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Determining proper motor, propeller to use for *copter?


#1

Hey there,

I’ve successfully built a little RC/robot car and while working on it, I’ve been tempted to also work on a ‘simple’ quadcopter; or at least, to determine feasibility.

One problem I keep bumping my head up against is … how do you know what motor/props you can get away with? On the one hand, there are some existing projects that work so one can just crib those parts … for instance, get some 7A brushless DC motors, and some Turnigy 10-18A ESC’s to drive them, and build or buy a flight controller, and away you go; those are fairly sizable motors, and I’m not sure what props you’d put on there, but you could likely get it all to work.

For my project, I’d like to entertain the idea of using cemented lego (or perhaps balsa wood if weight is a huge issue) for the X-frame, and then put small motors and props on there (maybe some foam circles to avoid damaging anyone/thing it crashes into). As ‘small as I can get away with’, and still buildable without many mechanical skills. Maybe try to have a framer thats 6" or 8" across, with some little 2-3" props on there… as opposed to big 6" props on a 24" frame or whatever. Essentially, I’d like to create a flying box … just a little quad that cna lift off, putter around, and land; no high speed needed, no high performance. I’ve picked up a gyro chip and some bits, so I Can try to have some fun with it, keep it balanced etc, but msotly I want to be able to fly up, and point a camera down to take a short clip or do live streaming flying (like in my car.)

I assume you start with a rough weight calculation; given the frame, pcb’s, guess at motor weights etc, you can get a full weight of X poudns/kg.

Given weifght X, how do you determine what sort of motor and amperage you need? and what props to get the necessary lift to get off the ground for a *copter?

I’ve experience with brushed motors, but resources seem to suggest brushless is the way to go for these projects; to keep my life easy, I may go with these $10-$13 Turnigy ESC’s (can get them at Hobby King.) Given 4 of them, and say $50 + shipping, thats pretty good and greatly simplifies the design of the flight controller.

The ESC family I’m looking at it:
hobbyking.com/hobbyking/stor … aff=588847

Other options are welcome! But if it helps to have a ESC to narrow in on motors, theres somethign to work with.

Please help :slight_smile:

jeff


#2

You will have better luck with the forum on diydrones.com, but the rule of thumb for motor current is 1 ampere per ounce of model. rcuniverse.com/forum/3d-elec … print.html


#3

Lol, … an amp an ounce, your plane wont’ bounce… cute :slight_smile:

I’ll check the other forum … thanks for the tip!

I’m sure theres a few more rules of thumb for conversion of rpm/amp to prop size and so on as well.

jeff


#4

After thinking a bit about it, I realized that the one AMP/ounce rule can’t be generally true as it doesn’t specify the battery voltage. I checked around and found some posted experimental data for a quadcopter that suggest a general rule of thumb of one WATT per ounce, which makes a more sense. code.google.com/p/ro-4-copter/wi … erformance

One the other hand here is an interesting discussion suggesting that motors provide 10 grams of thrust per watt (or about 3 watts/ounce) diydrones.com/forum/topics/impos … technology

Obviously you could do a lot worse, so these are very rough estimates.


#5

It cannot be all about the motors; given the wide variety in prop styles - pitch, lengths, material, sharpness and so forth … there must be a handy table or formula or somethign to work out particulars beyond how it attaches to an axle. I’ll look into it more … Theres too many paramaters for my small brain/experience – larger props are more efficient and harder to change due to momentum, so you typically want ‘as small a prop as you can get away with’.

I’ll get some datasheets, perhaps this is all a ‘well known’ thing that I’ve somehow missed :slight_smile:

Thanks for your tips so far!

jeff


#6

From what I’ve read, matching the airframe weight, motor, rpms and propeller size & pitch for optimum flight time and efficiency is still a black art. Most propeller-driven commercial aircraft have variable-pitch propellers, which is a critical feature but not generally available to hobbyists.


#7

I spent a year or so tweaking a small r/c helicopter called a Piccolo, always trying to make it lighter and able to fly longer. The biggest improvement was with the battery pack and its weight. I switched from using a nicad pack to a Kokam lithium polymer pack which extended my flight times from 8 minutes with the nicad, up to 25 minutes with the lithium polymer.