AE 451 Aeronautical Engineering Design IPropulsion and Fuel System Integration

Prof. Dr. Serkan Özgen

Dept. Aerospace Engineering

December 2017

Propulsion system options

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Propulsion system options

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Jet engine integration

• Engine dimensions:

L=Lnominal(SF)0.4

D=Dnominal(SF)0.5

W=Wnominal(SF)1.1

SF =Treq/Tnominal

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Turbofan engine dimensions

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Inlet geometry

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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.

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Inlet geometry

• There are four basic types of inlets:– NACA inlet: reduced wetted surface area and weight but poor in

pressure recovery.

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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.

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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 betterpressure recovery but has higher cowl drag and mechanically morecomplex. Suitable for M>2.0.

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Inlet geometry• Ramp inlet: Uses the shock pattern produced by a wedge. Suitable for

M<2.0.

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Inlet location

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Inlet location

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Inlet location

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Inlet location

• Inlet must not be placed where it can ingest a vortex off thefuselage or separated wake from the wing, the inlet flowdistortion can stall the engine.

• The nose location offers the inlet a completely clean airflow, but requires a very long internal duct, which is heavy withhigh losses, requires high volume.

• The chin inlet has the advantage of a nose inlet with a shorterduct. It is particularly good at high α because the fuselageforebody helps to turn the flow into it.

• Nose landing gear must not be placed ahead of the inlet.

• Another problem is foreign object ingestion.

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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.

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Capture area calculation

• Multiply capture area/mass flow by mass flow of the engine.

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Propeller engine integration

• Propeller size:

• Function of propeller: take shaft power from the engine andconvert it into thrust power. Propellers achieve this with someinevitable losses.

𝜂𝑝 =𝑇𝑉∞𝑃

< 1

• Propeller efficiency ↗ as diameter of propeller ↗ due to loweraccelaration through the disk. The larger the propeller, thehigher the mass of air processed by it.

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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 higherthe propulsive efficiency.

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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, otherwisecompressibility 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 increasingthe diameter and/or increasing the number of blades.

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Propeller tip speed

𝑉𝑡𝑖𝑝,𝑠𝑡𝑎𝑡𝑖𝑐 = 𝜋𝑁𝑑, 𝑁: 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑠𝑝𝑒𝑒𝑑, 𝑑: 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝑝𝑟𝑜𝑝𝑒𝑙𝑙𝑒𝑟

𝑉𝑡𝑖𝑝,ℎ𝑒𝑙𝑖𝑐𝑎𝑙 = 𝑉𝑡𝑖𝑝,𝑠𝑡𝑎𝑡𝑖𝑐2 + 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, 𝑉𝑡𝑖𝑝,ℎ𝑒𝑙𝑖𝑐𝑎𝑙 < 700 𝑓𝑡/𝑠 during takeoff.

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Blade diameter

• Two blades: 𝑑 = 1.74 ℎ𝑝,

• Three blades: 𝑑 = 1.64 ℎ𝑝

• Four + blades: 𝑑 = 1.54 ℎ𝑝

• The propeller diameters from the two approaches arecompared and the smaller of the two is selected.

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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.

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Propeller location

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Propeller location

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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 efficiencybecause 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 propellerclearance during takeoff or landing.

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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 toasymmetrical thrust enlarging the size of the vertical tail and the rudder.

• The propeller tip must be at lesat 9″ of the ground at allattitudes.

• The crew compartment should not be located within ±5o of thepropeller disc.

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Propeller location

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Propeller location

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Engine size estimation

• An existing engine can be scaled using scaling equations:

• 𝑋𝑠𝑐𝑎𝑙𝑒𝑑 = 𝑋𝑛𝑜𝑚𝑖𝑛𝑎𝑙𝑆𝐹𝑏 , 𝑋: 𝑤𝑒𝑖𝑔ℎ𝑡, 𝑙𝑒𝑛𝑔𝑡ℎ, 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟.

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Engine size estimation

• Alternatively, statistical models can be used.

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In-line engine

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Radial engine

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Fuel system

• Fuel system includes:

– Fuel tanks only components effecting overall aircraft layout.

– Fuel lines.

– Fuel pumps.

– Fuel management controls.

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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 thestructure.

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 twofuselage bulkheads.

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Fuel system

• The required volume of the fuel tanks is based on total requiredfuel calculated during initial sizing.

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Fuel system

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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 forintegral fuselage tanks.

– 77 % for wing and 83 % for fuselage of the volume is usable in bladdertanks.

– Allow for 3-5% extra volume to account for fuel expansion on hot days.

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