I remember reading about vortices somewhere awhile ago, and about they can sometimes improve the air flow and thus also improve thrust, stability, quiet flight, etc.


I noticed that a majority of the Quad's and multirotors have the booms (the arms that hold the motors to the fram) go to the motor and not any further.


Now, I'm no aeronauticle engineer, but I think it would be possible to induce vortices by extending the boom up to the tip of the propellors.


Additionally, this might even make it easier on the motor because the force load on the blade would be more even when the blade tips go over the boom.


Ok... think of the propellor as being very flexible and like a sheet of paper. If you apply a force to it, it will bend to react to the force. When the propellor is spinning over the open air, the force is pretty consistant and thus the load is consisitant.


However, when the propellor spins over a the boom, the pressure is higher on the blade, because the boom is within 2 diameters of the blade. This ends up with the blade flexing and pushing against the axle of the motor (at least so I think).


Has anyone considered this before? If so, what where your conclusions? 

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Put the propellor on the other side of the arm and then it is not an issue, it is just much more efficient!!!

Wouldn't the blades still be pulled towards the arms though?


ok... how did you come by this conclusion then? By experiment, some article, a book...?


If so, I'm curious how it went down.

I did cover that in detail on 2.0.38 help and suggestions somewhere around page 6.

Denny's reply in 2.0.38 help and suggestions


Ok, thanks for pointing to that, very informative and should be useful for future builds!


The problem I have with my current configuration is that if I did flip the motors over, the prop's would be below the C.M., since it wound up in the center of the arm hub.


I'm thinking that this would be a very bad thing, because the craft would then act like an inverted pendulum...


By the way, which arm profile do you prefer (and why)?

  • Square tubing, parallel to prop's disc
  • Square tubing, 45 degree to prop's disc
  • Round bar or tubing

Oval tubing 

That's a pretty specialized part I'm afraid... which means that it would be good for anybody that has a big bank account.


I saw a drag comparison chart some time back and saw that a square profile has a similar drag to circular profiles, provided that it is at a 45 degree angle to the wind.


But the problem with getting it at a 45 or using round stock is that it can be tricky mounting the motor on it.

what about making the section of the boom under the propwash an airfoil. look and the support structures on alot of aircraft and any rod/mount/whatever will be a teardrop shape to cut down on resistance.

That's what Denny was hinting at with the oval tubing, although I think an airfoil might make matters a bit worse with rotations along the ground plane.


The reason I prefer round arms (besides the fact that they're easy to machine) is that they have a drag coefficient that's consistent from any angle.


I think the primary function of motor placement is to have it below the arm and reduce the overhang from the motor mount plate to the prop. As aerodynamic drag is a square law this accounts for about 5% better efficiency in overall battery duration.

The direction that I am working in at this moment is to incorporate a thrust vectoring element into the flight dynamics. A lateral movement is normally initiated by a bank in that direction. But with either fixed or moving vanes below each thrust disc it is possible to make that movement and keep the model level.  

In the end it will be a trade off-   weight vs. stability.  

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