i have an os 46fxi, and about 5 times in the last 2 days the prop falls off. the first time it did it the guy said it was a back fire that loosend the nut. but it keeps doing it. almost like my starter is just unscrewing it. ive been torquing the nut very hard and dont think thats an issue. i am getting worried cuz i dont want it to come off under full power. this cant be a common thing. anyone have any ideas on what my problem might be. or and ideas on what i can do to prevent future tragedies.

I have taken this out as it seems like i gave out my opinion that another member thinks is wrong and I got jumped on for it.

I have two things you need to look into. First, make certain that the hardened steel washer that is part of the engine is located at the front of the prop, away from the engine. The prop nut must bear against it. In this manner, the back of the prop is against an engine washer that has serrations like teeth, that lock the prop to the engine. I have seen cases where the hardened washer was placed behind the prop which negated the purpose of the serrated washer behind the prop. Second, you say you really tighten the prop nut. Well I have big, powerfull hands and have always used the hobby X type of prop wrench. After having more than a few props come off like you have, I stopped using the X prop wrench, and instead use a 10 inch adjustable wrench. True, this is overkill and I do not recommend this to you. You may damage the prop or even damage the engine. I am very carefull and now my prop never gets lose and I have never caused any damage. Try tightening your normal way, then use a larger wrench and tighten it a little more. Get some experience in that manner.

the washer and nut are on properly. the prop backs up to a spinner plate that backs up to the engine part with the serations. the back of the spinner plate is serated but i did note there was not a whole lot of friction between the engine and the back of the spinner plate. what im calling the spinner plate is the back part to wich i screw the spinner onto.

Could it be that the nut is bottoming on the threads before it actually starts squeezing the prop. You mentioned that the friction between the engine and spinner backplate does not seem like it's enough so I am assuming that after you torqued the nut you were able to turn the spinner backplate.

What make is the prop?

Is your starter cranking the engine the wrong way. Saw this years ago and it turned out that the starters positive and negetive leads were reversed swithing the connection to the battery soleved the problem. Just sometnhing I remember from a while ago

bubba, its not slipping after i torque the nut down. i was just pressing the plate against the engine and noticed that it does not like lock in. but with the nut tight there is no slippage. put the nose cone back on. hit it with the starter and the prop falls off. wierd.

paul, that was the first thing we checked

Maybe you are over priming it?(hydraulic lock)

Try starting it without the spinner, the backplate of the spinner may be shot. Or get a spinner shaped propnut.
Also, with wood props, you need to tighten them every flying session.

ok im not thinking too well to day so bear wit me if this makes little sence or has already been said... but is i read it (good old niquil here....) the spinner back plate is right up against the back of the serated engine back plate? make sure that you have the propeller instaleld correctly into the spiner back plate (u no t hose 2 little plastic pegs where the screws go through? well they transfer the power of the engine to the porpeller. if they are not installed correctly the propeller may come off. also if the propeller spins AT ALL once you have tightened the prop nut on then odds are your hitting the end of the threads before you hit the prop and your crankign that nut down on the engine crank shaft (bad) to fix this make sure you have that extra little washer behind the nut and possibly add another to make sure you have thread left behind the nut so that it will seculrly hold the prop on. and ive also reversed the leads on my starter beore... wow that really hurt..... dont do that....

can the back of the spinner back plate get wore out? is it possible that hydrolock spun the plate in the first place stripping the little ridges on it. and then would it be a matter of just getting a new back plate?

The last time I saw this happening, the student had failed to ream his propellor.

If you don't ream the hole in the prop, you have to "Thread" it on to the prop shaft. Then you can tighten the nut all you want, but you won't compress the prop hub at all. It will hang up on the threads. The prop must slip over the prop shaft, all the way to the backplate.

If you have reamed the prop, and it slips over the prop shaft all the way to the backplate, please ignore!

Best wishes,
Dave Olson

If the spinner backplate isn't serrated enough to grip the propeller, you'll have problems keeping everything tight. A good solution is to glue two pieces of sandpaper back-to-back, with the abrasive out. Cut it to about a 1" diameter, put a hole in the center for the propshaft, and use it to add grip between the spinner and the prop.

Helps a lot!

actually i was reffering to where the back plate meets the engine.

And I'm betting you're not getting the prop tight enough. You really have to clamp down on it, and don't be afraid of tightening it, it's pretty difficult to do damage, although you CAN over-compress a wood prop.

If you have a spinner backplate, then there's no way your prop nut is bottoming out. (Running out of threads.)

Tighten it down real good, then tighten it some more.
Dennis-

Use a 6 inch, or longer, box wrench of the proper size. Do NOT use a crescent (adjustable) wrench. You will end up damaging the nut with the adjustable. It will need a metric wrench to fit properly or a socket would also work, preferably a 6 point. Use the proper tools and your equipment will last longer. Japanese motors need metric tools.

I never use anything but my 4 way wrench- I just tighten it as tight as I can with that on plastic (APC- MAS) props, not problem.

Sounds like hydrolock, or a worn out spinner backplate.

DB's right you can overtighten a wood prop, what I was refering to was to check the tightness before each flying session.

To Crashem......If the starter is cranking the engine the wrong direction then the nut would never get lose. The correct cranking direction, counterclockwise, is the direction to also unscrew the nut. Thus the problem. The nut must be tightened to a sufficiently high torque that the starter cannot losen it while the engine is being started.

A little "medium strength" loctite helps too.

Do not use the "high strength" or "sleeve retainer" type!

What ever happened with this? Did MrBass find the cure? We're all waiting to hear!

Dave Olson

Here's a word of caution. I had this happen to me on an OS .46 LA last weekend. It's happened before and the prop just spins harmlessly off the front. This time it spun off the front, then apparently hit the ground and bounced up and hit me on the side of the nose. Hurt like heck and left a cut. Fortunately I was wearing glasses, or I might have caught it in the eye. I went ahead and took my cheap plastic spinner off and had no problems after that.

-Scott

I just wanted to know if the problem has been solved? I would prefer it to be my prop that fell off and not my wings

well the prop fell off again so i took the advice and tightend the heck out of it. since then i have not had a problem. thanks for all the good advice

If the prop came off at a high RPM, and you were unlucky enough
to be in the way you may think differently.... Just a tip, nothing more:-)

I agree if all is tight and tightened down good and with the use of
the "double nut" as the lock nut, your prop should stay on..

I can't believe the comment above saying 2 strokes won't detonate, they do it quite readily. Who ever thinks that has had little or no experience with our 2 stroke engines. To much nitro, to high a compresion, flooding, to lean a setting--all can and will cause backfires to occur. The most common cause it setting the mixture to lean. Using to hot a plug might also contribute to backfires, especially if you are using high nitro content fuel or have a high compression head. Once an engine is properly set up, it is easy to start it by flipping the prop backward very hard, the resultant backfire reverses the direction and the engine will often run up nicely.

I decided to remove my opinions and will sit back and just read what is being said here on rcu from now on