Wing
The wing , also known as wing or wing or hydrofoil for water , is a component of a vehicle whose main task is to generate dynamic lift . The function of the wing is to generate a sufficiently large force perpendicular to the direction of flow by influencing the flow around it. This force is the lift that keeps an airplane in the air or lifts a hydrofoil (or sailboat , surfboard , wakeboard, or kiteboard ) out of the water.
The wings of aircraft are usually equipped with flaps that can be used to influence the flight position, lift or drag. In large aircraft, engines are usually attached to them, and the fuel tanks are also located in the wings. The distance between the left and right wing tips is called the wingspan .
Working principle
Wings generate momentum
The prerequisite for the generation of lift by airfoils is movement in a suitable fluid (such as air or water) that has the properties of mass , viscosity and at least to a certain extent incompressibility .
Airfoils with a suitable profile and angle of attack deflect the inflowing fluid ( downwash ); this creates a force acting perpendicular to the flow. The deflection transfers an impulse to the fluid. According to Newton's first law , this change in direction of the flow downwards requires a continuously acting force. According to Newton's third law ( actio and reactio ), an equal and opposite force, the lift, acts on the wing.
Parameters of the lift force of a wing
The mass of deflected air per unit of time depends on its density, the size (area) of the wings and the flight speed: the faster the aircraft flies, the more air is deflected in the same time. The acceleration of the deflected air mass depends on the flight speed and the angle of attack of the wing.
With constant air density, wing size and constant angle of attack, the lift force is proportional to the square of the flight speed: Both the deflected air mass per unit of time and its vertical acceleration increase proportionally with the flight speed. At twice the airspeed and otherwise the same flow of air, both the amount of air accelerated downwards and its speed double. This means that the lift is quadrupled.
However, since the deflection speed is included in the square of the drive power required for this, the power required to generate lift is inversely proportional to the airspeed and the size of the wings. This means that the higher the airspeed or the larger the wings, the lower the propulsion power required for lift. (However, this is less than the total drive power required for the flight, see below).
Flow resistance of wings
The mechanism of action described above is part of the induced air resistance : it removes the energy required for this in the form of flow resistance from the flow system providing lift . In principle, this part of the induced air resistance cannot be eliminated because it physically takes into account the law of conservation of energy and momentum.
Another form of induced drag is caused by tip vortices at the wing tips: This creates a pressure balance between overpressure under the wing and underpressure above the wing. This creates a tip vortex around the longitudinal axis of the aircraft at each wing tip, the kinetic energy of which is withdrawn from the flow system that generates lift and is thus lost unused. The tip vortices can be reduced by a high aspect ratio (= ratio of the wingspan to the mean wing depth), but in principle cannot be completely eliminated with finite wings. The winglets on the wing tips of modern aircraft also serve to reduce this form of resistance by partially preventing pressure equalization across the direction of flight (and thus the formation of eddies). It should be noted that the total vortex strength of the tip vortices cannot be influenced by winglets because of Helmholz's vortex law . According to Kutta-Joukowski's theorem , a reduction in the vortex strength would also mean a reduction in the total lift of the aircraft. However, winglets can have a positive influence on the lift distribution by skillfully shifting the vortices and thus reduce the induced air resistance. It is also possible to use winglets to positively influence the flight characteristics in the lower speed range.
In addition to induced drag, other forms of drag increase the power requirement of an aircraft:
The frictional resistance on the surface of the wing brakes the aircraft by converting kinetic energy into thermal energy in the boundary layer . The frictional resistance (or shear stress resistance) depends on whether the flow is laminar or turbulent . It can be reduced by a high surface quality (smoothness) by keeping the flow more laminar, but not completely eliminating it. Riblets can also reduce frictional resistance.
The form or pressure resistance comes about because the pressure on the front and back of a body is different. Where the flow turns into turbulence - generally at the trailing edge of the wing, but e.g. B. also on the edges of landing flaps and ailerons etc. - a braking suction is created, which corresponds to the cross section of the stall. The form resistance can be minimized by a sensible choice and careful shaping of the wing profile.
The wave resistance finally comes into play in supersonic flight : Here the supersonic impact of the air particles on the front of the aircraft induces a cone-shaped propagating shock wave ( Mach cone ), which is perceptible on the ground as a sonic boom.
The flow resistance (and thus the power required to overcome it) increases with the square of the airspeed. Together with the power requirement for the generation of lift, which is inversely proportional to the flight speed, this results in a certain speed for each aircraft, depending on the design, at which - in relation to the flight time - the energy requirement for level flight is lowest. In relation to the flight route , however, the minimum energy consumption is at a significantly higher speed, since the aircraft then has to be held in the air for the same distance less long. The speed with the lowest energy consumption per route is called the cruising speed .
Stall
The angle of attack required to generate lift increases at low speeds: Since more air mass is deflected in the same period of time at higher speeds and the amount of vertical acceleration also increases, a smaller angle of deflection is sufficient to generate the same lift. Conversely, the slower the aircraft flies, the more the angle of attack must be increased.
The Coandă effect on the upper side of the wing can only ensure that the flow is present up to a certain angle of attack, which is dependent on the profile shape, surface quality and Reynolds number , which is usually around 15-20 °. Beyond this angle of attack, the flow breaks away from the surface. This causes a drastic increase in the form resistance, at the same time the greater part of the lift collapses, since the profile in this flow state can no longer effectively deflect the air flow on the upper side of the wing, but essentially only swirls it. The airspeed at which the flow breaks off due to the increased angle of attack is called stall speed or stall speed ; the resulting flight condition, in which the aircraft sags and can only be controlled to a very limited extent, is the stall . The stall speed is thus the lowest speed at which an aircraft can just stay in the air; it depends on the design and in practice ranges from approx. 20 km / h ( paraglider ) to approx. 300 km / h (fast jet aircraft without activated landing aids).
The stall speed in an aircraft depends on the weight and the load factor , i. H. from the additional acceleration that occurs, for example, when turning. In addition, the stall speed ( true air speed ) increases with lower air density. However, the indicated air speed is the same, since the mechanical instruments are also influenced to the same extent by the air density.
profile
The profile is the cross-section of the wing in the direction of flow. The shape of the profile serves on the one hand, as much as possible at lift as little flow resistance to achieve, and on the other to the greatest possible angle of attack range without stall to allow. Different profiles are used for this depending on the construction (purpose, speed range, surface load ) .
Wing layout
In the early days of aviation, the shape of the wing floor plans was based on the shape of a bird's wing, as the curved profile was initially important. Otto Lilienthal (curvature) and Hugo Junkers (profile thickness) made decisive contributions to the wing profile. Today's wings have a variety of different shapes. As a rule, they are elongated and taper in the outer area (tapering) in order to achieve a better lift distribution and thus a lower induced air resistance .
In more modern commercial aircraft, they change into so-called winglets . Due to the lower air pressure on the top of the wings, the air flows from the bottom to the top at the tips. This creates air eddies that continue, among other things, in the dreaded wake vortices. The winglets improve the distribution of the tip vortices, thus reducing the loss of energy caused by the wake vortices and making the aircraft more economical in consumption. Contrary to what is often assumed, the strength of the vortex cannot be changed at constant speed, as it is directly connected to the creation of lift. The winglets can only improve the lift distribution and thus the geometry of the vortices.
Supersonic aircraft often have delta wings , the leading edges of which are usually straight, but in extreme cases can also be curved several times, such as. B. the “ Ogival ” wing of the Concorde . Delta wings are better adapted to the effects of supersonic flight than the otherwise commonly used trapezoidal wing. When flying at supersonic speed, compression shocks occur . These are areas in which the pressure of the surrounding fluid, i.e. the air, increases sharply. Some of these shocks spread around the aircraft in a shape that matches the sweep of the wing. (The higher the desired airspeed, the more the wing has to be swept.) When flying at supersonic speed, there is a (oblique) impact on the leading edge. When flying at transsonic speed, a (vertical) impact occurs on the upper side of the wing, behind which the speed of the air flow suddenly falls into subsonic speed, which results in a reversal of some fluid mechanical effects. If you combine these different effects on one wing due to an incorrect wing configuration, they can eliminate each other. A homogeneous flow velocity onto the leading edge of the wing is obtained if this is adapted to the flow itself. The sweep reduces this speed with the cosine of the arrow angle and leads to a loss of lift. Another disadvantage is that, in addition to this normal speed, there is also a tangential component that increases accordingly. This causes the boundary layer to float towards the outer wing area. As a result, the boundary layer is thickened and the flow can detach at the wing tips. This reduces the aileron effectiveness.
In addition, a number of other shapes are possible, for example ring-shaped wings ( ring wings ), which, however, have so far only been implemented in model and experimental aircraft .
In particular in aircraft with jet propulsion (“jet aircraft”), the wings are often angled backwards in the shape of an arrow to enable supersonic flight. A number of military aircraft that were constructed in the 1960s and 1970s can use a variable geometry to adjust the sweep of their wings in flight ( swivel wings ) in order to adapt them optimally to the respective speed.
In 2008 a team of researchers (Fish / Howle / Murray) tested a wing shape in the wind tunnel, based on the model of the front fins of the humpback whale, with a wavy front edge. This enabled lift to be increased by up to 8 percent compared to an otherwise identical wing with a straight leading edge, while at the same time reducing air resistance by up to 32 percent. The angle of attack at which the stall occurred was 40 percent higher. The reason for these good performance data lies in the introduction of energy into the flow through the corrugated leading edge (similar to vortex generators ).
arrangement
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Depending on the height at which the wings are attached, aircraft are divided into low- wing aircraft (the wings sit flush with the lower edge of the fuselage), middle- wing aircraft (medium height), shoulder - wing aircraft ( flush with the upper edge of the fuselage) and high- wing aircraft (wings above the fuselage). Airplanes in which the horizontal stabilizer is arranged in front of the wing are called duck or canard airplanes, airplanes in which the horizontal stabilizer is arranged behind the wing are called hang gliders. Modern wide-body aircraft are designed as low-wing aircraft, with the two wings being connected to the fuselage via a wing center box .
Most modern aircraft have a wing half on each side of the fuselage. In the first decades of aviation , double-deckers with two wings on top of each other were common, and in isolated cases even triplane was built. Today biplanes are only built for aerobatics . There are also aircraft with only one wing, without a tail unit . These are called flying wings or tailless. Airplanes with two or more wings arranged one behind the other ( tandem arrangement ) remained a rarity. As a further variant, there is the closed wing -Tragfläche that in practice so far only in model airplanes and ultralights Sunny is used.
The wing position is roughly characterized by the shape of its front view. It can be straight , have a more or less pronounced V-position or present itself as a gull wing .
drive
Unlike the wings of the animals, which generate propulsion and lift, wings only provide lift. The propulsion must be generated by separate engines . At the beginning of aviation experiments were carried out with wings that imitate the flapping of the birds' wings and thereby generate propulsion. However, these constructions (swing planes or ornithopters ) turned out to be unsuitable for man-carrying flying and so far have only been successfully implemented in model flying .
The only practicable solution to a combination of propulsion and lift in the wing is to let the wings rotate about a vertical axis. In this case one speaks of a rotor blade (see helicopter ).
More functions
The wings of modern aircraft also fulfill a number of other functions:
- They contain large fuel tanks , e.g. T. self-sealing
- They carry a variety of flaps for control, e.g. B. ailerons , spoilers , trim tabs
- You have buoyancy aids
- Thanks to their elastic construction, the wings are also the “suspension” of the aircraft and absorb vertical forces such as air eddies
- On many large aircraft they form the suspension for the engines (mostly in the nacelles underneath)
- On some aircraft with retractable landing gear, they are used to hold the landing gear.
- In the 1920s, the German aircraft manufacturer Junkers used the wing tips ( wing roots ) to accommodate passengers
See also
literature
- D. Anderson, S. Eberhardt: How airplanes fly. Sport Aviation, February 1999.
- David Anderson, Scott Eberhardt: Understanding Flight . 2nd Edition. McGraw-Hill, New York et al. 2009, ISBN 978-0-07-162696-5 .
- GK Batchelor: An introduction to fluid mechanics. Cambridge University Press.
- H. Goldstein: Classical Mechanics. Academic Sciences, Wiesbaden.
- Ernst Götsch: Aircraft technology. Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8 .
- J. Hoffren: Quest for an improved explanation of lift. AIAA 2001-0872.
- Henk Tennekes: Hummingbirds and Jumbo Jets - The Simple Art of Flying . Birkhäuser Verlag, Basel / Boston / Berlin 1997, ISBN 3-7643-5462-3 .
- K. Weltner: flight physics. Aulis Verlag Deubner, Cologne 2001, ISBN 3-7614-2364-0 .
- R. Wodzinski: How do you explain flying in school? Attempt to analyze various explanatory models. Plus Lucis didactics, 1999.
Web links
- D. Anderson and S. Eberhardt: A Physical Description of Flight
- Holger Babinsky: Flow over aerofoils. University of Cambridge, Department of Engineering, 2003, accessed April 7, 2018 .
- Peter Junglas: Generally understandable explanation
- NASA: Beginner's Guide to Aerodynamics Here is u. a. a computer wind tunnel in which one can change the main parameters of a profile and observe the effects on the flow field and forces.
- Klaus Weltner: Two articles on Buoyancy and Bernoulli
- Rita Wodzinski: How do you explain flying in school? Attempt to analyze various explanatory models (Acrobat PDF; 288 kB)
- Representation of flow behavior on a wing
Individual evidence
- ↑ Babinsky (2003): Flow over aerofoils , YouTube: "Flow over aerofoils"
- ↑ a b A Physical Description of Flight
- ↑ Detailed description of the Concorde wing construction
- ↑ Hydrodynamic flow control in marine mammals ( Memento from October 16, 2015 in the Internet Archive )
- ^ How airplanes fly. ( Memento from May 30, 2018 in the Internet Archive )
- ^ A Physical Description of Flight. ( Memento from February 23, 2018 in the Internet Archive )
- ↑ How do you explain flying in school? (PDF; 295 kB)
