Final answer:
Air speed indicators are designed for slow and medium speed aircraft flying on VFR. They measure the velocity of the aircraft's airspeed.
Step-by-step explanation:
The devices that are designed to be used by slow and medium speed aircraft flying on Visual Flight Rules (VFR) are air speed indicators. Air speed indicators measure the velocity of the aircraft's airspeed. These devices are commonly used in aircraft to provide pilots with information about the aircraft's speed in relation to the surrounding air.
Figure 12.7(b) shows a version of this device that is in common use for measuring various fluid velocities. This device is frequently used as an air speed indicator in aircraft. It functions by measuring the difference in static pressure and dynamic pressure created by the movement of air around the aircraft.