ORIGINAL: greggles47
G'day JB,
Let me give you a little info the other contributers seem to assume that you already have.
1. Highest priority...join a club. This has many benefits, including a free advice source (some of it is good advice), Safety issues - it's not a good idea to fly on your own. Insurance from you membership, important in case of any mishaps.
2. Running motors without props is not a good idea (in case you hadn't already guessed). Not from the fact that the revs are much higher, more from the fact that the is NO COOLING!! Overheating you motor WILL turn it to scrap in almost no time.
Air flow over motors and correct lubrication are absolutely essential. Good advice and the companionship found in clubs makes everything much more enjoyable,
Now go and enjoy your new hobby.
Regards
Greggles
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Running it without a prop will ruin the engine, in most, but not all, instances.
As Hobbsy pointed out, his Fox .74 was okay, but the Fox .74 is a very robust engine. Most less robust engines will suffer some kind of damage, usually to the crankpin or connecting rod.
Never try to run any engine without some kind of load on the crankshaft, such as a flywheel or a prop. As Greggles mentioned, the engine needs to be cooled properly too. A prop does this adequately, in most instances.
Welcome to modeling and don't let a few mistakes shake you from pursueing the hobby. It is loads of fun and you will make lots of friends. Join a club, as was previously mentioned.