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Military Aircraft Thrust Reverser Application and Exhaust Nozzles
This extended section of the book can be found on the Web site www. cambridge .org/Kundu and discusses important considerations involving typical military aircraft thrust reversers (TR) and exhaust nozzles. Associated figures include the following.
Figure 10.28. Military aircraft nozzle adjustment scheme (from [19])
(a) Mechanism for nozzle adjustment
(b) Individual petal movement
Figure 10.29. Supersonic nozzle area adjustment and thrust vectoring
10.10 Propeller
Aircraft flying at speeds less than Mach 0.5 are propeller-driven, larger aircraft are powered by gas turbines, and smaller aircraft are powered by piston engines. More advanced turboprops have pushed the flight speed to more than Mach 0.7 (e. g., the Airbus A400). This book discusses conventional types of propellers that operate at a flight speed of less than or equal to Mach 0.5. After introducing the basics of propeller theory, this section concentrates on the engineering aspects of what is required by aircraft designers. References [16], [18], and [22] may be consulted
Figure 10.30. Aircraft propeller
for more details. It is recommended that certified propellers manufactured by well – known companies are used.
Propellers are twisted, wing-like blades that rotate in a plane normal to the aircraft (i. e., the flight path). The thrust generated by the propeller is the lift component produced by the propeller blades in the flight direction. It acts as a propulsive force and is not meant to lift weight unless the thrust line is vectored. It has aerofoil sections that vary from being thickest at the root to thinnest at the tip chord (Figure 10.30). In rotation, the tip experiences the highest tangential velocity.
A propeller can have from two blades to as many as seven or eight blades. Smaller aircraft have two or three blades, whereas larger aircraft can have from four to seven or eight blades. Propeller types are shown in Figure 10.31 with associated geometries and symbols used in analysis (see Section 10.10.1). The three important angles are the blade pitch angle, в; the angle subtended by the relative velocity, <p; and the angle of attack, a = (в – <p). Also shown in the figure is the effect of both coarse and fine propeller pitch, p. When a propeller is placed in front of an aircraft, it is called a tractor (Figure 10.21a); when it is placed aft, it is called a pusher (see Figure 3.47). The majority of propellers are the tractor type.
Blade pitch should match the aircraft speed, V, to keep the blade angle of attack a producing the best lift. To cope with aircraft speed changes, it is beneficial for the blade to rotate (i. e., varying the pitch) about its axis through the hub to maintain a favorable a at all speeds. This is called a variable-pitch propeller. For pitch variation, the propeller typically is kept at a constant rpm with the assistance of a governor, which is then called a constant-speed propeller. Almost all aircraft flying at higher speeds have a constant-speed, variable-pitch propeller (when
Figure 10.31. Multibladed aircraft propellers operated manually, it is в-controlled). Smaller, low-speed aircraft have a fixed pitch, which runs best at one combination of aircraft speed and propeller rpm. If the fixed pitch is intended for cruise, then at takeoff (i. e., low aircraft speed and high propeller revolution), the propeller is less efficient. Typically, aircraft designers prefer a fixed-pitch propeller matched to the climb – a condition between cruise and takeoff – to minimize the difference between the two extremes. Obviously, for highspeed performance, the propeller should match the high-speed cruise condition. Figure 10.32 shows the benefits of a constant-speed, variable-pitch propeller over the speed range.
The в-control can extend to the reversing of propeller pitch. A full reverse thrust acts as all the benefits of a TR described in Section 10.9. The pitch can be controlled to a fine pitch to produce zero thrust when an aircraft is static. This could assist an aircraft to the washout speed, especially on approach to landing.
When an engine fails (i. e., the system senses insufficient power), the pilot or the automatic sensing device elects to feather the propeller (see Figure 10.30). Feathering is changing в to 75 to 85 deg (maximum course) when the propeller slows down to zero rpm – producing a net drag and thrust (i. e., part of the propeller has thrust and the remainder has drag) of zero.
Figure 10.32. Comparison of a fixed – pitch and a constant-speed, variable – pitch propeller
Windmilling of the propeller is when the engine has no power and is free to rotate, driven by the relative air speed of the propeller when the aircraft is in flight. The в angle is in a fine position.
