1.
Wing
design
Before
a wing is designed, its mission has to be determined. What type of aircraft
will this wing be attached to? Will it need to operate at high altitudes with
thin atmospheres? Will it have to carry heavy loads? Will it need space to
mount the engines? How much fuel will we want to store inside? How fast or
agile will the aircraft need to be? The list of potential specifications is
long and highly complex.
During
the wing design process, eighteen parameters must be determined. They are as
follows:
1.
Wing reference (or planform) area (SW or Sref
or S)
2.
Number of the wings
3.
Vertical position relative to the fuselage
(high, mid, or low wing)
4.
Horizontal position relative to the fuselage
5.
Cross section (or airfoil)
6.
Aspect ratio (AR)
7.
Taper ratio (λ)
8.
Tip chord (Ct)
9.
Root chord (Cr)
10.
Mean Aerodynamic Chord (MAC or C)
11.
Span (b)
12.
Twist angle (or washout) (αt)
13.
Sweep angle (Ʌ)
14.
Dihedral angle
15.
Incidence (iw) (or setting angle, αset)
16.
High lifting devices such as flap
17.
Aileron
18. Other
wing accessories
One of the necessary tools in the wing design process is
an aerodynamic technique to calculate wing lift, wing drag, and wing pitching
moment. With the progress of the science of aerodynamics, there are variety of
techniques and tools to accomplish this time consuming job. A variety of tools
and software based on aerodynamics and numerical methods have been developed in
the past decades. The application of such software packages–which is expensive
and time-consuming – at this early stage of wing design seems un-necessary.
Instead, a simple approach, namely Lifting Line Theory is introduced. Using
this theory, one can determine those three wing productions (L, D, and M) with
an acceptable accuracy.
·
Number
of Wings
One
of the decisions a designer must make is to select the number of wings. The
options are:
1.
Monoplane (i.e. one wing)
2.
Two wings (i.e. biplane)
3. Three
wings
·
Wing
Vertical Location
One
of the wing parameters that could be determined at the early stages of wing
design process is the wing vertical location relative to the fuselage
centerline. This wing parameter will directly influence the design of other
aircraft components including aircraft tail design, landing gear design, and
center of gravity. In principle, there are four options for the vertical
location of the wing. They are:
·
Dihedral
Angle
When
you look at the front view of an aircraft, the angle between the chord-line
plane of a wing with the “xy” plane is referred to as the wing dihedral . The
chord line plane of the wing is an imaginary plane that is generated by
connecting all chord lines across span. If the wing tip is higher than the xy
plane, the angle is called positive dihedral or simply dihedral, but when the
wing tip is lower than the xy plane, the angle is called negative dihedral or
anhedral.
·
High
Lift Device
One
of the design goals in wing design is to maximize the capability of the wing in
the generation of the lift. This design objective is technically shown as
maximum lift coefficient (CLmax).
The
application of the high lift device tends to change the airfoil section’s and
wing’s camber (in fact the camber will be positively increased). This in turn
will change the pressure distribution along the wing chord.
At
the airfoil level, a high lift device deflection tends to cause the following
six changes in the airfoil features:
1.
Lift coefficient (Cl) is increased,
2.
Maximum lift coefficient (Clmax) is
increased,
3.
Zero-lift angle of attack in changed,
4. Stall angle is changed,
5. Pitching moment coefficient is changed.
6. Drag coefficient is increased.
7.
Lift curve slope
is increased.
·
Type of Wing
1. Rectangular Wing
The
rectangular wing, sometimes referred to as the “Hershey Bar” wing in reference
to the candy bar it resembles, is a good general purpose wing. It can carry a
reasonable load and fly at a reasonable speed, but does nothing superbly well.
It is ideal for personal aircraft as it is easy to control in the air as well
as inexpensive to build and maintain.
2. Elliptical Wing
The
elliptical wing is similar to the rectangular wing and was common on tail-wheel
aircraft produced in the 1930s and 40s. It excels however in use on gliders,
where its long wingspan can capture the wind currents easily, providing lift
without the need for a lot of forward momentum, or airspeed.
3.
Swept Wing
The
swept wing is the “go to” wing for jet powered aircraft. It needs more forward
speed to produce lift than the rectangular wing, but produces much less drag in
the process, meaning that the aircraft can fly faster. It also works well at
the higher altitudes, which is where most jet aircraft fly.
4. Delta Wing
The
delta wing advances the swept wing concept, pulling the wings even further back
and creating even less drag. The downside to this however is that the aircraft
has to fly extremely fast for this wing to be effective. This is why it’s only
found on supersonic aircraft (aircraft that fly faster than the speed of sound)
such as fighter jets and the Space Shuttle orbiter.
2.
Swept
vs unswept (sweep)
Swept wing cut down on
drag caused by turbulence at the wingtips. But the real advantage of swept wing
s comes in supersonic flight. The configuration cuts down on wave drag by
redistributing the shockwaves along the plane’s aerodynamic profile. They are
ideal for these high-speed conditions,less drag, yaw stability, roll stability,
less induced drag, delay of stall Unfortunately, they do not allow for heavy
payloads at lower speeds. Swept wings are also inefficient and burn too much
fuel to stay aloft, which reduces the range of the aircraft,
Unswept wings are
efficient at low speeds, providing a great amount of lift compared to the
amount of induced drag exerted on the plane. Unswept wings are very bad at
dealing with wave drag.
3.
Root
chord dan tip chord
Tip Chord (Ct) is the
chord at the tip of an airfoil, measured parallel to the plane of symmetry, and
at points where straight leading or trailing edges meet the curvature at the
tip. In variable-sweep wings, the tip chord is measured when the sweep is
minimum.
Root Chord (Cr) is the
chord of an airfoil measured from its leading edge to the trailing edge at its
root.
In addition, since the
tip chord is smaller than root chord, the tip Reynolds number will be lower, as
well as a lower tip induced downwash angle. Both effects will lower the angle
of attack at which stall occurs. This will result in the tip may stall before
the root. This is undesirable from the viewpoint of lateral stability and
lateral control.
4.
AR
(Aspect Ratio)
In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its
breadth (chord). A high aspect ratio indicates long,
narrow wings, whereas a low aspect ratio indicates short, stubby wings.
Aspect ratio (AR)10 is defined as the ratio between the wing span; b (see
figure 5.31) and the wing Mean Aerodynamic Chord (MAC)
For most
wings the length of the chord is not a constant but varies along the wing, so
the aspect ratio AR is defined as the square of the wingspan
b divided by the area S of the wing planform,
which is equal to the length-to-breadth ratio for a constant chord wing. In symbols,
This
equation is not to be used for the wing with geometry other than rectangle;
such as triangle, trapezoid or ellipse; except when the span is redefined. At
this point, only wing planform area is known. The designer has infinite options
to select the wing geometry. For instance, consider an aircraft whose wing
reference area has been determined to be 30 m2. A few design options
are as follows:
1.
A rectangular wing with a 30 m span and a 1 m
chord (AR =30)
2.
A rectangular wing with a 20 m span and a 1.5 m
chord (AR =13.333)
3.
A rectangular wing with a 15 m span and a 2 m
chord (AR = 7.5)
4.
A rectangular wing with a 10 m span and a 3 m
chord (AR = 3.333)
5.
A rectangular wing with a 7.5 m span and a 4 m
chord (AR = 1.875)
6.
A rectangular wing with a 6 m span and a 5 m
chord (AR = 1.2)
7.
A rectangular wing with a 3 m span and a 10 m
chord (AR = 0.3)
8.
A triangular (Delta) wing with a 20 m span and a
3 m root chord (AR = 13.33; please note that the wing has two sections (left
and right))
9. A
triangular (Delta) wing with a 10 m span and a 6 m root chord (AR = 3.33)
The effects of aspect ratio on various flight features
such as aircraft performance, stability, control, cost, and manufacturability :
1. From
aerodynamic points of view, as the AR is increased, the aerodynamic features of
a three-dimensional wing (such as CLmax, CDmin) are getting closer to its two-dimensional
airfoil section (such as Clmax, Cdmin).
2.
as the AR is increased, the wing lift curve
slope is increased
3. As
the AR is increased, the wing stall angle is decreased toward the
airfoil stall angle. For this reason, the horizontal tail is required to have
an aspect ratio lower than wing aspect ratio to allow for a higher tail stall
angle. This will result in the tail to stall after wing has stalled, and allow
for a safe recovery. For the same reason, a canard is desired to have an aspect
ratio to be more than the wing aspect ratio. For this reason, a high AR
(longer) wing is desired.
4. Due
to the third item, as the AR is increased, the wing maximum lift coefficient
(CLmax) is increased toward the airfoil maximum lift coefficient (Clmax).
5.
As the AR is increased, the wing will be
heavier.
6. As
the square root AR is increased, the aircraft maximum
lift-to-drag ratio is increased. Since
7. As
the AR is increased, the wing induced drag is decreased, since the induced drag
is inversely proportional
to aspect ratio. For this reason, a low AR (shorter) wing is desired.
8. As
the AR is increased, the effect of wing tip vortex on the horizontal tail is
decreased.
9. As
the AR increases, the aileron arm will be increased, since the aileron are
installed outboard of the wing. This means that the aircraft has more lateral
control.
10. As
the AR increases, the aircraft mass moment of inertia around x-axis will be
increased. This means that it takes longer to roll. In another word, this will
reduces the maneuverability of aircraft in roll
11. If
the fuel tank is supposed to be inside wing, it is desirable to have a low
aspect ratio wing. This helps to have a more concentrated fuel system. For this
reason, a low AR (shorter) wing is desired.
12. As
the aspect ratio is increased, the wing stiffness around y-axis is decreased.
This means that the tendency of the wing tips to drop during a take-off is
increased, while the tendency to rise during high speed flight is increased. In
practice, the manufacture of a very high aspect ratio wing with sufficient
structural strength is difficult.
13. A
shorter wing needs lower cost to build compared with a long wing. For the cost
reason, a low AR (a shorter wing) is desired.
14. As
the AR is increased, the occurrence of the aileron reversal is more expected,
since the wing will be more flexible. The aileron reversal is not a desirable
phenomenon for a maneuverable aircraft. For this reason, a low AR (shorter)
wing is desired.
15. In
general, a wing with rectangular shape and high AR is gust sensitive.
Several rectangular wings with the
same planform area but different aspect ratio
5.
λ
Taper Ratio
λ
Taper Ratio 
Taper
ratio (λ) is defined as the ratio between the tip chord (Ct) nd the root chord
(Cr). This definition is applied to the wing, as well as the horizontal tail,
and the vertical tail.
The
geometric result of taper is a smaller tip chord. In general, the taper ratio
varies between zero and one.
.
The
effect of wing taper can be summarized as follows:
1.
The wing taper will change the wing lift
distribution. This is assumed as an advantage of the taper, since it is a
technical tool to improve the lift distribution. One of the wing design
objective is to generate the lift such that the spanwise lift distribution be
elliptical. The significance of elliptical lift distribution will be examined
in the next section. Based on this item, the exact value for taper ratio will
be determined by lift distribution requirement.
2.
The wing taper will increase the cost of
the wing manufacture, since the wing ribs will have different shapes. Unlike a
rectangular planform that all ribs are similar; each rib will have different
size. If the cost is of major issue (such as for homebuilt aircraft), do not
taper the wing.
3.
The taper will reduce the wing weight,
since the center of gravity of each wing section (left and right) will move
toward fuselage center line. This results in a lower bending moment at the wing
root. This is an advantage of the taper. Thus, to reduce the weight of the
wing, more taper (toward 0) is desired.
4.
Due to item 3, the wing mass moment of inertia
about x-axis (longitudinal axis) will be decreased. Consequently, this will
improve the aircraft lateral control. In this regard, the best taper is
to have a delta wing (λ
= 0).
5. The
taper will influence the aircraft static lateral stability , since the taper usually
generates a sweep angle (either on the leading edge or on quarter chord line).
The
effect of the weep angle on the aircraft stability show in figure below
6.
Wing
Area
Wing area is the
projected area of the planform and is bounded by the leading edge and trailing
edge and wing tips. The wing area is not the total surface area of the
wing(total surface area includes both upper and lower surface). The Wing area
is the projected area and is almost half of the total surface area.( take the reference wing area to be that of the trapezoidal
portion of the wing projected into the centerline.)
7. Twist
Wing twist is an aerodynamic
feature added to aircraft
wings
to adjust lift distribution along the wing.
Often, the purpose of lift redistribution is to ensure that
the wing tip is the last part of the wing surface to stall,
for example when executing a roll
or steep climb; it involves twisting the wingtip a small amount downwards in
relation to the rest of the wing. This ensures that the effective angle of
attack is always lower at the wingtip than at the root, meaning the
root will stall before the tip.
Twist
that decreases the local chord's incidence from root to tip is sometimes
referred to as washout. Washout is used to
control the spanwise development of the stall. Insufficient washout can cause
dangerous roll-off at the stall.
Twist that increases the local incidence from root to tip is
less common and is called wash-in. When the tip incidence and
root incidence are not the same, the twist is referred to as geometric twist.
However, if the tip airfoil section and root airfoil section are not the same,
the twist is referred to as aerodynamic twist.
Wing twist can also,
rarely, refer to the deflection of the wing when it is made of insufficiently
stiff materials.
8.
Chord
Chord
chord refers to the imaginary straight line joining the
leading and trailing edges of an airfoil.
The chord length is the distance between the trailing edge and the point
on the leading edge where the chord intersects the leading edge. The point on
the leading edge which is used to define the chord can be defined as either the
surface point of minimum radius, or the surface point which will yield maximum
chord length.
The chord of a wing, stabilizer and propeller is
determined by examining the planform
and measuring the distance between leading and trailing edges in the direction
of the airflow. (If a wing has a rectangular planform, rather than tapered or
swept, then the chord is simply the width of the wing measured in the direction
of airflow.) The term chord is also applied to the width of wing flaps,
ailerons
and rudder
on an aircraft.
Most wings do
not have a rectangular planform
so they have a different chord
at different positions along their span.
To give a characteristic figure which can be compared among various wing
shapes, the mean aerodynamic chord, or MAC, is used. The MAC is
somewhat more complex to calculate, because most wings vary in chord over the span, growing narrower
towards the outer tips. This means that more lift
is generated on the wider inner portions, and the MAC moves the point to
measure the chord to take this into account.
9.
Span
The
wingspan of an airplane is the distance from
one wingtip to the other wingtip , is always measured in a straight line, independently
of wing shape or sweep. For example, the Boeing 777
has a wingspan of about 60 metres (197 ft).
10.
Basic
theory wing
Wing is an aerodynamic structure that
generates lift when comes into contact with moving air molecules i.e. wind. The lift is generated due
to the wing’s unique shape. It is curved on the upper surface and is almost
flat on the bottom surface. This unusual form causes the air to go faster over
the top than the bottom. This difference in speed results in a difference in
pressure between the top and the bottom of the wing which exerts an upward net
force on the wing. This upward force is called lift.
Each wing section has a certain airfoil
that could be categorized as either laminar or conventional the difference
between these two types of airfoils is discussed later in the section.
It was found that the elliptical shape gave
tha uniform air deflection along the entire span, which minimize the induced
drag. It was also determined that the relationship between span and lift was
constant.
11.
Basic
theory wing mengacu pada elliptical wing, mengapa?
Menurut saya, itu
dikarenakan bentuk elliptical ini membuat sayap mempunyai distribusi lift yang
natural, yang mengurangi efek dari tip stall membuat defleksi udara pada span
menjadi seragam yang menyebabkan berkurangnya induced drag. Hubungan antara
span dan lift juga konstan. Bentuknya pun (elliptical) sangat indah untuk
dilihat, terkesan sangat modis.
12.
Span
efficiency factor (e)
Span
efficiency factor (e) measures the departure of the loading from its elliptic
optimum for the inviscid induced drag of a finite wing, when e =1. The
condition for e = 1 is only that the circulation distribution along the
span be elliptic, which, for a wing with constant profile shape, can come from
planform geometry or from wing twist.
This
e may be referred to as the Oswald
efficiency factor, or sometimes as the span efficiency, even though it is not
the same as e in CDI
equation because it contains corrections not only from departures from elliptical
loading δ but also from finite AR and from the presumed parabolic shape
of the section lift–drag polar k.
Increase Span Efficiency (e) can reduced induced
drag :
o Wing Tips
§ Some
Improvement possible (~ 5%)
o Winglets and End Plates
§ Induced
Drag Decreased
§ Parasite
Drag Increased
§ Span
Extension Usually Superior
o Improve Wing Root Junction Flow
§ Poor
Junction causes large loss of span efficiency
