Roller Coaster Flight

by Dr. Richard Stimson

in The Kitty Hawk Years

When Wilbur made the first attempt to fly a powered machine on December 14, 1903, the Flyer rose in the air steeply to about fifteen feet, stalled and crashed into the sand. He blamed the crash on his unfamiliarity with the sensitive elevator control.

Forewarned, the four successful flights on December 17 were handled better but still resulted in roller coaster rides that could have flown farther if not for the sensitive elevator control.

This wasn’t the first time that they experienced problems using the elevator for pitch control. The elevator on their 1901 glider hardly worked at all. This was an unexpected result that they didn’t understand because they hadn’t experienced this problem with their previous year’s glider.

E. C. Huffaker and George Spratt, two of their visitors, suggested that the problem might be the reversal of the center-of-pressure on the wings. They had experienced similar problems in their own work.

The brothers didn’t think this was their problem because they were using a wing camber similar to what Otto Lilienthal had used successfully and they didn’t think he had run into this problem.

At this point in time the Wrights didn’t understand the true nature of the center-of- pressure on a curved wing and what was happening as the angle of attack changed.

Here is a brief explanation. When a flat surface is at right angles to a stream of air, the center of this pressure lies exactly at the center of the surface. As the angle of the surface is reduced (smaller angle of attack) the center-of-pressure moves forward toward the leading edge. It continues moving forward as the angle is reduced until the surface is parallel to the stream of air. The center-of-pressure is now directly against the leading edge.

The behavior of a curved surface operates differently. As the angle of attack decreases, the center-of-pressure moves towards the leading edge as before.

However, when the angle of attack nears zero, the center of pressure reverses and moves rearward on the surface. In the case of a cambered airplane wing, this movement of the center-of-pressure forces the rear of the wing upward and consequently the airplane into a sudden dive.

The Wrights decided to test the wing of their glider to determine what was happening. They removed the top wing of their glider and affixed two lines to the front edge. They flew the wing as a kite under a variety of wind conditions.

They observed that in light winds the wing pulled upward on the lines indicating that the center of pressure was in front of the center of gravity and the leading edge was forced upward.

In stronger winds the wings were forced down at a smaller angle of attack and pulled down on the lines. This demonstrated that the center of pressure had moved behind the center of gravity toward the trailing edge of the wing.

The brothers now knew that the control problem with the 1901 glider was caused by the sudden reversal of the center of pressure at low angles of attack. These sudden reversals required frequent movements of the forward elevator to maintain stability in pitch.

The back-and-forth center of pressure travel produces what is called a “pilot induced oscillation,” in which the pilot’s efforts to control pitch might actually make it worse.

Fortunately the Wrights had made the wings on the 1901 glider so that the camber could be easily adjusted. They reduced the camber from 1 in 12 to 1 in 19. The glider handled much better after the change.

The problem of pitch control returned in 1903. Part of the problem this time was that the elevator was hinged at its center. With this configuration, the airflow forced the elevator to sharply deflect on its own after only a small movement away from the neutral position. Once the oscillation started it was very difficult for the pilot to regain control of pitch, resulting in a roller coaster ride that ended with a premature crash into the sand.

The brothers had lengthened the lever that operated the elevator by four inches after Wilbur’s control problems on December 14th. It did provide 65% more leverage but it is doubtful that it did much good.

Despite their problems on December 17, 1903

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