The accuracy of the compressor map representation is very important for precise cycle calculations. Remember, that 1% error in compressor efficiency can mean - dependant on engine type - up to 1% error in specific fuel consumption! It can easily happen, that such an error is introduced into the cycle calculation by use of a low quality compressor map. Both when the data are scattered or when there are not enough speed lines in the map the interpolation of data is prone to errors.

The compressor characteristics are needed in performance calculation programs in a special format. To produce this format "by hand" is a cumbersome task that takes quite much time because of the scatter in the data. Moreover the data from the compressor rig test usually are not evenly distributed over the tested speed range or the distance between speedlines is big. Interpolation and extrapolation of the measured data are required.

The program Smooth C is a tool that quickly produces high quality compressor characteristics from measured data. Such characteristics can be used not only for performance calculations. One can also evaluate small differences between different compressor types, for example the effects of interstage bleed, Reynold's number, tip clearance, restaggered blades and vanes, distortion etc.

However, Smooth C is not only a valuable tool for dealing with measured data. It can also be used to check the quality of any compressor map. One can use for example existing compressor maps as input. Various crossplots offered by the program allow a judgment whether the map is a reasonable description of compressor physics or not. Deficiencies can be corrected and physical meaningful interpolation and extrapolations of the map are possible.

Instead of genuine measured data - which seldom are available outside industry and research facilities - one can also take data from figures published in literature. Even relativized compressor map data can be used as input.

The program is suited not only for normal compressor characteristics but also for so-called "split" characteristics that are used to describe low pressure compressors of turbofan engines. Pressure ratio and efficiency in these compressors are different for the inner (core) and outer (bypass) stream. For one (total) mass flow there are two pressure ratios and two efficiencies.

The map smoothing procedure works as follows: Speedlines in terms of pressure ratio against mass flow are shown on the screen. The user can modify the lines with the help of the mouse. Efficiency is drawn also as a smooth line passing through the test data. The engineering judgment of the user defines the output. Nothing is prescribed by the program.

There are many alternatives built into the program. Among others you can check mass flow against speed, efficiency against pressure ratio or against mass flow. Instead of efficiency you can also use specific work and plot it over pressure ratio, mass flow or speed squared.