Part 5

What is called "route stability" describes the condition of straight flight. The balloon must point directly in its (independent) course. This involves the use of a steering rudder, and, in addition, of fixed vertical planes, which, on the principle of the vertical partitions of Voisin, probably give some automatic steadiness to the course. To avoid the difficulty or impossibility of holding the head up to the wind at high speeds, an _empennage_ or feathering tail is a feature of all present balloons. The empennage of the _Patrie_ (page 77) consisted of pairs of vertical and horizontal planes at the extreme stern. In the _France_, thirty-two feet in maximum diameter and nearly 200 feet long, empennage planes aggregating about 400 square feet were placed somewhat forward of the stern. In the _Clément-Bayard_, the empennage consisted of cylindro-conical ballonets projecting aft from the stern. A rather peculiar grouping of such ballonets was used about the prolonged stern of the _Ville de Paris_.

Rudders and Planes

The dirigible has thus several air-resisting or gliding surfaces. The approximately "horizontal" (actually somewhat inclined) planes permit of considerable ascent and descent by the expenditure of power rather than gas, and thus somewhat influence the problem of altitude control. Each of the four sets of horizontal rudder planes on the _Zeppelin_, for example, has, at thirty-five miles per hour, with an inclination equal to one-sixth a right angle, a lifting power of nearly a ton; about equal to that of all of the gas in one of the sixteen compartments.

Movable rudders may be either hand or motor-operated. The double vertical steering rudder of the _Ville de Paris_ had an area of 150 square feet. The horizontally pivoted rudders for vertical direction had an area of 130 square feet.

Arrangement and Accessories

The motor in the _Ville de Paris_ was at the front of the car, the operator behind it. This car had the excessive weight of nearly 700 pounds. The _Patrie_ employed a non-combustible shield over the motor, for the protection of the envelope: its steering wheel was in front and the motor about in the middle of the car. The gasoline tank was under the car, compressed air being used to force the fuel up to the motor, which discharged its exhaust downward at the rear through a spark arrester. Motors have battery and magneto ignition and decompression cocks, and are often carried on a spring-supported chassis. The interesting _Parseval_ propeller has four cloth blades which hang limp when not revolving. When the motor is running, these blades, which are weighted with lead at the proper points, assume the desired form.

Balloons usually carry guide ropes at head and stern, the aggregate weight of which may easily exceed a hundred pounds. In descending, the bow rope is first made fast, and the airship then stands with its head to the wind, to be hauled in by the stern rope. For the large French military balloons, this requires a force of about thirty men. The _Zeppelin_ descends in water, being lowered until the cars float, when it is docked like a ship (see page 84). Landing skids are sometimes used, as with aeroplanes.

The balloon must have escape valves in the main envelope and ballonets. In addition it has a "rip-strip" at the bottom by which a large cut can be made and the gas quickly vented for the purpose of an emergency descent. Common equipment includes a siren, megaphone, anchor pins, fire extinguisher, acetylene search light, telephotographic apparatus, registering and indicating gages and other instruments, anemometer, possibly carrier pigeons; besides fuel, oil and water for the motor, and the necessary supplies for the crew. The glycerine floated compass of Moisant must now also be included if we are to contemplate genuine navigation without constant recourse to landmarks.

Amateur Dirigibles

The French Zodiac types of "aerial runabout" displace 700 cubic meters, carrying one passenger with coal gas or two passengers with a mixture of coal gas and hydrogen. The motor is four-cylinder, sixteen horse-power, water-cooled. The stern screw, of seven feet diameter, makes 600 turns per minute, giving an independent speed of nineteen miles per hour. The machine can remain aloft three hours with 165 pounds of supplies. It costs $5000. Hydrogen costs not far from a cent per cubic foot (twenty cents per cubic meter) so that the question of gas leakage may be at least as important as the tire question with automobiles.

The Fort Omaha Plant

The Signal Corps post at Fort Omaha has a plant comprising a steel balloon house of size sufficient to house one of the largest dirigibles built, an electrolytic plant for generating hydrogen gas, having a capacity of 3000 cubic feet per hour, a 50,000 cubic foot gas storage tank, and the compressing and carrying equipment involved in preparing gas for shipment at high pressure in steel cylinders.

Balloon Progress

The first aerial buoy of Montgolfier brothers, in 1783, led to the suggestion of Meussier that two envelopes be used; the inner of an impervious material to prevent gas leakage, and the outer for strength. There was perhaps a foreshadowing of the Zeppelin idea. Captive and drifting balloons were used during the wars of the French Revolution: they became a part of standard equipment in our own War of Secession and in the Franco-Prussian conflict. The years 1906 to 1908 recorded rapid progress in the development of the dirigible: the record-breaking _Zeppelin_ trip was in 1909 and Wellman's _America_ exploit in October, 1910. Unfortunately, dirigibles have had a bad record for stanchness: the _Patrie_, _République_, _Zeppelin_ (_I_ and _II_), _Deutschland_, _Clément-Bayard_--all have gone to that bourne whence no balloon returns.

It is gratifying to record that Count Zeppelin's latest machine, the _Deutschland II_, is now in operation. During the present month (April, 1911), flights have been made covering 90 miles and upward at speeds exceeding 20 miles per hour with the wind unfavorable. This balloon is intended for use as a passenger excursion vehicle during the coming summer, under contract with the municipality of Düsseldorf.

At the present moment, Neale, in England, is reported to be building a dirigible for a speed of a hundred miles per hour. The Siemens-Schuckart non-rigid machine, nearly 400 feet long and of 500 horse-power, is being tried out at Berlin: it is said to carry fifty passengers.[A] Fabrice, of Munich, is experimenting with the _Inchard_, with a view to crossing the Atlantic at an early date. Mr. Vaniman, partner of Wellman on the _America_ expedition, is planning a new dirigible which it is proposed to fly across the ocean before July 4. The engine, according to press reports, will develop 200 horse-power, and the envelope will be more elongated than that of the _America_. And meanwhile a Chicago despatch describes a projected fifty-passenger machine, to have a gross lifting power of twenty-five tons!

Germany has a slight lead in number of dirigible balloons--sixteen in commission and ten building. France follows closely with fourteen active and eleven authorized. This accounts for two-thirds of all the dirigible balloons in the world. Great Britain, Italy, and Russia rank in the order named. The United States has one balloon of the smallest size. Spain has, or had, one dirigible. As to aeroplanes, however, the United States and England rank equally, having each about one-fourth as many machines as France (which seems, therefore, to maintain a "four-power standard"). Germany, Russia, and Italy follow, in order, the United States. These figures include all machines, whether privately or nationally owned. Until lately, our own government operated but one aeroplane. A recent appropriation by Congress of $125,000 has led to arrangements for the purchase of a few additional biplanes of the Wright and Curtiss types; and a training school for army officers has been regularly conducted at San Diego, Cal., during the past winter. The Curtiss machine to be purchased is said to carry 700 pounds of dead weight with a sail area of 500 square feet. It is completely demountable and equipped with pontoons.

THE QUESTION OF POWER

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.

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.

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.

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.

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.

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.

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.

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.

Resistance of Aeroplanes

The chart on page 24 (see also the diagram of page 23) shows that the lifting power of an aeroplane increases as the angle of inclination increases, up to a certain limit. The resistance to propulsion also increases, however: and the ratio of lifting power to resistance is greatest at a very small angle--about five or six degrees. Since the motor power and weight are ruling factors in design, it is important to fly at about this angle. The supporting force is then about two pounds, and the resistance about three-tenths of a pound, per square foot of sail area, if the velocity is that assumed in plotting the chart: namely, about fifty-five miles per hour.

But the resistance _R_ indicated on pages 23 and 24 is not the only resistance to propulsion. In addition, we have the frictional resistance of the air sliding along the sail surface. The amount of this resistance is independent of the angle of inclination: it depends directly upon the area of the planes, and in an indirect way on their dimensions in the direction of movement. It also varies nearly with the square of the velocity. At any velocity, then, the addition of this frictional resistance, which does not depend on the angle of inclination, modifies our views as to the desirable angle: and the total resistance reaches a minimum (in proportion to the weight supported) when the angle is about three degrees and the velocity about fifty miles per hour.

This is not quite the best condition, however. The skin friction does not vary exactly with the square of the velocity: and when the true law of variation is taken into account, it is found that the _horse-power_ is a minimum at an angle of about five degrees and a speed of about forty miles per hour. The weight supported per horse-power may then be theoretically nearly a hundred pounds: and the frictional resistance is about one-third the direct pressure resistance. This must be regarded as the approximate condition of best effectiveness: not the exact condition, because in arriving at this result we have regarded the sails as square flat planes whereas in reality they are arched and of rectangular form.

At the most effective condition, the resistance to propulsion is only about one-tenth the weight supported. Evidently the air is helping the motor.

Resistance of Dirigibles

If the bow of a balloon were cut off square, its head end resistance would be that given by the rule already cited (page 19): one three-hundredth pound per square foot, multiplied by the square of the velocity. But by pointing the bow an enormous reduction of this pressure is possible. If the head end is a hemisphere (as in the English military dirigible), the reduction is about one-third. If it is a sharp cone, the reduction may be as much as four-fifths. Unless the stern is also tapered, however, there will be a considerable eddy resistance at that point.

If head end resistance were the only consideration, then for a balloon of given diameter and end shape it would be independent of the length and capacity. The longer the balloon, the better. Again, since the volume of any solid body increases more rapidly than its surface (as the linear dimensions are increased), large balloons would have a distinct advantage over small ones. The smallest dirigible ever built was that of Santos-Dumont, of about 5000 cubic feet.

Large balloons, however, are structurally weak: and more is lost by the extra bracing necessary than is gained by reduction of head end resistance. It is probable that the Zeppelin represents the limit of progress in this direction; and even in that balloon, if it had not been that the adoption of a rigid type necessitated great structural strength, it is doubtful if as great a length would have been fixed upon, in proportion to the diameter.

The frictional resistance of the air gliding along the surface of the envelope, moreover, invalidates any too arbitrary conclusions. This, as in the aeroplane, varies nearly as the square of the velocity, and is usually considerably greater than the direct head end resistance. Should the steering gear break, however, and the wind strike the _side_ of the balloon, the pressure of the wind against this greatly increased area would absolutely deprive it of dirigibility.

A stationary, drifting, or "sailing" balloon may as well have the spherical as well as any other shape: it makes the wind a friend instead of a foe and requires nothing in the way of control other than regulation of altitude.

Independent Speed and Time Table

The air pressure, direct and frictional resistances, and power depend upon the _relative_ velocity of flying machine and air. It is this relative velocity, not the velocity of the balloon as compared with a point on the earth's surface, that marks the limit of progression. Hence the speed of the wind is an overwhelming factor to be reckoned with in developing an aerial time table. If we wish to travel east at an effective speed of thirty miles per hour, while the wind is blowing due west at a speed of ten miles, our machine must have an independent speed of forty miles. On the other hand, if we wish to travel west, an independent speed of twenty miles per hour will answer.

Again, if the wind is blowing north at thirty miles per hour, and the minimum (relative) velocity at which an aeroplane will sustain its load is forty miles per hour, we cannot progress northward any more slowly than at seventy miles' speed. And we have this peculiar condition of things: suppose the wind to be blowing north at fifty miles per hour. The aeroplane designed for a forty mile speed may then face this wind and sustain itself while actually moving backward at an absolute speed (as seen from the earth) of ten miles per hour.