The character of airflow basically depends on the shape of the airfoil section and on the phenomena occurring at the boundary layer, which is the thin layer of air adjacent to a solid surface such as an airfoil over which air is flowing and which is distinguished from the main airflow by distinctive flow characteristics of its own set up by friction. The flow in the boundary layer (Fig.1) where the thickness of the layer is shown greatly exaggerated may be laminar or turbulent. In laminar flow the velocity distribution in the layer shows a steady increase from zero at the surface of the airfoil: more particularly, the wing of an aircraft: to a maximum corresponding to the velocity of the main airflow.

An airfoil is a device which gets a necessary reaction from air moving over its surface. When an airfoil is moved through the air, it is capable of producing lift. For e.g.: Wings, horizontal tail surfaces, vertical tails surfaces, and propellers. The flow is relatively smooth and moves in layers parallel to the surface; hence the term laminar. In turbulent flow, the fairly regular motion of the laminar boundary layer is destroyed. The boundary layer undergoes transition; it becomes thicker and is characterized by large random motions (turbulence). These effects may give rise to separation, a term which denotes that the flow in the boundary layer detaches itself from the surface of the wing at the separation point and that immediately adjacent to the surface, flow even occurs in a direction opposite to the direction of the main flow (Fig.2).

The airflow around the wing occurs at the stagnation point and is laminar up to the transition point, where turbulence sets in the latter point is located near the point of minimum pressure, approximately where the wing has its greatest thickness. Normally the turbulent boundary layer separates itself from the trailing edge of the wing, where eddies develop. If this separation occurs too far forward toward the leading edge, there is serious loss of lift and an increase in drag.

This is liable to happen when the angle of attack exceeds the critical value known as the stalling angle or when the airspeed becomes too low. Some aircraft, especially sports planes, are equipped with a stall-warning device which may consist of a short triangular plate or a length of wire fitted to the leading edge of the wing (Fig.3). When the angle of attack becomes dangerously large, separation of the airflow commences at this plate or wire. There is an immediate loss of lift, which warns the pilot that he is approaching the stalling angle.

Regions of the wing where laminar separation is liable to occur may be provided with device for producing turbulence (Fig.4). The resulting turbulent flow adheres better to the surface than the laminar flow and premature separation is thus prevented. Such devices may, for example, take the form of small projecting plates which break up the laminar flow. Swept wings are provided with so-called fences, which are plates or vanes placed parallel to the main airflow and prevent flow (and separation) in the direction from wing root to tip, this subsidiary flow being promoted by the sweep of the wing.

A similar effect is obtained by forming the leading edge of the wings with sawtooth notches (Fig.6). At the tail of the aircraft, interference of the boundary layers of the horizontal and the vertical stabilizers produces interference drag. To diminish this, the so-called T tail has been developed, in which the horizontal surfaces are placed at the top of the vertical fin (Fig.7), while the junction of these components is provided with a fairing i.e., a streamlined casting designed to reduce drag.

Steering an aircraft in three directions is effected by means of: the rudder, which guides the aircraft in the horizontal plane, the elevator, which controls the pitch i.e., makes the tail go up or down, and the ailerons which control the rolling motion of the aircraft by their differential rotation. The rudder is attached to a vertical stabilizer, while the ailerons are set at the trailing edges of the wings (Fig.5).

Sometimes the horizontal and vertical stabilizers are not provided with separately movable attachments i.e. elevator and rudder, but can each be moved as a whole so as to alter the angle of attack. The trailing edge of the rudder may be provided with a small subsidiary rudder called a trimming tab (Fig.7) by means of which the pilot adjusts the trim of the aircraft i.e., the condition of static balance in pitch during rectilinear flight, with the main control surfaces seeking their neutral positions.

Further adjustments are achieved by means of flaps, these being control surfaces which serve to control the speed by increasing the drag and thus acting as a brake or to increase the lift or aid in recovery from a dive. The slat (Fig.8) is a movable auxiliary airfoil running along the leading edge of a wing; in a normal flight it is contact with the latter, but it can be lifted away to form a slot at certain angles of attack, so that air flows through the slot and reenergizes the boundary layer on the low-pressure upper surface of the wing. The plain wing flap (Fig.9) increases the curvature of the wing, with the result that the lift is improved and the angle of attack at which separation occurs is increased, so that the airspeed can be reduced without stalling.

This is important in connection with the take off and landing of high-speed aircraft. An improved form is the slotted flap (Fig.8); the flow of air through the slot between the flap and the wing gives a further increase in lift without separation of the boundary layer. In contrast with other types of flap mentioned, the split flap (Fig.10) serves to reduce the pressure on the suction face of the wing, whereby an increase in the lift is likewise achieved. Landing flaps (Fig.9) serve primarily to slow down the aircraft for landing; they breakdown the airflow around the aircraft and thus function as brakes. Such flaps are sometimes called spoilers, more particularly when installed on the underside of the wing.

A special dynamic device for boundary-layer control is the jet flap which consists of a flat jet of air expelled at high velocity from a narrow slot at the trailing edge of the wing and which exercises an action similar to that of an ordinary flap. The same principle is embodied in the blown flap (Fig.11), an ordinary trailing edge flap in which separation from the upper surface is delayed by blowing. This principle is also applied to elevators (Fig.12). Another modern control method, still in progress stage, consists in keeping the airflow laminar by sucking in air from the boundary layer through numerous small holes.