Better measure for airfoils

Citation

(2001), "Better measure for airfoils", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 1. https://doi.org/10.1108/aeat.2001.12773aab.008

Better measure for airfoils

Better measure for airfoils

Keywords: LMI Selcom, Gas turbines, Blades, Measurement

The task of measuring gas turbine airfoils for aircraft engines is problematic because of the complex, compound curves and critical small radii on leading and trailing edges of the compressor and turbine blades. Adding to this measurement challenge is the need to check hundreds and sometimes thousands of points on each blade.

To tackle this problem, LMI Selcom of Detroit developed a laser-based, computer controlled measuring system specifically designed for airfoil metrology. The Programmable Automatic Contour System uses fast, accurate, rugged, non-contact laser-based measuring sensors to profile surface points. The company claimed that the PACS system is used by several major engine manufacturers and their component suppliers.

Key to the system's performance is use of an industrial sensor, based on dual viewing triangulation principles. These non-contact, no-moving-part sensors provide surface measurement accuracy to 0.00005 inch.

Since airfoils are manufactured by many different processes and are often coated with surface protection materials, the reflectivity of surfaces to be checked can vary significantly, which is often a problem for simple lasers. The LMI industrial sensors are said to overcome these difficulties with built-in automated gain control, which automatically compensates sensor operation for any surface reflectivity over a range of 100,000 to 1 (from dull black to very shiny), more than enough to cover production surfaces. The tiny probe size of the laser beam (0.001in.) eliminates point of tangency errors, which occur when a physical probe does not touch a surface perpendicularly, so that the small edge radii on compressor blades can be measured more accurately.

PACS mounts the sensor on high accuracy positioning tables. The tables are mounted in a cylindrical design (one rotary and three linear axes) since blade surfaces are roughly cylindrical in base form. The controller provides inspection path planning control of measuring and data analysis display.

The PACS systems have the ability to download computer databases to compare CAD files with actual parts, to inspect parts without a database, or to apply reverse engineering to unknown parts. Users can select manual point-to-point, automatic, or on the fly scanning; concave and convex surfaces can be scanned at rates up to 20 points per second. Reporting software provides the ability to plot curves, compare actual to nominal, compare surfaces to tolerance bands, and determine calculated parameters, such as lean, twist, bow, and displacement at various sections.