This slide gives technical definitions of a wing's geometry, which
is one of the chief factors affecting
airplane lift and drag.
The terminology used here is used throughout the airplane industry today
and was mostly known to the Wright brothers in 1900. Actual aircraft wings are complex
three-dimensional objects, but we will start with some simple
definitions. The figure shows a wing viewed from three directions;
the upper left shows the view from the top looking down on the wing,
the lower left shows the view from the front looking at the
wing leading edge, and the right 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. This airfoil is a modern,
thick airfoil, which is slightly different from the thin
airfoils used by the Wrights and shown below. The terminology, however, is the same.
__Top View__
The top view shows a simple rectangular wing geometry, like that
used by the Wright brothers. 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 to trailing edges is called
the **chord**. The ends of the wing are called the **wing
tips**, and the distance from one wing tip to the other is called
the **span**. The shape of the wing, when viewed from above
looking down onto the wing, is called a **planform**.
For a rectangular wing, the
chord length at every location along the span is the same. For most
modern aircraft, the chord length varies
along the span, and the leading and trailing edges may be
**swept**. The **wing area** 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.*
The **aspect ratio (AR)** of a wing is defined to be the square of
the span **(s)**divided by the wing area **(A)**.
**Aspect ratio** is a measure
of how long and slender a wing is from tip to tip. For a rectangular
wing, this reduces to the ratio of the span to the chord length **(c)**:
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 or thick chords
(like the Space Shuttle). Gliders have a high aspect ratio because
the drag of the aircraft depends on this
parameter. A higher aspect ratio gives a lower drag, a higher
lift to drag ratio,
and a better
glide angle.
__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**
if the tips are higher than the root or the **anhedral angle** if
the tips are lower than the root.
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 usually
have the wing tips lower than the roots giving the aircraft a high roll
rate.
The Wright brothers designed their
1903 flyer
with a slight anhedral to improve the aircraft
roll
performance.
__Airfoil Geometry__
A cut through the wing perpendicular to the leading and trailing
edges will show the cross-section of the wing. This cross-section is
called an **airfoil**, and it has some geometry definitions of its
own as shown at the lower right. 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** the upper surface is a reflection
of the lower surface and the mean camber line will fall on top of
the chord line. But in most cases, the mean camber line
and the chord line 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 non-dimensional or
"percent" type of number. Airfoils can come with all kinds of
combinations of camber and thickness distributions.
Here is a photo of a model of the Wright brother's 1903 wing.
You are looking out towards the right wing tip and
the cloth skin has been removed from the wing so that you can view
the construction. The **spars** are long, heavy pieces that run
from wing tip to wing tip along the leading edge and near the middle
of the wing. The **ribs** are attached to the spars and the ribs
produce the airfoil shape.
The Wrights
developed the final geometry for their wings by
testing
small models in their
wind tunnel in 1901.
They used mechanical
balances
to measure the
lift and
drag for their
wing models.
The resulting airfoils were very thin, with a slight camber. Both the upper
and lower surfaces were curved.
*NOTICE: The upper and lower surfaces of the Wright airfoils are
nearly the same length; the lower surface is not flat like many modern
low speed airfoils. This is a perfect example
which shows that the popular theory of
lift generation
found in many textbooks is completely wrong! The upper surface doesn't have
to be longer than the lower surface to generate lift. The lift occurs because
the airfoil turns the flow of air and both the lower and
upper surface contribute to the turning.
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**Activities:**
**Navigation..**
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page
- http://www.nasa.gov
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