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 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 manned 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 conquered before any heavier-than-air vehicle can climb into the air in controlled flight. Three main forces are involved: thrust, drag, and lift. Thrust (or thrust force) is produced by the propeller-, jet-, or rocket-driven engines of aircraft. 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 the viscosity of the air through which it moves. The air resistance produced by any moving vehicle's shape, particularly any asymmetry, 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 making aircraft wings very long and as narrow as possible. 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 they stay ahead of the plane. 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 the angle at which they meet the air that passes by them causes the air to flow much more rapidly past the upper airfoil surface than past its lower surface. 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, air eddies form atop airfoils and decrease the lift enough to make an aircraft stall and drop toward the ground. 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. They automatically move forward when the aircraft slows down, increasing the lift available by adding to the camber.

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 hyper-sonic (at least five times the speed of sound). Subsonic aircraft are propeller driven, 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 not compressible, so its density is unchanging 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, all 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 1920's, 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 1960's 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 1960's, 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. For example, if it takes 5,000 horsepower to fly a particular aircraft at 500 miles per hour (804 kilometers per hour), it would require 70,000 horsepower to attain an airspeed of 1,000 miles per hour ,kilometers per hour).

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 being the most important aspect, and this leads to a great many variations on the straight-wing aircraft with which most people are familiar. For example, many jet aircraft (especially fighter warplanes) have "gull wings" that extend upward at a very steep angle and bend at a chosen point to become parallel to the ground. In other cases, the airfoils are raked sharply backward to combine much better camber with advanced streamlining. 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 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 helps minimize drag and optimize lift when aircraft operate at any given thrust.

Context

Modern understanding and actualization 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, 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 middle of the nineteenth century. These early aircraft 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 (for example, 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 development of jet and then 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. 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, use of wing camber, and high-lift device enhancement. 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

Bibliography

Allen, John E. Aerodynamics: The Science of Air in Motion. New York: McGraw-Hill, 1982. This carefully crafted book discusses many aerodynamic principles, including lift, drag, and thrust. It also includes some important historical information on aviation. The text and useful diagrams clarify many issues important to the understanding of heavier-than-air flight.

Covert, Eugene E., and C. R. Jam. Thrust and Drag: Its Prediction and Verification. New York: American Institute of Aeronautics, 1985. This carefully done book discusses many topics related to thrust and drag. It also clarifies many issues important to understanding drag and denotes other issues essential to understanding most aspects of flight.

Hanle, Paul A. Bringing Aerodynamics to America. Cambridge, Mass.: MIT Press, 1982. This interesting book combines a description of the development of aerodynamics in Europe and the United States with sound exploration of the basic principles of lift, thrust, and drag, as well as numerous other important aircraft flight attributes.

Kuethe, Arnold M., and Chow Chuen-Yen. Foundations of Aerodynamics: Bases of Aerodynamic Design. New York: John Wiley & Sons, 1986. This thoughtful, complex, and well-illustrated book explains aerodynamic concepts thoroughly and carefully as well as exploring many aerodynamic design principles. It is aimed at the technically advanced individual but is worthwhile for exploration by all interested readers.

Robinson, J. Lister. Basic Fluid Dynamics. New York: McGraw-Hill, 1963. This straightforward book explains fundamental fluid mechanics. Its precepts and description of the related principles set the stage for understanding the basis of aerodynamic concepts such as drag, lift, boundary layers, laminar flow, and turbulence.

Smetana, Frederick O. Computer Assisted Analysis of Aircraft Performance and Control. New York: McGraw Hill, 1986. Covers many elements of computer analysis of aircraft operation and the application of computers to the study of aerodynamic issues.

Snow, Theodore P., and J. Michael Shull. Physics. St. Paul, Minn.: West, 1986. This college physics textbook covers many of the issues related to the properties of fluids that are so essential to aerodynamic principles. Particularly useful is chapter 11, which discusses Bernoulli's work and Mach numbers and includes a depiction of airfoil properties.

Wegener, Peter. What Makes Airplanes Fly? History, Science, and Applications. 2d ed. New York: Springer, 1997. This interesting book touches all bases, dealing with aviation history, aerodynamic theory, and airplane properties and adaptations that have yielded modern aircraft.