Chicago, September 25, 1901
I learn from various sources that some of the Engineers were much interested by the address which you gave us. When will you give us the manuscript? Do not be afraid of making it too technical.
Since you were here I have received Mr. Huffaker's notes taken during your experiments. Shall I send them to you?
In looking over an old number of the Revue de l'Aéronautique I find that after describing his machine as containing 14 square meters (151 sq. ft.) with an arching of 1/12, it says:
Experiment showed that in calm air, the wings being practically horizontal, it followed a trajectory inclined 9° below the horizon, with a velocity of 9 meters per second [20.12 miles pr hour.] Previous experiments of Mr. Lilienthal showed that for similar surfaces, with an angle of 9°, the vertical component of the air pressure was 0.80 of the pressure on the same surface, struck at right angles with the same speed. Admitting for the pressure the formula P = 0.13V2, where all is expressed in kilograms, meters and seconds, we find in this actual case a vertical force of 118 kg., while the whole weight, machine and aeronaut, was really 100 kg.
Now 100 kg. is 220 lbs., and if the reported glide is correct we have with Lilienthal's formula:
while by the Duchemin formula:
Now this seems to corroborate Herring's statement that he descended in calm air at a trajectory of 10° with a speed of 22 miles an hour, and the puzzle remains why all the glides approximate an angle of 10° irrespective of the wind velocity. Can you throw any light on this?