Introduction to Aeronautical Engineering

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Introduction to Aeronautical Engineering

EGN1007: Engineering Concepts and Methods

Introduction to Aeronautical Engineering

The Bottom Line

In normal flight a light
aeroplane derives
its forward motion
from the
thrust

provided by the
engine
-
driven
propeller.


If the aircraft is maintaining a
constant height,
direction and speed

then the thrust force will
balance the air's resistance
(drag)

to the
aircraft's motion through it. The forward motion
creates an airflow over the wings and the
dynamic pressure changes within this airflow
create an upward acting force or
lift
, which will
balance the force due to gravity


weight



acting
downward.

The Bottom Line

Thus in normal
unaccelerated flight
the four basic forces
acting on the aircraft
are approximately in
equilibrium
. The pilot
is able to change the
direction and
magnitude of these
forces and thereby
control the speed,
flight path and
performance of the
aeroplane.

The property of resisting any
change in motion:
INERTIA

The
mass

of a body is a measure of its
inertia


i.e.
its resistance to being accelerated or decelerated
by an applied force increases with mass. The unit
of mass we will be using is the kilogram [kg].







The air also has mass and thus inertia and will resist
being pushed aside by the passage of an aeroplane.
That resistance will be felt as pressure changes on
the aircraft surfaces.



Airspeed depends on Inertia

An aircraft in flight
is 'airborne' and
its velocity is
relative to the
surrounding
air
,
not the Earth's
surface.

When the aircraft encounters a sudden change in the ambient air
velocity


a transient gust


inertia comes into play and
momentarily maintains the aircraft velocity relative to the Earth or


more correctly


relative to space. This momentarily changes
airspeed and imparts other forces to the aircraft.
(The fact that inertia
over
-
rides the physics of aerodynamics is sometimes a cause of
confusion)
.


Direction of forces are relative
to the flight path


Although we said that lift acts
vertically upward with
thrust and drag acting
horizontally, this is only
true when an aircraft is in
straight and level flight. In
fact, lift acts perpendicular
to both the flight path and
the lateral axis of the
aircraft, drag acts parallel
to the flight path and
thrust usually acts parallel
to the longitudinal axis of
the aircraft.

The lift equation


Aerodynamicists have found it convenient to resolve
that resultant force into just two components, that
part acting backward along the flight path is the wing
drag and that acting perpendicular to the flight path is
the lift. The amount of lift, and drag, generated by the
wings is chiefly dependent on:



Lift, Pressure, and Angle of Attack

Is there a way to calculate the lift and drag?

The lift equation

The amount of lift, and drag, generated by the wings is
chiefly dependent on:




the angle at which the wings meet the airflow or flight
path,


the shape of the wings particularly in cross section


the
aerofoil
,


the density (i.e. mass per unit volume) of the air,


the speed of the free stream airflow i.e. flight
airspeed,


and the wing plan
-
form surface area.


A
v
C
Lift
L
2
2
1


The lift equation

A
v
C
Lift
L
2
2
1


The values in the expression are:





(the Greek letter rho) is the density of the air,
in kg/m³




is the flight speed in meters per second


A

is the wing area in square meters


C
L

is a dimensionless quantity


the lift
coefficient. Mostly depends on the
ANGLE

of
attack and the
SHAPE

of the wing.

Angle of attack and the lift
coefficient


The diagram shows a
typical C
L

vs. angle of
attack curve for a
light aeroplane not
equipped with flaps
or high
-
lift devices.
From it you can read
the C
L

value for each
“aoa”, e.g. at 10
°

the
ratio for conversion of
dynamic pressure to
lift is 0.9

The lift equation: An Example

Calculate C
L

for the an 408.2 kg aircraft
cruising at 6500 feet at 97 knots ( 1 knot


0.5148 m/s). The wing area is very
close to 8 m²:




• lift = weight






= 1.0 kg/m³ (the approximate density
of air at 6500 feet altitude)


A
v
C
Lift
L
2
2
1


The Drag Equation

A
v
C
Drag
D
2
2
1


The drag equation is
similar to the lift
equation with

the exception that we
have a DRAG
COEFFICIENT

rather than a LIFT
COEFFICIENT. As C
L
depending on

“aoa”, the
C
D

depends
on the SQUARE of the
“aoa”.

We can make
this assumption based
on graphical data.

What effect does decreasing
speed have?

So the result of decreasing airspeed, while maintaining
straight and level flight, is an increase in the lift
coefficient; and that has two contributors


the
shape

of the wing and the
angle of attack


As the pilot can't change the wing shape (unless she/he extends
flaps) the angle of attack must have changed. How? By the pilot
adjusting control pressure to apply an aerodynamic force to the
aircraft's tailplane (
or some other control surface
) which has the
effect of rotating the aircraft a degree or so about its lateral axis.


Drag

Without the needed thrust, weight has more influence
than lift and pulls the airplane toward the ground.
Helping the force of weight is drag.
Drag is present
at all times and can be defined as the force which
opposes thrust, or, better yet, it is the force which
opposes all motion through the atmosphere and
is parallel to the direction of the relative wind


INDUCED DRAG
:
Newtonian &
Pressure Induced

Induced drag is the unavoidable by
-
product of
lift and increases as the angle of attack
increases

Newtonian or DYNAMIC

DRAG is caused by the

INERTIA of AIR.

Pressure Induced Drag

occurs when the “aoa”

Is too large and the air

Flow becomes turbulent.

PARASITE DRAG


There are also skin
-
friction drag and form drag, which
are referred to as parasite drag. All drag other than
induced drag is parasite drag.


Skin
-
friction drag is caused by the friction between
outer surfaces of the aircraft and the air through
which it moves. It will be found on all surfaces of the
aircraft: wing, tail, engine, landing gear, and fuselage


Putting it all together: Lift and
Drag

The LIFT/DRAG ratio can be found by
taking the lift coefficient and dividing by
the drag coefficient.

D
L
C
C
ratio
D
L

/
The L/D ratio is a measure of EFFICIENCY!!!

L/D Ratio

The
tangent

of the glide angle is equal to
the vertical height (h) which the aircraft
descends divided by the horizontal
distance (d) which the aircraft flies across
the ground.

L/D Ratio

What good is all this for aircraft design?


d
h
C
C
D
L
L
D
D
L








tan
tan
1
tan
From the last equation we see that the higher the
L/D, the lower the glide angle, and the greater the
distance that a plane can travel across the ground for
a given change in height

L/D Ratio

Because lift and drag are both aerodynamic
forces, we can think of the L/D ratio as an
aerodynamic efficiency

factor for the aircraft.
Designers of gliders and designers of cruising
aircraft want a high L/D ratio to maximize the
distance which an aircraft can fly. It is not
enough to just design an aircraft to produce
enough lift to overcome weight.
The designer
must also keep the L/D ratio high to maximize
the range of the aircraft.