Newbie Q's on flying saucer - RC Groups
Apr 17, 2012, 10:42 AM
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# Newbie Q's on flying saucer

Hi, a friend of mine is doing his first year in aerospace engineering. He gets to keep a handful of motors and wanted to build a toy single-rotor or contra-rotating prop flying saucer. This project strictly isn't remote control but I just want to ask some questions about the power system and pointers to the right direction.

The motors are brushed and was bought from http://www.jaycar.com.au/productView...SUBCATID=983#1.

The propellers are GWS DD 7x3.5. We used a static thrust calculator and got that in order for one prop lift 240g, it needs to spin at 9000RPM.

We are using non-rechargeable batteries and not lipo. It's going to be tethered so carrying the batteries around in the backpack would not be the problem.

I read on a lot of pages that the RPM is controlled by the voltage of the battery, but from trial and error, I've found that with a new battery (some unknown brand) with 10.4V (multimeter) spins the prop slower than using an old 9V (measured to be 8.4V) Duracell battery. The load is the same. So does the voltage or current control the RPM of a motor?
 Apr 17, 2012, 12:20 PM Registered User The actual voltage UNDER LOAD, i.e. while the motor is running, controls the speed. So you need to measure those voltages WHILE the motor is running not before the battery is connected to anything. All batteries will drop some voltage as you load them up. Different batteries drop by different amounts e.g. if you are trying to pull too much current out of the battery its voltage will drop a lot. Steve
 Apr 19, 2012, 03:17 AM Registered User Thanks. We just did a quick test and the battery is a 4.5V but the motor under load is pulling less than 4.5V. Is there a way to force the battery to pull more voltage and increase the RPM?
 Apr 19, 2012, 04:15 AM Registered User Not with the same load on the motor. If you want more voltage use a battery that starts off with a higher voltage or a bigger battery that keeps its voltage better under load. Alternatively using a smaller propeller will be less of a load and so will not drag the voltage down as much. That should give you more RPM but probably with less thrust because the prop is smaller. It all depends what you're actually trying to achieve. Steve Last edited by slipstick; Apr 19, 2012 at 04:21 AM.
Apr 19, 2012, 10:19 PM
Quote:
 Originally Posted by slipstick Not with the same load on the motor. If you want more voltage use a battery that starts off with a higher voltage or a bigger battery that keeps its voltage better under load. Alternatively using a smaller propeller will be less of a load and so will not drag the voltage down as much. That should give you more RPM but probably with less thrust because the prop is smaller. It all depends what you're actually trying to achieve. Steve
The internal resistance of the battery is what reduces the effective voltage at the terminals of the battery as you draw more current....

This is one reason LiPo's are so well liked - they typically handle pretty good (relatively speaking) current draws.

The prop will cause the current of the motor to go up based upon its load as well - do you have a way to measure the current? Maybe use a bench supply with voltage and current meters?
 Apr 20, 2012, 07:29 AM Registered User I connected the multimeter in series with the motor and the battery. Is there a way to control the signal into ESC without using a receiver? Say if I want to have it tethered, not RC. Is there a page that tells what sort of signals get out of the receiver? Sorry if the questions sound a bit stupid. Last edited by coaxialnewbie; Apr 20, 2012 at 07:35 AM.
 Apr 20, 2012, 08:04 AM Registered User Just use a servo tester. The ESC signals are exactly the same as servo signals. Steve
Apr 20, 2012, 11:06 PM
Registered User
Awesome. So with this servo tester for example, http://www.ebay.com.au/itm/3CH-ESC-S...ht_2825wt_1139, it has 3 pins, each for ins and outs. How do you connect these to the brushed motors?

I just watched a hobbyking video on a servo tester,
 HobbyKing - Turnigy 3-Mode Servo Tester (4 min 52 sec)
, they still uses a ESC for the motor. So did you mean the receiver signal is the same as the servo signals? Does this mean I will still need to have brushed ESC?
Last edited by coaxialnewbie; Apr 20, 2012 at 11:27 PM.
 Apr 21, 2012, 12:16 PM Registered User Yes, servo tester output = receiver signal output. Wire your servo tester to ESC then ESC to motor.
 Apr 24, 2012, 07:00 AM Registered User Awesome, the servo testers can take in 4.8V-6V. 1 cell lipo has 3.7V and 2 cell lipo has 7.4V. So do I need to use a voltage regulator? For most inexpensive 5V regulators, they can pass through 1A maximum. I see a lot of tables and charts of motors perform at 20A, 60A, etc. If the motor does pull 20A, would it burn the 5V regulator?
 Apr 24, 2012, 07:32 AM Registered User If the ESC has a BEC then it will supply the 5V to the servo tester just the same it would have supplied the receiver. Steve
 Apr 26, 2012, 02:55 AM Registered User Thanks Steve. Just another confusion, if I have 2 motors, would I need to connect a battery pack to each motor or is it ok to use 1 battery pack to connect to an ESC with BEC and use it to power the servo tester and the other ESC also?
 Apr 26, 2012, 04:10 AM Registered User You seem to be making this project a lot more complicated than it started off. If the motors are brushed as you originally said then you only need one ESC to drive the two motors. OTOH if you want to control the motor speeds separately then you will need two ESCs AND two servo testers. In either case you only need one battery which is connected to the ESC or ESCs. Steve
 Apr 27, 2012, 03:44 AM Registered User Ah, stupid of me I didn't know 1esc can control 2 brushed motor if they are travelling at the same speed .