Is this a common occurrence? I had problems on my Sig Kadet and now with my Avistar using the same engine, OS 40 with a 10 x 6 prop. I went from a Master Screw to the Top Flite Power Point wooden props. I have been told to go to APC since it is a better prop. I really tighten the nut down to where I think I am crushing the prop. Should I use lock nuts or Lock Tite or is this just a maintenance thing?

I had succeeded in starting the brand new engine at home all by myself. Then the first time I ever went to the field to fly, some guys were kind enough to help me get started in this great hobby. The prop came off and hit me in the chin, requiring five stitches, what a bleeder.

you might have the wrong nut. it shouldn't throw it that much if at all.

take the engine to the hobby shop and get a thrust washer and nut for that engine. you could also get a spinner nut.
use of blu locktight will also help.

Please do not use locktite on a prop nut. This turns in to an ugly mess. The single cause for the prop coming off is that you simply aren't getting the prop nut tight enough when you tighten it down. This is very common, so don't give yourself a hard time over it. And I am sorry to hear about you needing stitches. First of all, if you are using one of those 4-way combo wrenches that is used to remove the glow plug please promptly deposit it in your toolbox and leave it there. These are bad for doing the prop nut because it's next to impossible to get the nut tight enough with one of these. The best thing to use is a box end wrench of the appropriate size for your prop nut. Also get a glove, or a good thick rag, to put on your hand that you are holding the prop with. This is done so that you don't cut your hand on the prop as you tighten it. Too many people are worried about getting the nut too tight and stripping the threads out. Don't worry about this because the prop shaft and nut are both made out of steel and will handle the pressure you put on it. With the wrench (and the glove or rag to protect you hand) tighten the nut as tight as you can. What I like to do is use the prop to help with leverage in tightening the prop down. Once you it tight as you can then grasp the prop and the wrench in the gloved hand and squeeze them together, this will use the leverage of the prop to get it a bit tighter. Do this and the prop won't come off any longer.

As a side note, if you are using wood props you have to check your prop nut and re-tighten periodically. This is because the wood will compress under the prop nut and become loose.

Ken

Did the engine throw the prop while you were starting it?

first dont use locktite on your prop nut.
secondly what type of wrench are you using,not a multi pourpose,hobby type,the one for glow plugs,a box end wrench will give you the best leverage.
also check to see if the nut is bottoming out on the threads of the propshaft.
if it is not then really crank it down tightly,the nut and propshaft is hardened steel it can take some tourque.
if it is then the prop will not tighten, and you just need to add a washer as a added spacer on the nut side, so the nut will have enough threads to bite and tighten properly.
you can also use a double jamb nut like on 4stroke engines but usually a properly tightened single nut will do for almost all 2st engines.

First off, if you have it as tight as you say, it should not fly off. If it does, the usual suspect is that the engine is running lean - which will cause a backfire - which will throw a prop no matter how tight it is.

Sometimes a prop can come off if you have extra washers holding it on. The washers slip like bearings no matter how tight you have the nut.

Thread locking compound is not recommended and shouldn't be needed.

Secondly, NEVER stand in front of the engine once it is running! ESPECIALLY (and I can't emphasize this enough) if the engine is running up above an idle!!!

I can't believe how many seasoned fliers I have seen that will stand in front of their airplane and bring the throttle to full-bore!

I am one of those people who disregards a lot of safety rules (Like Red Green says, "Safety, because we have to" ) But common sense should tell you that if that prop is going to come off, it's not going to go BEHIND the engine - and THAT'S where you (and everyone in the area) should be!

Are you Sure you're not running out of threads; try holding the crank with a decent wrench and see if you can turn the prop by hand.

Why does a prop turn opposite of the right hand threads on the screw? Seems that would save a lot of problems.

Congratulations on joining us and welcome to this wonderful addiction, hobby, and vice. I'm sorry that your first day was so rough, hopefully you won't need stitches again. Anyway, I agree with everyone's comments....especially your last comment, LokiDog. That's another really common reason that props fly off....no, not because they loosen the nut themselves, but because a starter will often loosen a nut for you. If you were using an electric starter, it's very possible that that's what caused it. I've had 4 or 5 nuts come loose on me. Only one shot the prop off, and I saw it coming. I was about 4ft away, it was headed right for me....I moved and it barely nicked me. If you're starting it with an electric starter, get a spinner or a spinner AND jam nut. If you get a spinner, you won't need the jam nut.

I hope this helped, and I hope you have better luck next time out.

wood props sre hard to tighten simply due to humidty causing the wood to swell....then shrink when it's a little drier and the nut is loose....switch to APC and tighten it down to where the prop leaves a mark in your hand.....I've seen lots of wood props loosen but rarely a plastic/glass prop

4-5 in all the time you have been doing this? I have started my engine maybe 30-40 times and have had at least that many. Always use the spinner and electric starter but I am going to try the jam nut also.

You really don't need a jamb nut on there unless it's a 4-stroke engine. Just get the prop nut tight like we described about and you'll be fine. It's be at least 8-9 years since I've thrown a prop on any plane that I own (including 4-strokes). If you get the nut tight on the prop you won't throw them.

Ken

In almost 30 years I've never thrown a prop. I have had them come loose and spin on the shaft though. You are NOT getting them tight, that's all there is to it. Also be sure to use the proper washer under the nut that came with the engine. Wooden props crush down and occasionally need re-tightening. I don't use them at all anymore though as they damage too easily and become unsafe.

I have only had one prop tossed off an engine, it was A four stroke that ended up needing A new needle valve assembly, I had fired it up, moved behind the plane and tried to run it up. Back fired and tosed the prop into my shop and right through the wing of A new Up-Roar that was getting maidened the next day. A very close call for me.
Just do everything Ken said and get rid of that wood prop. I have my students use Master Air Screws then I have them go over to APC. If the prop is tight and your engine isn't set too lean there should be no problem with A thrown prop.
Watching another old time pilot standing in front of his plane and reaching over the prop to twist the high end while he is operating the throttle stick too sends shivers down my spine. Fire it up and get behind it!!!

In bigtim's post is something you definitely need to check.

I've thrown props. Luckie I was to standing a little bit behind and to the side and didn't go WOT.
Never that less the props and spiner still took off.
As a matter of fact it was the third tank of trying to break in an engine.
As a matter of fact i think it was a wooden zinger.

It's was bascailly from the electric starter torquing it loose.
I think I might had flooded the engine too, before hand.
New engines are tight at TDC

Okay, so, get rid of the wooden prop and tighten the SOB. I will try that. I don't think I am bottoming out on the threads and I think the engine is not lean, but I will have someone listen the next time I am flying.

When an engine is too lean, it backfires and slows down the shaft and the prop continues and then the nut loosens just a little and that is a cause?

OK, I've read all the responses and here is the straight dope.

Yes, wood props do compress and they are sensitive to humidity. But they are no more prone to being thrown than any plastic prop if checked for tightness. Once they have been installed for a long time, the tendency to loosen decreases. Of course if you break props very often (more than 2 or 3 over the life of the model), then they may not be for you.

A 4-way wrench is plenty for torquing down the prop. Even when I was a kid, with kid muscles, that was all it took.

Two cycle engines just don't throw props when adjust slightly lean, or even a lot lean. Four cycle engine do throw props when operated lean. While it is very easy to tell when a two cycle engine is lean by ear, four cycle engines really need to be set by a tach.

Throwing a prop or having the prop get loose on a two cycle engine is due to the engine back-firing while starting. This is caused by having too much fuel in the engine. It is so common to over-prime and then hit the engine with the starter. The inertia of the prop, starter, and spinner runs into the back-fire of the engine. This results in the prop getting loose or being thrown.

The proper proceedure is to prime the engine (usually by choking the intake and pulling over), then with the engine off of compression, hooking up the glow plug and pulling the engine over by hand through compression. If it is over-primed, you will feel a bump as the piston compresses the fuel/air mixture.

That bump is what causes the prop to come off when the starter is engaged.

Continue to pull the engine through the compression until the bump stops. At this point, most engines have a period of bump on one pull, no bump on the next. If starting with a chicken stick, pull it through the next bump, and flip the engine on the next. 90% of the time, it will start with one flip. If using an electric starter, use that after the bump. Remember, that until it is pulling through with a bump, then no bump, then it is still over-primed.

Once you start using this technique, you will have no more problems with throwing props. You can work on getting a good idle next.

This is one of those posts that I'm going to print out and put into my notebook.... Last weekend, I made the mistake of getting "in front" of the prop -even though I was a good 5 feet from it- the veteran who was standing nearby almost instinctively B*tch slapped me! Now I know why..... I'm really sorry to hear about the stitches, having had WAY more than my fair share of stiches in my head/face/lip due to one thing or another, I can feel your pain and it seems it NEVER stops bleeding! But just a little one-way-or-the-other-factor in play and it could have been your eyeball on the end of that prop (god, that makes me cringe just TYPING it!!!! [:'(] )

I also agree with everyone about not using the blue loctite -> unless you NEVER plan to get it off again and heaven help you if you should put it on JUST a little unevenly > a rotating mass becomes a nightmare to balance ( I was using this stuff on a project at work a while back and it threw the rotation of the part ALL of out whack and we almost sheared a hardened steel bolt trying to get the dang nut off!!! ) It was enough of a bad experience to where I don't even mess with the stuff any more. It was a gooey mess to start with and it ended with a PITA

Give that man a cigar!!!!!

Please reread MinnFlyer's post as post # 6 is darn good advise as are the others !


Do Not use lock tite... you should be able to figure that one out !
Do Not use sandpaper disks... they wear/sand out the prop drive hub and so on !!

1. How ever with using APC Props I have found them so hard that nothing short of a couple of thin leather washers (one on each side of the prop) would allow them to stay put. Thin like in glove leather works best. Only use them once then discard and cut new ones if you must.

1a. Double nut as the 4 stroke folks do !

2. When using any spinner trim them to fit allowing no spinner edge to touch the prop except the rear spinner back plate itself. Plastic spinners can get old and brittle causing them to explode !! as can old composite props, if nicked or split dispose of them by sawing them in half with a hack saw at home. I have seen trash divers pull them out of the trash at the flying site..... !!

3. Do a (walk around ) safety inspection each time you prepare to fly not just when you get to the field ! Real Pilots Do This Everytime ! Set a routine and stick to it !


Fly Safe **

Cheers

GSNUT

Are your Props balanced ?

Is the Whole size proper for your Shaft ?

Use a Box end wrench as Ken said !

Bob

my props fell off when there was too much fuel in the cylinder, it caused a hydraulic lock so the prop fell off.

in this case you unscrew the glow plug and swing the prop few turns.

other things, do you have prop washer?

do you have plastic spacer between the spinner back plate and the shaft, some spinners need that?

as mentioned, instructor is the best resource.

good luck
Alex

For what its worth I have had an aluminum propnut/spinner adapter on one engine for 6 years now using a wood propeller and have never had it come loose.

When the prop falls off my plane it is because it is overprimed. As stated above there is too much fuel in the cylinder.