I am about to embark on constructing a 1/4 scale O-2 Birddog (commonly known as a Cessna 337 Skymaster) from scratch. The Skymaster is an in-line twin engine airplaneone in the front and a pusher at the back of the fuselage. The theory was that in case one engine failed, the plane could be flown on the remaining engine without having to fight with the yaw caused by losing an engine on a wing mounted system.

The wing span will be a few inches over 9 ft. and the plane will be a brick with wings. I can envision lots of hardwood and plywood and servos. I am wondering what to power the plane. I am leaning toward an OS 120AX. That motor will spin a 15" or 16" prop, which would be pretty close to scale. However, I am wondering if two of those would get the thing off the ground. Should I look for a beefier engine? I don't want to lay out $500.00 for two motors only to find out that they are not big enough, not to mention the work needed to rebuild the cowlings and engine mounts. One of my limitations, however, is the availability to find a pusher prop for the rear engine.

Thanks for the help
Tim

As you get closer to finished and can get an estimate of all up weight, maybe you can make a determination on the engines?

Hello Tim; I'm thinking that two of those four stroke weed wacker engines would be pretty scale. I think the Cessna could handle the weight. This might sound unusual. I have never seen an AX 120 used as a pusher, no reason why not I suppose. Our local fire fighter service uses those planes as spotters, they are common here.

Two gas (weed whacker) engines may be your only way to go. At least with the pusher engine. Rear engines can get hot when ran static for any length of time, and if your plane is scale and rear cowled I can't see that helping matters any. With the weed whacker engine it may work if you keep the cooling fan/housing and place diverters/ducts in the cowling to help the fresh air moving along, keeping in mind air flow when it's in flight.

Pusher props small to large 2,3 or 4 blades:

http://www.zingerpropeller.com/index.htm

I know a 120AX will pull a 13-15 pound scale plane with ease, but most people who build this plane make the front engine a little bigger and the back smaller. prop selection is important because the back one has accelerated air feeding the blades so it usually needs a little more pitch.