Method for reducing turboprop noise

An apparatus comprising:

• an aircraft, said aircraft cruising at an altitude greater than about 20,000 feet above sea level and an airspeed greater than about 200 knots true airspeed;
• a propeller for providing thrust to said aircraft;
• a gas turbine engine for providing power to said propeller, wherein said engine is a model T56, model 501 or model AE 2100;
• and
• a means for increasing the distance between said gas turbine engine and said propeller;
• wherein the propeller has at least four propeller blades, the engine has an offset inlet duct, a portion of each propeller blade passes in front of the inlet duct, and the portion has a chord length and a trailing edge, said increasing means spacing apart the trailing edge from the inlet duct such that the minimum distance from the trailing edge to the inlet duct is greater than about one fourth of the chord length and less than about the chord length.

An apparatus comprising:

• an aircraft, said aircraft cruising at an altitude greater than about 20,000 feet above sea level and an airspeed greater than about 200 knots true airspeed;
• a propeller for providing thrust to said aircraft;
• said propeller having a plurality of propeller blades;
• a gas turbine engine for providing power to said propeller, said engine having a compressor, wherein said engine is a model T56 or a model 501;
• an offset inlet duct for providing air to the compressor of said engine;
• and
• a means for increasing the distance between said inlet duct and said propeller blades;
• wherein a portion of each said propeller blade passes in front of said inlet duct, and the portion has a chord length and a trailing edge, said increasing means spacing apart the trailing edge from said inlet duct such that the minimum distance from the trailing edge to said inlet duct is greater than about one fourth of the chord length and less than about the chord length.

A method for spacing a propeller relative to an inlet for a gas turbine engine, comprising:

• determining a cruise speed for an aircraft powered by a rotating propeller, a gas turbine engine driving the propeller, the propeller having a plurality of propeller blades;
• placing the propeller such that a portion of each propeller blade passes in front of an offset inlet duct providing air to the engine;
• and
• establishing the relative spacing of the offset inlet duct and the propeller, such that the spacing from the offset inlet duct to the trailing edge of the portion of the propeller blades is greater than about one fourth of the chord length of the portion, and less than about the chord length of the portion.

A method for placing a propeller relative to an inlet for a gas turbine engine, comprising:

• flying an aircraft at a cruising speed, the aircraft being powered by a propeller, the propeller being driven by a gas turbine engine, the propeller having a plurality of propeller blades;
• providing air to the compressor of the gas turbine engine through an offset inlet duct;
• rotating the propeller such that a portion of a propeller blade passes in front of of the offset duct;
• and
• positioning the offset inlet duct relative to the propeller such that the minimum distance from the offset inlet duct to the trailing edge of the portion of the propeller blades is greater than about one fourth of the chord length of the portion, and less than about the chord length of the portion.

An apparatus comprising:

• an aircraft propelled by a propeller, said aircraft cruising at an altitude greater than about 20,000 feet above sea level and an airspeed greater than about 250 knots true airspeed;
• a gas turbine engine for providing power to said propeller;
• an inlet duct having flowing through it a total airflow;
• an engine duct in fluid communication with said inlet duct and with said gas turbine engine, said engine duct flowing more than about fourty percent of the total airflow and less than about sixty percent of the total airflow;
• and
• a scavenge duct in fluid communication with said inlet duct, said scavenge duct flowing a portion of the total airflow.

A method for flowing air in an inlet duct of an aircraft, comprising:

• passing a plurality of propeller blades in front of an offset inlet duct;
• flowing a total airflow into the inlet duct;
• splitting the offset inlet duct into an engine duct for providing air to a gas turbine engine and at least one other duct;
• and
• flowing more than about fourty percent of the total airflow and less than about sixty percent of the total airflow through the engine duct.