Elasticity of the Air - Sunday, April 5, 1903
The History: The Wrights' friend and colleague Dr. George Spratt writes Wilbur today in response to Wilbur's letter of March 28th. [You can click on the thumbnails below for a larger version and read the letter.]
The Letter George Spratt Wrote Wilbur Wright
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Spratt clarifies to Wilbur that in his last letter he was referring to different airfoils [wing shapes] operating under different conditions. Spratt explains to Wilbur, "A surface moving at (a) speed, the same surface at (2a) speed and a similar surface X 2 (twice the area) at (a) speed are 3 dissimilar [different] propositions. .... I do not think you allow the elasticity of the air to have an important enough place in your theories."
Spratt shares with Wilbur that he has been "anxiously awaiting" a letter from Octave Chanute. He asks Wilbur, "Has he been away for the winter?" [In fact, Chanute has been traveling.]
George Spratt of Coatsville, Pennsylvania had served as one of Octave Chanute's assistants at the Kill Devil Hills camp during the 1901 testing. The Wrights enjoyed Spratt's company, appreciated his hard work at camp and have become friends. On October 17, 1902 Spratt took photographs of the Wrights gliding including this one.
One of the pictures George Spratt took of
Orville Wright (left) and Dan
Tate (right) launching the Wright 1902 glider
off the east slope of the Big Hill, Kill Devil Hills, North
Carolina on October 17 1902.
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What does elasticity mean? Let's use the example of a rubber band. You can stretch the rubber band to make it longer. When you stop stretching the rubber band it returns to its original shape. When referring to on object like a rubber band, the word "elasticity" means the material is able to recover its original size or shape after deforming it (changing its shape).
However, the word elasticity applied to a gas (like air) means that the gas can expand or compress (change size and volume) but not necessarily to its original size and shape. Even though Spratt uses an example of a piece of rubber, when he uses the word "elasticity" he is referring to this last definition.
Perhaps you have heard of the word "aeroelasticity". The word aeroelasticity is derived from the prefix "aero" meaning atmosphere or gas and the word "elasticity". Aeroelasticity is the study of the interactions between elastic objects in a gas stream and the resulting aerodynamic forces. Aeroelasticity affects materials in nature like trees and grass swaying in the wind, creating sound in a musical instrument as well as the design of flexible components of air and spacecraft.
George Spratt is on the "right track". He is correct; two identical airfoils will generate different lift and drift [drag] forces as the air's speed changes.
While the Wrights tested various shapes and collected drift [drag] and lift data, they did not vary the speed of their wind tunnel. During their 1901 wind tunnel tests the Wrights were more interested in comparing the lift and drag of different shapes, to determine how one shape was better than another.
Letter courtesy Wright State University.
