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Why do wing shapes vary across different types of aircraft?

Why do Wing Shapes Vary Across Different Types of Aircraft?

CHUNG Yan-wa
March 2024

How lift is generated on a wing?
Figure 1
Figure 1    Airflow around an aircraft wing.
Figure 1 illustrates the airflow around a typical aircraft wing as it is dashing on a runway, with the blue lines representing the paths of air particles as they whoosh over and under the wing during flight. This clever design of the aircraft wing lies in the curvature on the top surface, which narrows the cross-section of the wing’s upper part (line segment CD) compared to the cross-sections at the inlet and outlet of the wing (line segments AB and EF). The air passing over the top surface of the wing, due to its narrower cross-section, must flow faster than the air passing through the wider sections at inlet and outlet to maintain mass conversation. This is similar to the principle of squeezing a water hose, where water sprays out faster when the hose is constricted. The fast-moving air over the top surface of the wing creates a region of lower pressure. According to the Bernoulli's principle, the pressure difference creates lift, helping the aircraft to take off.
Tapered versus rectangular wings
Figure 2 Top view of tapered wings. Figure 3  Top view of rectangular wings.
Figure 2
Top view of tapered wings.
Figure 3
Top view of rectangular wings.

When an aircraft generates lift and flies, especially around the wingtips, small vortices are formed (see The “Tail” of an Aircraft). These vortices are actually thieves to steal the aircraft’s drive. Therefore, wing designs with gradually narrowing tapered tips (Figure 2) can reduce the wing’s surface area, resulting in fewer vortices being formed. Overall, the drag due to vortices will be reduced, making the flight more efficient. However, the production costs for tampered wings can be more expensive, which is one of the reasons why training aircraft still use the traditional rectangular wings (Figure 3).
Swept-back wings
The airflow passing over the aircraft wing, due to its specific design, can achieve a speed greater than the aircraft’s own speed. As a result, before the aircraft reaches the speed of sound, the airflow surpasses the speed of sound and generates shock waves. The immense drag caused by these shock waves decelerates the aircraft.
Figure 4
Figure 4    Top view of a swept-back wing.
Swept-back wings deflect part of the airflow over the wings to delay the formation of shock waves. From Figure 4, it can be observed that the airflow (arrow A) impacting the wing is separated into two components. Part of it crosses the wing (arrow B), while the other part flows sideways along the wing span (arrow C). This separation effectively reduces the speed of the airflow crossing the wing, delaying the formation of shock waves. Furthermore, swept-back wings help maintain the aircraft's straight flight.
Let's discuss other considerations in designing aircraft wings later.