This invention relates generally to aerospace vehicle structures and more particularly to materials and designs for improving ice shedding characteristics from such structures.
All aircraft include various “leading edge structures”, i.e. exposed surfaces that face the direction of flight. These surfaces include, for example, parts of the fuselage, wings, control surfaces, and powerplants.
One common type of aircraft powerplant is a turbofan engine, which includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a flow of propulsive gas. A low pressure turbine driven by the core exhaust gases drives a fan through a shaft to generate a propulsive bypass flow. The low pressure turbine also drives a low pressure compressor or “booster” which supercharges the inlet flow to the high pressure compressor.
Certain flight conditions allow for ice build up on the leading edge structures, and, in particular, the fan and booster flowpath areas of the engine. These areas include the blades, spinner cone, and static vane and fairing leading edges. The FAA requires certification testing at these flight points to demonstrate the ability to maintain engine thrust once the ice sheds from the various components and ingests into the engine.
One particular leading edge structure of interest is the engine's fan splitter. The splitter is an annular ring with an airfoil leading edge that is positioned immediately aft of the fan blades. Its function is to separate the airflow for combustion (via the booster) from the bypass airflow. It is desired for the splitter and other leading edge structures to have mechanical, chemical, and thermal properties such that ice build up and shed volume is minimized during an icing event. This in turn minimizes risk of compressor stall and compressor mechanical damage from the ingested ice.
Prior art turbofan engines have splitters made from titanium, which is known to provide favorable ice shed properties. The downside of titanium is the expense and weight when compared to conventionally treated aluminum. However, aluminum is believed to behave poorly in an aircraft icing environment. Examples of conventional treatments for aluminum include but are not limited to chemical conversion coatings and anodizing.
Leading edge structures can also be protected with known coatings that are referred to as “icephobic” or “anti-ice” coatings, for example, polyurethane paint or other organic coatings. These coatings have the effect of lowering adhesion forces between ice accretions and the protected component. While these coatings can improve ice shedding characteristics, their erosion resistance may be not adequate to protect leading edge structures from the scrubbing effect of airflows with entrained abrasive particles which are encountered in flight.