I am mainly interested in reducing drag in aircraft and I have an idea that I want to learn more about...
Can a superhydrophobic coating reduce drag in water-craft such as boats and submarines?
If that is the case, could a superhydrophobic coating decrease the drag coefficient of aircraft making them more aerodynamic? Or would some other material that's phobic to air be needed for this, and does it exist?
Superhydrophobic surfaces generally consist of a series of micro/nano scale pillars that increase the contact angle of water on the surface. Larger contact angle means water forms more spherical drops and can roll off more easily. Changing the chemical properties of the surface layer can also help change the contact angle.
These surfaces are optimized to effect the contact angle of water so will not have a significant effect on the drag of vehicles (except maybe boats) or radar detection.
However surfaces with other patterns/shapes of structures can have these effects. See riblets or sharkskin suits for reducing drag in aircraft/swimmers and moth-eye anti-reflection coatings, for light, not radar but that is probably just the size of features.
Common uses for super-hydrophobic coatings are in water repellant windscreens for cars and on waterproof jackets.
In short, whether there are long-term savings to be made by using super hydrophobic surfaces (SHS) on ships is still an open question.
There have been many claims, but the benefits have yet to be shown on real ships. There are many factors that come into play, such as durability of the surface, its cost, whether the surface must be cleaned in special ways etc. These will require testing over long periods of time.
There is some discussion of the potential of SHS in the paper:
Triantafyllou, Michael, "Science and Technology Challenges and Potential Game-Changing Opportunities ", Special Report 306: Naval engineering in the 21st century the science and technology foundation for future naval fleets, https://seagrant.mit.edu/publications/MITSG_10-36.pdf
A recent book also discusses SHS. See:
Perlin, Marc and Ceccio, Steven, "Mitigation of Hydrodynamic Resistance: Methods to Reduce Hydrodynamic Drag", 2015, 150 pp.
Be aware that there are quite a few people making unsubstantiated claims for drag reduction. My advice is to treat any claims of large performance gains with suspicion, and to keep an eye on developments reported by the International Towing Tank Conference (ITTC). http://ittc.info/index.php
(I'm sorry, but I can't help with your question about how SHS can be used to effect on aircraft.)
The viscous resistance of air on an aircraft is really tiny compared to the turbulent resistance caused the formation of eddies in the air behind the aircraft. For that reason, hydrophobicity of the surface is not really what we care about. An object travelling through air is to either be very small or be moving through the air very slowly in order to be experiencing only viscous resistance. All objects going through air experience linear resistance from air's vescosity below a certain speed and quadratic and quadratic resistance from turbulence above that speed and for any shape, the lowest speed an object of that shape can go and experience quadratic resistance varies as the reciprocal of the size of the object.
Actually, a substance doesn't have to be hydrophobic in order for water to roll off it. Any amorphous or monocrystalline solid etched nanosmooth, if water doesn't completely wet it, it will roll off because its receding contact angle will be so close to its advancing contact angle.
Airplanes do have other problems that are very worth solving. One such problem is supercooled water droplets in the air going into the engine and freezing and building up a thick layer of ice in it. There's an entire Wikipedia article about ways to prevent that problem at https://en.wikipedia.org/wiki/Ice_protection_system.
