Models with 4 motors counter rotating or not?

[Chris Walby]

Hi,

 

Question came up recently which got me thinking, then I ran out of ideas as to why it would be done differently to what I assumed. Twins it might be beneficial to have one motor rotating and the other counter rotating (viewed from front clockwise left, anticlockwise right) to reduce swing on the ground and P effect and assumed for a four motor model you would just have a pair of each on both sides. Two clockwise on one side and two anticlockwise on the other side.

 

However a clubmate has a electric four where the inner two go inwards direction and the outer two in the outwards direction, not quizzed him, but he must have done it for a reason/advice. 

 

PS- Not a clue how its been wired up on the batteries and ESC's, but for normal flight won't this arrangement really screw the airflow and balance up and not help yaw/roll?

 

Thanks for your time

 

Any suggestion as to what is the agreed way to do it and why the way the clubmate has? 

[Simon Chaddock]

Chris

The more motors the less is the net torque effect so counter rotating is not vital. The only excaption would be if you are close to the stall and low so you whack on full power to get out of trouble. The small net torque and the fact that the air flow over the wing from each prop is slightly squewed can cause a wing to drop quite significantly just when you don't want it. Counter rotating, ideally inward over the top, would avoid such a situation. Easy to do with electric if you can get the props so why not.

Note also that most if not all RC models have a onsiderably higher thrust to weight than full size so any motor generated effects tend to be amplified.

 

You have to decide if you intend to use a single battery or two. If you use two it is probaly wise to power the inboard pair from one and the outboard from the other. If one battery gives out before the other you will atleast have some power and still maintain a balanced thrust.

The simplest aarangemnt is to 'y' lead all four escs to the throttle channel. You may have to 'teach' each ESC in turn the upper and lower throtle limits so they all run equally accross the throttle range.

Also be aware that the ESC to motor wires can be almost any length whereas the battery to ESC wires cannot. With 'four on the wing' this tends to mean the ESCs endup close to the battery (in the fuselage?) so their cooling needs to be considered.

Have you flown a multi before?

 

There is no agreed way unless you are following scale like the Airbus A400.      

 

Edited August 25, 2021 by Simon Chaddock

[Chris Walby]

Thanks Simon, Its not my model as I tend to fly IC warbirds (some are twins), but as the clubmate was discussing the tip stall issue the conversation progressed on which motors are driven in which direction so I thought I would ask here.

It didn't look like it yawed much on takeoff, but he needed to hold right aileron in most of the time although that was in part due to a trimming issue and can be sorted post maiden.

 

Just seemed a bit odd the inner pair rotating (top towards) and outer (top away), although as you say if the inners are off one battery and the outers the other batter it would make sense (I think I would still go for both on each wing rotating in the same direction!). 

 

 

[Nigel R]

15 hours ago, Chris Walby said:

the outer two in the outwards direction

 

My layman understanding -

 

If the airflow goes inward over the top of the wing, then it contributes to reducing tip vortex and helps wing efficiency. Having the outer two rotate the other way seems counter productive to that goal.

 

I would have thought making both port rotate normal and both starboard rotate reverse would be better. In total the rotational effects are still cancelled out.

 

 

[Jon H]

When it comes to multi engine models i just have them all go the same way. With i/c i have little choice, but its also never been a problem so i just get on with it. 

[Robin Colbourne]

From the Airbus brochure for the A400M  A400M Brochure:

Quote

“Down between the engines counter-rotation”

• Produces a more symmetrical airflow over the wing,
which improves lift, aircraft handling and stability, as
well as allows a reduction in the structural weight of
the wing.
• Reduces the adverse yaw in case of an engine
failure and gives a 4% increase in the lift at low
speed.