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Place any chopper in the current beta ( AH9 and MH9 less subject to this due to lighter mass )
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Gain speed, 200kmh, roll ~45° to the right or left and pull up, no need to use the collective, just keep pulling up.
Enjoy infinite and perfect circles in the air, with 0 average speed loss, and fairly costant altitude.

You can do the same while going slower than 200kmh, but i found that the vehicle will just lose some altitude and have a massive acceleration, reaching over 200kmh in a couple seconds.

Doing all of the above while using the "Q" key, will make it possible to make orizontal loops, while gaining speed AND altitude.