These facts taken into consideration with my other experiments with large aeroplanes, demonstrated to my mind that it would not be a very easy matter to make a large and efficient aeroplane. If I obtained the necessary rigidity by making it of boards, it would be vastly too heavy for the purpose, while if I obtained the necessary lightness by making the framework of steel and covering it with a silk or cotton fabric in the usual way, the distortion would be so great that it would require altogether too much power to propel it through the air. I therefore decided on making a completely new form of aeroplane. I constructed a large steel framework arranged in such a manner that the fore and aft edges consisted of tightly drawn steel wires. This framework was provided with a number of light wooden longitudinal trusses, similar to those shown in [Fig. 76]. The bottom side was then covered with balloon fabric secured at the edges, and also by two longitudinal lines of lacing through the centre. It was stretched very tightly and slightly varnished, but not sufficiently to make it absolutely air-tight. The top of this framework was covered with the same kind of material, but varnished so as to make it absolutely airtight. The top and bottom were then laced together forming very sharp fore and aft edges, and the top side was firmly secured to the light wooden trusses before referred to. Upon running this aeroplane, I found that a certain quantity of air passed through the lower side and set up a pressure between the upper and lower coverings. The imprisoned air pressed the top covering upward, forming longitudinal corrugations which did not offer any perceptible resistance to the air, whereas the bottom fabric, having practically the same pressure on both sides, was not distorted in the least. This aeroplane was found to be nearly as efficient as it would have been had it been carved out of a solid piece of wood. It will be seen by the illustration that this large or main aeroplane is practically octagonal in shape, its greatest width being 50 feet, and the total area 1,500 square feet.
EXPERIMENTS WITH A LARGE MACHINE.
Upon running my large machine over the track ([Fig. 77]) with only the main aeroplane in position, I found that a lifting effect of 3,000 to 4,000 lbs. could be obtained with a speed of 37 to 42 miles an hour. It was not always an easy matter to ascertain exactly what the lifting effect was at a given speed on account of the wind that was generally blowing. Early in my experiments, I found if I ran my machine fast enough to produce a lifting effect within 1,000 lbs. of the total weight of the machine, that it was almost sure to leave the rails if the least wind was blowing. It was, therefore, necessary for me to devise some means of keeping the machine on the track. The first plan tried was to attach some very heavy cast-iron wheels weighing with their axle-trees and connections about 11⁄2 tons. These were constructed in such a manner that the light flanged wheels supporting the machine on the steel rails could be lifted 6 inches above the track, leaving the heavy wheels still on the rails for guiding the machine. This arrangement was tried on several occasions, the machine being run fast enough to lift the forward end off the track. However, I found considerable difficulty in starting and stopping quickly on account of the great weight, and the amount of energy necessary to set such heavy wheels spinning at a high velocity. The last experiment with these wheels was made when a head wind was blowing at the rate of about 10 miles an hour. It was rather unsteady, and when the machine was running at its greatest velocity, a sudden gust lifted not only the front end, but also the heavy front wheels completely off the track, and the machine falling on soft ground was soon blown over by the wind.
I then provided a safety track of 3 × 9 Georgia pine placed about 2 feet above the steel rails, the wooden track being 30 feet gauge and the steel rails 9 feet gauge ([Fig. 77]). The machine was next furnished with four extra wheels placed on strong outriggers and adjusted in such a manner that when it had been lifted 1 inch clear of the steel rails, these extra wheels would engage the upper wooden track.[8]
[8] Springs were interposed between the machine and the axle-trees. The travel of these springs was about 4 inches; therefore, when the machine was standing still, the wheels on the outriggers were about 5 inches below the upper track.
Fig. 77.—View of the track used in my experiments. The machine was run along the steel railway which was 9 feet gauge, and was prevented from rising by the wooden track which was 35 feet gauge.
