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Old 07-02-2008, 01:46 PM   #1 (permalink)
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Installment # 11, Part-4 Quotes

I tried this earlier and it blew up in my face.I'll try again.Little sleep,apologise if I seem cranky.

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The following are from the pioneers of aerodynamics.I've tried to let them speak for themselves.Those new to aerodynamics may gain insight from their words.

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1889,from, Otto Lilianthal's "Birdflight as The Basis of Aviation"

"Contrary to expectations,no disadvantage accrued (for wings) even from placing the thickening right at the leading edge,...it even appeared as though this shape possessed specially favorable properties as regards air pressure,namely,...very little retarding pressure,...but only when the thickening was in the front and not at the trailing edge."

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1910 from Alexander G. Eiffel ( Eiffel Tower designer-turned aerodynamacist )

"A body formed by a hemisphere,placed on the base of a 20-degree cone: With the point forward,the resistance is nearly that for a sphere.With the point to the rear,the resistance is reduced 1/2,and is approximately 1/12 of the resistance of a disc of (same) diameter."

This is the first citing I've found,where the effects of boat-tailing is actually isolated.Eiffel's "ice-cream-cone" has a rudimentary "classic" teardrop form.

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1935 from Assistant Professor,Karl D.Wood,Cornell University,

"The friction and eddy drag depend on the shape...cross section ( or profile ) and are called profile drag."

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1937 from Dr.Robert L. Daugherty,Caltech University,and Dr.Joseph B.Franzini,Stanford University,

"...the object of streamlining a body is to move the point of separation as far back as possible and thus to produce the minimum size of turbulent wake.This decreases the pressure drag,but by making the body longer so as to promote a gradual increase in pressure,the friction is increased.The optimum amount of streamlining,then,is that for whichthe sum of the friction and pressure drag is a minimum.....attention in streamlining must be given to the rear end,or downstream part,of a body as well as to the front....for streamlining against subsonic flow a rounded nose and a long tapered aftbody generally result in minimum drag."

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1946 from Hunter Rouse,Professor Emeritus,University of Iowa,

"...the process of streamlining -- i.e.,the easement of boundary curvature at the sides and rear--is entirely for the purpose of reducing the form effect upon the total drag.While the ideal limit of streamlining would involve the complete elimination of separation,it should be noted that the surface drag necessarily increases as a body is lengthened;in other words,Cd attains its smallest value when form drag and surface drag together are a minimum."

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1958 from L.M.Milne-Thomsom,author,Theoretical Aerodynamics,

"The problem of "streamlining" is to devise shapes such that the boundary layer will not breakaway and the wake will remain inconsiderable.(T)o delay the breaking away....the region where the fluid is moving against increasing pressure should curve as gradually as possible.......Good streamline shapes should be such that the breaking away point is as near as possible to the trailing edge."

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1963 from Kelly and Holcombe,General Motors Research Labs,

"Form drag is a function of basic body shape as created by the designer and influenced by the body engineer.Body shapes that minimize positive aerodynamic forces or pressure on the front of the vehicle and minimize negative forces or suction on the rear of the vehicle will exhibit low form drag.The tear drop or "streamline" shape suggested by this obviously does not fulfill the many requirements of passenger accomodation and salable appearance."

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1963 from Walter H.Korff,Lockheed Aircraft Corporation,

"....important to...streamlined automobile(s)....keeping the length reasonable for parking (is)(b)ob-tailing (which) affect (induced drag) if the "bobbing" occurs shortly after the start of the downward curve of the upper surface or top.Since the cut-off increases ...pressure drag slightly,and reduces the induced drag (lift),an optimum cut-off point can be determined for an automobile at which the two effects cancel each other.(T)his blunt ...aft portion will have no more drag than a full length streamlined body that curves down to the belly pan."

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1963 from H.Schmude,General Motors Corporation,

" If we were to design the cars 100 % streamlined,following all the aerodynamic principles,would not all cars look exactly alike and the designers have nothing to do?"

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1981 from Rich Taylor,Popular Mechanics Magazine,

" It is possible to build a car,complete with protruding tires and functioning air intakes,which approaches the teardrop in shape and has a Cd of around .10.But it is practical only at the Bonneville Salt Flats,where a car doesn't have to parallel park or go up a driveway."

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August,1982, from Alex S.Tremulis

"I am projecting Cx (Cd) values as low as 0.12.This is not unreasonable as I believe boundary control will be the key to avoiding premature boundary air layer separation."

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1997 from Anjelo Mojetta,author,SHARKS history and biology of the lords of the sea

" When the possible ideal shapes...were analysed in a wind tunnel,the conclusion was reached that bodies offering least resistance,including those of certain fish ( the origins of this discovery date to 1800 ) have elongated drop shape,in which the largest diameter is situated about a third of the way along,while the remaining two-thirds of the body tends to taper towards the rear."

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That's enough! It's not all but it should give you a sense about drag reduction.Summing up,get to your frontal area by about the first one-third of body length,then use the rest of the body to let the air ramp down to atmospheric pressure, without breaking away into turbulence.

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A parting thought.Where the minimum for form drag and skin friction drag occur,is with a teardrop form which is 2 1/2 times as long as it is wide.On the ground,and if pushed into soft mud,such that only the upper portion were still exposed,this form would describe the form of least drag.If you use this shape as a template,any vehicle which fits" under" this shape without "violating" it's boundaries,will have minimum drag.Also,when viewed from above,when the sides of the car also curve in at the back within the boundaries of the teardrop,you complete the picture of low drag.Everything between here and there is a function of practical limitations,or very clever engineering solutions.

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