Rise of Air Bubbles in Aircraft Lubricating Oils
Abstract : Lubricating and antifoaming additives in aircraft lubricating oils may impede the escape of small bubbles from the oil by forming shells of liquid with a quasi-solid or gel structure around the bubbles. The rates of rise of small air bubbles, up to 2 millimeters in diameter, were measured at room temperature in an undoped oil, in the same oil containing foam inhibitors, and in an oil containing lubricating additives. The apparent diameter of the air bubbles was measured visually through an ocular micrometer on a traveling telescope. The bubbles in the undoped oil obeyed Stokes' Law, the rate of rise being proportional to the square of the apparent diameter and inversely proportional to the viscosity of the oil. The bubbles in the oils containing lubricating additives or foam inhibitors rose more slowly than the rate predicted by Stokes' Law from the apparent diameter, and the rate of rise decreased as the length of path the bubbles traveled increased. A method is derived to calculate the thickness of the liquid shell which would have to move with the bubbles in the doped oils to account for the abnormally slow velocity. The maximum thickness of this shell, calculated from the velocities observed, was equal to the bubble radius.
Foaming and Aeration of Oils in Aviation Powerplant
The presence of foam or unduly aerated oil in the lubricating-oil systems of aircraft power plant can be very troublesome—more so in reciprocating engines than in gas turbines, since the rates of circulation are greater and the viscosity grade of the oil is higher. Consequently in this paper, which surveys present knowledge about oil foaming, reciprocating engines are the main consideration. In the conventional aircraft “dry-sump” oil system, a simplified diagram of which is shown in Fig. 1, oil is pumped from a supply tank to the bearings, and so on, by a so-called “pressure” pump. This oil then drains down into the sump and is picked up by “scavenge” pumps which return it to the supply tank. There are usually at least two scavenge pumps so that the oil is cleared away irrespective of the attitude of the aircraft.
Identification of Knock in NACA High-Speed Photographs of Combustion in a Spark-Ignition Engine
Report presents the results of a study of combustion in a spark-ignition engine given in NACA Technical Reports 704 and 727. The present investigation was made with the NACA high-speed motion-picture camera, operating at 40,000 photographs a second, and with a cathode-ray oscillograph operating on a piezoelectric pick-up in the combustion chamber. Photographs are presented showing that the origin of knock is not necessarily in the end gas. The data obtained indicates that knock takes place only in a part of the cylinder charge which has been previously ignited either by autoignition or by the passage of the flame fronts but which has not burned to completion. Mottled regions in the high-speed Schlieren photographs are demonstrated to represent combustion regions.