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What do I look for in a motor? torq/thrust/efficiency ?

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What do I look for in a motor? torq/thrust/efficiency ?


Old 08-08-2005, 11:38 PM
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Default What do I look for in a motor? torq/thrust/efficiency ?

After bad mouthing electric R/C aircraft for nearly 10 years, I am taking the plunge. The other night I had the opportunity to fly a Flatout Cap 580. The guy in my club who had it asked me if i wanted to give it a shot and WOW!!! These little planes are very, very fun! I went out and bought a Flatana, HS-55 servos, and a CC Phoenix 25 ESC the following week. I understand the Battery side of all of this but I am having some issues as far as understanding how to calculate the thrust a motor will produce given the voltage. Also, since I will be flying 3D with it, I am curious as to how to determine the torq of an electric motor. I know I can strap on a rimfire motor and go, but I enjoy knowing all of the figures and specs to help get the most out of my powerplant and just like my glow aircraft, I like to overpower a little ( or at least get the most torq ). So basically, what numbers do I need to pay attention to when shopping for a motor? How do I calculate thrust? How do I tell how much torq a motor has? Finally, how do I tell how efficient a motor is? I do know that I will be using a 3 cell LiPo pack although I don't know how many mAh is too much or too little. Thanks for the help!
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Old 08-09-2005, 06:50 AM
steve ypsi mi
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Default RE: What do I look for in a motor? torq/thrust/efficiency ?

here's a link that will answer most of your questions, look to the side tool bar and click on them
the phonix 25 will limit what size motor you will use, the 25 runs at 25 amps continoues and 40 burst, the more MAH you have will give you longer run times and a little more thrust, I find that 400 Mah more will give me 1 ounce more thrust on a 350 brushed motor
Himax and razor give the Rpms per volt, a 2015-5300 is a lighter motor with 5300 rpm times the volts used, 11.1 time's 5300=58830 rpm at the motor no load divided by gear redustion such as 5.3 into 58830=11100 rpm at the prop
for you the himax 2025-5366 is a torque motor and heavier with high RPM,, 2015 series are made for lighter planes no more than 20 ounces
click on the HiMax and Razor on this page , some of the motors like mega motors and some others have a number like 16-5-25 the last number means equeal to a gas size motor like 25 would be a .25 gas and the first one is motor size and winds in the motor

If this is your plane you can use a lot lighter motor like the 2015 Hi max 2015-5300, if you want more torque than speed they make 2015-4200 which will pull a larger prop such as a 1080 to 1180
http://www.electrifly.com/flatouts/gpma1111.html since it only weighs about 9 ounces, I would guess around 1500 to 1800 MAH will give you about 8 to 10 minutes flying times, just a guess
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Old 08-09-2005, 07:34 AM
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Default RE: What do I look for in a motor? torq/thrust/efficiency ?

Steve's given you several good options on a motor.

For a GP Flatout, a Phoenix 25 is absolute overkill. The Great Planes BL8 would be more than adequate. But, all you give up is a few grams of weight because the CC Phoenix 25 is extremely light for its capacity. Guaranteed you won't be drawing anywhere near 25 Amps on this plane; the LiPoly battery necessary to handle that kind of current would weigh as much as the plane ready to fly!

A 3D plane by nature is overpowered, so maybe for your first attempt it might be a good idea to go with the recommended setup.

With electrics, it's all about Volts and Amps and Watts. (Oh my! {lame Wizard of Oz reference}) These translate into torque, thrust and pitch speed at the motor, so we normally don't deal with those numbers directly. Given the plane's approximate finished weight with gear, you can easily figure out how many Watts is required to make the plane fly in a certain way. Put those Watts into an appropriately-sized propeller for the plane you're flying, and you're guaranteed success.

You can backwards-engineer thrust and torque once you have the Watts, RPM, and the efficiency of the motor. Unfortunately, most manufacturers don't give you that information in the advertising literature. 746 Watts is 1 Horsepower, Horsepower is Torque times RPM divided by 5656... It's pretty simple math, but only good for one setup. Change the prop size for example, and you have a whole different setup with different efficiency, different Watts, different RPM.

R/C tends to be more hands-on and less focused on the theoretical.
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