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The primary purpose of a lubricant is to reduce friction between moving parts. Because liquid lubricants Aircraft Wood and Structural Repair
or oils can be circulated readily, they are used universally in aircraft engines. In theory, fluid lubrication Airframe
is based on the actual separation of the surfaces so that no metal-to-metal contact occurs. As long as Cabin Environment Control System
the oil film remains unbroken, metallic friction is replaced by the internal fluid friction of the lubricant. Cleaning and Corrosion Control
Under ideal conditions, friction and wear are held to a minimum. Oil is generally pumped throughout Communication and Navigation
the engine to all areas that require lubrication. Overcoming the friction of the moving parts of the Electrical System
engine consumes energy and creates unwanted heat. The reduction of friction during engine operation
Engine Maintenance and Operation
increases the overall potential power output. Engines are subjected to several types of friction.
Fabric Covering
Fire Protection
Types of Friction Fuel System
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Weight and Balance
Figure 1. Two moving surfaces in direct contact create excessive friction Recent Posts
1 PROCEDURES FOR WEIGHING AN
AIRCRAFT
Friction may be defined as the rubbing of one object or surface against another. One surface sliding
General ConceptsThe most
over another surface causes sliding friction, as found in the use of plain bearings. The surfaces are not important reason for weighing an
aircraft is to find...
completely flat or smooth and have microscopic defects that cause friction between the two moving
Read more
surfaces. [Figure 1] Rolling friction is created when a roller or sphere rolls over another surface, such
as with ball or roller bearings, also referred to as antifriction bearings. The amount of friction created by 2 PLYWOOD SKIN REPAIRS - REPAIR
OF WOOD AIRCRAFT
rolling friction is less than that created by sliding friction and this bearing uses an outer race and an STRUCTURES
inner race with balls, or steel spheres, rolling between the moving parts or races. Another type of Plywood skin can be repaired
using a number of different
friction is wiping friction, which occurs between gear teeth. With this type of friction, pressure can vary methods depending on...
widely and loads applied to the gears can be extreme, so the lubricant must be able to withstand the Read more
loads.
3 PAINT SYSTEM COMPATIBILITY -
AIRCRAFT PAINTING
The use of several different types
of paint, coupled with several
Functions of Engine Oil proprietary...
Read more
In addition to reducing friction, the oil film acts as a cushion between metal parts. [Figure 2] This
4 FASTENERS USED WITH
cushioning effect is particularly important for such parts as reciprocating engine crankshafts and COMPOSITE LAMINATES -
connecting rods, which are subject to shock-loading. As the piston is pushed down on the power COMPOSITE HONEYCOMB
SANDWICH REPAIRS
stroke, it applies loads between the connecting rod bearing and the crankshaft journal. The load- Many companies make specialty
bearing qualities of the oil must prevent the oil film from being squeezed out, causing metal-to-metal fasteners for composite
structures and several...
contact in the bearing. Also, as oil circulates through the engine, it absorbs heat from the pistons and Read more
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cylinder walls. In reciprocating engines, these components are especially dependent on the oil for METHODS OF ILLUSTRATION -
AIRCRAFT DRAWINGS
cooling.
Applied GeometryGeometry is the
branch of mathematics that deals with
lines,...
Read more
Popular Posts
Aircraft Air Conditioning Systems
There are two types of air
conditioning systems commonly
used on aircraft. Air cycle air
conditioning is used on most
turbine-powered airc...
Types of Aviation Fuel
Figure 2. Oil film acts as a cushion between two moving surfaces Each aircraft engine is designed
to burn a certain fuel. Use only the
fuel specified by the manufacturer.
Mixing fuels is not permitted. T...
Oil cooling can account for up to 50 percent of the total engine cooling and is an excellent medium to
Control of Cabin Pressure -
transfer the heat from the engine to the oil cooler. The oil also aids in forming a seal between the piston Aircraft Pressurization Systems
and the cylinder wall to prevent leakage of the gases from the combustion chamber. Pressurization Modes Aircraft
cabin pressurization can be
controlled via two different modes
of operation. The first is the
Oils clean the engine by reducing abrasive wear by picking up foreign particles and carrying them to a isobaric mod...
filter where they are removed. The dispersant, an additive, in the oil holds the particles in suspension Aircraft Oxygen Systems and
and allows the filter to trap them as the oil passes through the filter. The oil also prevents corrosion on Components
Built-in and portable oxygen
the interior of the engine by leaving a coating of oil on parts when the engine is shut down. This is one systems are used in civilian
of the reasons why the engine should not be shut down for long periods of time. The coating of oil aviation. They use gaseous or
solid oxygen (oxygen generators)
preventing corrosion will not last on the parts, allowing them to rust or corrode. as suits the purp...
Aircraft Fuel System Indicators
The engine’s oil is the life blood of the engine and it is very important for the engine to perform its Fuel Quantity Indicating Systems
All aircraft fuel systems must have
function and to extend the length between overhauls. some form of fuel quantity
indicator. These devices vary
widely dep...
Requirements and Characteristics of Reciprocating Engine Contact Form
Lubricants
Name
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While there are several important properties that satisfactory reciprocating engine oil must possess, its
viscosity is most important in engine operation. The resistance of an oil to flow is known as its viscosity. Email *
Oil that flows slowly is viscous or has a high viscosity; if it flows freely, it has a low viscosity.
Unfortunately, the viscosity of oil is affected by temperature. It was not uncommon for earlier grades of Message *
oil to become practically solid in cold weather, increasing drag and making circulation almost
impossible. Other oils may become so thin at high temperatures that the oil film is broken, causing a
low load carrying ability, resulting in rapid wear of the moving parts.
The oil selected for aircraft engine lubrication must be light enough to circulate freely at cold
Send
temperatures, yet heavy enough to provide the proper oil film at engine operating temperatures. Since
lubricants vary in properties and since no one oil is satisfactory for all engines and all operating
conditions, it is extremely important that only the approved grade or Society of Automotive Engineers
(SAE) rating be used.
Several factors must be considered in determining the proper grade of oil to use in a particular engine,
the most important of which are the operating load, rotational speeds, and operating temperatures. The
grade of the lubricating oil to be used is determined by the operating conditions to be met in the various
types of engines. The oil used in aircraft reciprocating engines has a relatively high viscosity required
by:
1. Large engine operating clearances due to the relatively large size of the moving parts, the
different materials used, and the different rates of expansion of the various materials;
2. High operating temperatures; and
3. High bearing pressures.
Viscosity
Generally, commercial aviation oils are classified by a number, (such as 80, 100, 140, etc.) that is an
approximation of the viscosity as measured by a testing instrument called the Saybolt Universal
Viscosimeter. In this instrument, a tube holds a specific quantity of the oil to be tested. The oil is
brought to an exact temperature by a liquid bath surrounding the tube. The time in seconds required for
exactly 60 cubic centimeters of oil to flow through an accurately calibrated orifice is recorded as a
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measure of the oil’s viscosity. If actual Saybolt values were used to designate the viscosity of oil, there
would probably be several hundred grades of oil.
To simplify the selection of oils, they are often classified under an SAE system that divides all oils into
seven groups (SAE 10 to 70) according to viscosity at either 130 °F or 210 °F. SAE ratings are purely
arbitrary and bear no direct relationship to the Saybolt or other ratings.
The letter W occasionally is included in the SAE number giving a designation, such as SAE 20W. This
W indicates that the oil, in addition to meeting the viscosity requirements at the testing temperature
specifications, is satisfactory oil for winter use in cold climates. This should not be confused with the W
used in front of the grade or weight number that indicates the oil is of the ashless dispersant type.
Although the SAE scale has eliminated some confusion in the designation of lubricating oils, it must not
be assumed that this specification covers all the important viscosity requirements. An SAE number
indicates only the viscosity grade or relative viscosity; it does not indicate quality or other essential
characteristics. It is well known that there are good oils and inferior oils that have the same viscosities
at a given temperature and, therefore, are subject to classification in the same grade.
The SAE letters on an oil container are not an endorsement or recommendation of the oil by the SAE.
Although each grade of oil is rated by an SAE number, depending on its specific use, it may be rated
with a commercial aviation grade number or an Army and Navy specification number. The correlation
between these grade numbering systems is shown in Figure 3.
Figure 3. Grade designations for aviation oils
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Viscosity Index
The viscosity index is a number that indicates the effect of temperature changes on the viscosity of the
oil. When oil has a low viscosity index, it signifies a relatively large change of viscosity of increased
temperature. The oil becomes thin at high temperatures and thick at low temperatures. Oils with a high
viscosity index have small changes in viscosity over a wide temperature range.
The best oil for most purposes is one that maintains a constant viscosity throughout temperature
changes. Oil having a high viscosity index resists excessive thickening when the engine is subjected to
cold temperatures. This allows for rapid cranking speeds during starting and prompt oil circulation
during initial start up. This oil resists excessive thinning when the engine is at operating temperature
and provides full lubrication and bearing load protection.
Flash Point and Fire Point
Flash point and fire point are determined by laboratory tests that show the temperature at which a
liquid begins to give off ignitable vapors, flash, and the temperature at which there are sufficient vapors
to support a flame, fire. These points are established for engine oils to determine that they can
withstand the high temperatures encountered in an engine.
Cloud Point and Pour Point
Cloud point and pour point also help to indicate suitability. The cloud point of oil is the temperature at
which its wax content, normally held in solution, begins to solidify and separate into tiny crystals,
causing the oil to appear cloudy or hazy. The pour point of oil is the lowest temperature at which it
flows or can be poured.
Specific Gravity
Specific gravity is a comparison of the weight of the substance to the weight of an equal volume of
distilled water at a specified temperature. As an example, water weighs approximately 8 pounds to the
gallon; oil with a specific gravity of 0.9 would weigh 7.2 pounds to the gallon.
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In the early years, the performance of aircraft piston engines was such that they could be lubricated
satisfactorily by means of straight mineral oils, blended from specially selected petroleum base stocks.
Oil grades 65, 80, 100, and 120 are straight mineral oils blended from selected high-viscosity index
base oils. These oils do not contain any additives except for very small amounts of pour point
depressant, which helps improve fluidity at very low temperatures, and an antioxidant. This type of oil
is used during the break-in period of a new aviation piston engine or those recently overhauled.
Demand for oils with higher degrees of thermal and oxidation stability necessitated fortifying them with
the addition of small quantities of nonpetroleum materials. The first additives incorporated in straight
mineral piston engine oils were based on the metallic salts of barium and calcium. In most engines, the
performance of these oils with respect to oxidation and thermal stability was excellent, but the
combustion chambers of the majority of engines could not tolerate the presence of the ash deposits
derived from these metal-containing additives. To overcome the disadvantages of harmful combustion
chamber deposits, a nonmetallic (i.e., non-ash forming, polymeric) additive was developed that was
incorporated in blends of selected mineral oil base stocks. W oils are of the ashless type and are still in
use. The ashless dispersant grades contain additives, one of which has a viscosity stabilizing effect
that removes the tendency of the oil to thin out at high oil temperatures and thicken at low oil
temperatures.
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The additives in these oils extend operating temperature range and improve cold engine starting and
lubrication of the engine during the critical warm-up period permitting flight through wider ranges of
climatic changes without the necessity of changing oil.
Semi-synthetic multigrade SAE W15 W50 oil for piston engines has been in use for some time. Oils
W80, W100, and W120 are ashless dispersant oils specifically developed for aviation piston engines.
They combine nonmetallic additives with selected high viscosity index base oils to give exceptional
stability, dispersancy, and antifoaming performance. Dispersancy is the ability of the oil to hold particles
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in suspension until they can either be trapped by the filter or drained at the next oil change. The
dispersancy additive is not a detergent and does not clean previously formed deposits from the interior
of the engine.
Some multigrade oil is a blend of synthetic and mineral-based oil semisynthetic, plus a highly effective
additive package, that is added due to concern that fully synthetic oil may not have the solvency to
handle the lead deposits that result from the use of leaded fuel. As multigrade oil, it offers the flexibility
to lubricate effectively over a wider range of temperatures than monograde oils. Compared to
monograde oil, multigrade oil provides better cold-start protection and a stronger lubricant film (higher
viscosity) at typical operating temperatures. The combination of nonmetallic, antiwear additives and
selected high viscosity index mineral and synthetic base oils give exceptional stability, dispersancy, and
antifoaming performance. Start up can contribute up to 80 percent of normal engine wear due to lack of
lubrication during the start-up cycle. The more easily the oil flows to the engine’s components at start
up, the less wear occurs.
The ashless dispersant grades are recommended for aircraft engines subjected to wide variations of
ambient temperature, particularly the turbocharged series engines that require oil to activate the
various turbo controllers. At temperatures below 20 °F, preheating of the engine and oil supply tank is
normally required regardless of the type of oil used.
Premium, semisynthetic multigrade ashless dispersant oil is a special blend of a high-quality mineral oil
and synthetic hydrocarbons with an advanced additive package that has been specifically formulated
for multigrade applications. The ashless antiwear additive provides exceptional wear protection for
wearing surfaces.
Many aircraft manufacturers add approved preservative lubricating oil to protect new engines from rust
and corrosion at the time the aircraft leaves the factory. This preservative oil should be removed at end
of the first 25 hours of operation. When adding oil during the period when preservative oil is in the
engine, use only aviation grade straight mineral oil or ashless dispersant oil, as required, of the
viscosity desired.
If ashless dispersant oil is used in a new engine, or a newly overhauled engine, high oil consumption
might possibly be experienced. The additives in some of these ashless dispersant oils may retard the
break in of the piston rings and cylinder walls. This condition can be avoided by the use of mineral oil
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Principles, Requirements and Characteristics - Aircraft Engine Lubrication... https://www.aircraftsystemstech.com/2017/04/engine-lubrication-systems.html
until normal oil consumption is obtained, then change to the ashless dispersant oil. Mineral oil should
also be used following the replacement of one or more cylinders or until the oil consumption has
stabilized.
In all cases, refer to the manufacturers’ information when oil type or time in service is being
considered.
Reciprocating Engine Lubrication Systems
Lubrication System Maintenance Practices
Recommendations for Changing Oil
Requirements for Turbine Engine Lubricants
Turbine Engine Lubrication Systems
Turbine Lubrication System Components
Typical Dry-Sump Pressure Regulated Turbine Lubrication system
Pressure System
Typical Dry-Sump Variable Pressure Lubrication System
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Turbine Engine Wet-Sump Lubrication System Turbine Engine Oil System Maintenance
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