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new section: drag
Washout can also be used to modify the lift distribution and reduce the induced drag: this is not the primary consideration of this article.
= Drag =
== Introduction ==
A classic problem in hydrodynamics is called the “d'Alembert’s Paradox”. It describes the very bizarre result that, despite every care being taken and a wide range of cases and methods considered, theoretically anything moving in a fluid will have precisely, absolutely, identically zero drag. Any child who has waved a tennis bat will be able to tell that this is complete rubbish, but for hundreds of years fluid dynamists could not tell what had gone wrong.
Until in 1904, the superstar of modern engineering mechanics, Ludwig Prandtl, published the most important paper ever in fluid mechanics: ''On the Motion of a Fluid with Very Small Viscosity'', which established the theory of boundary layers as introduced earlier. Prandtl pointed out that the old assumption that, since the object moves sufficiently fast the viscosity effects can be neglected, does not apply in the immediate vicinity of a solid surface (i.e. within a boundary layer). It is necessary to consider the viscous effects in the boundary layer to predict correctly the drag on a body.
Viscosity is both beloved and hated in the field of aeronautics. On one hand, ''no lift can be generated without viscosity'', and on the other hand, viscosity creates all the drag that causes a glider the loss of altitude. The holy grail of glider design is to minimise the drag introduced for the same amount of lift, i.e. to maximise the '''lift-to-drag ratio (L/D)''' of the aeroplane, which is identical to the glide ratio of a glider. In short, less drag means a glider goes further, higher, and longer. It is, therefore, necessary to understand the physics of drag.
An aeroplane experiences three kinds of drag, namely '''skin-friction drag''', '''form drag''', and '''induced drag'''.
== Laminar and turbulent boundary layers ==
Unfortunately, before the discussion of drag creation, it is necessary to introduce the concept of laminar and turbulent boundary layers. In short, a flow can be either '''laminar''' or '''turbulent''', and going from laminar to turbulent is called the “'''transition'''”. This is most easily visualised by observing a burning cigarette in still air: the smoke initially goes straight up nicely (laminar), but after some distance the column suddenly goes unstable (transition) and the smoke wraps up into some unpredictable shape (turbulent flow).
When a boundary layer is laminar, there is less friction between the surface and the flow, but a laminar boundary layer separates easily. In contrast, a turbulent boundary layer causes more friction, but it is more reluctant to separate.
A laminar boundary layer can naturally transit into a turbulent one due to inherent instability, typically when the flow speed is sufficiently high. If natural transition does not occur, it is possible to use a “tripping device” to force premature transition at lower speeds.
== Drag forces explained ==
=== Form drag ===
Form drag, also known as '''pressure drag''', is the drag caused by the pressure in front of the body being higher than the rear pressure. This is most obvious for a “bluff body”, i.e. a body that does not have a streamlined shape. It is difficult to cycle or walk into strong head wind, because a human body is a bluff body with a lot of form drag. The pressure in front of you is greater than the pressure on your back, and this pressure difference tries to push you backwards. Going forward and you have to continuously fight against this form drag.
Form drag largely depends on the '''wake size''', which in turn depends on the geometry of the body. Objects such as spheres and cubes create very large wakes. An aeroplane is usually streamlined, which means it does not trail a significant wake behind it in flight. The form drag associated with an aeroplane is generally not dominating if not small.
A wake is created by boundary layer separation. Recall that a turbulent boundary layer is more reluctant to separate compared with a laminar one, so somehow making the boundary layer turbulent can reduce the form drag considerably. This is the reason why golf balls have dimples: the dimples can trip the boundary layer so that it is turbulent.
=== Skin-friction drag ===
Pulling a teaspoon out of a cup of tea can be effortless, but pulling it out of a jar of honey can be more difficult. This is because the teaspoon moving in honey experiences a significant skin-friction drag. Skin-friction drag is created by the viscous fluid sticking onto the solid object, and it largely depends on the '''surface area''' submerged (or in case of an aeroplane, indeed almost all the surface area). Unfortunately, streamlining a body to reduce form drag usually means creating a large surface area, so there is a subtle balance between form drag and skin-friction drag for the aeroplane designer to get right.
Most of the skin-friction drag on a glider comes from the wings. Recall that a laminar boundary layer causes less skin-friction than a turbulent boundary layer, so a lot of gliders use what is called “'''laminar flow aerofoils'''” which are aerofoils designed to keep the boundary layer laminar for as long as possible. The problem being laminar boundary layers separate easily, so the post hoc fix is to add a turbulent trip on the surface of the wing, just before the point where it would otherwise separate. Such a device can be found on the lower surface of the wing of DM (a CGC single seater).
=== Induced drag ===
Induced drag only exists in 3D: there is no 2D equivalent. When a lifting wing flies through the air, it is well known that a pair of '''tip vortices''' will be created behind the aeroplane. This is generally explained by considering the air escaping from the under side to the upper side around the tip: a more precise explanation is very involved and shall not be discussed here. However, by creating these vortices, extra kinetic energy is transferred to the air, which now has an additional swirl motion. This energy must come from the wing, and the result to the aeroplane is known as induced drag.
The magnitude of the induced drag coefficient is proportional to the lift coefficient to the second power, and '''inversely proportional to the aspect ratio of the wing'''. When the aspect ratio tends to infinity, the induced drag is zero. This is the second reason that, for aeroplanes whose drag is critical, high aspect ratio wings are desirable. Almost all gliders have large aspect ratio wings: the most extreme example being the ETA, which has a record-breaking glide ratio of 70:1.
=== Lift-to-drag ratio ===
It can be shown that, for a glider, the aeroplane lift-to-drag ratio is identical to the glide ratio. It just needs to be noted that the aeroplane lift-to-drag ratio is not the same as the wing L/D, because the fuselage contributes only to drag but not lift.
[[Category:Theory]]
