WIND TUNNELS OF NASA

 

Chapter 1 - Whirling Arms and the First Wind Tunnels

The Wright Brothers Put It All Together

 

[5] Wilbur (1867-1912) and Orville (1871-1948) Wright, operating from the unlikely background of bicycle manufacturers, built their first flying machine in August 1899. It was a simple, 5-foot span, unmanned biplane kite rigged so that it could be maneuvered by twisting or warping the wings (somewhat like birds do for control). Kite tests led to the construction of their first unpowered manned glider in 1900. Twelve test flights with glider No. 1 proved [6] that their pitch and roll controls worked. The glider, however, was generating far less lift and more drag than they expected.

To find out why their first glider did not perform as predicted, the Wrights set up a remarkably simple experiment using natural winds to compare the relative lifting forces of flat and cambered surfaces. In effect, they built an aerodynamic balance that showed unequivocally which of two test airfoils developed more lift. This "wind tunnel without walls'' confirmed the Wrights' growing belief that the accepted aerodynamic design tables they were using were seriously in error.

Sobered by these revelations, the Wrights increased the wing area of glider No. 2 to 290 square feet. The initial trial flights at Kitty Hawk disappointed them still further. The highly cambered wings created pitching movements that could not be controlled. After several near disasters, airfoil curvature was reduced, and the craft behaved much better.

The Wrights returned to Dayton with mixed feelings. Glider No. 2 had flown, but, from the standpoint of their expectations, the 1901 Kitty Hawk tests were a disaster. Their morale sagged. "Having set out with absolute faith in the scientific data, we were driven to doubt one thing after another, till finally after two years of experimentation, we cast it all aside, and decided to rely entirely upon our own investigations. "

They began with a comprehensive series of experiments with a wide variety of airfoils. In the short span of 3 months these tests produced the basic data needed for building their 1902 glider and the powered aircraft to follow. During this short span of time, the Wrights leapfrogged other aerodynamicists the world over.

The first tests were exploratory and utilized an unconventional testing machine: a bicycle with a third wheel mounted horizontally on the front of the frame. Two test shapes were mounted on the wheel, and the bicycle was pedaled rapidly (up to 15 mph) up and down the streets of Dayton. The airfoil being tested would produce a torque in one direction, but this was counterbalanced by an opposite torque from a reference shape. The rotating balance was brought into equilibrium by changing the airfoil's angle of attack. Data from the impromptu rig were crude, but they reinforced the Wrights' decision to reject existing handbook data. They had to write their own handbook, and for that they needed a wind tunnel.

The first tunnel consisted of a square tube for channeling the air, a driving fan, and a two-element balance mounted in the airstream. One balance element was a calibrated plane surface; the other was a cambered test surface inclined at the same angle but in the opposite direction. When the wind tunnel was brought up to speed, the vane-type balance turned one way or the other, thereby indicating the relative lifting forces. The preliminary results from the makeshift tunnel were so encouraging that the Wrights immediately built a larger and more sophisticated facility with a 16-square-inch test section. Here they obtained the critical data they needed for their first manned, powered aircraft.

They did make one mistake-they installed the tunnel's two-bladed fan upstream. Shields, screens, and a honeycomb grid did cut down the turbulence, but it was a curious lapse for the detail-conscious Wrights. Recognizing that their laboratory itself was the return path for the air rushing out of the tunnel test section at 25-35 mph, they forbade the moving of objects and people while taking data.


replica wind tunnel

 Working replica of the 1901 Wright brothers wind tunnel in the National Air and Space Museum. (Photo, National Air and Space-Museum)


 

[7] The heart of any successful wind tunnel is its balance system-the apparatus that measures the aerodynamic forces acting on the model. The Wrights built two balances-one for lift and a second for drag. The balances never measured actual forces; they simply compared test airfoils with reference airfoils or the forces on calibrated flat surfaces. This approach allowed the Wrights to rapidly pit one airfoil against another and select the best from many configurations.

The Wright brothers returned to Kitty Hawk in late summer 1902 to build glider No. 3. It was only slightly larger than the 1901 version, with a wing area of 305 square feet, a 32-foot wing span, and a weight of 116.5 pounds minus the pilot.

For straight-ahead gliding the craft worked well. The lift-to-drag ratio was approximately 8, a one-third increase over their earlier gliders. Pitch control was excellent, but turns were a problem. To turn, the plane had to be rolled in the direction of the turn. This was accomplished by warping the wings; that is, one wing panel would be twisted to increase the tip's angle of attack, while the other wing's panel would be twisted in the opposite direction. The high wing, however, created excessive drag and tended to wheel the craft in a direction opposite from that intended. The addition of a rudder linked to the wing-warping control solved this problem.

The famous 1903 Wright Flyer followed the 1902 glider design closely, except for the addition of twin counterrotating propellers 8- 1/ 2 feet in diameter driven by a 12-horsepower gasoline engine. Back again at Kitty Hawk, on the morning of December 17,1903, with Orville at the controls, the Fryer headed into a 20-mph wind. After a short run of 40 feet, it rose into the air under its own power and flew for 120 feet. Three more flights were made that morning, with the longest lasting 59 seconds and covering 862 feet on the ground, or about 1/2 mile in the air. The Flyer was slightly damaged on the last landing, and before repairs could be made a gust of wind turned it over and destroyed it. It never flew again.

The historic Wright Flyer has been rebuilt and is now on display at the National Air and Space Museum in Washington, D.C.


rebuilt Wright Flyer

 The rebuilt Wright Flyer in the National Air and Space Museum. The Apollo 11 capsule is in the foreground and the Spirit of St. Louis overhead. (Photo, National Air and Space Museum)


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