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Name: Scrinand
Status: Other
Grade: Other
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.


Bob Wilson.

Hi, Srinand.

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 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.

Ryan Belscamper

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