inlets and nozzles: design considerations

Inlets and Nozzles:Design Considerations

EGR 4347Analysis and Design of Propulsion

Systems

Subsonic Inlets:Major Design Variables/Choices• Inlet total pressure ratio and drag at cruise• Engine location on wing or fuselage• Aircraft attitude envelope• Inlet total pressure ratio and distortion envelope• Engine out windmilling airflow and drag• Integration of diffuser and fan flow path contour• Integration of external nacelle contour with thrust

reverser or accessories• Flow field interaction with nacelle, pylon or wing• Noise suppression requirements

Inlet Considerations

Proximity to Ground - FOD Proximity to nose gear - FOD Proximity to Gun and Missile Bays - Smoke Boundary layer buildup / diverter / stealth Hide compressor face - stealth Engine running during combat turn - access Vortex ingestion - strake wakes

Subsonic Inlets

Subsonic Inlets

Subsonic Inlets: Total Pressure Ratio

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Subsonic Inlets: Typical Airflow Requirements

Subsonic Inlets: Total Pressure Distortion

Subsonic Inlets: Drag

Subsonic Inlets: Diffusers

Subsonic Inlets: Boundary Layer Control

Inlet Integration

• Keep ducts as short as possible– reduces volume, reduces viscous losses– limits on turning flow without separation

• Keep offset ducts long enough to prevent separation

• Use the wing and fuselage to shield the inlet, reduce distortion

• Watch proximity to ground

Nozzle Design Considerations• Accelerate the flow to high velocity with minimum

total pressure loss• Match exit and atmospheric pressures as closely as

desired• Permit afterburner operation without affecting main

engine operation – requires variable-area nozzle• Allow for cooling of walls if necessary• Mix core and bypass streams of turbofan if necessary• Allow for thrust reversing if desired• Suppress jet noise and infrared radiation (IR) if

desired• Thrust vector control if desired

Nozzle Geometry

Gross Thrust Coefficient

• Exhaust velocity vector angularity

• Friction in the boundary layers

• Loss of massflow – leakage in nozzle

• Flow nonuniformities

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Nozzle Integration• Smooth boat tails (e.g., F-18)• Minimize interference between ...

– Nozzle and control surfaces– Nozzle and nozzle

• Pay attention to rotation geometry• Evaluate trades

– 2-D or round (axi) nozzles?– integrated into trailing edge?– shielded above? below? from the side?– thrust vectoring? reversing?