Your letter of the 19th May was received and read with much pleasure. We were glad to know that you remain well.
We have been very busy on a machine for Ft. Myer and as we are interrupted very much the work goes slower than we could wish but we hope to be flying before this month ends. About a week of the time will be consumed in traveling back and forth between Dayton and Washington to receive medals. The Dayton presentation has been made the excuse for an elaborate carnival and advertisement of the city under the guise of being an honor to us. As it was done in spite of our known wishes, we are not as appreciative as we might be.
After looking over the Ft. Myer machine we have decided that the trouble came in the following manner. One blade of the right propeller developed a longitudinal crack which permitted that blade to flatten out and lose its pushing power. The opposite blade not being balanced by an equal pressure on the injured blade put strains on its axle and its supports which permitted it to swing forward and sidewise a little farther than the normal position and at the same time set up a strong vibration. This brought the uninjured blade in contact with the upper stay wire to the tail and tore it loose, the end of the wire wrapping around the end of the blade and breaking it off. The blade which broke off was not the one which originated the trouble. The machine was pointing almost toward Arlington Cemetery at this moment, but swerved to the right where small trees made a bad landing place. Orville in the meanwhile had stopped the engine and attempted to turn to the left so as to land in the regular grounds, but found the tail inoperative, He therefore twisted the wings so as to present the left wing at the greater angle in order that the increased resistance might hold that end, of the machine back and face the machine up the field. The maneuver succeeded very well and the machine had faced back toward the derrick and descended a third or more of the distance to the ground without any indication of serious trouble. He next moved the lever to straighten the wing tips so as to go straight ahead, but the machine instantly turned down in front and made almost straight for the ground. He thinks the tail had fallen over on its side with possibly a slight negative angle and that, when he moved the handle which operates the wings and tail, the latter was twisted to a positive angle and received a pressure on the under side, which caused the plunge. He pulled the front rudder to the limit, but for a time the response was very slow. Toward the end something seemed to change and the machine began to right, but it was too late. The ground was struck at a very steep angle and with terrific speed. The splitting of the propeller was the occasion of the accident; the uncontrollability of the tail was the cause. The screws stand without injury more than twice the strains they receive in flight, when tested in the shop by the application of pressure at a point 20 percent from the tip to center. But we have concluded that the test was defective in one respect. The pressure of the air is applied to every portion of the blade. We applied the pressure to a little block laid at the center of the width of the blade on the concave side. This evidently tended to keep the blade from flattening and splitting along the inside (i.e., the concave side) in the test. It was this splitting that made the trouble at Ft. Myer. It was the consequent lessening of pressure due to change of angle that set up the vibrations which brought on the other consequences.
Now that we have located the trouble we are certain that its recurrence can be avoided. We are making a set of screws which are heavier at the weak point and will strengthen them with canvas all the way down on the concave side. Then we will brace the axle supports in such a way that no amount of vibration can enable the screws to reach the tail braces. I am glad it was no carelessness of Orville that brought about the catastrophe. It is so easy to overlook some trifling detail when setting up a machine under the conditions which existed at Ft. Myer, that I feared he might have failed to properly secure a nut somewhere; but everything was found tight, after the accident, except the fastening which was torn out when the screw struck the tail stay wire. In so far as the responsibility falls upon anyone I suppose it falls upon me as I did the testing of the screws. I see now that a better test would have been to turn the full power of the motor on to a single screw and thus give it an overstrain under conditions similar to those it undergoes in use. It is the first time we have ever had any indication of trouble with our propellers.
[P.S.] The end of the propeller which was split, was not broken off and had no mark to indicate that the split was due to a blow.