Theory behind upward angled arms / Booms and inclined motors

[tahoelight]

I see DJI s800 and a few other frames use an angled arm to both lift the motor above the center plate and to angle the motor inward. Does anyone know what this design is intended to accomplish? What are the pros and cons? I can't find and discussion on it.

 

[DucktileMedia]

On my fold-n-fly the arms are semi flexible. They dont flex enough to vibrate but they do seem to stabilize the heli a bit since they angle inward just a a hair. Since I am using the Hoversport, which has no autolevel, any stability seems to be based a lot on balance and inherent mechanical stability. I bet if I put the battery up top it would not be able to handle it well. At the same time, my friend tried adding a little angle to his large hex using the WKM and he said it flew like crap. He went back to flat and it was much more stable. I think these controllers are more than able to accomodate the lack of dihedral. On a bigger heli, I would think rigidity would be more of an issue than the angle.

 

[kloner]

Dihedrals main thing it does is increase stability by helping to self level by design. p-51, alot of the early piston fighters had it

That frame you cut for me had 3 degrees, it makes a difference but also creates less speed in my case. it got crazy mileage

 

[jcmonty]

Yup, dihedral is an passive way of stability in planes and in this case MRs. I don't really see the point on MRs since you are completely relying on your flight controller to perform dynamic control and stability. But it may provide some "resistance" to movement - commanded or uncommanded. The disadvantage is you are a bit less efficient because you are expending some of your lift in a horizontal vector.

 

[kloner]

the one i did had disco arms..... bet really stiff arms would be different than what mine was like

 

[PeterLester]

These comments seem to make a lot of sense, i.e., inherent mechanical stability precludes added electronic stability etc. And there is always a tradeoff between performance and stability as suggested abovre.

But there might be other reasons. Is it possible that the dihedral or tip washout also increases MR stability when the MR gets close to the ground in ground effect where there is a lot more air turbulence?

 

[DucktileMedia]

from watching a few s800 videos, I would say it doesnt help.

 

[kloner]

it would be fun to make one of them smoke things like at a wind tunnel and look at the diffrences in dihedral, coaxial and flats to get a better understanding of what goes on, how the air is blent

 

[gadgetkeith]

i would say its a way to increase the footprint area without actualy increasing the lengh of the arms themselves

look at the colums of air thust downward like the legs on a tripod etc the splay at the base creates a larger footprint ie better stability

probably less work for the fc to hold position over a fixed point

not sure how much this effects general manoverablity efficiency but must cause some amount of loss in efficiency when moving in any direction

also using the reverse principle to angle the arms down or tilting the motors outward from centre would have the reverse effect

creating a less stable craft in the hover but probably a lot more manoverable with faster direction changes etc

its all down to thust vectoring either increasing or decreasing the size of the air footprint the craft rides on