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### Topic: DC motor for rotor thrust (Read 2799 times)previous topic - next topic

#### dgonz

##### Dec 29, 2011, 07:11 am
I am using an Arduino Pro Mini 5V to control a bicopter. It should be pretty light overall, with the bulk of the weight coming from the batteries and motors.

I purchased two 9v dc motors and rotors for each, but upon testing they don't generate much thrust. I went with the 9v model because I assumed a 9V motor would be stronger than a 7V or 3V. Was I mistaken? What is the best way to determine whether or not two of these can lift the helicopter? Can I use more than nine volts to run the 9V motor, "overclock" it? Where can I get a good enough motor for my purposes?

Motors:

Thanks.

#### kf2qd

#1
##### Dec 29, 2011, 11:56 pm
For Small DC motors RPM is dependent on voltage. A 9 volt motor will require a higher voltage for more RPM. Thrust is generally related to RPM. Depending an the load - you can overvoltage a motor for more RPM, as long as the current draw is not too high.

At a higher voltage you will see less motor life, but it will still probably exceed your needs and the motors are relatively cheap.

You really need to look at RPM required, and then look at what current the motors will pull at those RPMs and then decide which motor to use.

A 3 volt motor may put out the RPMS required at 5 volts, but draw too much current. A 7 volt motor may put out the right RPM at 9 volts and the current be within limits, the 9 volt motor may need 12 volts for the proper RPM and the current will less than the 7 volt motor. Problem is - it it worth the extra hassle to put 12 volts onboard (and the weight) when a lower voltage battery will function on less weight. At the higher voltage you will waste more of your capacity in heat when you regulate it down for the 5 volt part of your system.

#### ajofscott

#2
##### Jan 02, 2012, 03:07 am
You won't drive the rotors directly anyway, 1200 rpm would be fast depending on pitch and diameter. One thing to remember about helis is high power fast delivery. You are flying on the prop so to speak. My suggestion is purchase a pair of main rotor shafts, the bearings, gearing and designed motors from a hobby shop specializing in RC Aviation. The rule of thumb is 122 Watt/lbm for fixed wing aircraft, the ratio is more than twice for a heli. The good RC helis are running at least 4-6s lipo packs ie running voltages 12-18V, flight gear is not run from the propulsion battery in all cases. Especially if your ESC does not have a switching regulator in its battery elimination circuit(BEC). Mechanical losses go up with speeds so it all ends up being a compromise on torques, power, and speed. Low speeds mean higher torques, more to cope with in flight, while high peeds and low torques make control easier the frictional losses in the rotor go up as the square of the speed change. Your local Chamber of Commerce should have some info on what AMA clubs are in your area, go visit them, talk with the heli pilots. Their help could make your endeavor a successful one.

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