Two weeks ago, I decided that I should try using a gas engine, because the batteries for the electric motor take too long to charge.
One week later, I had obtained an R/C airplane engine and all the accessories. After much trial and error, I was able to run the engine.
Today I built a new test rig that allows me to turn the engine from horizontal to vertical while it is running (this requires positioning the fuel tank, fuel lines, and carburetor on the axis of rotation). Using this rig, I determined that the engine can run and be started in a vertical orientation.
Photo of the rig. Detail. Accessories.
This contraption (including the 2.5 pound weight) can take off vertically. Of course, it's not a stable flying machine, so you have to hold on to it with one hand (which requires a certain amount of insanity).
The amount of lift and fuel consumption are both good.
The biggest problem is vibration. Every nut and bolt must be very tight; otherwise it will shake itself to pieces. (I bought some nut-locking glue, which I'll use on the final version.) Normally the engine is rigidly attached to a heavy airplane body, but for this helicopter, the engine should be the heaviest component, so there isn't much to damp the vibration.
Of course, this much vibration will be bad for recording video footage. I'll have to modify the helicopter body to isolate the camera as much as possible. Perhaps I can use a smaller engine to reduce the vibration.
The engine has other drawbacks: it is rather loud, it coats everything in fuel residue, it is a bit tricky to start, it requires many parts and accessories, and it requires draining the fuel tank and fuel lines after use.
Nonetheless, I think I'll continue to experiment with the engine.
One option would be to have two gas engines fixed in a vertical orientation and use two electric motors for forward motion and turning. Alternatively, I could place the engines on actuated pivots, as was the original design for the electric motors.
Next step: laser-cut an engine support structure.