ashfordeOU Thrust Required (flight-mechanics/performance/thrust-required)
Use when the task is the level flight performance analysis: the thrust required and power required curves versus airspeed, the minimum drag speed, the minimum power speed, and the minimum thrust.
Domain quick reference
- For level unaccelerated flight the lift equals the weight and the thrust required equals the drag. With the parabolic drag polar, CD = cd0 + k CL^2, and CL = 2 W / (rho V^2 S), the thrust required is T_req = 0.5 rho V^2 S cd0 + 2 k W^2 / (rho V^2 S), with weight W in newtons, wing area S in m^2, air density rho in kg/m^3, and speed V in m/s equivalent airspeed (EAS). The power required is P_req = T_req V. Compressibility is ignored; this is the usual low speed assumption.
- Minimum drag speed: V_md = sqrt((2 W / (rho S)) sqrt(k / cd0)), the speed where the parasite drag equals the induced drag. Worked: W = 650000 N, S = 122 m^2, rho = 1.225 kg/m^3 at sea level, cd0 = 0.02, k = 0.042 gives about 112.3 m/s EAS, with a thrust required of about 37.7 kN.
- Minimum power speed: V_mp = sqrt((2 W / (rho S)) sqrt(k / (3 cd0))), about 85.3 m/s EAS for the same case, where the induced drag is three times the parasite drag. V_mp = V_md / 3^(1/4).
- Maximum lift to drag ratio: (L/D)_max = 1 / (2 sqrt(cd0 k)), about 17.25 for the worked case, and the minimum thrust is T_min = W / (L/D)_max = 2 W sqrt(cd0 k), about 37.7 kN, which equals the thrust required at the minimum drag speed.
- Curve shape: the thrust required curve is U shaped. At 60 m/s the worked case needs about 71.3 kN (induced drag dominated), at 112.3 m/s about 37.7 kN, and at 140 m/s about 41.4 kN (parasite drag dominated).
- The curve is only flyable at or above the stall speed: below the stall the required lift coefficient exceeds CL_max and the points are not physical.
Workflow
- Collect the aircraft level flight inputs: the weight W in newtons, the wing area S in m^2, the air density rho in kg/m^3, and the full aircraft drag polar coefficients cd0 and k.
- Compute the characteristic points with minimum_drag_speed(...) and minimum_power_speed(...); the minimum drag speed is the reference for the curve.
- Sweep the speed range from the stall speed up to the desired cruise speed and evaluate lift_coefficient(...), drag_coefficient(...), thrust_required(...), and power_required(...) at each speed.
- Confirm the curve minimum: the thrust required at the minimum drag speed equals minimum_thrust(...), and the power required minimum sits at the minimum power speed.
- Report the curve and the characteristic values with maximum_lift_to_drag(...) as the efficiency reference.
Pitfalls
- Routing polar fitting here: estimating cd0 and k from measured points and the Oswald span efficiency belong to aerodynamics/drag-polars/drag-polar; this leaf consumes the polar coefficients, it does not fit them.
- Routing excess thrust here: rate of climb and excess power belong to climb-performance; this leaf stops at the thrust required curve and never subtracts the available thrust.
- Routing fuel here: specific air range, fuel flow, and sector fuel burn belong to specific-range; this leaf returns forces and power, not fuel.
- Routing energy here: energy height and specific excess power belong to energy-height.
- Routing installed thrust here: sea level static thrust, thrust lapse, and the thrust margin against the cruise drag belong to vehicle-design/sizing/engine-sizing; this leaf computes what the aircraft needs, not what the engine delivers.
- Routing the envelope here: the load factor envelope, stall speed boundary, and corner point belong to flight-test-operations/envelope/load-factor-envelope; that analysis lives in the n-V plane, this one in the T-V plane.
- Using mass where weight belongs: W in the formulas is the weight in newtons (mass times gravity); feeding mass in kg scales the thrust required by g.
- Mixing EAS and TAS: the speeds are equivalent airspeeds at the given density; at altitude convert the true airspeed to EAS with the density ratio before evaluating the curve.
- Using a wing-only polar: cd0 and k must cover the whole aircraft (fuselage, nacelles, interference), otherwise the thrust required is understated.
- Reading the curve below the stall: the parabolic polar points below the stall speed are not achievable in level flight; start the sweep at the stall speed.
- Ignoring the minimum power speed: for propeller driven aircraft the range and endurance speeds come from the power required curve, not the thrust required curve.
Behavior contract (gate 3)
The thrust required and power required logic is exercised by the gate 3 contract test: scripts/test_thrust_required.py against scripts/thrust_required_logic.py (stdlib unittest, offline). Run: python3 scripts/test_thrust_required.py
Compliance
- Standards referenced, not reproduced: FAR-25 and CS-25 performance requirements frame the level flight analysis for transport aeroplanes; the parabolic polar thrust and power required method is common performance methodology, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.