Drag racer needing a little help on aero.

[Blue]

Soap box cars & guys on bicycles benefit from aerodynamic aids.
Seen any square fish lately??
  Sid.

Go to the Monterrey Bay Aquarium and look at the "Open Ocean" tank.  10,000 mile/year tuna are pretty efficient...

My favorite is the King Makaral or "Wahoo".  60 mph when hooked.  Good shape.

[Reverend Hedgash]

Yes indeedy fish are slippery but how do they get the power to the ground?

dik

[Nexxussian]

My understanding of SPlitters is they come out horizontally from the car and not only help prevent air going under the car they help a bit with front downforce as the air coming down from the dam pushes down on it prior to going around the back.

This force has to come from somewhere though and that will be your engine so as usual in aero you need to make the decision whther it is worth the power loss for the downward force benefit.

rH+

Rev, I was a semi involved spectator on a race team up here that went racing with IMSA in the GTS1 category in the mid '90s (only a few race events, never the full schedule), they were allowed a splitter of a certain maximum dimmension (I forget how much, but not a whole lot).

In researching what was best, the member of the team in charge of that came to the conclusion that the drag penalty of a splitter would be difficult to measure so long as you kept is short enough (or made it stiff enough) so that it wouldn't need brace rods to the bumper.

Put another way, the 4 - 6 brace rods had several orders of magnitude more drag than the splitter (I don't recall it being very thick).

Of course at some point I'm sure you reach the ever popular "point of diminishing returns".

As I recall, there was a limit in the rules as to how far that lower surface could extend aft past he air dam too.

I have no idea where that gentleman was getting his information, and I have lost touch with him so I can't ask him.

All I know is the car went alot better than most people expected a car to do, that came  from here. (can't do that if doesn't stick in the corners )

[Reverend Hedgash]

Yep, sounds about right.

Not many corners in landspeed though so still I think having a splitter is not the best solution and if balance is out then getting more mass up front would be a better solution and keeping the air as clean as possible. i guess the equation is whether it is better to pay the penalty by keeping as much air out from under the car, or pay the penalty under the car with the mess that is under there.

Open the air around the car neatly, don't interupt it on the way through,  and close it again after the car neatly. Yep, like a fish.

As for the rods circles aren't good and I can understand the high drag your mate mentions. There is a ten fold drag reduction by making a circle into an aerofoil shape of the same thickness. We will be doing this to the Spirit of Sunshine's axles etc. as the gain is so good. It is like having ten times less axle in the wind and that is as significant as a very significant thing.

It's all great fun this aero stuff. I love it.

rH+

[Nexxussian]

True, there aren't corners in the mix out there on the salt, and a belly tank is a bit of a special shape for a car.

For them of those that aren't running one (ie everybody else that isn't, like me), nothing says the splitter has to be maximum length, they could make one that looked more like a structural stiffener at the lower edge of the air dam.

I'd like to try one on the #559, but we have yet to catch up with all the other stuff we've had to do to the car, and have any time left.   (insert violin music here  ).

I don't want it to make downforce, but drag reduction as well as lift reduction seems an excellent idea.

As for round shapes (exposed tubes) being bad for drag, I remember reading that angling the tube back relative to the ariflow returned a disproportionatly large reduction in drag up to 45 ish degrees, of course that won't work for an axle.

How are you streamlining yours, an eliptical fairing of some sort, or just tapered piece that hooks to the back (using the axle tube for the leading edge)?

[SPARKY]

just making a rectangle out of it helps--if I remember  Mr. K wants 4x the diam

[Reverend Hedgash]

Erik, I was thinking that there are a number of manufacturer's making aerofoil shapes for the architectural industry such as these:

http://www2.hunterdouglascontract.com/en-RS/suncontrol/sunlouvres/aerofins/aerofoils/index.jsp?t=specification

A simple mod would make these a nice teardrop shape for example but I am sure I have seen teardrops too.

My plan was to find just such an extrusion that could fit our axle and use that. The rear one I was thinking I could get it to be movable so I could vary its angle of attack to get more downforce if traction was breaking. In this case the drag gain would directly result in more power getting to the ground so should be worth it.

If I was losing power but not breaking traction then I would angle it as straight as possible.

The advantage of these are there fairly cheap, strong, and with a finish that would be hard to reproduce in our shed.

Hot Rodding at its best!

d

[SPARKY]

"racy" sailboat masts  might be a broken piece around a rigging shop

[Reverend Hedgash]

! Excellent thinking... I'll get onto it.

d

[Reverend Hedgash]

Not quite the pure teardrop but 20' for 125$ not a bad start!

http://www.dowsar.com/hardware.htm

The internal diameter of this would be slightly too small for us though... I'll keep looking.

Sorry to hijack your thread, carry on!

Dik

[Blue]

...As for round shapes (exposed tubes) being bad for drag, I remember reading that angling the tube back relative to the ariflow returned a disproportionatly large reduction in drag up to 45 ish degrees, of course that won't work for an axle.

Look at the shape in the "chord-wise" direction.  IOW, a circular cross section turned 45 degrees is actually elliptical to the air flow.  Take a paper towel tube and cut it at 45 degrees; place it on your desk flat; trace around the axis of flow: it's an ellipse.

Now extend the trailing edge to sharp and we have an airfoil.  It's that simple.

The elliptical trailing edges of swept sections like formula car suspension arms can also create an effective sharp trailing edge from the sweep of the arm creating span-wise trailing edge flow.

We need a separate topic for this...

[Leadfoot]

Blue, could you comment on what the aero effects on the rear of
a car usually below the bumper. I 've seen ribs, plates with vents on them, and almost every car that passes  you on the freeway have some attention from the factory in this area. What gives ?

[cold85]

thanks for the help guys we did not have time to fix problem during racing season. But in off season my buddy biggun got the car about 3inches lower front and back and mounted a new one pice frontend and chin spoiler. also moved turbo out front and closed the cowl hood off to the windshield.

[Cajun Kid]

LOOKS GOOD

[SPARKY]

Updates,  let us know how it goes, did it change the handling???