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Apr 23, 2019, 03:25 AM
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Discussion

3D printed folding prop hubs ?


Would it be feasible to 3D print prop hubs ?
Basicly it is just 2x pieces with holes on the sides for a nut and bolt, and in the center a semi-circle cutout (horizontal so that when screwed together, it will be tight against the shaft .

Something like this:
https://www.hacker-motor-shop.com/Ac...ding-Props.htm

The nuts and bolts will ensure no layer separation is possible. Pull will be across the bolt, so horizontal to the first layer if printed flat. Many outside walls will likely be the strongest. Will likely have to be printed in ABS to handle stress.

My guess is, that it should work, no problems. Any other opinions ?
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Apr 23, 2019, 03:36 AM
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Breitlingwings's Avatar
Sure it is. Here you have another version. PETG with a good cubic infill should be enough:

https://www.rcgroups.com/forums/atta...entid=11911371

Credits to Eclipson Airpanes.
Apr 23, 2019, 03:56 AM
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Quote:
Originally Posted by Breitlingwings
Sure it is. Here you have another version. PETG with a good cubic infill should be enough:

https://www.rcgroups.com/forums/atta...entid=11911371

Credits to Eclipson Airpanes.
Is this discussed in a thread somewhere ? link above is only to an attachment.

The ones on the page does not look well printed, and I would think printing it turned 90 degrees would be stronger. Here layers are not held together by the bolts. And my experience with PETG on even on indoor microquads etc is that it breaks too easy on impact. Slow force it easily handles, but a crash, and it breaks every time.
Apr 23, 2019, 04:08 AM
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Lynxman's Avatar
I have designed two folding props. The big one is for my pusher glider and the small one is for my pylon racer. Both work fine and are very strong. The big one can hold quite large blades as the tension plates are printed flat, which makes them extra strong. It has seen a year of frequent use. It has an internal fan to extract air from the motor, which is why it has a hoe in the end.
Apr 23, 2019, 05:29 AM
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Thread OP
Props I am not sure I dare.
Assume a 10" prop, 4S (16V) and 800KV motor, and we have a tip speed of 5*2.54*2*pi*16*800/1000 = 1021 m/s = Mach 2.98.

But the hub has much lower speed,
Apr 23, 2019, 05:39 AM
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Lynxman's Avatar
That doesn't make sense, the hub load is what you need to worry about, not the tip speed of the blades. Your math is also wrong. A 10" prop on 4S (16 V loaded) with zero motor slip (100% efficiency) gives a tip speed of 171,5 m/s, or 617,4 km/h. A motor with 20% slip, as is normal with 80% efficiency, would give proportionally lower speed.

The 8 g blades on this pulls with a force of about 589 N (or 60 kgf) on the hub if the CG of the blade is at the half point at 10240 rpm (80% efficiency).

I fly mine with a 665 kv motor on 4S. Giving a load of 407 N (41,5 kgf). This is nothing to worry about. It's 10". I even tried suspending my body weight from the tension members.
Last edited by Lynxman; Apr 23, 2019 at 05:46 AM.
Apr 23, 2019, 05:41 AM
FlyMike
mmilan's Avatar
Very nice Lynxman. Are you willing to share your STL files?
Apr 23, 2019, 06:00 AM
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Ah yes, KV is Rounds per minute per volt. Not per second.

But with the printed prop, I assume a 10" will stick out 5" to each side.
Then multiply by 2*pi*25.4 mm/inch * 16V * 800KV / 1000mm/m / 60/s/m
5*2*pi*25.4*16*800 / 1000 /60 = 170 m/s.

There will be some centrifugal force from the props, and they will puch forward in movement direction. About 1.6kg, and split in 2 that is 0.8kg/side.

Centrifugal force if we say 10g prop, 125cm radius (approx 5"), 170 m/s is 2.3kN, this is worse than worst case. That is like 230 kg. Of course the weight is distributed. So if we say the weigth is half way in (and thus half speed), then we hit 1.2kN, which is still 100kg.

Say haft the weight at same distance (6.25cm), and we have half the pull.

So it is linear, and there can be a pretty heavy centrifugal pull. Now I know one reason why the make the cross section smaller and smaller towards the tip. I guess most props has 75% of the mass within the first 25% of the prop radius.
Apr 23, 2019, 06:10 AM
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Lynxman's Avatar
Yes, you use the radius at the CG of the prop blade, which is about half way out, so about 64 mm in the case of my blades, which is what I used in my calculations.


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