Weed-eater Giant (with video)

I’ve always wanted to fly a gas plane. The smell of the fule and the sound of the tiny engine idling fascinated me. When I was really young, one of our neighbours flew nitro, which I think really inspired me. Resently I got to thinking I would like to try it out but the high price was intimidating, so I went and talked to our grounds keeper (I live on a boarding academy campus) and asked him if he had any weed-eaters that he didn’t need any more. He thought for a moment and then said he had two of them I could choose from. As soon as I could I ran over to the shed where the weed-eaters were kept and fired them up. The one ran nice and the other, not so nice, so I went with the one that ran nice—I suppose that was the logical thing to do. It was a 21cc Echo gt-225.

I did a bit of research and found that a few other people had done this before, but there was very little documentation. So, using this very helpful “Rules of Thumb” page I found on RC Groups I came up with my dimensions: Rules of Thumb

Wingspan: 8ft (96in)

Wing thickness: 2in

Cord: 16in

Wing Area: 1440 sq in

Fuselage: 6ft (72in)

Leading Edge of wing from back of prop: 14.5in

Horizontal Stabilizer:

SA: 360sq. in

L:30in

W:12in

Elevator: 3 in wide (SA: 90sq. in)

Vertical Stabilizer:

SA: 144sq. in

H: 16 in

L: 10in

Rudder: 3 in wide (SA: 48sq. in)

My next problem was trying to figure out what prop size to order from Hobby King. I did some more research and found a few people using 15-18in props, so I decided that I’d go with 16X6 props. I added three of them to my cart, tossed in five 35g servos and hit confirm order. 

While my order was shipping, I turned to my weed-eater and begun tearing it apart. Slowly but surly the plastics came off until all that was left was a tiny little engine: 

Now on one side of the engine there was a clutch and on the other there was the pull-start mechanism. I unthreaded both of them and then went to the shop on our campus and rummaged through a box of metric nuts and bolts (the thread on the crank-shaft was metric). My challenge now, was to find a way to attach the prop to the shaft. Sounds simple, but I found it was far more difficult.  

In the end, I decided to try welding three of nuts together in a row. I threaded half of the three nuts onto the motor shaft and then thread into the other half, a bolt through the prop and therefore held the prop to the motor.

Next I asked our maintenance manager if I could have a 2x8ft. sheet of pink insulation foam. He agreed and I set to work cutting the trailing edge with a hand-saw, and then I cut a 45deg. angle with the table-saw on the leading edge. I rounded it off with sandpaper and she looked great! I cut a notch with the table-saw on the CG mark and glued a oak spar into the wing. cut some ailerons into the wings, and popped on the servos and, there it was. 

The fuselage was a 2x2 that I cut at 6 feet long and then drilled big holes all the way down the length of the body to reduce weight. I then used the four bolts that held the pull-starter to the motor to bolt the engine to the body. I bent an aluminium piece for the gear and drilled holes in that too, just to lighten it up.

I bolted the wing to the body and to two aluminium angle pieces I screwed to the side of the fuselage. The original fuel tank from the weed-eater I bolted just under the wing: 

For the horizontal stab I used foam-board doubled over. and the Vertical stab was just one layer thick. I cut control surfaces into them and it was ready to fly!

I gave the prop a flick and she fired up! 

And lift off!

Final aproch...

Touch down!

Back on the ground, safe and sound! I learned a ton from this project and I hope it inspires others to try some far-out things they have been thinking about doing but never got around to trying. If you have any questions about the spesifics of the build don't hesitate to leave a comment!

Happy flying!