Rotor direction

D

Damien Glassy

Member
I just started planning to build a quad rotor as model for an even larger one. In an "X" configuration- each opposing rotor spins in the same direction. Does it matter if the two forward rotors spin CW and the aft rotors spin CCW? Eventually I plan to power the rotors with a single gas engine and use pitch control.
 
Left Front = CW
Right Front = CCW
Left Rear = CCW
Right Rear = CW

They can be opposite of this but the configuration has to be the same.

when I had trouble remembering which way they all went I just remembered that it looked like it was pulling food into its mouth and butt and throwing it out both sides.
 
Quads fly on the premise of apposing centrifugal forces to maintain stability. They use speed variation between all motors to give tilt in a specific direction. Eg. Want to flip forward you decrease throttle on the front two motors and increase on the back two. Want to yaw CCW direction then you turn up the motors running in that direction and decrease the other two. With these 2 simple processes the quad can move in all 4 directions(Vertical, Pitch, Roll, and Yaw). Typically there is a combo of CW and CCW on each side. No matter what side you look at it has a CW on one side and CCW on the other. The order may not be important as long as your flight controller is programmed to deal with it. Symmetric is probably the most efficient.
 
To get your yaw movement without a tail which helicopters have, you will need to have both CW and CCW motors.

These are the two ways you can arrange those motors and props without any problem (just change some settings in your flight controller) :

CW CCW
CCW CW

or

CCW CW
CW CCW

However if you were to put two CCW or two CW beside each other then you would yaw when you pitched or rolled away from the pair of motors, meaning to maintain stability and desired controls you'd need to put 1 or two motors on a pivot (like with an odd number of motors).

So this won't work because you would yaw to the right as you went forward (the front props go slower than the rear to move you forward):

CW CW
CCW CCW
 
Thank you very much for your explanation. Now suppose i have 4 rotors all spinning at the same speed. Cw in front, ccw in back. Can i use pitch control only to change yaw, pitch, roll etc, assuming that my control system properly translates the motor speed variations into the corresponding pitch adjustments? Eventually i'm looking to build something that can transport a much larger payload. use maybe 100 hp.
 
GJH's explanation kinda says no and I agree. same axis has same direction, not front and back. Yaw would be impossible in my thinking. You would flip or spin every time you tried to yaw. What you are trying to do sounds unneeded to me. What makes quads special is the 4 or more independent motors. You cant use one motor without also having an unbelievably complicated clutch system. Why do you keep wanting to ignore us telling you how it needs to be set up to work? We cant tell you how to make your idea work because as far as we know it wont and theres no need to change what works. There are already quads that can carry hundreds of pounds of cargo. A gas engine doesnt have the response time needed to make the thousands of adjustments a second it takes to keep a quad in the air. Youre trying to redesign the helicopter and theres no need for that cause there are better ways now. If you are carrying payloads you want multiple redundant motors not one. There are drones with 30+ motors that can still fly if they loose up to 10 motors. I think im done with this thread.
 
DrAbsurd said:
GJH's explanation kinda says no and I agree. same axis has same direction, not front and back. Yaw would be impossible in my thinking. You would flip or spin every time you tried to yaw. What you are trying to do sounds unneeded to me. What makes quads special is the 4 or more independent motors. You cant use one motor without also having an unbelievably complicated clutch system. Why do you keep wanting to ignore us telling you how it needs to be set up to work? We cant tell you how to make your idea work because as far as we know it wont and theres no need to change what works. There are already quads that can carry hundreds of pounds of cargo. A gas engine doesnt have the response time needed to make the thousands of adjustments a second it takes to keep a quad in the air. Youre trying to redesign the helicopter and theres no need for that cause there are better ways now. If you are carrying payloads you want multiple redundant motors not one. There are drones with 30+ motors that can still fly if they loose up to 10 motors. I think im done with this thread.
Click to expand...
Don't give up now Doc, it's just getting interesting
 
Please read on.. Boeing has sponsored a competition (GoFly) for the private sector for submitting ideas and working full size models for a personal flying machine. Points are awarded for various aspects (size(has to fit in a 8.5' cube), noise, flight time(longer the better), speed, has to fly 6 laps around a 1 mile course, and other factors) of the device with the grand prize being a million dollars. I realize that electric power/controls are very desirable, there just isn't enough power density in modern batteries to power a 700 lb vertical thrust vehicle for an hour carrying a 200 lb person. That leaves a gasoline fuel (engine @ 200 hp)with electric motor redundancy) connected to four rotors running at the same speed and using general speed and pitch control for flight maneuvers. My team has come up with a power train that works like I described. However, I still have options to change it if the numbers warrant. I am not trying to reinvent the heli, just trying to make progress into personal flight. I am new to the particular physics of quad copters, thus my questions for people who know.......
 
A tricopter will fly with all rotors spinning in the same direction.

Tricopter-layout.png

Screen-Shot-2015-10-08-at-2.04.12-AM.png
 
Damien Glassy said:
I appreciate your reply, but it doesn't tell me why it has to be so.... As an engineer, I need to know why....
Click to expand...


:rolleyes: ... My understanding of the aerodynamic principles of quads says there's no difference between
motor speed adjustments of fixed pitch rotors versus pitch adjustments of fixed speed motors.
So, YES you can drive the props with a gas engine and no you do NOT need any clutches. ;)

YOU DO need to adhere to the opposing motor spins criteria though, as that is required as a function of
anti-torque correction to maintain yaw stability. Pitch and roll reactions are self evident, but yaw operates
entirely differently and is less intuitive in nature. o_O
Motor pairs with identical spin directions when given more speed (or prop pitch) overcome the otherwise existing
counter rotational torque balance and allow the quad to spin in the opposite direction of the selected motor pair.
In other words, increasing the "prop bite" of (only) the clockwise props will cause counter clockwise yaw movement. :)

Now, having established the control scheme as viable, I'd offer a suggestion that would be a "nice touch" :
As a passenger carrying craft, you don't want the occupants being tilted forward looking at the ground in
forward flight. The quad (and hence the passenger compartment) can easily stay perfectly level in forward
flight if the arms the front and rear motors are mounted upon are able to pivot. This has already been done
with several quad models and there's Youtube videos showing them in operation. :D
 
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I seen a few YouTube vids lately with reversed motor direction & reversed props. Makes sense aswell, because the props are spinning the branches out & you don't get green goo all over the camera. Gonna try on a new build I'm about to finish. Building these 130's with my sausage fingers is like surgery.
 
It did cross my mind a while back but it was on a try it later idea. Then I seen another one recently explaining about keeping the gunk off the camera.
People saying it flies better but I can't see that. But if it solves the other 2 problems I'll be changing them all over.
 
I just tried with all reversed & it went nuts with a mad wobble.
As soon as I reversed all motors & changed props back, all was OK.
Had to remap the motors because 1&2 where mixed, that was a first for me & realised along the way that everytime I killed a motor port on a crappy naze32 I could ha just used 5or 6. Oh well.
Im still learning that's for sure.
I'll probably forget that & relearn it in the future.
 
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