Wright Airplane Disaster

by Dr. Richard Stimson

in The Military Airplane

On September 17, 1908, Orville crashed with Lt. Selfridge as passenger during a flight at Ft. Myers, Virginia. Selfridge died soon after and Orville was left with injuries that pained him throughout the rest of his life.

They were circling the parade ground when, on the beginning of the third circle as they were headed toward the wall of Arlington Cemetery at about 100 feet off the ground, Orville heard a slight tapping at the rear of the machine.

He turned and looked behind him, but couldn’t see anything. Sensing something was wrong, he decided to cut the power as soon as he completed his turn toward the crowd. Suddenly, he heard two thumps, followed by violent shaking. He struggled with the controls as the machine dropped toward the left, causing the nose to drop. The machine hit the ground at full speed and nosed over, burying Orville and Selfridge.

The respected Scientific Journal published an article, Lessons of the Wright Aeroplane Disaster in their September 26, 1908 issue. The article, including my comments, follows below.

“Seldom has there occurred a more pitifully tragic disaster than the sudden fall of the Wright aeroplane, involving the death of that promising young officer Lieut. Thomas E. Selfridge, and inflicting shocking injuries on the talented inventor, Orville Wright.

That the disaster should have occurred at the culmination of a series of brilliant flights, and on the eye of winning that prize of government recognition for which the Wright brothers had striven, unaided, through long years of patient toil, renders the disaster extremely pathetic, and accentuates that world-wide sympathy in which the Scientific Journal so sincerely shares.

But although the accident is deplorable, it should not be allowed to discredit the art of aerospace navigation. If it emphasizes the risks, there is nothing in the mishap to shake our faith in the principles upon which the Wright brothers built their machine, and achieved such brilliant success.

The defect was purely of structural detail. The breaking off of the blades of the propeller of an airship is comparable to bursting the tire on an automobile. In each case there is the danger of an upset; but in neither should the accident be taken to indicate that the principles and design of the whole machine are at fault.”

Comment: One of the propeller blades did break off although that is not what caused the crash. Here is what really happened.

The right blade flattened when it developed a longitudinal crack. That started a sequence of events.

The blade then lost enough power to cause unequal thrust between the two blades. The resulting vibration is what Orville heard as a light tapping noise.

Next in the sequence of events was that that the vibration loosened a stay wire fastened to the tube that housed the propeller axle. The axle moved enough to bring the undamaged propeller blade in contact with the upper stay wire attached to the vertical rudder in the tail.

The wire broke and wrapped itself around the propeller blade, breaking it off, causing the loud thumping sound. That was the broken blade seen flying from the machine.

The broken blade, however, was not the cause of the crash. It was the vertical rudder that had been loosened by the loss of the stay wire. It caused the Flyer to first swerve right toward the cemetery, then to the left, so that it was heading north up the field.

At this point Orville moved the wingwarping lever to the right to straighten the wings and at the same time moved it forward to move the vertical rudder to the right in order to glide to the ground. The problem was that the rudder, without its upper stay wire, was so tilted to the horizontal that it functioned more as an elevator. This sent the Flyer into a fatal dive and ultimate crash.

Orville had been forewarned of possible trouble when on September 9, a propeller developed an 18 1/2-foot split. Orville had to have Loren ship two new blades from Dayton. The new blades had the same chord but were two inches longer.

The Scientific American continued: “Nevertheless, it must be admitted that if the demand for absolutely first-class design and material is strong in the automobile, it is doubly so in the aeroplane.

Judged by the nature of the work it has to do, and in view of the tragic penalties which may attach to the breakage of any one of its delicate and nicely calculated parts, it would seem that a broader margin of safety should be allowed in cutting down the size and weight to secure the necessary lightness.

The supporting planes (wings) with their fragile wooden struts and hair-like wires, constitute a trussed bridge, whose strength, like that of a chain, is no greater that the strength of its weakest link.

Should a single strut or wire snap, the whole fabric must collapse. Similarly, the equilibrium of the whole structure is so sensitive to disturbance, that any sudden change in the opposed forces, such as was occasioned by the snapping of one of the two propellers, must instantly upset the delicate poise, and change the aeroplane suddenly, from a self-sustaining machine to an inert mass, subject to the destructive force of gravity.

The lessons of this particular case are, first, that wood is too uncertain a material to safely endure the complicated stresses due to thrust, high centrifugal force, excessive vibration, or the possibility of contact with the machine to which a propeller is subjected; and, secondly, that the distribution of the thrust between two propellers, placed on either side of the center of gravity, constitutes, as this terrible accident has too clearly shown, a constant invitation to disaster.

Should one propeller break, become loose, or be disconnected from its chain drive, the whole power of the engine becomes concentrated at a point several feet to one side of the center of resistance of the machine, with the result that it becomes immediately unmanageable, and is driven violently from its path; whereas the breaking of a single, centrally-placed propeller would have no greater effect upon the control than would the simple stopping of the motor.

Undoubtedly, it was the inevitable confusion created by the breaking of the propeller on the vertical rudder wire that caused the disaster; for although Wright made a gallant effort to bring the machine back to control, stopping his motor, etc., the horizontal rudders appear either to have failed or to have been pulled in the wrong direction; the aeroplane, after partially righting, taking a sudden and steep plunge to the ground.

Perhaps the most important lesson of all, however, is, that, to render the aeroplane reliable, some method of automatic control of both lateral and horizontal stability must be devised. This control should automatically hold the rudders and plane tips in the requisite position for equilibrium, any deviation therefrom being made separate manual control.”

Comment: The Wrights ignored the free advice. Wilbur was in France at the time of the accident. When he returned and had time to examine what had happened, he stated, “The splitting of the propeller was the occasion of the accident; the uncontrollability of the tail was the cause.”

In June 1909, they tested a replica of the failed 1908 propeller in a barn behind Loren’s house. The first test blade cracked after less than two minutes running. They concluded that the propeller had a weak spot on the concave side that allowed the blade to flatten and split.

The blades were redesigned and made heavier at that point and canvas was added down their concave sides. Also, the tubes supporting the propeller axles were braced so that any vibration would not cause the propellers to reach the wires bracing the vertical rudder in the tail. The problem was solved.

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