Netters,
The way to get a good rough estimate of the effect of lengthening the fuselage
is to "borrow" someone's weight and balance numbers from a flying KR that has
your engine you intend to use. Once you see their numbers, then scale the
materials that would be necessary to extend the fuselage, and weigh the
materials necessary to make the tail feathers. Some of the netters have
recorded the weights of their final assemblies prior to installation. Again,
these would be for estimates only. Take the original numbers, record with a
tape measure the additional fuselage materials distance from the chosen datum
line (most I have seen use the spinner; I use the firewall). Then add this to a
basic weight and balance form, and subtract the tail feathers from their
standard location, and add them at the "new" arm or distance from the datum
line. Empty weight will only increase by the additional materials to extend the
fuselage, and the arm will increase on the same weight tail feathers
(empennage). This will give you the new CG location with the extended frame.
Once you see where the CG moved to, you can then calculate how far forward you
will have to locate the engine to move the CG back to the desired point on the
plane, usually one inch forward of the main spar empty.
Basic Empty Weight Arm or CG Moment
568# 20.92
11880
Rt and Lt mains had 270# each, at an arm of 15 inches each, moment of 4050
each:
Rt Main 270# 15
4050
Lt Main 270# 15
4050
Tailwheel 28# 135
3780
add weights and moments and divide for the CG.
Now take the additional material estimate say 5# total paint and all
and add it to the tailwheel at its new arm.
270 15
4050
270 15
4050
33 153
5049
573 22.95
13149
Original CG was at almost 21 inches, while the additional material moved
the CG aft to almost 23. Measure the arm of the engine from the datum line, and
the calculate its installed weight and then solve the equation for the desired
CG by inserting the numbers into the equation and multiplying for the moment
result. Then subtract from the present moment and you have how far the engine
must move to restore the CG to the desired point.
573 21
12033 so 13149-12033 = 1116. This is how much the engine
moment must move forward. In my case the installed weight of the VW is
approximately 230# so the center of the weight resides at -18 inches forward of
the firewall now. This weight added this far back will need to be balanced by
moving the engine forward 4.85 inches. (1116 / 230 = 4.85). Check out how far
forward Steve Trent had to mount his turboprop engine due to it being 100#
lighter than a VW, in a KR2.
Colin Rainey KR2(td)
[email protected]
Sanford, Florida
FLY SAFE!!!!
