xii. 3 LANGLEY MEMOIR ON MECHANICAL FLIGHT 



245 



The reason for building the engine with five cylinders instead of some other 

 number, and for arranging them radially on a central drum using only one .•rank 

 pin may not appear quite obvious. The advantages gained by such a construe 

 tion, however, are very great, and may be briefly summed up as follows: 



First, since in a gas engine of the four-cycle type there is only one explo 

 sion in each cylinder every two revolutions, and the crank shaft and crank pin 

 therefore are loaded only one-quarter of the time for each cylinder, it is obvious 

 that by having four cylinders arranged radially around a central drum the 

 load on the bearings of a single crank shaft and crank pin may be kept very 

 uniform. However, with four cylinders thus arranged it is impossible to have 

 the cylinders explode and exert their effort on the crank at uniform intervals 

 in the cycle, it being necessary to have the cylinders explode in the order of 

 1, 3, 4, 2, 1, etc., thus giving intervals between explosions of 180 degrees, 90 de- 

 grees, 180 degrees, 270 degrees, etc., or to have them explode in the order of 1, 

 3, 2, 4, 1, etc., thus giving intervals of 180 degrees, 270 degrees, 180 degrees, 90 

 degrees, etc. On the other hand, with any odd number of cylinders the explo- 

 sions will occur at equal intervals in the cycle. With three cylinders they will 

 explode in the order of 1, 3, 2, 1, etc., or at equal intervals of 240 degree-. 

 while with five cylinders they will explode in the order of 1, 3, 5, 2, 4, 1, etc., 

 or at equal intervals of 144 degrees. It is therefore seen that there is a great 

 advantage in smoothness of operation and uniformity of torque of the engine 

 through having an odd number of cylinders instead of an even number. 



Second, it is readily apparent that the greater the number of cylinders, 

 provided the number is an odd one, the more uniform the torque will be, and 

 it would seem at first that seven cylinders would therefore be better than five, 

 since the uniform intervals between explosions with seven cylinders would In- 

 only 103 degrees (approximately). The advantage gained, however, through 

 seven cylinders instead of five is largely, if not completely, counterbalanced by 

 the added number of parts and the difficulty of providing sufficient circumfer- 

 encial width for the connecting-rod shoes on the crank-pin bearing, even with 

 the improved construction of this bearing already described. There is consid- 

 erable fluctuation of the torque in each revolution of the engine with five cylin- 

 ders, but this fluctuation of torque is more easily smoothed out by the use of 

 very light fly wheels than by increasing the number of cylinders, and thus 

 adding to the complication of the engine. 



Third, the strongest point in favor of the radially arranged cylinders is the 

 reduction in weight and complication which it permits. The crank shaft is re- 

 duced to the very minimum, there being only one crank pin with two main 

 bearings which can, without any difficulty whatever, be kept absolutely in line 

 with each other and thus prevent binding and loss of power. Again, the use 

 of a single-throw crank not only reduces the cost and weight of the crank it- 



