Hello to you all,

I'm working for my PPL and I've got a question...
Can someone explain to me what's the relation between oil temperature, oil viscosity and oil pressure please ?

Thanking you in advance.

 

https://youtu.be/PMuJltveP9s

 

Viscosity describes the thickness of the oil.

Thicker oil (higher viscosity) is harder to force through a system (pipes, bearings, filters, etc) than thinner oil, so the back pressure through the system increases the pressure of the oil supply from the pump. Difference between pumping molasses vs. water.


Temperature affects viscosity:

Colder oil behaves "thicker" (increased viscosity).

Warmer oil behaves "thinner" (reduced viscosity).

 

Viscosity is resistance to flow. Temperature does affect viscosity, but how it affects it depends on the oil.

In straight-grade oils that have a single numerical viscosity rating (eg 100W), generally speaking the oil gets thinner (lowers in viscosity) as the temperature rises. This is not true for multi-grade oils, in which the viscosity is lower at lower temperatures, and gradually increases to the maximum value of the oil. These are displayed as two numbers (eg 20W-50), with the lower, first number representing the cold viscosity and the second higher number representing the operating value at temperature (still a variable).

Where a viscosity value is given, such as 100W, it's the same viscosity as 50W oil. 60W and 120W are oils for radial engines; they're the same viscosity. With lower values, such as a 50 weight oil, 25 is not the same value. Always verify that the specific oil that's being put in the engine is acceptable for that engine, and acceptable with regard to what's in the engine. Some manufacturers require that if a different type, grade, or manufacturer of oil are not acceptable; if one wishes to put in a quart or pint of something different, the engine must be drained and flushed. Always verify.

Lower viscosity oil tends to have lower oil pressure, but that's not always the case. For example, multi grade oil has lower viscosity when cold, but may have higher oil pressure until oil flow has been established. Oil pressure is partially a function of temperature, but a function of the resistance to flow within the oil system in the engine. Sometimes a correlation can be made between temperature and pressure to verify that one is really seeing a low pressure condition or high temperature state (high temp, look for lower pressure, etc), but don't count on it. Systems compensate, and there are things that can occur in the engine (such as debris trapped at the relief valve) that can cause pressure drops or incorrect temperatures, that aren't associated with low oil, low pressure, or high temperature).

What is important is an abnormal condition; if you see high oil temperature or low pressure, regardless of the actual cause, it's a good time to get on the ground and sort it out there. Both high temperature and low pressure (and high pressure, incidentally) can be warning signs of very serious conditions that could lead to engine failure or worse; oil is the life blood of the engine and tells a lot about the engine.

 

Consider a stick of butter. It melts at high temps and becomes a solid at low temps - that's viscosity.

Oil manufactures can engineer melting point and thickness into their products to match machine requirements.

Check this: Lubricating Oil Properties used in Aircraft Engines

Go forth and conquer the skies.

 

Some oils get thicker at low temperatures, or experience an increase in viscosity. Others do not, and are engineered to be less resistant to flow (lower viscosity rating) at lower temperatures to allow them to flow when cold. Hence, multi-viscosity, or multi-grade oil.

 

Eons ago flying feeder for FedEx in a Convair I walked into the line shack of Indy Beechcraft, looked around and said, "I'll buy all the red can oil you have." It was Christmas madness and cold. The 6 five gallon cans we carried in back were the consistency of peanut butter.

 

They all get thicker in the cold, it's just a question of how much. Multi-vis mitigates that effect.

 

Why are synthetics not used in piston powered aircraft?

 

Multi-weight or multi-grade thickens as it warms; 20W50 starts at 20W, for example, and increases to 50W as the temperature increases; at operating temperatures, it performs with the properties consistent with 50W oil.

Many years ago, it was common to use fuel injection to the oil tank or crank case to thin the oil with fuel at or just after shutdown.

Some oils have become difficult to get. About the only radial oil available now is Aeroshell 120, and it needs ordered by the barrel (which is fine, because it's not uncommon to take 3-5 gallons of oil per engine per hour on a large radial). In the past, when working around the country, we had to order the oil in advance and have it shipped to the area we'd be working, because there was near zero chance anyone would have it. I usually carried a couple of drums aboard, too.