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Originally Posted by JulianEdgar
Honestly, there is a lot of misinformation discussed here (and of course some excellent information as well). I sometimes think that people don't even stop to think for a moment before accepting advice / statements that have been made. Then they apparently become gospel - weird.
- Streamlined bodies don't develop lift? What, then, is an aircraft wing?
- Airflow over the curved upper surfaces of a car doesn't develop lift? Really, when you can see in any pressure measurement diagram of a car all that top surface lift?
- Percentage drag reductions can be precisely quantified as fuel economy gains? And that's apparently irrespective of the car?
- Kamm tails (as opposed to the use of simple separation edges) are somehow great for drag? Really? What's an example, and don't give me those K cars from the 1930s that now measure at 0.37!
- There is a perfect streamline template shape? And this is irrespective of the thickness of the boundary layer (which will be car-dependent)?
- Box cavities are a panacea? And that despite almost no mention in the technical literature?
- Coast down tests are accurate? And that's when even the experts hate them (unless they're using very sophisticated equipment).
- Full wheel covers are always best for low drag? There are specific, recent tech papers than contradict that.
It's a bit frustrating when you see so many people putting in work and enthusiasm to modify the aero of their cars when there is so much poor advice being given out.
Note that I am not throwing the baby out with the bath water - there is some excellent information here, and some very good testing (especially mileage over long periods). And that's great. But it's so frustrating reading older threads where suggestions are made, I think to myself 'that won't work' (or - often - 'the difference will be so small, forget it'), and then after a lot of time and effort, the OP says 'that didn't make any difference'.
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*Look to THEORY OF FLIGHT,by Richard Von Mises, DOVER Publications, for an introduction to the concept of the streamline body, conceived in 1907,by Frederick W. Lanchester, who was one of only two people on Earth with a true command of aerodynamics, Ludvig Prandtl being the other.
*Airfoils have nothing to do with streamline bodies.
*And again, if you will re-visit 'THEORY OF WING SECTIONS Including a Summary of Airfoil Data, Appendix-IV, by Abbott and Von Doenhoff, you'll be re-acquainted with 118-families of airfoils, all of which are zero-lift, depending on angle of attack, also addressed in Von Mises work.
*Wait until you see the pressure profile over a streamline body and it's zero-lift before you condemn them.
*Yes, if properly gear-matched, a 10% drag reduction will approximately relate to a 5% improvement in constant velocity,55-mph, highway driving (not to be confused with any EPA Highway testing protocol);for any vehicle which has undergone only a shape change.
*As to 'Kamm' tails, technically, it would be 'Koenig' tail, as Reinhard Koenig-von Fachsenfeld holds the patent for the 'K' truncated tail. Kamm is given the credit, but he's really known for his low-drag cooling system. And yes, in the context of the model studies conducted at FKFS, the K-tail might as well be considered ideal.FKFS starts with the entire streamline body, then begins lopping off sections, documenting the drag change as a function of the degree of truncation.'Verjungunsverahaltnisses'. Hucho states that this is the only path to low drag, and the premise of aerodynamic streamlining.
* The Langenburg Castle K-car is not Kamm's K-5,Cd 0.23 car, it is the K-3.It was Jerry Sloniger of ROAD & TRACK,August,1982,page 66, who tracked down the proper nomenclature of these cars. Kaselbach is incorrect in his data presentation. Kamm's car was Cd 0.23,diesel-powered,overdriven,85-hp,and top speed of 113-mph.HOT ROD Magazine featured photos of the K-5 in 1963,depicting tuft-testing.The flow is virtually ideal up until it reaches the rear radii of the tail, something not depicted in in the FKFS drag tables.
*There is a perfect streamline template shape. Hucho gave us all the critical elements for its construction in his 2nd Edition of 1987.At 1/3rd-scale,it returned Cd 0.1201 at DARKO,with compromised wheel fairings. It's a sure thing. You don't have to worry about boundary layer or anything else.It's pre-tested, at any scale you choose, up to 250-mph.
*Would you like to show me where I've ever implied that box-cavities are a panacea?
* I have only recommended that people approach CAR and DRIVER, and see if they'll conduct the coastdowns at Chrysler Proving Grounds, E.Chelsea, Michigan.They know the SAE protocols,have scientific-grade weather system,and optical fifth-wheel, as well as access to Chrysler's all-wheel scales, a dedicated test-track, where they conduct the rapid, back-to-back coastdowns, plus custom software for the data reduction.Top speed is conducted on the 8-mile oval, with only stop-watch,and no aerodynamic excrescences of any kind. They tested my CRX in 1991,afer the speed record at Bonneville. Call for current pricing and availability.
*As to the wheelcovers, your messengers have yet to re-test previously-measured static structures on a rolling-road wind tunnel, and publish the discrepancy between the two readings. I've yet t see any bona fides,prima facie evidence for the superiority of aspirated covers,except in specific cases.And curiously, when GM,Ford,or Volkswagen set out to establish land speed records, their choice of wheelcover is the MOON disc, even though they have access to virtually any 'secret-weapon' design. And to the rotating wheel supremacy issue, Dennis Semanaitis,of ROAD & TRACK,August,1982.p.35,reported:'[Rotating wheel] Tests have shown that wheels and tires of an ordinary passenger car generate added drag of 0.005 or less....The idea of a moving belt has been explored,though most believe its complication outweigh any potential improvement in realism.'
Hucho, in 1987, states,'The rotation of the wheels isusually not taken into consideration in wind tunnel testing; testing is carried out with the wheels stationary.When the wheels are integrated into the body the roration of the wheels appears to have negligible influence upon the forces and moments acting upon the vehicle (Second Edition, page 419).