1 — Two Kinds of Drag
Drag is the force that resists the aeroplane’s motion through the air, and thrust must balance it to hold speed. It comes in two families that behave in opposite ways with speed — and understanding that opposition is the whole point of this lesson.
- Parasite drag is the resistance of simply pushing the airframe through the air. It has nothing to do with making lift, and it grows with speed.
- Induced drag is the unavoidable by-product of making lift. It is largest when the wing is working hardest — slow and at a high angle of attack — and it shrinks as you speed up.
2 — Parasite Drag
Parasite drag is everything that resists motion but produces no lift. It has three parts:
- Form (pressure) drag — the wake left by the shape of the airframe; reduced by streamlining.
- Skin-friction drag — the air rubbing along the surface, set by surface area and smoothness.
- Interference drag — extra turbulence where components meet, such as the wing–fuselage junction.
All three rise with dynamic pressure, so parasite drag grows with the square of the speed: double the speed and you roughly quadruple it. This is why parasite drag dominates at high speed and is what ultimately limits your level cruise speed.
3 — Induced Drag and Wingtip Vortices
Induced drag is the price of lift. A wing makes lift by holding higher pressure below and lower pressure above — but at the wingtips that pressure difference has an escape route. The air spills around each tip from below to above and rolls up into a wingtip vortex:
Pressure is higher below the wing and lower above it. At the tips the air spills from below up and around to the top, rolling up into two trailing wingtip vortices. These vortices tilt the lift slightly rearwards — that rearward component is induced drag. It is greatest at low speed and high angle of attack, and trails behind as wake turbulence, a hazard to following aircraft.
These trailing vortices create a downwash that tilts the wing’s lift vector slightly rearwards. That backward-leaning component is induced drag. Two consequences follow:
- Induced drag is greatest at low speed and high angle of attack — exactly when the wing is working hardest to make enough lift — and falls off quickly as you speed up.
- A longer, narrower wing (high aspect ratio) keeps the tips farther from the lift-making centre, weakening the vortices and reducing induced drag. Winglets do the same job — which is why gliders have long thin wings and many airliners wear winglets.
4 — The Total Drag Curve
Add the two together and you get the most useful graph for performance flying. Drag the speed and watch parasite drag (rising) and induced drag (falling) trade places:
The lowest point of the total curve is VₘD, the minimum-drag speed, and at that speed
parasite and induced drag are equal. VₘD matters because:
- It is (very nearly) the speed for best glide range and maximum endurance — the most air miles, and the most time, per unit of thrust or fuel.
- Below
VₘDyou are on the back of the drag curve (“region of reversed command”): to fly slower in level flight you need more power, because induced drag climbs faster than parasite drag falls. Approaches are flown here, which is why a low, slow, dragged-in approach can need a lot of power.
5 — Ground Effect and Why This Matters
Close to the ground — within about a wingspan — the surface interferes with the wingtip vortices and downwash, so induced drag falls and the wing becomes temporarily more efficient. That is ground effect. It can let an overloaded or too-slow aeroplane lift off and then sink back as it climbs out of it, and it lengthens the float in the flare.
All numbers and curves here are illustrative; your aircraft’s real best-glide and approach speeds come from its POH/AFM.
