Drag and Lift

• Type of physical science: Drag and Lift, Aerodynamics, Fluid dynamics, Classical physics
• Field of study: Mechanics

Drag and lift are two opposing forces associated with the fluid-dynamic principles that enable heavier-than-air aircraft to move through the air. The understanding of these forces is of great importance in the design and optimization of all such aircraft.

Overview

The concepts of drag and lift are intimately associated with aerodynamics, the aspect of fluid dynamics that studies the movement of bodies through the air, and are essential in actualizing heavier-than-air flight. Without an understanding of these two concepts, which relate to the movement and safe operation of heavier-than-air aircraft, Wilbur and Orville Wright would not have been able to design and create the first crewed aircraft, a biplane with two fixed wings. If scientists’ understanding of drag and lift had not continued to evolve, later aircraft, including jets and hypersonic aircraft, would not have been actualized.

The force of gravity must be overcome by lift before any heavier-than-air vehicle can climb into the air in controlled flight. Four main forces are involved: thrust, drag, lift, and weight (gravity). Thrust (or thrust force) is produced by the aircraft’s propeller-, jet-, or rocket-driven engines. It propels an aircraft forward as long as the vessel’s overall design allows it to achieve an applied thrust that exceeds the drag (or drag force) caused by air resistance including viscosity and pressure effects. The air resistance produced by any moving vehicle’s shape, produces drag, which diminishes the vehicle’s speed. The ratio of thrust to drag can often be greatly increased by carefully streamlining an aircraft so that the drag due to shape is minimized.

The four main types of drag are friction drag, form drag, induced drag, and drag caused by shock waves. Friction drag is due to the thin layer of air that is closest to the aircraft, the boundary layer. The boundary layer flows over the surface of an aircraft, and where it encounters any irregularities in this surface, the smooth (laminar) flow of the air is replaced by an irregular airflow, called “turbulence,” that causes friction drag. Turbulence is minimized by making all aircraft surfaces as smooth as possible to maximize the laminar flow of the air in the boundary layer. Form drag occurs when the boundary layer breaks away from a moving aircraft because of the production of a great many swirling eddies. It is countered by the maximum streamlining of aircraft and by the placement of a large number of vortex generators on modern aircraft wings. These generators are tiny airfoils that stick up in rows set atop the main wing. They produce many very small disturbances in the boundary layer that act to minimize the amount of air breaking away from the aircraft. Induced drag is caused by the lift force that actually enables an aircraft to leave the ground. Inescapable swirling streams of air (each a vortex) are generated behind the aircraft. The main vortices that form behind wings produce the most induced drag, so drag’s effects are often minimized by increasing the wing’s aspect ratio (longer wings relative to their width). Drag engendered by shock waves is seen only in supersonic and hypersonic aircraft, which fly faster than the speed of sound. Shock waves, which are air-pressure disturbances, travel at the speed of sound. They do not cause drag in a subsonic aircraft because shock waves do not form at subsonic speeds. Drag induced by shock waves is minimized by the extreme streamlining of supersonic and hypersonic aircraft that includes sharply pointed noses and thin, swept-back wings.

Lift (or lift force) is the key to flight because it enables an aircraft to rise into the air. Without a source of appropriate lift, only very fast-moving ground vehicles can be created. Lift operates upward, at a right angle to the forward motion of a vehicle. It is supplied by an aircraft’s airfoils (mostly its wings but also tail assemblies). The airfoils are designed so that their shape and angle of attack cause pressure differences between the upper and lower surfaces, contributing to lift. This makes the air pressure above the airfoil much lower than the air pressure below it and produces the amount of lift needed to raise any properly designed heavier-than-air aircraft into the air.

For the airfoil to properly split the air that passes by it, the airfoil must have a rounded leading (front) edge and a much sharper trailing (rear) edge. For the airflow around an airfoil to be asymmetric and create higher air pressure beneath the airfoil than above it, the airfoil is curved (cambered) and designed to meet approaching air at an appropriate angle (the angle of attack). In addition, the airflow from above and below an airfoil must merge smoothly as the air leaves its trailing edge if an aircraft is to fly safely.

The angle of attack, which can be changed somewhat by altering the aircraft’s position in space, is very important. Angles of attack of up to 15 degrees enhance the lift produced by airfoils, enabling very fast climbs and rapid, in-flight positional modifications. Once the angle of attack becomes too steep, airflow separation occurs over the airfoil, reducing lift and potentially causing a stall. Stalls usually result in crashes unless the angle of attack is very quickly diminished to a value that ensures adequate lift.

Airfoils must be able to handle the slow flight speeds of takeoff and landing as well as the more rapid speeds of regular flight, so aircraft have assemblies called “high-lift devices.” Two important high-lift devices are flaps and slats. A flap, or hinged portion of the back of each airfoil, fits smoothly in line with the airfoil during flight but may be lowered on takeoff or landing. Such flap lowering increases aircraft camber, furnishing the extra lift needed on takeoff and slowing aircraft ground speed upon landing. Aircraft slats are hinged sections placed at the front tip of each airfoil. Slats may deploy automatically or manually and increase lift primarily by delaying airflow separation.

Aircraft are designed differently for flight at various speeds: subsonic (below the speed of sound), supersonic (at and up to two times the speed of sound), and hypersonic (at least five times the speed of sound). Subsonic aircraft may be propeller-driven or jet-powered, most supersonic aircraft are jet airplanes, and hypersonic aircraft are usually space vehicles. The speed of sound varies with the density of the air through which aircraft travel, so aerodynamics experts and aircraft designers use the ratio of the airspeed attained to the speed of sound to more accurately designate aircraft speed. This ratio is called the “Mach number.” Subsonic flight occurs below Mach 1. At these speeds, the air is only slightly compressible, though density changes are generally small, and subsonic flight is relatively simple.

At Mach 1—and above it—the air density, air pressure, and air temperature effects that occur complicate flight issues. To counter these effects, many supersonic (and hypersonic) aircraft fly at much higher altitudes than do subsonic aircraft. This allows them to take advantage of the lower air density at those altitudes. However, any high-altitude flight requires additional design changes, such as cabin pressurization, which have their own disadvantages. Originally, the main problem in attaining Mach 1 (“breaking the sound barrier”) was the very variable shock waves that throw aircraft about, cause extreme drag, and require aircraft modifications that minimize in-flight position instability.

Applications

An understanding of the forces of drag and lift must be applied to the fabrication, the movement, and the safe operation of every heavier-than-air aircraft both in conceptualization and in actualization. The first such vehicle was developed by Wilbur and Orville Wright at the very beginning of the twentieth century. It was a manned biplane that flew by virtue of the lift that was created by paired airfoils joined by braces. The Wright brothers’ and other early heavier-than-air aircraft were much slower than was desired because of the excessive drag created by the biplane airfoil arrangement. By the 1920s, biplanes had begun to be replaced by single-wing aircraft (monoplanes), which were faster and more maneuverable. More advantageous lift-drag combinations were partly responsible for the increase in speed and maneuverability. Almost all modern aircraft are monoplanes. The few biplanes still in existence are mostly antiques, often found in aircraft museums, and copies made by modern flight hobbyists.

The majority of aircraft used worldwide until the 1960s were propeller-driven, with thrust derived from a number of engines located in the vessels’ fuselages and airfoils. The main modifications made to improve monoplanes were the alteration of airfoil size, shape, and camber and the streamlining of airfoils, engines, and fuselages. These changes reduced overall drag and enhanced lift ability, yielding ever-increasing airspeeds.

Development of jet aircraft, very common by the 1960s, required many other alterations to diminish drag—especially shock-wave-induced drag—to maximize lift, and to produce acceptable airspeeds and maneuverability. A jet engine is necessary to exceed Mach 1 because the huge horsepower increases needed to drive an aircraft at such speeds are just not possible with the propeller-driven aircraft engine.

Once an optimum propulsion system has been designed to give the desired thrust, aircraft manufacturers apply a combination of appropriate fuselage and airfoil design, streamlining, added vortex generators, and numerous high-lift devices that optimize lift and overall aircraft performance. Airfoil shape is seen as important for optimizing the lift-to-drag ratios, while the wing planform is important for maintaining drag at high speeds—this leads to a great many variations on the straight-wing aircraft design with which most people are familiar. Many jet aircraft (especially fighter aircraft) have delta wings that enable the aircraft to deliver an optimal performance at supersonic speeds. In other cases, the airfoils are raked sharply backward to reduce drag at high speeds and improve aerodynamic efficiency. This maximizes the lift force obtained while minimizing drag.

Tail airfoils also vary widely as designers seek to optimize camber, lift, and flight stability. Usually, a single fuselage makes up the entire body of an aircraft, maximizing the aircraft's streamlining, optimizing the associated lift, and minimizing the inherent drag. In addition, aircraft are made as light as possible (usually through the use of light aluminum to replace much denser steel). This weight decrease is very useful in minimizing fuel use per unit traveled. It also improves fuel efficiency and flight performance at a given thrust.

Context

Modern understanding and development of heavier-than-air flight was made possible in part by Sir Isaac Newton’s seventeenth-century theory of air resistance. This theory explained the behavior of the forces that occurred between objects and the fluids in which they were immersed. Even more crucial was the work of Newton’s contemporary, the Swiss scientist Daniel Bernoulli. Bernoulli showed that the pressure of a fluid decreases as its speed increases. Their early work in fluid dynamics helped enable the actualization of heavier-than-air flight in the early twentieth century. These early flying machines were gliders flown by German aeronautical engineer Otto Lilienthal and others. The gliders were very enjoyable but were unsuited for the day-to-day commercial and warfare uses envisioned for aircraft.

However, the actualization of gliders allowed the concepts of lift and drag to be explored and refined. Several experimenters, such as American Samuel Langley, soon produced small operational models of functional powered aircraft. However, it was not until 1903 that the first full-sized, manned, motor-powered aircraft was flown, the propeller-driven biplane created by Wilbur and Orville Wright. Clumsy as it was, its appearance caused practical aviation to be viewed as a sure thing. It quickly became clear that biplanes, regardless of how they were modified, were of quite limited use because of their very low maximum airspeeds and high fuel use.

Biplane operation during World War I and careful thought about their limitations led to the realization that diminished drag could be achieved by using monoplane airfoils. These airfoils soon dominated practical aircraft design. Continued advances in aerodynamics resulted in improved means of diminishing drag and the later development of jet and rocket propulsion.

Modern aircraft development uses computer modeling techniques to assess variables and compare new designs with known aircraft. In addition, wind tunnels are used to examine the in-flight properties of aircraft models and full-sized aircraft prototypes. Developments include NASA’s testing of laminar-flow wing designs that have the potential to maintain smoother airflow, reducing drag and improving fuel efficiency by about 10 percent. Advancements also include the use of machine learning to design airfoils and predict airflow patterns more quickly and accurately than traditional computational methods. Regardless of the propulsion system used, the optimization of aircraft function will always require advances in the methodology of drag minimization to ensure both maximum lift and airspeed. This requires ever-enhanced streamlining and devices such as variable-sweep airfoils that can be adjusted to straight or swept-back positions in flight, the use of wing camber, and high-lift device enhancement. Advanced designs include morphing wings that can change camber and shape during flight to improve lift and reduce drag under varying conditions. Future aircraft needs are sure to be met by redesign after careful study of the principles of aerodynamics, especially drag and lift, followed by suitable computer models, wind-tunnel testing, and the development of other tools needed to study the modified aircraft.

Principal terms

AIRFOIL: A wing or some other device (such as a tail) that enables heavier-than-air aircraft to fly

ANGLE OF ATTACK: The angle an airfoil makes with the air flowing past it; a change in the angle of attack will increase or decrease an aircraft’s lift force

BERNOULLI PRINCIPLE: The discovery that the pressure of fluids decreases as their speeds increase, a principle essential to flight of heavier-than-air aircraft

BOUNDARY LAYER: The thin layer of the air immediately adjacent to an aircraft moving through the atmosphere

CAMBER: The curved shape that enables an airfoil to function optimally

DRAG: The aerodynamic force that counters lift, slowing airspeed and lowering the ability of aircraft to remain in flight

LIFT: The force produced by movement of air around an airfoil that enables aircraft to leave the ground and remain in flight

MACH NUMBER: The ratio of the airspeed attained to that of the speed of sound under a given set of conditions

THRUST: The force provided by propeller engines, jet engines, and rocket engines that enables forward and upward aircraft motion

WING PLANFORM: An aircraft's wing shape, as seen from above

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