dean van winkle wrote:

Ref John Slade's comments on the Lyc at 310 lbs and the 13B at 260 lbs install

ed weight.  To keep the same aircraft CG, the 13B would have to move forward, no
t aft.  The goal is to have the same installed total moment (weight x distance t
o engine CG (
arm)) with the 13B as with the Lyc.

Use the firewall as your fixed datum point rather than the aircraft CG which i

s a floating point.  For purposes of comparison assume the same prop weight and
location, same cowl, and same engine mount weights.  Also assume that the Lyc wi
th all access
ories weighs 310 lbs with the engine CG 24 inches (arm)  forward of the firewall
 for a total moment of 7440 inch lbs. To give the same moment, the CG of the 13B
 with all accessories and redrive would fall 7440/260 = 28.61 inches (arm) forwa
rd of the fir
ewall.  To be more precise, the weights and arms of the water and oil radiators
with fluids should be summed into the 7440 inch lb moment which would result in
the engine moving back slightly closer to the firewall.

Dean
RV-9A
89 13B NA

See the last message Dean. Pusher not tractor.

Paul Lamar

Ken Powell wrote:

Paul,
I have talked to many people over the last year about Rotary
CG's and most want to keep the prop flange of the redrive in the
same location as the Lyc prop flange.  You MUST perform the CG
calcs using the CG of the engines, not just put the prop flanges
at the same point.  Dean has it right.  If you put the prop
flange in the same location as the Lyc the engine CG will move
toward the firewall because the 13B is longer when you add the
redrive.  Most people are coming out OK anyway because the
weight of the coolant, radiators, and oil coolers are being
mounted in the forward part of the cowling and this offsets the
weight of the rotary being moved aft (tractor airplane).

Ken Powell

 
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