Actually the Drone engines were hugely popular at first. They pretty much dominated the control line competition events for about 2 years. Just about everyone had to have a Drone engine to compete. Plus everyone else wanted one for sport flying too. I remember guys telling me how they would see a fellow show up, fuel up his Drone powered plane and go flying, refuel and fly, and so on. No problems no hassles. All the while everyone else was struggling to get their cantankerous spark ignition engines to run so they could make one flight for the day, at least. They would give up and get a Drone engine too.

But alas the advent of the glow engine hit the market and just took over almost completely in rapid order. But I think for 1947,1948, and 1949 maybe into 1950 the Drone engines were the engine to have if you wanted to fly at the time.

That was back in the good old days, when a flyer could go to the local pharmacy with their fuel container and the pharmacist would top it off with ether for you. You would add the oil to the container and the pharmacist would pour in the ether from his big container. Nowadays ether is tightly regulated.

Unfortunately when glow engines took over they really took over and dominated the market almost completely. Pushing out gasoline spark ignition engines and model diesel engines.