Propel me

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Lesson 13 Propel me
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In a helicopter, you can move in any direction or
you can rotate 360 degrees
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How do helicopters work? The wings of a
normal airplane obtain upward lift forces from
the air as the airplane moves forward through the
air. That's because the shape and angle of the
wings is such that air flows faster over the top
surface of each wing than under the bottom
surface. A helicopter spins its wings around in a
circle so that they move through the air even
when the helicopter itself is stationary. Normally
, these rotating wings are called blades. Again,
the air flows faster over each blade than beneath
it and there is a net upward pressure force on
each blade. These upward forces support the
helicopter and they also allow it to tilt
itself--by adjusting the angle of each blade as
the blades turn, the helicopter can obtain twists
from the air so accelerates forward, backward, or
toward the side.
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The Main RotorA helicopter's main rotor is the
most important part of the vehicle. It provides
the lift that allows the helicopter to fly, as
well as the control that allows the helicopter to
move laterally, make turns and change altitude.
To handle all of these tasks, the rotor must
first be incredibly strong. It must also be able
to adjust the angle of the rotor blades with each
revolution of the hub. The adjustability is
provided by a device called the swash plate
assembly.
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• The swash plate assembly consists of
  two plates -- the fixed and the rotating swash
  plates
• shown above in blue and red,
  respectively.
• The rotating swash plate rotates with the drive
  shaft (green) and the rotor's blades (grey)
  because of the links (purple) that connect the
  rotating plate to the drive shaft.
• The pitch control rods (orange) allow the
  rotating swash plate to change the pitch of the
  rotor blades.
• The angle of the fixed swash plate is changed by
  the control rods (yellow) attached to the fixed
  swash plate.
• The fixed plate's control rods are affected by
  the pilot's input to the cyclic and collective
  controls.
• The fixed and rotating swash plates are connected
  with a set of bearings between the two plates.
  These bearings allow the rotating swash plate to
  spin on top of the fixed swash plate.

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The cyclic changes the angle of attack of the
main rotor's wings unevenly by tilting the swash
plate assembly. On one side of the helicopter,
the angle of attack (and therefore the lift) is
greater.
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The Forces At Work
There are many forces at work when a helicopter
flies, and many are specific to helicopter
flight. We all have learnt about lift, drag,
gravity, and thrust
In order to spin the shaft with enough force to
lift a human being and the vehicle, you need an
engine. This arrangement works really well until
the moment the vehicle leaves the ground. At that
moment, there is nothing to keep the engine (and
therefore the body of the vehicle) from spinning
just like the main rotor does. So, in the absence
of anything to stop it, the body will spin in an
opposite direction to the main rotor. To keep the
body from spinning, you need to apply a force to
it. The usual way to provide a force to the body
of the vehicle is to attach another set of
rotating wings to a long boom. These wings are
known as the tail rotor. The tail rotor produces
thrust just like an airplane's propeller does. By
producing thrust in a sideways direction,
counteracting the engine's desire to spin the
body, the tail rotor keeps the body of the
helicopter from spinning.
The tail rotor has a 6 to 1 rotational ratio to
the main rotor system.
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Autorotation
Most people think that a helicopter will fall
like a rock and the rotor system will stop once
the engine fails. This is a totally false
assumption. A helicopter can continue to fly
without any power from the engine. "Autorotation"
is the term used for "Gliding" a helicopter down
after the engine fails or the throttle is
retarded to the idle position. If you look at a
rotor blade from the tip of the blade toward the
root, you will see it will twist laterally. At
the tip of the blade, the leading edge may point
down while at the root of the blade, the leading
edge may point up. This allows different regions
of the blade to perform different tasks, one of
which is Autorotation. The outer portion of the
blade, when the collective is lowered all the way
to what is called "Flat Pitch", will drive the
rotor system as the aircraft glides downward,
increasing or maintaining the speed of the rotor
system. The rotor system is driven normally by a
centrifugal clutch which is positively engaged
while the engine supplies power, but disengages
when power is removed. The rotor system "Free
Wheels", and continues to spin. The air
travelling upward through the rotor system
continues to drive the system and maintain rotor
RPM.
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Notes
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A rotary motion is the easiest way to keep a wing
in continuous motion. So you can mount two or
more wings on a central shaft and spin the shaft,
much like the blades on a ceiling fan. The
rotating wings of a helicopter are shaped just
like the airfoils of an airplane wing, but
generally the wings on a helicopter's rotor are
narrow and thin because they must spin so
quickly. The helicopter's rotating wing assembly
is normally called the main rotor. If you give
the main rotor wings a slight angle of attack on
the shaft and spin the shaft, the wings start to
develop lift. In order to spin the shaft with
enough force to lift a human being and the
vehicle, you need an engine of some sort.
Reciprocating gasoline engines and gas turbine
engines are the most common types. The engine's
driveshaft can connect through a transmission to
the main rotor shaft. This arrangement works
really well until the moment the vehicle leaves
the ground. At that moment, there is nothing to
keep the engine (and therefore the body of the
vehicle) from spinning just like the main rotor
does. So, in the absence of anything to stop it,
the body will spin in an opposite direction to
the main rotor. To keep the body from spinning,
you need to apply a force to it. The usual way
to provide a force to the body of the vehicle is
to attach another set of rotating wings to a long
boom. These wings are known as the tail rotor.
The tail rotor produces thrust just like an
airplane's propeller does. By producing thrust in
a sideways direction, counteracting the engine's
desire to spin the body, the tail rotor keeps the
body of the helicopter from spinning. Normally,
the tail rotor is driven by a long drive shaft
that runs from the main rotor's transmission back
through the tail boom to a small transmission at
the tail rotor. What you end up with is a
vehicle that looks something like this
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INTELLIGENT IGNORANCE Desire can create
'Intelligent Ignorance' and with this a lot of
things can happen. You've no doubt heard the
story about the bumble bee. It's a classic
example of what intelligent ignorance is all
about. If you have done any reading into the
science of aerodynamics you will know that the
bumble bee cannot fly. His body is too heavy and
his wings are too light. It is impossible for him
to fly. But you see the bumble bee doesn't read
- the bumble bee flies! Henry Ford was a
classic case of intelligent ignorance. He was a
man with a limited education. He had made a
fortune with the Model T and then the Model A and
then one day he had a wild idea. He conceived an
idea of building a V8 engine. Mr. Ford was not
an engineer so he called his highly paid staff
together and said "Gentlemen, I want you to build
me a V8 engine". They explained to him in no
uncertain terms that the V8 was an engineering
impossibility, that it couldn't be done. He
said "Well, I understand that, but we've got to
have one, and I want you to build one. What's
more I want you to build it right now!" The
story goes that they spent quite a few dollars
making a half-hearted effort and came back a few
months later to say to Mr. Ford, "Like we said
Mr. Ford, the V8 is an impossibility - it cannot
be done." Mr Ford said, "Gentlemen, obviously
you don't understand. We've got to have a V8
engine and you've got to build one, so go build
it!" Well this time the engineers went out and
they spent a little more money and they stayed a
little longer. They finally came back and said
"Mr. Ford, it just can't be done." This time
Mr. Ford hit the ceiling! He said "Gentlemen,
obviously I am not communicating my message to
you. The V8 engine is going to be built and
you're going to build it for me. So go out there,
and this time I don't want you to come back until
you tell me the news you know I want to hear! You
are going to build a V8 engine". They built the
V8 engine! Henry Ford had that intelligent
ignorance. He once said, "I'm looking for a lot
of men with an infinite capacity for not knowing
what can't be done." We need more people who
don't know what can't be done. They are the ones
who will go out there and do it. Henry Ford had
the desire! Find your desire, set your goals,
take action - and make your dreams a reality
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It takes both hands and both feet to fly a
helicopter! One hand grasps a control called the
cyclic, which controls the lateral direction of
the helicopter (including forward, backward, left
and right). The other hand grasps a control
called the collective, which controls the up and
down motion of the helicopter (and also controls
engine speed). The pilot's feet rest on pedals
that control the tail rotor, which allows the
helicopter to rotate in either direction on its
axis.
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