streamliner designs

[Blue Foam]

I realize that nobody knows me here, so I expect some disagreement with the following:

First, below the velocity at which we encounter compressability (~500 mph at Bonneville) the following factors, in order, are the source of drag of LSR vehicles that I have seen:
1. Separation
2. Wetted area
3. Laminar flow percentage

Separation is easy to spot once someone has made the conceptual leap that it exists.  Look at any vehicle from the back and throw water at it. If it hits anything but a highly swept surface, there is separation.  There are other sources, namely abrupt pressure recovery areas.  We have to have wheels, but the treatment of the rear face of the tires is the largest source of curable separation.  Second is the aft bumper - chute area.  The rule:  aft facing blunt areas are the largest drag contributor.  This is well understood in aerospace, but only the best LSR vehicles have covered their parachutes with proper fairings.  Stagnation (another form of separated flow) occurs on all of those forward facing abrupt transitions, like canopies.  Make the canopy part of the aerodynamic shape of the vehicle, not an add-on.

Wetted area is next.  Size matters, smaller is better.  This begs the question that the original poster had of fineness ratio (length over cross section).  The lowest drag form with no protuberences (wheels) has a fineness ratio of about 2.5 (a football).  A 2.5 body has drag of less than 20% of a vehicle with a fineness ratio of 9 (typical LSR) with identical volume.  Some of that comes from more laminar percentage, most of it from a reduction in wetted area.  As a consequence, a low fineness ratio vehicle with more frontal area and less wetted area will have less drag than a high fineness ratio vehicle with more wetted area.  The low fineness ratio will also allow more volume for less drag.  Frontal area has no, repeat no, direct relation to drag.

Read that twice. 

The relation of "frontal area" to drag died in aerospace science about 46 years ago with the Navy's towed mine detector experiments.

Finally, laminar run can reduce drag by significant amounts.  It requires a positive pressure gradient to the transition point and TOTAL smoothness.  Canopy joints, wheels, pesky access panels (no matter how well sealed) trip the boundary layer to turbulent.  For LSR, it's far easier to fix the separation and the fineness ratio.

Bottom line, minimizing separation, minimum wetted area, and a fineness ratio of 4 to 5 should yield the lowest drag.  If a tail is needed for stability, put it on a boom like a proper tail and leave the body fineness ratio blunt.  Supersonic has a completely different set of rules.

[sockjohn]

  Frontal area has no, repeat no, direct relation to drag.

I believe you meant to say no direct relation to coefficient of drag. 

If not, you're wrong unless they rewrote all the physics books. 

Believe it or not, it is often easier to make a large vehicle with low cD than a small vehicle. 

[tortoise]

  Frontal area has no, repeat no, direct relation to drag.

I believe you meant to say no direct relation to coefficient of drag.

I don't think so. The point being made is that to enclose a given volume of stuff, the lowest drag form is not the longest and narrowest, but a fatter one with less surface area. The problem with this, of course, is that the short wheelbase is liable not to be very stable-handling. The Nebulous Theorem cars finesse this problem cleverly by being essentially two fatter bodies, one at each end of the car, connected by a real skinny, low surface area tube. This principle could be carried quite a bit further.

[sockjohn]

  Frontal area has no, repeat no, direct relation to drag.

I believe you meant to say no direct relation to coefficient of drag.

I don't think so. The point being made is that to enclose a given volume of stuff, the lowest drag form is not the longest and narrowest, but a fatter one with less surface area. The problem with this, of course, is that the short wheelbase is liable not to be very stable-handling. The Nebulous Theorem cars finesse this problem cleverly by being essentially two fatter bodies, one at each end of the car, connected by a real skinny, low surface area tube. This principle could be carried quite a bit further.

Maybe I'm too dense to get what he's saying, but every physics book and aerodynamics book clearly states the power to overcome drag is proportional to (frontal area)*(coefficient of drag)*(velocity cubed).  If this weren't true, there would be no sense in cramming your body in an awkward narrow confined space. 

The evolution has been from short and fat to long and skinny, and until I see the trend reverse highly doubt the claims.  A pair of rather famous Indians seems to prove my point   Note I'm not saying that you have to have a long vehicle to have a low cD or low drag, but it sure is difficult to have low overall drag with large frontal area.

One nice thing about long vehicles as opposed to short and stubby is that it's easier to separate the engine from the driver compartment with a good firewall, as opposed to trying to share the same space. 

The human powered vehicle (HPV) Mango is a really good example minimal surface area, but they had to rework the front steering to make it have even ok handling from what I understand. At least it had minimal cross section area to get blown around in the wind.

Scroll down to the Mango bike, at 6 foot tall I'm amazed a person can squeeze in this, much less turn their legs to pedal
http://www.recumbents.com/WISIL/whpsc2002/resultstuesday.htm

http://www.recumbents.com/WISIL/whpsc2002/photos_tues/whpsc2002-demo-mango.jpg
Good sense of the size here:
http://www.recumbents.com/WISIL/whpsc2002/photos_tues/whpsc2002-demo-mango2.jpg

The yellow HPV in back is the Barracuda, an older but very successful HPV, the blue HPV in front is a camera bike, no canopy to see thru (camera is in the white tail fin).
http://www.recumbents.com/WISIL/whpsc2002/photos_tues/whpsc2002-demo1.jpg

[tortoise]

Maybe I'm too dense to get what he's saying, but every physics book and aerodynamics book clearly states the power to overcome drag is proportional to (frontal area)*(coefficient of drag)*(velocity cubed).  If this weren't true, there would be no sense in cramming your body in an awkward narrow confined space.

But sometimes you can decrease cD by 11% by increasing frontal area by 10%.  Thanks for the HPV link.

[Rex Schimmer]

Blue Foam,
Everything you have said is correct for the condition of complete laminar flow over the entire streamline body. If you have this condition the main contributor to drag is the friction between the streamline bodies skin and the fluid and also the pressure drag generated by the fact the the boundary layer is thickening as you go toward the rear of the streamlined body and as the streamlines combine at the end of the body there is a region of low pressure that is the thickness of the combined boundary layers. So in this case, complete laminar flow, shortening the length of the body will reduce the total drag by reducing the surface area, "wetted area" of the body and by reducing the depth of the boundary layer at  the rear of the body thus reducing the generated pressure drag and a decrease in the frontal area will have almost no affect as long as the streamline shape has complete laminar flow front to rear. For bodies in high density fluids, i.e. water laminar flow can be maintained at relatively high speeds because the fluid density is high which make the Reynolds number fairly low.

BUT!!! ALL OF THIS DOES NOT APPLY TO LANDSPEED RACING CARS!!!
BECAUSE: LANDSPEED RACING CARS DO NOT HAVE LAMINAR FLOW!!! All of them are operating at Reynolds numbers that are in the multiple millions! At the speeds that they are running actual laminar flow probably only exist on the very front few inches of the body! The rest is turbulent flow! and if we are good at body design it will be attached, and with turbulent flow the amount of drag generated by skin friction on the "wetted area" is actually reduced and the major contributor to drag is the ability of the body to maintain attachment of this turbulent flow. Any place that it becomes unattached pressure drag will be generated.

Rex

[JackD]

" Theoretical projects produce theoretical results."

NEXT ?

[John Nimphius]

Rex

I've been reading what you guys have been saying long enough that I think even I'm beginning to understand it.

John N

[Stan Back]

I don't.  Probably good we got a Street Roadster.

[Robin UK]

It's also worth taking a long hard look at the underbody design of the JCB car and reading what they found during their CFD research. You can and should do all you can to reduce profile and skin friction drag but it all needs to considered in conjunction with rolling drag. JCB found that 60% of their aero drag was skin, 40% profile drag. With a design that was essentially zeroed in terms of downforce (to make sure it didn't fly but also didn't absorb power overcoming downforce) attention turned to rolling drag. What they found was that rolling resistance was in fact higher than their overall aero drag! So you can have the cleanest body shape in the world and still not perform as you might expect. Part of that rolling resistance is the friction of tyres on salt (which you need for drive and control) but a significant part of it turned out to be something that Ron calls 'spray drag' - designers of record breaking boats know all about this. Essentially it's the mixture of salt, dust and disturbed air under the car that increases drag and also affects the ability of the tyres to get a good clean grip with minimal drag from the surface. The air flowing under the car accounted for about half the total aero drag as well as trying to control the spray drag. The carefully sculpted underside and slightly raised nose were all designed to minimise this affect. As Ron says, the photographers are interested in the top view but the real technical challenge is on the underside!  After the successful runs he was the one poking around underneath, delighted to find that it was as clean as a whistle.

Robin  

[tortoise]

JCB found that 60% of their aero drag was skin, 40% profile drag.

. . . with turbulent flow the amount of drag generated by skin friction on the "wetted area" is actually reduced and the major contributor to drag is the ability of the body to maintain attachment of this turbulent flow.

Am I misreading Rex or does Robin's statement contradict his?

[JackD]

Lets start with Carl's truck.
Why was it so fast ?

[PorkPie]

Lets start with Carl's truck.
Why was it so fast ?

Power, weight, round on the front and clean rear end (last version)

[JackD]

Lets start with Carl's truck.
Why was it so fast ?

Power, weight, round on the front and clean rear end (last version)

YUP!

[Rex Schimmer]

What Robin said regarding the 60/40 split of the total drag on the JCB car is a statement of the aero efficiency of the JCB car, i.e. the body shape maintains attached flow over most of its length therefore the only drag is the friction drag of the turbulent air going over the skin of the car and it has very little pressure drag (that is the drag caused by the creations of unattached turbulent air, vorticies, behind the car which are at high velocity and therefore low pressure) So yes on a very "aero" car skin friction can be the major component of the total vehicle drag. Viscous friction between the skin and the air is reduced by attached turbulent air flow BUT it is not eliminated.


To go fast at B'ville is done by either having a car like Stan's roadster and applying lots of good old American horse power or by trying to "cheat" the wind and build a more streamlined car. Of course the really fast guys, do both and are the guys hold the records.

Rex