Chapter 2 - A Heritage Lost and Regained

Europe's Second Generation of Tunnels


[11] Gustave Eiffel next proceeded to build a bigger, faster tunnel based on his Champ de Mars design. Located at Auteuil, it boasted a test section 2 meters (6.56 feet) in diameter and a wind velocity of 32 meters per second (105 feet per second). A smaller tunnel sharing the same drive motor reached a wind speed of 40 meters per second ( 131 feet per second)- the fastest tunnel built as of 1912. In addition to the customary research on airfoils, propellers, and so on [12] Eiffel carried out the first wind tunnel tests of complete aircraft configurations, that is, wings, fuselage, tail, and landing gear, in model form. For example, a model of the famed Nieuport fighter was tested for power requirements, stability and control, and pressure distributions. Eiffel applied a "coefficient of enlargement" to predict flight characteristics of the full-scale aircraft. Without doubt, the systematic tests of various French military aircraft designs led to the outstanding performance of French aircraft during World War I.

Whereas the French tunnel was directed more toward practical aircraft design, Prandtl's second-generation tunnel at Gottingen contributed more to basic wind tunnel design. In fact, most of the world's large wind tunnels built over the last half century have been based on the Gottingen second-generation model. Prandtl incorporated two features that have now become standard: (1) a stilling chamber just upstream of the test section where flow disturbances can die out, and (2) a contraction cone at the entrance to the test section. This section helps create a uniform air velocity across the tunnel test section and also reduces turbulence. The performance of the 1916 Prandtl tunnel was outstanding in terms of size (7.35-foot test section), wind speed (170-feet per second), and flow quality (i.e., lack of turbulence). Too late to contribute much to the German war effort, it nonetheless launched a new era in tunnel design and provided a timely base for U. S. aeronautical research, which the war had proven woefully inadequate.

Great Britain's second-generation tunnel went into operation at the National Physical Laboratory (NPL) in 1918-also too late for the war. The salient feature of the NPL design was size: the tunnel configuration consisted of two 7 x 7-foot tunnels united into a single 7 x 14-foot test section. Called a ''duplex tunnel," it was ideally suited for tests of models of complete aircraft configurations. At this stage in wind tunnel development, one of the primary contributions of the NPL was a sophisticated means of supporting the model and measuring the forces and turning moments along and about the plane's three axes. NPL scientists claimed they could detect force changes as small as 1/10 000 pound. This represented an improvement of four orders of magnitude in 50 years.

drawing of Prandtl's second generation wind tunnel

 Prandtl's second generation wind tunnel was built at Gottingen in 1916. It is a model for modern wind tunnels. Starting from the test section, the tunnel expands slowly in cross section as the air moves clockwise around the circuit, through the fan, and around the corners. Just before the test section containing the model, the air enters a stilling chamber where tunnel-generated turbulence is allowed to die out. Finally, the low-speed air is accelerated to a contraction cone or nozzle-a unique feature of this tunnel. The nozzle was a major advance in making the air velocity at the entrance of the test section uniform.