Some decisions about aero....

From "Australian Bellytank" on landracing.com 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...


AND REX RESPONDED>>>>>>

Reverend,
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

The Tender Hearts at the Retreat Hotel Sunday 3rd July 2011

I had a weird feeling on the way to the gig......there's been a bit of confusion about what time we were on....whether we were to play in the bar or in the lounge....all I could think to myself was "we'd better be playing", I have to be honest it's a habit for me, if a gig is waved under my nose the itch has to be scratched....once I've loaded my gear into the car there's no turning back, that was my only concern.

We got to the pub and set up, Swamplands were playing in the bar, we were to play in the lounge for a 7pm start.

Jody arrived , she was still jet-lagged after getting back to town from the UK 48 hours before, " It got to 4 O'Clock this arvo and I just went bleugh!"...she looked asleep.

We set up, so did Julian the house mixer...we kicked off, the crowd was light and we were playing so fast I felt like it was one of those dreams where you can't run....

Second song we got it together, we sounded pretty good on stage, the foldback was loud but that was good....

By the end of the set we'd put away a few beauties, pity there was practically no-one there to hear it....

During the break Julian sat down and said "um, there's been a bit of a mix-up"...it turns out that there was another band booked for a set after us and that we were supposed to play one hour long set....we had a quick pow-wow ans decided to play another short set so Seamus and Fergus could start by 9.

 The second set was tight, I goosed a few chords, alzheimers we'll put that down to. All up the set was good, the crowd was bad....But, those who were there seemed to think we put on a pretty good show...it's a great little band and I love playing with them, pity hardly anyone saw it.

Next gig 26th August at the Lomond, 36hrs after I get back from the US, lets see how MY jet-lag is....

The T-Bones rock the Lomond.

And so it was...the first time the T-Bones proper had played together in quite some time. Miles our drummer was away the last time we played and Michael Barclay filled the seat , and I think the two times before that Alics had other gigs on her mind and so Charlie's daughter Loretta Wilde played bass.

It was great "gettin the band back together". It was a good crowd , we started loud and kept it up....I took my long time stalwart the white Elite Tele with the Bigsby and the Strat neck and naturally took the Musicman RD210 to plug it into, they're a tight combo and I don't use them often enough since I've been playing standard Tele's and using the Fender Vibrolux . Charlie had taken my Fender Concert 2x10 home after our last gig because his 4x10 Concert had dropped a valve.He brought it back and I was left with no alternative but to use it and the Musicman in tandem. The guitar just sang.

We played a couple of newish ones which is always great ...The Murray River song and one that I can never remember the name of , but it's in G if that's any help. We played All about LOve off "Low Down" the last album too.

In what was a very rare sight we had a dancefloor of all men, yeah it was strange, the two hottest items were two largish guys who'd given it a proper shove, they waltzed, and for a bit there shimmied their way through a couple of numbers.....heaps of people were dancing later but it was hilarious to see these two bigfella's, a lone "expressive" and a couple of other guys who, god love 'em, couldn't dance for shit and didn't care.

We finished at 12.30 after a final set that included Seventeen, Fightin at the Pub, Goodbye Loser and Train... A couple in their early twenties walked in just as we started Goodbye Loser ....a deconstructionist country stagger, they looked appalled at the scene before them ...the first verse with it's disjointed rhymes, the ragged drunken hillbilly guitar riffs and the assembled dancers.....but by the time the first chorus was over they'd caught on and were shreiking with laughter.....yep, the T-Bones, the best cold crowd band I've ever seen......

We ended the night after I'd paid "respect to Tony Hargreaves the traditional mixer of the Lomond Hotel"...
he had a toothless grin from ear to ear..." my favorite pub rock band, I dunno, I was singing along with every song and there not a bad one amongst 'em"

We'd had dinner with some of the Sunshine locals who hightailed it early closely followed by some of my workmates but those who stayed and those who turned up later like Beanpole and his son Bede went home with their ears ringing safe in the knowledge that Melbourne indy original country rock is alive and well.

Tomorrow the Tender Hearts at the Retreat........

Gigs for the weekend 1-3rd July 2011...

This weekend is bustin out all over with hot-rockin-country-rock

On Friday we have the T-Bones at the Lomond . Now, that is Lomond ,pron. "Low-Mond" not "Le-Monde" that's because it is named after Loch Lomond in Scotland not something else in France. It was my father who gave me a curious look when I made the  faux-pas on that one....er hang on faux-pas?..Le-Monde?Sacre Bleeeeeuuuughghghghghghgh....

Anyways, the T-Bones aren't cheese eating surrender monkeys nor are they haggis hiding hammer throwers. They are simple folk raised by and large in rural areas....except of course those amongst them who were raised on communes, and those who were raised in Scotland, yeah Scotland...huh?

Aye, We hae a wee scottish lassie oan bass an' We loove 'er...........

Which is just as well ....for me at least because pretty much any music I'm involved in also involves Alics....

..........like The Tender Hearts, Jody Galvin writes the toons, plays acoustic and sings 'em........Alics plays bass, Mikaela plays drums and I get to play electric....which is just as well , I'm not much chop on anything else. The Tender Hearts are kickin off at the Retreat Hotel at about 5 on Sunday arv....

Just like the T-Bones I weedled my way into the Tender Hearts because I love the songs they write....... them's good, real good.

Now, there is another matter . I began recording a new solo album in November 2008, then ten days later I got myself a busted leg . That turned out to be a bit of a distraction. It wasn't until last year that I managed to convince my ducks to line up....I went to Michael Thomas and said "help". Over five days we thrashed adding all the guitars, keyboard ,bass and vocals to the drums that Miles had recorded years before.There are fifteen songs some that came from the Overnight Jones era, some from the Purple Hayseeds and some from FourDoorShitBox....and a couple of newish ones. So, that was last year.Last week I went and visited Craig Pilkington and said "help".

By my estimation the mixing and the mastering of the recording will be done in a few months....So, by about this time next year I should be ready to release it.


I will gladly accept all suggestions for the name of this hotly anticipated vanity project, yes even yours Johnny.

So , this Friday :T-Bones.

Sunday: Tender Hearts.

Sometime in the indeterminate never never a new solo album from me, James Stewart.

Quit ya mopin' let's crack this open.....................

Here we go my first post on the new "High Salt Diet" blog........ I might miss a few gears, I might set a record....Who knows?

This is going to be an uneasy mix of Landspeed racing, music and sometime personal views on lots of different things.....kind of like me.....

There'll be lots of ellipses...........


...and opinions,

there may also be some coarse talk.....

Let's just touch the hot lead on the battery and see what happens......