More on DARPA Submersible Airplane

Here are the main challenges identified by DARPA:

DARPA identifies five technical challenges. First is the diametrically opposed weight requirements between an aircraft that needs to be light to fly and a submarine than needs to be heavy to submerge. DARPA believes it should be possible to meet these requirements by controlling lift and volume - potentially by generating downforce on the wing and flooding spaces in the airframe to submerge.

The second challenge is to design a shape that works in air and water despite the difference in density. Here the key is that flows are similar in both fluids if something called the Reynolds Number (Re) is the same. DARPA believes it is possible to design a platform that operates at the same Re, airborne and submerged, but at radically different speeds: 100-400kt in air and 5-18kt in water.

Challenge three is structural - aircraft are pressure vessels with thin skins; submarines have thick skins to withstand crushing loads: the forces are applied in opposite directions. But DARPA believes this problem is dramatically reduced by limiting operating depth to the minimum required to avoid perturbing the surface.

Fourth is wing location. Traditionally seaplane wings are placed high to avoid the waves and shield the engines. Lowering the wing increases ground effect while sea-skimming. But the wing needs to be below the water to generate the downforce required to submerge. DARPA sees a range of possibilities ranging from two separate wings, one for air and one for water, to a single morphing wing able to change its height, area and airfoil.

The final (?) challenge is the powerplant, which must work submerged as well as airborne. Aircraft engines are light; submarine powerplants are heavy. One option is a single engine with air-independent and air-breathing modes. Another is dual powerplants, one breathing air and one running off batteries, fuel cells or DARPA's Aluminum Combustor. The agency believes the shallow depth required will allow use of a snorkel to provide air to the engine when submerged._AviationWeek_via_Wired

These are engineering challenges which may well require advances in materials science for their optimal solutions.

It is one think for DARPA to commission such vehicles for top secret, covert operations. It is quite another for Airstream or Winnebago to build them as RVs for the general public. At this time, I am willing to compromise and substitute a "surface effect" flier for a true aircraft, as long as the machine can go submerged for days at a time, and travel over the surfaces of roads, water, snow, mud, ice, rough ground, etc.

In an Obamanation, we will want to retain the freedom of travel in the fullest sense possible. Excessive limitations are for primitive and between levels humans, Obama zombies. We strive for something better.