CG:
I am a beginner, and I know a few things abount engines. This thread started about " can I run an engine without a prop?. It progressed into 4 cycle engines and lawnmowers, I know, but the fact is still there. Do not run a 2 stroke or 4 stroke model engines without a prop, if you do run it, you will be paying for repairs. Rotary engines neeed the counter balance.
I will maintain, it is a beginner question, and it got more involved, but the question was answered in the first 3 or 4 posts.

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I like to use a filter between the tank and the carb!

OK, I'm going to throw a spanner in the works here and say that a 2 stroke (not a 4 stroke) glow engine can be run without a prop or a flywheel but only under certain circumstances and I don't recommend anyone try it. It's already been said that sometimes if you break both blades off a prop then the engine will keep running but at extremely high revs. That's called a shaft run and can quickly break things in an engine. The time I had one it split the disc valve and loosened the screws holding the crankshaft housing to the crankcase. Without a flywheel the engine can only keep running if the revs are high enough for the crankshaft to act as a flywheel and have enough angular momentum to turn the engine through compression, the same as a flywheel can do at much lower revs. This is probably in the area of 15,000 revs or so.

Way back in the dim dark ages some racing (unthrottled) engines were run in by starting them with an old wood prop, tuning the mixture extremely rich then shearing off the prop blades with a broom handle. Well I said don't try this at home . The revs would instantly jump very high but the peak revs were then controlled by richening the mixture even more if necessary. The engine would stay at a safe rev and wouldn't overheat because the very rich mixture let the fuel do all the cooling on the inside.

In effect this is how some of the car guys are starting to run in their engines. They start them as normal at idle then slowly open the throttle until the wheels start spinning then richen the mixture to lower the revs, open the throttle a bit more, richen etc until they're at full throttle but with the engine so rich it can't rev very high. That's also a shaft run but it can be done at lower revs because of the heavy flywheel.

I've seen Controline combat planes shed their props, prop nuts and washer, rev so high you think they were going to explode and then shut down. Don't think they ever survived.

Thanks, folks. I hope you understand where this was going. It was beginning to turn in to a nasty.. well, you get the idea.

Back to the subject of the OP. (That's Original Post, FLAP).

Ever seen one of those very fast aircraft with a one blade prop? Very odd, but they do run and do fly. They get way up there in RPM, too. I can't quote the numbers, but I've heard them run. They end up sounding like a very loud electric motor or something. Very odd, but very fast. From what I understant, the single blade prop cuts back on the drag caused by a two or multiple bladed prop?

I wonder what happens when THEY throw a prop. Those engines don't last long, either. But, like dragster motors, they do the job for a short time then it's either rebuild or replace time.

CGr.

Those speed models with the single blade prop have a counterbalance weight hidden inside the spinner so in fact they're perfectly balanced. As they'd want to be at 35-40,000revs . They need the single blade because the engine has to rev extremely high and a 2 bladed prop would have to be too small a diameter to make the thrust needed at the same revs.

There's some photos and a video [link=http://www.flyrc.org.uk/record/control_line.htm]here[/link] of one of these single blade CL speed models setting a world record at just over 208mph....with a .15 engine. If you watch the video then turn up the sound and listen to what happens when the pipe cuts in .

sweet jesus! that thing sounded like a mosquito from hell! I bet that guy was dizzy after that.