The present invention relates to an augmented turbofan engine, and more particularly, to backup control means for limiting fan overspeed during augmented operation of such an engine.
A gas turbine engine which drives a fan and causes a portion of the fan airflow to bypass the gas turbine, or core, is typically referred to as a turbofan engine. In a mixed flow gas turbine engine, the fan airflow and the turbine discharge are allowed to mix. Turbofan engines may include a variable area exhaust nozzle for developing the desired thrust. Such variable area exhaust nozzles are often employed on turbofan engines having thrust augmentation means, such as an afterburner or preturbine injection. Further information on such turbofan engines can be found in U.S. Pat. No. 4,128,995, entitled "Means and Apparatus for Stabilizing an Augmentor System," issued Dec. 12, 1978, to P. D. Toot, and in U.S. Pat. No. 4,128,208, entitled "Exhaust Nozzle Flap Seal Arrangement," issued Dec. 5, 1978, to E. W. Ryan, et al. Each of these patents is hereby incorporated into reference in the present application.
A conventional primary speed control system employed on mixed flow augmented turbofan engines includes control of fan speed by modulation of the main engine fuel flow and control of fan operating point, i.e., fan pressure ratio which is related to fan performance and required surge margin, by modulation of a variable area exhaust nozzle. This results in two control loops which are typically powered by a single engine electrical supply means, such as an engine-driven alternator. When fan speed is controlled in such a system, core engine speed will, through aerodynamic coupling, seek the steady-state value required to provide the necessary energy to keep the fan rotor at a governed speed. A hydromechanical core speed governor is typically employed to limit the core speed to a safe value. This value is slightly in excess of that which the core will achieve when operating normally in the previously described typical fan speed/fan operating point control mode.
This conventional control system is generally designed such that loss of electrical power thereto will result in reversion from speed control of the fan to speed control of the core engine and in failure of the variable area exhaust nozzle to a full closed position. Such a failure condition is desired in order to make available adequate thrust under the loss of electrical power condition. If the electrical power loss occurs while at a maximum nonaugmented power setting, speed control reverts to the core engine with an attendant slight increase in core speed. Fan speed, through aerodynamic coupling, will seek a value commensurate with the energy delivered to the fan turbine and the degree of fan throttling by the exhaust nozzle. The nozzle failure position is full closed so the fan will be throttled slightly more than during normal control system operation and fan speed will consequently seek a steady-state value well within its maximum speed capability.
Augmentor fuel flow control of such engines is also typically powered by the previously mentioned engine electrical supply means which may, for example, comprise an engine-driven alternator. The augmentor fuel control system is normally designed to establish the minimum level of augmentor fuel flow in the event of electrical power loss. This design is necessary for the case in which electrical power loss occurs while operating in the augmentation regime. However, to avoid throttling the fan into a surge condition, the system must be designed such that, at electrical power loss, augmentor fuel flow is reduced from a predetermined level toward a minimum level at a rate faster than the variable area exhaust nozzle is closed to minimum area, e.g., full closed.
The need to reduce augmentor fuel flow at a rate faster than exhaust nozzle closure creates a potential for serious fan overspeed. This overspeed potential exists during that portion of the augmentor fuel flow and exhaust nozzle area transient when the nozzle area is larger than normal for the particular augmentor fuel flow. When this condition exists, fan throttling is much less than normal and the energy available at the fan turbine is slightly higher than normal. This results in an acceleration of the fan to an excessive speed, i.e., a fan overspeed condition. Such fan overspeed conditions are highly undesirable as such conditions generally result in equipment damage and poor performance.
Accordingly, it is a general object of this invention to provide improved control means for an augmented turbofan engine.
It is another object of this invention to provide backup control means for limiting fan overspeed in augmented operation during loss of electrical power conditions.
It is another object of this invention to provide such backup control means which is lightweight and which utilizes already existing components of a primary speed control system.