Equilibrium
In steady straight and level flight lift equals weight and thrust equals drag. Any imbalance produces an acceleration: excess thrust accelerates or climbs, excess lift curves the flight path upwards.
Two kinds of drag
Parasite drag grows with the square of speed — skin friction, form and interference drag. Induced drag, the price of producing lift, falls as speed increases. Their sum has a minimum, and that speed is where the aircraft flies most efficiently and glides furthest.
Excess power, not excess lift
A common misconception is that climbing requires more lift. In a steady climb lift is actually slightly less than weight; it is excess thrust — excess power — that sustains the climb.
Key points
- Steady flight: lift = weight, thrust = drag.
- Total drag is minimum where induced and parasite drag curves cross.
- Climb performance comes from excess power, not extra lift.
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