Design of the Wing
definitions:
Airfoil: the shape or design of a wing
Laminar flow airfoil: a specially designed airfoil that is commonly used on modern airplanes. It has the thickest part of the airfoil at 50% of the chord, rather than 25% of the chord. You will read more about this later in this section.
Planform: The shape of the wing as seen from directly above. The wing may be rectangular, eliptical or delta shaped.
Aspect ratio: The aspect ratio is the relationship between the length or span of the wing and its width or chord. It is found by dividing the span by the chord. Gliders have high aspect ratios, and Cessnas have small aspect ratios.
Airfoils are specially designed for the purpose that they will serve. Airfoils designed for fast airplanes will be thin and streamlined. An aircraft designed for carrying cargo at low speeds will have a thicker airfoil that will produce more lift.
Laminar Flow Airfoils
Commonly used on modern airplanes is a recent development called the "laminar flow airfoil". Originally the laminar flow airfoil was designed to make airplanes that fly faster. This type of airfoil is thinner than conventional airfoils, the leading edge is more pointed, and the uppper and lower surfaces are almost symmetrical. The most important difference between a laminar flow airfoil and a conventional airfoil is where the thickest part of the wing is. In a conventional airfoil, the thickest part of the wing is usually at about 25% of the chord (close to the leading edge). In a laminar flow airfoil, the thickest part of the wing is at about 50% of the chord, halfway between the leading and the trailing edge.
You will remember from the last section that the laminar layer is the layer of air that flows smoothly over the first part of the wing. The laminar flow airfoil is designed so that the air flows smoothly over the wing for longer than with the conventional airfoils. The transition point (where the laminar flow becomes the turbulent layer) is moved more towards the trailing edge. Dragis reduced with this type of airfoil.
Planform
Planform is the shape of the wing when seen from directly above. The shape of wings changes from airplane to airplane. Some have rectangular shaped wings, others have elliptical (rounded rectangles) wings. Others, like fighter jets, have delta wings.
The aspect ratio is a way of expressing the planform view of a wing. Rather than say that the wing is narrow or wide, we can use numbers to describe the aspect ratio of the wing. The aspect ratio is the relationship between the span (length) of the wing and its chord (width). It is found by dividing the span by the chord.
A wing with a high aspect ratio will generate more lift and less induced drag than a wing with a low aspect ratio. Gliders have high aspect ratios to take advantage of this fact.
pg 8 blue book pic
Angle of Incidence
The angle of incidence is the angle at which the wing is permanently inclined to the longitudinal axis of the airplane. The angle of incidence can never be changed because the way that the wings are attached can never be changed. The angle of incidence is decided while the airplane is being built, depending on what the needs of the aircraft will be.
The angle of incidence affects flight visibilty (how well you can see out the front window), take off and landing characteristics (how it handles in the air) and the amount of drag that the airplane experiences.
Wing Tip Design
Specially designed wing tips are sometimes able to help control drag and wing tip vortices. Vortices decrease lift, increase drag and can cause instability during flight at low airspeeds and high angles of attack.
Wing tip tanks allow for greater range in flight (because they carry extra fuel) but they also help keep the air from flowing from the bottom of the wing to th top of the wing, and therefore help prevent vortices.
Winglets, small vertical winglike surfaces attached to the wing tip, alos help prevent vortices.
Wash-In and Wash-Out
This has nothing to do with the teenage tendancy to noyt wash their clothes! In fact, wash-in and wash-out is when the wing is designed so that it appears to be twisted from tip to root. The actual amount of twisting is slight, but is enough to have an effect on the flight characteristics of the airplane.
Wash-in is when the angle of incidence towards the root of the wing (part close to the fuselage) is greater than the angle of incidence at the wing tip. This is good, because it helps prevent sudden stalling. The section of wing nearest the wing root will stall first. This is good because the section closer to the wing tips is unstalled, and the ailerons, located on the outer part of the wing, are able to be used.
Wash-out is when the angle of incidence is decreased, and wash-in is when the angle of incidence is increased.
Wing Fences
Wing fences are fin like vertical surfaces that are attached to the upper surface of the wing. They are used to keep the air flowing properly over the wings, and provide better slow speed handling and stall characteristics.
Slats
Slats are extra airfoils that are attached to the leading edge of the airfoil. When the airplane flies at high angles of attack, the slats automatically move ahead of the wing, because the low pressure above the wing is enough to pull them out to position. When the angle of attack is lower the greater air pressure pushes tha slat back into the wing.
When the slat is out, it has a lower angle of attack than the rest of the wing. This allows air to flow smoothly over the wing and helps to smooth the turbulence that is created on the main wing.
Slats help to improve lateral control.
Slots
Slots are special passageways built into the wing close to the leading edge. At high angles of attack air is able to flow through the slot and out over the wing. This extra flow of air helps to smooth out the turbulence found at the transition point.
Spoilers
Spoilers are devices that are attached to the wing. They are used to increase drag and decrease lift. Normally, a spoiler is a long piece of metal that is arranged lengthwise along the wing.
Spoilers interupt the flow of air over the top of a wing, and create drag. Spoilers may be used on both wings at the same time, or on one wing at a time. A single open spoiler on a wing has the same effect as the ailerons. The open spoiler causes drag, and a decrease in lift, and causes a turn towards the wing with the spoiler open. In some airplanes, spoilers have replaced the ailerons completely.
Flaps
Flaps are devices that are attached to the inner trailing edge of the wings of some aircraft. Flaps are able to be adjusted from inside the cockpit. When extended, they increase the camber of the wing. This gives better take-off performance, permits steeper approach angles and slower approach and landing speeds. Flaps also increase the drag of the airplane.
Question about something in this section you don't understand? Email the instructor:
efcarter@island.net