THE problem of ring-sticking in aviation engines is unusually complicated due to the lack of standard test methods and to the difficulties in obtaining reproducible data on full-scale engines. In fact, the immediate problem is largely one of developing suitable test equipment and methods. As soon as such a method is developed, coordination of activities by different groups becomes possible, and the problem will be well on its way toward solution.
In common with most lubrication problems, ring-sticking involves the inseparable trio of variables, namely, oil characteristics, engine-design factors, and operating conditions. During the early stages of aviation, ring-sticking was largely the result of the use of oils of inferior stability, such as vegetable oils and blends of these oils with mineral oils. Since that time there has been such a rapid improvement in oil stability and such a rapid increase in engine horsepower that engine-design factors are of much greater importance than previously. Aviation oils are now so stable at high temperatures that the older type of ring-sticking seems to have passed entirely out of the picture.
As far as oil characteristics are concerned, the ring-sticking of today in high-output aviation engines is tied up with so-called “oiliness” as well as stability. Although ring-sticking in many of these engines probably can be minimized by alterations in design, present indications point to the necessity for using compounded oils of high oiliness and stability if satisfactory ring performance is to be obtained in engines of higher horsepower per cubic inch displacement than those in use at present.