The pitch attitude of the aircraft determines the forward speed, and power determines the altitude.
For instance, when flying a Robinson helicopter:
~ a magnetic compass mounted on the windshield in front of the pilot can be referenced to the horizon to provide very accurate pitch information
~ holding the top of the compass on the horizon will cause the helicopter to fly along at about 60 knots = 111 km/h
~ holding the compass an inch below the horizon will cause it to fly at 70 knot
~ holding the compass an inch above the horizon will cause it to fly at 50 knots

Attitude flying typically takes less than 1/2 second for the helicopter attitude to change.
If the pilot knows the attitude required for the desired airspeed,
the cyclic can be moved until the fuselage moves to the desired pitch attitude,
and the pilot can be fairly sure the aircraft will accelerate/decellerate to the commanded airspeed within the next 10-15 seconds.
After 10-15 seconds, the pilot can verify that the airspeed indicator shows the desired speed.
If the airspeed indicator indicates a speed a few knots away from the desired speed,
the pilot can make a small attitude change to bring it exactly to the desired value.

Problems using the airspeed indicator to control the speed of the aircraft
~ there is a large time lag from the time that the pilot moves the cyclic control until the helicopter finally stabalizes at the commanded speed
~ this lag can be tens of seconds, and would make it very difficult to command the correct cyclic position, because feedback takes so long to occur

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