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Inlets and Nozzles: Design Considerations. EGR 4347 Analysis 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

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Inlets and nozzles design considerations

Inlets and Nozzles:Design Considerations

EGR 4347

Analysis and Design of Propulsion Systems


Subsonic inlets major design variables choices
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


Inlets and nozzles design considerations

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 total pressure ratio
Subsonic Inlets: Total Pressure Ratio


Subsonic inlets typical airflow requirements
Subsonic Inlets: Typical Airflow Requirements


Subsonic inlets total pressure distortion
Subsonic Inlets: Total Pressure Distortion




Subsonic inlets boundary layer control
Subsonic Inlets: Boundary Layer Control


Inlet integration
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
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



Gross thrust coefficient
Gross Thrust Coefficient

  • Exhaust velocity vector angularity

  • Friction in the boundary layers

  • Loss of massflow – leakage in nozzle

  • Flow nonuniformities


Nozzle integration
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?