REFLECTOR: TR: Prop flange distance

[Al Gietzen]

Thanks a lot for your replies.
This bring me to another interrogation?

the majority seems to be between 41 and 42 inches mine is 47 1/2 but the
distance from the center of the blade axix to the prop flange is 4 3/4
inches which is bringing me to a total distance of 52 1/4 from the firewall.
It would be interesting to compare this distance. Will I be too far ?

Gilles;

I was mistaken on the cowl length of 42 on the standard - I think it was
closer to 40".

But I'll add this for your new query.  My prop flange on the 20B is 41 1/2"
from the firewall.  I added a 6" prop extension to get the prop further away
from the cowl and wings in the belief that it would improve prop efficiency
(it was also to give me a 7" flange with 3/4" lugs and 1/2" bolts for the
higher power).  The prop hub is about 4.5 - 5" thick, so center of the blade
would then be about 50" from the firewall.

Based on speed and fuel burn, my plane is very efficient - and the rotary
would not be expected to have a better specific fuel burn.  It could be that
allowing me to reduce the width of the cowl for better air flow into the
prop, and moving the prop further back, are positive factors.

Another consideration is CG.  I could move the prop back because the Catto
fixed pitch only weighs about 16 1/2 pounds.  Putting a 50 lb. constant
speed back that far could have an adverse effect on CG.

FWIW,

Al