Believe it or not the blimp is the most

streamlined shape you can have for subsonic speeds.

Jim Brewer wrote:

Paul,
I'm no engineer, so help me out here with some references that defines

the

"blimp" shape and why your claim would be so.

Jim Brewer

This was discovered and confirmed years and years ago purely by many
trial and error experiments.  It is a combination of what is called
form drag and skin friction drag. Form drag has to do with the shape
of the object and skin friction with the total surface area of an object.

There are two optimum shapes. One is based on the minimum drag for
a given enclosed volume. The other is based on the minimum drag for a

given

frontal area. They both look like blimps. Frontal area is not the shape of

the front of the object. Frontal area is technically defined as the maximum
cross sectional area of a given shape. The max frontal area of an object can
be two thirds back along its length but that may not be the lowest drag

position.

The bible on this sort of thing was written by Dr-Ing. Sighard F Hoerner
called Fluid-Dynamic Drag and published by the author in 1965. I think it
is out of print but you may be able to find a used copy on the Internet.
My new copy bought from the author in the late 60's is signed by Dr.
Hoerner himself who is now deceased. BTW Klause Savier claims he dated
Dr. Hoerner's daughter at one time :-)

Paul Lamar

Tom Kendall wrote:

Some other real world examples of the "blimp shape":  Submarines (c'mon
guys, you didn't think I'ld let that one slip by did you?) and Ships.

The "blimp" shape is a teardrop:  Rounded (hemispherical seems to work best)
front and gently tapered tail that trails off to nothing.

The "pointy" bow of a ship is great for cutting through waves where the
density varies greatly (like the difference between air and water) every few
seconds.  But deep down, where it is always submerged is the inevitable
hemispherical "bulb".  In naval ships, the bulb houses sonar equipment...
gets it out in front in undisturbed and quiet water.  In merchant ships it's
just another extension of a ballast or cargo tank.  But hydrodynamically
they do the same thing:  Allow the water to flow streamline itself into the
least drag streamlines for any given speed.  Really helped out the fuel
economy of the big tankers when they added the bulbous bow!

The submerged sterns of all subs and ships taper off gradually to leave
minimum wake vortexes (i.e. wasted energy).  I gotta believe that until we
get into very high velocity regimes, streamlining of aircraft has a lot of
analogies to displacement hulls in water.

TCK

P.S.  One of the first things that every submarine sailor is taught is that
on a submarine, the pointy end is aft and the round end is forward.  Kinda
opposite of what their surface type air-burning counter parts believe.

In aircraft the fastest piston engine airplane in the world
is the F8 Bear Cat. Rare Bear. Very blimpy. Also the P47 Jug was no slouch
in the speed department either.

Paul Lamar

The speed of the radial engine fighters of WWII also prove the effectiveness of External Diffusion, as Bill Freeman was so kind to teach us last year.  It is interesting to look at the aerodynamics of the Reno racers for clues on how to go faster.  I will be giving this subject some real study when I get further along on my RV-4.  At this time I am giving some thought to using a fastback much like the Harmon Rocket uses.

Ken Powell

 
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