Expert: Paul Soderman - 4/6/2009

Question
Hello, and thanks very much for your willingness to answer questions. I know
that with watercraft, it can be beneficial to put an extremely low-friction
surfactant on the exterior surface, or to texture the hull with tiny diamonds
or lines not unlike sandpaper -- as humans have recently learned from
imitating the configuration of sharkskin.

Would it be a good idea to try doing a similar thing on the outside of an
aircraft, perhaps to get less parasitic drag, more efficient de-icing, and less
trouble with dust and rain? And I have heard about something called
"leading-edge flow control." Are the two ideas similar? For all I know, it may
be that aerodynamic flow has little enough in common with hydrodynamic
flow, so that this crude idea of mine might never work.

To be clear,  I am asking this question as part of a contribution I want to
make to a realistic role-playing game. So the aircraft in question does not
really exist, but if you are interested, it is a manned production version of the
FestoŽ 'Air Ray concept', except it is in the form of a EXTREMELY heavy lift
Class 4 ekranoplan (a wing in ground-effect aircraft that can also fly over
land at reduced efficiency. "My" aircraft is for military disaster relief, bigger
even than Hughes' Spruce Goose, or even the proposed Boeing Pelican. To
allow for this, it is a turboprop with six engines like an Antonov Mriya,
wingbeat flight as an ornithopter to balance the extra weight, and lifting gas
cells. There are also ground effect skirts to move as a hovercraft, so no
landing gear.

Thanks for your time and attention, I look forward to your reply.

Answer
Julian
Hydrodynamics and aerodynamics are essentially the same thing.  The main differences are the fluid densities and velocities, which effect the Reynolds number - a parameter that relates viscosity and inertia. So what is good for a shark at low speed in water may or may not be good for an aircraft at high speed in air.  I am not familiar with the data, but many aerodynamicists know about sharkskin and have tried various ways to doctor surfaces to reduce parasite drag, but aircraft skins are still smooth so I assume that is optimum in most cases.  There may be something out there waiting to be discovered, but I don't know what it is.  Incidentally, low speed devices such as golf balls do have rough or dimpled surfaces to control the boundary layer and reduce drag.

Leading-edge flow control is entirely different.  Slats or micro jets are used at leading edges to help the flow make it over the airfoil nose without flow separation.

So your imaginary plane has large wings with three turboprops each side and the wings flap.  That might be an interesting structural/dynamic problem.  

Glad you are interested in aero.
Paul