NACA TRANSONIC AND SUPERSONIC COMPRESSOR RESEARCH: 1945-1955

The need to use axial, instead of centrifugal, compressors in order to attain high levels of thrust in aircraft gas turbine engines had become increasingly clear by the end of World War II.29 Unlike centrifugal compressors, however, axial compressors were proving to be difficult to design with consistency. The base point in aero­dynamic design technology that had emerged by 1945 allowed efficient axial compressor stages to be designed30, but only under the restriction that the aerodynamic demands made on the compressor remained modest. The design method in question was based to a considerable extent on empirical data from tests of some airfoil profiles in cascade31 over a limited aerodynamic range. Specifically, the pressure-rise, turning, and thermodynamic losses had been determined for these airfoils in cascade as functions of incidence conditions in two-dimensional wind – tunnel tests. Compressor blades were then formed by selecting and stacking a sequence of these airfoil profiles radially on top of one another, as if the air flows through the blade row in a modular series of radially stacked two-dimensional blade passages. Achieving more ambitious levels of compressor performance was going to require this method to be extended, if not modified, and this in turn was going to require a substantial research effort, including extensive wind-tunnel tests of a wider range of airfoils in cascade. The engine companies conducted some research to this end – e. g., P&W carried out their own wind-tunnel airfoil cascade tests. Never­theless, the main body of research fell to government laboratories like the National Gas Turbine Establishment in England and the National Advisory Committee for Aeronautics in the U. S.

The applied research program on axial compressors carried out by the NACA in the decade following World War II was especially important in advancing the state of the art. This program involved a number of diverse efforts, most of them located at the Lewis Flight Propulsion Laboratory, in Cleveland, though a few at the Langley Aeronautical Laboratory, in Virginia, as well. While this research program deserves a historical study unto itself, we will confine ourselves here primarily to results that ended up contributing crucially to the design of the first successful turbofan engines. We say “ended up contributing” because none of this work appears at the time to have been aimed at the design of turbofan engines. The goal throughout was to advance the performance of axial compressors in what were then standard aircraft gas turbines.