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Wednesday, 13 July 2011

The high ride , or the low ride......height.

Some decisions about aero....

From "Australian Bellytank" on published Jan 2006

I am writing this reply to you and to whoever else is interested or wish to chuck their two cents in.

Apologies for the usage of the high falutin metric system.

For those who missed it Rex pointed to a page in Goro Tamai's "The Leading Edge" discussing the manner by which drag quickly increases below a certain ride height and cited some figures which seem to threaten our design's efficiency.

I must admit that this area has had both the good Doctor Goggles and myself ponderous as to the correct direction to take so I went out and found a copy of the book. Below is a history as to how the bottom is as it is at present.

Our main Aerodynamics text to date has been  "Race Car Aerodynamics" by Joseph Katz, Ph.D which I can recommend for its readability and information.

The difference between the two texts is that Katz's book focuses on methods of maximising downforce due to aero effects whilst minimising drag whereas Tamai's focus is about eliminating downforce altogether.

Tomai's book is written primarily for  solar cars where energy conservation is a priority and traction issues marginal. As the title suggests Katz's focusses on racecars which have different criteria (eg cornering and acceleration issues).

Upon reading Katz and understanding the traction difficulties on salt,  we found ourselves considering all sorts of thoughts of ground effect devices but for a number of reasons were reticent to embrace them.

Firstly we did not wish to destroy the traditional style of the belly tank so we were reticent to go too far with a diffuser etc.

Secondly, we are building against the clock so we wish to keep it simple for its first years out then can tune the shape against a base model.

Finally it is a bit of a black science when you don't possess a wind tunnel and you could do a lot of work that makes the Dodge thing slower.

We were cheered up by the fact that the So-Cal Lakester belly tank wannabe (So-Called belly tank?) turned up sporting a similar method that we considered. Rex mentioned that it hasn't performed well yet but in its defence Bonneville has seemed to be pretty rough of late and not conducive to ground effect technology. Neither is several inches of water  so the jury is out for me on that one.

My understanding of how it works by looking at it is a splitter at the front to stop air creeping under and provide some downforce at the front, then has a difuser at the back to accelerate the air that is underneath so that a low pressure zone is acheived by the Benouli affect pulling it down. It doesn't seem to have skirts so I would imagine that it sucks more in from the side rather than the car down causing drag inducing vortices... correct me if I am wrong. (anyone GM?)

Anyway, for the above reasons we have decided to not have any ground effects for March and have addressed the issue of traction by making it HEAVY. (No replies please Propster).

Our intention was to get the car as low as possible for stability and to acheive the goal of having the axles on the centre line this put the diff in the widest point of the car and meant body panels could sit over each appendage minimising cutouts, oh yeah it looks cool too.

A number of cars at the salt are really low and perform well so we assumed that the drag due to ground proximity was minimal. For example one car we have enjoyed watching develop is John and Paul Brougham's belly tank which has put in multiple 200mph runs over the past couple of years at Gairdner and is very low (see image). Admittedly it is a little TOO low as it bottoms out a bit at the moment but larger Goodyear Eagles are on their way! Both John and Paul have been very helpful in providing us with much info along the way.

The Brougham tank does have a curved base though where we are proposing a flat base. Our favorite tank was the Hooper tank (the flat head killer) (see image) and part of its charm is the low flat base, as is Xydias' original SoCal and we made the decision early to go down this path.

So what height is the best height?

On page 118 of Tomai is a graph outlining the best ground clearance heights for certain shapes to ensure lack of drag due to ground effect.

Rex indicated the row entitled, ?torpedo shape with an oval width / height of 1.25 and length/height of 3.6. has a H/l of  0.3 min. to 0.05 ; that is a minimum ground clearance of 126mm to 210mm for ours (our car being 4.2m metres long.)

Currently we are around 40mm so that looks way under.

But our w/h is 0.88 / 0.81 = 0.92 and l/h is 4.2 / 0.884 =4.75 and extremely tapered. Tomai's calcs are predominantly to be used for a solar car of width of 2metres and 400 to 700mm thick and of fairly uniform shape for the length.

A better zone of the graph therefore to look at is:

?Torpedo with flat bottom with various cambers and width=height? which is worked out as a ratio of height to breadth at a Hmin ratio of 0.15.

For our car that makes 121.5mm (a whole 3.5mm lower!!!!) but at least it confirms that we are at the lower end of the scale.

The fact the whole shape is tapering I assume will lower the impact as well....?

The mention of "Camber" refers to the amount the centreline axis of the shape is above the chord from tip to toe expressed as a percentage. (Bloody ?camber,? couldn?t the nerds have used a term not already in the automotive lexicon???)

The curve of the centerline of our car caused by the extra tank on top and chopped bottom  helps counter the drag caused by proximity to the ground  by increasing the distance traveled over the top of the car and hence similarly accelerating it helping equalize pressure.

The book says the ideal amount is between 3% and 6%. I worked out ours to be 4.7% (yay). Apparently the best shape to have is a slight ?S? shape in this camber. Ours is that slight S shape so that seems helpful.

Drag versus downforce?

It would seem to me though if we are running a car with more horses than we are putting to ground through traction issues on salt then ground effects are the way to go. The racecar book says that the downforce via diffusers et al is a cheap payoff, and we can overcome the extra drag by the extra efficiency of getting power to ground.

If we find that we are only just pulling top gear (or heaven forbid worse!!!) then reducing drag to the utmost becomes paramount and then up she goes!

This will be one of the many possible learning things in March should all go well.

Sorry for the lengthy post Rex (trying to get those points) but I wanted to answer your query with a complete reply and to prompt some discussion on this and maybe even assist someone else...


Thanks for the great reply! I certainly understand the pressures of trying to get something ready for the race, it seems that they will start the race if you are there or not! So you bust Acura to make it happen. As far as ground affects go, I did talk to the young aero engineer that claims responsibility for the new So Cal tank and he claims that the ground affects on that car have no effect on the drag. Well, he was claiming to be an "aerodynamicist" and I am just an old engineer that likes to "diddle" in the aero stuff BUT nothing is free! If you generate any kind of aero lift, up or down, then you have a coefficient of lift which means that you have to supply some kind of HP to make it happen. Yes maybe the So Cal car has a small Cl but it does have one and it does take power to overcome and with HP limited cars that can be the difference between just going fast and going fast enough to have the record. The one think about the "sun cars" discussed in the "Leading Edge" is that they are all very restricted on HPs and so they really do alot to make sure that they have everything low drag and very low lift. It makes a big difference as to the aero detail you need to look at if you have a 2000 HP Hemi in the back.

My favorite tank lakester is Seth Hammonds old car, look at the ground clearance, lots of it! and that car holds more records in more classes with more drivers, than just about any car.

Good luck on making the March meet and let us know how you do!!!!!

Rex Schimmer

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