At the Private tier we said lift is a deal: turn air down, get pushed up. This article is about the invoice that arrives with every pound of it — because the same downwash that buys your lift also, unavoidably, charges you drag. Understanding how that bill is computed explains some of the least intuitive behavior an airplane has, including the corner of the envelope where flying slower demands more power.
Where the bill comes from
A real wing has tips, and the tips leak. Underneath the wing the pressure is higher; on top it's lower; at the tip those regions meet, and the high-pressure air curls up and around the end of the wing into the low-pressure zone. As the handbook puts it, the air curling up around the tip "combines with the downwash to form a fast-spinning trailing vortex" — one streaming from each wingtip, the same vortices that make wake turbulence a thing.
The vortex system's effect on the wing itself is what matters here: it bends the local airflow downward before and as it reaches the wing. The wing therefore flies through air that is already descending slightly — and since lift is always perpendicular to the air it's actually meeting, the whole lift force tilts aft. The tilt is small, but lift is enormous — it's carrying the airplane — so a small rearward lean of a huge vector is a real force pointing backward. That component is induced drag. It isn't friction and it isn't the price of pushing the fuselage through air; it's the geometric cost of manufacturing lift with a finite wing. No lift, no induced drag. More lift, more.
Why slow flight is expensive
Here's the counterintuitive part. At high speed, the wing meets a lot of air per second, so it only needs to deflect it gently — small angle of attack, downwash mild, lift vector nearly vertical, induced drag trivial. Slow down and the river of air thins; to keep making the same lift, the wing must turn what's left much harder. Big angle of attack, steep downwash, lift vector leaning well aft. For level flight, induced drag grows roughly with the square of how much you slow down — halve your speed and this one drag species roughly quadruples.
Total drag is this curve added to its mirror image: parasite drag (skin friction, form, and interference drag) grows with speed squared, induced drag shrinks with it. The sum is the familiar U-shaped drag curve with a sweet spot in the middle — best glide lives there — and a rising wall on both sides.
The back side of the power curve
The left wall is where the physics starts charging like a casino. Slow below best glide and total drag starts growing as speed decreases. Slow a little further — past the bottom of the power-required curve, down around the speed for maximum endurance — and the throttle's logic inverts outright: holding altitude at a slower speed now requires more power, not less. Slower still, more power still. Instructors call it the back side of the power curve, or the region of reversed command.
For instrument pilots this is not trivia. A draggy, low-speed configuration on a coupled approach, a level-off flown ten knots slow, an icing-thickened wing quietly demanding a higher angle of attack for the same lift — all of these push you toward the region where the airplane responds to sloppiness by sinking, and the correct response to sinking is nose down and power, not the instinctive pull. The airplane isn't misbehaving; it's presenting the induced-drag bill.
Reading the bill elsewhere
Once you know what induced drag is, you see its fingerprints all over aviation. Gliders and airliners wear long, slender wings — high aspect ratio — because spreading the same lift across more span means gentler downwash and smaller tip losses; winglets fake extra span for the same reason. Heavy airplanes generate more of it at any speed, which is why weight stretches takeoff rolls beyond what mere heft explains. And in the flare, the ground itself interferes with the downwash and vortex system, briefly slashing the bill — that's Ground Effect: The Float, Explained, the float, seen from one tier up.
The wing never works for free. It works on credit, and induced drag is the interest — modest when you're fast, usurious when you're slow, and always, always collected.