aeronautics Aeronautics is the science or art involved with the study, design process, design, and manufacturing of air flight-capable machines, and the techniques of operating aircraft and rockets within the atmosphere. While the term originally referred ...

, the thickness-to-chord ratio, sometimes simply chord ratio or thickness ratio, compares the maximum vertical thickness of a wing to its chord. It is a key measure of the performance of a wing planform when it is operating at

transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and Supersonic speed, supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach numb ...

speeds. At speeds approaching the

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. More simply, the speed of sound is how fast vibrations travel. At , the speed of sound in a ...

, the effects of

Bernoulli's principle Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...

over curves on the wing and

fuselage The fuselage (; from the French language, French ''fuselé'' "spindle-shaped") is an aircraft's main body section. It holds Aircrew, crew, passengers, or cargo. In single-engine aircraft, it will usually contain an Aircraft engine, engine as wel ...

can accelerate the local flow to

supersonic Supersonic speed is the speed of an object that exceeds the speed of sound (Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...

speeds. This creates a

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

that produces a powerful form of drag known as

wave drag In aeronautics, wave drag is a component of the aerodynamic drag In fluid dynamics, drag, sometimes referred to as fluid resistance, is a force acting opposite to the direction of motion of any object moving with respect to a surrounding flu ...

, and gives rise to the concept of the

sound barrier The sound barrier or sonic barrier is the large increase in aerodynamic drag and other undesirable effects experienced by an aircraft or other object when it approaches the speed of sound. When aircraft first approached the speed of sound, th ...

. The speed at which these shocks first form, critical mach, is a function of the amount of curvature. In order to reduce wave drag, wings should have the minimum curvature possible while still generating the required amount of lift. So, the main reason for decreasing the blade section thickness to chord ratio is to delay the compressibility effect related to higher Mach numbers, delaying the onset of a shock wave formation. The natural outcome of this requirement is a wing design that is thin and wide, which has a low thickness-to-chord ratio. At lower speeds, undesirable

parasitic drag Parasitic drag, also known as profile drag, is a type of aerodynamic drag that acts on any object when the object is moving through a fluid. Parasitic drag is defined as the combination of '' form drag'' and '' skin friction drag''. It is named a ...

is largely a function of the total

surface area The surface area (symbol ''A'') of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the d ...

, which suggests using a wing with minimum chord, leading to the high

aspect ratio The aspect ratio of a geometry, geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side—the ratio of width to height, when the rectangl ...

s seen on

light aircraft A light aircraft is an aircraft that has a Maximum Takeoff Weight, maximum gross takeoff weight of or less.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 308. Aviation Supplies & Academics, 1997. Light aircraft are use ...

and

regional airliner A regional airliner, commuter airliner or feeder liner is a small airliner that is designed to fly up to 100 passengers on short-haul flights, usually feeding larger carriers' airline hubs from small markets. This class of airliners is typically ...

s. Such designs naturally have high thickness-to-chord ratios. Designing an aircraft that operates across a wide range of speeds, like a modern

airliner An airliner is a type of airplane for transporting passengers and air cargo. Such aircraft are most often operated by airlines. The modern and most common variant of the airliner is a long, tube shaped, and jet powered aircraft. The largest ...

, requires these competing needs to be carefully balanced for every aircraft design.

Swept wing A swept wing is a wing angled either backward or occasionally forward from its root rather than perpendicular to the fuselage. Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigated in Ge ...

s are a practical outcome of the desire to have a low thickness-to-chord ratio at high speeds and a lower one at lower speeds during

takeoff and landing Aircraft have different ways to take off and land. Conventional airplanes accelerate along the ground until reaching a speed that is sufficient for the airplane to takeoff and climb at a safe speed. Some airplanes can take off at low speed, this b ...

. The sweep stretches the chord as seen by the airflow, while still keeping the

wetted area In fluid dynamics, the wetted area is the surface area that interacts with the working fluid or gas. * In maritime use, the wetted area is the area of the watercraft's hull which is immersed in water. This has a direct relationship on the overal ...

of the wing to a minimum. For practical reasons, wings tend to be thickest at the root, where they meet the fuselage. For this reason, it is common for wings to taper their chord towards the tips, keeping the thickness-to-chord ratio close to constant, this also reduces

induced drag Lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, in aerodynamics, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...

at lower speeds. The

crescent wing A crescent shape (, ) is a symbol or emblem used to represent the lunar phase (as it appears in the northern hemisphere) in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hindu iconography, Hind ...

is another solution to the design to keep a relatively constant thickness-to-chord ratio.

References

Further reading