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Prop Nut: Is there such thing as TOO tight?

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Old 08-16-2010, 02:28 PM
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R-Duhb
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Default Prop Nut: Is there such thing as TOO tight?

I have a Dubro aluminum spinner prop nut on an O.S. FS40. Yesterday was the first time running the engine and I threw the propeven after I THOUGHT the damn thing was tight. I haven't run the engine again but I've tightened the spinner down to the point where outer ring of the spinner is bitting into the prop. I actually think I can get it even tighter, but I'm not sure how tight is too tight. First time using this type of spinner nut, so I'm just curios. Thanks!!
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Old 08-16-2010, 02:35 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Yes, there is such thing as too tight!

If you threw the prop on a 4-stroke, you are probably running it too lean
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Old 08-16-2010, 03:12 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?



So, what is the rule of thumb for tightening a spinner prop nut? And what can happen if the prop nut is TOO tight?

Thanks!!

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Old 08-16-2010, 03:20 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Too tight is bad. If the cone of the spinner rubs against the prop, it could cause the prop to wear and fail
(read - "shed a blade") while the engine is running. Nothing about the spinner except the backplate should EVER touch the prop.

After snugging the prop nut down, you shouldn't need more that another half turn at most to hold any prop.

There is only one cause of throwing props - the engine is backfiring and slinging it off. Richen your high speed needle.

Hmmm..."spinner prop nut". Do you have a 2-piece full spinner or just a cone-shaped prop nut?
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Old 08-16-2010, 03:54 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

It's just the cone shaped spinner.

http://www.dubro.com/hobby/ecatalog/dubro/00052.htm
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Old 08-16-2010, 04:09 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

R-Duhb, if the outer ring is digging into the face of your prop it is too tight. I tried one of the Dubro nuts on a .61 two stroke. I "thought" I had it tight but it came off in flight. I now use the Higley or Great Planes "acorn" nuts that can be tightened with a regular wrench.
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Old 08-16-2010, 04:10 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

OK, after you snug it down finger-tight, tighten about 1/2 turn more.
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Old 08-16-2010, 04:26 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Four strokes are usually double nutted but if you are too lean like Mike mentioned they can still toss the prop when they back fire. I haVE USED SINGLE PROP NUTS WITHOUT PROBLEMS BUT WITH ENGINES THAT HAVE BEEN RUN AND IN TUNE. Over tighten?? I think the threads would strip on the nut before I got it over tight. I have often used the nuts with the washer behind them. I ran out and took a photo of an OS 1.20FS I'm putting through testing before I put on a spinner. This one is double nutted. There is the washer, standard FS nut then the spinner nut. I use a piece of piano wire about five inches long to tighten them, it's a little bent but you know if it starts bending you have tightened it good enough. If the engine is rich enough it woun't back fire and toss the nuts and prop, start fat and work into the lean.
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Old 08-16-2010, 05:48 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

ORIGINAL: R-Duhb

.........I've tightened the spinner down to the point where outer ring of the spinner is bitting into the prop. I actually think I can get it even tighter, but I'm not sure how tight is too tight.
If you are using a wood propeller, and you are crushing the wood, then it is too tight.
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Old 08-16-2010, 07:55 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

I purchased a 1/4-20 Dubro prop nut recently that was incorrectly tapped. The prop nut thread was correct in pitch but the diameter was too large. It was a sloppy fit on my prop shaft, and I doubt it would have held on had I used it.

I contacted Dubro and they sent me a replacement, also tapped wrong. I suspect Dubro produced a whole batch of these. I recommend that you check yours for proper fit on your prop shaft.
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Old 08-16-2010, 08:07 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Tighten it until you strip the threads then back it off a quarter of a turn.
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Old 08-16-2010, 10:38 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

I started it again today. This time there were no problems. Had safety glasses on JUST IN CASE. Ran engine rich.
Thanks everyone!!
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Old 08-17-2010, 07:38 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Not "rich", you ran it with the "correct" mixture

A 4-0stroke is not going to put out the RPM of a 2-stroke. That's one of the common mistakes people make. They have less RPM, but more POWER!
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Old 08-17-2010, 08:48 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

less rpm but more thrust in comination with the prop....

for similar displacement a 4stroke produces less power than a 2 stroke because it cannot get to the rpm..

confusing power with thrust is a common mistake people make
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Old 08-17-2010, 08:59 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

I do NOT agree with the statement that if the prop gets lose while starting the engine it is because it backfired. Maybe it did, maybe it did not. I'm curious, who here realizes that when you turn the prop in the normal direction to start the engine, counter clockwise, that you are turning it in the direction to loosen the prop nut? If the prop nut is not sufficiently tight to resist being loosened by the action of the starting force, then the prop may loosen and if the engine also started then things can get worse. The "X" wrench most of us have used to tighten the prop nut is absolutely worthless unless you have exceptionally strong wrists. I use a long adjustable wrench and hope I don't strip something. To not cause damage takes a great deal of experience. For those with little experience, consider looking up a bolt/nut torque manual. I recall many years ago intentionally over tightening 1/4-20 NC and 1/4-28 NF bolts till they snapped to get a "feel" for how much force it took. I have never damaged an engine, and it seems like it has been over ten years since I had a prop nut get loose while starting the engine, ever since I started to use a long adjustable wrench. So, who here realizes that starting the engine can loosen the prop nut? The natural question then is "why don't they use a left handed thread?" I know, but you can figure that one out.
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Old 08-17-2010, 09:23 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

They can't use a left-hand thread because then the engine would loosen it on every power stroke (Sort of like an impact hammer).

As it is, the engine actually helps to keep the prop tight - which is exactly why a backfire will spin the prop off in a heartbeat.
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Old 08-17-2010, 10:09 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Actually, I did consider the fact that the starter is turning the spinner prop nut in the direction that would loosen it.  I figured that along with the backfire and the damn thing just not being tight enough led to the prop throw.  An alternative to an electic starter is always a chicken stick. 
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Old 08-17-2010, 10:14 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

It doesn't matter how tight the prop is, if an engine backfires, the prop will come loose 99% of the time. The double nut will not even prevent it.

What the double nut is for is to keep the prop from leaving the shaft if or when it DOES come loose.
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Old 08-17-2010, 11:36 AM
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Default RE: Prop Nut: Is there such thing as TOO tight?

It is simply a physics phenomena:

"Rotational inertia:
Another form of inertia is rotational inertia (moment of inertia), which refers to the fact that a rotating rigid body maintains its state of uniform rotational motion. Its angular momentum is unchanged, unless an external torque is applied; this is also called conservation of angular momentum. Rotational inertia depends on the object remaining structurally intact as a rigid body, and also has hidden practical consequences."

(Copied from: http://en.wikipedia.org/wiki/Inertia)


Due to the higher radius of the CG of each blade (compared to the radius of the washer and nut), the rotational inertia of the prop is high enough to continue rotating, disregarding the sudden change of the direction of the rotation of the engine shaft during a back fire.

During starts, the nut starts unbolting only when the starting torque that is applied on it by the starter is higher than the one created by the friction force of the threads.
To overcome that friction force requires an unbolting torque a little bigger than the torque applied for bolting in (static friction is higher than dynamic friction).
As we all know, locating the shaft right after the top center, gives the starter a better impulse to overcome the resistance of the first compression; hence, less chance to unbolt the nut.
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Old 08-17-2010, 12:21 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

My money is on Minniflyer, the only time I've ever had a prop come off was due to backfire. I've even seen where, when two small pins lock the prop to the backplate, the two pins were sheared off with the force of the backfire on a 1.20 four stroke.
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Old 08-17-2010, 12:38 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Is there such thing as TOO tight?
Having pulled the threads off prop nuts AND shafts I can say without hesitation that there is a "too tight" outcome.
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Old 08-17-2010, 03:01 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

Yep! Too tight can damage. Some years back a friend overtightened the prop on his Webra Speed .61 and broke the crankshaft in the threaded portion.
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Old 08-17-2010, 04:10 PM
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Default RE: Prop Nut: Is there such thing as TOO tight?

I think it's more a point of common sense, everything has a limit. If you broke a crank then that really went past all points. Last winter I was having a problem with one of my YS engines, an old 1.20NC. I had it running OK on the ground but as soon as the plane lifted off there was a small poop, a poop, not a pop. I killed the engine and as it was loosing RPMs I watched the front of the spinner fall off then the addapter nut then the prop nut and washer. When I landed the prop and spinner back plate were the only things left on the crank and they were getting close to falling off. I just followed the trail of parts and started twisting needles some more until I got it right. There is a fine line between being set correctly and too lean. I crossed that line just a little.
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Old 02-03-2018, 05:48 AM
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Default Tourque

I have a os fs 40 it has a 1/4-28 nut and crank shaft. If using a poly/glass prop what are the tourque requirements. Thanks
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Old 02-04-2018, 08:34 PM
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Default

If you have good friction between the prop and drive washer, all it needs is to be tightened until you feel the torque start to rise signficantly. That rise in torque indicates you've started compressing the prop and/or stretching the end of the crank.
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