Flying Boat

Overview

The Flying Boat is a four-engine amphibian whose fuselage is itself a boat. Instead of a tube sitting on wheels, the lower body is a Boat Hull shaped to plane across water, take off from it, and land back on it. Wheels are still fitted, but they are a Amphibious Landing Gear that retracts fully clear of the hull so the aircraft can also use a runway. The aircraft can serve coastal towns, island groups, lakes, and rivers that have no airfield, and still put down on a paved strip at the far end if one exists.

The layout follows from the water role. The wing is a high Wing mounted parasol-fashion above the hull, braced by an external Wing Strut on each side. Sitting the wing high keeps the four Turboprop Engine units and their Propeller discs well above the spray thrown up on takeoff, because salt water in an engine intake or pitted into a propeller blade is destructive. The tail is set high for the same reason, with the Horizontal Stabiliser carried near the top of the Vertical Stabiliser out of the wash.

The Planing Hull

The Boat Hull does two jobs that pull against each other. At rest and at low speed it is a displacement boat, floating on the buoyancy of its sealed volume. Above a certain speed it must stop pushing water aside and start riding on top of it, planing on a small wetted area near the stern so that drag falls far enough for the wing to take over and fly the aircraft off. The hull shape is a deep V-bottom: the Bottom Plating meets at a sharp Keel and rises to a hard Chine Strake on each side. The V softens the slam when the hull drops onto a wave, and the chine is a sharp lengthwise edge that flings the bow wave sideways and down, away from the propellers and flaps.

Structurally the hull is heavier than an ordinary fuselage of the same size. A line of Hull Frame members sets the cross-section, Longeron stiffeners run fore and aft, and the Bottom Plating below the chine is thick aluminium plate because it takes repeated water impact. Above the chine the lighter Side Plating forms the topsides. The hull is divided by six Watertight Bulkhead walls into separate compartments, so that a single holed section, from striking a log or a reef, floods only its own bay and the aircraft stays afloat. Whatever water does get in is cleared by the Bilge System, a set of Bilge Pump units drawing each compartment down through a Bilge Valve that stops one flooded bay from filling the others.

The Step

The single feature that separates a planing hull from a boat hull is the step. The hull bottom is not a smooth curve from bow to stern; it has an abrupt transverse break, the Forebody Step, where the bottom jumps up by a few centimetres. As the aircraft accelerates and the hull rises onto the plane, the water flow separates cleanly at this edge instead of clinging to the whole length of the bottom. Without it, the rising water would hold the hull down by suction and the aircraft could never unstick from the surface. A Step Vent feeds air into the low-pressure cavity that forms just behind the step, which keeps the flow separated and stops the hull porpoising, a violent pitching oscillation that can throw the aircraft back onto the water. A second, shallower Afterbody Step further aft trims the wetted length during the planing run and reduces drag in the final seconds before liftoff.

Spray and the Floats

Spray management runs through the whole design. The bow wave at takeoff is a sheet of water moving fast, and any of it reaching a propeller erodes the blades, while water on the wing or flaps spoils lift at the worst moment. The Chine Strake is the first defence, peeling the sheet away. The high parasol wing and the raised Engine Pylon mountings keep the Propeller Blade discs out of the thrown water, and each blade still carries a metal leading edge against the spray it does meet. The Air Intake for each engine sits high and contains a Water Separator that swirls droplets out of the airflow before the Compressor.

On the water at rest a flying boat wants to roll onto one wingtip, because the hull alone is a narrow, tippy boat. The fix is a Stabilising Float under each wingtip. In flight and at speed the floats ride just clear of the surface, adding little drag; the moment the aircraft slows and a wing drops, that side's Float Hull touches the water and its buoyancy levels the aircraft. The floats are carried on a Float Strut pair bracing them to the wing.

The Amphibious Gear

Being amphibious means the hull has to be watertight even though wheels poke through it. The Amphibious Landing Gear is a retractable wheeled set: a Main Gear Leg leg folds up into the hull side on each side and a steerable Nose Gear Leg retracts forward. For a water landing the gear is pulled completely inside and a Gear Bay Door closes flush over each bay so the hull bottom is smooth and sealed. For a runway landing the legs come down and lock, the Oleo Strut on each absorbs the touchdown, and the Brake System with its Carbon Brake stacks stops the aircraft. The cost of getting this wrong is severe: gear down on water flips the aircraft, gear up on a runway destroys the hull. A Gear Position System system therefore cross-checks the leg positions against whether the crew has selected a land or water arrival and warns before either mistake.

Engines and Systems

Four Turboprop Engine units give the power to drag a heavy hull up onto the plane and the redundancy to fly home on three if one fails far out over water. Each is a free-turbine turboprop: the Gas Generator Turbine drives only the Compressor, while a separate Power Turbine turns the Reduction Gearbox and the propeller, so the propeller can feather and reverse independently. Reverse pitch through the Pitch-Change Mechanism backs the aircraft off a beach. The Fuel System feeds all four engines from integral wing tanks with a Crossfeed Valve to balance the load. The Flight Controls keep mechanical cable runs to the surfaces, and a Water Rudder drops from the stern to steer the hull at low speed on the water, where the air rudder has no flow over it. The Cabin Interior converts between Passenger Seat rows and a cargo load on a reinforced Cargo Floor, loaded through a side Cargo Door from a dock.