Transcript click to save-helicopter lift

INTRODUCTION
A helicopter is an aircraft that is lifted and
propelled by one or more horizontal rotors, each rotor
consisting of two or more rotor blades.
A helicopter works by having its wings move through air
while the body stays still. The helicopter blades are called
main rotor blades. During flight there are four forces on the
helicopter and those forces are lift, drag, thrust and weight.
Technical Terms

Bernoulli' principle :This principle states that as the air
velocity increases, the pressure decreases; and as the
velocity decreases, the pressure increases .
 Airfoil : is technically defined as any surface, such as an
elevator, rudder, wing, main rotor blades, or tail rotor
blades designed to obtain reaction from the air through
which it moves
 Angle of Attack :is the acute angle measured between the
chord of an airfoil and the relative wind.
 Angle of Incidence :is the acute angle between the wing's
chord line and the longitudinal axis of the airplane.
(usually manufacturer had built the aircraft with the wing
has some degrees to the horizontal plane or airplane
longitudinal axis
 Blades : The blades of the helicopter are airfoils with a
very high aspect ratio ( length to chord ). The angle of
incidence is adjusted by means of the control from pilots.
 Swash Plate Assembly : The swash plate assembly
consists of two primary elements through which the
rotor mast passes. One element is a disc, linked to the
cyclic pitch control. This disc is capable of tilting in
any direction but does not rotate as the rotor rotates.
 Transmission : The transmission system transmits
engine power to the main rotor, tail rotor, generator
and other accessories
Description of lift on an airfoil
In a helicopter, the structure making flight possible is
the airfoil - a surfaced body that responds to relative motion
between itself and the air with a useful, dynamic reaction
known as lift. The term airfoil, refers to the rotary wing, and
more specifically means the curvature, or camber, of the
blade.
As the diagram indicates, the thick end of the section is
known as the leading edge. The small tapering end is the
trailing edge. The distance between the leading edge and
the trailing edge is known as the chord of the airfoil.
The rotary wing blade in a helicopter
is
asymmetrical, that is it has a curvature that changes along
the entire length of the chord. If the blade were
symmetrical, then the chord line would be a straight line
from the leading to the trailing edges. Since the curvature
changes constantly in rotor blade, the result is that the
chord of the blade also changes. When computing the chord
line of this type of blade, an average or mean aerodynamic
chord (MAC) becomes apparent .
When a blade (airfoil) is moved through the air, a
stream of air flows over and under it. The blade is designed
so that the flow of air will be smooth and will conform to the
shape of the moving blade. If the blade is set at the proper
angle and made to move fast enough, the airflow will
support the weight of the blade. This is the nature of the
action that enables rotary wings to furnish enough lift to
sustain the helicopter in flight.
Hence by applying bernoulli’s theorem we can
observe that lift is produced by a lower pressure created on
the upper surface of the helicopter’s wings compared to the
pressure on the wing's lower surfaces, causing the wing to be
lifted upward. The special shape of the rotor (airfoil) is
designed so that air flowing over it will have to travel a
greater distance and faster resulting in a lower pressure area
thus lifting the wing upward.
Lift equation
Lift depends upon:
(1) shape of the airfoil
(2) the angle of attack
(3) the area of the surface exposed to the airstream
(4) the square of the air speed
(5) the air density.
Where,





L is lift force,
ρ is air density,
v is air speed over the airfoil,
A is wing area, and
CL is the lift coefficient at the desired angle of attack
Lift in an established flow
 Established flow may be considered as steady, laminar
&
incompressible flow.
 As fluid never crosses a streamline in a steady flow; hence mass
is conserved within each streamtube.
 One streamtube travels over the upper surface, while the other
travels over the lower surface; dividing these two tubes is a
dividing line that intersects the airfoil on the lower surface,
typically near to the leading edge.
 The streamline leaves the airfoil at the sharp trailing edge, a
feature of the flow known as the Kutta condition.
 This image shows the streamlines over a NACA 0012 airfoil
of the real flow. The flow approaching an airfoil can be
divided into two streamtubes, which are defined based on the
area between two streamlines.
 The upper stream tube constricts as it flows up and around the
airfoil, a part of the so-called upwash. From the conservation of
mass, the flow speed must increase as the stream tube area
decreases. The area of the lower stream tube increases, causing
the flow inside the tube to slow down. It is typically the case
that the air parcels traveling over the upper surface will reach
the trailing edge before those traveling over the bottom.
 From Bernoulli's principle, the pressure on the upper
surface where the flow is moving faster is lower than
the pressure on the lower surface. The pressure
difference thus creates a net aerodynamic force,
pointing upward and downstream to the flow direction.
 The component of the force normal to the free stream is
considered to be lift; the component parallel to the free
stream is drag. In conjunction with this force by the air
on the airfoil, by Newton's third law, the airfoil imparts
an equal-and-opposite force on the surrounding air that
creates the downwash.
Principle of Helicopter Flight
Helicopter Lift is obtained by means of one or
more power driven horizontal propellers which called Main
Rotor.
When the main rotor of helicopter turns, it
produces lift and reaction torque. Reaction torque tends to
make helicopter spin. On most helicopters, a small rotor near
the tail which called tail rotor compensates for this torque.
On twin rotor helicopter the rotors rotate in
opposite directions, their reactions cancel each other.
MAIN ROTOR
 The lifting force is produced by the main rotor . As
they spin in the air and produced the lift. Each blade
produces an equal share of the lifting force. The
weight of a helicopter is divided evenly between the
rotor blades on the main rotor system. If a helicopter
weight 4000 lbs and it has two blades, then each blade
must be able to support 2000 lbs. In addition to the
static weight of helicopter ,each blade must be accept
dynamic load as well . For example, if a helicopter pull
up in a 1.5 time the gravity force, then the effective
weight of helicopter will be 1.5 time of static
helicopter weight or 6000 lbs. due to gravitational
pull.
 The tail rotor in normally linked to the main rotor via a system of
drive shafts and gearboxes .Most helicopter have a ratio of 3:1 to
6:1 . In most helicopter the engine turns a shaft that connected to
an input quill in the transmission gearbox.
Torque Reaction
If you spin a rotor with an engine, the rotor will
rotate,but the engine and helicopter body will tend to rotate in
opposite direction to the rotor. This is called Torque reaction.
Newton's third law of motion states , " to every action there is an
equal and opposite reaction" . The tail rotor is used to
compensates for this torque and hold the helicopter straight.
Dissymmetry of Lift
All rotor systems are subject to Dissymmetry of
Lift in forward flight . At a hover , the lift is equal
across the entire rotor disk . As the helicopter
gain air speed , the advancing blade develops
greater lift because of the increased airspeed and
the retreating blade will produce less lift , this
will cause the helicopter to roll .
In order to overcome this problem blade flapping
is done.
Dissymmetry of lift in helicopter aerodynamics refers to an uneven
amount of lift on opposite sides of the rotor disc.
The dissymmetry is caused by differences in relative airspeed between
the advancing blade and the retreating blade.
Blade Flapping
Dissymmetry of lift is compensated by blade
flapping ,because of the increased airspeed and
lift on the advancing blade will cause the blade to
flap up and decreasing the angle of attack . The
decreased lift on the retreating blade will cause
the blade to flap down and increasing the angle
of attack . The combination of decreased angle of
attack on the advancing blade and increased
angle of attack on the retreating blade through
blade flapping action tends to equalize the lift
over the two halves of the rotor disc.
Flight Control
Swash plate assembly :
Its primary component is the swash plate,
located below the rotor head. This swash plate
consists of one non-revolving disc and one
revolving disc mounted directly on top. The
swash plate is connected to the cockpit control
sticks and can be made to tilt in any direction,
according to the cyclic stick movement made by
the pilot, or moved up and down according to
the collective lever movement.
 The Collective Control :.
 The collective control is made by moving a lever
that rises up from the cockpit floor to the left of
the pilot's seat, which in turn raises or lowers
the swash plate on the main rotor shaft, without
tilting it. This lever only moves up and down and
corresponds directly to the desired movement of
the helicopter; lifting the lever will result in the
helicopter rising while lowering it will cause the
helicopter to sink
 The Cyclic Control :
The cyclic control works by tilting the swash plate
and changing the pitch angle of a rotor blade at a
given
point
in
the
rotation.
As the pitch angle changes, so the lift generated
by each blade changes and as a result the
helicopter becomes 'unbalanced', and so tips
towards whichever side is experiencing the lesser
amount of lift. Thus with its help, helicopter can
move right or left , backward or forward.
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