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Motor Constraints

#1
Trying to Find Motor

Hi all,
This is my first post on this forum (any forum for that matter) and I am pretty new to this rc business. I am currently building a hovercraft for a competition, and am having trouble finding a motor that meets the given constraints. The motor has to be brushed and the current must never exceed 9 volts. This motor will be for the thrust, so I bought an APC 5 5 pusher prop for the back (it seemed like the best option). What are some options that I have for this motor? Please try and keep relatively cheap (I hope to stay under $20-25). Thanks!
 
Last edited:

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#2
Decent sized brushed motors are a bit harder to source these days -- at that scale it's better to go with brushless -- but if the rules require brushed, then brushed it must be.

Quick look on Hobby king . . . only looks like one is in your ball-park:

https://hobbyking.com/en_us/mabuchi-rs-540sh-6527-brushed-motor-90w.html

Everything else they sell is either way too small, or geared down too much.

That being said . . . this motor is on the fast side for it's power -- spec sheet says 23400 RPM @ 9.6v, so it's roughly 2437kV. For direct drive, it'll throw a small prop.

Not sure about your "current must never exceed 9 volts" spec -- volts is voltage ("pressure" behind the flow), Amps is current (how fast the electrons are flowing). You set the max voltage by your battery supply(so "must never exceed" for voltage seems a weird spec), then throttle the current using your ESC.

The APC 5x5P is a bit of a problem, though . . . fortunately, props are cheap. For direct drive on this motor, on 2S lipo (tops off at 8.4v) you're looking at about 11A of current, which is over this motor's 8.2A max load (and the 9A requirement?). It's close, but not close enough. Dropping the pitch will get you to a good sweet spot -- a 5x3" prop like one of these would be a better pick. This prop would draw from that motor just over 7A at full throttle on a 2S lipo.

In reality, you want the shallower pitch anyways -- the higher the pitch the faster the prop needs to move through the air to work well. starting off from a dead stop, the shallower pitch will work better than a steeper one, at least until they get up to speed . . . but "up to speed" for this RPM is around 50mph . . . which you'll never get to, so shallower will perform better than steeper.



Don't forget, you'll need a prop adaptor and a BRUSHED speed controller (10A minimum) of some sort. For a brushed motor, you can substitute a relay or switch to give it full power on/off, but a brushed speed controller is cheap and will allow you to precisely throttle your motor.
 

JimCR120

Got Lobstah?
Site Moderator
#3
You also might be interested to know that Peter S. built a hovercraft (though different specs) that might give you ideas.
FT Hovercraft

I wish you well on your endeavor.
—Jim
 
#4
Thanks

Decent sized brushed motors are a bit harder to source these days -- at that scale it's better to go with brushless -- but if the rules require brushed, then brushed it must be.

Quick look on Hobby king . . . only looks like one is in your ball-park:

https://hobbyking.com/en_us/mabuchi-rs-540sh-6527-brushed-motor-90w.html

Everything else they sell is either way too small, or geared down too much.

That being said . . . this motor is on the fast side for it's power -- spec sheet says 23400 RPM @ 9.6v, so it's roughly 2437kV. For direct drive, it'll throw a small prop.

Not sure about your "current must never exceed 9 volts" spec -- volts is voltage ("pressure" behind the flow), Amps is current (how fast the electrons are flowing). You set the max voltage by your battery supply(so "must never exceed" for voltage seems a weird spec), then throttle the current using your ESC.

The APC 5x5P is a bit of a problem, though . . . fortunately, props are cheap. For direct drive on this motor, on 2S lipo (tops off at 8.4v) you're looking at about 11A of current, which is over this motor's 8.2A max load (and the 9A requirement?). It's close, but not close enough. Dropping the pitch will get you to a good sweet spot -- a 5x3" prop like one of these would be a better pick. This prop would draw from that motor just over 7A at full throttle on a 2S lipo.

In reality, you want the shallower pitch anyways -- the higher the pitch the faster the prop needs to move through the air to work well. starting off from a dead stop, the shallower pitch will work better than a steeper one, at least until they get up to speed . . . but "up to speed" for this RPM is around 50mph . . . which you'll never get to, so shallower will perform better than steeper.



Don't forget, you'll need a prop adaptor and a BRUSHED speed controller (10A minimum) of some sort. For a brushed motor, you can substitute a relay or switch to give it full power on/off, but a brushed speed controller is cheap and will allow you to precisely throttle your motor.
Thanks for the help! I really appreciate it.