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Perhaps a combination: you could simply measure speed, and a change in altitude.
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You can place meters for all axes, and then do the math to calculate the true velocity direction and value, no matter if you're upsidedown or whatever. The velocity meter method is less reliable because it will only measure in 1 dimension (which greatly depends on the orientation of the vehicle, the surface the meter is placed on). this might be a problem for some vehicles, especially ones that are capable of switching directions multiple times within 1/4th of a second If you're doing the altimeter method, then keep in mind how many ticks you're measuring over, as that forms a sort of "average" (so if you did 10 tick measurement, that's 1/4 of a second being measured). That "change in altitude" value will only be greater if you're moving faster. With this positive or negative value, you can add it onto the performance of all your thrusters So whether you're trying to fly up or down, turning or whatever, all the thrusters change according to the behavior of the change in altitude.
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Slap a speedometer on your vehicle in velocity mode (pointed up/down) or simply measure altitude (over time, so altitude - altitude from a few ticks ago, for change in altitude over time)
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