Aerodynamics
Aerodynamics is probably the
first subject that comes to mind when most people think of Aeronautical or
Aerospace Engineering.
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Aerodynamics is essentially the application of classical
theories of “fluid mechanics” to external flows or flows around bodies, and the
main application which comes to mind for most aero engineers is flow around
wings.
The wing is the most important
part of an airplane because
without it there would be no lift and no aircraft. Most people
have some idea of how a wing works; that is, by making the flow over the top of the wing go faster than the flow over the bottom we get a lower pressure
on the top than on the bottom and, as a result,
get lift. The aero engineer
needs to know something more than this. The aero
engineer needs to know how to shape the wing to get the optimum combination of
lift and drag and pitching
moment for a particular airplane
mission. In addition
he or she needs to understand how the
vehicle’s aerodynamics interacts with other aspects of its design and
performance. It would also be nice if the forces on the wing did not exceed the
load limit of the wing structure.
If one looks at enough
airplanes, past and present, he or she will find a wide variety of wing shapes.
Some aircraft have short, stubby
wings (small wing span), while others have long, narrow wings. Some wings are
swept and others are straight. Wings may have odd shapes at their tips or even
attachments and extensio
ns such as winglets. All of these shapes are related to
the purpose and design of the aircraft.
In order to look at why
wings are shaped like they are we need to start by looking at the terms that
are used to define the shape of a wing.
Here is the viedo about the Primary Flight Control Surfaces
Figure 1.1: Airfoil Terminology
A two dimensional slice of
a wing cut parallel to the centerline of the aircraft fuselage or body is
called the airfoil section. A straight
line from the airfoil section
leading edge to its trailing
edge is called the chord line. The length of the
chord line is referred to as the chord. A line drawn half way between the
airfoil section’s upper and lower surfaces is called the camber
line. The maximum
distance between the camber line and chord line is referred to as the airfoil’s
camber and is usually
enumerated as a percent of chord. We will see that the amount
of airfoil camber
and the location of the point of maximum camber
are important numbers in defining the shape of an airfoil and predicting its
performance. For most airfoils the maximum camber is on the order of zero to
five percent and the location of the point of maximum camber is between 25% and
50% of the chord from the airfoil leading edge.
When viewed from above the aircraft the wing shape or planform is defined by other terms.
Figure 1.2: Wing Planform Terminology
Note that the planform
area is not the
actual surface area of the wing but is “projected area” or the area of the
wing’s shadow. Also note that some of the abbreviations used are not intuitive; the span, the distance
from wing tip to wing tip
(including any fuselage
width) is denoted
by b and the planform area is given a symbol of “S” rather than perhaps
“A”. Sweep angles are usually given a symbol of lambda (λ).
Another
definition that is based on the planform
shape of a wing is the Aspect Ratio (AR).
AR = b2/S.
Aspect ratio
is also the span divided
by the “mean” or average
chord. We will later find that aspect
ratio is a measure of the wing’s efficiency in long range
flight.
Wing planform shapes may
vary considerably from one type of aircraft to another. Fighter aircraft tend
to have low aspect ratio or short, stubby wings, while long range transport
aircraft have higher aspect ratio wing shapes, and sailplanes have yet higher wing spans.
Some wings are swept while others are not. Some wings have triangular or “delta”
planforms. If one looks at the past 100 years
of wing design
he or she will see an almost
infinite variety of shapes. Some of the shapes come from aerodynamic
optimization while others are shaped for structural benefit. Some are shaped the way they are for stealth, others for
maneuverability in aerobatic flight, and yet others just to satisfy their
designer’s desire for a good looking airplane.
Figure 1.3: Some Wing Planform
Shapes
In general, high aspect
ratio wings are desirable for long range
aircraft while lower
aspect ratio wings
allow more rapid roll response when maneuverability
is a requirement. Sweeping a wing either forward or aft will reduce its drag as
the plane’s speed approaches the speed of sound but will also reduce its efficiency at lower speeds.
Delta wings represent a way to get a combination of high sweep
and a large area. Tapering
a wing to give it lower chord
at the wing tips usually gives somewhat better performance than an untapered
wing and a non-linear taper which gives a “parabolic” planform will theoretically give the best performance.
In the following material we
will take a closer look at some of the things mentioned above and at their
consequences related to the flight capability of an airplane.
Before we take a more
detailed look at wing aerodynamics we will first examine the atmosphere in
which aircraft must operate and look at a few of the basic relationships we
encounter in “doing” aerodynamics.