Special Types of Aircraft Developed for Marine Use
RUNNING ON THE STEP is the expression used to describe the hydroplaning of a seaplane on the surface of the water. Before hydroplaning, the floats plough through the water, build up a bow wave and gradually climb up the “hump” so formed. The floatplane illustrated is an Airspeed Queen Wasp.
WHEN the designer of the first seaplane produced his crude machine, he can have had but little idea that this type of craft was destined to capture the world’s speed record, which stood early in 1938 at 440·6 miles an hour. Locomotion through the water is leisurely compared with locomotion on land; yet a seaplane flying over the water is the fastest vehicle in the world.
Seaplanes may be divided into two principal classes: floatplanes and flying boats. The amphibian is a type which can alight on or take off from either land or water, and so may act as a landplane or as a seaplane.
Irrespective of their class, all seaplanes must have certain qualities in common. For example, all seaplanes must have sufficient buoyancy and stability while on the water to enable them to meet every contingency likely to occur in ordinary usage. They must be able to ride to moorings safely. They must be easily manoeuvrable when taxying. They must be able to take off from and alight upon the water without “porpoising” or “ballooning” into the air (see Fig. 1, below). And they must be able to be beached when required for the purposes of storage, examination and overhaul.
As their class name suggests, floatplanes are fitted with floats (called pontoons in America). Formerly built of wood, floats are now generally built of one of the alloys of aluminium. Each float is divided into watertight compartments, so that the accidental holing of a plate will not flood the whole float.
FIG. 1. Movements to be avoided by the pilot of a seaplane when taking off or alighting on the water.
Except that it is fitted with floats instead of wheels, a floatplane is in every way identical with a landplane of similar type. Many floatplanes can be changed into landplanes by the simple process of substituting a land type of undercarriage for the float undercarriage.
Floatplanes are generally fitted with two floats. These floats are attached and braced to the underside of the fuselage by struts and wires.
These two floats provide all the seagoing requirements of the craft - buoyancy, stability and shock absorption.
The buoyancy provided by the full displacement of the floats must approximately equal twice the fully loaded weight of the complete floatplane.
The floats must be long enough to provide fore-and-aft stability on the water for the whole machine. To give stability against rolling, they must be mounted on struts raked well out on either side of the centre line of the machine.
Unlike the main wheel struts of a land-plane, which are fitted with oil cylinders and metal or rubber springing to take the shock of contact with the ground, all the float struts of a seaplane are solid. Shock is absorbed by the floats themselves; the bottom of each float is fashioned in a Y-shaped curve, so that when the seaplane alights on the water the V cuts the water gently. The curves must be accurately designed to give this necessary property to the float. The hull of a flying boat must carry out a similar duty.
FLYING BOATS AND FLOATPLANES from aircraft carriers of the Royal Navy during exercises in the Mediterranean. This photograph illustrates the two types of seaplane. In the foreground are two Supermarine Walrus amphibian flying boats. On water these craft can alight on their hulls, the floats on the wings serving only as stabilizers. To alight on land the amphibians let down wheels. The Fairey III F’s and Ospreys are floatplanes, and are fully supported on the water by their main floats.
When at rest on the water all seaplanes have their flotation gear (floats or hull) partly immersed. When the engines are opened out for the take-off, the floats plough through the water and build up a bow wave; when the seaplane has gathered sufficient speed it climbs over its own wave and so hydroplanes or skims along the surface of the water.
Certain technical expressions are used to distinguish these conditions. The point when the seaplane climbs from the partly immersed to the hydroplaning condition is called “the hump”. The subsequent hydroplaning is frequently referred to as “running on the step”.
If the bottom of a seaplane float or hull is examined it will be seen that the sweep from bow to stern is broken in one, and sometimes in two places by a step up from front to rear.
This step allows air to get under the float or hull surface when the seaplane hydroplanes over the surface of the water, and thereby reduces resistance. It also enables the designer to determine as accurately as he can where the centre of water pressure will affect the seaplane.
Both these points are important, the first obviously so, for it will enable the seaplane to accelerate faster and thus get into the air quicker. Far more power would be required to take off with a stepless float. So, with an efficient hydroplaning surface, a seaplane can rise into the air with less horse-power or with a greater load.
The second point is not less important, because it affects fore-and-aft stability. If the centre of water pressure is too far aft, the seaplane will tend to nose dive under the water. If the centre of water pressure is too far forward the seaplane will try to leave the water before it has reached flying speed. The ideal seaplane must do neither of these things. It should ride smoothly and
steadily over the surface and yet be responsive to the pilot’s touch on the controls.
The flying boat is essentially a marine type of aeroplane designed to be moored on the water - if necessary for long periods - and to ride to moorings in the same way as a ship. Its hull provides the buoyancy and fore-and-aft stability that are required on the water, and in addition offers accommodation for the crew and passengers; but by itself the hull is not stable against rolling. Fins must either be provided on the sides of the hull or small floats must be fitted to the wing tips to give protection to the wings when rolling takes place.
There is often more accommodation in a flying boat than in a landplane of similar type, merely because the flying boat’s hull is generally deeper than the landplane’s fuselage. The reason for this difference lies in the conditions in which each type operates.
With a landplane taking off from level aerodromes a small clearance for the airscrews is sufficient; but when a seaplane takes off from roughish water considerable waves are thrown out from the hull before the hump is reached; unless there is a reasonable clearance between the rising wave and the tip of the airscrew at the lowest part of its disk, water and spray may be thrown or sucked into the airscrew. This produces severe vibration which is detrimental to the airscrew and to the engine, and in a lesser degree to the whole aircraft; it also interferes with the effective thrust of the screw by causing a reduction in its speed.
THE FLOATS OF A FLOATPLANE are generally broken into one or two steps up from front to rear. This allows air to get under the float when the seaplane is hydroplaning over the surface of the water during the take-off. The Airspeed Queen Wasp is a radio-controlled target aircraft fitted with an Armstrong Siddeley Cheetah engine.
To avoid this, designers generally allow greater airscrew clearance for seaplanes than for equivalent types of landplane.
Moreover, the undercarriage of the landplane must provide a definite clearance between the body and the ground, so that the clearance for the airscrews can be measured from below the fuselage. As the flying boat’s hull is, however, partly immersed in the water when at rest, the clearance height for the airscrews must be measured from the waterline, which is some way up the hull. Thus to get equivalent airscrew clearance, a flying boat’s hull must be deeper than a landplane’s fuselage by a distance equal to the landplane’s ground clearance plus the immersed portion of the flying boat’s hull.
Flying boats have to meet conditions for navigation on the water which have been established by the oldest form of large-scale transport in the world - waterborne transport. When the flying boat is at rest or moving on the water it comes in the same category as an engine-driven ship. It must give right of way to sailing ships and conform to all regulations affecting marine craft. On the other hand, the aeroplane on land is provided with its aerodrome where no other established form of transport interferes with it.
FINS OR SEAWINGS are fitted instead of wing-tip floats to this Martin 130 transoceanic flying boat, to provide the necessary lateral stability when the aircraft is resting on the water.
TAKING OFF FROM SOUTHAMPTON WATER. The Imperial Airways flying boat Capella, in the same way as all her sister Empire flying boats, is fitted with outboard, or wing-tip floats. In the background are two troopships, distinctively painted white with a blue band round the hull.
It is customary to carry life-saving jackets or lifebelts in flying boats, and it is standard practice to divide the interior of the hull into a number of separate compartments, each provided with a watertight door. Thus, damage to one part of the hull need not swamp the complete craft if the watertight doors are closed or can be shut in time. Because of the additional weight the flooding of one compartment would almost certainly make it impossible for the flying boat to take off from the water. Despite that disadvantage, however, the flying boat would be able to taxi or to ride the sea in safety until help could reach it.
In flight, there is no real difference in the handling of seaplanes and landplanes of comparable size. But the handling of both classes of seaplane on the water requires a different technique from the handling of landplanes on the ground. The Royal Air Force maintains special schools for instruction in seaplanes. When Imperial Airways changed from landplanes to the Short Empire flying boats to operate the main trunk air routes of the British Empire, it was necessary for their pilots who were to handle the new flying boats to go through a course of special instruction in seaplane work.
The man who has been a landlubber all his life must become acquainted with the rules for navigation at sea if he is to handle seaplanes.
If a pilot has been used only to the handling of landplanes on solid ground he has to learn to handle his new craft in widely varying conditions. The sea may have a calm mirror-like surface which makes alighting extremely difficult. Or there may be a tidal river where the water moves at eight knots. Other variations may include winds and short, steep waves, or no wind and long, smooth rollers. For every one of these conditions the pilot must adjust his methods for taxying, taking off and alighting.
In this constant change of conditions, and in the pitting of judgment and skill against them, the pilot finds the seaplane much more fascinating than the landplane.
Because the surface in contact with the air is much larger than the surface in contact with the water, a seaplane, with the engines throttled or shut off, always rides head to wind. So, when steering an out-of-wind course over the water, the pilot has to use considerable rudder or (if the seaplane is multi-engined) more pull from the engines on one side than on the other, to counter-balance the effect of the wind. Moreover, when steering across tidal or river waters, he must make allowance for the drift caused by the current.
THE EMPIRE FLYING BOAT floats on her hull, the floats attached to the wings giving lateral stability on the water. The Cambria is shown here at Foynes, Eire, being prepared for one of the experimental flights across the Atlantic in 1937. The Cambria has a span of 114 feet, a length of 88 feet and a height, to top of fin, of 31 ft 10-in. The wing area is 1,500 square feet.
Perhaps the most difficult condition of all for manoeuvring on the water is encountered with wind and tide travelling in the same direction when the pilot is attempting to come up to a slipway at right angles to the wind.
The art of handling seaplanes on the water is similar to the handling of small sailing craft, with one great difference. With sailing craft the sails can be lowered, or the amount of sail exposed to the wind can be reduced; but with seaplanes the pilot cannot reduce the wing or body surface. The seaplane pilot is therefore forced to handle his craft as a yachtsman would be forced to handle a yacht if her sails were permanently fixed.
At one time the floatplane was the more common type of seaplane. But as the size of aircraft has increased, the flying boat has increased in numbers and is now of more importance than the floatplane.
There are still certain types of floatplane used in transport aviation - by Italy, Germany and Canada, for example - but most regular seaplane air routes now use the flying boat. The floatplane is still, however, an integral and important section of all naval air forces. By the use of interchangeable floats and wheels the same types of aeroplane may be operated as landplanes from aircraft carriers with large flying decks and as floatplanes from warships which have no flying deck.
In the Royal Navy, however, the Walrus amphibian flying boat, fitted with a single engine, has challenged the supremacy of the floatplane even in this field.
Across the Pacific Ocean
Large flying boats are now in use on many of the great international trunk air routes of the world. It would have been almost impossible to have inaugurated the “China Clipper” air service between San Francisco and Manila (Philippines) by any other means, for the Pacific air stations , beyond Hawaii - Midway, Wake and Guam Islands - are small islands which do not afford the facilities needed for passenger air services with large landplanes. They are, however, suitable calling points for flying boats.
Along the waterways of the British Empire, the main arteries of communication and the principal centres of commerce, the trunk services run by Imperial Airways are planned to operate entirely by flying boats. The regular services between England and Durban,
Singapore, Australia and New Zealand are all to be operated by flying boats. The future for the big flying boat on the world’s air routes seems assured.
The work that seaplanes have accomplished in the past has been wide and varied. In war they have been used for reconnaissance, bombing and submarine spotting. For long oversea reconnaissance the flying boat is ideal. Meals can be cooked and served during flight and the crew can live on board as comfortably as in a small ship.
In Canada seaplanes have made important peaceful patrols from the lakes, reporting fires and collaborating with miners and explorers.
By using the tropical rivers in British Guiana, a floatplane ambulance some years ago saved valuable lives when white men far in the jungle went down with blackwater fever. In two hours they were transported to hospital in Georgetown, formerly a journey of three weeks by canoe.
British R.A.F. flying boats have flown to and round Australia, and in December 1937 a number left Plymouth for a flight to Sydney, N.S.W., and back.
The limitation of the seaplane begins only when the water (river, lake or sea) ends and where the land becomes arid for more than a thousand miles. In the seaplane the old and fine tradition of the sea is carried into the air.
INTERIOR ACCOMMODATION ON TWO DECKS is allowed by the depth of hull in a transoceanic flying boat. This American aircraft is the Boeing 314 four-engined flying boat. On the main or passenger deck there is sleeping accommodation for forty passengers. There is a crew of eight, including two stewards The Boeing 314 has a span of 152 feet and a length of 109 feet.