An Aeroplane Equipped with the Light Boat Hull Shown, in this Figure is Known as a "Flying Boat". It Differs from the "Seaplane," as the Floats or Pontoons of the Latter Do Not Enclose the Passenger and Pilot.
CHAPTER XX. AERONAUTICAL MOTORS.
General Notes. It is assumed that the reader understands the principles of the automobile motor and its accessories, for a minute description of gas-engine principles does not fall within the scope of this book. If more information is desired on this subject, the reader is referred to the author’s "Practical Handbook of Gas, Oil and Steam Engines." Only those features peculiar to aeronautic motors will be discussed in this chapter.
Aeronautic Requirements. The principal requirements of an aeronautic motor are light weight, low oil and fuel consumption, reliability and compactness. The outline as viewed from the shaft end is also very important, for the motor must be mounted in a narrow streamline body. The compression pressures are much higher than those employed on auto motors, and the speed is generally lower. With one or two exceptions the four-stroke cycle has been universally adopted.
Aeronautic service is a severe test for the motor. From the start to the finish of a flight, the aeroplane motor is on a steady grind, loaded at least to 75 per cent of its rated power. The foundations are light and yielding and the air density varies rapidly with changes in the altitude. As the fuel and oil require an expenditure of power for their support, the fuel consumption becomes of great importance, especially in long flights. Because of the heavy normal load the lubricating system must be as nearly perfect as it is possible to make it.
A motor car runs normally at from 10 to 25 per cent of its rated horsepower, while the aero motor may develop as high as 75 per cent to 100 per cent for hours at a time. A car engine of 672 cubic inches displacement is rated at 65 horsepower, while the same size aero engine has a rating of 154. On the basis of normal output, this ratio is about 7 to 1, and taking the weight of the aero motor as one-half that of the auto type, the true output ratio becomes 14 to 1. Up to the time of a complete overhaul (50 hours), and at 100 miles per hour, the average distance traveled by the aero motor is 5000 miles. The equivalent motor car mileage is 25,000, and the duration is about 1000 hours. This suggests the necessity for improved materials of construction. Even on the present aeronautic motors the fiber stress in the crank-shaft ranges from 120,000 to 140,000 pounds per square inch against the 80,000-pound stress used in auto shafts. The crank case of an aeronautic motor must be particularly rigid to withstand the stresses due to the light mounting, and this demands a higher grade metal than that ordinarily used with automobiles. Unlike the car engine, quality comes first and price is a secondary consideration.
Cooling Systems. Both the air and water cooling system is used, the former for light fast aeroplanes such as speed scouts, and the latter for the larger and more heavily powered machines. Even in some types of speed scouts the air-cooled motor has been displaced by the water-cooled, owing to the fact that the air-cooler cannot be built satisfactorily for outputs much greater than 110 horsepower. By increasing the revolutions of the stationary water-cooled type an increase in power may be had with the same cylinders, but in the case of the rotary air-cooled type the speed is limited by the centrifugal forces acting on the cylinders.
A 6-Cylinder Hall-Scott Motor Installed in a Martin Biplane.