# Engine Airframe Integration > Use when you must account for how an engine behaves once it is installed on an airframe: compute installed thrust from uninstalled gross thrust minus intake momentum (ram) drag, nacelle and pylon drag, and bleed and accessory power extraction losses, and reconcile the thrust-drag bookkeeping convention with the airframe drag count. Produces the per-term installation loss split, the installed thrust lapse and misalignment effects, and the performance verdict that feeds aircraft sizing and FAR-25/33 certification framing. Trigger: installed thrust, installation drag, ram drag, intake momentum drag, nacelle drag, pylon drag, thrust-drag bookkeeping, engine-airframe integration. - Skill: `ashfordeou/engine-airframe-integration` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/engine-airframe-integration` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/engine-airframe-integration/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Integrations & APIs - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/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 = 0.5*rho*V0^2*Cd_nac*A_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 = 0.5*rho*V0^2*Cd_pyl*A_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 0.5*rho*V0^2. ## 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.