Basic structure of an airplane
Lesson 3: AIRFOIL CHARACTERISTICS
chord Line, AIRFOIL CHARACTERISTICS, Camber, Angle of Attack, Angle of Incidence, Dihedral, Anhedral, Static Pressure, Total Pressure, BERNOULLI’S
AIRFOIL CHARACTERISTICS.
Chord and Chord
Line.
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The wing chord,
figure 1-3, is the distance from the leading edge (forward) to the trailing
edge (aft) of the wing, parallel to the fuselage. It is measured in a direct
manner as opposed to around the upper or lower surfaces of the wing. If the
wing is not rectangular in shape, the chord length will vary at different
points on the wing. The average of these chords, or Mean Aerodynamic Chord, is
used for balance calculations.
Camber.
The term camber is
analogous to curvature. Camber is defined as the ratio of the greatest distance
between the chord line and the mean camber line (next paragraph) to the length
of the chord. In general, the greater the amount of camber and the thicker the
airfoil, the greater the coefficient of lift will be. A high camber and large thickness
usually causes the airfoil to stall gradually at high angle of attack. Conversely,
a thin, flat airfoil loses lift abruptly at low angles of attack.
Mean Camber Line.
Figure 1-3 shows an
imaginary line drawn from the leading edge to the trailing edge of the wing
that is equidistant from the upper and lower surfaces of the wing. Used to
determine the curve of the wing.
ANGLE OF ATTACK (AOA).
Defined as the
angle between the relative wind and the chord of an airfoil, (Figure 1-4). AOA
directly affects the amount of lift being produced on an airfoil and managing
AOA is extremely vital to aircraft performance. This is accomplished by both
the aircrew in flight, as well as by aircraft design.
Figure 1-4, Angle of Attack
ENGINEERING
DESIGNS.
Aerodynamic
engineers use a number of design parameters to obtain certain effects from an
airfoil. These can be manipulated to allow for different aircraft types or
functions.
Angle of Incidence.
This is the angle
between the chord line and the longitudinal axis. Engineers determine the angle
of incidence, figure 1-5, using two factors. First, the aircraft fuselage
should be moving through the air at a zero angle of attack. Secondly, the wing
should be at its maximum lift ratio for the average operating condition.
Figure 1-5,
Angle of Incidence
Dihedral and
Anhedral.
These terms refer
to the angle between the wing chord plane and the lateral axis. See figure 1-6.
The purpose of the dihedral angle is to affect roll stability when the aircraft
is in a skid or slip such as while in a banked turn. The dihedral will cause an
increase in the angle of attack on the low wing since the wind is hitting that
wing more on the underside. This will cause more lift on the low wing that will
tend to return the aircraft to straight and level. Thus, the dihedral design
promotes lateral, or roll stability.
Figure 1-6,
Dihedral angle on a KC-135
While a dihedral configuration angles the
wing tips up from the fuselage, an anhedral configuration angles the wing tips
down. This creates the opposite effect of dihedral by decreasing lateral
stability, making the aircraft more likely to roll. Even though it is less
stable, some aircraft are designed with anhedral wings and/or flight control
surfaces to increase maneuverability. The F-16 in figure 1-7 utilizes a
slightly anhedral design on its wings and a very noticeable anhedral on the
aircraft’s tail section. These together give it incredible maneuverability, but
at the expense of inherent stability.
Figure 1-7, Anhedral
angle on an F-16
AIR PRESSURES
EXERTED ON AN AIRFOIL.
Lift is a result of pressure exerted on the surfaces of the wing of an aircraft. In aerodynamics, there are two kinds of pressure: Static and dynamic.
Static Pressure (Ps). Very simply, static pressure is the pressure exerted on a surface by unmoving air. Static pressure decreases as dynamic increases.
Dynamic Pressure (Pd). Dynamic pressure is exerted when the airfoil or the air around it begins to move. Dynamic pressure increases with the velocity of the air that moves over the top of the airfoil.
Total Pressure (Pt). Total pressure is the sum of static and dynamic pressure. It remains constant in a given set of conditions.
- Formula: Pt = Ps + Pd
BERNOULLI’S
PRINCIPLE.
Daniel Bernoulli, a
Swiss scientist, advanced the following theory in 1738: As the velocity of a
gas or liquid increases, the pressure decreases; and as the velocity decreases
the pressure increases. See figure 1-8. This principle allows engineers to
accurately predict such forces as lift and drag. A venturi tube, a tube with
flared ends and a constricted middle, illustrates this principle. However,
certain assumptions must be made for the purposes of the experiment:
The gas
- Is incompressible
- Has a constant rate of flow
- Passes through the tube without turbulence
Obviously, the
amount of air entering the tube equals the amount of air exiting the tube. If
the area is the same at each end, then the speed of the air will be the same at
each end. At the throat of the venturi, where the area is smaller, the velocity
must be higher in order to pass the same amount of air as at the ends. Because
air has mass, a force must be exerted to accelerate it. The force in the
venturi tube comes from a difference in the static and dynamic pressures. The
pressure difference at the narrow section of the venturi will be greater than
that at the entrance. The relationship between velocity and pressure is known
as Bernoulli’s principle.
In simple terms,
the relationship states that the dynamic pressure will gain any loss in static
pressure because the total pressure remains constant. The increased velocity at
the throat of the tube causes the loss in static pressure, which results in an increased
dynamic pressure. This differential pressure is the foundation for lift produced
by an airfoil.
A wing is designed
so that the distance from the leading edge to the trailing edge of the wing is
different around the upper and lower surfaces. It is further across the top of
the wing than it is across the bottom. As air moves across a wing, it must move
faster across the top surface than across the bottom surface in order to
maintain its integrity at the trailing edge. If you apply Bernoulli’s
principle, the slower moving air on the bottom of the wing creates more
pressure than the faster moving air on the top. The net effect is called lift.
Figure 1-8, Bernoulli’s
principle illustrated
FACTORS AFFECTING
LIFT.
Angle of Attack.
If you recall, the
angle of attack is the angle between the relative wind and the chord of an
airfoil. As the AOA increases, lift increases with it until a point of
diminishing return is reached. At this critical point, the wing begins to stall
and lift decreases dramatically. This phenomenon is explained in detail in the
section on Boundary Layer Separation.
Airfoil Shape.
As camber, or
curvature increases, so does the lift created by the airfoil. This property is
used when employing flight controls. Flight controls increase or decrease lift
in a given direction thus causing the airfoil to move in that direction.
Air Density.
As air density
increases, lift increases and vice-versa. Air density is dependent on
atmospheric conditions such as humidity and temperature. Increased humidity
lowers air density and as a result, decreases lift. Temperature also affects density
– higher temperature lowers density and therefore lifts generation.
True Airspeed.
As an aircraft sits
still, the only pressure exerted on the airfoil is static pressure. In order to
achieve a static pressure differential, the aircraft must move through the air
to create dynamic pressure. As the airspeed increases, the static pressure
differential on the airfoil increases resulting in increased lift. Similar to
AOA, there is a point where the effect of TAS is negligible.
Wing Area.
For a given airfoil
shape and aircraft weight, the greater the wing area then the greater the
amount of lift produced by that airfoil. This is a result of fewer pounds per
unit of area of wing surface.
AIRFLOW ABOUT AN
AIRFOIL.
Any discussion
about airflow must begin with the subject of boundary layers. Before reading
on, be sure you understand skin friction as introduced in this study guide’s
section on drag. The boundary layer is that thin layer of air in direct contact
with the surface of an airfoil.
Laminar and
Turbulent Airflow.
Because of skin
friction, which exists between the airfoil and the boundary layer, the boundary
layer is subject to very powerful decelerating forces. In order to maintain
airflow, fast flowing layers of air must pass over slower layers beneath them.
If this flow pattern is smooth, the boundary layer is said to be laminar. On
the other hand, if the air is unable to travel smoothly the flow is said to be
turbulent.
Boundary Layer
Separation and Stall.
The usual tendency
is for airflow to remain laminar from the leading edge until it begins to grow
turbulent at a point near the trailing edge. The point at which the turbulence
begins is considered the start of boundary layer separation. See Figure 1-9.
Increasing the AOA until it reaches 16 degrees will continuously increase the
lift produced by a conventional shaped airfoil. Beyond 16 degrees, the airflow
over the upper surfaces is subject to forces too great to allow laminar flow.
When the BLS transition point occurs far enough forward on the upper surface of
the airfoil the result is separation to a point where it decays lift. At this point,
the airfoil is said to stall. Beyond this stall angle, called the critical
angle of attack, the airfoil suddenly produces insufficient lift to keep the
aircraft aloft and the results can be disastrous.
Figure 1-9, Boundary
Layer Separation
Boundary Layer Control.
A variety of
devices have been invented to ensure that the airflow remains laminar. Though
there are more, we will discuss some of the more common ones here.
Slots and Slats.
Slots and slats are
secondary flight control surfaces located on the leading edge of the wing. They
actually allow air to flow through the leading edge of the wing in certain
areas. This helps to maintain laminar airflow at a higher AOA.
Wing Fences.
Wing fences, figure
1-9, are vertical, fin-like structures on the upper surface of the wing. They
help control the airflow over the flaps and near the wing tips. On swept wing
airplanes, they are located about two-thirds of the way out towards the wing
tip and prevent the drifting of air toward the tip of the wing at high angles
of attack. On straight wing airplanes, they control the airflow in the flap
area. In both cases, they give better slow speed handling and stall
characteristics.
Figure 1-9,
Wing Fences
Vortex Generators.
The vortex
generator, figure 1-10, is a small vane set at an angle to the airflow on the
upper surface of the wing, although not an airfoil with camber, the resulting
rotation of air produces a high drag phenomena, transfers energy downstream.
This reduces overall drag and buffeting and maintains boundary airflow. Vortex
generators are also placed on the aft fuselage and on the empennage to reduce buffeting.
Figure 1-10,
Vortex Generators
Control Surface
Effects
As an aircraft moves through the air, the
pilot makes inputs to the flight controls through various means. As the control
surfaces are manipulated, the aircraft moves about any of three axes of
rotation or most likely a combination of the three. These axes all intersect at
the center of gravity of the airplane. See Figure 1-11.
LONGITUDINAL AXIS.
An imaginary line extending from the nose
to the tail of the airplane. Motion about this axis is called roll.
LATERAL AXIS.
The lateral axis is
the horizontal reference line passing from wing tip to wing tip. It is
perpendicular to the longitudinal axis. Movement about this axis is called
pitch.
VERTICAL AXIS.
The vertical axis
is the reference line that is perpendicular to both the other axes. Rotation
around this axis is called yaw.
