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I have a quadcopter and it has a flip function (as do many other quadcopters).
I was wondering how this works. Say I am flipping it right (clockwise), I assume the motors on the left spin much faster than those in the right. If so, how does the quadcopter not loose too much altitude when doing this?
The trick is speed. The reason why multirotors don't lose (very much) altitude while performing quick mid-air aerobatics is because of the speed of the move and the skillful management of throttle by the pilot.
You're correct in presuming that a multirotor performs a flip by changing the speed of the motors on each side of the craft. Freestyle pilots usually make use of near-total stick deflection and extreme rate profiles targeting ~600-1200 deg/s of angular velocity. The fact that the flight controller speeds up motors on the side which needs to rotate "up" and slowing down those on the side which needs to rotate "down" ensures that the move occurs as fast as possible because all four motors contribute to the torque which rotates the multirotor.
The other factor which helps the multirotor not descend too much is the skillful management of the throttle during the move. By lowering the throttle to near idle while the craft is inverted, the pilot minimizes the amount of thrust the craft generates which acts to pull it down towards the ground. As I discuss in my answer to "What's the purpose of airmode and idle up?", the idle up and airmode features (of the RC transmitter and flight controller respectively) help ensure that the multirotor is in control during these times of low-throttle.
Because of these two factors, the duration in which gravity has the power to suck the craft towards the ground is often too little to matter, and the multirotor flips in-place.
Of course, there's nothing preventing you as the pilot from slowing down the flip and performing an "O"-shaped loop instead of a flip in-place. This can be done by easing off the roll/pitch axis and turning down rate profiles.
You are correct that the motors on one side speed up.
The reason the drone doesn’t lose too much altitude is that the motors speeding up on one side, and the resultant force of gravity on the other side, create a torque about the pitch (or roll, depending on if it’s a flip or roll) axis.
In other words, as one side goes up, the other side goes down.
This can be difficult to visualise, so here is something you can try in real life:
Take a ruler and balance it with a finger at either end. Remove one of the fingers and you will see that by the time the ruler has completed one full roll, it has lost altitude.
However, try it again but this time, as you remove one finger, raise the other one sharply. You’ll see that the ruler barely loses any altitude by the time it has completed a full roll.
