The size of the aircraft wing is determined based on the necessary trade-off between differing requirements at various phases of flight. Large (surface area) wings generate greater lift and minimize takeoff and landing speeds and distance. On the flip side, large wings create more drag, which is particularly detrimental to aerodynamic performance in cruise conditions.

Since commercial airliners spend most of their operating time in cruise conditions, significant emphasis is put on optimizing cruise performance. Therefore, aircraft wings are designed with necessary wing extensions (particularly slats and flaps) to optimize wing performance at different phases of flight.

Lift-to-drag ratio

Lift and drag are the two aerodynamic forces acting on an airfoil (cross-section of a wing) during flight. The lift force is perpendicular to the oncoming airflow, whereas the drag force acts opposite to the aircraft’s relative motion. At a steady-state level flight, lift force must equal the weight of the aircraft (to maintain a constant altitude), and drag force must be less than the thrust generated by the engines (to keep the aircraft moving forward).

KLM Boeing 777-200ER
Photo: Vincenzo Pace | Simple Flying

The lift-to-drag ratio (L/D) is the ratio of the lift generated at a given speed and drag incurred due to the aircraft's movement through the air. It is noteworthy that both the lift and drag forces vary with aircraft's speed, angle of attack, and altitude, among other parameters.

Get the latest aviation news straight to your inbox: Sign up for our newsletters today.

Takeoff

The takeoff speed is much lower than the aircraft's cruising speed. Therefore, a greater surface area is required to lift the aircraft off the ground. To achieve that, leading-edge slats and trailing-edge flaps are deployed. These devices increase the surface area of the wing while providing the necessary curvature to the wing.

With the flaps extended at take-off speeds, the total drag on the aircraft becomes greater. The total drag is a combination of parasite drag and lift-induced drag. While parasite drag is negligible at low speeds, lift-induced is very high.

Climb

During the climb, the airspeed and altitude increase. Flap extensions are reduced, and landing gears are retracted. Subsequently, the total drag decreases with a decrease in lift-induced drag and air density. The climb angle is controlled to achieve the necessary lift while minimizing the effects of lift-induced drag.

American Airlines Boeing 777-223(ER) N759AN (2)
Photo: Vincenzo Pace | Simple Flying.

A controlled climb angle also ensures laminar flow over the wing. As the aircraft gains altitude, the total drag is further reduced, and the aircraft speed is just shy of the cruise speed. With continuously varying parameters, lift and drag coefficients are balanced to optimize the L/D at the climb.

Cruise

Cruising at a constant altitude, the lift force generated by the wings roughly equals the aircraft’s weight. Similarly, with a constant speed, the thrust generated by the two engines equals the total drag force acting on the aircraft.

The drag at steady-state cruise conditions determines the amount of fuel the aircraft will burn. At this stage of flight, the lift-induced drag is approximately 5% of the total drag. The transonic drag is generated due to supersonic speeds on the upper side of the wing. In most modern aircraft, supercritical airfoils substantially reduce transonic drag.

EVA Air Boeing 777-35E(ER) B-16707
Photo: Vincenzo Pace | Simple Flying

Descent and Landing

During descent and approach, trailing-edge flaps are deployed to increase lift at slower speeds. This, in turn, increases the total drag, which helps in reducing the speed. The L/D during descent is similar to that during the climb. However, as the aircraft speed reduces, total drag increases. Landing gears are typically deployed minutes before touchdown to delay the added parasite drag.

What do you think about the lift-to-drag ratio at different phases of flight? Tell us in the comments section.