The Aeroplane Theory Flashcards
What are the main functions of oil?
If a small film of oil separates two metal surfaces it will prevent them from rubbing together. Without oil there would be friction forces, causing very high temperatures to develop quickly in the metal, with extreme wearing of the metal surfaces, and, probably, mechanical failure.
Lubrication - The oil film will allow the two metal surfaces to slide one over the other without actually touching each other. There will be only low friction forces and, consequently, high temperatures in the metal are avoided. Engine components subject to high loads, such as the bearings at either end of the connecting rods, especially the crankshaft bearings, are cushioned by a layer of oil and the mechanical shock on them is reduced.
Cooling - The pistons absorb a lot of heat from the combustion chambers and are cooled by oil splashed or sprayed onto them from below.
Cleaning - Oil circulating through an engine can carry away dirt and other foreign material, thereby reducing abrasive wear on the moving parts of the engine. This contamination is removed by the oil filter. If the filter is not kept clean it may block, causing dirty oil to bypass the filter and circulate within the engine’s lubrication system. Dirty oil has poorer cooling and lubricating qualities and so the engine will suffer.
Sealing - Oil also provides a seal. This prevents the compressed gases (fuel/air) escaping past the piston rings into the crankcase.
Explain what a dry and wet sump is:
A dry sump engine has scavenge pumps that scavenge oil from the sump attached to the lower part of the engine casing and pump it back into the oil tank, which is separate from the engine. It is usual to have a dry sump oil system on aerobatic aircraft that commonly find themselves in unusual attitudes.
A wet sump engine has a sump in which the oil is stored. Most light aircraft engines are wet sump engines.
Map out a typical oil system diagram:
What may a high or low oil temperature reading indicate?
High oil temperature readings may indicate:
- Clogged oil line
- Low quantity of oil
- Blockage in the oil cooler
- Malfunctioning temperature gauge
Low oil temperature readings may indicate:
- Incorrect type of oil being used during cold weather operations
- Malfunctioning temperature gauge
What may a high or low oil pressure reading indicate?
High oil pressure readings may indicate:
- Failure of the pressure relief valve
Low oil pressure readings may indicate:
- Insufficient oil
- Leaks
- Failure of the oil pump
- Problems within the engine, such as failing bearings
- Oil pressure relief valve (PRV) stuck open
What happens when oil becomes too hot?
Oil does not lubricate properly
High air density improves aircraft performance. Which factors will increase the air density?
- Low humidity
- Low altitude
- Low temperatures
What is humidity?
The mixture of gases that we call ‘air’ consists mainly of nitrogen, oxygen and water vapour. The amount of water vapour in the air is called humidity. Because water molecules are very light, a high humidity will cause the air density to be slightly less, but no account of this is taken in performance charts.
Just how much water a parcel of air is capable of holding depends on its temperature - warm air can hold more water than cold air. If a parcel of air is holding 70% of its maximum capacity of water vapour, it has a relative humidity of 70%. If it cools, its amount of water does not change, the relative humidity increases.
If the parcel of air cools unit its capacity to hold water vapour is equal to that which it is actually holding, its relative humidity is 100% and the parcel of air is said to be saturated. Any further cooling will cause some of the water vapour to condense as water droplets and form clouds, fog or dew.
What is relative humidity?
Relative humidity is defined as the amount of water vapour present in a parcel of air compared to the maximum amount that it can support at the same temperature.
If it evaporates, the air is ‘dry’ (low relative humidity) and can absorb more water vapour. If the perspiration remains as beads of sweat, the relative humidity is high and the air cannot absorb the extra moisture.
What is the International Standard Atmosphere (ISA) mean sea level (MSL) pressure?
1013.2 hPa
What is the ISA MSL temperature?
15° Celsius
At what rate does pressure decrease?
1hPa per 30 feet
At what rate does temperature decrease?
2° Celsius per 1,000 feet
