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icarus46
10-18-2011, 06:04 PM
I have an electric motor and currently the housing is made from balsa and a few pieces of hardwood. This will go in a short nosed WWI bird and I need a lot of weight up front for the plane to balance correctly. I have limited room in the cowl for weight so I wanted to maybe try something.
If I duplicated the current motor housing (2 3/4" long) and made it out of a piece of .085 wall metal tube, would this have any interference with the magnets or any other function of the motor or the electronics?
Heres a pic of the current mount
Thanks for the help.

CNY_Dave
10-18-2011, 09:23 PM
As long as you are far enough away from the motor that you don't feel the magnets dragging on a nail at that distance, I'd say the ill effects would be zero.

Other than making sure the motor is cooled, of course.



Dave

kyleservicetech
10-18-2011, 09:40 PM
I have an electric motor and currently the housing is made from balsa and a few pieces of hardwood. This will go in a short nosed WWI bird and I need a lot of weight up front for the plane to balance correctly. I have limited room in the cowl for weight so I wanted to maybe try something.
If I duplicated the current motor housing (2 3/4" long) and made it out of a piece of .085 wall metal tube, would this have any interference with the magnets or any other function of the motor or the electronics?
Heres a pic of the current mount
Thanks for the help.

Question: What kind of motor is it? An outrunner brushless motor, or a brush type motor?

I assume you're planning to make up that 0.085 metal wall tube to fit OVER the outside of the motor? If you've got an outrunner motor where the outside of the motor rotates with the shaft, obviously that metal tube has to have clearance with the motor. Otherwise the whole thing would be spinning. And that would lead to balance problems.

If you've got a brush type motor, then placing that metal tube directly over the motor housing would not be a problem. In fact, years ago, Astroflight did exactly that with their brush type 05 motor. Astro placed a metal shell over the existing motor shell, improving the magnetic performance, making that 05 motor into a 15 motor.

I've also done that with some of those old brush type Astro motors. It made a slight improvement by reducing RPM, and increasing torque. The same thing would apply to a brushless inrunner motor.

icarus46
10-19-2011, 07:12 AM
Question: What kind of motor is it? An outrunner brushless motor, or a brush type motor?

I assume you're planning to make up that 0.085 metal wall tube to fit OVER the outside of the motor? If you've got an outrunner motor where the outside of the motor rotates with the shaft, obviously that metal tube has to have clearance with the motor. Otherwise the whole thing would be spinning. And that would lead to balance problems.

If you've got a brush type motor, then placing that metal tube directly over the motor housing would not be a problem. In fact, years ago, Astroflight did exactly that with their brush type 05 motor. Astro placed a metal shell over the existing motor shell, improving the magnetic performance, making that 05 motor into a 15 motor.

I've also done that with some of those old brush type Astro motors. It made a slight improvement by reducing RPM, and increasing torque. The same thing would apply to a brushless inrunner motor.
The motor is brushless and an outrunner. There will have to be some clearance for it to spin in the housing. I was curious if this would have any ill effects on the perfomance of the motor surrounded by the steel or metal tube housing if I make one for it.
I know I need to get the weight as far forward but I don't like the idea of the cowl weighing 24oz. either. Anyone have a bipe where they did put all the weight in the cowl or did you attach some of the weight to the firewall to distribute the weight more evenly?
I realize by doing this also makes the plane heavier but will it make it less resistant to nose overs on landings?

kyleservicetech
10-19-2011, 06:35 PM
The motor is brushless and an outrunner. There will have to be some clearance for it to spin in the housing. I was curious if this would have any ill effects on the perfomance of the motor surrounded by the steel or metal tube housing if I make one for it.

What you've got to watch out for with a metal tube surrounding the outrunner motor is magnetic drag. What can happen is that spinning motor housings magnetic field will be rotating inside your shell, and will be inducing currents into it. A good quality motor will have very little magnetic field escaping from its outside housing, so if you provide perhaps 1/8 inch clearance between the motor and your shell, that should be quite sufficient.

That 1/8 inch clearance is both for what ever stray magnetic fields might exist, plus providing cooling for that motor. Some motors, such as several varieties of the Hacker motors have built in fans for this purpose. This magnetic drag drops off very rapidly with distance. If it could be built, a clearance of 1/1000 inch would likely cause problems due to this effect.

icarus46
10-19-2011, 06:56 PM
Thanks Kyle. These were my thoughts also which is why I posted the question. The motor is attached to a 1/4" thick piece of ply that slips in and out of the balsa and ply housing. I think I will just make a steel plate to replace the ply one to give it some extra weight in the nose and hopefully remove some from the cowl.

kyleservicetech
10-19-2011, 07:00 PM
Thanks Kyle. These were my thoughts also which is why I posted the question. The motor is attached to a 1/4" thick piece of ply that slips in and out of the balsa and ply housing. I think I will just make a steel plate to replace the ply one to give it some extra weight in the nose and hopefully remove some from the cowl.

Sounds good. It's always a pain in the *** to add dead weight to an electric model. Nice thing about quality motors and batteries, you've got enough horses in the power system to make up for it.

DennyV

Dr Kiwi
10-20-2011, 02:29 AM
As DennyV says... some motors have high magnet field strength "escaping" from the bell! This Littlescreamer might not be the best outrunner to spin inside a metal casing!

kyleservicetech
10-20-2011, 02:45 AM
As DennyV says... some motors have high magnet field strength "escaping" from the bell! This Littlescreamer might not be the best outrunner to spin inside a metal casing!

LOL
If you ever have access to those very high powered cobalt or similar magnets that can lift over 200 pounds, this can be demonstrated quite easily. (If you have access to a shop full of aluminum bar stock)

I've got several rare earth magnets that will lift some 400 pounds on a one inch thick steel block. Take that magnet, place it against a piece of 1/2 by 2 inch by a foot or so of aluminum flat stock, and RAPIDLY slide that magnet along the aluminum flat stock.

The magnet will be generating a lot of short circuit current in that aluminum piece when you quickly move the magnet. It will provide a very noticeable drag while doing so. I did some very rough measurements of the amount of current in that piece of aluminum, it was somewhere around 1000 amperes. At about one millivolt or so.

Or, if you have access to a rare earth button magnet, try dropping it inside an aluminum tube several feet long. Samething will happen. (Obviously don't use steel tubing LOL)

kyleservicetech
10-20-2011, 02:48 AM
As DennyV says... some motors have high magnet field strength "escaping" from the bell! This Littlescreamer might not be the best outrunner to spin inside a metal casing!

Good grief. Made in China???

icarus46
10-20-2011, 07:02 AM
Thanks for the help fellas. I'm glad I asked about this first. Let me ask you this, if a metal object doesn't stick to the motors casing, it would then be okay to encase an outrunner in a housing made of metal?

If everything goes well on the first flight, to lessen the dead weight, I'm considering using pvc for the housing which is heavier than balsa and ply.

kyleservicetech
10-20-2011, 07:16 AM
Thanks for the help fellas. I'm glad I asked about this first. Let me ask you this, if a metal object doesn't stick to the motors casing, it would then be okay to encase an outrunner in a housing made of metal?

If everything goes well on the first flight, to lessen the dead weight, I'm considering using pvc for the housing which is heavier than balsa and ply.


IMHO, if you've got 1/8 inch clearance, and very little magnetic flux escaping from the motor casing, you should be OK. As an example, those magnets I've got that can lift 400 pounds on a one inch thick flat steel block, will drop off to perhaps 20 pounds with a 1/8 inch gap.

CNY_Dave
10-20-2011, 03:18 PM
It's not if something will stick to the case, as the field drops off very very quickly.

If you can detect no pull/drag on a nail at the distance you want to put the case, you are pretty much good to go.

If you wanted to check, do a static test and measure RPM or thrust (or just do a gut-check) both with and without the casing.


Any call out there for an eddy-current or lorentz-force dyno? Could just measure the voltage. That'd be a neat project.


Dave

kyleservicetech
10-20-2011, 06:49 PM
Any call out there for an eddy-current or lorentz-force dyno? Could just measure the voltage. That'd be a neat project.


Dave

I do recall some people making a dyno with a strong magnet, and a disk of aluminum. The motor spins the aluminum disk, and a strong magnet is placed near that aluminum disk to provide drag.

Only limitation on how much watts that dyno can absorb is how hot that disk will get.