Wednesday, September 12, 2012

HOW IT WORKS: GLIDERS

by Aman Habtemariam

Introduction
A glider is a special kind of aircraft that has no engine. There are many different types of gliders. The
simplest gliders are paper, balsa wood or Styrofoam toy gliders. These gliders use the basics of aerodynamics.
Another form of glider is hang-glider, which is made with cloth wings and minimal structure,
like a piloted kite.
However this report is about sailplane gliders, with standard aircraft parts, construction and fl ight control
systems, but no engine. The wright brothers invented this kind of gliders in the year 1900 to 1903.
It is a combination of structural parts and units, which is independent with each other. The arrangement
of separate parts and units which make up the entire glider are subject to certain laws of construction.
The shape of the glider and its structural solutions are found on the basic requirements of
aerodynamics, strength, production and operation. Therefore the report shows what gliders are; how
glider functions and their technical description.


What are the components of glider? and what are their functions?
Gliders are designed with a smooth skin to allow the plane to slip more easily through the air. They are
constructed from aluminium with structural aluminium skins. The rivets and seams are not used, because it
produces additional drag, which tends to decrease performance. Therefore fi berglass and carbon fibre are
quickly replacing aluminium.


Glider can be divided into three main parts:
1. Fuselage (body): Gliders has no engine taking up space, they are sized around the cockpit, which carries
two people. This is considered to be the main body of a glider. It is cambered and we attach the wing in
the middle section, where the camber is maximum.
2. Wing: It is the most essential part of a plane. When air fl ows, due to the curvature of its upper and lower
parts lift is generated, which is responsible for balancing the weight of the plane, and the body can thus fl y.
The wings of a glider are longer and narrower than those on conventional aircraft. The slenderness ratio of
the wings is: (span of the wing) 2 / area of the wing.
The wings have very high aspect ratio, so their span is very long compared to the width, because the drag
created during the lift can have a signifi cant portion of the drag on a glider. And the only way to solve it is to
increase the slenderness ratio. Therefore they produce less drag for the amount of lift they produce.
The airfoil of a wing, which is a cross sectional shape of the wing consist of leading edge - front edge; trailing
edge - back edge; chord line - line connecting these two edges; camber line - a joining line of the two
edges of an airfoil equidistant from the upper and lower surfaces. The centre of pressure, which is the point
in airfoil where the lift is concentrated upon is located at C/4, where C is the chord length. It must coincide
with the centre of gravity, which is the point where the weight of the glider acts. To do that, we must add
some weight at the nose.


3. Control surfaces: Gliders use the same control surfaces as other conventional planes to control the direction of flight. The ailerons and elevator are controlled using a single control stick between the pilot’s legs.The rudder, as in conventional aircraft, is controlled using foot pedals.

The control surface consists of:
Ailerons: Controls the Rolling motion of the glider. It is placed on opposite directions on each side of the
glider. If the pilots want to roll the plane to the right he will use his control stick to the right. This causes the
left aileron to defl ect down, creating more lift on this side and the right aileron to defl ect up, creating less lift
on this side. This causes the glider to rotate about its long axis.
Elevator (horizontal stabilizer): It is a movable horizontal wing structure on the tail and it controls the pitch of
the glider, by pointing the nose of the glider up or down.
Rudder (vertical stabilizer): It is a movable vertical wing structure on the tail and it controls the yaw of the
glider, by pointing the nose of the glider from left to right or visa versa.
To understand the more about the movements look at the orientation of the glider.
4. Landing gear: By reducing the size of the gear, the size of a glider could be reduced. It consists of a single
wheel mounted just below the cockpit.
5. Cockpit: It consists of:
A. Altimeter- indicates altitude;
B. Air-speed indicator- tells how fast you’re going;
C. Variometer- tells what the air around you;
D. Radio- communication with other planes or ground;
E. Control stick- controls the control surfaces
F. Tow rope release knob- disengage the tow rope


What are the orientations of a glider?
Any aircraft, including gliders will rotate about its centre of gravity, a point which is the average location
of the mass of the glider. To defi ne the orientation of the glider, we must fi rst defi ne the three dimensional
coordinate system through the centre of gravity with each axis perpendicular to the other two axes, by the
amount of rotation of the parts of the glider along these principal axes.
Yaw axis: It is perpendicular to the wings with its origin at the centre of the gravity and directed towards
the bottom of the aircraft. Yaw motion is a movement of the nose of the aircraft from side to side. Rudder is
used to make this motion possible.
Pitch axis: It is perpendicular to the yaw axis and is parallel to the wings with its origin at the centre of the
gravity and directed towards the right wing. Pitch motion is an up or down movement of the nose of the
glider. Elevators are used to make this motion possible.
Roll axis: It is perpendicular to the other two axes with its origin at the centre of gravity, and is directed
towards the nose of the glider. So the rolling motion is an up or down movement of the wing tips. Aileon are
used to make this motion possible.
Aerodynamic forces
There are four aerodynamic forces acting on a glider in fl ight and these are:
1. lift: upward force generated by wings
2. Gravity: downward force, which is the weight of the glider
3. Drag: backward force, which is the resistance of air
4. Thrust: forward motion



How does a glider stays in the air without an engine?
For the lift, wing of a glider plays an important role; it must produce enough lift to balance the weight of the
glider. The faster the glider goes the more lift the wings make and eventually it will produce enough lift to
keep it in the air. But the wings and the fuselage produce drag, and they produce more drag the faster it
fl ies. The glider has to generate speed by angling it downward and trade altitude for speed, because it does
not have engine to produce thrust. By doing this it will fl y fast enough to generate the lift needed toe support
its weight.



The performance of the glider is measured by glide ratio, which could reach to 60:1. The glide ratio tells
us how much horizontal distance a glider can travel compared to the altitude it has to drop. If you compare
it with a commercial jetliner, its glide ratio is somewhere around 17:1. But glide ratio is not the only factor
involved in making the glider stay in the air. So how do they do it?
A glider is dependent on nature, it will slowly descend with respect to the air around it, but what if the air
around it is moving upward faster than the glider was descending? For example if a glider descends at 1
m/s and air around the glider is rising at 2 m/s, the glider is gaining altitude.
There are three main types of rising air used by glider pilots increase their fl ight times:
1. Thermals: As the sun heats the air on the ground it will expand and rises. Therefore parking lots, dark
ploughed fi elds and rocky terrain, are a great way to fi nd thermal columns. Pilots look also for newly forming
clouds or large birds soaring without flapping their wings, which can also be signs of thermal activity. So
glider will follow the thermal direction until they reach their desired altitude.
2. Ridge lift: It is created by wind blowing against mountains, hills or other ridges. As the air reaches the
mountain, it will redirect upward and form a band of lift along the windward side of the slope.
3. Wave lifts: It is similar to ridge lift and it is created on the leeward side of the peak by winds passing over
the mountain instead of up one side and with the help of this lift gliders could reach an altitude of more than
10.700 meters.


What is airfoil? How is it designed?
Airfoil is surface on a glider that produces lift. Even though there is different kind of airfoils, all produce lift
in a same way. This is the main section of glider and fundamental principles of aerodynamics are used to
design the airfoil.
While in flight three kind of forces are applied- lift, drag, weight and thrust:
Lift opposes the downward force of weight and is produced by the dynamic effects of airstream acting on
the wing. Lift is produced when there is decreased pressure above and increased pressure below an airfoil.
Drag is the force that resists the movement of the glider through the air. There are two kind of drag: parasite
drag- aircraft surface is interfered, which causes with the smooth airfl ow around the glider to slow down;
Form drag- turbulent wake causes separation of airfl ow form the surface of a structure. A great streamlined
airfoil designs reduces form drag by reducing the amount of airfl ow separation.
Weight is the third force that acts on a glider. Weight affects the vertical lift through the centre of gravity of
the glider. Gravitational force will act to move the glider through the air since a portion of the weight vector
of a glider is directed forward.
Thrust is the forward force and in the case of unpowered gliders have an outside force, such as a tow
plane, winch or automobile to launch the glider.
Therefore the airfoil is design using the fundamental understanding of the aerodynamic forces. The airfoil is
streamlined and the end are rounded as far as possible in order to reduce drag, the weight of the model is
kept as minimum. When the drag is more than the lift, then the glider will loss lift and will result into a stall,
where the weight of the glider cannot be supported any longer.

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