Dramatic drag comparison
 

The following is an anatomical drag comparison between a circular cylinder section and a NACA laminar wing section.
The two bodies are shown in true size relationship to one another.
The laminar wing section is 167-X longer than the cylinder.
They have identical drag.
http://i1271.photobucket.com/albums/...titled5_14.jpg
(we don't need no stinkin' streamlining):p
PS, the table is from 'Boundary Layer Theory,' by Hermann Schlicting,7th-Edition

Last two digits for NACA airfoil designations indicate maximum thickness as a percentage of chord.

21% of 167 is 35.1.

So this NACA airfoil has the same drag as the cylinder in spite of having a 35 times larger frontal area.

Change that circle to a square and see how much smaller it'd have to be.

Either way, good lesson.

Just need to quote this next time the "Can I increase frontal area AND reduce drag?" question comes up.

Now I understand, thanks aerohead. CD and frontal area much more dependent on cd, especially when it gets low enough.

regards
mech

Clarify for me, by "drag" do we mean CdA and not just Cd? The airfoil has identical CdA? Seems increadible. That's not the case right? This is just identical Cd, yes?

Drag is the force pushing back on the body as it moves through a fluid. it is directly proportional to CdA as long as fluid density and velocity are held constant.

Exactly! My travel trailer build will have a much lower CDa because of this. More can be less, much less :)

That's stunning.

I remember seeing add-on fairings to make the cables on one's ultralight aircraft less of a drag, but they didn't significantly increase the frontal area, they weren't much more than stiff tapes to add to the cable.

This shows that one could replace the cables with, for instance, spars and still have no net gain while picking up lots of strength.

A little reality check on the math though....

35 times the frontal area? That means it has 1/35 the drag coefficient, right?

1/35 = .029

IF the coefficient for the round shape was 1 (it must be less, but follow along), the coefficient for the airfoil is .029? Seems too low, even for a section of a chord? Someone has some numbers, right?

It must?

Aerohead would have to give any exact numbers from his source... But Cd of an infinitely long cylinder moving at low speeds through air is typically given as 1.2. 1/35th of that is .034. That's in the right realm for a streamlined body.

Compare this to the old video Darcane posted in this link:
http://ecomodder.com/forum/showthrea...rag-25378.html

From that, at 5:49:
Ten times. That is quite a bit less than the NACA wing shape, but the wing shape used there did not have the concave tail section.
Would that alone make for such a big difference?

cables
 

Here's a selection of shapes for fairing circular sections
http://i1271.photobucket.com/albums/...d2/8-10-13.jpg

square
 

According to Hoerner,a square section of infinite length,crosswise in the flow would have Cd 0.20.
Korff has data on circular sections,I'll have to get that from home.

wing shape
 

Hoerner chose a laminar wing section for the comparison.It's entire boundary layer is laminar under flight conditions,leading to very low surface friction drag.

clarify
 

These shapes are in 2-dimensional flow and their drag is based upon wetted area.
We'd use the data for wings,struts,tails,rudder,landing gear,pylons,bracing wires,some forms of auxiliary tanks.

numbers
 

When the chord length of the airfoil section equals 167X the diameter of the wire,the drags are identical.
Bear in mind that we might be considering a length of crude circular wing support on an ultralight aircraft to a length of wing on a Lear Jet.
It's 2-dimensional flow,so not real helpful for automotive applications,however it does illustrate potentialities with respect to streamlining.

realm
 

Perhaps Abbott and von Doenhoff lists the drag table for this particular airfoil section.
Walter Korff offered a drag value for structural sections as a function of a hundred feet of length at 100-mph.We can probably reverse-engineer some values from that.

I already have a wing shaped sleeve over my car's antenna stalk, but it is a bit out of shape.
I'll use this as a template to craft a new one from hard foam and thick alu foil.

Maybe some fairings for suspension members (at least what can be covered by a belly pan?

Apparently careful design has allowed the much thicker concave airfoil to maintain the same drag as the thinner airfoil in the video which was not necessarily chosen for max efficient width.
.
Can you imagine the extreme efficiency of an electric Vetter style motorcycle with that pure airfoil top view. Even in the current form of the Vetter fairing which has been made to universal and practical application the gas bikes are over 150 mpg in mixed riding and Hershner's electric streamliner is pushing 300 mpgE.

Thanks again for your post. you reminded me my truck has two aftermarket antennas in addition to the standard antenna.
I'd like to keep the CB radio, but the one on the roof is no longer in use and of course is the hardest to remove.
I can make some sleeves though. :thumbup:

For a wire antenna mounted at the front of the roof and angled backward, the airflow would see an ovoid shape. The airflow near the base of the antenna would also have an upward component that would accentuate that effect, although the upper part of the antenna would likely be in free (horizontal) airflow.

motorcycle
 

We'd never be able to use a laminar section on a motorcycle.They're only for 'flight' conditions,tens of thousands of feet above ground level where there is zero atmospheric turbulence.
The Earth has a turbulent boundary layer which can be measured in kilometers of thickness.Any 'laminar' airfoil operating at ground proximity would be subjected to this ambient turbulence and immediately transition to a full turbulent boundary layer,destroying its potential for laminar BL performance.
Craig Vetter's fairing design is about as low drag as we can expect in the real world.

drag of circular cylinder sections
 

*subcritical Reynolds number drag= Cd 1.2
*supercritical Reynolds number drag = Cd 0.40
The data is cited in Hoerner's book "AERODYNAMIC DRAG",and the actual research is from the German DVL,1934
*With a "optima" fairing (not shown),the drag is as low as:
^ Cd 0.35 subcritical
^ Cd 0.05 supercritical

Which brings me to another question. Which streamlined shape would be the best compromise for efficiency assuming frequent off axis wind vectors in the real world? A standard NACA airfoil? Or is there another formula that would be better in rough and off axis air?

Since all your compromise is with functionality, it may be hard to reduce to a formula. Such formula would need a term for 'the front wheel should have open spokes to reduce cross-wind reactions'.

I propose a squircle.
http://ecomodder.com/forum/member-fr...perellipse.jpg
Except unrotated, so the body is narrow at your head and the road surface, and widest through your knees and shoulders.

The front view is not in question and normally turns out to be an oval with a flat truncation under the middle of the bike. But doesn't even have to be vertically symetrical in any way. The top view is the critical consideration for a two wheeled streamliner since most of the air goes around the sides. Unlike a car which has more air going over the top.

At an equal ride height the flat-bottomed oval will ground strike before the circle when you lean it over. The squircle-based body could lean to 45°.

I would expect the formula to provide asymmetry in all three axes, with a 30/70 split in the top and side views. My open-wheel race car design has that, and a lowered equator.

http://ecomodder.com/forum/member-fr...11-5-38-12.png

Because it isn't a leaner, it also has a flat truncation on the bottom as you suggest. Every vertex is geometrically defined to as many decimal places as one cares to use.

best compromise
 

The lowest drag airfoil is depicted in the center image in the table below.It has a length to thickness ratio of 3.93:1,or 25.5% thickness.
http://i1271.photobucket.com/albums/...rohead2/-2.jpg
You've got to have the facility to use body English to keep the bike balanced,which means you'll have to lean 'outside' the body at times,like Craig and Allen have been doing.
http://i1271.photobucket.com/albums/...itled12_13.jpg
'Off-axis' would have a specified engineering target.There isn't a one-size-fits-all' solution.
Enclosed bikes are inherently unstable in gust and unless your willing to go to a trike,you'll have to be physically interactive with your riding environment if you're going to survive.
Unless you have a fortune to give the GALCIT folks at Cal Tech University,for design testing and engineering,shoot for zero-yaw streamlining with an eye on your ability to shift your body around.

Ignoring cross-winds, in a sit-in bike you have to veer left to initiate a rightward turn. It's kind of like drifting.

That's an interesting bike design. I'd like to see someone get honor and offer.

On my screen the overall fineness ratio is about 3:1. Exclude the high-tail stinger and the front wheel fairing and it's 2.5:1.

Back to the original topic. This also shows the insignificance of skin drag.

skin drag
 

yes,with automobiles,the surface friction contributes 9-12% of the overall aerodynamic drag,depending on body design.
The Coefficient of surface friction drag is 0.003.
On automobiles,the wetted area is estimated at 10X-frontal area,yielding Cd 0.03 average skin friction drag,based upon frontal area of the car.
If you could smooth the finish beyond that of glass it wouldn't make any difference to skin drag.
It all has to do with air viscosity and Reynolds number effects.
Since drag coefficients on bikes begin at Cd 0.9 they're absolutely dominated by separation induced pressure drag and until the bikes are streamlined,skin friction is essentially meaningless.