University of Virginia Library

Some Data on Bird Power.

The most difficult case is purposely selected. For if we assume that the bird has previously acquired an initial minimum speed of seventeen miles an hour (24.93 feet per second, nearly the lowest measured), and that the air was rising vertically six miles an hour (8.80 feet per second), then we have as the trend of the "relative wind" encountered:

6/17 = 0.353, or the tangent of 19° 26'.

which brings the case into the category of rising wind effects. But the bird was observed to have a negative angle to the horizon of about 3°, as near as could be guessed, so that his angle of incidence to the "relative wind" was reduced to 16° 26'.

The relative speed of his soaring was therefore:

Velocity = [square root] 172 + 62 = 18.03 miles per hour.

At this speed, using the Langley co-efficient recently practically confirmed by the accurate experiments of Mr. Eiffel, the air pressure would be:

18.032 X 0.00327 = 1.063 pounds per square foot.

If we apply Lilienthal's co-efficients for an angle of 6° 26', we have for the force in action:

Normal: 4.57 X 1.063 X 0.912 = 4.42 pounds.

Tangential: 4.57 X 1.063 X 0.074 = -0.359 pounds, which latter, being negative, is a propelling force.