I want to add a motor or small engine to my bike as an assist to help me climb slopes. I do not need continuous assist but just a push to get over the hump. Thinking of adding a model aircraft engine - these have very hi speeds, so if I gear it down I can get some useful torques to push me uphill. Is this feasible, thinking of a 3.5cc or 5 cc engine with a speed around 15,000RPM.

 

I want to add a motor or small engine to my bike as an assist to help me climb slopes. I do not need continuous assist but just a push to get over the hump. Thinking of adding a model aircraft engine - these have very hi speeds, so if I gear it down I can get some useful torques to push me uphill. Is this feasible, thinking of a 3.5cc or 5 cc engine with a speed around 15,000RPM.

We've gone through this already. It wouldn't work.
You need at least 200W motor. Typical power for ebike motors are 350-500W.

 

Maybe you could use one of these:

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It's got 30 pounds of thrust and it is only 11" long, 3.9" in diameter, and about 3 pounds.

Of course there are some downsides. It will go through a gallon of fuel in about 10 minutes, it spins p to about 127,000 RPM at full throttle, oh, and it costs $2,395.00. When you can buy one. Which right now you can't. And the company has no estimated date for availability. But otherwise it's a perfect solution.

 

I want to add a motor or small engine to my bike as an assist to help me climb slopes. I do not need continuous assist but just a push to get over the hump. Thinking of adding a model aircraft engine - these have very hi speeds, so if I gear it down I can get some useful torques to push me uphill. Is this feasible, thinking of a 3.5cc or 5 cc engine with a speed around 15,000RPM.

I had 2.8cc and 3.5cc motors that could barely push the tiny dune buggy it was mounted in good luck and, how does a gas hobby motor mounted to a bicycle relate to an electronics forum?