081 Principles of Flight topic guide
Induced Drag and Wingtip Vortices
Lift is a byproduct of forcing air downward, and a finite wing cannot do that job neatly. Near each wingtip, the higher pressure beneath the wing spills around into the lower pressure above it, rolling the airflow into a trailing vortex that grows stronger the harder the wing is working. Downwash and wingtip vortices are two views of the same physical process: the wing changing the momentum of the air passing over it.
That trailing vortex system costs the wing something in return, in the form of induced drag, and it leaves a genuine hazard behind the aeroplane for anyone flying through the wake. Both effects share the same driver, the lift being produced relative to the wing's span, which is why the conditions that maximise induced drag also make the wake most dangerous.
How the vortex costs the wing drag
The trailing vortices induce a small downward flow, called downwash, over the wing itself, which tilts the local airflow direction slightly nose down relative to the free stream. Because the lift vector stays perpendicular to the local airflow, not the free stream, that tilt rotates part of the lift vector rearward, and the rearward component is induced drag. It is a real force, generated purely as the geometric price of producing lift with a wing of finite span.
Induced drag depends on the square of the lift coefficient, and at a constant weight in level flight, lift itself is fixed, so induced drag becomes inversely proportional to speed squared. Halving speed at constant lift multiplies induced drag by 4, and doubling speed divides it by 4, which is the opposite behaviour to parasite drag, which rises as speed squared. Their sum is the total drag curve, and the speed at which the two are equal is the minimum drag speed, Vmd.
What changes the vortex strength and the wake hazard
Anything that raises the lift coefficient the wing must produce raises both induced drag and the vortex strength together, since both trace back to the same circulation around the wing. Weight, aspect ratio and configuration are the usual exam levers, and each has a clear, single direction of effect worth fixing in memory.
- Heavier aeroplane: more lift needed at any given speed, so more induced drag and a stronger wake.
- Higher aspect ratio: the same lift is spread over more span, so weaker tip vortices and less induced drag per unit of lift.
- Winglets: recover some of the tip vortex energy, acting in some ways like added span without the structural weight of a longer wing.
- Wake turbulence is worst behind an aeroplane that is heavy, slow and clean, flaps and gear retracted, because that combination demands the highest lift coefficient.
Worked example
Worked example: induced drag when speed is halved
Not quite. The correct answer is C.
Correct answer: C
- A. This scales induced drag linearly with the change in speed, rather than with the square of speed that actually governs it at a constant lift.
- B. This assumes induced drag depends only on the lift coefficient, forgetting that at a constant lift the drag also depends on speed squared, and speed has changed considerably here.
- C. Correct: at constant lift, induced drag is proportional to 1 divided by speed squared, so halving speed divides the speed squared term by 4, which multiplies induced drag by 4.
- D. This applies a cubed relationship, the one that links speed to power required, rather than the squared relationship that links speed to induced drag force.
Step by step
- At constant lift, induced drag is proportional to 1 divided by speed squared, since the lift coefficient, and the constants tied to the aeroplane's aspect ratio and span efficiency, do not change here.
- Halving speed divides the speed squared term in that relationship by 4.
- Dividing the denominator by 4, while the numerator stays fixed, multiplies induced drag by 4.
- So induced drag exactly quadruples, whatever its value was at 200 kt.
- Sanity check: this is exactly why induced drag dominates at low speed and why the total drag curve rises steeply again below Vmd, the reverse of parasite drag's behaviour with speed.
Common mistakes
Assuming induced drag falls as speed falls
Slower flight needs a higher lift coefficient to hold the same lift, and induced drag actually rises sharply at low speed. Confusing this with parasite drag, which does fall with speed, loses marks on any total drag curve question.
Cubing the speed ratio instead of squaring it
The cubed relationship belongs to power required, not to the induced drag force itself. Applying it to a straightforward induced drag question compounds the error and produces a distractor built for exactly this confusion.
Ignoring aspect ratio and winglets when a stem changes the aeroplane
Induced drag for a given lift also depends on aspect ratio and span efficiency, so a question that changes the type of aeroplane, not just the speed, cannot be answered from the speed relationship alone.
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Last reviewed July 2026