Private Pilot · Unit 2 · 25 min

Drag and the Drag Curve

The lift lesson promised that drag would get its own treatment. This lesson covers the two families of drag, why one grows with speed while the other shrinks, the curve their sum traces, and the reduction in drag near the runway that shapes the last few feet of every landing.

Two kinds of drag

Drag is the force that resists the airplane's movement through the air, acting parallel to the relative wind. Everything about the airplane contributes to it, but every contribution belongs to one of two families. Parasite drag is the resistance of simply pushing an object through the air; it has nothing to do with producing lift, which is how it earned the name. Induced drag is the penalty of producing lift: it exists whenever the wing is producing lift, and it cannot be separated from the lift that causes it.

PHAK Ch 5, "Drag"

Parasite drag

Parasite drag divides into three types, and each one names something a designer, a mechanic, or you can influence:

• Form drag comes from the airplane's shape and the airflow around it. Air separates to pass a body and rejoins behind it; the more turbulent that rejoining, the more drag. It is the reason a streamlined fairing has a fraction of the drag of a flat plate of the same frontal area, and the reason cowlings and wheel pants are shaped the way they are.
• Interference drag arises where two airstreams meet, such as the junction of wing and fuselage. The merged flow is more turbulent than either stream alone, and the effect is worst where surfaces meet at perpendicular angles. Fairings over these junctions exist to ease the intersection.
• Skin friction drag is the resistance of air rubbing along the airplane's surface, which under a microscope is rough no matter how smooth it looks. Designers counter it with flush rivets and smooth finishes. You counter it by keeping the airplane clean and waxed; dirt on the skin is drag you can remove with a rag.

One behavior unites the whole family: parasite drag grows with the square of airspeed. Double your speed and the airplane pushes through four times the parasite drag. This is the drag that dominates when you fly fast.

PHAK Ch 5, "Parasite Drag"

Induced drag and wingtip vortices

Induced drag originates at the wingtips. A lifting wing carries higher pressure on its lower surface than on its upper surface, and at the tip the two surfaces meet, so the pressure difference drives a flow there: air moves outward along the lower surface, around the tip, and inward toward the low pressure on top. Combined with the airplane's forward motion, this circulation rolls the air into a wingtip vortex trailing behind each tip, rotating counterclockwise off the right tip and clockwise off the left when viewed from behind.

The vortices strengthen the downwash behind the wing. Recall that lift acts perpendicular to the relative wind: the added downwash tilts the local relative wind at the wing slightly downward, so the lift vector, remaining perpendicular to it, leans slightly aft. That rearward component of lift is induced drag. Energy also goes into the rotation of the vortices themselves, and that energy loss appears as additional drag.

Induced drag grows with angle of attack. Fly slowly and the wing needs a high angle of attack to make enough lift, the pressure difference across the tip grows, the vortices strengthen, and induced drag rises. The relationship is the inverse of parasite drag's: induced drag varies inversely with the square of airspeed. Halve your speed and induced drag roughly quadruples. This is the drag that dominates when you fly slow.

PHAK Ch 5, "Induced Drag" and "Wingtip Vortices"

The drag curve

Adding the two families together across the speed range produces the chart below. At low speed, induced drag is by far the larger component. At high speed, parasite drag is. Between the two extremes, their sum, the total drag, reaches a minimum.

The minimum has a name. The lift-to-drag ratio (L/D) measures the wing's efficiency: how much lift it produces for the drag it costs. The ratio peaks at one specific angle of attack, written L/D_max, and flying there produces the required lift for the least possible total drag. You saw this on the coefficient chart in How a Wing Generates Lift: the green L/D curve peaks at a single angle of attack. Fly slower than that point and induced drag rises; fly faster and parasite drag does.

L/D_max has a direct application. The best glide speed printed in your handbook is the airspeed that flies the wing at L/D_max: minimum drag, and the farthest forward travel for every foot of altitude spent. When an engine failure has you gliding, that number comes straight off the drag curve.

PHAK Ch 5, "Lift/Drag Ratio"

Ground effect

Induced drag also depends on the wing's height above the surface. Within about one wingspan of the ground, the surface restricts the airflow pattern around the wing: the downwash and the wingtip vortices have less room to develop, and weaker vortices mean less induced drag. This is ground effect, and it strengthens rapidly as the wing gets lower: at one full wingspan above the surface the reduction in induced drag is about 1.4 percent, at one quarter of a span it is 23.5 percent, and at one tenth of a span it is 47.6 percent.

You will feel ground effect in both directions. Landing, the airplane descends into it: drag falls just as you flare, the airplane floats, and excess approach speed turns into runway consumed. Taking off, the airplane climbs out of it: an airplane lifted off early in ground effect may settle back to the runway when the full induced drag returns a few feet up, before it has the speed to climb without the cushion. Both behaviors have the same cause: induced drag returning as the wing leaves the cushion.

PHAK Ch 5, "Ground Effect"

Check your understanding

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