In the year 1810, a steam engine weighed something over a ton to
the horse-power. This was reduced to about 200 pounds in 1880. The
steam-driven dirigible balloon of Giffard, in 1852, carried a complete
power plant weighing a little over 100 pounds per horse-power; about
the weight of a modern locomotive. The unsuccessful Maxim flying
machine of 1894 brought this weight down to less than 20 pounds. The
gasoline engine on the original Wright machines weighed about 5 pounds
to the horse-power; those on some recent French machines not far from 2
pounds.
Pig iron is worth perhaps a cent a pound. An ordinary steam or gas
engine may cost eight cents a pound; a steam turbine, perhaps forty
cents. A high grade automobile or a piano may sell for a dollar a
pound; the Gnome aeroplane motor is priced at about twenty dollars a
pound. This is considerably more than the price of silver. The motor
and accessories account for from two-thirds to nine-tenths of the total
cost of an aeroplane.
A man weighing 150 pounds can develop at the outside about one-eighth
of a horse-power. It would require 1200 pounds of man to exert one
horse-power. Considered as an engine, then, a man is (weight for
weight) only one six-hundredth as effective as a Gnome motor. In the
original Wright aeroplane, a weight of half a ton was sustained at the
expenditure of about twenty-five horse-power. The motor weight was
about one-eighth of the total weight. If traction had been produced by
man-power, 30,000 pounds of man would have been necessary: thirty times
the whole weight supported.
[Illustration: THE GNOME MOTOR
(Aeromotion Company of America)]
Under the most favorable conditions, to support his own weight of
150 pounds (at very high gliding velocity and a slight angle of
inclination, disregarding the weight of sails necessary), a man would
need to have the strength of about fifteen men. No such thing as an
aerial bicycle, therefore, appears possible. The man can not emulate
the bird.
[Illustration: SCREW PROPELLER (American Propeller Company)]
The power plant of an air craft includes motor, water and water tank,
radiator and piping, shaft and bearings, propeller, controlling wheels
and levers, carbureter, fuel, lubricating oil and tanks therefor. Some
of the weight may eventually be eliminated by employing a two-cycle
motor (which gives more power for its size) or by using rotary
air-cooled cylinders. Propellers are made light by employing wood or
skeleton construction. One eight-foot screw of white oak and spruce,
weighing from twelve to sixteen pounds, is claimed to give over 400
pounds of propelling force at a thousand turns per minute.
[Illustration: ONE OF THE MOTORS OF THE ZEPPELIN]
The cut shows the action of the so-called "four-cycle" motor. Four
strokes are required to produce an impulse on the piston and return
the parts to their original positions. On the first, or suction
stroke, the combustible mixture is drawn into the cylinder, the inlet
valve being open and the outlet valve closed. On the second stroke,
both valves are closed and the mixture is highly compressed. At about
the end of this stroke, a spark ignites the charge, a still greater
pressure is produced in consequence, and the energy of the gas now
forces the piston outward on its third or "working" stroke, the valves
remaining closed. Finally, the outlet valve is opened and a fourth
stroke sweeps the burnt gas out of the cylinder.
[Illustration: ACTION OF THE FOUR-CYCLE ENGINE]
In the "two-cycle" engine, the piston first moves to the left,
compressing a charge already present in the cylinder at _F_, and
meanwhile drawing a fresh supply through the valve _A_ and passages
_C_ to the space _D_. On the return stroke, the exploded gas in _F_
expands, doing its work, while that in _D_ is slightly compressed, the
valve _A_ being now closed. When the piston, moving toward the right,
opens the passage _E_, the burnt gas rushes out. A little later, when
the passage _I_ is exposed, the fresh compressed gas in _D_ rushes
through _C_, _B_, and _I_ to _F_. The operation may now be repeated.
Only two strokes have been necessary. The cylinder develops power twice
as rapidly as before: but at the cost of some waste of gas, since the
inlet (_I_) and outlet (_E_) passages are for a brief interval _both
open at once_: a condition not altogether remedied by the use of a
deflector at _G_. A two-cycle cylinder should give nearly twice the
power of a four-cycle cylinder of the same size, and the two-cycle
engine should weigh less, per horse-power; but it requires from 10 to
30% more fuel, and fuel also counts in the total weight.
[Illustration: ACTION OF TWO-CYCLE ENGINE]
The high temperatures in the cylinder would soon make the cast-iron
walls red-hot, unless the latter where artificially cooled. The
usual method of cooling is to make the walls hollow and circulate
water through them. This involves a pump, a quantity of water, and a
"radiator" (cooling machine) so that the water can be used over and
over again. To cool by air blowing over the surface of the cylinder is
relatively ineffective: but has been made possible in automobiles by
building fins on the cylinders so as to increase the amount of cooling
surface. When the motors are worked at high capacity, or when two-cycle
motors are used, the heat is generated so rapidly that this method of
cooling is regarded as inapplicable. By rapidly rotating the cylinders
themselves through the air, as in motors like the Gnome, air cooling is
made sufficiently adequate, but the expenditure of power in producing
this rotation has perhaps not been sufficiently regarded.
[Illustration: MOTOR AND PROPELLER
(Detroit Aeronautic Construction Co.)]
Possible progress in weight economy is destined to be limited by the
necessity for reserve motor equipment.
The engine used is usually the four-cycle, single-acting, four-cylinder
gasoline motor of the automobile, designed for great lightness. The
power from each cylinder of such a motor is approximately that obtained
by dividing the square of the diameter in inches by the figure 2-1/2.
Thus a five-inch cylinder should give ten horse-power--at normal piston
speed. On account of friction losses and the wastefulness of a screw
propeller, not more than half this power is actually available for
propulsion.
The whole power plant of the _Clément-Bayard_ weighed about eleven
pounds to the horse-power. This balloon was 184 feet long and 35
feet in maximum diameter, displacing about 100,000 cubic feet. It
carried six passengers, about seventy gallons of fuel, four gallons of
lubricating oil, fifteen gallons of water, 600 pounds of ballast, and
130 pounds of ropes. The motor developed 100 horse-power at a thousand
revolutions per minute. About eight gallons of fuel and one gallon of
oil were consumed per hour when running at the full independent speed
of thirty-seven miles per hour.
The Wellman balloon _America_ is said to have consumed half a ton of
gasoline per twenty-four hours: an eight days' supply was carried. The
gas leakage in this balloon was estimated to have been equivalent to a
loss of 500 pounds of lifting power per day.
The largest of dirigibles, the _Zeppelin_, had two motors of 170
horse-power each. It made, in 1909, a trip of over 800 miles in
thirty-eight hours.
The engine of the original Voisin cellular biplanes was an
eight-cylinder Antoinette of fifty horse-power, set near the rear
edge of the lower of the main planes. The Wright motors are placed near
the front edge. A twenty-five horse-power motor at 1400 revolutions
propelled the Fort Myer machine, which was built to carry two
passengers, with fuel for a 125 mile flight: the total weight of the
whole flying apparatus being about half a ton.
[Illustration: TWO-CYLINDER OPPOSED ENGINE.
(From _Aircraft_)]
[Illustration: FOUR-CYLINDER VERTICAL ENGINE
(The Dean Manufacturing Co.)]
The eight-cylinder Antoinette motor on a Farman biplane, weighing 175
pounds, developed thirty-eight horse-power at 1050 revolutions. The
total weight of the machine was nearly 1200 pounds, and its speed
twenty-eight miles per hour.
The eight-cylinder Curtiss motor on the _June Bug_ was air cooled. This
aeroplane weighed 650 pounds and made thirty-nine miles per hour, the
engine developing twenty-five horse-power at 1200 turns.