Feature

50 Years Ago: Tests in a New NASA Wind Tunnel Help Save an Industry
04.09.10
 
On Sept. 29, 1959, Braniff Flight 542, a new Lockheed L-188 Electra Turboprop, fell out of the sky on a trip between Houston and Dallas.

All 29 passengers and five crew members died in the crash at Buffalo, Texas.

Lockheed Super Electra

Lockheed L-188 Electra.

On March 17, 1960, Northwest Orient Flight 710, also a Lockheed L-188 Electra, was on a trip between Chicago and Miami when it crashed into a field eight miles east of Cannelton, Ind.

All 63 people on board died.

Why?

The answer was found 50 years ago in a new facility at NASA's Langley Research Center: the Transonic Dynamics Tunnel.

"The Lockheed Electra was the first in a series of turbo propeller airplanes," said Bill Reed, who worked on that first outside test of the new facility.

"Apparently (Lockheed engineers) got an indication that (the crashes) had something to do with a propeller and the 'cell' (engine and cowling) that were attached to the wing. They come off the airplane. To test that concept they came to NASA."

The TDT had just come on line as the renovated 19-foot Pressure Tunnel. The idea was sold in a memo to then-Langley center director Floyd Thompson, and $10 million later the center could offer a facility that could test the aeroelastic qualities in an aircraft flying at transonic speeds.

The medium for most tests was Freon 12, which is four times heavier than air and so allowed scaling of models and flying conditions.

Work in TDT wind tunnel

Semi-span model in TDT wind tunnel. Credit: NASA/Sean Smith

The TDT got a quick evaluation from Norm Land and Tom Foughner, who were doing research on a theory developed by Carson Yates concerning 45-degree swept-back wings on an airplane.

"I was third author on the paper, but it was a good assignment because I was able to do both the testing part and work with (Yates)," said Foughner. "The main thing was the transonic speed we could get in this tunnel was important, because problems occurred at the transonic speed."

The tests also helped determine the maximum size of a model that could be tested in the TDT, as well as how it should be mounted. It was also something of a validation for the data generated in the new wind tunnel because data was already available.

The Lockheed situation was another story.

"Other companies in the aircraft industry were there," said Foughner. "It was like the UN. Each had a group, their flutter experts or aeroelasticity experts, and they all came and helped Lockheed to figure it out. "These airplanes crashing was bad news to everybody in the aviation industry."

The primary players were Boeing and Lockheed, with a great deal of input from the Federal Aviation Administration, as well as NASA.

Commercial flight was considered somewhat exotic in the late 1950s and early '60s, when people still rode busses or trains, and when gasoline for their cars was only 25 cents a gallon.

"The industry realized that this wasn't just Lockheed's problem," said Foughner. "They had to solve the problem for everybody."

Reed remembered the intensity of the testing.

"The cooperation was outstanding," he said. "It was around-the-clock testing. We had cots down in the calibration lab for the engineers to take off."

Work in TDT wind tunnel

Model installation in the TDT wind tunnel. Credit: NASA/Sean Smith

Lockheed provided a one-eighth scale model of the Electra for the tunnel and modified the cells for testing, which was done in air, rather than Freon.

Foughner and Reed, then young engineers for two-year-old NASA, worked on the test. Foughner monitored an oscillograph and read time histories "with about 20 other people," he said.

Reed worked on a phenomenon that came out of the tests called "propeller whirl flutter."

"It was discovered here and had not occurred before in the history of aviation," he said. "The reason for it was probably that these propellers were larger and the engines were flexibly mounted to the wing. Theories were developed to predict it. I was heavily involved in that for a number of years."

From those tests came answers.

"The remedy was to make the structure redundant, so that any single element that failed, any structure attaching the engine to the wing, there was enough remaining structure to be stable," Reed said. "Also, there was a damping added to the mechanism between the engine and the wing. They also added stiffeners to the wing itself."

A review of the propeller whirl flutter found some history.

"Before we had this theory, the industry didn't know much about the phenomenon," Reed said. "It didn't even have a name.

"At MIT, back in 1937, they predicted the possibility of this happening, but it had never happened before, so they had kind of given up worrying about it."

And then the demand for air travel increased, and airplanes grew to meet that demand.

"The large propellers and the powerful engines, the turboprop system was new," Reed said. "The features of that system was what precipitated the possibility."

He continued to work on propeller whirl flutter, often using a model crafted in his home workshop.

Once the problems with the Lockheed L-188 were fixed, it was renamed the "Super Electra." It remained in use as an airliner through the millennium, and its design was used in building the P-3 Orion, the foundation of the Navy's airborne anti-submarine warfare effort.

The Transonic Dynamics Tunnel had carved its niche in aviation in its very first year.

"Larry Lofton (an early supporter of the TDT and, later, Director of Aerodynamics at the center) is often quoted somewhere as saying the value of that test in terms of money far exceeded the cost of modifying that tunnel," Reed said. "One test."

One important test for an entire industry.

 
 
Jim Hodges
The Researcher News
NASA Langley Research Center