Large Model Association Forum

Today, following a fourth flight with a new DLE 170 in my Skybolt, I noticed the prop' move when de-rigging.

All six 6mm prop' bolts were sheared, some bent (see pic'). The prop was only held in place by the spinner..........

I have never seen the like on a 3W, ZDZ, Zenoah, or whatever. I suggest those with these engines replace the bolts with H.T. ones as a matter of course.

This does nothing to allay my suspicions about 'Chingy-Chango' metallurgy.................

K.

Hi Keith.
Will take note of that as I am about to purchase one of these. What size of prop are you running on it out of interest. Anything about the engine that will be of help to me that you can say.

Simon

Simon Willey wrote:Hi Keith.
Will take note of that as I am about to purchase one of these. What size of prop are you running on it out of interest. Anything about the engine that will be of help to me that you can say.

Simon

Simon - the recommended 32 x 10 .....

Keith

I have seen this several times with different makes of motor with 6 bolt hubs and the clue to the problem is actually in your email. "after the 4th flight "

The cause of the bolts sheering seems to be compression of the prop hub after the initial tightening which allows the bolts to very slightly slacken. The hammering effect of the power stroke puts a sheer load on the bolts and one by one they fail. If the model is on the ground and you hear a faint "ticking" sound always be very wary as that is the sound the motor makes if one or more bolts have snapped.

Glad all ended well and you managed to extract the bolts but I have seen prop drivers so damaged by the broken bolts that extraction is not possible.

Always check the tightness of prop bolts after initial motor runs and every time after a model has been laid up for any length of time.

John

This is not unusual, Had it happen to me recently on my bulldog racer with 275cc 3W. Carbon prop flew off in the air after only 10 minutes flying and having tightened the 6 bolts prior to flight. Landed ok but a bit long and did a bit of damage to under carriage. Any way this was a mystery until a club member found the prop and alli clamping plate some while later. The bolts had sheered and the back surface of carbon prop appeared to have been affected by heat. A German flyer I met at Euroflugtag confirmed that this was a common problem with this engine due to heat transfer to the drive hub. Wooden props do not suffer this problem, the theory being that the compressed wood keeps the tension on the bolts and wood will stand the heat without softening as the epoxy resin did in the carbon prop. So an engineering friend has suggested a ply spacer (thin) placed between carbon prop and hub to isolate the heat and form a compressed cushion. Wood prop should be ok. Any thoughts or suggestions from you lot would be welcome. As a side issue the second magnet in the hub came out and I found it stuck to the exhaust stub ! .

This is more common than we realise in anthing with a large diameter and small clamping area, i rebuilt a watercooled campervan engine for a friend in South Africa, a few thousand miles later the bolts holding the flywheel on sheared when his wife changed down to the wrong gear, on inspection it was seen that the bolts must have loosened causing less clamping effect and then bang one loose flywheel, and one broken camshaft.

I also am aware of this happening on full size, so simple way to solve is to use spring washers or similar, these need to be replaced every time the prop is changed or taken off, instead of ply a layer of PTFE could be better, of course it would be best if the bolts could be wired in place as on full size, also a bit of thread lock may be called for.

It is also critical that the prop is drilled very accurately with very tight tolerance so no slack in the bolt holes as this can make the issue worse as the prop is able to move under load, this movement then causes obvious problems. On full size a walk around is part of the preflight perhaps a simple one needs to be done in this case, some of the best rc flyers I know keep log books for the plane and i feel that this may be called for for larger models, a prop falling off in flight holds it own dangers but a propless plane holds even more danger for those on the ground and in the larger sizes flown by those on this site I consider prop security to be part of a normal check, i fully realise how hard this is under a spinnner, but rather the pain of a quick spinner removal than a serious problem.

I cannot claim to have a great history in this and am as bad as the rest so perhaps I need to be just as aware of this and take appropriate precautions, prop failure on full size A/C has claimed the lives of two of my friends so always been an issue I have been extra cautious of when I flew full size, now something I need to be more aware of on my RC planes.

Thanks for posting this at least it has made some people consider the problem and hopefully will help us keep the props where they belong.

Thanks, chaps.......

John describes the process very succinctly - these big boxer's really pack a punch on the power stroke! But I was taken aback that 6mil bolts were sheared. On my 3W's the bolts are just 5mil and - yes - I've had them loosen, but shearing the lot - that was a surprise. I have to admit to the fact that I neglected to check tightness after the first flight ....... and they were made tight, initially. But, as John says, initial compression of a wood prop' does occur, and the bolts need to be 'nipped-up' However, one has to be careful not to overdo this as you can end-up just compressing the prop' more and more - till you think you could almost go all the way through it!

One additional point. The bolt shards - much to my delight (and concern) - were quite slack in the hub and easily removed with a pair of fine pliers. I just wonder about the bolt-thread mating on this engine....... On my other engines, there's a resistance when screwing in (implying good mating) and I believe it's this which has prevented problems before.

A smear of 'removeable Loctite' or an easy solution, thick cyano' on the threads, will be the order of the day on the replacements........
K.

I've seen first hand this happening when the flyer was using a carbon fibre spinner and backplate. the resin in the backplate softened, and eventually the whole lot departed the front of the engine...sheared bolts...no damage upon landing and nobody in the line of fire of the errant prop.

I check prop bolts before every flying session.

Cheers,
Dave.

If you're compressing the fibers of a wooden prop then you're doing something seriously wrong - once that starts to happen, throw the prop in the bin as that's all it's good for.

How many of you use a torque wrench with these multi-bolt hubs?? no-one by the sound of it - get one and use it! properly, every time the prop is fitted!

It has been mentioned about using locking wire as a safety feature - unless you know what you're doing - don't! you can cause even more problems when you get it wrong.

Spring washers.... arrgghhhhh philistines!! use proper flat washers under the bolt heads and torque the bolts correctly in sequence.

30 years in aeronautical engineering teaches good practices - I'm passing them on - use them!


Mark.

What is considered appropriate torque on the bolts in this application?

Having been a single bolt Zenoah runner for the past 17 yrs without issue........I'm just about to start running a pair of DA's, so I'm now a little concerned!!!

Phil

Phil Clark wrote:What is considered appropriate torque on the bolts in this application?

Having been a single bolt Zenoah runner for the past 17 yrs without issue........I'm just about to start running a pair of DA's, so I'm now a little concerned!!!

Phil

I have three DA 150cc motors and not had a problem with either wood or carbon on these. The 3W 275 is a different beastie.

1. Black cap bolts (not Plated).
2. Accurately fitted.
3. checked regularly.
4. Torqued correctly.
5. Lock tight'ed.
6. ?????

Pat

Silver cap heads are generally stainless steel, SS is not very tolerant of high torque applications, Black cap heads are made from high tensile high carbon steel, these are the ones for us, i modify lots of differant ones at work for the aircraft industry, tough as old boots to turn,

Phil Clark wrote:What is considered appropriate torque on the bolts in this application?

Phil, recommended torque for a 1/4" bolt through a wooden prop is 12-14Nm (metric are not usually used in full sized aviation so I'm having fun trying to find smaller metric bolt values).

Mark.

I totally agree i am a phillistine and what i know can be writen on the head of a pin .

If my prop was only spinning at 2800 RPM I would be happier, if I had some lovely heavy duty well made steel bolts to Tourque so much the better and if I had only listened in school even better still but is it not very interesting that even after years of fastening wheels on trucks with all sorts of special nuts bolts and indicators the things always come loose, the reason me thinks is that thing that often goes on in my head, a lovely bit of vibration, vibration causes all sorts of problems some of which we understand and some which my mind have yet to grasp but the good old fashioned springy thingy even if of the single split variety seems to resist things shaking ratling and rolling loose ( see even elvis had an understanding of vibration ) now i do know that on my bigger toys with some lovely wire wrapped and rolled did the job but this was applied with a special tool and inspected by a trusted AMO and the bolts were tightened with a tourqey thingy that clicked and tutted just like my mother in law when i spilt gravy on her tablecloth.

I also must admit that on most flywheels, nay say all of them i have yet to find a springy thingy doing the job to stop the dreaded creep of loose bolts, but then i do also think that spot or Loctite does wonders to help resist the loose trouser syndrome ( wrong train of thought again )

It would be great if we could have bolts and things made to full size A/C spec with lots of log books and early retirement due to excessive less than half life use just in case it happened to snap and drop that poor joystick shaker in a pile on the ground ( yes i do know how important that all is and bad maintenace costs lives ) I truly do think we could apply better technology to our planes and I also wish that the bolts nuts and components I use are actually made in the same place with tools that produce fasteners which actually fit without doing an Elvis impersonation, I also wish we produced more her and imported less and therefore had stricter control over what our importers shovel in our direction, but untill the tax on imports means we start to make some decent parts here again i will use springy things, Lock em up tite and electric glue if needs be to keep things where i want them to stay,

so from a Phillistine to an engineer long may the slide rule reign.

I have been following this thread with interest and I have to say that John is right about the initial compression of the wood fibres on our propellors. The principal of compressing the wood on a full-size propellor would indeed be cause for concern, but our props are much smaller and will be subjected to much greater stress in order to keep them fastened to the engine. Having used multi bolt hubs for a number of years, I have found it to be good practice to remove the prop before leaving for home. This ensures that the prop is regularly checked for its integrity. Another thing to mention is the effect that "tracking" the prop can have on vibration etc. I always check the run-out of the prop by resting a screwdriver (or similar) against the top of the engine cowl and let the prop just brush against it . . . then rotate the crank 180 degrees and check the other blade. By tightening the bolts accordingly it is easy to adjust this so that both blades run on the same track. I dont claim to be an expert in aeronautical sciences, but I can speak from experience with large model engines. I have also suffered Keiths fate with sheared bolts. These were original 3w bolts and they all sheared flush, but this was due to not checking the tightness of the the bolts regularly. Since then I have adopted the above practice without incident.

Perhaps if we had a scrutineer to check such things at the start of the day problems like loose props would be picked up on and corrected before more damage was done. just a thought.

Scrutineering whats that ? thought that was scrapped ! Oh now I remmember that was the fellow who checks all the wiggly bits and sees nothing dangles dangerously, may even find a loose thing or two, no need for him the lawyers say, better to have the pilot/owner/builder check the model as then only he is to be blamed when it falls on someones head, i think we should pack all the Tort lawyers next to the field in the line of fire and see how many parts land on them.

but this is about loose props the scruti thingy is on another topic entirely so stick to subject loose props have nothing to do with that do they ?

On full size wooden props, do they sleeve the bolt holes, so the bolts pinch down on the sleeve and not the wood? (a bit like servo mounts).
Steve

hi guys running a DLA 112 had this happen yesterday , prop shheered of 6 mins in to flight got model down safe on inspection the woodrough key sheered also whats the answer anybody know apart from binning the sh*te thanks for your time,
regards and happy landings

Steve Mansell wrote:On full size wooden props, do they sleeve the bolt holes, so the bolts pinch down on the sleeve and not the wood? (a bit like servo mounts).
Steve

Hi Steve, sorry but I missed this post.

No, the bolts clamp the prop directly. Well, they use a either a large 'prop' washer or spreader plate between the bolt and the prop.

Another thing to remember is that the prop and prop bolts are inspected on a regular basis.