TM 1-407, Aircraft Induction, Fuel and Oil Systems, 1941: Section 3 - Oiling Systems

SECTION III: OILING SYSTEMS

15. General.-a. Although the complete oilingsystem of an aircraft power plant includes the lubrication system of the engine, only the external units of the system are shown in oiling-system diagrams of specific types of aircraft. For that reason this section deals only with the external units of the system. The internal engine lubrication system is treated separately under another heading.

 b. The oiling system performs two functions. It provides for an adequate oil supply, the amount determined by the fuel system capacity, and it incorporates a means of cooling the hot oil discharged from the engine. It is designed and located in the aircraft to furnish oil by gravity to the inlet side of the engine pressure pump during practically all positions of flight except inverted.

16. Oiling-system units.-a. General.-The major units in an oiling system include the supply tank, the necessary piping and connections, the oil temperature regulator assembly, the oil temperature gage, and the oil pressure gage. Some modern oil systems also incorporate an oil dilution system.

b. Supply tank.-The tanks are usually constructed of aluminum, aluminum alloy, or stainless steel and are of such design as to permit installation in the aircraft as close to the engine as possible. The ideal tank location is on the engine side of the fire wall with its center approximately 20 inches above the engine oil pump when the aircraft is in its ground position. Oil tank capacity should provide for 1 gallon of oil for each 11 gallons of fuel for air-cooled engine installations, and 1 gallon of oil for each 14 gallons of fuel for liquid-cooled engines. These figures do not include the oil in the piping, regulator, and engine, which usually totals from 1 to 3 gallons.

(1) The tank outlet is usually located in its lowest section in order to permit complete drainage while the aircraft is in ground position or in normal flight attitude. The outlet is so arranged that with the tank filled to one-half its normal capacity, it will not be uncovered in any normal flight attitude. The tank inlet line from the oil regulator enters the top of the tank and is of the same size as the outlet line. The two vent outlets lead from the top of the tank to the engine crankcase. The filler and cap unit are conveniently located for servicing and checking the oil supply in the tank.

FIGURE: 14-Flexible oil hose connection.

(2) The supply tank used in conjunction with an oil dilution system incorporates a hopper or metal tube installed inside the tank in a vertical position, with the top of the hopper approximately onehalf inch from the top of the tank and the base of the hopper extending down inside of the sump to within one-half inch of the bottom. The minimum oil flow area from the tank into the bottom of the hopper must be greater than the area of the oil tank outlet line. The hopper has a volume of 1 to 2 gallons of oil, depending on the oil flow capacity of the engine.

c. Tube fittings, oil lines, and drain cocks.-There are comparatively few fittings used in the plumbing of aircraft oiling systems. Other than a few street-ells and oil-gage-fittings, the system is made up of large piping and flexible hose connections. The supply tank is usually secured to the engine mount in a more or less rigid frame. It necessarily follows that there is vibration, which is violent at times, between the fixed oil system units and the engine. This vibration is naturally concentrated in the oil system tubing and is conducive to fatigue failure in piping. This effect is considerably reduced by the use of flexible hose connections.

(1) Rubber hose is used for the flexible connections and is clamped to the tubing or fittings with standard hose clamps (fig. 14). The length of the hose exposed to the oil flow should not be less than 1/4 inch or greater than the tube diameter. Each hose connection used on the oil line connecting the scavenging pump outlet to the oil cooler is secured by four standard hose clamps because of high pressure exerted by the flow of cold oil when the engine is first started.

(2) Piping may be of copper or aluminum tubing. The wall thickness of copper tubing should not be less than 0.040 inch and of aluminum tubing not less than 0.050 inch. The ends of oil pipes are raised to insure a more positive attachment of the hose connections. Avoidance of excessively tightened hose clamps insures a properly installed flexible hose connection. Oil lines are painted with a band of yellow paint near each end for identification purposes.

(3) A drain cock (fig. 15) is installed to drain the system. One inlet of the cock is from the tank and the other from the engine pump, both draining into a common outlet. The cock is manually controlled by rotating a handle from the vertical position, which

FIGURE 15.-Oil system drain cock.

opens the cock, to the horizontal position, which closes it. When closed the cock handle is locked in position to prevent movement. In an oil dilution system, an auxiliary valve is incorporated in the drain cock to provide a means of mixing a certain quantity of fuel with the oil.

d. Oil temperature regulator.-There are a number of sizes of oil temperature regulators in service to control the operating temperature of the oil. The operating temperatures must be kept within certain limits to control viscosity for proper lubrication.

(1) The oil temperature regulator, commonly known as the oil cooler, is placed in the oil system between the oil outlet of the engine and the oil inlet of the supply tank. It consists of a built-up core section surrounded by a metal jacket. A thermostatically controlled valve is incorporated in the unit to control the flow of oil automatically either through the core or around the shell, depending upon the temperature of the oil. The oil is cooled by the air moving through the core tubes while the oil passes around the tubes. Oil coolers are usually cylindrical in shape and are so designed that the relief valve assembly regulates the amount of oil forced through it to the supply tank. Various diameters of different core areas are used, depending upon the amount of oil circulated through the particular engine installation.

FIGURE 16-Oil temperature regulator with thermostatic valve open.

(2) A thermostatically operated valve is incorporated in oil temperature regulators to control the passage of oil automatically through the cooler. When the oil scavenged from the engine is comparatively cool, the thermostatic valve (fig. 16) is open, permitting the oil to flow around the core jacket to the top of the supply tank without flowing through the core. When the oil reaches a normal temperature, the thermostatic valve (fig. 17) closes and directs the oil through the core for cooling purposes. Although it is possible to vary the operating temperature of the oil slightly by incorporating a number of gaskets in the thermostatic regulator valve assembly, it must be assumed that these valves are nonadjustable. They are usually designed to operate and shunt the oil through the cooler when

FIGURE 17-Oil temperature regulator wit h thermostatic valve closed.

the oil reaches a temperature of approximately 60° C. A thermometer bulb is installed in a well incorporated in the inlet line from the supply tank to the engine pump. It registers the inlet oil temperature on the temperature gage in the aircraft cockpit.

(3) Where a shutter assembly is installed in the air exit side of the regulator core, it may be operated to maintain desired oil temperatures. Where there is no provision for a shutter assembly, it is permissible in cold weather to blanket the core area partially with fiberboard or other suitable material in order to obtain proper oil temperatures.

e. Pressure gage.-The oil pressure gage in the aircraft cockpit is connected to the pressure line of the lubrication system in the engine. A restricted orifice is placed in the oil pressure gage line at the engine end  The gage line may be filled with instrument oil in cold weather, giving a comparatively rapid and true indication of oil pressure during engine warm-up.

17. Typical aircraft-oiling -system.-a. By referring to the oiling system diagram (fig. 18), a general idea of the circulation of the oil to and from the engine may be obtained. Assuming that the supply tank has been serviced to its proper level, the oil flows by gravity through large tubing to the inlet side of the engine-oil pressure pump, which forces the oil under pressure through the internal lubricating system. The engine scavenging pumps return the oil from the engine-oil sumps to the top of the supply tank through the oil-temperature regulator. In most instances the crankcase is vented through suitable piping to the top of the supply tank. Two vent lines are used and are located so that at least one vent is open in all normal flight attitudes.

FIGURE 18-Typical aireraft-oiling system.

b. The thermometer well, for the installation of the thermometer bulb, is incorporated at a tangent in the oil piping between the outlet of the tank and the engine oil pressure pump; therefore, the indicator gives a temperature reading of oil entering the engine. The reading obtained at this point is more valuable than one taken from some other point in the oil return line.

c. The drain valve is also located in the tank to engine pressure pump piping at the lowest point to permit complete drainage of the tank and engine. It should be noted that the drain cock does not drain the oil from the oil temperature regulator, which must be drained separately by removing the plug located at the lowest point on the assembly.

18. Oil-dilution system.-a. Due to difficulties experienced in starting aircraft engines in cold weather, an oil-dilution system has been developed to dilute the oil immediately before the engine is stopped when a cold start is anticipated. Inasmuch as the high cranking torque of a cold engine is due to the high viscous drag of the oil, particularly between the pistons and cylinder walls, it is evident that a decided thinning of this oil immediately before the engine is stopped will greatly reduce the cranking torque and facilitate subsequent starting.

b. By referring to figure 18, the units comprising the oil-dilution system are shown, and with the exception of the hopper in the supply tank may be readily incorporated in any aircraft-oiling system. A line is connected from the fuel pressure line to a special Y drain cock in which a spring loaded poppet valve is installed. The valve is operated manually from the cockpit, and before the engine is stopped in cold weather a small amount of fuel is allowed to enter the oil-in line at this point by holding the dilution control open for a, short time with the engine operating. This operation permits the diluted oil to replace the heavy oil throughout the entire engine, thereby facilitating starting in cold weather. Inasmuch as some of the diluted oil is returned to the hopper in the supply tank during the last minutes of operation, this diluted oil will be the first oil used at the next start, thereby insuring a more positive flow to the engine pump. A typical hopper installation is shown in figure 19.

(1) Because use of the oil-dilution system increases the combustible vapor in the discharge from the engine crankcase breather, a pipe is installed to conduct these vapors from the breather to the cowl line.

(2) With oil dilution, there is a tendency for the diluted oil from the tank to seep into the engine crankcase and into the cylinders when the engine is at rest. The only satisfactory solution to this problem is the use of a check valve installation in the strainer assembly in the engine. This valve is spring loaded so that the normal gravity pressure of the oil cannot cause seepage into the engine; however, it opens readily under the influence of pump pressure when the engine is in operation.

19. Operating instructions.-a. When servicing an oil tank, no attempt should be made to fill the tank completely, since a definite air space must be provided to accommodate expansion. In most tanks, the filler cap is so located that the correct level is automatically established. In some tanks a level cock is also provided to permit an accurate check of the filling operation.

b. During engine warm-up on the ground it is most important to check carefully the operation of the lubrication system. When an engine is first started, the oil pressure reading will generally be very high due to the high oil viscosity. However, even at high pressures the rate of oil circulation may be quite low, so extreme care is necessary to prevent engine wear. In general, the engine r. p. m. should be kept low until a rise is observed in oil temperature, indicating

FIGURE 19-Typical hopper tank.

proper circulation. Take-off can be safely accomplished after only a moderate period of ground operation, provided the oil pressure indication is steady and within the proper range, and the engine is otherwise satisfactory. Unusually quick take-off is possible with airplanes having oil dilution systems. Prolonged periods of ground operation are definitely harmful to most high output aircraft engines.

20. Maintenance.-The principal points to be checked in the maintenance of oiling systems may be enumerated as follows:

a. The tank should be inspected for proper mounting and for signs of leaks and dents.

b. The oil-temperature regulator supports should be checked and the core inspected for restrictions.    I

c. The oil-dilution valve linkage will sometimes bind and fail to operate properly unless carefully adjusted.

d. The oil tubing must be properly installed and supported, the hose connections must be in good condition, and the clamps should be tested for tightness.

e. Screens and filters accumulate a large amount of dirt and sludge and are, therefore, cleaned periodically.

f. All plugs and drains must be carefully checked for leakage and for proper safetying.