Thrust and Power Required
From drag to thrust
In unaccelerated level flight thrust equals drag and power required equals drag times true airspeed. Thrust required therefore mirrors the total drag curve; power required is that curve multiplied by speed, which shifts its minimum to a lower speed than the minimum-drag speed. This is why a propeller aircraft achieves maximum endurance at a speed below that for maximum range.
Candidates must be able to sketch both curves and identify the region of excess thrust and excess power. Excess thrust determines climb angle; excess power determines rate of climb. A question that asks for the steepest climb wants maximum excess thrust, while the fastest climb wants maximum excess power.
Climb relationships
For small climb angles, sin γ ≈ (T − D)/W and rate of climb ≈ (P_available − P_required)/W. Both thrust and power available fall with speed in different ways for jets and propellers, so the speed for best climb angle and best climb rate differ between engine types. For a propeller aircraft, maximum rate of climb occurs near the speed for minimum power required; best angle of climb near the speed for minimum thrust required.
Traps and a worked example
Traps: equating thrust with power; assuming best angle and best rate of climb occur at the same speed; and ignoring the effect of weight on climb performance.
Rule of thumb: rate of climb in feet per minute is approximately 101 × excess power in kilowatts divided by weight in kilograms, using consistent units.
Worked example: an aircraft of weight 12 000 N has 3000 N of thrust and 1800 N of drag at 60 m/s. Excess thrust is 1200 N, so sin γ = 0.1 and the climb angle is about 5.7°. Excess power is 1200 × 60 = 72 kW, giving a rate of climb of 72 000 / 12 000 = 6 m/s, about 1180 ft/min.