Symptoms typical of an aerated hydraulic system plagued the F-104 aircraft for many years. (To understand what fluid aeration is and how fluids become aerated, see chart below) Aeration was caused by system components with built-in chambers and passages wherefree air (bubbles) collected when system fluid was saturated or at rest with no pressure acting on the fluid.

The steering-damping unit of the nose landing gear on the F-104 aircraft is a typical hydraulic component. It extracts energy from hydraulic fluid by restricting flow between two chambers and thus damps nose wheel oscillation. If the fluid is aerated, the energy extracted diminishes until it reaches a level where the damping energy is below the unbalancing force of the wheel and shimmy sets in.

Several types of air extraction devices then available were tested over a span of years. Unfortunately, they were not effective in the pressure environment of the aircraft system. Finally basic studies were undertaken. Theories were explored which led to the construction and testing of a laboratory model of an air-oil separator which stopped the nose wheel shimmy.

Theory of operation

This air–oil separator operates on the principle of lowering the pressure (to a desired vacuum) in a chamber containing small quantities of hydraulic fluid continually bypassed from a pressurized dynamic system. The gases thus released from the fluid are collected and prevented from being returned to the system. The gases are stored for removal at convenient intervals when the hydraulic system is at rest.

Wire mesh screen separates bubbles

Initial attempts at actually constructing the air-oil separator involved using a 10-micron (nominal) wire mesh filter screen as a barrier because of its ability to resist the passage of air. This resistance is demonstrated by the familiar bubble test used on filter elements wherein the element does not pass an air bubble until the pressure differential is at least 9 inches of water. This premise was correct in that air did not pass through a 10-micron filter screen as long as the air was in the form of small bubbles. In other words, free air did not pass through the filter screen when the air content of the fluid was well above the saturation point.

How fluids become aerated

Air can enter a hydraulic system in many ways and, according to Henry’s Law, will be dissolved in a fluid in proportion to the pressure acting on the fluid. Thus, considerable quantities of air can be dissolved in a system that has a pressurized reservoir because the pressurizing media (air or a gas) is in direct contact with the fluid surface. Additional quantities of air may be introduced by aircraft servicing equipment such as hydraulic ground test stands, which also use air-pressurized reservoirs.

When the system is at rest and unpressurized, air in excess of that which can naturally be dissolved at the zero pressure condition is released as free air.