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...

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