Basic structure of an
airplane
Lesson 2: PRINCIPLES OF AERODYNAMICS
Aerodynamics
As we learned before, the word
“aerodynamics” is derived from two Greek words.
To some, speaking about aerodynamics is still Greek. Aerodynamics is
most certainly a language of its own so
we must have a working knowledge of terms associated with aerodynamics.
Aerodynamics: CHARACERISTICS
OF AIR.
The air that passes over an airfoil will
have a direct affect on the ability of
the airfoil to produce lift. As a result, an aircraft’s capacity to carry fuel, cargo, passengers, and munitions is
affected. Also, variables in the makeup of air
can affect the distance required for takeoff and landing.
Density.
Although related to pressure, density
measures the mass of air per cubic foot
and is directly related to lift production by an airfoil.
Increased Density = Increased Lift
Pressure.
The pressure exerted by the atmosphere at
a given point. Its measurement can be
expressed in several ways. However, when used in aerodynamics the force per unit area is in inches or
millimeters of mercury (Hg). Also known as
barometric pressure. Barometric pressure and density are directly
related.
Increased Pressure = Increased
Density
Temperature.
Defined as the measure of molecular motion
or the degree of heat of a substance. It
is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is
also the degree of hotness or coldness. In
surface observations, it refers primarily to the free air or ambient
temperature close to the surface of the
earth. Of importance to our discussion, is that increased molecular motion (associated with increased ambient temperature)
means each air molecule occupies more
space. Therefore, fewer molecules can occupy a given volume, and we see that temperature is inversely related to
density.
- Increased Temperature = Decreased
Density
Humidity.
Defined as the amount of water vapor
present in the air. Water vapor
molecules are actually lighter than air molecules and will thus displace
air molecules. This reduces the air
density and means humidity and density are inversely related.
- Increased Humidity = Decreased Density
RELATIVE MOTION.
The motion that exists when one object
changes its position in respect to the
position of another object, even though one or the other may remain stationary with respect to a third
object. Relative motion is important in many
applications; for example, a generator produces electricity by creating
relative motion between a magnetic field
and a conductive material. With aircraft, we must create relative motion between an airfoil and a body
of air in order to create lift. From an
aerodynamic standpoint a minimum amount of relative motion (i.e. airspeed)
is required to create and sustain enough
lift to hold the aircraft aloft. From a navigational perspective, the aircrafts motion relative to
the earth (ground speed) takes on
importance as we then concern ourselves with traveling from point A to
point B.
INDICATED AIRSPEED
(IAS).
IAS is the airspeed shown on an
airspeed indicator. It is a measurement
derived by taking the difference between STATIC
pressure and IMPACT pressure. Static pressure is the air pressure
exerted on an object, such as the static
pressure port on an aircraft, when the air is not moving. Thus, the static ports on an aircraft are located
in an area out of the direct airstream. Impact
pressure is the pressure exerted by the air that impacts a pressure
sensor (Pitot tube) on the aircraft as a
result of forward movement. The Pitot tube is then of course located where it will be in the direct
airstream.
TRUE AIRSPEED (TAS).
At sea level, indicated airspeed gives us
an accurate measure of relative motion
between the aircraft and the airstream (TRUE AIRSPEED). However, as altitude increases and the air
becomes thinner, the IAS becomes less
accurate. For example, at sea level a TAS of 440 MPH will equal an IAS
of about 440. At 20,000 feet, a TAS of
440 MPH will have an IAS of about 360. TAS adjusts IAS for a given temperature and pressure. An air data
computer receives inputs on actual
temperature and pressure readings outside the aircraft and makes the
appropriate calculations to correct the
given IAS to a TAS. Thus, TAS is a measure of the velocity of the relative motion of an aircraft.
AIRFOIL.
By design, an airfoil, figure 1-2, is
built to obtain a reaction upon itself
from the air through which it passes. In most cases, it is designed to
produce lift. There are other functions
for airfoils as well, such as propellers, which are airfoils that produce thrust. In addition to wings and empennages,
even some aircraft fuselages are
considered airfoils. They produce limited lift on some heavy aircraft
and a significant amount of lift for
fighters. Finally, flight control surfaces are airfoils that produce a reaction, which allows the aircraft to be
moved about an axis of rotation. This provides
controllability and maneuvering of the aircraft.
RELATIVE WIND.
Relative wind is simply the air moving
past an airfoil. Its velocity and
direction are directly equal to and opposite of the aircraft’s flight path.
Figure
1-2, Airfoils
FLIGHT PATH.
The path the center of gravity would
follow in flight. Can be changed by
forces acting upon the aircraft (such as a crosswind), in addition to directional changes resulting from flight
control inputs. The center of gravity is located at the intersection of the pitch, roll, and
yaw axes for a given aircraft.