Wing Design

Laminar and conventional airfoils

Two types of airfoils commonly used are the laminar and conventional airfoils; as a rule, the laminar foil is faster, but the cost is more adverse stalling characteristics.  The two types differ with respect to location of the maximum camber—while the maximum camber on a conventional airfoil is located 25% behind the leading edge, the laminar maximum camber is located at 50% chord.  On the laminar foil, a greater portion of the upper camber is dedicated to laminar airflow; there is therefore less surface friction drag.  The cost of this, however, is that the transition or separation point jumps rapidly forward at the approach of a stall.  Additionally, the laminar foil is more susceptible to surface contamination.

Angle of incidence

The angle of incidenceis defined as the angle between the chord line and the longitudinal axis of the aeroplane; designers select an angle that provides optimum lift/drag ratio.

Washout

Washout is a design trait that pacifies or softens the stall characteristics of an aeroplane whereby the wings are twisted such that the wing tips have a lower angle of incidence than the wing root.  This means that the entire wing will not stall simultaneously; instead, the stall will progressively move from the roots to the tips.  Since the wing tips are the last to stall, the ailerons will remain effective longer during the stall.

Stall Strips

Stall strips are triangular strips placed on a portion of the leading edge of wing; they also have the effect of pacifying the stall characteristic of an aircraft—instead of the entire wing surface stalling uniformly, stall strips create a two-phase stall whereby those portions of the wing behind the strips stall first as the angle of attack is increased.

Airfoil Variation

Airfoil variation is in fact span-wise airfoil variation whereby a thin high-speed airfoil is designed near the roots, and a thick low-speed airfoil near the tips.  The result is that the high-speed roots stall before the low-speed tips—again, this prolongs aileron control.

Wing Fences

Wing fencesare vertical fins that are attached to the upper wing surface and serve the function of reducing the out-flow of air over the upper camber and therefore reduce induced drag.

Winglets

Winglets3 are vertical wing-like surfaces attached to the wingtips; they serve the function of inhibiting the development of wingtip vortices, and therefore reduce induced drag.

Slots and Slats

These are two leading-edge devicesused to enhance lift in high angle of attack attitudes.  Slots are passageways located just aft of the wing’s leading edge, through which air flows through the wing and becomes laminar flow along the upper surface without having to transition over the leading edge; turbulent flow over the upper surface is therefore reduced by slots.

In contrast, slats are auxiliary airfoils attached to the leading edge and which move ahead of the main airfoil at high angles of attack and enhance laminar flow; the enhancement of laminar flow is caused by the reduced angle of attack of the auxiliary airfoil, when compared to the main airfoil.  The space between the two airfoils is considered to be a slot.

Spoilers and Speed Brakes

Spoilers are designed to spoil lift and increase drag in the portion of the upper wing surface where they are located.

Spoilers may be designed only to operate during roll movements, in which case they are referred to as roll spoilers.  On some aircraft such as the Mitsubishi MU-2, only roll spoilers create roll as there are no ailerons; on the Dash 8, two roll spoilers are automatically activated on the down-going wing to assist aileron deflection at speeds below 140 KTS, while only one roll spoiler operates on the down-going wing at airspeeds greater than 140.4

Spoilers may be used symmetrically on both wings during flight or during touchdown to produce decreased lift and increased drag; if used in the air they are referred to as flight spoilers, while if used on the ground at touchdown, they are referred to as ground spoilers.

In contrast, speed brakes are not designed to undermine or spoil lift, but are instead simply designed to increase drag; speed brakes can be mounted on the fuselage or the wing, and incorporate plates that extend into the airflow.  Unlike spoilers, speed brakes do not increase the sink-rate of an aircraft, but simply decrease airspeed.

Flap Variations

There are six types of flaps commonly used:

Stability

Aeroplane movement is based on three axes: the vertical (normal) axis, the lateral axis, and the longitudinal axis; all three axes pass through the aircraft C of G; stability is defined as the tendency of an aircraft to return to, stay at, or move farther from its original attitude after it has been displaced.  There is positive, neutral, and negative stability, and stability is separated into static and dynamic categories

Longitudinal

Movement around the longitudinal axis is roll, which produces bank, and is produced by the ailerons; longitudinal stability (of the axis) is provided by a nose-heavy design and a negative-lift tail.

Lateral                                                                                                                                 

Movement around the lateral axis is pitch, and is produced by the elevator; lateral stability (of the axis) is provided by dihedral, which lowers C of G relative to the lifting surfaces (wing tips are positioned higher than the wing roots); in short, the downward moving wing has greater lift than an upward moving wing.

Directional

Movement around the vertical (normal) axis is yaw, and is “controlled” by the rudder; adverse yaw takes place during rolling movement because of aileron drag, and is controlled by rudder; stability of the vertical axis is referred to as directional stability, and is produced by the vertical stabilizer and sweptback (sweepback) wing design.