Wing profile
In aeronautics, wing profile, airfoil or simply profile, refers to the shape of the cross-sectional area of an element, which when moving through the air it is capable of creating around itself a pressure distribution that generates lift.
It is one of the most important considerations in the design of lifting surfaces such as wings, or other similar bodies such as turbine or compressor blades, propeller or rotor blades in helicopters, and stabilizers.
Depending on the purpose pursued in the design, the profiles can be thinner or thicker, curved or polygonal, symmetrical or not, and even the profile can vary along the wing.
However, this concept is not limited only to aircraft, since every object has a characteristic profile, whose shape can:
- To present greater or less resistance to progress in a fluid; therefore, a greater or lesser ease of movement in that fluid.
- Generate dynamic forces on the same, of greater or less intensity in conjunction with the displacement of the object in the fluid in which it is located.
Basic notions
When immersing a blunt body within a fluid current, there is always a force that pushes the submerged body. Let's imagine that we vertically introduce a plank of wood into a river. The profile in this case will be a rectangle, which is the section of the plank. We will observe that the force that drags said plank downstream is small when we face the narrower face to the current, and the drag is great if we face the current with the wider side. This force that pushes in the direction of the current is called resistance or drag. We observe that this drag varies as we rotate the plank with respect to a longitudinal axis, that is, as we vary the angle formed by the section of the plank with the direction of the current. This angle is called the angle of attack.
When the fluid current impinges on the plank with a certain angle of attack, in addition to the aforementioned drag force, another force appears that does not have the direction and sense of the current, but a direction perpendicular to it. This force perpendicular to the direction of the current, which also depends on the angle of attack, is called lift and can be many times greater than the drag force. In applications where we want a smooth stream to "push" with the greatest possible force to a solid, this solid shall be designed to have the proper shape and angle of attack to achieve maximum lift and the least possible drag. The shape of the airfoil substantially influences the lift and drag forces that will appear. The plank in the example, with a rectangular profile, proves to be inefficient from an aerodynamic point of view, since effective profiles normally have much less drag and enormous lift. To do this, they usually have the area facing the current rounded (leading edge), and the opposite area sharpened (trailing edge or trailing edge). >).
Usually the aerodynamic characteristics of an airfoil are found by testing airfoil models in an aerodynamic tunnel (also called wind tunnel) or in a tunnel or canal hydrodynamic. In them, the lift and drag are measured by varying the angle of attack and the conditions of the fluid current (usually the speed of this), and they are taken to some graphs of the characteristics of the profile.
The first models of profiles tested in wind tunnels arose from sections of frozen fish. Since the middle of the s. XX there are important published catalogs that define the geometry of a profile and its aerodynamic curves. During the First World War, the tests carried out in Gottingen contributed to the design of the first modern profiles, until after the Second World War, the National Aeronautics Committee (NACA), predecessor of the current one, took over in the United States. NASA, which has developed most of the profiles used today. However, the aerodynamic characteristics of some profiles used in military aviation remain top secret.
Parts and regions of a profile
- Deletion of attack (leading edge).- It's the front of the elar profile. It is called a “battle of attack” because it is the first part that takes contact with the air current, causing it to bifurcate towards the intrados and the extrados.
- Clear exit (trailing edge).- Also called “running ground.” It corresponds to the point where air currents from the intrados and extrados converge and leave the profile. Although in most graphics it is drawn sharply, it is not always like that, having in some cases a square finish.
- Intradós (lower surface).- Generic term that denotes the inner part of a structure. In a surface profile corresponds to the lower part of it.
- Extrados (upper surface).- Also called “transdos”, is a generic term that denotes the outside part of a structure. In a surface profile corresponds to the top of it.
- Maximum curvature region.- Area of a surface profile between the absciss (axis X) of the starting point of the attack edge and the absciss of the maximum curvature.
- Region of maximum thickness.- Area of a surface profile between the absciss of the starting point of the attack edge and the absciss of the maximum thickness.
Geometric parameters of the profiles
The geometric characteristics of a profile have a great impact on its aerodynamic characteristics. These can be listed as follows:
1. Leading edge radius.- Defines the shape of the leading edge and is a value that significantly influences the stall. Geometrically it corresponds to the radius of a circle drawn as follows:
- It must be tangent to both the intrados and the extrados.
- Its center must be located in a tangent to the origin of the middle curvature line
Its length is measured in % of the value of the chord, oscillating between values:
- Very small (Next to 0).- It generates a fairly sharp (damaged) attack edge, which can cause early detachment of the limit layer. Ideal for supersonic flight.
- 2 % of the rope.- It generates a more obtuse (down) attack edge.
2. Chord (chord).- Corresponds to the straight line that joins the leading edge and the trailing edge. Its value is a particular characteristic of any profile.
3. Mean camber line (mean camber line).- It is an equidistant line between the extrados and the intrados. Define the curvature of the profile as follows:
- If this falls above the rope (as in the figure), it is said that the profile has positive curvature
- If this falls below the rope, it is said that the profile has negative curvature.
- If this falls above and also below the rope, it is said that the profile has a double curvature.
4. Maximum camber (maximum camber).- Corresponds to the maximum distance between the line of average curvature and the chord. The value of its ordinate and the position of this ordinate is usually expressed in % of the chord length. A typical value for this is 4% of the chord.
5. Thickness (thickness).- The thickness is a segment drawn from a referential point of the profile. There are two ways to express this concept, as shown in the figure:
- American Convention.- The thickness is traced perpendicular to the middle curvature line.
- British Convention.- The thickness is traced perpendicular to the string line.
These two forms result in two segments of different lengths.
6. Maximum thickness (maximum thickness).- Corresponds to the maximum possible length of the thickness of a surface profile.
The value of its ordinate and abscissa as position value is generally expressed in % of the length of the chord, oscillating between the following values:
- Ordered equal to 3 % of the rope, for very thin profiles (supersonic flight).
- Typical values: Sorted equal to 12 % and abscised equal to 30% of the rope.
- Ordained equal to 18 % of the rope, for thick profiles (low speed flight).
Classification of profiles
- According to form:
- Asymmetric (with curvature)
- Symmetrical
- According to its characteristics:
- Laminar flow (perfiles designed to maximize the percentage of laminar flow in the limit layer)
- High-sustainment (profiles with comparatively high support coefficients)
- Autostables (profiles designed to generate a neutral or approximately neutral angular moment)
- Supercritics (they are optimized to minimize the scale of shock waves generated by wing at transonic speeds)
- STOL (short take off and landing = Deployment and landing short, have slats (a portion of the attack edge that deploys to redirect the air to the upper surface of the wing) usually fixed, and flaps usually placed under the exit edge, as separate wings with a string of a fraction of the main wing. This results in a drastically higher loss angle than that of a common profile, and therefore in a drastically lower loss rate on aircraft using them)
- According to design orientation towards a range of operating speeds:
- Subsonics
- Transonics
- Supersonics
Other data
- Viscosity: property of the fluids by which they present resistance to deformation speed.
- Limit layer: distance from the surface of the profile, to the point where the speed is identical to that of the air-free current.
- Laminar cap: considered the profile of a plane, when the air movement is performed in an orderly way, in parallel layers, we obtain a laminar circulation and therefore a laminar limit layer.
- Turbulent cap: in it the movement of particles is not in the form of parallel layers, being chaotic, passing the air molecules from one layer to another moving in all directions.
- Attack angle: can be positive, negative or neutral.
- Aerodynamic force: it is the result of the conjunction of forces acting on the profile. By breaking down this force on the flight direction, it gives the "L" (perpendicular force to the open air current) and the "D" resistance (force parallel to the open air current).
- NACA ratings.
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