Lift and drag in depth

The lift coefficient, the drag polar, L/D max, and the region of reversed command.

18 min read · Advanced aerodynamics

After this lesson you can

  • Name the four terms of the lift equation and say which one the pilot actually controls in flight.
  • Explain why induced drag and parasite drag move in opposite directions as speed changes, and locate L/DMAX on the curve.
  • State why L/DMAX is a fixed angle of attack but a variable airspeed, and predict how a change in weight moves it.
  • Recognize the region of reversed command and fly the correct correction (power, not pitch) when caught in it.

At the private level, lift is mostly there to explain why the airplane flies at all. At the commercial level, it becomes something you calculate, and that shift changes how you fly: you stop chasing a number on the airspeed indicator and start managing an angle of attack (AOA), with every speed in the flight manual falling out as a consequence of that AOA and the airplane's weight.

Nearly everything in this lesson orbits a single point on the drag curve: the angle of attack that produces the maximum lift-to-drag ratio, L/DMAX. It is the point of minimum total drag, the best glide angle, the maximum-range speed for a propeller airplane, and the dividing line between the two ways an airplane behaves when you pull the power back.

The part that surprises people, and that shows up on the written test, is this: L/DMAX happens at one specific angle of attack, but the airspeed at which you fly that angle of attack changes with weight. A heavier airplane glides exactly as far as a lighter one flying the same profile - it just has to do it faster.

The rest of this lesson

  1. 1.The lift equation, and what you can actually change in flight
  2. 2.Two drags that move in opposite directions
  3. 3.L/DMAX: one angle of attack, not one speed
  4. 4.Climb speed: why a propeller airplane is not a small jet
  5. 5.The region of reversed command
  6. 6.Ground effect, quantified
  7. 7.What to remember

These 7 sections, their figures, the checks along the way and the end-of-lesson quiz come with the CPL track. Paid once, yours for good.

See what the CPL track costsEvery module 1 is free to read, no account, no card.

Key terms

Try to recall each definition before turning the card.