A/C SYS 2017

the most common form of buffeting is that of the tail, which can experience severe vibrations when it is invested by the turbulent wake of the stalled wing

an elastic aircraft in an accelerated flight condition may experience loads which are much greater than those predicted assuming a perfectly rigid structure. External loads, such as gust and landing loads, excite oscillations of the structure. The additional inertia forces associated with these oscillations can produce dynamic load overshoots.

1. Pressure Dome
2. Cabin Floor
3. Frame
4. Stringers
5. Skin

3.05 m/s (10 ft/s) at maximum landing weight (MLW)

the higher its value, the shorter the landing gear compression required to absorb the vertical kinetic energy at landing

• + lower weight (nose gear is much heavier)
• + less drag (if the gear is not retractable)
• + more space available in the aircraft nose (particularly if the gear is retractable)
• + better suited for operation on rough fields
• - laterally unstable when rolling
• - heavy braking may bring the danger of tipping the aircraft over on its nose

• the aircraft does not need to be aligned with the runway (a crab landing is possible)
• it is much easier for the pilot to keep it on the centreline, even with strong side gusts
• offers better visibility for the pilots during taxiing
• easier loading and unloading of the aircraft (and more comfort for the passengers) thanks to the nearly horizontal floor
• low drag during take off acceleration
• is more forgiving when landing with insufficient or no flare (the aircraft tends to reduce the angle of attack after touchdown, and is less prone to bouncing back in the air)

• + lower weight
• + less drag (when gear is extended)
• + used when not enough space for langing gear
• - difficult take-off and landing
• - In crosswind conditions, care must be taken not to land on one of the outrigger gear legs, which are not designed to take substantial landing loads.

23,000 kg (although for safety reasons it would be better to fit twin wheels)

23,000 - 68,000 kg (This configuration is sometimes found also on heavier aircraft with a take-off mass of up to 100,000 kg)

90,000 - 180,000 kg

• spring type
• bungee type
• oleopneumatic shock absorbers (oleo struts)

• gear legs are elastically deformed under load
• typical of small general aviation aircraft
• The vertical kinetic energy is transmitted gradually to the structure
• practically no damping effect, except for the energy dissipated by lateral friction of the tires (when the gear deflects, the wheels move sideways)
• aircraft tends to bounce back in a hard landing

• the main gear legs are hinged at the side of the fuselage and connected by rubber bungees (bundles of rubber
  strands packed in a cloth tube) that stretch as the gear deflects
• common in early light aircraft
• In addition to the lateral friction of the tires, some energy is dissipated by rubber internal friction and hysteresis

• efficiently dissipate the vertical kinetic energy at landing and smooth out the bumps on uneven terrain
• ombine a spring effect, created by a variable chamber filled with compressed air or nitrogen, with the damping effect of a piston which forces oil through small orifices
• The three most common configurations are: the telescopic gear, the triangulated gear, and the levered (or trailing link) gear

• the main structure of the gear leg is formed by the oleo strut itself
• simpler design and has a lower number of components
• must provide the full required amount of compression, and be sized to take the whole lateral and longitudinal loads
• quite heavy
• The bending associated with these loads can cause serious wear on the oil seals

• conceptually similar to the bungee type: when the gear leg deflects, an oleo strut is compressed
• the wheel lateral and longitudinal loads are carried by the solid gear legs, and a shorter and smaller oleo strut can be used
• Maintenance is easier (oleo strut can be replaced or overhauled without removing the wheel assembly and the landing gear leg)
• configuration is usually a little heavier than the telescopic one
• the gear deflections causes lateral friction on the tires, which shortens their life

• variant of the other two configurations
• the wheels travel slightly backwards when the oleo strut is compressed; this makes it easier for a wheel to overcome a bump
• very robust
• heavier than the telescopic leg type
• well suited to aircraft that operate on aircraft carriers or to short take off and landing aircraft designed to take off and land from unprepared runways

• A torsion link keeps the alignment between cylinder and piston, and prevents it from dropping out of the cylinder.
• Airborne: the air (or nitrogen) pressure in the inner cylinder, and the weight of the wheel, tire and brakes, causes the shock absorber to fully extend, and the oil fills the piston cavity.
• Touchdown: the ground reaction pushes the piston inside the cylinder, and the oil is forced to flow into the snubber tube through the metered orifice, then into the inner cylinder through the flapper valve.
• The flow pressure losses caused by the metered orifice provide the compression damping effect. When max. compression is reached, the increased air pressure in the cylinder tries to extend the shock absorber by pushing the oil back into the piston, but the flapper valve closes and the oil is forced to flow back slowly through the tiny oil return holes.
• The extension damping caused by the pressure drop through these holes is generally higher than the compression damping. This prevents a rapid extension of the strut, which would make the aircraft bounce back into the air after a hard landing.

The ground pitch and roll angles that may occur in operation. At these angles there must be no contact between any part of the aircraft and the ground.

• Pitch angle: at least equal to the angle of attack at lift off plus a reserve (ca. 10° - 15°)
• Roll angle: at least 8 for large transport aircraft, and up to 15 for light aircraft

the flow which passes through the actuator disc of a propeller.

as the product of thrust and aircraft speed

1. the distribution of thrust on the propeller disc is not uniform
   1. Most of the thrust is generated in the outer part of the blade (rotational velocity component is higher)
   2. The energy lost in these tip vortices corresponds to the mechanical work needed to overcome the induced drag of the blades.
2. Other sources of energy loss are the parasite drag of the rotating blades, of the spinner and of the hub, and the angular momentum of the slipstream behind the propeller.

\(\pi*n*D\)

• n = propeller speed in revolutions per second
• D = propeller diameter in meters

the advance ratio J and the blade angle \(\beta\)

When the propeller pitch is set to a value that reduces drag to a minimum (in the event of an engine failure)

the fuel lower heating value [J/kg]

• A small fraction is used to overcome internal friction in the engine and to drive accessories, such as the fuel pump and the oil pump.
• The rest is left in the exhaust gas thermal energy. Indeed, the exhaust gas temperature of this engine at maximum static thrust was higher than 1000C.

A jet engine with no compressor and turbine

• sometimes called engine feed pumps
• are used to boost the fuel pressure in the engine feed lines. One of the reasons for this is to prevent aeration and cavitation at high altitudes

the presence of air bubbles in the fuel lines that could cause an engine flame-out

• the combination of low pressure, relatively high fuel temperature and high engine demand causes the formation of vapour bubbles in the fuel
• drastic reduction in fuel flow to the engine
• can cause a flame-out
• For this reason, engine manufacturers usually impose the requirement that the fuel feed pressure must remain at least 5 psi above the true vapour pressure at all times.

generation, control, transfer and utilization