Beefing it up for a 582
When this aircraft was purchased as a partially
completed kit, it came with a new Rotax 503. The opportunity came
along to swap the 503 out for a mid time Rotax 582 and the owner
took it. It looked like a straightforward swap, powerplant weights
are almost identical, & the FSII motor mount tube and mounts
appear identical to the MKIII motor mount tube and mounts. But
not so.
The Kolb company was not approving of such a modification,
and inquiry as to why revealed that the motor mount tube for the
Firestar is .049 wall thickness whereas the MKIII is .065 thickness.
Also, the rear fuselage members are also one tubing thickness
thinner than the MKIII, even though they are externally the same.
Ergo, a full throttle climb out that punched into
a good rowdy thermal could possibly result in the extra torque
of the 65 HP 582 twisting that thinner wall tube & cause the
engine to wring itself free of the aircraft.
Taking the rear upper fuselage tubes and rear spar
carry through with it
Hmmm...
Bummer...
So here's what we did. We added a completely separate
substructure to stabilize and support the extra torque of the
582. The weights of the 582 and the 503 are within a few pounds
of each other, that is not a factor. The focus is to prevent the
torque of the 582 from twisting anything.
Below is the rear brace. It ties into the bolt that
goes through the boom tube and the rear of the fuselage and extends
to the rear motor mounts. It is 5/8" X .065 and also serves
to mount the radiator.
Here is the bottom half of it.
Below are the forward mounts. The FSII has a large
bracket of unknown intent located at the juncture of the upper
fuselage tubes and the rear diagonal fuselage tubes, at the back
edge of the cockpit door area. We drilled a 3/16" hole in
that bracket and then ran a structural steel tube from there to
the front motor mounts. The bottom end of that tube has a flat
bracket which is bolted to the original fuselage juncture point
bracket, the top end has a flat bracket which is bolted to the
outside end of the motor mount plate.
The forward tube is 1/2" X .065 steel tubing,
and after the installation was complete, we had a mechanical engineer
come and look at the finished product, his opinion was that the
extra horsepower of the 582 could not possibly twist the motor
mounts on the center tube.
So - even though this mod is neither sanctioned
or approved by Kolb - we are satisfied that as long as we fly
the aircraft within the weight and airspeeds originally set forth
for the aircraft, we should have no problems.
Next question - How does the 582 improve performance?
Well - this particular FSII is a bit on the porky side. The owner
wanted a lot of bells and whistles, and bells and whistles add
weight. Consequently this particular FSII probably weighs at least
120 pounds more than the factory demonstrator FSII, so if you
have the seen the factory demo with a 503 flying, this one flies
about like it.
Which is not bad. Not bad at all. Pretty great,
actually...