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Author Topic: noseblock design for sidethrust  (Read 710 times)
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billdennis747
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« on: June 14, 2017, 03:09:45 AM »

Experts please. I now fly my duration models right/left and like it. My only concern is friction losses with the 3+ degrees sidethrust needed. Is there a better system than a simple wire in tube bearing? Would very short lengths of tube soldered front and back inside a larger tube yield better results?
I don't have a lathe.
thanks
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ZK-AUD
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« Reply #1 on: June 14, 2017, 05:59:59 AM »

I too will be interested when the real engineers get going on this topic,  but it seems to me that no matter what type of bearing is involved you still have the issue of the force vector.

Follows that the answer to minimizing any losses is the bearing that will result in the least friction.  I suspect that this would be a miniature ball race at each end of the shaft and I think that modern F1Bs use something like this. 

I often make up a cartridge bearing these days using a peck nylon bearing in each end of an aluminium tube, suitably reamed to take the shaft  Whether this results in less friction that a brass tube right through I could not say.

Prop hook on the end of a universal joint?
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billdennis747
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« Reply #2 on: June 14, 2017, 06:13:55 AM »

Ivan Taylor is using small ball bearings and I'm waiting to see how it goes. It strikes me that the shaft will bear hard on one side of the tube at its inner end, and a bearing would help there. Maybe also the shaft will try to flex and bear on the outer side of the bearing tube halfway along. Perhaps a  tinplate front and rear bearing with clearance in between would be better than my brass tube.
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Mark Braunlich
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« Reply #3 on: June 14, 2017, 10:52:55 AM »

Friction force, the force resisting motion in the direction of movement, in this case a torque force resisting rotation, is dependent on the coefficient of friction for the two surfaces / materials in contact and the normal force pushing the two parts together.  Surface area of contact doesn't come into the equation.   That's what I was taught in school anyway but that was a LONG time ago.
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Mark
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« Reply #4 on: June 14, 2017, 02:49:06 PM »

Following Mark's reasoning the way to go is to reduce the coefficient of friction which would seem to point back to the ball races
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Rich Moore
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« Reply #5 on: June 14, 2017, 03:21:09 PM »

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Surface area of contact doesn't come into the equation.

Sounds like revision hasn't finished... Yep, it doesn't sound right at first glance, but a bigger surface area just spreads the force out, it doesn't increase it.
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danberry
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« Reply #6 on: June 14, 2017, 08:11:46 PM »

Good golly.
Oil the shaft and pick better air.
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gossie
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« Reply #7 on: June 14, 2017, 08:46:40 PM »

On my larger rubber jobs with right thrust, over the past 14 or 15 years I've used rivets front and rear of the noseblock with a domed head about 1/4in long that take in old talk, piano wire that would be about 13swg.....i.e. thicker than 14swg and thinner than 12swg..........I'm lost on the modern piano wire sizes.

I put a drop of oil front and rear on the shaft, and all seems to run smoothly.
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Re: noseblock design for sidethrust
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Mike Thomas
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« Reply #8 on: June 14, 2017, 09:26:18 PM »

13swg is very close to 3/32".
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gossie
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« Reply #9 on: June 14, 2017, 10:22:23 PM »

13swg is very close to 3/32".

Hi Mike, I thought 12swg was 3/32"
I know it's been tough to bend, but it sure does the job.
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gman
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« Reply #10 on: June 15, 2017, 04:44:23 PM »

Bill, I'm no expert but I would have thought that the extra aerodynamic drag much exceeds the bit of friction resulting from a few degrees extra right thrust. Anyway, if you like the trim, it works and allows you to concentrate on picking lift then that's worth any minor inefficiency. There was a small piece in the short lived Australian Vol Line by George Matherat about bearings. Basically he used a (say) 14swg tube crimped each end with pliers onto a 16swg shaft. Hence reduced contact area but maintaining the bearing length. I'll dig out the sketch and email it to you.
Although, in theory, friction is independent of area try spinning a length of wire in a full length of brass tube.....
Gavin
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gman
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« Reply #11 on: June 15, 2017, 06:32:45 PM »

That should have read "short lived Australian Vol Libre", drat that pesky auto-connect!
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Hepcat
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« Reply #12 on: June 16, 2017, 10:06:31 AM »

I have no special knowledge so I offer a few random thoughts.  I can’t trust my memory these days so I turned to the Internet and can confirm that there is nothing wrong with George’s memory; the coefficient of friction is the significant item, not the area.  It reminded me of something else I had forgotten; that static and moving  coefficients are often different.  Example if you want to push a heavy box along the floor it may be very difficult to start it moving but much easier when it is.  I suspect our propeller shaft are in a comfortable region of not having starting problems and not going too fast.
Next random thought. A ball race does seem obvious but all ball races are not the same. I can remember from my ball race days that to carry end loads one uses a sloppy fit bearing and for radial loads a close fit bearing where the ball radius and the track radius were nearly the same.  So, although the prop shaft is basically a radial loading the fact that the sidethrust is trying to pull the inner race oblique may increase friction if the bearing is the close tolerance one normally used for radial loads.
Next random thought2.   I assume we take the side load as the end load times the sine of the sidethrust angle which means the sideload is about a twentieth of the end load.  But what is the end load?  I know when starting to stretch wind the end load is enormous but what is it whilst the motor is unwinding?
Random thought3. A prop shaft bearing runs for about four minutes in a competition, three rounds and a flyoff.  If you are lucky you probably manage a competition a month so in a good year your bearings probably run for less than half an hour so I don’t think wear is likely to be a problem.  This argument suggests that short bearings, either end of a tube (or the noseblock) is the way to go (and also that soft materials are acceptable).
Random thought4. I started using ‘tin’ bearing at front and back of a noseblock some years ago and they worked. More recently I have done indoor models with just a 1/32nd ply disk, with a 0.02” hole in the middle, stuck on the front and the back of the noseblock and it has been excellent.  Now I don’t know how much this can be scaled up but I think it is worth a try.  Some woods are very good as bearing material having a low coefficient of friction and often being self lubricating.  Lignum vitae is the favoured wood for bearings and is used for large things like ship’s propeller shafts and famously  by Harrison in his chronometers.  I think it would be worth trying a 16 swg hole in a piece of //16th ply on the front and back of a noseblock.
John
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