reversing motor rotation (direction)

[broncomech]

Multiple newbie questions:

I am working on a semi scale model that has twin engines with counter rotating props to reduce torque effect. This may not be a factor on the model but it is a major limitation on the real aircraft to operate it with both props turning the same direction. If posible I would like to duplicate the engine prop configuration just in case the model is sensative to torque effect.

Is it posible to operate two motors simultaniously from one speed control?

Is it posible to have the motors turn oposite directions (both propshafts rotate inboard)?

If no to the first question how would you operate two motors simultaniously?

[Pipistrelli]

A brushless ESC is a very complex piece of kit. I would not recommend that it is used to control 2 motors because it does have inbuilt sensors to monitor what a specific motor is doing.

The way to control 2 brushless motors is to use 2 ESC's in parallel via a 'Y' lead to the receiver. This may not be perfect but should work.

Motor direction is usually changed by reversing any 2 phase wires, so this bit is easy.

Torque problems on a twin should be minimised because both motors are offset from the fuselage centre line anyway.

The biggest difficulty you have is finding 2 identical props that rotate CW & CCW. When these are running at flight speed, who will notice anyway?

[broncomech]

Thank you for the information.
Is there some place that I could find a schematic for the wireing?
I am guessing the phase swapping is done at the connection between the motor and ESC but pictures would help get the concept through my thick
grease monkey skull :)

[Pipistrelli]

I've not seen any schematics anywhere, but it should be fairly simple:

Plug a Y lead into channel 3 (throttle) of your receiver and then plug in 2 identical ESC's to the 2 spare sockets of the Y lead. This is your control circuit for 2 motors. You will need 2 flight packs to power these circuits seperately (or a 2 way feed from the battery to the ESC's).

The motors are wired up as per the attached photo's, battery to esc, esc to Rx(via a Y lead in your case) and three remaining wires to be connected to the motor.

Swap any 2 wires and the motor direction reverses. I use 1mm pin connectors so that I can swap gear around, this should be clear in the 1st photo.

[broncomech]

Ok now I (see) understand connection process.

Thanks for the photos.

Dave

[rat1932]

There is a three page article in the Nov 2004 issue of Fly RC Mag. on this subject. I have not read it (no application at present), but it looks like most questions would be answered in it. The author list five simple rules:
1. Use one controller per motor.
2. Use Y-connectors to run all the throttle leads together into the receiver's throttle channel.
3. Only one BEC-equipped controller can be used to power the receiver and servos at a time. Disable the BEC on all additional ESCs.
4. When using a single battery pack, its current rating must equal or exceed the sum of the current ratings of all the motors.
5. You can use separate battery packs for each motor. or for pairs of motors, even if they are controlled by the same throttle output. The risk is that one of the packs may run down before another, taking the motor (or set of motors) with it.
Bob

[broncomech]

Hey Bob,
Thank you for the info, I will have to pick up that issue.
All of my motor experiance is with brush motors so this is all new to me,
still trying to get my brain wraped around 3 phase DC

[Roy_Oetting]

Hi Broncmech,
The reason for counter rotating props on a twin isn't to compensate for torque. They rotate in opposite directions to minimze asymmetrical thrust.
The prop blades produce more thrust when moving downward because of the increased angle of attack. If both props turned in the same direction, CCW as observed from the front , the right props center of thrust is further out on the wing. In the event the left engine stopped it would require more rudder power to control yaw. In order to get sufficient yaw control a higher speed must be maintained. This is called Vmc. Vmc is based on critcal engine out which in the above case is the left engine. Vmc is considerably higher than stall speed but it is the speed that a twin engine plane must sustain for safety. Planes having counter rotating props will have a lower Vmc which makes the plane inherenently safer.(the props would be moving downward on the fuselage side of the engine - CW for the starboard engine and CCW for the port engine) Other methods would be a large rudder, two rudders, off-set thrust lines. These three methods reduce performance.
Roy

[broncomech]

Hey Roy,
You got me :) asymmetrical thrust is the correct term.
The limitations in the flight manual stop just short of saying do not fly this aircraft with two like rotation engines.
My concern is that when scaleing down, existing aerodynamic instability often becomes magnified. The published tendancy to have pronounced yaw with application of high power on the full scale could be all but uncontrolable, or no effect at all on a 1/10th scale.
Better over cautious than rebuilding, I do enough airframe repair at work, this project is for fun :)

Dave

[Roy_Oetting]

Hi Dave, From the stand point of accuracy it would be nice to have counter rotating props. Tom Hunt had props made for his Flying Styro P-38. I think the P-38 was the first plane to have counter rotating props.
Roy

[jimbo]

I think the P-38 was the first plane to have counter rotating props.
Roy
hehe... that honor actually belongs to the first successful airplane. :lol:
http://quest.arc.nasa.gov/aero/wright/photos/wfet.jpeg
http://quest.arc.nasa.gov/aero/wright/photos/wfet.jpeg

[Roy_Oetting]

Jimbo,
I can't argue with that. I pass by a stainless full size version of one of the flyers on a regular basis. the chain on one side passes thru tubes to reverse the direction. I knew that! I'm glad I added the "I think" to my comment. May be it was someone before the Wright brothers. Just kidding.
Thanks, Roy

[broncomech]

Hey Roy,
You got my curiosity cranked up and I started searching for info on the P38's prop config. Yes it does have counter rotating props but Lockheed didn't spin them the conventional direction see photo.
http://www.alaska.faa.gov/fai/images/Aircraft/P38-b.jpg
I haven't found any good articles on the power package but it would be interesting to know why they chose that configuration, might also explain how the devil's tail ended up being forked :)

Dave

[Roy_Oetting]

Hi Broncomech,
You got my curiosity up so I started looking.
http://www.wpafb.af.mil/museum/engines/eng1.htm
All the photos I saw had the props turning in the opposite direction from what I earlier stated except the XP-38.
I don't know why but I have a friend who probably knows. Since it's after mid-night I think I better wait a few hours to ask him.
Roy