Engine Airframe Integration

Engine-Airframe Integration (propulsion/engine-airframe/engine-airframe-integration)

Use when the task is the interface discipline between the engine and the airframe: translating the uninstalled (cycle) thrust of the propulsion family leaves into installed thrust at the airframe, and accounting for the installation drag and power off-takes that the aircraft performance model must carry.

Domain quick reference

Units are SI: mass flow in kg/s, velocity in m/s, thrust and drag in N, power in W, area in m^2, density in kg/m^3.

• Uninstalled gross thrust Fg = mdot_e*Vj + (Pe - P0)*Ae: the nozzle exit momentum plus the pressure term; mdot_e is the exhaust flow (captured air plus fuel), Vj the jet velocity.
• Intake momentum (ram) drag D_ram = mdot_0*V0: the momentum of the captured stream that the engine must supply; mdot_0 is the captured air flow, V0 the flight velocity.
• Uninstalled net thrust F_uninst = Fg - D_ram: the cycle bookkeeping carried by the turbofan-cycle and nozzle-design leaves. The ram drag is already netted here, never subtracted again.
• Nacelle drag D_nac = 0.5rhoV0^2Cd_nacA_nac: external skin friction and pressure drag on the cowl, boat-tail, and cooling flow exits, written against a reference area A_nac.
• Pylon drag D_pyl = 0.5rhoV0^2Cd_pylA_pyl: the strut that carries the nacelle off the wing or fuselage, sized on its frontal area.
• Bleed loss dF_b = mdot_b*(Vj - V0): bleed air taken from the compressor for anti-ice, pressurization, or cooling removes its specific-thrust contribution from the propulsive stream.
• Accessory loss dF_a = P_ext/V0: shaft power drawn by generators and gearboxes costs propulsive power, so roughly dF_a = P_ext/V0.
• Installed thrust F_inst = F_uninst - D_nac - D_pyl - dF_b - dF_a, and the installation loss fraction is 1 - F_inst/F_uninst.
• Thrust vector misalignment theta trims the axial component to F_axial = F_inst*cos(theta).

Worked anchor (mdot_0 = 100 kg/s, mdot_e = 102 kg/s, Vj = 600 m/s, V0 = 250 m/s, fully expanded nozzle, rho = 0.36, Cd_nac = 0.35, A_nac = 1.2 m^2, Cd_pyl = 0.30, A_pyl = 0.5 m^2, bleed 1.5 kg/s, accessory 500 kW): Fg = 61200 N, D_ram = 25000 N, F_uninst = 36200 N, D_nac = 4725 N, D_pyl = 1687.5 N, dF_b = 525 N, dF_a = 2000 N, F_inst = 27262.5 N, loss fraction 24.7%.

Workflow

1. Fix the flight point and engine reference: V0, rho, captured flow mdot_0, exhaust flow mdot_e, jet velocity Vj, and the nozzle pressure term (Pe - P0)*Ae.
2. Form the uninstalled terms with gross_thrust and intake_momentum_drag, then uninstalled_net_thrust. This is the cycle result the propulsion family leaves hand over.
3. Estimate the external drag with nacelle_drag and pylon_drag at the flight dynamic pressure.
4. Estimate the power off-takes with bleed_thrust_loss and accessory_thrust_loss.
5. Sum the bookkeeping with thrust_drag_summary and read the per-term ledger plus the installation loss fraction.
6. Fold misalignment in with axial_thrust and carry the installed thrust into the aircraft performance model (installed thrust lapse, climb and cruise sizing).

Pitfalls

• Confusing installed with uninstalled thrust: the turbofan-cycle family leaves quote cycle (uninstalled) thrust; this leaf subtracts the installation losses the airframe actually feels.
• Double-counting ram drag: F = mdot*(Vj - V0) already contains the intake momentum drag subtraction, so subtracting D_ram again charges the installation twice.
• Treating the intake as a thrust source: the ramjet-inlet leaf recovers ram pressure for a ramjet, while here the intake is purely a momentum drag term on the captured stream.
• Mixing gross and net nozzle thrust: the nozzle-design leaf computes ideal gross thrust with the pressure term; the installed bookkeeping nets ram drag and external drag against it.
• Carrying the air-breathing bookkeeping into rocket staging: the rocket-staging leaf has no captured air, no ram drag, and no intake; its installation losses are thrust-structure and boat-tail drag, not intake losses.
• Netting bleed and accessory losses twice: they appear once as power off-takes here, not again as airframe drag items.
• Mixing mdot_0 (captured air) with mdot_e (exhaust, includes fuel) in the momentum terms.
• Dropping the pressure term when the nozzle is not fully expanded, or using V0^2 instead of the dynamic pressure 0.5rhoV0^2.

Behavior contract (gate 3)

The installed thrust bookkeeping is exercised by the gate 3 contract test: scripts/test_engine_airframe_integration.py against scripts/engine_airframe_integration_logic.py (stdlib unittest, offline). Run: python3 scripts/test_engine_airframe_integration.py

Compliance

• FAR-33 (engine certification) and FAR-25 (airframe certification) are referenced, not reproduced: US government work (public domain); the installation bookkeeping is common propulsion methodology, summary only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.