Help with transportation superhighway

[aerohead]

Originally Posted by Bicycle Bob

Do you have a reference showing attached flow on such an abrupt shape?

According to "the Theory of Wing Sections" by Abbott & Von Doenhoff, the NACA 66021 has a cd of .0035, but the 66009, twice as fine, gets .003. Even the 63 series is only listed at up to 21% thick, although I think I did OK with a 64025 for a strut.

For a good example of balancing volume with frontal area, I'd look at the Zeppelins. It is unfortunate that so much data on shapes pertains to wings. The earlier NACA series 0010-35 shape, with 10% thickness and a continuous convex curve to the back edge, more like a zeppelin outline, got to .003 cd, but had trouble with pitch instability as a wing.

To modify a pure shape for running near the ground, the bottom is squished in proportion to how low it is. This produces a wing shape, but the lift can be cancelled by a bit of rake and the venturi effect underneath. The overall effect on drag is not as bad as the addition of wheel exposure. Successful LSR cars are not jacked up. One such HPV was made, but it embarrassed the builders.

What I take away from their work,is that the wing sections have really low Cds,but they've got all the induced drag associated with lift.The other thing is that with aircraft,the wetted area is equally if not more important than Cd.Lots of skin friction and much interest in laminar flow design.-------- As far as bodies of revolution,I did take another look at Mair's boattail work,and if you look at his data table,his data infers that he achieved Cd0.035 with his body of revolution,with prolate elipsoid nose,and one-diameter length boat-tail@ 22-degrees.I think this is the lowest thing I've seen for free air.Hucho has this in his book.-------- Ernie's 5-1 fineness ratio for low drag in ground effect seems to still hold true.VW has found it to be the "sweet-spot",also Mercedes-Benz,with their record-car projects.Mair's body in ground-effect according to Hucho's relationship,would come in around Cd0.70,and with ther addition of wheels,would final out in Cd0.12 territory.With extreme attention to the under-carriage,they might shave it to Cd 0.10,as Honda achieved with Dream-2.

[Bicycle Bob]

Creating induced drag is the purpose of wings, with as little form drag and surface drag as possible. So, if you align a wing with the airflow, to eliminate induced drag (as in the Vomit Comet going over the top to produce weightlessness) you have a pretty well streamlined shape, with low enough form drag that the unavoidable surface drag is a significant part of the whole.

[aerohead]

Originally Posted by Bicycle Bob

Creating induced drag is the purpose of wings, with as little form drag and surface drag as possible. So, if you align a wing with the airflow, to eliminate induced drag (as in the Vomit Comet going over the top to produce weightlessness) you have a pretty well streamlined shape, with low enough form drag that the unavoidable surface drag is a significant part of the whole.

Bicycle Bob,when I posted last,I was shooting from the hip without my references with me,as is common for me.I got into Theory Of Wing Sections and found your airfoils.There was also a NACA 0010-35 that had Cd 0.035 @ a couple degrees each side of 0.At some time one of the pop sci magazines did an article about modern airliners and tossed around numbers as low as 0.002.-------------- In Schlichting's Boundary Layer Theory,he was big on NACA 63(4)-021,as it is a laminar flow wing,but it doesn't score numbers as low as the 66 series airfoils.--------------- Where I really missed the boat,is with Mair's numbers.I enlarged his graph and added resolution to the data points,and when extrapolating out to a full boattail,his body of revolution scores on the order of Cd 0.0225.---------- This conflicts with Hucho's portrayal at Cd 0.04.Perhaps the art reproduction in the book is off.I don't know.---------------- Going back to Aerodynamic Drag by Hoerner,the lowest drag he publishes,is for the ideal teardrop,at Cd0.04,which might take form as an airship or wingtip tank.A basic fuselage scored Cd 0.06.A 12% fuselage got Cd 0.066.15% fuselage got Cd 0.05.Gee Bee R-1 I estimate at Cd 0.044.37% teardrop Cd 0.04.Mair body of revolution with prolate ellipsoid nose an full 22-degree boattail an 39.6% dia/length = Cd 0.0225.---------- The best float I could find was Cd 0.056.Flying boat hull Cd 0.063.Torpedo Cd 0.35.50-kg bomb Cd 0.11.------------ The wing sections are slick,but its two-dimensional flow and packaging kinda weird.--------- I don't know what to make of Mair's form(secret-sauce streamlining).--------- If we take Cd0.04 as the minimum for three-dimensional flow in free-flight,at a fineness ratio of around 2.5 to 1 we get 5-to-1 due to ground-effect mirroring and Cd 0.08.Adding wheels,Cd jumps to 0.13.Fairing in the wheels and we get back to Cd 0.10.the theoretical minimum.I don't know if Ernie can break Cd 0.10.If you introduce any angle of attack,then induced drag eats into overall drag,and at 200-mph,which I think was Ernie's target,would stability not be a problem if lift is varying as the square of the velocity?

[Bicycle Bob]

The wing sections were selected for types that had a fairly consistent center of pressure at varying angles of attack, to reduce the need for compensating forces from the tail surfaces. When designing for streamlining, the variable lift figure (side force in a crosswind) is an unwanted by-product, but it matters less just where it is centered; it is almost inevitably too far forward. We are now seeing blowover accidents moving from hydroplanes, to Funny Cars, to Le Mans, and side winds have always been a problem for streamlined HPVs. Since better streamlining almost always produces more potential for lift, I think it behooves us to move the masses forward as much as possible, and consider tail fins to restore directional stability. If you hit a patch of wet glare ice in a strong crosswind, you should get shoved over, not spun around.
The silver lining is that a crosswind can actually reduce drag. Someday, we might see rear-wheel steering to let cars angle into the wind like sailboats.

[aerohead]

Originally Posted by Bicycle Bob

The wing sections were selected for types that had a fairly consistent center of pressure at varying angles of attack, to reduce the need for compensating forces from the tail surfaces. When designing for streamlining, the variable lift figure (side force in a crosswind) is an unwanted by-product, but it matters less just where it is centered; it is almost inevitably too far forward. We are now seeing blowover accidents moving from hydroplanes, to Funny Cars, to Le Mans, and side winds have always been a problem for streamlined HPVs. Since better streamlining almost always produces more potential for lift, I think it behooves us to move the masses forward as much as possible, and consider tail fins to restore directional stability. If you hit a patch of wet glare ice in a strong crosswind, you should get shoved over, not spun around.
The silver lining is that a crosswind can actually reduce drag. Someday, we might see rear-wheel steering to let cars angle into the wind like sailboats.

Yes,everyone seems to emphasize keeping the CG ahead of CP,and if you have to add some rear fin,to do it.Bonneville closes the course at around 7-mph crosswind.I think it's the same at El Mirage.Battle Montain in '05 had some wind we had to wait on.Varna is better up than down!

[nwbabybronco]

"Ecotopia" by Ernest Callenbach is worth your time. Bicycle Bob has similar ideas. See also Curitiba, Brazil.

[Ernie Rogers]

Originally Posted by Bicycle Bob

<SNIP> The silver lining is that a crosswind can actually reduce drag. Someday, we might see rear-wheel steering to let cars angle into the wind like sailboats.

Bob, you are a prophet!

Isn't this the thread I started about a super-efficient car? It was postulated to be attached to a rail. (Or rails.) Cross-wind lift is probably okay and should augment propulsion. (The lift vector has a forward component.)

This minimum drag issue is of such fundamental importance that I think somebody has to help us get it settled. It seems to me that a CFD program is the best way to find the answer. And, based on what we have seen here so far, a good study (though brief) should be suitable for publication in a journal. Any takers? Should be good for a master's thesis, don't you think?

Ernie Rogers

[aerohead]

Originally Posted by Ernie Rogers

Bob, you are a prophet!

Isn't this the thread I started about a super-efficient car? It was postulated to be attached to a rail. (Or rails.) Cross-wind lift is probably okay and should augment propulsion. (The lift vector has a forward component.)

This minimum drag issue is of such fundamental importance that I think somebody has to help us get it settled. It seems to me that a CFD program is the best way to find the answer. And, based on what we have seen here so far, a good study (though brief) should be suitable for publication in a journal. Any takers? Should be good for a master's thesis, don't you think?

Ernie Rogers

Ernie,a number of months ago,a member posted that MIT had solved the Navier Stokes equations for
the 3-dimensional coordinate matrix.If true,CFD would be able to predict actual 3-dimensional flow for any real form.I don't know if anything like this would exist outside the academic arena,however,it sounds like the kind of tool you might use to model your architecture.------ Wind spectra within an urban environment might play havoc with the dynamic range of results but short of building a scale model and subjecting to empirical testing,I can't imagine an alternative to the 3-d CFD.Perhaps you could apply for a grant to fund a graduate study.Let us know how things shake out and good luck!