ashfordeOU Turbofan Off-Design (propulsion/turbofan/turbofan-off-design)
Use when the task is turbofan performance away from the design point: corrected mass flow and corrected speed at the engine inlet, altitude net thrust with the ram drag penalty, SFC altitude and throttle behavior, throttle-setting sanity, and the fan and core component matching verdict.
Domain quick reference
- Corrected mass flow (SI, kg/s): m_dot_c = m_dot * sqrt(T_ref / T) / (P / P_ref), with m_dot the physical mass flow in kg/s, T the inlet total temperature in K, P the inlet total pressure in Pa, and the standard-day references T_ref = 288.15 K, P_ref = 101325 Pa. At the reference condition m_dot_c = m_dot; a hot day (T above T_ref) lowers m_dot_c at fixed physical flow, and a low inlet pressure raises it.
- Corrected speed (rpm): N_c = N * sqrt(T_ref / T), with N the physical rotor speed in rpm and T in K. At T = T_ref, N_c = N. The correction removes the temperature effect on the speed of sound so that operating points stay comparable.
- Net thrust at altitude: F_alt = F_SL * (rho / rho0) * f_mach - D_ram, with F_SL the sea-level static rating in N, rho/rho0 the density ratio, f_mach a ram and Mach recovery factor (about 1 at low speed, lower at high Mach), and D_ram the ram drag. Ram drag is the momentum of the captured air: D_ram = m_dot * V0 with V0 the flight speed in m/s; it is subtracted from gross thrust to give net thrust.
- SFC altitude and throttle behavior: SFC_alt = SFC_SL * (rho / rho0)^k with k about 0.1 to 0.2 in a quick model; SFC improves (falls) with altitude because the cold air raises the cycle efficiency, and it rises again at low throttle. At sea level static the factor is 1.
- Throttle-setting sanity: the throttle fraction of the maximum rating runs from idle (about 0.05) to max continuous (1.0); below idle the combustor is unstable, above max the rating is exceeded.
- Component matching verdict: the fan and core corrected-flow deltas from their matched operating points must both stay inside a band (default plus/minus 10 percent); a component outside the band is off-design and the operating point must be reworked.
- FAR-33 (14 CFR Part 33) sets the certification context for engine ratings and operating limitations; the corrected-parameter and scaling relations above are common off-design performance practice.
Workflow
- Correct the operating point: corrected_mass_flow and corrected_speed from the physical inlet flow, temperature, and pressure with the standard-day references.
- Scale the thrust to the flight condition with net_thrust_altitude(sea_level_thrust, rho, rho0, mach_factor, ram_drag) and compare it with the aircraft demand at that altitude.
- Apply the SFC altitude factor with sfc_altitude_factor(sfc_sea_level, rho, rho0) for the cruise fuel-flow estimate.
- Sanity-check the throttle fraction with throttle_verdict; rework the schedule if the verdict is below-idle or over-throttle.
- Judge the fan and core position with component_matching_verdict(fan_matching, core_matching); a non-matched verdict flags which component must be reworked.
- Report corrected flow, corrected speed, altitude net thrust, cruise SFC, and the matching verdict.
Pitfalls
- Mixing corrected and physical quantities: the map and matching relations use corrected flow and corrected speed, so physical values must be corrected before any comparison across operating conditions.
- Forgetting the ram drag: at cruise the ram drag is a large part of the gross thrust; scaling sea-level thrust by density alone overstates net thrust.
- Reversing the temperature ratio: the corrected flow and speed both carry sqrt(T_ref / T), never sqrt(T / T_ref); the hot-day direction is a decrease in both corrected values.
- Treating the SFC altitude exponent as a fixed value: 0.1 to 0.2 is a quick-model range, not a rated figure; the engine program supplies the measured or cycle-derived value.
- Assuming thrust scales with pressure ratio instead of density ratio: the density ratio drives the mass-flow and thrust scaling in the quick model.
- Calling the matching verdict at a single component: fan and core must both be inside the band; one in-band component does not make the point matched.
- Passing zero or negative inputs, or a throttle fraction outside the idle-to-max range; the module raises ValueError instead of returning a nonsense number.
Behavior contract (gate 3)
The off-design relations are exercised by the gate 3 contract test: scripts/test_turbofan_off_design.py against scripts/turbofan_off_design.py (stdlib unittest, offline). Run: python3 scripts/test_turbofan_off_design.py
Compliance
- Standards referenced, not reproduced: FAR-33 is US government work (public domain) and covers engine ratings and operating limitations, not off-design performance methods; the corrected-parameter and scaling relations are common engine performance methodology, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.