Aircraft regulations require designers to consider flight conditions that contribute to ice formation and ice accumulation on critical portions of the aircraft. Leading edges of wings and engine nacelles can be particularly susceptible to ice formation, and require active ice protection in many aircraft designs. With respect to aircraft engines, accumulated ice can break away from the lip of the nacelle inlet, and enter the engine. Ice entering an engine can damage an engine's fan blades, or other critical engine components. Ice formation and accumulation on a nacelle inlet lip also can restrict airflow, thereby hindering engine performance. Accordingly, ice protection systems are needed for wing leading edges and engine nacelles in general, and for nacelle inlet lips in particular.
Various systems and methods are known for minimizing and eliminating ice accumulations on critical surfaces of aircraft. For example, the airport crews commonly spray an ethylene glycol de-icing solution on accumulated ice on aircraft wings while the aircraft are on the ground preparing for departure. Alternatively, some aircraft are equipped with pneumatically actuated bladders along leading edge surfaces of their wings that can be periodically inflated to shed accumulated ice. Many jet aircraft direct hot gases from their engine compressors onto the wing or nacelle inlet leading edges to melt accreted ice. Though such hot gas systems can generally be effective, such systems are not available for aircraft that do not have jet engines, or aircraft that do not have sufficient hot air capacity for such purposes.
Another method of preventing and/or eliminating ice from aircraft leading edges employs resistance-heating elements positioned along an aircraft's leading edges. Such electrothermal systems use various types of electric heating elements that are affixed on a surface structure of an aircraft. For example, the heating elements may include metallic electrodes arranged in a serpentine pattern, and affixed to a substrate that is attached to a surface structure of an aircraft. Other similar systems use ribbons or sheets of electrically conductive material as heating elements. Such systems commonly include heating elements that are intermittently spaced along an aircraft surface in a manner such that individual heaters can be selectively energized. Because most aircraft have limited available electrical power, the individual heating elements or sets of heating elements can be sequentially energized to conserve the amount of power consumed at any one time during a heating cycle.
An airplane's airframe and engines produce varying amounts of audible noise during takeoffs and landings. For example, an aircraft's engines typically operate at or near maximum thrust as the aircraft departs from or approaches an airport. Aircraft engine fan noise can be at least partially suppressed at the engine nacelle inlet by a noise absorbing liner. Such liners are provided inside of and proximate to the nacelle inlet. These liners can convert acoustic energy into heat, and typically consist of a porous skin supported by an open-cell honeycomb matrix. The open-cell matrix provides separation between the porous skin and a non-perforated backskin. Some have postulated that the partially open cells of the liner create a Helmholtz resonant effect that absorbs sonic energy, and thereby effectively suppresses at least a portion of the generated engine noise. Government regulators often mandate aircraft engines with reduced noise signatures, and as a result, aircraft manufacturers, airline companies, and airport communities frequently demand such engines on aircraft.
Though electrothermal systems can be effective in preventing ice formation or shedding ice from various sensitive areas of aircraft, such systems generally do not provide for noise attenuation, such as is beneficial at the lip of an engine nacelle inlet. Conversely, prior art noise attenuation systems for aircraft generally do not provide ice protection for leading edges of the aircraft. Accordingly, there is a need for an electrothermal ice protection system for the leading edges of aircraft that also includes noise attenuation capability. In particular, there is a need for an electrothermal ice protection apparatus for a nacelle inlet noselip that is capable of attenuating at least some engine fan noise.