Chapter 4 - Propellers to Jets: The Impetus of World War II

The War-Time Tunnels at Ames


[32] Moffett Field, the site of the new NACA Ames Laboratory, was located 40 miles south of San Francisco. A cadre of experienced wind tunnel designers had been moved from Langley to the West Coast to oversee the construction of the new test facilities. The main theme at Ames was ostensibly high-speed aerodynamics, but the overriding military need of the moment was the testing of new aircraft designs at moderate speeds (about 250 mph) on an urgent basis. There was no time and no need for inventing new aircraft and airfoils.

In May 1940 construction began on two 7 x 10-foot wind tunnels patterned after the 7 x 10-foot Atmospheric Wind Tunnel built at Langley in 1930. The two Ames tunnels were identical in design, with a closed-throat, single-return circuit operating at atmospheric pressure. Airspeeds in the test section reached about 250 mph. An air-interchange tower provided the necessary cooling. The first tunnel was completed in March 1941; by fall of that year both were responding to a flood of military requests. The tunnel staffs went on two- and three-shift operations to accommodate the new aircraft designs.

Despite the press of war work, the engineers at the 7 x 10-foot tunnels were able to eke out some original research results. For example, the programs investigating propeller slipstream effects and air inlets pioneered new technology, as did a novel effort to predict the flying qualities of aircraft through coordinated wind tunnel tests and actual flight research.


arial view of the 2 wind tunnels at Ames

 The two wartime 7 x 1O-foot wind tunnels built at Ames. The test sections are in the blocklike buildings on the left-hand sides of the air circuits.


[33] This latter led to a system of wind tunnel testing that predicted those aircraft parameters which would best satisfy specific flight parameters, such as maneuverability, stick forces, response of controls, and so on. This empirical link between what a pilot desires in a full-scale plane and what the wind tunnel can test at the model stage was considered one of the outstanding research contributions of Ames Laboratory during World War II.

The third wind tunnel built at Ames was a giant- 16 feet in diameter at the test section-that represented a major advance in wind tunnel design and construction. Operating near the speed of sound (about twice the speed of the 7 x 10-foot tunnels), the 16-foot tunnel required a 27 000- horsepower electric motor-the most powerful tunnel drive system in operation anywhere. The tunnel configuration was fairly conventional, with a closed-throat, singlereturn circuit. The completion of the 16-foot highspeed facility was opportune: early December 1941, just a few days before Pearl Harbor.

The new American fighter aircraft about to go into active service desperately needed large-scale testing at the speeds attainable in the 16-foot tunnel. Many craft, now classics to airplane buffs, went through the facility: the Lockheed P-38, the Bell P-39, the Curtiss P-40, the Republic P-47, and the North American P-51. All these planes were pushing toward Mach 1 and were encountering problems unknown at lower speeds. During the war, the new Ames tunnel operated with three shifts per day, often 6 days per week, in order to get these aircraft ready for active duty.

The P-51 Mustang posed a typical development problem. During early flight tests a strange rumbling noise emanated from the bowels of the aircraft. Because this might presage some sort of structural failure, the cause had to be found. Designers decided that tests in the 16-foot tunnel might discover the source of the vibrations faster than actual flight tests. Consequently, the outer portions of the P-51 wings were removed and the aircraft fuselage-plus-wingstubs was squeezed into the 16-foot tunnel. It was a close fit, but the ploy worked. The first runs traced the rumbling to the belly scoop. Merely lowering the leading edge of the scoop until it was outside the fuselage boundary layer immediately eliminated the vibrations. This modification was incorporated in over 14 000 P-51s manufactured during the war.

A more serious problem plaguing aircraft designers in the early 1940s was whether the newly developed NACA laminar flow airfoils would improve performance at high speeds. Soon after the 16-foot tunnel's...


Ames 16-foot wind tunnel

 This photograph, taker June 10, 1942, illustrates the size of the Ames 16-foot wind tunnel. Note the diminutive automobiles and the two story building in front.


North American XP-51B

 [34] A North American XP-51B with outer wing panels removed undergoing tests in the Ames 16-foot high-speed tunnel.


....inauguration, NACA aerodynamicists ran tests on full-scale wings. For the first time, they obtained data on full-size wings at high speeds and high Reynolds numbers.

The fourth war-time wind tunnel built at Ames departed from the high-speed theme that was supposedly Ames' reason for being. In fact, this new tunnel could not generate test section velocities beyond a paltry 230 mph. Yet this low-speed tunnel was an invaluable addition to NACA's repertoire of tunnels because it was big: 40 x 80-feet at the test section. It was big enough to handle all but the largest bombers and transports-with their engines operating. The low airspeeds did not matter because the purpose of the tunnel was to examine the takeoff and landing characteristics of aircraft. These two periods of flight are extremely sensitive in terms of lift, drag, and stability. Full-scale tests in the 40 x 80-foot tunnel led to seemingly small improvements that actually meant a great deal in aircraft operations. For example, after tunnel tests, the Douglas XSBD-2 dive bomber was provided with a modified wing-flap system that lowered landing speeds from 90 to 84 mph. When landing on carriers, these few miles per hour gave the pilot much better control and, in addition, significantly reduced the energy that had to be absorbed by the carrier's aircraft arresting gear.

The technical challenge of the 40 x 80-foot tunnel was its sheer physical size. The facility covered 8 acres, and the air circuit was just over 1/2 mile long (2700 feet). Six 40-foot-diameter fans, each powered by a 6000-horsepower electric motor maintained airflow at 230 mph or less (these are still tornado velocities). Construction began in late 1941, the mammoth construction task sorely taxing the resources of the new center. Two and a half years later, in dune 1944, the 40 x 80-foot full-scale tunnel went into operation.

In later years this tunnel became the primary facility for investigating the flying characteristics of fullscale helicopters and vertical takeoff and landing (VTOL) aircraft. In the case of the VTOL craft, the tunnel tests explored the critical flight regime where the craft makes the transition from powered lift at low forward speeds to wing-borne lift at high speeds.

Inadvertently, the 40 x 80-foot tunnel also helped study the structural failures of advanced helicopter rotors and new VTOL aircraft. In each instance of unplanned failure, tunnel damage was slight, and the facility was back in operation quickly. Since the tests were well instrumented, the causes of failure were soon found, leading to successful modifications.

So successful was the 40 x 80-foot tunnel in testing full-scale aircraft that 35 years after its initial startup, tunnel power was increased to 135000 horsepower, raising the maximum speed to about 330 mph (Mach 0.45). Modifications were begun to incorporate a new leg with an 80 x 120-foot test section. The largest fighter-bombers, helicopters, and VTOL/STOL aircraft will be accommodated here.