Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/294

§ 186 units, for the pressures in kilos per square metre can be obtained with a sufficient degree of approximation by multiplying by five the figures given in Tables X. and XII.

§ 187. Comparison with Actual Measurements.—The portent of the preceding sections may be illustrated by a few examples from Nature.

The herring gull, according to the system of measurement adopted by the author and subsequently explained, carries its load at the rate of about 1.3 to 1.4 pounds per square foot; its $$n$$ value is 7. Referring to Table X. ($$\xi =$$ .02) we find this load corresponds to 38 feet per second or about 26 miles per hour, which is probably a fair approximation to its actual speed.

The albatros carries about 3 pounds per square foot, and has an $$n$$ value of 12; referring to the Table we find the corresponding velocity to be about 50 feet per second or slightly over 84 miles per hour, which again is probably not far from the truth.

If now we take the case of a dragon-fly: an example weighed and measured by the author (Fig. 115) gave a result, from a planimeter measurement of the whole wing surface, of .68 grammes on 3.5 square inches, which is .062 pounds per square foot. The “$$n$$” value may fairly be taken as about = 4.

Referring to the Tables and taking $$\xi =$$ .02, we have for pterygoid form the corresponding velocity = 9.1 feet per second, or according to Table XII., considering the wings as planes, the velocity should be from 13 to 15.6 feet per second, according as $$\xi$$ is taken as .02 or .01.

Unfortunately no scientific measurement of the flight of this