
Geometry Definitions of
Wing
This slide gives technical definitions
of a wing's geometry, which is one of the chief factors affecting airplane lift
and drag. The terminology is used throughout the airplane industry and is also
found in the FoilSim interactive airfoil simulation program developed at NASA
Glenn. Actual aircraft wings are complex threedimensional objects, but we will
start with some simple definitions. The figure shows the wing viewed from three
directions; the upper left shows the view from the top looking down on the wing,
the lower right shows the view from the front looking at the wing leading edge,
and the lower left shows a side view from the left looking in towards the
centerline. The side view shows an airfoil shape with the leading edge to the
left.
Top View
The top view shows a simple wing geometry, like that found on a light general
aviation aircraft. The front of the wing (at the bottom) is called the leading
edge; the back of the wing (at the top) is called the trailing edge. The
distance from the leading edge to the trailing edge is called the chord, denoted
by the symbol c. The ends of the wing are called the wing tips, and the distance
from one wing tip to the other is called the span, given the symbol s. The shape
of the wing, when viewed from above looking down onto the wing, is called a
planform. In this figure, the planform is a rectangle. For a rectangular wing,
the chord length at every location along the span is the same. For most other
planforms, the chord length varies along the span. The wing area, A, is the
projected area of the planform and is bounded by the leading and trailing edges
and the wing tips. Note: The wing area is NOT the total surface area of the
wing. The total surface area includes both upper and lower surfaces. The wing
area is a projected area and is almost half of the total surface area.
Aspect ratio is a measure of how long and slender a wing is from tip to tip. The
Aspect Ratio of a wing is defined to be the square of the span divided by the
wing area and is given the symbol AR. For a rectangular wing, this reduces to
the ratio of the span to the chord length as shown at the upper right of the
figure.
AR = s^2 / A = s^2 / (s * c) = s / c
High aspect ratio wings have long spans (like high performance gliders), while
low aspect ratio wings have either short spans (like the F16 fighter) or thick
chords (like the Space Shuttle). There is a component of the drag of an aircraft
called induced drag which depends inversely on the aspect ratio. A higher aspect
ratio wing has a lower drag and a slightly higher lift than a lower aspect ratio
wing. Because the glide angle of a glider depends on the ratio of the lift to
the drag, a glider is usually designed with a very high aspect ratio. The Space
Shuttle has a low aspect ratio because of high speed effects, and therefore is a
very poor glider. The F14 and F111 have the best of both worlds. They can
change the aspect ratio in flight by pivoting the wingslarge span for low
speed, small span for high speed.
Front View
The front view of this wing shows that the left and right wing do not lie in the
same plane but meet at an angle. The angle that the wing makes with the local
horizontal is called the dihedral angle. Dihedral is added to the wings for roll
stability; a wing with some dihedral will naturally return to its original
position if it encounters a slight roll displacement. You may have noticed that
most large airliner wings are designed with diherdral. The wing tips are farther
off the ground than the wing root. Highly maneuverable fighter planes, on the
other hand do not have dihedral. In fact, some fighter aircraft have the wing
tips lower than the roots giving the aircraft a high roll rate. A negative
dihedral angle is called anhedral . Historical Note: The Wright brothers
designed their 1903 flyer with a slight anhedral to enhance the aircraft roll
performance.
Side View
A cut through the wing perpendicular to the leading and trailing edges will show
the crosssection of the wing. This side view is called an airfoil, and it has
some geometry definitions of its own as shown at the lower left. The straight
line drawn from the leading to trailing edges of the airfoil is called the chord
line. The chord line cuts the airfoil into an upper surface and a lower surface.
If we plot the points that lie halfway between the upper and lower surfaces, we
obtain a curve called the mean camber line. For a symmetric airfoil (upper
surface the same shape as the lower surface) the mean camber line will fall on
top of the chord line. But in most cases, these are two separate lines. The
maximum distance between the two lines is called the camber, which is a measure
of the curvature of the airfoil (high camber means high curvature). The maximum
distance between the upper and lower surfaces is called the thickness. Often you
will see these values divided by the chord length to produce a nondimensional
or "percent" type of number. Airfoils can come with all kinds of combinations of
camber and thickness distributions. NACA (the precursor of NASA) established a
method of designating classes of airfoils and then wind tunnel tested the
airfoils to provide lift coefficients and drag coefficients for designers.
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