my 8" prop extension in relation to the cowl.

Vance Atkinson
EAA Tech and Flt Advisor
COZY N43CZ
VEZ N3LV
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