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Seaplanes

Seaplanes are classified as flying boats or floatplanes. Flying boats land on their bellies in the water, and typically incorporate small floats called sponsons on their wings for roll stability while on the water. Float planes, on the other hand, typically feature a pair of large, parallel sausage-shaped floats attached to their undersides by struts. Floatplanes are generally high-wing, and thus easier to dock than flying boats, because their wings don't interfere with the dock. The float plane design also provides the flexibility of being able to mount floats on existing land plane designs. This flexibility saves time and money, because an entirely new airplane need not be designed -- merely a means of attaching floats to an existing design.

Seaplanes acquired great utility in the years before World War II, when few land airports existed. Seaplanes, as opposed to land planes, could alight anywhere suitable smooth water existed. The large flying boats of the era plied the airways from Europe to Africa, around the Americas and across the Pacific to the Far East. Seaplanes still enjoy popularity in areas lacking land airports, such as Canada and Alaska. In addition, the sheer joy of landing and taking off from water attracts innumerable aficionados, who fly a wide variety of seaplanes.

In 1898 inventor Wilhelm Kress made the first powered seaplane take-off from the waters of Lake Tullnerbach in Austria. Although the machine later sank, it became clear that seaplane pilots (pilote d'hydravion) had a five year headstart over their land based counterparts. With the success achieved by the aeroplane, inventors began to consider the possibility of combining the aircraft with the boat, for use on the sea. Hugo Matullath of New York seems to have been the first to suggest this, but beyond filing his specifications nothing was done to put his ideas into practical effect. The invention of the hydroaeroplane is due to Glenn Curtiss who, in 1908, fixed floats to his aeroplane as safety devices.

Before the Wright brothers made their famous "first flight" on Dec. 12, 1903, another aviator named Samuel P. Langley attempted two powered flights launched from a houseboat in his Langley Aerodrome. These attempts occurred on Oct. 7 and Dec. 8, 1903. Unfortunately, both ended in premature water landings in the Potomac River. Langley died in 1906 without again attempting flight. The Langley Aerodrome was consigned to storage, where it languished until 1914. During that year the Smithsonian Institute sponsored a study of its airworthiness. The Aerodrome was taken out of storage and shipped to the Curtiss factory at Hammonsport. Glenn Curtiss undertook its restoration, and made several modifications, including mounting it on floats. It was on 28 March 1910 that the first seaplane soared into history. Launched from the shores of the small, picturesque town of Martigues, France, the plane successfully managed a flight of about a mile and a half, flying just a few feet above the water. The visionary brainchild of French aviator Henri Fabre, the aircraft was given the name of 'Le Canard'-literally, 'the duck'. Fabre added the floats as an integral part of the machine with the express idea of rising from the sea. Fabre used a 50 HP Gnome rotary engine to power his plane, which at that time, was revolutionary. Constructed of an ash frame and then covered with a cotton material, the plane seemed unlikely to be capable of flight.

One year later and thousands of miles across the ocean, it was another aviator, this time an American by the name of Glenn Curtiss, who forever revolutionized the aviation industry. Inventor of the 'hydroaeroplane', Curtiss will always be remembered for his flying boats and his contributions to the dawning of naval aviation prior to the First World War. The first Curtiss seaplane, flown on January 26 th, 1911, was actually a biplane fitted with floats-an ingenious solution to the problem of taking off from water. Later, he went on to perfect the 'flying boat', literally a boat with wings.

During the morning of May 29, 1914 Curtiss flew the Aerodrome 150 feet, and landed softly on the water. This was accomplished despite the addition of 350 pounds of weight, caused by the floats and their supports. Thus, it could be argued that the first aircraft design capable of engaging in controlled, powered flight, was a seaplane.

Seaplanes are aircraft designed to take off and land on the surface of the water. Aircraft of this type fall into two distinctly different categories. One category consists of conventional land planes that are mounted on floats, sometimes called pontoons, in place of a conventional landing gear with wheels. The other category consists of a basically different type of aircraft in which the lower part of the fuselage is shaped somewhat like a boat and which, at rest and low speed, floats on the surface just as a boat does - hence the term "flying boat."

Early aircraft utilized construction techniques resulting in high drag coefficients. Struts, wires, and braces in the airstream resulting in high drag designs. Thus, the amount of drag introduced by a hydrodynamic step contributed relatively little to the overall drag of the craft. However, modern aircraft are much more carefully designed to eliminate drag and have highly efficient aerodynamic designs. In such a design, a float or hull step may comprise a significant source of drag.

In general, the amphibious aircraft is more complex than its land based counterparts due to the dual mission. Water landing requires the aircraft to be engineered to be water tight and withstand the added stress of water impact and docking. The propeller must be protected from the spray. The aircraft must have a safe method for boarding on both land and water. Desirably the aircraft would be designed so as to be able to use a standard boat dock and thus avoid the extra expense of building a special boarding facility. It also is desirable that the aircraft complexity does not degrade aerodynamic performance when compared to comparably sized land based planes.

These design objectives must be met in such a manner that the aircraft is stable on the water, on land and in air. Further, there is the consideration of designing the aircraft so that it is convenient and safe for the pilot and passengers, and yet is integrated so that the various aerodynamic, structural and design features blend together to provide an overall practical, efficient and economical aircraft.

The seaplane hull is an integral part of the fuselage of a flying boat or the floats on a floatplane. The design criteria of sponsons or floats mounted outboard on a flying boat which, arising from their displacement, commonly provide flying boats with lateral static stability, contrasts to the design criteria for seaplane hulls and this invention does not necessarily relate to their design.

The use of a hydrodynamic step in a water hull for aircraft use is attributed by some to Glenn Curtiss. Early floatplanes designed by Curtiss had difficulty in becoming airborne, regardless of the amount of power applied or wing surface provided. It was discovered that the float or hull of the craft would tend to "stick" to the water surface due to the laminar flow of the water. The simple solution was to introduce a drag-inducing (and vortex generating) step at the rearward portion of hull or float to reduce or eliminate water adhesion and allow the craft to separate from the water flow. Unfortunately, this same step design also generates air vortices once the craft is airborne, thus contributing to aerodynamic drag.

Since the inception of seaplanes, hull development has revolved around the planing hull concept, so much so that the hydrodynamic form to achieve efficiency when in displacement mode, that is to say below planing speed, has been generally ignored. By combining aspects of design established in racing catamarans and passenger catamaran ferries and innovating to enable such hulls to achieve seaplane design criteria, new and advantageous seaplane hulls may be developed.

In plan form, conventional hulls almost universally have a resemblance to an elongated rain-drop with a rounded bow and an afterbody trailing to a point. Though this form is aerodynamically very efficient, it is wholly unsuited to motion on the surface of water. It creates a large bow-wave which creates high drag, and the Coanda effect draws it powerfully down into the water preventing it from taking-off in addition to creating a powerful nose-up pitching tendency. This is in contrast to the comparatively sharp bows and fine bow entry angles of recent catamarans.

The object of conventional seaplane hull design has been to minimise these hydrodynamic penalties while incurring as mild structural and aerodynamic penalties as possible. To prevent the Coanda effect sucking the hull down into the water, hulls are generally fitted with a transverse abrupt "step" below the centre of gravity of the seaplane to separate the water flow from the hull. To enable them to climb over the bow wave, they feature hard edged forebody chines and powerful planing surfaces from the bow back to the step. The said forebody chines and the step are angled steeply across hydrodynamic and aerodynamic streamlines and create substantial vortices which both damage directional stability and create substantial drag. The result is undesirable qualities from the point of view of hydrodynamic loading and hydrodynamic resistance during take-off. They demand an aircraft with high power-to-weight ratio and result in limited payload capacity for a given size of aircraft.

Variable geometry hulls, hydrofoils, air blowing and other devices have been used to try and maintain a clean aerodynamic hull form. However, they all have limitations and add considerably to the weight and complexity of an aircraft. None of these attempted solutions have proved Certifiable and/or commercially viable though various ideas have been tested in prototype and experimental form.

Some improvements however have been demonstrated by the use of high length-to-beam ratio hulls and this was researched in some depth prior to 1950 and is well reported in "Development of High-speed Water-based Aircraft", by Earnest G Stout in the Journal of the Aeronautical Sciences Vol. 17 August 1950. This discusses tests on hulls with length/beam ratios of up to 12 though the advantages indicated were little exploited as there has since been little seaplane development work. The U.S. Navy's flying boat XP5Y-1 (first flown in 1950) had a length/beam ratio of 10. However, it had all the features of a conventional seaplane.

The top speeds of offshore racing multihulls and passenger catamarans approximately doubled in the 25 years to 1994. These hulls have also demonstrated major sea-keeping advantages, but despite the demonstration of the advantages of this type of hull, no seaplane has incorporated this type of design. The appropriate use of these types of hull for seaplanes is not obvious as there are two potentially problematic factors in their application to seaplanes. Firstly, they have a large wetted surface area at high speed which results in high frictional drag when approaching take-off speed. Secondly, there appears to be a widespread assumption amongst aircraft designers that the Coanda effect remains problematic on any hull without a central step.



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