Constant speed unit

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A constant speed unit or CSU is the device fitted to a propeller to automatically change its pitch so as to attempt to keep engine speed constant. Most engines produce their maximum power in a narrow speed band. The CSU can be said to be to an aircraft what the automatic gearbox is to the motor car: The engine can be kept running at its optimum speed no matter what speed the aircraft is flying through the air.

The advent of the CSU had another benefit: It allowed the designers of aircraft engines to keep ignition systems simple - the automatic spark advance seen in motor vehicle engines is much simplified in aircraft engines.

Note that not all propeller-driven aircraft have CSU's. The smaller the aircraft the less likely is a CSU. E.g. neither the Cessna 152 nor the 172 has a CSU whereas the 182 and 206 both do.

Three methods are used to vary the pitch: Engine oil pressure is the usual mechanism used in commercial aircraft and the Continental and Lycoming engines fitted to light aircraft. Alternatively or additionally centrifugal weights may be attached directly to the propeller as in the Yak-52. Increasingly, especially on small modern engines such as the optional CSU for the Rotax 912, the pitch is changed electrically.

A pilot requires some additional training and, in most jurisdictions, a formal signoff before being allowed to fly aircraft fitted with a CSU. Interestingly CSU's are not allowed to be fitted to aircraft certified under the USA Light-sport Aircraft regulations.

[edit] See also

• Variable pitch propellers, of which the CSU is but one type.