Chapter 5 - The Era of High-Speed Flight

An Exercise in Wind Tunnel Complexity


full-sized jet engine

 [71] A full-sized jet engine installed in the Lewis Unitary Plan 10 x 1O-foot supersonic wind tunnel.


With the Unitary Plan tunnels in place, it is an opportune time to stand back and take a look at what 60 years of NACA/NASA wind tunnel development had wrought. Most designers recall with nostalgia the good old days of NACA wind tunnel No. 1 of 1920 vintage. In a simple, nonreturn circuit largely constructed of wood, a 200-horsepower electric motor driving a wooden propeller generated air speeds as high as 90 mph (Mach 0.1). The complete wind tunnel with its test chamber and attached shop area was housed within a small laboratory building that also served as the return passage for the air.

From the day of the Red Baron with his scarf streaming in the breeze to the modern-day fighter pilot in his pressurized G-suit, the design of wind tunnels underwent a similar increase in sophistication. Nowhere is this transition more evident than in the NACA wind tunnels of the Unitary Plan. These three NACA facilities, which came on line in 1955, represented a landmark in wind tunnel design by any criterion-size, cost, performance, or complexity.

The Langley Unitary Plan wind tunnel is the smallest of the three Unitary facilities insofar as test section size and power are concerned. But the complexities of the drive system and air ducting to attain speeds of Mach 5 provide an interesting contrast with NACA wind tunnel No. 1-a facility of equivalent test section size. As one knowledgeable visitor was heard to comment, "It looks more like an oil refinery under a roof rather than a wind tunnel."

It is interesting to compare the concentration of power relative to test section area. The NACA wind tunnel No. 1 with its 200-horsepower drive, required only 10 horsepower per square foot to attain Mach 0.1. The Langley Unitary Plan tunnel, in comparison, demands 6250 horsepower per square foot at Mach 4.6 and 10 atmospheres pressure.

Nowhere in wind tunnel design has change been more rapid than in the area of data acquisition systems. In wind tunnel No. 1 all raw data were collected manually and then laboriously reduced over a period of days (and often weeks) after the test was run. As late as 1940, substantial amounts of wind tunnel data were collected manually. Forces were often measured on commercial platform scales emblazoned "Honest Weight. "

In contrast, the electronics-filled control room is the heart of the Langley Unitary Plan tunnel operation. Data on pressures, forces, and temperatures are collected remotely and automatically from 85 separate data channels. Each channel is sampled at rates as high as 64 data points per second, incomparably faster than a human could record them. The resulting raw data are processed through on-site computers to reduce the data to coefficient form-lift, drag, pitching moment, and so on. These reduced data are then presented almost instantaneously on TV screens or printed out on automated plotters in real time. Despite the manifest differences in sophistication, complexity, and power, both wind tunnels-NACA No. 1 and the Langley Unitary Plan facility-made aeronautical history. Their different levels of technology accurately mirrored the machines they were testing.


 (Overleaf) The Langley Unitary Plan supersonic wind tunnel, completed in 1955.


view of wind tunnel structure

engineer inspecting angle fins

technician inspecting wind tunnel component


technician inspecting internal surfaces of wind tunnel

crosscut view of wind tunnel model

scientist at the ctrol panel of wind tunnel

engineer observing model test section

technician inspects wind tunnel test model