used for turbofan engines whereas Nf is mainly used for turboprop or turboshaft engines. In many cases, even for free turbine engines, the N1 and N2 may be very similar. Ns refers to the speed of the reduction gear box (RGB) output shaft for turboshaft engines. In many cases, instead of expressing rotor speeds (N1, N2) as RPM on cockpit displays, pilots are provided with the speeds expressed as a percentage of the design point speed. For example, at full power, the N1 might be 101.5% or 100%. This user interface decision has been made as a human factors consideration, since pilots are more likely to notice a problem with a two- or 3-digit percentage (where 100% implies a nominal value) than with a 5-digit RPM. See also Pump-jet Rocket engine Turboprop – a gas turbine engine used to turn propellers Turboshaft – a gas turbine engine used for helicopters References Citations Cited sources Jet engines de:Strahltriebwerk