aeronautics Aeronautics is the science or art involved with the study, design, and manufacturing of air flight–capable machines, and the techniques of operating aircraft and rockets within the atmosphere. The British Royal Aeronautical Society identifies ...

, the relative wind is the direction of movement of the atmosphere relative to an

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...

or an

airfoil An airfoil (American English) or aerofoil (British English) is the cross-sectional shape of an object whose motion through a gas is capable of generating significant lift, such as a wing, a sail, or the blades of propeller, rotor, or turbine. ...

. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere. Close to any point on the surface of an aircraft or airfoil, the air is moving parallel to the surface; but at a great distance from the aircraft or airfoil, the movement of the air can be represented by a single

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

. This vector is the relative wind or the ''free stream velocity vector''.Clancy, L.J. (1975), ''Aerodynamics'', Section 5.2, Pitman Publishing Limited, London The angle between the chord line of an airfoil and the relative wind defines the

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...

. The relative wind is of great importance to pilots because exceeding the critical angle of attack will result in a stall, regardless of

airspeed In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: * Indicated airspeed ("IAS"), what is read on an airspeed gauge connected to a Pitot-static system; * Calibrated a ...

In freefall

Relative wind is also used to describe the airflow relative to an object in freefall through an atmosphere, such as that of a person's body during the freefall portion of a

skydive Parachuting, including also skydiving, is a method of transiting from a high point in the atmosphere to the surface of Earth with the aid of gravity, involving the control of speed during the descent using a parachute or parachutes. For ...

or BASE jump. In a normal skydive, the vertical descent of the skydiver creates an upward relative wind. The relative wind strength increases with increased descent rate. The relative wind is directly opposite the direction of travel. When a skydiver exits a forward-moving aircraft such as an aeroplane, the relative wind emanates from the direction the aeroplane is facing due to the skydiver's initial forward (horizontal) momentum. As

aerodynamic drag In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding fl ...

gradually overcomes this forward momentum and

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

simultaneously attracts the skydiver downward, the relative wind alters proportionally into an upward (vertical) direction. This creates an arc of travel for the skydiver similar to water flowing from a low pressure hose held horizontally and creates a variation in the

angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''Side (plane geometry), sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), vertex'' of the angle. Angles formed by two ...

of the relative wind from horizontal to vertical. When exiting from a forward-moving aircraft (as distinguished from a hovering aircraft, such as a balloon or a helicopter in hover mode) during a normal belly-to-earth skydive, the skydiver must arch his body in the direction of travel which is initially horizontal. If the skydiver continues to arch, his belly will gradually alter pitch until he is belly-to-earth. This section of the jump is commonly referred to as "the hill". Relative wind differs from the wind in

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...

in that the object (''e.g..'', the skydiver) moves past the air, as opposed to the air moving past the object.

References

Aerodynamics {{aviation-stub