AE 451 Aeronautical Engineering Design I Propulsion and Fuel System Integration Prof. Dr. Serkan Özgen Dept. Aerospace Engineering December 2017
Propulsion system options 2
Propulsion system options 3
Jet engine integration Engine dimensions: L=L nominal (SF) 0.4 D=D nominal (SF) 0.5 W=W nominal (SF) 1.1 SF =T req /T nominal 4
Turbofan engine dimensions 5
Inlet geometry 6
Inlet geometry An inlet must produce: A high pressure recovery (1% loss in inlet pressure recovery results in 1.3% loss in thrust). Deceleration so that the Mach # at the engine entrance is 0.4-0.5. Low drag. 7
Inlet geometry There are four basic types of inlets: NACA inlet: reduced wetted surface area and weight but poor in pressure recovery. 8
Inlet geometry Pitot inlet: works well subsonically and fairly well at low supersonic speeds. However, the normal shock produced will reduce pressure recovery so it is not suitable for prolonged operation above M=1.4. 9
Inlet geometry Conical (spike or round) intake: exploits shock patterns created by the supersonic flow over a cone. The spike inlet is lighter and has slightly better pressure recovery but has higher cowl drag and mechanically more complex. Suitable for M>2.0. 10
Inlet geometry Ramp inlet: Uses the shock pattern produced by a wedge. Suitable for M<2.0. 11
Inlet location 12
Inlet location 13
Inlet location 14
Inlet location Inlet must not be placed where it can ingest a vortex off the fuselage or separated wake from the wing, the inlet flow distortion can stall the engine. The nose location offers the inlet a completely clean airflow, but requires a very long internal duct, which is heavy with high losses, requires high volume. The chin inlet has the advantage of a nose inlet with a shorter duct. It is particularly good at high α because the fuselage forebody helps to turn the flow into it. Nose landing gear must not be placed ahead of the inlet. Another problem is foreign object ingestion. 15
Inlet location For a low bypass ratio turbofan: vertical distance of the inlet from the ground should be > 80% of inlet height. For a high bypass ratio turbofan: vertical distance of the inlet from the ground should be > 50% of inlet height. 16
Capture area calculation Multiply capture area/mass flow by mass flow of the engine. 17
Propeller engine integration Propeller size: Function of propeller: take shaft power from the engine and convert it into thrust power. Propellers achieve this with some inevitable losses. η p = TV P < 1 Propeller efficiency as diameter of propeller due to lower accelaration through the disk. The larger the propeller, the higher the mass of air processed by it. 18
Propeller engine integration For the same thrust, a larger diameter propeller requires a smaller velocity increase accross the propeller disc. Smaller the velocity increase in any propulsive device, the higher the propulsive efficiency. 19
Propeller engine integration Two constraints for the propeller: The propeller tip must clear the ground when the airplane is on the ground. Propeller tip speed should be less than the speed of sound, otherwise compressibility effects will ruin the propeller performance. At the same time, the propeller must be large enough to absorb engine power. The power absorption of the propeller is increased by increasing the diameter and/or increasing the number of blades. 20
Propeller tip speed V tip,static = πnd, N: rotational speed, d: diameter of propeller V tip,helical = 2 V tip,static + V 2, V : flight speed At sea level, the helical tip speed of a metal propeller < 950 ft/s At sea level, the helical tip speed of a wooden propeller < 850 ft/s If noise is a concern, V tip,helical < 700 ft/s during takeoff. 21
Blade diameter Two blades: d = 1.7 4 hp, Three blades: d = 1.6 4 hp Four + blades: d = 1.5 4 hp The propeller diameters from the two approaches are compared and the smaller of the two is selected. 22
Blade diameter Advantages of two blades: Less weight, Higher efficiency due to higher diameter. Advantages of three or four blades: Smaller diameter, shorter landing gear, Less severe compressibility effects, Higher efficiency due to better power absorption. 23
Propeller location 24
Propeller location 25
Propeller location Alternatives: Tractor: puts the heavy engine to the front, shortening the forebody. This allows a smaller tail area and improves stability. Also provides a ready source of cooling air and the propeller is placed in undisturbed air. Pusher: it reduces skin friction drag because the airplane flies in undisturbed air. It also reduces wetted area by shortening the fuselage. The inlow caused by the propeller accelerates the air over the fuselage, delaying flow separation. However, the pusher configuration has reduced propeller efficiency because it encounters disturbed air off the fuselage, wings, etc. Since the heavy engine is at the rear, the tail must be larger for stability. The pusher propeller may require a longer landing gear for propeller clearance during takeoff or landing. 26
Propeller location Alternatives: Wing mounted: is normally used for multi-engine designs. Reduces fuselage drag by removing the fuselage from the propeller wake. However, introduces engine-out controllability problems due to asymmetrical thrust enlarging the size of the vertical tail and the rudder. The propeller tip must be at lesat 9 of the ground at all attitudes. The crew compartment should not be located within ±5 o of the propeller disc. 27
Propeller location 28
Propeller location 29
Engine size estimation An existing engine can be scaled using scaling equations: X scaled = X nominal SF b, X: weight, length, diameter. 30
Engine size estimation Alternatively, statistical models can be used. 31
In-line engine 32
Radial engine 33
Fuel system Fuel system includes: Fuel tanks only components effecting overall aircraft layout. Fuel lines. Fuel pumps. Fuel management controls. 34
Fuel system There three types of fuel tanks: Discrete: fuel containers that are separately fabricated and mounted in the aircraft by bolts. Normally, used only for general aviation and homebuilt airplanes. Bladder: is made by stuffing a shaped rubber bag into a cavity in the structure. Available fuel volume is decreased by 10% because of rubber thickness. Self sealing. Integral: cavities within the airframe structure that are sealed to form a fuel tank. May be created simply by sealing a wing-box or cavities between two fuselage bulkheads. 35
Fuel system The required volume of the fuel tanks is based on total required fuel calculated during initial sizing. 36
Fuel system 37
Fuel system Rules of thumb: 85 % of the volume measured to the external skin surface is usable for integral wing tanks. 92 % of the volume measured to the external skin surface is usable for integral fuselage tanks. 77 % for wing and 83 % for fuselage of the volume is usable in bladder tanks. Allow for 3-5% extra volume to account for fuel expansion on hot days. 38