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Author Topic: Thrust Bearing  (Read 1141 times)
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awilder
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« on: October 20, 2016, 04:47:10 PM »

Could someone give me some advice on making a thrust bearing (aka prop shaft holder) used in models like the "Hangar Rat".  I've used small tubing to hold the prop shaft, but I would like something that allows for adjustment, like adding down thrust.  What material would work best?  And what's the best tool for making the holes for the prop shaft?

Thanks
Andrew
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hastf1b
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« Reply #1 on: October 20, 2016, 06:22:09 PM »

Some older, maybe helpful.

Heinz
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ram
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« Reply #2 on: October 20, 2016, 07:43:27 PM »

See also:

http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=21249.msg195246#msg195246
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OZPAF
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« Reply #3 on: October 20, 2016, 07:54:25 PM »

The nose bearing I use is similar to that shown on the plan and in the link in the previous post.

It is 1/32" aluminium bent as shown and drilled for the prop 0.03" prop shaft. Select a sharp drill for this and it should be tight on the shaft.

The sample on my 30 yr old rat shown.

You adjust the thrust lines by bending the angled tab on the front - keeping the front and rear legs parallel.

John
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awilder
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« Reply #4 on: October 20, 2016, 08:42:17 PM »

Thank you! All three replies are extremely helpful.  I couldn't find many examples on the internet except for some small bearings used for very light F1D type planes.  I was worried about choosing a metal thin enough to adjust, but not so thin the rubber motor would deform it.  

Thanks!
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Hepcat
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« Reply #5 on: October 21, 2016, 07:56:20 AM »

I suppose I don’t make many models and so this only comes up every year or so and I don’t remember making many.  Often it is an item that can be rescued from a model that is being replaced.  Normally I have used a conventional one made from piano wire (guitar string), ‘U’ shaped with a round hole at the front and a pigtail at the rear.
On my A6 I did something different which is in the photograph below and has worked very well. It is just a piece sawn from the plastics handle of a disposable razor.  A hole is made at the front and rear ends and I managed to cut a slot to the rear hole so that the wire was a ‘click’ fit and so the propeller and shaft could be removed.  You will notice that a thin piece of balsa packing is interposed between the bearing and the motor stick.  One can easily cut through the packing and then replace the bearing, twisted for sidethrust or with packing adjusted for downthrust.  At this point I should like to mention that models should not normally require large thrust offsets.  If they appear to, it will often be a poor choice of rubber motor or a CG badly out of position.
Finally, although it is embarrassing to confess my parsimonious bodging, I do have trouble making small holes.  Small drills always seem too expensive for me to buy but I did notice some years ago that carpet shops usually have pieces of 16 g wire, about four inches long with a loop at one end and a very sharp point at the other.  They are very strong steel and I think they are stuck into carpets to stop them unrolling.  I have acquired several and find then invaluable for starting holes in thin steel, plywood, plastics and the like.  After a sharp rap on the loop end and bit of wiggling I can usually get one of my ‘rat tail’ files to finish the hole.
John
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« Reply #6 on: October 21, 2016, 08:01:17 AM »

Earlier this year, I made a couple of videos that highlighted this exact thing:

part #1
https://www.youtube.com/watch?v=quldvcskTZI

part #2
https://www.youtube.com/watch?v=jNItBKvkS_A

--george
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Tapio Linkosalo
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« Reply #7 on: October 21, 2016, 08:21:05 AM »

I was worried about choosing a metal thin enough to adjust, but not so thin the rubber motor would deform it.  

For Hangar Rat sized models I have made the bearing from 0.5mm (0.02") thick mild steel, some 4 mm (.16") wide. The material is such used in hardware stores to put together stacks of material (wood, pipes etc.) for transport. Many use some sort of plastic, but the heavy duty stuff is still steel.

The bearing needs to be strong enough that it does not deform under any load. For adjusting you can always use two pliars to hold the opposite ends and bend as required. The balsa fuselage is too weak to bend the bearing against!

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awilder
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« Reply #8 on: October 21, 2016, 09:46:03 AM »

At this point I should like to mention that models should not normally require large thrust offsets.  If they appear to, it will often be a poor choice of rubber motor or a CG badly out of position.

I wondered about this.  Are the effects of changing the center of gravity the same or different than changing downthrust?  In other words, how is adding or removing a little clay to the nose affect the plane as opposed to raising or lowering the angle of the prop shaft?
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cavelamb
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« Reply #9 on: May 28, 2017, 07:10:30 PM »

They are different things.

Adding weight to the nose (or tail?) is a correction to the balance point - the center of gravity of the model.
It's a static thing.  Balance HAS to be right (or at least close) for the model to fly at all.

Changing the thrust line is a dynamic thing.  As long as the thrust is there it tends to pull the nose down.
But once the thrust unwinds, the model will respond nose high.
Assuming the model is properly balanced to begin with.



At this point I should like to mention that models should not normally require large thrust offsets.  If they appear to, it will often be a poor choice of rubber motor or a CG badly out of position.

I wondered about this.  Are the effects of changing the center of gravity the same or different than changing downthrust?  In other words, how is adding or removing a little clay to the nose affect the plane as opposed to raising or lowering the angle of the prop shaft?
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