Aircraft and Submarine Shape
Location: Outside U.S.
Country: Great Britain
Date: September 2006
I am doing an experiment on why aircraft and
submarines have different nose cone designs. I live in London.
High speed aircraft usually have conical nose designs whereas
submarines have epilliptical ones. If water is just a medium like
air (albeit denser) why the difference? I need some help on how to
create experiments to measure air resistance of the two nose designs as
well as coming up with a way of creating an experiment in which I can
observe the flow patterns in water (like a wind tunnel but with water
instead of air).
I am no aerodynamics engineer, but I can suggest at least two
key differences between slow speed submarines moving in
water, and high speed aircraft.
The first is that air is compressible and water is not. This
is the a very fundamental difference. The second is that high
speed aircraft approach or exceed the speed of sound. Thus,
they generate shock waves that a slow submarine in water
never encounters. A very important aspect of the design of
high speed aircraft is to manage the generation of transonic
and supersonic shock waves that form at all leading edges. A
sharp nose cone helps to reduce the severity of the shock
wave. You will notice that slower speed aircraft that do not
approach the transonic region, are generally designed with
more rounded noses.
The answer to your question is that air and water have more
differences than just density. In a perfect fluid where the
molecules have no inherent friction, fluid motion can be described
using equations called potential flow equations. Of course no fluid
is perfect, they have a property called viscosity, that varies from
fluid to fluid (viscosity of a fluid can also change dynamically,
but I digress).
Anyhow, different fields have developed different ways to measure
how closely the fluid they are interested in approaches the
properties of a perfect fluid. In aerodynamics, we use the
Reynold's number, which is V * l * density / viscosity. The
Reynold's number is like a ratio of how much each type of property
influences the fluid flow.
In ship design, I believe the primary number of interest is called
the Froude number (though they also use the Reynold's number), and
is a slightly different calculation more useful for ship
design. Since my experience is in aero, I am not sure what a good
text would be to refer you to for references on that (perhaps
someone with ship design experience could chime in here), but a
basic aerodynamics text usually talks about this in the first few
chapters. A search on amazon.com turns up "Fundementals of
Aerodynamics" by John Anderson - this is a good aero book, but you
might get more useful information about this sort of thing from a
basic fluid mechanics text. Usually the first few chapters talks
about fluid properties and their influence. Again, searching on
Amazon turns up several. Your local library should have several of
them that you can browse through to get more information.
One other thought - This is kind of out there, but you might also
look for a book on hovercraft design. I became aware of how these
factors overlap and influence shape during a design project at
school where we had to do the conceptual design for a
hovercraft. It is influenced by both of the above factors, and they
interact in unexpected ways.
David Brandt, P.E.
Well, I can help with the last part. You need to set up a little
channel for water to flow through, like a river. (A water pump will
keep your water moving from one end to the other.) In a wind
tunnel, they use streams of smoke to show the manner in which air
flows over a surface. For your water channel, I would recommend
using a number of pieces of submerged thread. Make sure the sides
of your tank/channel are smooth though, or else you will get far too
much turbulence from that to make the whole project worthwhile.
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Update: June 2012