Total Drag - ATPL 081 — Principles of Flight

The previous lesson promised a curve with a minimum, built from one drag that falls with speed and one that rises. This lesson builds it. It gathers every drag met so far into one classification, adds the last missing member, and then reads the total drag curve for the two facts that matter operationally: the speed where drag is least, and the uncomfortable behaviour of the region below that speed.

Two families of drag

Every part of an aeroplane's drag belongs to one of two families:

Induced drag (the previous lesson's subject) — the price of lift: it falls with $1/v^2$ in level flight, and would vanish if the wing stopped lifting.
Parasite drag — everything else: the drag the airframe would generate even if it produced no lift at all, and it rises with the dynamic pressure: with $v^2$ .

Total drag is simply their sum:

One naming knot to untie, because three lessons have used three words. The two-dimensional lesson called a section's friction-plus-form drag its profile drag. Parasite drag is the same idea grown to the whole aircraft: the profile drag of wing, fuselage, tail and nacelles, plus one new member that only appears when those parts are joined together. Some textbooks say parasite, others say profile or zero-lift drag for the whole-aircraft figure: the exam's split is the one above: everything is either induced or parasite. (A third family, wave drag, appears only near the speed of sound and belongs to the high-speed module.)

The members of parasite drag

Parasite drag has three members, two of them already familiar:

Friction drag — the two-dimensional lesson's skin friction, summed over the whole aircraft: air stuck to the skin shearing against the layers above it. It grows with the total wetted area, with speed, and with surface roughness: every rivet head, seam, bug and ice grain counts, which is why the same airframe measurably slows down when it gets dirty.
Form (pressure) drag — the unrecovered pressure behind anything the flow cannot close around: high pressure on the nose of a part, missing pressure in its separated wake. Its magnitude follows the shape (streamlined versus blunt), the frontal area presented to the flow, and how much of the flow separates, which is why exposed parts that cannot be retracted are faired wherever the drag saved repays the weight added.
Interference drag — the new member. Where two bodies join (wing and fuselage, pylon and wing, strut and skin) their flow fields collide: each body accelerates the air the other must then fight through, the combined adverse pressure gradients trigger separation in the corner that neither part would suffer alone, and the junction produces MORE drag than the two parts measured separately. Its magnitude follows the angle and abruptness of the junction, which is why wing roots wear smooth fillets: the curved blending panels that ease the two flows past each other.

Parasite drag and speed

Parasite drag needs no new law: it rides the dynamic pressure directly. The parasite drag coefficient of a given configuration is essentially constant (shape does not change with speed), so the drag force follows $q = \tfrac{1}{2}\rho v^2$ : double the speed, four times the parasite drag. Set that against the previous lesson's induced law (half the speed, four times the induced drag) and the two families pull the total in opposite directions. Slow flight is dominated by induced drag, fast flight by parasite drag; somewhere between the two there must be a speed where the total is least.

The total drag curve

Plot both families against indicated airspeed and add them, and the whole story appears on one graph. The induced curve falls away with $1/v^2$ ; the parasite curve climbs with $v^2$ ; their sum (the total drag curve) is high at both ends and lowest in between. The bottom of the curve is the minimum drag speed, $V_{md}$ , and it sits exactly where the two curves cross: at $V_{md}$ , induced drag equals parasite drag, each contributing half the total. This is the same flight condition the polar already marked: minimum drag in level flight is the maximum $C_L/C_D$ point, the origin tangent of the previous lesson, now located on a speed axis instead.

Mass and altitude move the curve

Two variables reshape the graph, and the exam asks for both. Gross mass works through the induced family: more weight demands more lift, and induced drag grows with $W^2$ , while parasite drag, indifferent to lift, stays put. The induced curve rises at every speed, so the total curve rises everywhere, but the growth is concentrated on the slow side, so the crossing point (and with it $V_{md}$ ) moves to a higher speed. A heavy aircraft carries more drag at every speed, and the speed where its drag is least is faster.

Pressure altitude is the subtler one, and the axis decides the answer. Against IAS, the graph does not change: indicated airspeed is dynamic pressure in disguise (the first lesson's calibration), parasite drag follows that same dynamic pressure, and at a given IAS in level flight the wing needs the same $C_L$ regardless of altitude, so both families, and the total, are unchanged. Against TAS, the whole curve shifts right: producing that dynamic pressure in thinner air requires a higher true airspeed (the IAS–TAS gap of the first lesson), so the same drags simply happen at higher true speeds. Same curve, relabelled axis.

Speed stability

The curve's two slopes behave very differently under a disturbance, and this is the operational heart of the lesson. Fly level with thrust set equal to drag (taking the set thrust as constant with speed, a good approximation for a jet) and let a gust nudge the speed without the pilot touching anything. Above the minimum drag speed the slope is positive: lose a few knots and drag falls below the set thrust, the surplus accelerates the aircraft, and the speed returns by itself. That is the speed-stable region: the right side of the curve, where cruise lives, and the aircraft does its own speed-keeping. (This same curve returns in the flight-mechanics module as the thrust required for level flight, where it is set against what the engines can deliver.)

Below the minimum drag speed the slope is reversed, and so is the behaviour. Lose a few knots there and drag rises above the set thrust; the deficit decelerates the aircraft further, which raises the drag again: a divergence that continues toward the stall unless the pilot adds thrust or lowers the nose. This is the speed-unstable region, flown routinely on the back side of the drag curve during slow approaches: a windshear, a gust decaying, or turbulence can start the slide, which is why a slow approach demands active thrust management and why the industry's stabilised-approach rules exist. Around the bottom of the curve lies a neutral band where the curve is nearly flat: a small speed change barely changes the drag, so the speed neither recovers nor diverges: it just stays where the disturbance left it.

What the exam asks

This lesson's questions live on the graph: expect to read, shift and interpret it. The overview, following the learning objectives:

State that total drag consists of parasite drag and induced drag

Describe the types of drag included in parasite drag

Describe form (pressure) drag and the factors that set its magnitude: shape, frontal area, separation

Describe interference drag and its factors: junction flows, cured by fillets

Describe friction drag and its factors: wetted area, speed, roughness

Describe the relationship between parasite drag and speed ( $\propto v^2$ )

Explain the total drag–speed graph and its constituent components

Indicate the minimum drag speed: where induced and parasite drag are equal, the best $C_L/C_D$ point

Describe the effect of gross mass on the graph: total up, $V_{md}$ to a higher speed

Describe the effect of pressure altitude on the drag–IAS graph (unchanged) and the drag–TAS graph (shifted right)

Describe speed stability from the graph, and the non-stable, neutral and stable IAS regions

Explain what happens to IAS and drag in the non-stable region when speed suddenly decreases, and why that can occur: gusts, windshear, decaying headwind on a slow approach