In level flight at a given weight the wing must always produce the same amount of lift. But the drag the aircraft has to push through changes dramatically with speed — and not in a straight line. Slow down enough and drag actually starts going up again. That U-shape is the single most important graph in aerodynamics for an exam pilot.
The two drag families
Total drag splits into two components that behave in opposite ways with airspeed.
1. Induced drag — the cost of making lift
Induced drag is the drag that results from the production of lift. It is caused by the deflection of the airflow downwards behind the wing. — Oxford ATPL Principles of Flight (2020), Ch.4
Newton's 3rd Law — the fundamental confusion cleared:
You learned "air pushes wing UP." That's correct — that's the lift force. But here's the other side of the coin: by Newton's 3rd law, if air pushes the wing UP, the wing must push air DOWN. These are the same force, just opposite perspectives:
- Air's view: "wing is pushing me down"
- Wing's view: "air is pushing me up"
Both are true simultaneously.
How this creates induced drag:
When the wing pushes air DOWN (behind the aircraft), it creates downwash in the wake. This downward-moving air affects the local airflow right at the wing. The local airflow gets tilted downward. Since lift is always perpendicular to the local airflow, when the airflow tilts downward, the lift also tilts backward (backward = toward the tail, opposite to the direction of flight). The backward component of that tilted lift is induced drag.
Why slow speed is catastrophic — and high speed is cheap:
Here's the mechanism: the angle of attack controls how hard the wing pushes air down.
- At slow speed: Wing needs a steep angle of attack to generate lift → pushes air DOWN very hard → creates strong downwash → local airflow tilts significantly → lift tilts back significantly → HUGE induced drag
- At high speed: Wing needs only a shallow angle of attack to generate the same lift → pushes air down gently → creates weak downwash → local airflow barely tilts → lift barely tilts back → tiny induced drag
This is why:
- Induced drag rises inversely with the square of airspeed (Di ∝ 1/V2).
- Halving your speed quadruples induced drag (1/2² = 1/4 means 4× as much drag). Lethal at slow speeds.
- Doubling your speed quarters it (2² = 4 means drag drops to 1/4). This is why high-speed cruise is efficient.
- Induced drag dominates at low speed where angle of attack is steep (final approach, holding, go-around, climb-out).
2. Parasite drag — the cost of pushing through air
Parasite drag is independent of lift production. It is the sum of skin friction drag, form (pressure) drag, and interference drag. — Oxford ATPL Principles of Flight (2020), Ch.4
In plain terms: even a non-lifting body has to shove air out of the way. That cost goes up as the square of speed (the "1/2 ρ V2" you see everywhere in dynamic pressure).
- Parasite drag rises with the square of airspeed (Dp ∝ V2).
- Doubling your speed quadruples parasite drag.
- Parasite drag dominates at high speed (think cruise, high-speed dash).
The total drag curve
Add them together and you get a U-shaped curve. Where the falling induced curve crosses the rising parasite curve, total drag is at its minimum.
That speed has a name: Vmd — minimum drag speed.
Why Vmd is the most important speed on the chart
At Vmd, induced drag and parasite drag are equal. This is where drag is least, and where several other "best" speeds sit very close to each other:
- Maximum endurance (jet): flown at Vmd. A jet's fuel flow is roughly proportional to thrust, and at Vmd thrust required is least — so fuel flow is least.
- Maximum range (jet): flown slightly faster than Vmd, at Vmdr — the speed where the line from the origin is tangent to the drag curve. Typically about 1.32 × Vmd.
- Best glide (engine-out): flown at Vmd. Maximum lift-to-drag ratio occurs there, giving the flattest glide angle.
- Holding speed: ICAO holding speeds are chosen near Vmd for the holding altitude.
For a propeller aircraft the picture shifts — power required is what matters, not thrust. The minimum-power speed Vmp sits about 0.76 × Vmd, slower than Vmd. So:
- Maximum endurance (prop): flown at Vmp (slower than Vmd).
- Maximum range (prop): flown at Vmd.
In a piston-engined aircraft, maximum endurance occurs at the speed for minimum power, Vmp. Maximum range occurs at the speed for minimum drag, Vmd. — Oxford ATPL Principles of Flight (2020), Ch.6
The "wrong side" of the drag curve — speed instability
Below Vmd the aeroplane sits on what instructors call the back side of the drag curve. Here the curve slopes the wrong way: as speed drops, drag rises, which slows you further, which raises drag again. This is the classic low-speed instability you see on a poorly flown approach — you have to keep adding power just to hold the speed.
Above Vmd the curve is well-behaved: a small disturbance slowing you down lowers drag, so you naturally accelerate back. That's why approach speeds for transport aircraft are always set comfortably above Vmd.
Quick check
- A speed below Vmd ⇒ more thrust to hold it (back side of the drag curve).
- A speed above Vmd ⇒ less thrust to hold it (front side, normally stable).
- Doubling speed at high speed ⇒ 4× parasite drag, induced drag almost gone.
- Halving speed at low speed ⇒ 4× induced drag, parasite drag almost gone.
- Heaviest weight ⇒ Vmd shifts to a higher speed (more lift needed ⇒ more induced drag ⇒ the curves cross further right).
Common mistakes
- Confusing Vmd with stall speed. Vmd is a drag minimum, not a lift minimum. Vmd is well above Vs for most aircraft.
- Assuming jet best-range is at Vmd. It isn't — range is a distance problem, so you fly slightly faster (Vmdr) where speed/drag ratio is best.
- Treating induced drag as a "high-speed" problem. It's the opposite — induced drag bites hardest at slow speed and high angle of attack. That's why Va is set well below the level-flight cruise speed.
- Ignoring the back-side trap during a go-around. A poorly trimmed go-around at low speed is on the wrong side of the curve — you need power now, not a small power adjustment.
Why it matters for exams and interviews
- DGCA CPL/ATPL Principles of Flight has multiple direct questions on Vmd, Vmp, and the shape of the drag curve every paper.
- Airline interview tech questions love "what speed do you hold for max endurance?" and "what side of the drag curve are you on at approach speed?".
- Real-world fuel planning and engine-out diversions both depend on knowing exactly where you are on the curve.