Thrust, power and level flight

What does the bottom of the drag curve mean?

In straight, steady, level flight, lift equals weight and thrust equals drag. At low speed, induced drag is high because a large lift coefficient is needed; at high speed, parasite drag rises approximately with speed squared. Their sum is the familiar U-shaped drag curve. Power required is drag × TAS, so its minimum occurs at a lower speed than minimum drag.

Balance the forces

In straight, steady, level flight, lift equals weight and thrust equals drag. At low speed, induced drag is high because a large lift coefficient is needed; at high speed, parasite drag rises approximately with speed squared. Their sum is the familiar U-shaped drag curve. Power required is drag × TAS, so its minimum occurs at a lower speed than minimum drag.

At a fixed altitude and speed, thrust above drag initially accelerates the aircraft if height is held. If speed is held, excess power can support a climb. Both descriptions spend the same energy surplus. A power-available curve must be compared with power required, not with a drag curve in newtons.

Find the intersections

Where available thrust equals required thrust, level flight can be steady. The high-speed intersection bounds maximum level speed for that thrust setting, unless Mach, IAS, temperature or structural limits intervene first. A slow-side intersection can be speed-unstable: slowing increases required power and promotes further slowing without corrective action.

With increasing altitude, available thrust or power usually falls. Maximum level IAS generally decreases while maximum TAS can initially rise and later fall, depending on the engine and aerodynamic limit. It is unsafe to infer a universal maximum-TAS trend from the IAS trend. Maximum endurance is useful for waiting; maximum range is useful when fuel must cover distance.

Can you make the performance decision?

Four original questions. Work from the stated conditions and review the reasoning.