Tip velocity is a term used for the speed at which a rotor rotates in feet per second or in miles per hour. It is used as part of engineering workup studies on nearly anything with a propeller or turbine blade. It is particularly important when setting up wind turbines, as tip velocity directly contributes to the sound they make and how much power they generate.
Observe the object rotating. With a stopwatch handy, count how many times the blade turns in 10 seconds, then multiply by 6. This will give you the revolutions per minute or rpm. (For very fast moving rotors, you may be better off referring to the owner's manual.)
Find the length of one rotor. You can do this by measuring the rotor with a tape measure after turning the device off, or by looking at the owner's manual for the device you're using.
Calculate the circumference the rotor blade travels in one revolution through by doubling the length of one rotor, and multiplying by pi (3.14159). For example, a fan with a 6-inch blade would travel through a circumference of 6 x 2 x 3.14159 = 37.69908 inches.
Multiply the circumference by the revolutions per minute derived earlier. For example, if that fan were doing 120rpm, the velocity of the tip would be 120 x 37.69908 = 4,523.889 inches per minute.
Divide inches per minute by 12 to get feet per minute. In this example, it would end up being 376.9908 feet per minute.
Multiply feet per minute by 60 to get feet per hour. In our example, 376.9908 x 60 = 22,619.448 feet per hour.
Divide feet per hour by 5,280 to get miles per hour. Our sample fan would have a tip velocity of at 22,619.448/5,280 = 4.283 miles per hour.