The mysterious X dimension

Flying Safety, August, 2002 by Chad Hogan, Craig Pessetto

* X-dimension vs. aircraft gross weight.

* X-dimension vs. pressure.

* X-dimension vs. both pressure and gross weight.

Some technical data may only give aircraft gross weight vs. X-dimension, like the KC-135. Care should be exercised when using gross weight to determine X-dimension, because the aircraft center of gravity may not be in the center of the aircraft, depending upon the loading condition. Using gross weight as the only method for checking strut servicing is also prone to error because of binding effects.

Aircraft like the F-16 check X-dimension during preflight by using strut pressure. Using X-dimension vs. pressure is a good way to check the X-dimension, because aircraft CG or strut binding effects are of little or no concern. With the F-15, gross weight vs. X-dimension is given for preflight checks, and pressure vs. X-dimension is also given for other instances when a pressure gauge is available (see figure 1). If the proper procedure is not clear, crews will try to service F-15 struts using gross weight and pressure simultaneously. This usually does not work because of CG and strut binding effects. Pressure or gross weight should be used individually but never together. Pressure readings always produce the most accurate results.

Basic Conceptual Physics Of Strut Operation and Proper Servicing Of A Shock Strut

Now that we have discussed the design and operation of a shock strut, we would like to focus on the physics of operation and proper servicing of a shock strut. A question maintainers always seem to ask landing gear engineers is, "Why doesn't the landing gear ever seem to extend to the proper X-dimension?" It is a common frustration that landing gear engineers are constantly answering. The typical short answer to this question is, "The strut hasn't been serviced properly." This answer is not intended to be an accusation but a statement of physical laws. The following information is an attempt to explain these physical laws and how they relate to getting a proper X-dimension. For simplicity, this discussion omits the transient effects of altitude and temperature.

The three basic main drivers of strut servicing are gross aircraft weight, X-dimension and internal strut pressure. Once a strut is serviced with fluid and gas, it is a closed fluid and gas system, and some intuitive things happen. X-dimension decreases when the aircraft gets heavier, and increases when the aircraft gets lighter. A heavier aircraft produces higher internal strut pressure than a lighter aircraft. As the volume of the internal strut chamber decreases due to external forces, internal pressure increases (i.e., pressure is inversely proportional to volume). Regardless of what is going on outside of the strut, in a closed system, the internal strut pressure is affected only by changes in strut internal volume.

Once a strut is considered a closed system, physical laws dictate that a certain X-dimension correlates to a volume of gas in the strut, which always produces a corresponding pressure. X-dimension and internal strut pressure are only loosely related to gross aircraft weight because of center of gravity (CG) and binding effects. For example, assume a half --compressed strut has an X-dimension of eight inches and an internal pressure of 725 psi. No matter how many times you extend and compress that strut (if proper gas-to-fluid ratio exists), or how you load the aircraft, an X-dimension of eight inches will always produce an internal pressure of 725 psi, because the internal volume of the strut defines the strut pressure. Considering this fact, it is a simple thing to pull a strut out of an aircraft, compress it, record X-dimensions vs. pressure and then put the data on a chart. A sample curve is shown in chart 1. For simplicity, the three charts in this discussion disregard aircraft weight and consider only pr essure vs. X-dimension.