DISTRIBUTION OF WEIGHT - AEROPLANE or AIRPLANE.
Figure 6 illustrates the relative proportions of the total weight taken up by the various sections into which it is convenient to divide the component parts. The figures apply to an average com mercial aeroplane of moderate size. The distribution of weight varies from one machine to another.
The "structure" includes a great deal which is not part of the essential structure of the aeroplane. A change in the specified strength affects about 6o to 7o% only of the nominal structure weight.
weight of the power plant depends mainly on the desired maximum speed of the aeroplane, and on the weight of the engine and its equipment per brake horse-power. There is generally a reduction in this item if an air-cooled engine is used instead of a water-cooled engine.
The weight of fuel and tanks is dependent on the speed and duration of flight specified, and on the standard of fuel economy reached by the engine. The latter factor varies little for modern engines, both air- and water-cooled engines now giving one brake horse-power hour for about 1- lb. of petrol.
The crew and equipment tend naturally to absorb a progressively smaller proportion of the total weight as the size of the aeroplane increases.
The paying load, being the balance after the above items have been accounted for, is from 13 to 20% of the gross loaded weight of the aeroplane. For slower aeroplanes it may reach a much higher percentage. Compared with other means of transport this is a high proportion. Both a passenger railway train and an ocean liner carry a much smaller percentage of paying load. That these means of transport are able to offer lower fares than aircraft is due partly to the higher running costs of an aeroplane per passenger mile, but mainly to the fact that the volume of passenger traffic is as yet small compared to that of other common carriers.
The essential need for economy of weight in aircraft is due, not only to the fact that but one-fifth of its weight is available for paying load, but to the higher cost of carrying this load by air, as compared with other ways. A saving in the weight of some part of the aeroplane of r% of the whole allows a reduction of the cost of transport by about 5%.
