Induced Drag

What is Induced Drag?

Induced drag is the aerodynamic resistance that results from generating lift. When a wing produces lift, air flows from the high-pressure area beneath the wing to the low-pressure area above it, especially around the wingtips. This creates wingtip vortices and downwash, tilting the lift vector rearward and generating a drag component known as induced drag.

How is Induced Drag Created?

• Induced drag is a direct byproduct of lift.
• The stronger the lift (higher angle of attack or heavier aircraft), the greater the induced drag.
• Wingtip vortices are the main culprit, as they cause air to spiral behind the wing, increasing downwash and shifting the total reaction force rearward.

Induced Drag vs Parasite Drag

• Induced drag dominates at low speeds and high angles of attack.
• Parasite drag (form, skin friction, interference) dominates at high speeds and is unrelated to lift.
• As speed increases, induced drag decreases rapidly, while parasite drag increases with the square of speed.

Induced Drag and Wing Design

• High aspect ratio wings (long and slender) produce less induced drag than low aspect ratio wings (short and stubby), as they reduce the strength of wingtip vortices.
• Devices like winglets, wing fences, and tip tanks help minimize induced drag by disrupting vortex formation.

Graphs and Formulas

• On the CL-CD (aeroplane polar) graph, induced drag increases with the square of the lift coefficient (CL).
• The total drag coefficient (CD) is given by: CD = CPD + kCL², where CPD is the parasite drag coefficient and k is a constant related to wing shape and aspect ratio.
• The induced drag coefficient is approximately proportional to CL² divided by aspect ratio.

Speed and Induced Drag

• At low speeds, induced drag is high because a large CL is needed to maintain lift.
• As speed increases, required CL drops, and induced drag falls sharply.
• The minimum total drag occurs at the speed where induced and parasite drag are equal (minimum drag speed, VMD).

Mass and Induced Drag

• Heavier aircraft require more lift at a given IAS, resulting in higher induced drag.

Induced Drag Symptoms and Causes

• Noticeable power requirement increase during slow flight, approach, or climb.
• Greater sensitivity to changes in angle of attack near stall.

Reducing Induced Drag

• Fly at higher speeds (lower angle of attack) when possible.
• Use high aspect ratio wings or wingtip devices.
• Take advantage of ground effect during takeoff and landing.