[Illustration: Fig. 68.--The De la Grange machine on the ground and about to make a flight.] [Illustration: Fig. 69.--The De la Grange machine in full flight and very near the ground.] In the following calculations, I have assumed that the machine has the higher speed--40 miles per hour. I have been quite unable to obtain any reliable data regarding the angle at which the aeroplanes are set, but it would appear that the angle is about 1 in 10. The total area of the two main aeroplanes is 321·4 square feet. A certain portion of the lower main aeroplane is cut away, but this is compensated for by the forward horizontal rudder placed in the gap thus formed. The two rear aeroplanes forming the tail of the machine have an area of 128·57 square feet. The area of all the aeroplanes is, therefore, 450 square feet. As the weight of the machine is 1,000 lbs., the lift per square foot is 2·2 lbs. Assuming that the angle of the aeroplanes is 1 in 10, the screw thrust would be 100 lbs., providing, however, that the aeroplanes were perfect and no friction of any kind was encountered. Forty miles per hour is at the rate of 3,520 feet in a minute of time, therefore, (3,520 × 100)/33,000 = 10·66 H.P. If we allow another 10 H.P. for atmospheric resistance due to the motor, the man, and the framework of the machine, it would require 20·66 H.P. to propel the machine through the air at the rate of 40 miles per hour. If the motor actually develops 50 H.P., 29 H.P. will be consumed in screw slip and overcoming the resistance due to the imperfect shape of the screw. The blades of the De la Grange screw propeller are extremely small, and the waste of energy is, therefore, correspondingly great--their projected area being only 1·6 square feet for both blades. Allowing 200 lbs. for screw thrust, we have the following: 200/1·60 = 125 lbs. pressure per square foot on the blades. If we multiply the pitch of the screw in feet by the number of revolutions per minute, we find that if it were travelling in a solid nut it would advance over 70 miles an hour. By the Eiffel tower formula P = 0·003 V², a wind blowing at a velocity of 70 miles per hour produces a pressure of 14·7 lbs. per square foot on a normal plane; therefore, assuming that the projected area of the screw blades is 1·6, we have 1·6 × 14·7 = 23·52 lbs., which is only one-fifth part of what the pressure really is when the screws are making 1,100 turns a minute. It is interesting to note that the ends of the screw blades travel at a velocity of 414 feet per second, which is about one-half the velocity of a cannon ball fired from an old-fashioned smooth bore. [Illustration: Fig. 70.--Farman’s machine in flight].