Drag, Drag and More Drag

[xcarguy]

Many of us are now running full roll cages on our Sevens, and for good reason. But just how much drag do these round tubes produce? This is an old video, but the conclusions are just as valid today as when they were first realized:

 

 

. . . . . . :rofl:

Edited August 21, 2014 by xcarguy

[DeanG]

I believe that if you clean up the aerodynamics on a 7 you no longer have a 7. You get something like this

 

http://www.ebay.com/itm/181496687932?item=181496687932&pt=US_Cars_Trucks

 

which, by the way, I would love to have in my garage.

[xcarguy]

Yep, I wouldn't mind having that to keep the Storker company.

[yellowss7]

Wow Shane that was very informative. I'm now wondering how to form an airfoil extention for the roll cage tubing. :svengo: Tom

[xcarguy]

Tom,

 

You read my very thoughts. This is what in looking at. Got my wheels turning:

 

http://www.uflyit.com/streamline_fairings.htm

[yellowss7]

That makes it pretty easy to tryout. Cost is very reasonable. Now to go out and measure my rollcage diameter. Thanks. :seeya:

[Kitcat]

Dont know how they measured it, but the guys who built my car said the addition of the cage dropped the top speed considerably.

[xcarguy]

Dont know how they measured it, but the guys who built my car said the addition of the cage dropped the top speed considerably.

 

Mike,

 

I've heard/read on several occasions that round bar is one of the worst things you can hang out in the wind. The lack of taper (teardrop shape) on the back side of the bar (when exposed to the relative wind) causes the resultant airflow to stall (in a since) on the back side of the bar. This causes a low pressure area (suction), know to us speed jockeys as drag. Here are a couple of videos that can illustrate better than I can with words. The first is a symmetrical airfoil (watch first) and the second is round tube (cylinder). On the second video, notice the vortices directly behind the trailing edge of the cylinder; that's the dreaded area of drag. Essentially, the faster we go, the more we try to suck ourselves backwards! :ack:

 

Airfoil:

 

Cylinder:

[NVP66S]

I'm not an aerodynamicist by trade, but I hang out with some, and I've done a bit of aircraft design. If you are seriously interested in drag reduction, I highly recommend getting a copy of "Fluid Dynamic Drag" by S.F. Hoerner. It's extremely well written and has lots of experimental results. And if you're a nerd, it's a fun read.

 

For ground vehicles, drag is referenced to frontal area (airplanes refer more often to wing area) and drag force = 1/2 * rho * v^2 * Cd * Area. So drag goes up with speed squared and linearly with Cd.

 

Cd for a cylinder at typical car conditions (Mach, altitude, velocity, etc) is about .9 to 1.0. This is terrible. In the real world with end effects, fairing imperfections, fasteners, etc, adding a fairing can lower Cd to 0.2 or better.

Dan

[xcarguy]

I'm not an aerodynamicist by trade, but I hang out with some, and I've done a bit of aircraft design. If you are seriously interested in drag reduction, I highly recommend getting a copy of "Fluid Dynamic Drag" by S.F. Hoerner. It's extremely well written and has lots of experimental results. And if you're a nerd, it's a fun read.

 

For ground vehicles, drag is referenced to frontal area (airplanes refer more often to wing area) and drag force = 1/2 * rho * v^2 * Cd * Area. So drag goes up with speed squared and linearly with Cd.

 

Cd for a cylinder at typical car conditions (Mach, altitude, velocity, etc) is about .9 to 1.0. This is terrible. In the real world with end effects, fairing imperfections, fasteners, etc, adding a fairing can lower Cd to 0.2 or better.

Dan

 

 

Dan,

 

I was wondering when you would chime in. That's encouraging numbers regarding wrapping round bar with fairing. A couple of us on here are considering rising to the challenge to see if we can indeed cheat the elements. :cheers:

[yellowss7]

Shane, I thought of another item that needs a fairing. The spring and shock assembly. I remember reading that the inboard suspension on Croc's car had significant impact. I wonder if we can get a fairing around the spring/shock assembly??? How are your aluminum fabrication skills???:seeya: Tom

[xcarguy]

Shane, I thought of another item that needs a fairing. The spring and shock assembly. I remember reading that the inboard suspension on Croc's car had significant impact. I wonder if we can get a fairing around the spring/shock assembly??? How are your aluminum fabrication skills???:seeya: Tom

 

Tom,

 

That is another area I've been contemplating. I have a couple of ideas, but, my fab skills have to catch up with my imagination. :jester: One idea is to cover the A-arms individually and simply leave the shock exposed. The other is to fab rounded covers that attaches to the 'lips' (fairings) on the sides of my hood and extend outward, and backward, far enough to cover the majority of the suspension components without interfering with steering and suspension travel. Yep, I need an English wheel and a planishing hammer . . . . . and I need to learn how to use both. :smash:

[xcarguy]

Here a few photos that easily illustrate the coefficient of drag for different shapes (note the difference in drag coefficients between cylinders and airfoils). Also attached is a photo of a cross section of an aircraft round wing strut (think roll bar tube) covered with streamlined tubing. Theory is that if one can streamline any portion of an exposed roll bar, then that portion of the roll bar will be more aerodynamic and produce less drag.

 

. . . . . .

[Automoda]

Now help me out here.

 

I thought the ultimate subsonic aero shape was essentially the shape of a bullet-- Blunt nose, smooth sides, then a flat backside. Some car company made a hideous prototype that looked like that and said it was essentially impossible to improve on that shape.

 

Of course, some bullets use that shape, such as a .45, while others like a .223 have a pointy nose. Boat-tail bullets are sort of a specialty round, so I always assumed they were for supersonic loads.

 

So what about the bullet shape? Judging from the stuff I've read above, it'd be among the WORST shapes.

[jlumba81]

I was looking underneath a subaru the other day and they had little gurney flaps to divert air around the rear suspension.

[NVP66S]

Now help me out here.

 

I thought the ultimate subsonic aero shape was essentially the shape of a bullet-- Blunt nose, smooth sides, then a flat backside. Some car company made a hideous prototype that looked like that and said it was essentially impossible to improve on that shape.

 

Of course, some bullets use that shape, such as a .45, while others like a .223 have a pointy nose. Boat-tail bullets are sort of a specialty round, so I always assumed they were for supersonic loads.

 

So what about the bullet shape? Judging from the stuff I've read above, it'd be among the WORST shapes.

 

Bullets are not really low subsonic drag. The blunt aft end gives them stability, as the ability to hit the target is as important as the velocity. Don't get too greedy or you will suffer in crosswinds or during cornering. Turn that nice slender shape 30 degrees and OMG! (Hey, Shane, how draggy does your plane get at 12 degrees AOA during the landing flare?) Another feature of bullets that is not analogous is that most of them are supersonic. Supersonic drag reduction doesn't use the same bag of reduction tricks.

 

A better example is to look at land speed record cars.

[xcarguy]

Pretty draggy, Dan. Pretty doggone draggy.

[subtlez28]

I remember a thread similar to this... I thought it was on here.

 

One thing I did not see mentioned is the product mentioned on the other thread. A plastic device that clips on the rear of the roll bar to create the taper.

[xcarguy]

subtle,

 

The link is in a post on the first page of this thread. This is what's on the table for consideration:

 

http://www.uflyit.com/streamline_fairings.htm

[ecarte56]

Thanks for starting another interesting thread..

 

So - my front and rear roll cage hoops have combined frontal area of 210 sq in. For 150 mph and Cd = 1 , theory says that I would have 68 lbs of drag, which would use up 28 WHP. Might be enough to effect balance too. If the center of roll cage pressure was 40 inches off the ground and 18 inches ahead of the rear wheels, cage drag would transfer 29 lbs from the front to the rear tires.