Army

Variable pitch turbine blades

Tunable blade mechanism maintains high aerodynamic performance and optimal thermal design for gas turbine engines operating under partial-load conditions

Energy Other

Gas turbine blades of conventional rotorcraft turboshaft engines are optimized to operate at nearly a fixed speed and a fixed incidence angle. If the operating condition of the engine changes, then the flow through the turbine may need to be guided to a more optimum direction.

(a) pitching the rotor and stator blades both by 0 deg, and (b) pitching the rotor and stator blades both by 15 deg.

One way to do this is with variable turbine nozzle geometry. But this standard method has some disadvantages including increased weight and complexity, as well as a limited operating range since the nozzle vanes can only be turned to a certain point before severe flow incidence angles disrupt the rotating blades downstream.

The Army has been researching the use of incidence tolerant blades in combination with variable speed power turbines in rotorcraft to optimize engine performance across a range of power. This is particularly important for rotorcraft as maintaining high fuel efficiency is a challenge.

A careful balance must be achieved with the fuel burn penalties associated with variable speed engine capability, and the gains made by slowing the main rotor speed substantially (to 51 percent) of take-off speed as required to maintain high propeller efficiencies at cruise flight speed.

While these previous approaches of incorporating variable nozzle vane geometry and incidence tolerant blading can increase the operating range of a turbine to some extent, further optimization and performance improvements could be achieved by articulating the rotating blades of the turbine in coordination with stator nozzle vanes. To this point, Army scientists have developed a mechanism and method to articulate the pitch angle of rotating gas turbine blades and stator vanes for variable speed applications that always maintain incidence angles optimized for maximum aerodynamic performance.

A ratcheting type of base-disk is used for each blade to fix the blade angle orientation after articulation of the blade to the desired position. (a) is a side view and (b) is an angled view.

Within this adaptable articulating blade assembly the inner portion of the blade airfoil base mating with turbine rotor disk is housed with an actuation device that is used to change the pitch angle of each rotor blade from its base. This rotation changes the geometry of the blade angle with respect to the incoming flow incidence angle.

By pitching the rotor blades in coordination with the stator nozzle vanes, the flow incidence angles can be maintained within the optimum range for improved aerodynamic performance. Potential benefits to future military and commercial aviation gas turbine engines include highly aerodynamically efficient turbine blades, reduced noise, and vibration, reduction of the need for active blade cooling and thermal barrier coatings, increased fuel efficiency, power density, and the ability to fly faster and longer.

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