Teardrop bellytank downforce question

[Dr Goggles]

Brad, re-read Dr Goggles thread here and there is much discussion pertaining to your questions.

Rex, Reeeeeeeeex!......ReeeeeeeeeeeeeeX!!!!!!!!


man, Rex's computer must be broken or something.....the "flat bottommed keep it low" gunslinger is strutting up and down the street and Rex "keep it round and keep it up" Schimmer ain't even outa bed yet.....



If it's any consolation Brad and I have a coherent email discussion on the go already.

For the benefit of all , what sensible discussion there is about flat versus rounded car bottoms on the Australian Bellytank Thread" is all in the first few pages....2 and 3 mostly.

Cheers .

[Jack Gifford]

... stop calling it a "tanker"...

Shame on you, Brad!

[Jack Gifford]

...
Most of the early tanks had swing axles...

To me, "early" would be forties and fifties (and maybe sixties)- 'most every tank had solid axles front and rear in those days.

[Jon]

Seeing as Rex isnt around I will say "Keep it round and up in the air", I'm a bike guy though.........

As Dr Goggles said being round and fairly symetrical the "lift" produced will be pretty even in all directionsat 90 degrees to the surface; Up, Down and Sideways will all be fairly equal other than a small ground effects influence.

cheers
jon

[Tman]

I am in the get it on the ground camp

[Nortonist 592]

I wonder did Burke and Xydias contemplate lift and other forces before they built their tanks?

[Rex Schimmer]

Well I finally got out of bed, thanks for the wake up call Dr. G, and can only comment on this subject based upon my old friend, Daniel Bernoulli, who's law , when interpreted for this situation, says that air that is at a higher velocity is at lower pressure than air that is at a lower velocity when flowing around a moving body. That being said we need to consider the air flow around a typical wing tank when in close proximity to the ground. The air stream hits the nose of the tank and splits, the air that goes over the top does accelerate in velocity because it is required to move upward to go around the tank, that air is not restricted from moving by anything except the atmosphere above it. The air that is going below the tank is restricted by the ground plane which means that it must accelerate to a higher velocity than the air going over the top therefore it is at a higher velocity and therefore a LOWER pressure i.e. you are generating down force.

Several things can happen to a tank related to ground clearance. If you have the tank very low this can cause the air flow along the centerline to begin to interfere with the boundary layer of air at the surface which can completely restrict air flow beneath the tank. This will typically cause the generation of large vorticees that can  interrupt the hopefully attached air flow on the rear sections of the tanks lower half and cause separation of the air flow in this area which will increase greatly drag. As Dr Goggles says I do not like flat bottoms as they are great generators of vorticees all of which generally increase drag. The minimum ground clearance for various shapes as published by Katz, Tamia, McBeath et all are not something that they just pulled out of their collective a$$es, they are based upon both mathematical and experimental data that is proven and reliable. For a "torpedo" shape (typical tank) the minimum is 3 to 5% of the total length of the shape. i.e. if your tank is 100 inches long then the minimum ground clearance should be between 3 to 5 inches.

There certainly are flat bottom cars that are fast, witness Poteet and Main, Bobby Moore, Jack Costello's cars but there are also some round bottom cars that are pretty fast, Seth Hammond, Al Teague, Charles Nearberg,  Dennis Varni, the Contrivance Engineering modified roadster. As far as I am concerned the big advantage of a flat bottom car is that they are alot easier to build.

Rex

[Brad54]

With a symmetrical shape lift will be generated due to ground effect. With equal amount of air flow around all sides of the vehicle the pressure going under has the ground to 'push off of'.

IMO having the nose apex lower then center is ideal by decreasing the amount of air going under and increasing the amount over. This will balance the higher pressure going over with the lower pressure + ground effect will help make the vehicle aero neutral (no lift or df) and will make a more stable vehicle at any speed.

Good luck....


~JH

That was excellent info. Thank you. Exactly what I was looking for.

-Brad

[Brad54]

Brad, you may be overthinking this. Markley Bros and Johnson/Shipley went very fast with no aero mods to speak of. Jack Kelly`s tank was no slouch either. What do your plans entail? Blown KB of V4F?

I've got a tank, a friend has a garage full of '25-'28 Chevy 4 blocks.
If you can't make power with the engine, you have to find MPH other places.

-Brad

[Brad54]

Well I finally got out of bed, thanks for the wake up call Dr. G, and can only comment on this subject based upon my old friend, Daniel Bernoulli, who's law , when interpreted for this situation, says that air that is at a higher velocity is at lower pressure than air that is at a lower velocity when flowing around a moving body. That being said we need to consider the air flow around a typical wing tank when in close proximity to the ground. The air stream hits the nose of the tank and splits, the air that goes over the top does accelerate in velocity because it is required to move upward to go around the tank, that air is not restricted from moving by anything except the atmosphere above it. The air that is going below the tank is restricted by the ground plane which means that it must accelerate to a higher velocity than the air going over the top therefore it is at a higher velocity and therefore a LOWER pressure i.e. you are generating down force.

Several things can happen to a tank related to ground clearance. If you have the tank very low this can cause the air flow along the centerline to begin to interfere with the boundary layer of air at the surface which can completely restrict air flow beneath the tank. This will typically cause the generation of large vorticees that can  interrupt the hopefully attached air flow on the rear sections of the tanks lower half and cause separation of the air flow in this area which will increase greatly drag. As Dr Goggles says I do not like flat bottoms as they are great generators of vorticees all of which generally increase drag. The minimum ground clearance for various shapes as published by Katz, Tamia, McBeath et all are not something that they just pulled out of their collective a$$es, they are based upon both mathematical and experimental data that is proven and reliable. For a "torpedo" shape (typical tank) the minimum is 3 to 5% of the total length of the shape. i.e. if your tank is 100 inches long then the minimum ground clearance should be between 3 to 5 inches.

There certainly are flat bottom cars that are fast, witness Poteet and Main, Bobby Moore, Jack Costello's cars but there are also some round bottom cars that are pretty fast, Seth Hammond, Al Teague, Charles Nearberg,  Dennis Varni, the Contrivance Engineering modified roadster. As far as I am concerned the big advantage of a flat bottom car is that they are alot easier to build.

Rex

THANK YOU!
I'll have to read this sixteen or seventeen times before it sticks, but it's definitely the kind of info I was looking for.

Now here's another question: When discussing flat versus round, how much of a flat spot changes things?

Where's the relationship between round and ride-height come into play? Also, symmetry should come into play somewhere as well.

-Brad

[krusty]

    Well, our car (#1429 and various other class change #s) most recently # 4829 C/GRMR, is based off of the Contrivance modified roadster. It has a 20" wide bellypan running from the nose to the tail (on a 175" wb car), but the rest of thebottom is flat. It has a 3/8" steel plate welded to the frame from front to rear. I guess it is a hybrid! Runs straight and fast 258+ exit so far, record at 256+ at 4370# with fuel and driver. Drivers (4 so far)have not mentioned any bad behavior  (back end moving around, front end feeling light or wandering, etc). The front ride height (as measured to the flat bottom) is 7 3/4"; the rear is 9". No strakes or splitter lips on the sides or nose, etc. Haven't really explored the %age of flat to curved, etc. (and really, the bottom of the belly pan is flat, too).  I need to review the windtunnel data from 3 years ago to see if I can find a reason for its success.    I guess the answer I have is I'm not smart enough to explain everything that's happening (and that there's more than one way to skin a cat...)       vic

[Rex Schimmer]

l am attaching some pics of the original Contrivance Engineering modified roadster that Krusty (Vic) is talking about. Note that it  has a lot of ground clearance especially compared to the "standard" gutter and downspout modified roadsters that are running, it also has nice rounded lower edges that blend the sides to the bottom and also a rounded belly pan section that runs pretty much from the front to the back of the underside. The nose is rounded and made to make the air go down the sides of the car and it is running a 23 T body, and there is a good reason for this also. This car is so different than the "standard" modified roadster and it is some what "homely" BUT it is also the most important thing FAST!!! This car holds all of the gas modified roadster records from E to AA and by my thinking it is because they did the aero right and part of that aero combination is the amount of ground clearance that they run.

Vic I stopped by your pits at Bonneville and you were in such a $hitty mood I ran off!!

[Glen]

Some of the tanks have a flat bottom due to the small tires they run in the 4 cyl and flat head classes. The big bore fast lakester/tanks can use taller tires.

[krusty]

    Rex - Sorry! Was it on Saturday after we had blown up 2 new engines?!!  Or it might have been Sunday, Monday, Tuesday, and/or Wednesday until we were finally in impound with my rookie driver!  I can be "a bit" grumpy... when things aren't going my way...       vic     PS: Best to find me before tech, I guess (although this year I had dynoroom Mike and his kid so it wasn't too bad).

     Thread hijack complete.

[krusty]

     That last pic is a very good one; it shows what the bottom of ours looks like too, although I haven't made my rear exit as much of a "nozzle" as Neal has. Ours just rises up to a 1" opening along the last 18"or so. Also, our body, a '27 (and there's a reason for that, too), has very sharp edges where the flat bottom meets the sides. If you look more closely, I think you'll see that Neal's does, too.      vic