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Run Without Prop?

NunavutPA-12

FRIEND
67.8N,115.1W CYCO Canada
I have my prop off for inspection/re-paint (done). My hangar is open (no door).

In order to do the compression check, can I run the engine without the prop just enough to warm it up, or is it essential to have that extra "flywheel effect" of the prop?

I'd rather not have to pull the airplane out of the hangar because it's a very tight fit to put it back in with another 'plane in the way.

All this won't happen until the weather is warmer, of course (-40 today).
 
It would be erratic and out of control like a lawn mower without a blade. Not to mention a chance for some internal damage.

Craig
 
I have my prop off for inspection/re-paint (done). My hangar is open (no door).

In order to do the compression check, can I run the engine without the prop just enough to warm it up, or is it essential to have that extra "flywheel effect" of the prop?

I'd rather not have to pull the airplane out of the hangar because it's a very tight fit to put it back in with another 'plane in the way.

All this won't happen until the weather is warmer, of course (-40 today).


I think it would be a cool thing to watch. Be sure a video of the panel showing the tach is made. I bet that a 0320 would go over 5000 RPM before it started coming apart and would most likely do it with only throttle cracked a little.

Keep us posted on your progress.
 
And yet lightweight composite props work just fine.

I'd do the compression test cold but rotating the crank will be a treat.
 
Good thing I asked! Many thanks.

Curious, though - I guess the "flywheel" on the engine is not very effective. I have actually run a lawnmower without a blade and it doesn't fly apart because the engine has an effective flywheel, and I can idle my truck without doing any damage. I guess the "flywheel" on the O-320 is just a place to put the ring gear.
 
The weight of the prop isn't as or flywheel isn't as important as the air load/resistance the prop places on the engine. Picture a prop being driven to the feathered position bogs the engine down, in contrast the prop being driven to a flat pitch position will result in a overspeed. So u could bet on a overspeed without a prop.
 
How were you planning on holding the engine from spinning while you do the test. I put on thick gloves and take a deep breath when I grab the prop and pour the air to it.

Jerry
 
A question for you that say you can't run an engine without a prop. Are you saying that you can't just start one up and idle it to warm it up without damaging it? I understand why you would not want to pour the throttle to it as it will over speed but run the thing at a low rpm and your engine is damaged? From what? I'm not trying to argue anyones point just want to understand what you guys are saying.

Jerry
 
I would be concerned about burning up the cylinders over anything else. Even at an idle there is quite a bit of air flowing over the cooling fins.

TIm
 
Many years ago I used to fly an Enstrom helicopter. The engine didn't have a fly wheel, just the ring gear for starting and to drive the alternator. It had a propensity to break the starter housing when it kicked back during start. Also it had a very high idle rpm. I don't remember what it was. It ran just fine without any load on it. There was an idler pulley for a clutch to the drive belt and it did have a cooling fan.
 
The Robinson R-22 helicopters use an O-320 and also start with no load on the engine....just make damn sure you had that throttle closed...same with the bell47 sorta, they have a centrifugal clutch that starts engaging right away but easy to spin it right up to 3200+ rpm if your not careful with the throttle...YMMV
 
How were you planning on holding the engine from spinning while you do the test. I put on thick gloves and take a deep breath when I grab the prop and pour the air to it.


Jerry

Good point/! I guess I didn't think that far ahead! :lol:
 
How were you planning on holding the engine from spinning while you do the test. I put on thick gloves and take a deep breath when I grab the prop and pour the air to it.

Jerry
Evidently I must be unique. I can do a compression check by myself. I turn the prop until the impulses click and then back it up a little. Then I make sure everything is out of the way, including me, of the prop and apply the air. Three out of four times, the prop never moves and I get my reading---hopefully 70 something over 80.:lol:
 
Evidently I must be unique. I can do a compression check by myself. I turn the prop until the impulses click and then back it up a little. Then I make sure everything is out of the way, including me, of the prop and apply the air. Three out of four times, the prop never moves and I get my reading---hopefully 70 something over 80.:lol:

I do mine by myself without feeling special, but I hold the prop in case it comes off tdc. They spin quite well with all the bottom plugs out and 80lbs on one cylinder. Since I do mine cold I also may need to set the rings which means I have to come off tdc and give a good push up the compression stroke till she rests back on tdc. Still I never let go of the prop and think there would be some difficulty doing the test without a prop on. That's the way I was taught and may need to learn another method - possibly.

Jerry
 
I've seen reading IMPROVE after a FLIGHT.

Don't recall readings that were better cold then hot.

Seen a few unusual ones after brief ground runs.

Bottom line if it's good cold it should be good after a flight.

Deill a couple holes in a 2 x 4 to replace prop for the Comp Ck.
 
Thanks for all the good advice. Yes, it makes sense that "...if it's good cold it should be good after a flight." If I can check the compression cold and don't have to run the engine then I'll just put the prop back on.

One question though: the 'plane has been sitting in the hangar for the last four months and it will be six months by the time I do the compression check. The cylinder walls will probably be pretty dry. Should I try to get a bit of oil in the cylinders before I do the check?

Just thinking the dry cylinders may give me a lower reading than if they are oiled up.
 
If its gonna be another 6 months before you do the compression check, put the prop on and do it right. Am i missing something?

John
 
If its gonna be another 6 months before you do the compression check, put the prop on and do it right. Am i missing something?

John

Sorry - I meant it would be six-months TOTAL, since it last flew, by the time I do the check. It won't be possible to take the plane out of the hangar until June as it's such a pain to get it back in again.

You may be right - I can do the compression check as the last item on the annual when I take it out of the hangar for the float season, prop on, sitting on the trailer.
 
doing a compression test on O-470 a few years back, momentary lack of concentration cost me 8 stitches in the top of my head
 
doing a compression test on O-470 a few years back, momentary lack of concentration cost me 8 stitches in the top of my head

Years ago (why do my posts usually start like that?) I bought my own Snap On differential compression gauge and had a mechanic teach me how to use it to keep track of the O-300 in the 172. I remember he told me to always bring the pressure up slowly while "feeling" for top dead center. As you first bring a little pressure on, move the prop back and forth through say 10* and feel for either "side" of top dead center. Average this distance and bring the pressure up a little more. Re feel TDC and the increase pressure again until you know exactly where it is. I never had a prop move after setting it up like that. Caveat: The O-300 is a small 6 cylinder so the pressure from any piston would be much less than a 470. Anyway that's how I was taught and thought I would pass it along but maybe I was taught wrong. Comments?
 
doing a compression test on O-470 a few years back, momentary lack of concentration cost me 8 stitches in the top of my head
You were lucky it wasn't worse... much worse. With over 3/4 of a ton pushing on that piston it doesn't take much OFF-DTC to lose it... along with some body parts.
 
I bought a plane from a guy that had a prop separation in flight. He said the first thing he responded to was the instant change in thrust and W&B and the resulting change in control feel. By the time it registered that the engine was over-speeding it was too late, and that only took a few seconds. He said it was incredibly noisy and very confusing. I can only imagine.
 
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