As for tuninng purposes like setting low idle screw and on some low mixture, can an engine run without a prop for any time or will it over heat right away.

Well I never ran an engine without a prop but like my last post my spinner came loose as did the prop at full throttle while running some fuel through it and the Rpms sky rocketed in seconds which caused my engine to smoke, I hit the engine kill within seconds after I heard the rpms scream and the temp was around 280 or so which is ok on my 55ax but I killed the engine before it could overheat. I am fairly new to the hobby but I would assume not running it without a prop because the engine is not turning over anything and the rpms will get very very high, I could be wrong but with my last issue I would assume that is not a good idea. Jason

Never ever run it without any load, unless you really need a new engine, also there is no way to tune an engine with no load applied as the tune will change with load.

What he said.

The only time to run an engine without a prop is if you want a new engine. Running without a prop is a sure fire way to destroy the engine.

Ken

It will turn so many rpms that it sounds like a whistle almost, and then it will simulate a grenade. I would use a prop.

I've never tried it, but I don't think one of our engines would run without a prop. I think it's the flywheel effect of the prop that carries the engine through the compression stroke. Has anyone done this? Every time I've seen an engine throw a prop the engine has stopped instantly.
Paul

While most sport engines generally stop with a prop strike, racing engines usually continue to run. If shut off quickly, the damage is limited to a overstressed rod. But allowed to run, the much higher rpm beats the bushings out of the lower end of the rod and feeds the metal through the top end of the engine. This results in the engine stopping very quickly and the rotational energy being absorbed by the case. A grenade is good description.

A lawn mower won't start without a blade on it. I doubt you could start an RC engine without a prop. Has anyone tried?[]

I've started a lawnmower engine without a blade.

I smacked one inverted one time. The plane popped back up. The prop went from a 11x6 to a 2x6. The engine turned about 50,000 for about 1/10 of a second. I could not tell you if it died from overheat or no flywheel or what. I really thought it had thrown a rod or something. Once I fixed the plane it ran fine.


David

when I was a noob I tried to start a 52 2stroke with a slowfly electric prop and it wouldnt run. I was stupid becouse if it did run then the slowfly prop would have exploded! I then put on an apc 11x7 and it fired right up. Guess the slowfly prop was too light to act as a flywheel.

Not at all true. Lawn mower engines have a built-in flywheel (and governor) and will run just fine without a blade. Nearly all lawn tractors start and run with the blades disengaged. RC engines will indeed run without a prop but, as pointed out, they will not run long.

GaGeebees: I do not agree with your theory at all. I had and still have a 4 stoke gas engine mounted on an Airens lawnmower, which by the way has an 8 HP engine on it. One day I decided to sharpen the blade, took it off, then started the engine. It did not start , but after a few try's it did. It did not sound right, so I shut the engine off after 2 minutes. That 2 munites cost me about $175.00, I had to replace the crank, piston, and other parts as well. That happened 3 years ago.
Never Ever run any engine 2 stoke or 4 stoke without the proper load applied to it. It will destroy the engine in short order.
Paul

I think the replies above including mine were to state that a lawnmower engine could be started without a blade, but anything above 1500rpm will do some serious damage as in your case. Most newer lawnmowers will go right to a certain rpm as there is no throttle control and that would be bad if no blade is on, most gokart engine will rev freely with no load until the centrifugal clutch kicks in and that's usually around 1500-2000rpm, even when i used my 440cc snowmobile on the microsprint we ran there was only a centrifugal clutch on the end of the crank so there was no load until 3500 rpm, now when you lose a chain at high rpm there is usually some damage done.

You could probably start a glow engine with no prop but as the first question was related to tuning there is no way to tune a glow engine without load.

Think about the guys with boats, you can't tell me a boat prop is a very big load on a glow engine ( out of water ), but i also don't see them running wide open on the bench to tune their engines.

I believe that most if not all small boat engines have a flywheel, and a flywheel acts like a load. and engine needs some type of load or control.

Yep minn your right, i now remember the pc where my starter belt was, it was a small flywheel, so i recind that last statement.

I only know because a friend and I took apart my parents old 75hp chrysler outboard and we had a hard time getting the flywheel off.

Jon

FLAPHappy (or Paul) - Four stroke lawnmower engines do not need a blade on them to run properly. That's not a theory, it's a simple fact. Unlike our glow aircraft engines, they have a governor to keep them from over-revving. The 13 HP Kohler engine on my John Deere will run all day without the blades turning in any way. The engine is happily free-wheeling until I engage the PTO. Sorry about your Airens but your theory for why it broke down doesn't hold water.

gageebees, my lawnmover is a rotary lawnmower. The blade is a counter balance, It has to be mounted on the engine. Without it, it is very difficult to start and if it doe's start it will run rough and damage the engine. I took it to the repair shop and told them what happened, and the mechanic told me never run this engine without the blade. All I can say is $175.00 later, the engine runs like new again.
Paul

I made a terrible mistake. I made a generalized statement. I know better. I should not have said that lawn mower engines would not start without a blade. I should have said that we once bought a used mower that looked great but had no blade. We tried off and on for two days to start it but could not. We than called a friend who owned a small engine repair shop and explained the problem and described the mower. He got quite a laugh out of it. Told us he would be glad to donate the parts to fix it. Showed up in just a few minutes with a used blade that he bolted to the crankshaft of the engine. It started right up and ran pretty well. I understand that larger engines have flywheels and will run without a blade, but there are some (at least one) low horsepower engines that will not start without the blade to act as load. That certainly is a better comparison to an RC engine than a 20 hp riding mower engine or a 75 hp outboard engine.

Put a flywheel on it and start it on low throttle. Providing you do not over rev it you will have no problem BUT it will overheat if there is a significant amount fuel being used. approx 80% of fuel is turned into waste heat. Some sort of fan cooling would be needed for extended runs. 30 - 60 seconds no problem,

Millions of model car engines can not be wrong!

Yeah, definitely not a good generalization. It's like comparing apples to oranges... except apparently if you own an Airens rotary lawnmower

Gaggebees, a rotary mower engine operates like a 2 or 4 stroke engine. The prop or blade must be attached to the crankshaft before running the engine. I am not mixing apples and orange here, but let me say none of the model engines that I know of to not have a clutch and flywheel on airplane engines.. If they did, the you can discuss PTO's, that's the oranges. A designed rotary engine has to have a load on it, just like a 2 stoke model airplane engine. If you run it without a prop(airplane engine) or a rotary lawnmower, you will destroy the engine period. If you have a rotary mower, or a freind that doe's, take the blade off and try to start it.Have your wallet handy, because you will be going to the repair shop.
Usuallyl the engine will not start, but persitence, in cranking it over, it will fire up. Then the damage occurs. Read the posts earlier on this same thread, run an engine without a prop, you will be buying a new engine!. I am not argueing this point any longer, go ahead and run a 2 stroke airplane engine without a prop or a rotary designed lawnmower engine without a blade, you'll get the picture. They both work on the same principle. They rotate directly on the crankshaft ,What ever floats your boat........

I thought this was an aircraft beginners forum? This subject is more suited for "The Clubhouse" where, as described, "If it doesn't fit in any other category and is about general RC stuff then post it here at the Clubhouse."

http://www.rcuniverse.com/forum/forumid_190/tt.htm

Why don't you guys take this over there.