THERE ARE FIVE METHODS of altering the level of an airship. The use of swivelling propellers is the most powerful of these, but this method stops the forward movement of the ship. The four other methods of level adjustment are the use of elevators, the movement of weights, the discharge of gas or ballast and the adjustment of air in the balloon. Part of the elevator control wheel is seen to the right in this illustration of the control car of the British airship, R28, built in 1918.
THE airship pilot claims to be more of a scientist than the pilot of heavier-than-air craft and his period of training is longer. A candidate must qualify for a free balloon pilot’s certificate before he can become a qualified airship pilot.
The airship, unlike the aeroplane, has two lifts, aerostatic and aerodynamic. Aerostatic lift is that obtained from the volume of gas with which the ship is inflated; aerodynamic lift is that obtained by the action of the air pressure upon the elevators as the ship is forced forward by her engine power. As aerostatic, rather than aerodynamic, lift is the principal lift of an airship, it is essential that the airship pilot should have a thorough grasp of this subject.
Aerostatics includes a knowledge of the behaviour of gases under varying degrees of pressure and temperature. A vessel containing any substance will take the place of or displace a certain fixed quantity of air. For example, if a ball is put into what is called an empty bucket, but what is in reality a bucket full of air, there will be less air in the bucket than there was before by exactly the amount which would fill the space taken up by the ball. The amount of air so displaced can be weighed. The weight of air varies, but in general we may consider that, at normal pressure, air weighs 75 lb for 1,000 cubic feet; in other words, air in a room 10 feet square and 10 feet high weighs 75 lb.
Air sufficient to fill a balloon of 1,000 cubic feet will weigh the same amount as air in a room of 1,000 cubic feet. If this balloon is filled with coal gas instead of air, the gas in the balloon will weigh only 37½ lb, as coal gas weighs about half as much as air. If this gas-filled balloon is placed in the outside air, it will displace a quantity of air weighing 75 lb. As the gas itself weighs only 37½ lb it will be lighter by 37½ lb than the air that it displaces.
If the balloon itself weighs more than 37½ lb, the balloon, when full of coal gas, will stay on the ground. If it weighs exactly 37½ lb it will stay in the air at any height at which it is put. If it weighs less than 37½ lb it will rise unless held down.
If the balloon is filled with hydrogen, the hydrogen in the balloon will weigh only about 5 lb, as this gas weighs only about one-fifteenth of the weight of air. Therefore the balloon filled with hydrogen will weigh 70 lb less than if it were filled with air, and it will rise if the balloon itself weighs less than 70 lb.
The weight of a column of the atmosphere forty miles high and one square inch thick is about 15 lb, which is therefore the pressure per square inch of the atmosphere. The weight of air varies directly as the pressure, as all gases expand if the pressure is reduced and contract if the pressure is increased.
THE LANDING CREW waiting for the Goodyear airshipPuritan to reach the ground. This photograph, taken at Chicago on the occasion of the Exposition in 1930, shows how an airship may be manoeuvred into a small space. The Puritan, built in 1928, with an original volume of 96,000 cubic feet, was later increased to 123,000 cubic feet. She has two engines of 293 horse-power, a cruising speed of 53 m.p.h., and carries six passengers with a crew of two. By June 1936 she had flown 407,704 miles and carried 44,398 passengers without an accident.
As an airship rises, the pressure of the air diminishes, and a barometer in the balloon will constantly fall, that is, register a lower pressure. At the same time the gas inside the airship will have less pressure on it from the outside and will therefore try to expand. As the ship continues to rise, this expansion of the gas goes on until the gas bags, or envelope, are full. Then the ship is at or near what is known as “pressure height”.
A further rise of the ship means greater gas expansion and the gas now forces itself through the gas valves, escaping into the atmosphere. Loss of gas means loss of lift, and the pilot must be able to calculate how much he may lose and still be able to land his ship in safety and under control. Again, as the ship rises, her lifting power constantly decreases, because the volume of air she displaces, though remaining the same, is lighter, and the lifting power of the gas is the difference between its own weight and the weight of an equal volume, of the surrounding air.
There are other considerations that effect the lift of an airship. In rain, or when the atmosphere is damp, the amount of moisture taken up by or remaining on the ship can be considerable. The temperature of the gas and of the atmosphere must be taken into account. All gases expand about part for every degree Fahrenheit of rise of temperature and contract similarly if the temperature is lowered. On a cold, clear winter’s day, therefore, the lifting power of an airship will be considerably more than on a hot summer’s day.
The height to which an airship can ascend is limited by the amount of ballast on board to compensate for the gas which, as we have seen, must be discharged as the ship rises.
Measurement of Drift
It will be realized from the foregoing how important a thorough knowledge of aerostatics is to the airship pilot, so that he may at all times be able to calculate the lift of his ship in varying conditions.
Anyone hoping to command an airship must be a good navigator, with a sound knowledge of meteorology. He must be able to do all that the navigator of a seagoing vessel has to do, but the work is intensified. Speeds are greater and the drift imparted to an airship by the wind is nearly always far more than that given to a surface vessel by current or tide.
The force and direction of the wind, which changes with various heights, must be regularly observed. These changes will affect the drift of the airship.
The instrument used for measuring the angle of drift consists of a telescope pointed directly downwards, with a row of parallel black lines scratched on the glass. These lines can be turned until objects on the ground appear to move directly along them. A reading can then be taken of the angle between the longitudinal axis of the ship and the course over the ground.
The speed over the ground is not at present measurable by instruments, but must be calculated. This is not difficult as long as the sun or moon is shining. It is merely a matter of observing the ship’s shadow, which is the same length as the ship herself, and noting the time it takes for objects on the ground-to pass from one end of it to the other. Simple arithmetic does the rest.
MOBILE MOORING MASTS are used to tow large airships into their sheds. Guy wires attached to the after part of the airship are used to guide the tail. This photograph shows the U.S. Navy airship Macon being towed into her shed at Akron in 1933. Smaller airships are today towed into their sheds by a smaller type of mast.
Without a shadow the calculation is rather more complicated. The speed developed by the ship’s engines is ascertained by a log similar to that of an ocean-going vessel. This log consists of a little propeller attached to an airship-shaped body trailed on a cable beneath the ship. The propeller drives an alternating-current dynamo, and the number of frequencies developed is proportional to the speed through the air. When this has been calculated the force of the wind must be added, or subtracted, to obtain the ground speed.
The velocity of the wind is ascertained by measuring the angle of drift, changing the direction by 45° and then making a second measurement. This process is sufficient for learning the exact ground speed. Altitude is measured by an altimeter in the form of an aneroid barometer. It differs from the ordinary weather-glass only in what is written on the scale; instead of having the normal indications such as “Fair”, “Change”, or “Stormy”, it is graduated in feet above sea level. The fault of this instrument is that it records not only the height but also changes in atmospheric pressure. If the ship flies from a high-pressure area into a low-pressure area, the barometer will register a greater height than that at which the ship is flying. In the earlier days of airships this sometimes led to dangerous situations, which were averted only by the pilot’s knowledge of aerostatics and meteorology.
Today the pilot has a scientific instrument which checks his altitude with great accuracy. The instrument is the Behm echo-sounder, and was developed for aviation by the inventor of the deep-sea sounder, Dr. Alexander Behm, of Kiel, Germany. The echo-sounder is based on the speed of sound. An air-pressure whistle blows, automatically setting a stopwatch going; the echo from the ground is then automatically recorded by an indicator on a scale which gives the precise altitude. As it is independent of the human element, it is a most reliable instrument.
There is no prescribed route for an airship flight. Like the captains of the old sailing ships, the airship pilot must always consider how he can reach his destination most quickly. He travels not only by charts and maps, but also by weather charts, and navigates
“meteorologically”. Head winds are to be avoided, and a detour often saves time. In ordinary life on land, as well as at sea, stormy weather is called “bad” or “unfavourable” weather. To the airship pilot the wind is the all-important factor. He does not mind rain or wind, provided he has a following wind, which helps the airship on her way.
The science of meteorology is so highly developed today that the airship pilot knows exactly, and is trained to take advantage of, the laws of wind currents. Often the wind at a higher altitude is not so strong as that at a lower altitude, and its direction may be more favourable. Wrong conclusions about the weather may cause hours of delay which cannot be recovered. A clever exploitation of favourable winds and the avoidance of depressions can save much time.
How the Gyroscope Helps
Air navigation has become an exact science. The gyroscope alone offers the pilot an immensely greater degree of accuracy than he could obtain with the magnetic compass. The gyroscope shows him changes in direction of as little as one-tenth of a degree, and enables the helmsman to adjust the rudder to compensate for discrepancies as small as three-tenths of a degree. Formerly, with a magnetic compass alone, the helmsman could steer within five degrees only.
Wireless direction reports are a great aid to navigation, especially when visibility is bad. An airship pilot must have a working knowledge of radiotelegraphy and be able to receive and transmit messages. He must also be able to read and send messages by flash lamp, flag and semaphore, and he must understand flag hoists made by all ships at sea.
Although it is unlikely that an airship pilot would be called upon to repair an engine in the event of a breakdown, he must understand the working and construction of all engines used in airships. He must be able to dismantle, reassemble and time any airship engine; and he must be able to determine the probable cause of engine failure.
Such knowledge enables him to appreciate just what his engines can do, and he therefore takes good care of them, only calling for their full power for a few minutes at a time when absolutely necessary.
The pilot must know the ship he is to fly, her design and construction, and so he studies the materials of which envelopes, gasbags and outer covers are made, the stresses and strains of wires and frames, and the construction and adjustment of gas valves. Thus he learns what his ship will stand in various conditions of flight and what he can expect of her.
THE CAPTAIN OF A CRUISER, urgently required on shore, being taken on board the airship NS3, in the North Sea, in 1917. The NS3 was built in the same year for work with the Grand Fleet. The NS1, first of this type, made a flight of 49½ hours and covered 1,500 miles, but later ships carried out flights of 100 hours’ duration.
Pupils learning to pilot airships graduate from small non-rigid ships to large rigid ships. In the pilot’s seat of a small ship are to be found the various instruments, valve levers and controls. The instruments fixed on a dashboard include a pressure gauge, which records the pressure inside the envelope, an altimeter, and a statoscope, which, being more delicate than the altimeter, gives immediate indication of any rise or fall of the ship. There are also an inclinometer, which tells the trim of the ship in a fore-and-aft direction, an air speed indicator, a spirit compass, an engine revolution indicator, and an oil pressure gauge. On the starboard side are the wheel for the elevators and the water ballast release. Stirrups, to which the port and starboard rudder wires are attached, are fixed immediately beneath the dashboard; and by means of these the pilot steers the ship with his feet.
The shed doors are opened at that end which is most suitable according to the direction of wind. A handling party under the direction of a qualified pilot then arrives. This party is divided into three sections: those on the car, those on the fore guys and those on the after guys. The handling party is now posted, the crew are in the ship and the ballast bags are removed. The ship’s lift is now tried and she is “ballasted up” until she is a little light.
Sometimes a ship is brought out stern first, sometimes bow first, according to circumstances. If the ship is coming out stern first, the pilot in charge of the handling party takes a position at the bow of the ship. In this way he has the ship and every man under his eye all the time as the ship is brought out. This is important.
He gives the order, “Walk ship astern”, and controls operations so that the ship is kept central and in an exact line with the axis of the shed until she is completely clear. Once clear, the ship must be kept head to wind, otherwise she becomes more or less out of control, but by continual orders “Stern to port” or “Stern to starboard” the ship’s head is kept in the right direction.
“Stand Clear of the Propellers”
The position on the landing ground from which to begin the flight is left to the discretion of the pilot in the ship. When this point has been reached the ship is again ballasted up, the engines are started and the pilot assures himself that everything is ready to let go. He then gives the signal by raising both arms. The pilot in charge of the handling party calls, “Stand clear of the propellers”; a whistle blows; the handling party must all let go together; and the ship begins to rise. The pilot puts his elevators “up” and opens out his engines. As the ship gathers speed she climbs at an increasing angle. The gas pressure will increase according to the rate of climb, and the pilot must either release air from his ballonets (which maintain the shape of the envelope), or gas from the envelope should the ballonets be empty. If it is necessary to release gas, this will entail a loss of lift of 1/30 of the ship’s gross lift for every 1,000 feet of rise.
A NON-RIGID AIRSHIP of the U.S. Navy being brought out of her shed at Lakehurst, in New Jersey Airships of this type are helium-inflated, and have a capacity of 210,000 cubic feet. They are used for coastal patrol. The ground crew move the airship to a suitable point for taking off. The engines are then started, and at a signal the men let go of the handling guys. The airship rises, the engines are opened up, the elevators are put up and the airship begins to climb.
When the desired height - say 1,000 feet - has been reached, the elevators are eased down to a horizontal position and the ship will fly on a level keel. If she does not fly level, the trim will be adjusted by blowing air into the forward or after ballonet, according to whether she is nose heavy or tail heavy. It will be found necessary, in maintaining a fixed height, to be continually using the elevators. Rising currents, changes of temperature and a passage over water affect the ship’s path to a greater or lesser degree. The pilot must continually adjust the pressure. When this is being done air should be let into or out of the ballonets alternately, first the fore and then the after ballonet, to avoid altering the ship’s trim. With an airship there are five different ways of effecting alterations of elevation. The pilot must know the relative value of each and the circumstances in which each should be used.
The elevator is the normal means of correcting alterations of level. The filling of one ballonet more than the other causes a diminution of lift at the point where the fuller ballonet is situated and a corresponding increase of lift at the other ballonet. The envelope tilts downwards towards the fuller ballonet and follows an inclined path through the air.
There are two objections to this method. First, the higher the airship goes and the more empty the ballonets become, the less effective is the method until, when the ballonets are empty, it cannot be used at all. Secondly, the method is slow in operation and can be used only in the non-rigid type of airship.
In the rigid airship elevation is altered by moving weights, such as water ballast, fuel or freight forward or aft, and so affecting the trim of the ship as to cause her to be down by the head or stern.
The rise or fall of the airship can also be obtained by swivelling propellers. This is the most powerful, sure and direct method, and a direct rise or fall can be realized. It has the disadvantage that it stops the forward movement of the ship. Another method is by letting out gas or ballast. Letting out gas lessens the buoyancy and so causes the airship to fall. Letting out ballast has the opposite effect. These two methods, in emergency, are regarded as a last resort.
The pilot is trained to act promptly if the airship breaks down while in flight. No definite rules can be laid down to fit every emergency, and the pilot must take into consideration the attendant circumstances, such as weather, strength and direction of the wind, position of the ship, and the like.
Repairs in Mid-Air
The breakdown of one engine, or in a large rigid airship the breakdown of all motors except one, is not serious. The airship is still able to make a certain headway, and the pilot is therefore able to retain complete control of the ship while repairs are being carried out. It is seldom, if ever, that a breakdown is so bad that it cannot be repaired in mid-air.
If all engines fail, the airship becomes a free balloon and must be treated as such. As soon as possible a decision must be reached as to whether a repair can be effected in the air and how long it will take. Having made a decision, the pilot must decide whether to land at once or wait for the repair. It will now be realized why the airship pilot must first qualify as a balloon pilot.
If it should be decided to land, particular care must be used to keep the rate of descent as slow as possible. Over the sea the ship can be brought down and ballasted up to a sea anchor (see page 312). From here she can be towed to shore and moored in a sheltered place until repairs have been completed, when she would continue her voyage or return to her home station. Should she be over the land every effort must be made to land near a good natural shelter, where the airship is securely moored.
If the steering-gear or rudder lines break, and the damage cannot be put right, the airship can still be steered and landed by means of her engines. This is done by going ahead on one and astern on the other, that is to port or starboard as required.
The pilot, having completed his flight, now prepares to land, and here again the value of his knowledge is to be proved.
He begins by bringing the ship down to within two or three hundred feet of the ground and “ballasts up”. This is done to put the ship into correct trim for landing. The procedure is to put the ship head to wind, slowing down the engines until the airship has lost all way and remains stationary. The elevators are put in the neutral position, and the statoscope will at once indicate whether the ship is falling or rising, that is whether the ship is heavy or light.
AN EXPERIMENTAL LANDING on the deck of H.M.S. Furious, by the non-rigid airship SSZ 59 in 1918. This test was carried out in the Firth of Forth to try out the possibility of taking these small airships to sea with the Fleet. The test was a success, but further experiments were prevented by the signing of the Armistice in 1918.
The pilot may expect loss of buoyancy through having risen to an altitude which required the valving of gas, a rise in temperature, a fall in the barometer, rain, snow, and so forth. Gains in buoyancy may be expected from consumption of fuel and oil, fall in temperature, rise in barometer, a wet ship drying or discharge of ballast.
All these factors must be known to him and, with his knowledge of aerostatics, a simple calculation gives him the amount his ship is heavy or light. He then drops ballast or valves gas accordingly.
To make a good landing the ship must be slightly trimmed down by the nose and a little lighter than air, carrying a few degrees of “up” helm on the elevators. In this trim the ship can be forced down easily if the speed is increased or, alternatively, will rise slowly by her own buoyancy if the engines are slowed down.
The ship is now ready to land. On receipt of a signal from the landing party, which has been formed up on the ground with the men’s backs to the wind, the ship is slowly flown down until the trail rope can be dropped into the hands of the landing party. The engines are then eased right down and when the ship has been hauled down on the ground and is in the hands of the landing party they are stopped.
The airship is brought back into the shed in the same way as she was brought out. Large airships are brought out and in by means of a mobile mooring mast (see pages 308-12).
In the shed the pilot is trained in the care and maintenance of the ship on the ground. This really amounts to a thorough examination of the ship to ensure that she is always ready for flight. The pilot must personally superintend this examination and see that it is thoroughly carried out.
Having obtained all this knowledge in detail, and having made the required number of flights, the pilot has to pass a written examination. He is then given a certificate.
Duties of Officers and Crew
The pilot then acquires further experience as pilot of a non-rigid airship, from which, provided he shows the necessary promise, he graduates to the position of an officer on a large rigid airship, and thence to that of captain of such an airship.
Although the principle of airship flying remains the same, the captain of a large ship does not handle the controls himself, but takes command in the control car in much the same way as the captain on the bridge of a seagoing vessel. The handling of the helm,
elevators and the like is delegated to experienced men who work under the orders of the captain or officer who is at the moment in charge of the ship. The crew on board a rigid work in watches as on a ship at sea.
In each engine gondola there is always an engineer throughout the flight. He is on duty three hours and off six. Other men continually patrol the ship from bow to stern. They examine the gas bags for any sign of leakage, which must be reported at once and repaired. Control wires and surfaces, electric wiring and the hull itself are regularly inspected. Fuel and oil are being consumed and this alters the flying trim of the ship, so that water ballast must be pumped periodically from one tank to another to keep the ship in proper trim. Nothing, therefore can happen throughout the ship without its being observed immediately and put right.
As repairs to ship and engines can often be carried out during flight, there is on board a spare-part store, which may weigh over one thousand seven hundred and fifty pounds.
With an airship in flight it is possible to remove a damaged propeller and fit a new one.
MOORING AN AIRSHIP to a mast calls for skilful pilotage by the airship commander. This picture shows the British airship R 100 moored to the high mast at Montreal after her flight across the North Atlantic to Canada in 1930. This mast was similar to that built at Cardington, Bedfordshire, in 1928.