HYDRAULICS AND LANDING GEAR

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HYDRAULICS AND LANDING GEAR
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BASIC LAWS

• Hydraulics take advantage of the fact that
  liquids are flexible and incompressible.
• A liquid in a closed system acts like a flexible
  method of transmitting force.
• Pascals law states Pressure in an enclosed
  container is transmitted equally and undiminished
  to all parts of the container and acts at right
  angles to the enclosing walls.

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AIRCRAFT HYDRAULIC SYSTEMS

• Hydraulics are used for a variety of aircraft
  applications
• Brakes.
• Landing gear.
• Flight control.
• Flaps.
• Speed brakes.
• Nosewheel tillers
• Name the hydraulic systems on the C-172.

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HYDRAULIC FLUID

• A good hydraulic fluid is as incompressible as
  practical.
• Flows with minimum friction.
• Has strong lubricating properties.
• Resistant to foaming.
• Maintain its properties at high temperatures.
• Two commonly used fluids in aviation are
  MIL-H-5606 (red, petroleum-based) and Skydrol
  (purple, synthetic).
• Hydraulic fluid must never be mixed.
• Hydraulic fluid is flammable (5606).
• Caution hydraulic fluids are suspected to have
  adverse health effects including nerve damage.
  Avoid contact with skin.

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SYSTEM COMPONENTS

• Hydraulic pumps typically engine or electrically
  driven gear type pumps, which provide system
  pressure.
• Large aircraft will typically have more than one
  interconnected hydraulic systems with backup
  pumps in case of failure.

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SYSTEM COMPONENTS

• Axial-piston pumps achieve up to 98 efficiency
  and can maintain pressure between 1500 and 6000
  psi. May be constructed to pump at variable
  rates.
• A rotating swashplate creates the reciprocating
  motion for the pistons which pump the fluid.

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SYSTEM COMPONENTS

• Hydraulic motors utilize hydraulic pressure to
  provide mechanical power for flaps or landing
  gear.
• Hydraulic cylinders use pistons to translate
  hydraulic pressure into linear mechanical
  movement. (brakes, control surfaces, anything
  with relatively short travel.)
• Hydraulic lines deliver hydraulic power from
  pump to motor or actuator. (can be flexible or
  rigid.)
• Pressure gauge supplies the pilot with system
  pressure information.

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SYSTEM COMPONENTS

• Valves direct the flow of hydraulic fluid and
  control and regulate pressure.

light spring
seat
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SYSTEM COMPONENTS

• Actuators convert hydraulic pressure to
  mechanical motion to move components to a desired
  position.
• Reservoir stores adequate hydraulic fluid for
  system. Low-pressure air may be applied to
  minimize foaming.
• Standpipe a standpipe is typically designed into
  the reservoir to guard against system leakage. If
  the system develops a leak the pump will drain
  the reservoir to the level of standpipe intake.
  The remaining fluid will be available to
  emergency systems through a lower drain point in
  the reservoir.

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RESERVOIR AND STANDPIPE DESIGN
return
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ACCUMULATOR

• Absorbs the shocks due to rapid pressure
  variations in a hydraulic system
• Helps maintain a constant pressure within the
  hydraulic system
• Helps the hydraulic pump under peak pressure
  loads
• It is an emergency source of power (some braking
  systems have their own accumulator)

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ACCUMULATOR

• In order to store fluid (incompressible) under
  pressure, a compressible substance must be used.
• A gas is used to maintain fluid pressure in the
  accumulator. (usually nitrogen)
• This compressed gas exerts a force on the
  hydraulic fluid, holding it under pressure when
  the rest of the system is unloaded.
• There are three common types of accumulator
• Bladder
• Diaphragm (most common)
• Piston

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ACCUMULATOR
BLADDER
DIAPHRAGM
PISTON
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ACCUMULATOR

• Typically the accumulator is charged to one half
  of the system pressure.
• As the system fluid pressure builds it forces the
  gas to compress.
• The gas will compress until it equals system
  pressure.

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½ of system pressure
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ACCUMULATOR

• 1. The accumulator of an aircraft hydraulic
  system is pre-loaded to 1,000 psi. If the system
  normally operates at a pressure of 3,000 psi, the
  system pressure gauge and the accumulator gauge
  reading after engine start-up would be
  respectively
• a) 0 and 1,000 psi
• b) 3,000 and 4,000 psi
• c) 3,000 and 3,000 psi
• d) 4,000 and 1,000 psi

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ACCUMULATOR

• 2. In order to ensure smooth operation and
  provide reserve in the case of an emergency, most
  hydraulic systems on aircraft employ a pressure
  accumulator. The pre-charge on the accumulator is
  1300 psi and the operating pressure of the
  hydraulic system is 1600 psi while the system is
  rated to a maximum of 2100 psi. What is the
  pressure in the accumulator under these
  conditions?
• a) 1300 psi.
• b) Between 1300 and 1600 psi.
• c) 1600 psi.
• d) Between 1600 and 2100 psi.

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POWER PACK

• To simplify hydraulic systems on light aircraft
  system components may be combined into one
  assembly.
• A power pack contains the reservoir, electric
  hydraulic pump, control valves, and other
  essential components.

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OPEN-CENTER/CLOSED CENTER

• Open-center hydraulic systems allow hydraulic
  fluid to continue to flow through the selector
  valve and back to the reservoir after the desired
  work has been accomplished. The pump will run
  continuously with this type of system.
• Closed-center hydraulic systems trap the
  hydraulic fluid and the pump is usually shutdown
  between actuations. (B-200)

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Standpipe
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LANDING GEAR

• Landing gear comes in a variety of
  configurations.
• Fixed gear C-172
• Retractable gear B-95
• The two most common gear designs are tail dragger
  and tricycle.
• Retractable gear may be electrically actuated on
  light aircraft, but usually hydraulic on larger
  aircraft.

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LANDING GEAR

• Retractable gear have the advantage of reduced
  drag, but add weight and complexity to the
  aircraft.
• All retractable gear have an emergency method of
  extension. In the event of a system malfunction
  the gear may be extended through use of a hand
  pump (hydraulic) or crank (electric) or
  emergency hydraulic supply on larger aircraft.

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LANDING GEAR

• Retractable gear incorporate squat switches to
  guard against inadvertent gear retraction on the
  ground.
• Squat switches are subject to failure and should
  never be exclusively relied upon.
• Gear warning horns and annunciators warn the
  pilot of a retracted gear condition at low power
  settings or landing flap configurations.

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SHOCK STRUTS (OLEO)

• Shock struts are responsible for absorbing the
  forces during landing.
• The oleo is charged with hydraulic fluid and air
  (nitrogen).
• A torque link allows the oleo to move up and down
  while preventing rotation.

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KING-AIR 200
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An-225
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BRAKES

• Brakes on modern aircraft utilize hydraulics to
  apply force to a disc brake assembly.
• Large aircraft incorporate anti-skid and
  auto-braking systems to help control braking
  action.
• The momentum involve with large aircraft makes
  stopping a challenging task.
• The brake systems on these aircraft are subject
  to extreme forces.
• If limitations are exceeded brakes can fail or
  catch fire.

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TIRES

• Aircraft tires are subject to extreme forces.
• The tire must accelerate from stop to a high rate
  of rotation on touchdown, and then withstand the
  forces of maximum braking.
• Tires have max. speed limitations associated and
  are subject to failure above these speeds due to
  heat build up.

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TIRES

• Proper inflation is key to tire performance,
  check manual for appropriate numbers.
• Transport aircraft tires are usually inflated
  with nitrogen. Nitrogen holds less water than
  compressed air, so it is resistant to freezing at
  altitude, maintains pressure longer, and exhibits
  inert properties which prevent breakdown of tire
  materials.
• Some high performance tires are designed with a
  fusable plug (thermal plug) which melts and blows
  out, allowing the tire to deflate rather than
  blow out.
• Jet aircraft with fuselage mounted engines will
  have chine-type nose wheel tires which deflect
  water and contaminants away form the engines.

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CHINE-TYPE TIRE
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TIRE WEAR
UNDERINFLATION
OVERINFLATION
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