# Does the stability of a mass change if the center of gravity is above or below the point of thrust? [duplicate]

I was wondering if it makes any difference if the mass is balanced above or below the point of thrust. See image below for clarification:

In the image you can see two configurations. The red arrow / point in both represents the force / location pushing against the mass (hull of craft). This force can be an impulse or prolonged force. the blue arrows in both represent the upwards force of the rotors. And the green arrow represents the gravity in both.

I would imagine that balancing (keep the craft hovering) would be easier with the mass below the rotors since then gravity helps pulling the center of mass back under the center of the rotors. While if the center of gravity would be above the rotors and it would be pushed to an offset gravity would help make this offset worse.

So does it makes any difference for the stability of the mass (center of gravity) is above or below the point of thrust?

just to be sure config A represents the mass and center of gravity being above the point of trust, and in config B it is below the point of trust.

I am have no engineering or physics background so please explain this as simple to me if possible :)

Let me know if this is not the right place to ask and maybe suggest me where to ask this then.

• The vid is way better than the ASE responses lol. You should just post the video as an answer, or add it as an answer post to the linked answer. Jul 26, 2020 at 13:51

The most stable way to maintain a hover is center of mass below and center of drag above.

What confuses many people is how a "pendulum" reacts while being accelerated, and how it reacts when air drag stops acceleration.

Placing the center of drag above the CG is commonly seen on aircraft tails. This is one reason why "classic" aircraft tails are "upright". Wing dihedral serves the same purpose, to roll a side slipping plane back to level.

Once hover stability is achieved, one may reduce drag above or add drag below to reduce stability enough to allow controlled movement.

Leaving the Center of Gravity underneath will increase stability where when the object is not accelerating. But "pendulum stability" is more properly called "pendulum and drag stability", as in a parachute.

So, in your case, keep the triangle above with light materials., but have your weight below the rotors, and just use the rotors to lift and propel the aircraft. But if you want a separate (red arrow) source of propulsion, try to put the center of horizontal drag and CG in the same spot, and push there.

• Aircraft tails are upright so they don't plow up the runway. Jul 26, 2020 at 15:53
• Well, then maybe they can plow fields that way then, great idea! Jul 26, 2020 at 17:53