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Summary Airframes & systems, Electrics, Powerplant, Emergency equipment
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Airframes & systems, Electrics, Powerplant, Emergency equipment for ATPL-IR (H) Helicopter
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1. Stress fatigue and airframe designStress is defined as the forces divided by the cross-section.
Fail safe is damage tolerant through redundancy. Safe life is that fatigue is calculated by
cycles or flight hours.
The S/N curve graph represents the number of cycles against the force and show at what
values the ultimate stress will be catastrophic.
Aluminum is less dense than steel, good corrosion resistance and high strength and stiffness
but weaker, alloys are stronger but worse in corrosion resistance. Magnesium alloys is less
dense but bad against corrosion.
Oxidation (dry) is corrosion from its environment, electrolytic (wet) corrosion from water.
Truss construction is a box construction and carries the entire structural load. Monocoque is
that the structure is carried by the skin (circular). Semi-monocoque skin carries only part of
the load.
Doubler, extra skin. Stringers in length of AC (short). Longeron in length of AC (whole length).
Hard time maintenance is that components are replaced after some hours/cycles.
Maintenance can also be on-condition.
2. Principles of hydraulics
An active system has a pump, a passive system does not (force by the pilot). For an active
system an actuator, pump and valve are needed. The main pump is the EDP for example a
spur gear (low pressure) and electrical driven (back-up).
Valves with 4 ports have an inflow and outflow parts, if the valve is orientated so no fluid can
go in or out the piston is called hydraulic lock.
An open centered system is a low-pressure system where one actuator can be operated at
once. There is always pressure, when an actuator reaches its level it will be locked and the
hydraulic system is open again, when this fails, there is a PRV installed.
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,High pressure is 3.000psi and low pressure 2.000psi.
Pascal’s law is that the fluid has equal pressure on all sides on the container.
𝐹𝑜𝑟𝑐𝑒 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 ∗ 𝐴𝑟𝑒𝑎
𝐼𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 ∗ 𝐼𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑂𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 ∗ 𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑉𝑜𝑙𝑢𝑚𝑒 = 𝐴𝑟𝑒𝑎 ∗ 𝐿𝑒𝑛𝑔𝑡ℎ
3. Hydraulic valves and actuators
The PRV has a ball with a spring, if the pressure gets too high the ball will rise and the oil can
flow. The NRV also has a ball with a spring the fluid can only run from the ball side. Shuttle
valves allow oil supply from a secondary system if the primary system fails. A restrictor valve
narrows the flow, can be in one way (closes the return way) or 2 ways. Flow control valves
are after hydraulic motors to ensure an even flow rate, increasing flow make it close.
Pressure reducing valve is in the subsystems, as pressure increases it activates the system
(spring mechanism) and reduced the oil supply and introduces a flow back. Hydraulic fuses
make sure the oil supply is cutoff if it detects a leak. Priority valve if it senses not enough
pressure for primary systems it cuts for flow to the secondary systems.
Mineral fluids are flammable and made out of kerosene, most helicopters use this. Synthetic
base comes in 3 grades and is fire resistant and has a lower viscosity.
In an oil filter, the oil passes through a bold which has a filter installed, the output is in the
center of this filter, so all oil is filtered. There is a bypass valve installed for when the filter is
clogged.
4. Hydraulic pump and accumulators
The fixed volume pump is attached to the gearbox and has a constant speed and works
through angular displacement.
Accumulators are to store fluid under pressure, damp out fluctuations and allow thermal
expansion. They can come with a separator or a diaphragm in them or without.
A variable volume pump its output is not dependent on the pump speed, the volume of the
piston can be altered, by altering the swashplate, pressure regulation is helped by a cutoff
valve.
AC pumps are used as backup pump.
Direct reading gauges are fitted with a bourdon tube. Modern helicopters use pressure
transmitters via electrical signals.
A de-aerator tray allows air bubbles to escape from the fluid. The reservoir is pressurized to
prevent the oil will boil at higher altitudes and positive pressure to the pump. This is done by
bleed air.
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, 5. Landing gear and brakes
A torque link is fitted to keep the wheels aligned with the runway direction, there is a joint in
the middle to allow the oleo strut going up and down.
In retractable gear there are stays to prevent the gear from collapsing towards the opening.
Over center locks, lock the landing gear in place.
Brake fade is when breaking action decreases due to hot disks.
If hydraulics are lost the oil in the accumulator can provide 6 braking application.
6. Wheels, tires and flotation devices
Ply rating is the strength of the tire, the higher the number the higher the strength. Cross-ply
is that the layers are laid on top of each other with an angle. Radial ply is the layers are in the
travel orientation.
Creep is movement of the outer tire, on the creep mark you can see if it has moved, the max
is when the edges just touch each other.
Flotation device can be activated in the cockpit or automatically by contact with water.
7. Pneumatic systems
Pneumatic pressure can come from engines or bottled pressure.
Total loss systems use the air once then bump it overboard.
Engine bleed air reduces the power output.
8. Air conditioning
For AC there is a hot and cold stream of air that can be mixed.
A bootstrap system is a compressor driven air turbine. The by-pass valve reacts on
temperature. On the ground the ram air is provided by an electric fan, or CAU. When the air
passes over the turbines the pressure slows down, at the second turbine it is almost freezing.
Vapor cycle cooling (works like a fridge) and has a cycle of Freon (which changes from liquid
to vapor) and one with air. They are most often found in helicopters, and also work when on
ground.
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