What is actually a 'thrown prop'? Does one blade separate from the hub? Does this happen often? Does it happen less on 2 strokes?

I read somewhere about 4 strokes breaking props somehow, mostly when lean? Or when they 'kick back'?

Please clear this up as I probably don't know what I am talkng about.
Thanks

when a 4 stroke is lean and "detonates" its like a car backfiring for a split second the engine tries to run backwards thats just enough to spin off the propnut /spinner and sling the prop off the shaft it usually doesnt damage any thing unless flesh is in the way but running your 4 stroke any way than on the rich side of peak idle for extended periods of time will do damage to the engine
P.S. this is why it is NEVER wise to stand anywhere near the prop arc I believe this is the most broken safety rule in RC.

The engine backfires, causing the propnut to come loose. I've not seen a propnut actually come off, but I've heard of others who have experienced it.

I use double nuts, so ont nut turns against the other and has MUCH less chance of coming off.

--Sean

I have experienced it, in particular with a GP SS 40 powered by a 48 Surpass some years ago, that kicked the prop off in flight more than once.

Do you have any idea how long it takes to find a prop, spinner and prop driver in a field? I do!

The prop kicking tendency seems to disappear once the engine is run in. Until then, as advised, keep it on the rich side.

I experienced this once while flat-spinning toward the ground. I noticed that I could actually see my prop and then realized the engine had actually quit. Fortunately, I was able to land it rather than continue spinning to the ground. Once it stopped on the runway, the prop was slowly spinning. At first, I had NO idea what was up. Thought something inside broke, but the engine merely popped the nut loose and the prop was spinning on the shaft. Whew... and yes, I'm glad it didn't keep running which would have caused damage.

-s

I've seen on a Saito 150 throw a prop so violently that it threw the aluminum spinner into a trailer and actually made a small split in the trailers skin!
So never stand in the way of the prop!!!!!

Do you mean directly in front of, or in the prop arc?

I guess it would also be wise to close your legs while squatting in front of it. A violently thrown prop could castrate you pirate:

NEVER stand in front of the prop or the side of it! I learned the hard way, by trying to adjust an OS 61 from the front. It took 23 stitches in my two fingers! Put them right thru the prop trying to reach the high speed needle valve.

BTW, the spinner flew to the side and spun back of the plane about 30 ft to hit the trailer. The prop nuts (2 of them) shot ahead about 20 feet. Needless to say you would be speaking with a high voice after they dug the prop nuts out of you!

As soon has i have started an engine, I run BEHIND the plane, I have seen4 strokes engines throw the prop 3 times and this can be very dangerous as previously mentioned.
My experience (but this is only my opinion) is that propeler separation risk can be reduced by performing a long breaking in in flight in the rich side and also having the valves perfectly adjusted.

We don't have many castrate members in the club (I didn't really investigate closely but I supose). A member stupidly lost 3 fingers last season.
The man was a HIGHLY experimented modeler, he started his saito 180, went behind the plane, a other guy was holding the plane, he puted full throttle for needle valve adjustement, he was performing all that in a very safe way when his sunglasses fall from his nose, he had the reflex to try to catch them........ and his hand went directly in the prop.

It's been mentioned here that you should run a 4 stroke rich until it's broken in. Sorry, but no. You should ALWAYS run a 4 stroke on the rich side of peak performance, on both idle and high end settings. Even after many hours of running, a broken in engine can, and will toss the prop if given a lean condition, both on high end and acceleration. In fact, detonation is more likely to occur on a broken in engine due to higher compression/better sealing of the combustion chamber.

Cooler weather will increase the potential for detonation as well. If the engine was tuned to run reliably just shy of peak during warmer weather, cooler weather will put it on the lean side. Cooler air = denser air, which effectively means less fuel in the mix, resulting in a leaner condition.

I ran my Saito 150 all summer with no adjustments, set for "smokey" on high end with 20% lube in the fuel, and slightly rich on the low end. Cooler weather hit, and although I made the corrective adjustment to the high end needle, I neglected the low end needle, (My bad!) and while flying one morning I accelerated the engine quickly, and it shut down, loosening the spinner. Obvious low end mix problem, corrected by 1/8 turn richer on the low end needle.

I run just a DuBro aluminum conical hub/nut on that engine. Once you know your engine and how to safely handle it, have it properly tuned and can recognize the signs of potential detonation you can avoid tossed props completely, eliminating the need for multiple nuts if your application requires you to not use them.

Often the engine will "knock" or "ping" before it actually gets to the point of tossing the prop, giving you a good indication that it is too lean. This is a rather grey area, but when you know what your engine is "supposed" to sound like when it's running well, you'll notice these sounds. Obviously, on a reliable engine that has been tuned it will be due to borderline weather changes.

Pretty much, if you're going to run 4 strokes, you will end up tossing the prop at some point. Just stay behind the engine after starting, keep it pointed away from people and it will be fine. It's also a good idea to warm these engines up for a minute or so at idle or low speed before cramming the throttle to them.

Enjoy!

a long breaking in in flight in the rich side

by that I meaned more rich than rich

I think the Saito 150 is the king of prop kickers but I wouldn't trade mine for the world. Mine is an early one with the black cylinder and chrome rocker covers that has higher compression than the newer ones, I also have an early .80 like that. Both will unload a prop at the slightest hint of leaness. The gray area that Damnathius mentions is very narrow on them. I appreciate an honest engine and know that if it kicks a prop, it's my fault. By the same token my YS .53 FZ can shed one in a hurry too.

I was standing in back of a Saito 91 about two years ago and I thought just by the sound that it was to lean and about that time it slung the spinner and the prop. Part of the spinner struck me on the upper right side of my chest and it felt like someone gave me a pretty good punch. It had me doubled over for a few seconds to say the least...I felt like someone should at least come over and issue me a standing eight count.....

I've heard a few times about guys that have cobbled together a starter setup (using a standard starter) such that the plane could be held with both hands and pushed into the starter cone which would run as long as the plane was pushing on it. Once started you simply back the plane off. This way you're never in front of or to the side of the spinning prop. I've thought about it but haven't seen a plan for one. Anyone here seen or developed such a starter?

To high a nitro content will also increase the probablility of a backfire on a 4 stroke.

I have noticed that I hear detonation on some engines when glow heater is on plug . This happens most on OS engines with OS four stroke plug. I have seen them throw props at idle. This is using regular glow heater . Any ideas? big max 1935

Some of you probably saw my thread 2 weeks ago. This happened to me. I was starting a Saito .91. While advancing the throttle the prop nut was sent through the front of my spinner like a bullet, the prop followed. I was very thankful nothing was in front of the plane when this happened.

The result

Jim,

I ahve seen one that was built into a step-stool type tool box. It was configured to where the starter switch was by-passed. The starter slid on a rail of some sort and the cone protruded thru the side of the box. When the nose of the plane is pushed into the cone it would slide the starter into a button type switch thus oprating the starter motor. I don't have any drawings for you. Hope this helps. It sure seemed alot safer than conventional methods

Tazz

Hobbsy, I agree with you about the 150, but yours is probably worse since mine is a newer model, and most of the thrown props on that one were on the bench running it in and getting a feel for how it runs.

The Saito 91 seems to have that narrow grey area as well. I had one toss the spinner and prop at the top of a 200 foot climb on an Extra... The engine was running just great flat out on the level, and I ran about 2/3 of the tank out before it got that "sound", and decreasing RPMs as it ran through the climb. Before I could throttle it back, the prop was off and the engine stopped. I think the prop must have went at least 75 foot higher than the plane!

Notice the double, split jam nuts that Saito and everyone else uses on their 4-strokes. They are there for a reason. if your spinner (plastic or metal) does not allow the use of the factory-style locknuts, don't use it since that is an injury waiting to happen. With a 4-stroke, the operative word is "when" and not "if" when describing a bacfire.

Tru-Turn offers the proper locknuts to be used with their spinners, FWIW.

Jam nuts? No, I guess I missed those in my blind rush to get airborn.

Reason? Yeah, perhaps to thwart liability lawsuits brought on by those that shouldn't be handling this type of engine, much like many firearm manufacturers now supply a lock with their guns. Some people just can't be trusted not to hurt themselves or others with dangerous toys like 4 strokes and guns.

so will the jamb nut actually prevent the nut from coming off ??/

Yes, the prop will get loose and just freewheel and the engine will stop, when the engine goes lean combustion happens too soon, stops the engine quicker than all the rest of the stuff can stop. Also the serrated grip facing on the prop driver usually takes a sizable amount of material off of the back of the prop.

"Notice the double, split jam nuts that Saito and everyone else uses on their 4-strokes. They are there for a reason. if your spinner (plastic or metal) does not allow the use of the factory-style locknuts, don't use it since that is an injury waiting to happen. With a 4-stroke, the operative word is "when" and not "if" when describing a bacfire.

Tru-Turn offers the proper locknuts to be used with their spinners, FWIW."


AMEN!

Question: For those of you who have already had a prop sling due to a back-fire situation, are you reaming the props so they do not have to "thread" on to the prop shaft or do your props still require "threading" to get them on the shaft?

Just curious.

-s