The Wrights examined all the terms in the lift and drag equations. Some values—weight of the craft, wind speed, and wing surface area—could be directly measured, so the Wrights were confident of their accuracy. But the coefficients of lift and drag and Smeaton’s coefficient were drawn from the work of others. The brothers focused on these as the possible source of their gliders’ poor lift performance.

They recorded the angle of the wing to the airflow at which the air pressure on the wing shape balanced the air pressure on the plate. This is called the angle of attack.
Then they calculated what Lilienthal’s data indicated the angle should have been. The model wing required more than three times the angle of attack than Lilienthal’s data predicted.
 
The Wrights concluded that either Lilienthal’s data or Smeaton’s coefficient—or both—were in error.