People are often confused and discontented by the unexpected appearance of float aircraft that never existed in reality.
The truth is that hydroplanes were viewed differently in the 30s. In our contemporary world of cutting-edge technologies, we are used to planes on wheeled chassis, and the dense network of airfields covers most of the Earth. As such, there is little need for special hydroplanes, with some exceptions.
However, at that time the situation was entirely different. Airfields were few, and they usually consisted of a flat grass field, like a football field but bigger. Thus, hydroplanes were often deployed to open new lines, because they had no need for complex infrastructure. An inclined exit ramp (slip) to the water surface and a wheeled dolly were all that was needed. A service hangar was a welcome addition, but not entirely necessary. Transcontinental routes to Brazil and Africa were opened via hydroplanes.
Military affairs also comprised numerous tasks for both wheeled and floated aircraft. In this context, reconnaissance spotters were in particularly high demand. Such aircraft were needed over both the sea and the ground. Sometimes, regular planes were adapted for naval use; sometimes, planes were designed initially with two types of chassis in mind.
For example, the Hawker Osprey was initially planned to be used with both chassis types.
Even in cases when a floated version was not included in the original design, and the plane turned out to be exceptionally good and mass-produced, a new floating chassis could be added later. That was the case with the well-known U-2 (Po-2). Initially land-based, the plane saw such high demand that various floated variants emerged for use in Siberia.
Thus, the mounting of floats was not at all rare in those years. This method was not only designed but also described in specialist literature. We used the book by P.D. Samsonov “Concept and engineering design of hydroplanes” (Moscow. Moscow Aviation Institute. 1936. Chief editorial board of aviation literature).
Page 167 marks the start of the section titled accordingly—“Mounting of floats onto a land-based plane.”
In this case, the profile of the aircraft and the attachment points of the land chassis (the wheels and the construction between them and the plane) are known. Usually, naval chassis (floats) are attached to the same bearing members.
Let’s take a look at the transformation process for the abovementioned British Hawker Osprey.
This is how it looks before we start the process.
The main stages of making a floated plane from a land-based one:
- Floats are selected based on the weight of the plane. Different time periods had different styles. For the sake of simplicity, let’s assume that standard sets of floats are available, which would be true for the 1930s and later. For instance, floats produced by the EDO corporation were widely used.
- Wheel chassis is demounted (main undercarriage legs, tail skid or tail wheel, sometimes the nose undercarriage leg with a wheel).
- A new structure is added between the fuselage of the plane and the floats. To maintain optimal takeoff and landing performance, several conditions must be preserved.
- The clearance between the water surface and the propeller should be at least 0.6 meters.
- The clearance under the plane tail should be at least 1.5 meters.
- The sum of the takeoff movement angle (usually 8-9°, numbered 1 in the image), wing setting angle (2), and thrust angle towards the back of the float (generally assumed as +2°, numbered (3)) is equal to the difference between the full-lift capacity angle (4) and the safety angle (usually assumed as 1-3°, numbered (5)). The general idea of this rule is to make sure that when the plane takes off, the wing incidence is less than the maximum by that safety angle. This is done to prevent the plane from accidentally reaching beyond-stall angles of attack, which would result in falling into a spin and, thus, an inevitable crash.
- The horizontal distance between the center of mass of the hydroplane (marked (6)) and step on the floats should comprise about 20-30% of the average aerodynamic chord (AAC). AAC is a virtual entity that serves as a replacement for a wing or wings when dealing with plane balance issues. AAC is numbered (7), the distance is marked (8).
- The distance between the floats should ensure the dihedral stability of the plane on water. It depends on the mass of the plane, displacement of the float, and the surface of the float waterline. However, we won’t overload you with formulae.
- Since most of the side surface of the float turns out to be in front of the center of mass, this factor disrupts the longitudinal stability of the plane. That is why floated variants often get an extra 10% added to the area of vertical tail surfaces. Sometimes this is done in the form of fins under the tail, in other cases the area of the fixed vertical fin or the rudder is increased.
Thus, almost any land-based airplane weighing from 5-7 tons, and sometimes even more, can be set on floats. This plane will fly, although its flight performance will be slightly worse compared to the original. All this amounts to an increase in weight by 10-12%, a reduction of the top speed by 5-15%, and a drop in the altitude ceiling by 10-12%.
This is how our airplane looks once we’ve mounted the floats.
Aircraft engineering practice has numerous examples of setting fully-fledged land-based planes onto floats or plans to perform such conversion.
Below are some examples of such airplanes.
This plane was meant to operate from island bases while a proper airfield was being constructed. This design was influenced by the success of the Japanese floated fighters Nakajima A6M2 Rufe.
The British designed this variant for operation outside the airfield network, particularly in Norway, in case the Germans captured the airfields. However, the German assault on Norway was so swift that the need for this variant of the plane was eliminated.
We also managed to find some evidence that the British had plans to float the Hawker Hurricane as well.
The Germans did the same to their He.51.
Thus, when we faced the need to create lots of floated fighters and spotters for numerous ships of various tiers, and no such planes ever existed in reality, we took the liberty of setting many planes onto floats ourselves.
Among them, we find the ones below to be the most intriguing:
Ta.152C on floats
Polikarpov I-153 on floats
Curtiss XP-40S on floats
This way, we are able to find a suitable candidate for any ship that requires a floated fighter or spotter. And if there is no such real plane at all, even among land-based aircraft, we can design a plane from scratch.