The Meaning of Laminar

A very thin sheet of air lying over the surface of any wing in flight adheres to that wing’s surface. That sheet is called the Boundary Layer, and its adhesion to the wing is a consequence of air’s viscosity. As the wing moves forward, the boundary layer flows smoothly over the frontal curvature of the airfoil. This smooth flow is referred to as being laminar, and the boundary layer at this stage is called the Laminar Layer.

As the boundary layer approaches the centre of the wing, it begins to slow and becomes turbulent due to skin friction. At this point, known as the transition point, laminar flow transitions into turbulent flow, and the resultant flow becomes known as the Turbulent Layer. Turbulent flow is the opposite of laminar flow: laminar flow is smooth, while turbulent flow is rough.

At the transition point between laminar flow and turbulent flow, drag due to skin friction is relatively high. (Turbulent flow produces a thicker boundary layer, which means that more air molecules are being dragged along with the aircraft, thus generating more skin friction.) Also, an increase in aircraft speed tends to move the transition point towards the leading edge of the wing. An increase in the aircraft’s angle of attack tends to do the same thing. Thus, boundary layer control is a prime consideration for any airfoil design.

Maintaining laminar flow over the greatest possible part of the airfoil is foremost as a prerequisite of modern wing design. Laminar flow can be maintained by providing a negative pressure gradient over the wing. A negative pressure gradient is a design feature in which the pressure continuously drops from the leading edge to the trailing edge of the wing. This tends to suck the flow of air rearward, promoting laminar flow over the entire camber of the wing’s upper surface.

A well-designed laminar flow airfoil tends to be thickest at 50 percent of its chord. Combined with smooth fabrication methods, such a wing can produce laminar flow over 50 percent to 70 percent of the wing. When compared to more conventional airfoils – those being ones with their thickest points at 25 percent of chord distance – the transition point at which the laminar flow of air becomes turbulent on a laminar flow airfoil is, thus, rearward of that same point on more conventional airfoils.

With its thickest point further rearward along its camber, the laminar flow wing is usually thinner than a conventional wing. Typically, its leading edge is more streamlined, and its upper and lower surfaces are more symmetrical. The effect achieved by this design of wing is that of a reduction in drag since the laminar airfoil takes less energy to penetrate the air. The pressure distribution on the laminar flow wing is better distributed since the wing’s camber is more gradual than a conventional wing.

When flying airplanes with laminar flow wings, pilots must be more precise with control inputs. Abrupt changes in speed and angle of attack can cause large areas of the wing to transition from laminar to turbulent flow. This can cause large variations in the wing’s drag resulting in porpoising of the aircraft, especially at low speeds.

Furthermore, on a laminar flow wing, at the point of wing stall, the transition point moves more rapidly forward than it does on a conventional airfoil. Thus, a sudden increase in the angle of attack or in G loading can result in a violent stall. Pilots flying aircraft with laminar flow wings should be especially careful during landings so as not to approach with too high an angle of attack near the stalling speed.

Anything that impedes the smooth surface of the laminar flow airfoil will reduce its efficiency more than the same impediments will harm a conventional airfoil shape. Therefore, it is especially important to keep a laminar flow airfoil free of mud, dust, frost, bugs, or any other forms of debris that can disrupt the critical shape of its upper surface. Aircraft with laminar flow airfoils will usually get you through the air faster, but they’ll usually be touchier to fly.