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Author Topic: Electric to rubber with rubber prop  (Read 622 times)
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mick66
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« on: July 30, 2018, 01:47:04 PM »

Hi

This might sound stupid but I was wondering what set up you'd need to build say a jumbo scale rubber model like my Comet Aeronca Champ but with and electric motor but a rubber size prop.

See my build thread ... http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=16809.0
https://youtu.be/tA_Y3TahdL8

You'd be looking at say an 18" balsa rubber prop as used in the thread.  It would turn over at a similar speed so we're looking at a geared set up.  It would be for calm evening flying with micro RC ... more guided free flight but with the impression of a rubber model flying round.  I know I could use smaller electric type props but the conversions I've seen seem to have tiny props at high rpm  and fly way too fast.

Any ideas on a suitable motor, gearbox setup that would be ?

Cheers

Mike
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raggedflyer
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« Reply #1 on: July 30, 2018, 02:14:54 PM »

Shouldn’t the title be rubber to electric with rubber prop?
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mick66
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« Reply #2 on: July 30, 2018, 02:59:10 PM »

Haha .. so you were paying attention

Yeah ... I guess it should be but let's not get hung up on details

Cheers

Mike
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TimWescott
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« Reply #3 on: July 30, 2018, 04:18:45 PM »

Use one of these?  I did a conversion using one of their littlest gearboxes on a Comet Gull.  It flew fantastic with power on, but with power off the prop would drive the motor & gearbox and generate tremendous drag -- I toyed with trying to make a freewheel mechanism of some sort, but never got as far as implementation.

They have some pretty high ratio boxes, which would let you drive a big prop.
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strat-o
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« Reply #4 on: July 30, 2018, 05:06:10 PM »

I think an interesting challenge would be to do this without having to use a gear reduction.  The reason being gear reductions are often noisy, but ungeared electric motors are nearly silent.  Slightly off-topic to the original idea but you might be able to achieve this with several small outrunners combined like a multi-row radial.

Marlin
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USch
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« Reply #5 on: July 31, 2018, 10:02:31 AM »

You have the same problem I have to deal with the F1Q category (http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=23078.0)

A high torque, slow turning and light motor is out of the normal production of most every supplier. If you want a "decent" motorization the only one I found is the Hacker A10 with reduction gear. With decent I mean a brushless motor and a planetary gear without noise production as strat-o says. I run mine with a 13" x 7" propeller which is still less than I wanted. As long as I cannot find a better combination I will stick to this one.

Urs
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raggedflyer
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« Reply #6 on: July 31, 2018, 01:13:28 PM »

So what happens if we go back to basics....

When the outrunner revolution started people were adept at rewinding CD-ROM motors to match the performance needed. This normally meant increasing power and perhaps Kv rating. Is now the time to aim for a different goal - low Kv and high torque?

I had (still have) a need for a low speed / high torque motor of about 30 - 50w input power to run from a single Lipo cell for a non modelling project and looked at gimbal motors which seem to be low Kv but oddly don’t seem to rate power output the same way, nor do they seem to offer a very long life. Any ideas?
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DerekMc
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« Reply #7 on: July 31, 2018, 01:49:02 PM »

You have the same problem I have to deal with the F1Q category (http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=23078.0)

A high torque, slow turning and light motor is out of the normal production of most every supplier. If you want a "decent" motorization the only one I found is the Hacker A10 with reduction gear. With decent I mean a brushless motor and a planetary gear without noise production as strat-o says. I run mine with a 13" x 7" propeller which is still less than I wanted. As long as I cannot find a better combination I will stick to this one.

Urs

Omri Sirkis uses a Dualsky ECO 2212C kv 830 motor with the Reisenauer Micro edition 1/5 gearbox  and a 24x28 f1b prop
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Derek
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« Reply #8 on: July 31, 2018, 02:03:46 PM »

Ok, we to define a few things. Power is power regardless of the source. To fly like an 8 strand of 1/4 inch you will need about 40 to 50 wats per pound. If you want the initial burst you got you will need about 70 watts per pound.

Speed and rpm are not coupled. Adjust your speed with the pitch of the prop.

Winding a low KV motor will lower the current rating (thin wires burn up with added current).

Helical gear boxes are a lot quieter than the rubber flapping around in the fuselage.

In practical terms you will need a gear box,

This should do it on a single cell with an 18 x 18 prop. You will likely want to lay up your own light weight prop.
https://hackermotorusa.com/shop/hacker-brushless-motors/outrunners/a10-7l-4-41/

On 2 cells an 18x7 might work.

If using the GWS dual IPS motors I wouldn't be happy with the performance in a 54 inch Comet. The props would likely be too small for your application, 13 inch.
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USch
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« Reply #9 on: August 04, 2018, 03:16:26 PM »

This should do it on a single cell with an 18 x 18 prop. You will likely want to lay up your own light weight prop.
https://hackermotorusa.com/shop/hacker-brushless-motors/outrunners/a10-7l-4-41/

On 2 cells an 18x7 might work.


No, this will not work at all!
First of, the A10 with gear box is available with 3 winds
A10-7L + 4,4:1,   2200R/V
A10-9L + 4,4:1,   1700R/V
A10-13L+4,4:1,   1300R/V

So why choose the fastest one from the three?
 
All three motors are build to run on a 3 cell LiPo. Using 2 cells will give you some headache and with 1 cell it simply doesn't work. These motors, which have a outside diameter of 21mm and weight 20g without gear, are to small to start running with a big propeller. And not running means a short circuit which melts down the motor winding.

Actually I am using the A10-13L, a 13x7 prop and 3 cells. In the future I may change to a 14x9, but there is no way to use anything bigger!!!

Urs
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TimWescott
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« Reply #10 on: August 04, 2018, 03:27:10 PM »

So what happens if we go back to basics....

When the outrunner revolution started people were adept at rewinding CD-ROM motors to match the performance needed. This normally meant increasing power and perhaps Kv rating. Is now the time to aim for a different goal - low Kv and high torque?

I had (still have) a need for a low speed / high torque motor of about 30 - 50w input power to run from a single Lipo cell for a non modelling project and looked at gimbal motors which seem to be low Kv but oddly don’t seem to rate power output the same way, nor do they seem to offer a very long life. Any ideas?

Not oddly -- the usage is quite different, why should the rating system be the same?  I haven't looked at your gimbal motor ratings, but I've worked with torquer motors for full-sized gimbals; those were rated by how much torque you could generate continually, which boils down to a maximum current rating.  The motors were always running so far below their maximum speed that people pretty much didn't care about what power you could (theoretically) get out of them.

I suspect that if you reworked your gimbal motors with better bearings that they'd last a long time.  Then you just need to do some bench testing to see how much power you can run through them without them overheating.

I further suspect that if you just put in better bearings (for your purposes) and ran the thing at whatever their maximum rated current is, that you'd get reasonably long life -- they're probably rated for that current in an enclosed space, so if you provide them with airflow then they'll almost certainly run cooler.  You may even be able to significantly increase the current (and hence power and torque).
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raggedflyer
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« Reply #11 on: August 04, 2018, 03:57:08 PM »

Thanks Tim, i’ll have to revisit the gimbal motor specs again.

With regards to the other comment from someone else, about a motor with large propeller load not starting on a single cell could this not be circumvented by using a folder prop which had a mechanism to start with the blades folded but slowly open to the normal position in a torque limited fashion. Imagine a normal folder prop but with a slow release opening mechanism (silly putty?) so it extended over say 5s rather than 100ms.
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Konrad
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« Reply #12 on: August 04, 2018, 04:45:03 PM »

Urs the voltage that a motor is designed for mainly determained by the rpm limit of the rotor and the breakdown voltage of the wire insulation. These motor will run just fine on one cell 4.2v. As to start up load the mass of the prop was more to do with this than the aerodynamic load (pitch and diameter). This is why I said one might want to lay up ones own prop.  To use a single cell usually takes a dedicated single cell ESC. Even these ESC usually need "high kv" motors to commutate properly at start up without a timing sensor.

Yes, if I was using 3 and maybe 2 cells I would normally go with the lowest KV motor. But then again I wouldn't be trying to run a rubber band prop by an electric motor.
« Last Edit: August 04, 2018, 06:17:09 PM by Konrad » Logged

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raggedflyer
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« Reply #13 on: August 05, 2018, 11:21:41 AM »

At these low voltages the wire insulation is affected by temperature rise not dielectric strength. Temperature rise of the winding is related to motor load current. Wire is insulated by several coats of polymer, the better the insulation the higher the class of wire and maximum working temperature.

Exceeding the temperature limits can cause a ‘shorted turn’, increasing current further until the smoke is let out.
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