Teardrop calculation

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David Leikvold
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300

Post by David Leikvold »

Despite the Cd numbers, it went over 300mph with a flathead. For the most part it is very narrow and they have improved the shape over the rear wheels since they finished with the flathead. Now it runs a Dodge 4 cylinder and I think it went about 350mph this year. Still a beautiful shape and proves there's more than one way to skin a cat. And when viewed from the front it is very much wedge shaped which must help keep it straight and stop it swapping ends.
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nitro-nige
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Post by nitro-nige »

I think flatfire was an entirely different car.
Image

Makes sense to be tall and narrow for stabilty.
Hard to be 100% sure whether its aero, horsepower or good luck that makes any particular car fast.
David Leikvold
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Post by David Leikvold »

I think you might be right, the front is almost, but not exactly the same but everything behind the canopy is completely different. Does anyone know for sure? Still is a very nice car in either version. Engine bay shots of Flatfire on landracing.com show beautiful workmanship.
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Dr Goggles
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Location: Right behind you Chief !

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Post by Dr Goggles »

this is copied from landracing.com ( from this thread http://www.landracing.com/forum/index.p ... 821.0.html ), it says it all pretty well.....Howard Nafzger is a clear thinker........

I might as well throw in some thoughts. I have had good success at designing and building streamliners for the salt and I used a few basic principles for their design. Sorry if I repeat some things that have already been said.
1 Minimize the frontal area
2. Punch a hole in the air once and then leave the air alone. All the swoopy humps and bumps may fit you image of what is should look like but they create turbulence which is drag in disguise.
3. Make the nose nice and smoothly rounded like the front of a Boeing 747 for example.
4. A gradually tapered rear (top view) allows the flow to remain attached to the body (laminar) rather than going turbulent and draggy.
5. Bring the bottom of the body straight back from the belly pan. The kick-up in the rear creates a lot of turbulence(drag) and slows the car down. The cars with the big rooster tails are turbulent in the rear. A proper streamliner should have a rooster tail that is no higher than the top of the car.
6. A body length longer than necessary to get the proper aerodynamic shape and to include the required equipment does nothing but increase skin friction drag.
7. don't go overboard on a lot of nose sticking out in front of the front axle. It amplifies crosswind problems.
8. Remember that it takes more horsepower to get out of the air than it does to punch a proper hole in it.

Opinions will vary but these basic principles have worked well for me. Check out Charles Nearburgs performance as an example. I designed and built that car. Caveat: This is only one way and others may disagree.

Howard Nafzger


here's some pics of Howard's first liner which I think is now owned by Hawkwind's mate( :lol: ) Kent Riches the proprietor of Airtech Fairings.
(thanks Sum for the great site)

http://purplesagetradingpost.com/sumner ... ures2.html

check out the chute placement on the back of his car( once again , thanks Sum for the pic)
Image
...few understand what I'm trying to do , but they vastly outnumber those who understand why..
Rob
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Post by Rob »

Learned something interesting today whilst playing with Internetscooter's Java based airfoil analysis link. Perhaps answered my own question earlier regarding tank shapes deviating from the perfect teardop.

I plotted an airfoil that aproximated a P-38 bellytank and the calculated cd was 0.0179. When I lengthened the tank by a further 2 feet that figure dropped to 0.0163 or around a 9% decrease. Same frontal area but a lower drag coefficient.

Obviously there's an increase in drag fom the additional skin area but I have nothing to calculate that.

Thought it was worth sharing.
Rob
I owe, I owe, so off to work I go.
David Leikvold
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veeery interesting!

Post by David Leikvold »

Rob, when you extended the shape by two feet was that a parallel cylinder or did you increase the diameter of the cross section so you could plot some sort of slightly curved shape? The Broughans did the parallel thing and their car is no slouch so maybe it doesn't matter so much at the speeds we do. So did Lucky Keiser with his Sabre tank bike, which is mostly parallel tube. And if the Cd drops, so much the better! The next enhancements would be to have the windscreen/canopy shape as close to the teardrop shape as possible. I'd also make the canopy as low as possible so that your nose was about level with the tank body, that way you could still see out of it but wouldn't be wasting frontal area. I particularly like the air inlet on the new So-Cal speed shop lakester, it looks like it takes engine air from what might still be a high pressure area.

Doc / Dik, that number 5 on the list surprised me until I remembered Jack Costella leaves the underside of the tail flat. The JCB Dieselmax has a raised floor tail and has been wind tunnel tested to death. There must be more to it. Yours is interesting at the back also, is the flat floor shape on yours a teardrop and is that why you extended the underside of the tail?
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Rob
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Post by Rob »

G'day David,

What I did was extend the chord by 24" whilst maintaining a maximum depth of 3' so the shape was preserved. I guess I could sub in a parralel section but believe (as against knowing) that would hurt laminar flow and induce drag.

It's also interesting looking at the flows for the various shapes.

Have you read the Dieselmax book, it's an interesting read as are the accompanying photos?

Cheers,
Rob
I owe, I owe, so off to work I go.
David Leikvold
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Post by David Leikvold »

No, I haven't read the Dieselmax book, I should have a look around for it. Drysdales is nearby and they've got lots of good stuff. I read the Thrust book which covered the whole story up to and including SSC. It started with his first jet car, which was very ordinary, then the second car, Thrust, the gold one with one engine and finally covered Thrust SSC. It was a very good read. For no scientific reason I agree with your thought that curved is better than parallel, it just makes sense. I think the airflow would more likely stay laminar over a smoother shape. They only thing I'd also consider is making it oval in cross section (taller not wider) at its widest point just to save some frontal area.
Good, Fast, Cheap, pick any two!
David Leikvold
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Post by David Leikvold »

Oh duh! Any longitudinal slice through a teardrop will give a teardrop shape flat section. I'll just sneak away now before anyone notices :D
Good, Fast, Cheap, pick any two!
David Leikvold
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Department of Corrections

Post by David Leikvold »

More stuff not quite right. The JCB Dieselmax's aerodynamic design was all done by CFD because it made more sense than using a rolling road wind tunnel that wouldn't go fast enough and also having to use a scale model which would introduce errors of scale.
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gidge348
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Re: Transaxle=$$$

Post by gidge348 »

grumm441 wrote:
Rob wrote:
Dr Goggles wrote:It would work with a transaxle
Possibly, Audi, Porsche or Subaru, however, none of these items are really within the Jarman-Stewart ........
G


Just on the quiet if you are looking for a cheap strong transaxle,& if you see an old Citroen DS lying around the place have a look at the transaxle….

Only 4 speed cast iron casing REALLY short box and the same box used in the Maserati Merack, nearly bullet proof. Was popular in Lamborghini replicas behind warm small block Chevs till they went over to 5&6 speed Renaults etc.

If you find a dead Citroen the box is probably on worth $50 or so….?
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Reverend Hedgash
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Re: veeery interesting!

Post by Reverend Hedgash »

David Leikvold wrote:Doc / Dik, that number 5 on the list surprised me until I remembered Jack Costella leaves the underside of the tail flat. The JCB Dieselmax has a raised floor tail and has been wind tunnel tested to death. There must be more to it. Yours is interesting at the back also, is the flat floor shape on yours a teardrop and is that why you extended the underside of the tail?


The flat floor is indeed a teardrop shape in plan.

Our floor is rising at a few degrees to cater for the ever increasing boundary layer thickness as it moves towards the end of the car to avoid too much turbulence and drag at the bottom.

The underside of the rear of the tail is an existing condition of our tank. You must remember that our tank is upside down from what it was on the plane so this element was on the top originally. I think it would help marginally in pushing the centre of pressure towards the rear of the car like a fin would helping increase stability.

The making flat of our bottom was a holistic solution to a number of problems / opportunities:

    It enabled us to get the wheels on the centreline- important for us both visually and to get the cut lines in the right place for aero. The lower the body in relation to the wheels of course meant a smaller cross sectional area but also a lower centre of gravity, handy for reducing tipping tendency in a spin.
    Our tank is a slipper tank (attached to the end of the wing) and not a true bellytank and so had a asymmetric cutout here which we were able to clean up by cutting back to a flat bottom. (Most other bellytanks in the States use 2 tanks to make a clean shape to avoid the attachment profile problems, we only having one tank did not have that possibility.
    A big design issue was to locate our head/eyeline position in the best position so part of the chopping analysis involved how to best locate ourselves in the object as well as all the framing required. We didn't have a decent pipe bender and so mainly did straight rollcage work which works best with a straight bottom.
    The flat bottom maximised the usage of aero ideas we had at this point as mentioned above.
    Finally, the flat bottom simply looked right in our drawings of the thing and it often seems in this game if it looks right it could actually be right...


The front part of the floor is level until the break mark at the rear of the cockpit. This would mean if there was any venturi affect due to the rear as a kind of diffuser (which I doubt as the air would spill out the sides) it would operate more at the front than the rear keeping the nose down as the diffuser affect occurs where the air speeds up in the narrow section.

We kept it level here as we were trying to maximise the cockpit space as we are both over 6' tall

rH+
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momec
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Post by momec »

I've done some research myself and found my chest of drawers requires more horsepower to achieve the desired result regardless of how much social lubricant you use.
:?
Chris
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nitro-nige
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Post by nitro-nige »

I was researching Kamm back cars and found this.
Image

To me the rear end of the JCB (and a few other very fast cars) follows this to a degree.
The second example shows why it's so much harder to go fast with a production car based racer.
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Reverend Hedgash
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Post by Reverend Hedgash »

A kamm tail with vortex generators at its rear edge is apparently the ticket as the the vortex generators help stop the air curling in to the void behind the vehicle lessening turbulence. There is a sort of virtual tail created by doing this where the air behind the car is just carried along with the car rather than causing turbulence and low pressure and drag...

I was thinking of making some and putting it onto my Chev motorhome.

(Which is for sale if anyone is interesed in the beast... newish vortec chev v8, brakes all done, looking at offers around 15g...)

r H+
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