[DonRotolo]

Altimeters are not what you want to use - these are generally based on air pressure, or for very expensive versions on RADAR, but are not nearly sensitive enough to stabilize a helicopter.

Instead, consider using an accelerometer and a rate gyroscope. If I were going after this kind of project, I would probably want three axis versions of each.

The basic idea is to tell the controller what the readings on these sensors 'should' be, and then program it to control the helicopter such that the readings on the sensors 'are' where they need to be. The accelerometer is used for detecting large changes, and the rate gyro is used to detect small changes.

This is known as a closed-loop feedback system. There is a LOT of literature out there on this.

You have chosen an extraordinarily difficult project. A human with experience can have difficulty controlling a helicopter in hover; an autonomous system for the same might be considerably more complex than you think.

Not to discourage you - go for it! - but just have a realistic expectation that it's not easy.

Good luck,
Don