Turboran speed limiting control system

Abstract

The maximum speed of a turbofan engine varies in accordance with moderately complex relationships involving the temperature, the pressure or altitude, and whether or not the subordinate systems such as the anti-ice systems are operative. The maximum allowable rotational speed for the fan section of a turbofan engine increases with increased altitude or reduced pressure, and normally increases as the ambient temperature increases up to speeds limited by the centrifugal and other forces acting on the turbine blades. However, there is a maximum fan rotational speed above which the engine should not be operated, and there is a limit which decreases with increasing temperature above which the fan should not be operated regardless of the pressure. In addition, when the anti-ice or other subordinate systems are operative, these limits are reduced substantially. The present invention involves the electronic determination of the normal linear relationship of fan speed with temperature at a fixed pressure or at a series of fixed pressures, a determination of the maximum fan speed limit for each temperature, in terms of a function having a negative slope with increasing pressure, an electronic circuit arrangement for shifting from one characteristic to the other to avoid damage to the turbofan engine by running it at above-specification rotational speed; and also includes switching arrangements for changing operational limits upon operation of the de-icing or similar equipment.