Stability for Hovering Aircrafts

[MCurrie]

Saw a neat study on the news of how top heavy hovering objects/aircraft are more stable in flight than ones with a lower center of gravity. Check out the link!

http://www.tgdaily.com/sustainability-features/61387-hovering-isn-t-difficult-if-you-re-top-heavy

 

[colorex]

I don't really understand why, but it would be easy to test with a quadcopter.

 

[teflyer]

I think with a lower center of gravity, the pivot point for motion will be lower at the center of gravity, so the possibility of over correction when a stick for example moves left and right is higher. Also with a higher center of gravity, the torque required to move the stick, assuming you are balancing it on your hand, which is another pivot point, will be much greater than to move the stick with a lower center of gravity. A similar thing happens with hovering objects. They are balancing on the air with two pivot points, one at the center of gravity and one on the air, which might be the center of pressure???. The further apart those two points are, the more stable the object is.

 

[colorex]

With a low center of gravity, the aircraft will be harder to get to move. With a high CG, it will be harder to return to the original position. With a centered CG, these forces are balanced, making it easier to move and to hover.

What I do know for sure is that longer arms give much more stability. The longer the arm, the better resolution your actions get, but also slower reaction times.

 

[teflyer]

I think the thing about the experiment is that they tested in an "oscillating column of air", I assume vertically. In that case, I think that the "bugs" are acting more like rockets as weird as that sounds. and one of the principles of rocketry is a high center of gravity relative to the center of pressure to assume maximum stability. Also, if you notice in 3D planes, the center of gravity is high reltive to the control surfaces, excluding ailerons. I think similar principles apply.

With airplanes, since they are moving laterally and their point of lift is at the wings, they are like pendulums being carried up. With a low CG, they are able to self stabilize very easily. But maybe I am wrong. After all, I do not know that much on aerodynamics.

 

[colorex]

Rockets physics is kind of like a stone with a streamer. The weight is at the top, the rest just follows.

However, for acrobatic planes the CG will always be centered on the plane. This is what makes trainers easier to fly and acrobatic planes more maneuverable.

 

[Ak Flyer]

Rockets physics is kind of like a stone with a streamer. The weight is at the top, the rest just follows.

However, for acrobatic planes the CG will always be centered on the plane. This is what makes trainers easier to fly and acrobatic planes more maneuverable.

Actually the CG on 3D planes moves around per pilot taste. I keep the cg way back on my indoor plane that hovers all the time, makes the tail work better to.

 

[ananas1301]

What I thought of is that if you attach weights to each end of the arm you´ll leave the CG on the same spot but in theory it should be more stable as the motors would have to compensate firstly more and secondly the Multicopter will be much more stable on any outer influences such as wind or something.

 

[teflyer]

Here is another article on the hovering research at NYU from Physical Review Letters
Floats Like a Pyramid

 

[colorex]

Looking at those pictures I'm having a hard time believing that the high CG fly best!

 

[teflyer]

I guess you have to consider that the air is generally only moving in the vertical direction up and down. Thus it is not exactly "flying" than rising and falling with the oscillations of air, which is why I brought up the point on rockets that seemed to somewhat fit. And since the air is pushing against the bottom of the pyramid, the contact force of the air on the pyramid will be pointing down from the base of the pyramid, much like a rocket.