Flyer Fatality
Version 2.1
Copyright ©2017 by Paul Niquette. All rights reserved.
September 17, 1908, Orville Wright conducted a demonstration flight in the Wright Flyer Model A for the U.S. Army at Fort Myer, Virginia. His passenger was Lt. Thomas Selfridge. 

Flyer III

After the machine completed four laps over the parade grounds at 150 feet in elevation, witnesses saw a "sizable fragment fly off into the air."  Orville Wright described the ensuing events in a letter to his brother Wilbur...

On the fourth round, everything seemingly working much better and smoother than any former flight, I started on a larger circuit with less abrupt turns. It was on the very first slow turn that the trouble began. ... A hurried glance behind revealed nothing wrong, but I decided to shut off the power and descend as soon as the machine could be faced in a direction where a landing could be made. This decision was hardly reached, in fact I suppose it was not over two or three seconds from the time the first taps were heard, until two big thumps, which gave the machine a terrible shaking, showed that something had broken. ... The machine suddenly turned to the right and I immediately shut off the power. Quick as a flash, the machine turned down in front and started straight for the ground. Our course for 50 feet was within a very few degrees of the perpendicular. Lt. Selfridge up to this time had not uttered a word, though he took a hasty glance behind when the propeller broke and turned once or twice to look into my face, evidently to see what I thought of the situation. But when the machine turned head first for the ground, he exclaimed 'Oh! Oh!' in an almost inaudible voice.

Lt. Selfridge died in the crash and Orville Wright's injuries required months for recovery.  The tragedy was a first in aviation history: passenger fatality in powered flight.



An abundance of literature about the 1908 crash
is available on the Internet.  Solvers will find some accounts to be questionable.  Contemporaneous analyses -- even official investigations -- were based on incomplete technical knowledge at that time about key aviation realities. 

For example, the management of asymmetrical thrust as explored
in Pilot's Nightmare...
Asymmetrical thrust -- a good engine on one wing working against the drag of a dead engine on the opposite wing -- can flip the plane onto its back.  Twin-engine flight training is dominated by the pilot learning critical reflexes -- "step on the good engine!" (with the correct rudder-pedal) and "never to turn into a bad engine!"
...was not appropriate for any of the Wright Flyers, inasmuch as each had one 'good engine' driving two propellers.  Practice for its failure would merely cause the machine to revert to the familiar status of a glider without manifesting the hazards of asymmetrical thrust. 


The Flyer Fatality puzzle will parse Orville Wright's letter for the sequence of events and develop independent explanations for them. 

Let us begin with...
I suppose it was not two or three seconds from the time the first taps were heard, until two big thumps, which gave the machine a terrible shaking, showed that something had broken.
...and examine this post-crash photograph...

Broken Blade
Orville Wright being carried from the crash scene by stretcher, with the bottom of the Wright Flyer wing in the background, from which we are able to confirm that the right blade was splintered and broken.
Wright Propeller Blade
The Wright propeller blade was designed as a twisted, wing-like airfoil, which was carved from laminated planks. Unlike modern propellers, the blade broadens toward the tip.  Each blade element, in effect, 'flies' in a circle and develops incremental lift, contributing to axial thrust according as the square of its radial location.  The local angle of attack was chosen to match the tangential velocity. 

The propeller blade was subjected to bending forces, which increased with radial distance from the drive axis.  Accordingly, failure of the blade probably began with splintering near the tip, which would explain the first taps from that initial imbalance.  Most likely, the final blade failure took place in two stages, manifested as the two big thumps...
Stage 1 The blade fragment folded forward under the influence of the 'lift' vector, which by design was pushing the whole Wright Flyer forward.
Stage 2 As that force became redirected radially inward, the blade fragment was
thrown outward by centrifugal force.
Separation of the 'sizable fragment into the air' abruptly increased the propeller imbalance, which thereafter gave the machine a terrible shaking.  At the same time, the engine would increase in speed with the reduction in blade drag on the right propeller.



Other explanations for those observations can be found in the literature.  Most postulate damage to the aft structure on the Wright Flyer Model A
inflicted by the blade fragment...
  • Impacting the vertical rudders themselves -- 'knocking them horizontal', in one account.
  • Cutting structural wires to the rudder assembly or breaking of struts.

The Wright Flyer was the first of many aircraft designs that exploit the Canard Advantage, which puts the elevator control forward-most on the aircraft rather than in the tail along with the rudder (see empennage).  Thus damage from the blade fragment could not have a direct effect on the Wright Flyer's pitch control at the beginning of the sequence of events.

The Flyer Fatality puzzle applies an immense resource -- Wright Plans & Drawings, which features 44 computer-generated drawings of the Wright Flyer, including...

View from rear of Flyer III with broken
                          blade
View from the rear of the Wright Flyer, here marked up for the Flyer Fatality puzzle to depict a break in the right propeller blade and its implications.

The
"sizable fragment" of the propeller blade necessarily began its trajectory on a tangent from a point in the propeller rotation that depended on the instant of its separationThe sketch above shows that for 2/3rd of the arc defined by the propeller tip, the fragment would have taken a trajectory away from the tail structure and rudder. The red part of the arc suggests that, with a 1/3rd probability, the rudders might be struck
  • However, solvers can readily surmise that, based on known dimensions and estimated time intervals, the blade fragment would pass well in front of the rudders.  
  • Meanwhile, striking struts or cutting guy wires in the tail assembly would hardly result in the vertical rudders being knocked into a horizontal condition to affect pitch.
  • Finally, cutting either or both of the rudder control wires would most likely result in harmless 'weather-cocking' into neutral position.
Let us conclude, then, that the propeller fragment did not play a rôle in the rest of the drama.



We are left with the following events to explain:
The machine suddenly turned to the right and I immediately shut off the power. Quick as a flash, the machine turned down in front and started straight for the ground. Our course for 50 feet was within a very few degrees of the perpendicular.
What is your explanation for the sudden turn to the right with power on followed by pitching straight down with power off?


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