Aircraft Oils and Greases Part 1 (Lubricants)

The Oracle @ Delphi #5 4/20/06 - Aircraft Oils and Greases Part 1 (Lubricants)

I've just spent the last two months learning everything that I already knew, everything that I forgot and then a bunch of new stuff about aircraft oils and greases, and of course I'm willing to share. A solid foundation of knowledge about oil and its role in the internal combustion engine will allow you, the pilot/owner of an aircraft to better choose an engine oil that is closely matched to the flying environment of the aircraft.

Lubrication

Hydrodynamic lubrication is when two moving parts create a wedge of lubrication that produces enough pressure within the film to separate the surfaces. A good example is the crankshaft turning in the bearing (see example). When the crankshaft turns in the bearing the oil is forced into an ever smaller area. This pressure squeezes the oil molecules together so they cannot move. The viscosity of oil at 300,000 PSI is equivalent to nylon.

Boundary lubrication occurs in locations that must be lubricated however, cannot use hydrodynamic. A good example is the piston rings when they stop at the dead center position. The surfactant and chemical properties of the lubricant determine the amount of wear.

Cleaning

In the combustion process there are lots of impurities left from the combustion process. If these impurities are left unchecked they will, sooner or later, form a thick sludge. This sludge will end up blocking the flow of oil. Dispersants are added to the oil to suspend these particles in the oil. If you burned an Ashless Dispersant oil in a test tube there would be no ash deposits remaining. Detergents in the oil prevent deposits from forming on the hot engine surface. Metallic ash deposits in a combustion chamber may form a hot spot resulting in pre-ignition.

Cooling

The thin coat of oil left on the cylinder walls transfers the heat from the rings to the cylinder walls, where it is passes to the atmosphere through the cylinder barrel's cooling fins. Oil also absorbs heat from other sources as well. For example when oil passes through a turbo charger part of the heat is absorbed by the oil and removed from the local area.

Viscosity

Viscosity is by far the most important property of engine oil. Viscosity affects oil consumption, engine cranking speed, wear rates and engine friction. Viscosity is the "thickness" or the resistance to flow. Oil's viscosity is dramatically dependent in temperature. Oil decreases in viscosity (thinner) with rising temperature. Because of the large range of environments that aircraft engines are used in there are many single weight oils as well as several multi-grade oils available. Single Weight Oils consists of 65, 80, 100 and 120. 65 Weight has the lowest viscosity and is intended for use in very cold environments (Outside air temperature below -15 ēC), 120 Weight is the highest viscosity and is intended for use in warm climates (Outside air temperature above 26 ēC). Multi Grade Oils such as 15W-50 are a blend of viscosity. It is intended to be an "All Season Oil", or in an instance where you do not fly enough to justify a summer and winter weight oil, or in the instance where you are flying from a warm to cold climates. As you might imagine, you cannot get something for nothing. First is that a multi grade oil will cost more. Second, the VI Improver (Viscosity Improver) which are used in multi-weight oils to thicken the oil will tend to brake down with time and heat (leaving the oil thinner and necessary), making frequent oil changes a must.

Corrosion

One of the big killers or general aviation aircraft is the under use of aircraft. The frequent operation of an engine will tend to drive moisture out of the oil. Further continuous use will also require more frequent oil changes, thereby removing acids suspended in the oil. For aircraft that are not as heavily used there are additives that can provide a measure of rust and wear protection. The AeroShell W100 Plus and the AeroShell W 15W-50 contain both an anti-wear and an anti-corrosion additive. Further both these products include the additive LW 16702 which is required on certain Lycoming engines.

Oil Change Interval

You should consider not only the engine time (hours), but the calendar time (Months) as well. In a low use scenario the need for frequent oil changes in aircraft is not caused by the oil wearing out, but rather by the oil becoming contaminated with by-product or combustion, dirt, water and unburnt fuel.

Final Notes

The use of straight mineral oils is generally used to engine break in. Once the engine is fully broken in then you should switch to the non-ash dispersant oils for general use.

The AeroShell Fluid F2 (http://www.aeroshell.com) should be used when you want to place the engine in long term storage.

As always the FAR, Operation manuals and technical publications take precedence over anything you might find here. Most of this information was gathered from the Shell publication as well as the Engineering Manual by Sacramento Sky Ranch. I would recommend both of these for your book shelf.