Aero design question
 

In the effort of designing a more slippery vehicle while still maintaining decent looks, is it possible to see a lower CD by offering a "channel" that would allow air to flow thru/under the vehicle?
I noticed the Loremo design is like this, the floor is higher up except at the tires. I'd be interested in having this channel be narrower but far taller, forming a "center console" between driver and passenger that is for example 12" wide and 18" tall..
Air would enter the grille area say 12" tall by 36" wide, and instead of just running into an engine, be routed thru the channel back under a rear mounted engine and out, pulling exhaust with it.
The squeezing of the channel would accelerate the air under the car and help provide a more balanced shape, as a generally slippery car shape tends to resemble a wing and can get dangerous at higher speeds.
I keep looking at all these sleek cars heading for the x-prize competition wondering if there is more to be gained by using such a method and getting rid of more frontal area....

Some racecars already have channels in the underbody panels. What you are talking about though would take up a lot of interior space.

Doesn't the channel on race cars fan out at the back, creating suction, to increase down force? That would be counter productive for MPG's. I could see a strait line back may improve things.

What your talking about sounds a lot like a ram-jet engine. Squeeze the air really tight and warm it up with the exhaust, see if you can get some push from it. I wanna say they did that with the P-51 mustang. Got a few extra MPH's out of it.

Sloping the back upward creates downforce just like a fastback creates lift but a fastback is more aerodynamic than a wagon back.

channel
 

So far,it looks like all the really low drag designs minimize internal flow as best they can.When duct work is necessary,it's held it to a minimum,and the air is always vented to a place on the body which maximizes drag reduction,not necessarily at the back of the car.----------------------- Your point about high speed stability is well taken,however,safe,zero-lift low drag designs have been hammered out now since the late 1980s.Pininfarina's CNR,Ford's Probe-IV,and GM's PNGV would be some examples you could look at.

The probe in one of my favorite shapes and if I was going to start with a rolling shell it would be one of those...but then theres plain old steel and heavy everything....nah.
Take a 1st generation Mazda Rx7 for example. The bullet shape of the front end was pretty decent, but the rear fastback of the car got loose at high speed...I raced one for several years both autocross and road racing and found that I had to change setups because the higher speeds got the rear end loose and low speeds understeer'd...
My goal I am trying to achieve is to reduce frontal area and therefore gain a few % on the designs already out there. Take the Avion for instance, look at it's front view and then take a big bite out of the bottom center of it....
Slowly tapering this channel smaller and shallower towards the back(and so it doesn't totally kill the passenger space), should yield a lower CD and help balance the effect of lift on the back of the car due to it's shape(fastback type shape). In the beginning I was hesitant about even posting because I am unsure if this can be answered without wind tunnel testing or computer testing of the shape itself.
FYI in high school I based my co2 dragster design on a similar concept and made basically a bullet shape at the cylinder with a pair of vertical triangles that picked up the axles. Frontal area was as little as it could be, and despite that I had doubled the side surface area by hogging out the center, it was the fastest by over 5mph of all the ones tested.
A couple of wood 1/12 scale models and a visit to my friend at the VRI program may be in my future..

Hi,

The crux of the matter is, that the whole body has to be aerodynamic, in order for it to work best. You can't solve the overall drag by channeling the air under the car; in order to make the outside look "better". The air will always drag on the parts of the car that are styled instead of engineered.

I think we need to change our ideas of what looks good?

I totally agree, but at the same time lets look at some alternatives. You configure the car to be like a jet cockpit with pass behind driver and narrow it up. Then you have a car that is prone to sidewinds and tipping over in a windstorm...
Crap my car is on it's side in the neighbors yard again lol...

There is going to have to be compromises made that will yield a practical car with acceptable mileage. Sure I could take a empty torpedo shell and mount 4 aero wheels on it and probably get 200mpg laying on my stomach looking out a clear nosecone, but hows that gonna work for old people, fat people, or spoiled people?
Wheres the cruise control and DVD player?

Flat and wide is stable, run air thru the parts of the car you can instead of around it...
I will post some scans of my drawings up once autocad gets here next week and I can get them transferred in..

oooohhhh i can't wait! I love this stuff! Haven't a clue about the mathematics of it though!

ollie

rkcarguy

To help us out for your aero concept is a narrowed design criteria of what you want to accomplish. This way your we could have an engine, powertrain, drive, passenger/cargo configuration & car size (i.e., city, subcompact, compact, mid, full, etc) classification defined. And what is probably the most difficult thing to settle is what do you define as 'decent looking'. Pure aero forms would either look biomorphic (fish-like) or mathematically-defined forms, it would look odd to most and people usually are accustomed to well defined proportions set by the car design industry. Besides the Loremo & RX 7, what other examples do you consider a 'decent' inspiration for your project. A 'clean' paper project is going to be a great project. Even a heavily modded existing platform chassis with a new 'kitted' aero shape would still be a great challenge and would be closely followed by this group, if you don't mind a design-by-'enthusiast'-committee.

Better designs with aero bodies
 

I could suggest to look at GTM-R cars for 'decent' design with some aerodynamic-tested shapes for inspiration. It was awarded 'Best Exterior Design' at last year's SEMA show. It has a lightweight tube-steel rolling chassis mated with a rear/mid-mounted V-8 but could see other engine/powertrain donors. Check their site for more info:

GTM Photogallery

Revenge shows new GTM-R ‘Super Car’ in Detroit - MotorAuthority - Car news, reviews, spy shots

Another inspiration (styling purposes) for great looking 'kit' car, power-mated with your Honda engines. Not pure aero because it is a roadster but an aero roof and aero studies could be added to the base. Pics of their racing coupe.

K-1 ENGINEERING - Fotogaléria

If you are really out for Cd having your passengers in tandem would go alongway towards that.

Then your design could be more torpedo like, lol.

Seats low slung angled somewhat heavily backwards( I think you wanted rear wheel drive? that might not have been something you were looking at) and the engine could be mounted behind and a little under the second seat.

Cooling you could use small fins on both sides like the Bentley Speed 8.

http://members.fortunecity.com/freec..._speed8_02.jpg

You would want to angle them so thay are pulling air from a lower point to a slightly higher one to increase downforce on both sides. OR instead of using it for cooling you could ram-air if you wanted to go that road.

The original designs for the Speed 8 called for the rear-view mirrors(side view mirrors) to be mounted on the wheel guard and the front to be slightly more tear than it is(which would put the fairings on the front right tire higher than the rear ever so slightly).

I would go with Tandem for coolness sake let alone the potential enormous gains you in Cd you might be able to reap from the much decreased frontal area and potential stability.

The GTM is actually pretty close, sans the wide flares and side intakes. The honda engine is pretty narrow and would also allow the passenger compartment to be further back.
Looking at the K-1 racing cars, those look awesome and despite a few things are pretty close. I'll take a roller please:)
My plan was to top mount the radiator almost horizontally and scavenge some air from the "channel" and route it the the radiator and out a grillework below the rear window.

The problem is with most of these cars, is that they are designed to provide alot of downforce for racing, but will probably be heavier at speed than not in motion. For a balance of mpg and safety, I think you have to have a _____# car that weighs the same on each wheel at 100mph as it does sitting in a parking lot.

You can design a car so that otherwise useless drag is turned into downforce

haha I'm sorry, I was not saying you should make it exactly like anything that races, but if you go similar and drop the wing you lose all the racing downforce and can add back as much as you need for stability at highway speeds.

I suggested those two examples for exterior design inspiration. It looks like you have a clean sheet design. You've already settled on your cooling configuration at a downstream horizontal plane, hopefully redirecting cool air from your underside flow channel. Not sure about steady flow because the upward sloping belly pan is a low pressure plane. Just like the sample, Bentley Speed8, the top roof scoop has high pressure flow for the engine air ingestion. Efficient cooling NACA inlets & scoops will need to be placed at high pressure surfaces. You also have an engineering target of the 'magical' zero lift at any/all speeds. For handling safety, agitated gusts, large-vehicle vortex turbulences, & crosswinds are held in check at higher speed limits by useful downforce. A lesson learned, just check this out.

http://www.youtube.com/watch?v=TtV1D...layer_embedded

I knew what this was before I even looked at this movie, I was watching this event on TV when it happened.
It'd be interesting to see how this car performed in wind tunnel testing, if it got "light" as speed increased to begin with. I think in general, a lighter vehicle must be smaller to obtain a density such that it won't be subject to the wind gusts, pressure waves from passing vehicles, etc. Many of these lightweight race cars that are very wide and low, create a near perfect shape of a wing and coming over a little knoll with a headwind is all it takes.
The amazing part was the driver walked away from that IIRC...
On a different subject, I found a local place near work that makes fiberglass bathtubs and shower stalls using the chopper gun sprayed fiberglass. While it's not optimum, I could certainly have them spray their stuff into a mold taken off of the aluminum bodywork to save some weight and make parts reproducable.

You would be amazed at what can be done to a normal car to get the Cd down.

Normal 1969 Dodge Charger was probably close to .40 plus Cd

http://www.roadragegraphics.com/imag...ge_charger.jpg

http://i13.photobucket.com/albums/a2...9chargercg.jpg

Throw on a new nose, flatten out the rear window and presto a .28 Cd car.

http://blog.webridestv.com/wp-conten...charger500.jpg

http://2.bp.blogspot.com/_L7_lLU74UC..._1969_02_m.jpg

Rear wing was that tall so you could open the trunk lid.

It looks like you are more comfortable with metal fab but wood is easier to work with. The basic plywood buck is soo much easier to create & modify. Any experienced FG fabricator are familiar with styling bucks and can assist you with the creation of your FG body sections. Even before creating these bucks, depending of your CAD/CAM experience all of your work could be kept in database & be tested with many different fluid flow programs. There are tube chassis design programs to help design a balanced, lightweight yet strong structure. A lot of your assumptions can be easily tested and a whole lot of fab time saved.

Car Body Buck in Plywood

Using sprayed in FG could be. . .risky. I've had FG cary body parts explode on the highway from rocks. It makes a lovely mess.

If you wanna go FG all you need to do is get a mold fabbed, put a liner down, lay Fiberglass cloth(Fiberglass Cloth in bidirectional weave) coat in resin and fixative and sit and wait. It heats up a little you wait 45 minutes pop it out sand, smooth paint bolt.

My first boss used to own a small company that made composite aircraft parts and small boats. They would coat the prototype with a waxy non-stick stuff, then lay a fiberglass mold over it and build up many layers. After it dried they would remove it, beef it up with a wood structure, and do some finish work such that the items done in the mold came out needing minimal handwork. I have never done this work myself but I understand it and know how it works. Fiberglass stinks and I have no desire to work with it myself unless I had a totally ventilated filtered room to work in wearing a haz-mat suit( I have neither). My machinist/fabricator work already cost me my lower back(reason I am at a desk now), and I'd like to retain a healthy respiratory system.

Ah, yes you could layer it. . .I forgot you said it was a bathtub. . .manufacturer? The woven cloth is not as bad as the spray by a long shot. I made a duplicate of my current front bumper in case it exploded too.

Although the current bumper is easily 2-3 times stronger. The weave gives the material MUCH more strength than spray but it takes more time than spraying it and heating. With most resins out there you don't have to heat it or vacuum pump, but if you want you can get pre-impregnated for the ultimate in strength-density.

^^ Or use carbon fiber cloth instead...but I'd like to drive this thing not sit and drool on it all the time and get pissed everytime I get a rock ding.
Hoping to have Autocad in the mail today when I get home....

only downside is CF is way more expensive. . .and its actually heavier. If you wanted to do the chassis CF is the way to go but for body FG I would recommend some form of very resilient paint lol. FG is as light as it comes and since body panels don't support weight. . .

Yeah. . .rocks are the bane of composites. . .or at least CF and FG. I'm still looking for something thats a much thicker paint that might be able to dissipate rock impact energy. . .

Vacuum formed tupperware body panels=Win:)

laugh laugh laugh FTW!

Rubberized undercoating could absorb some minor impact damage to composites. Wouldn't look so nice and would be heavy, but would probably work.

Autocad came wednesday and drawings are in progress. It's funny because so far it looks similar to a flatter 1st gen MR2...
I'm talking to a friend of mine who seems to think I'm not too far off taking a mold off of the original nose fabrication and using it to vacuum form ABS or Polycarbonate sheet....
I really like the K-1attacks nose and have styled mine similar however more neutral shaped as far as aero and no batmobile vents holes and stuff.
Getting headlights at 24" on center from the ground is proving to be the hardest part, because the whole thing is only 28" off the ground to the bottom of the glass....
I'm thinking I need some really wide thin headlights lol...

Try putting the headlights in pods, mounted on aerodynamic stalks. I think that would minimize the disturbance to the flow over the vehicle itself.

It's actually practical because the vehicle can use off the shelf projector headlights like these

Low beam http://www.rallylights.com/detail.aspx?ID=1800

High beam http://www.rallylights.com/detail.aspx?ID=1801

You could use a surplus military fighter "drop/belly" fuel tank as used in the attached photos. You may not want a V8 for power. :turtle:

Really narrow & aerodynamic.

Bellytank



List of external fuel tanks & specifications.
External Fuel Tank Product Index

In a heart beat! But where do you get drop tanks on the East Coast? I'm also still looking for an intact, but non-airworthy Cessna 150 fuselage to build a 3-wheeler out of, but running out of patience.

Another inspiration from New York Auto Show
 

Here's an interesting concept with plenty of the aero design criteria you are exploring. It's a PHEV concept but the lessons are put in place for you to study in your project.

AutoblogGreen

This is an immensely interesting discussion. botsapper asks some interesting questions related to realistic constraints on most such projects. You will probably have to start with some existing platform, otherwise the design and fabrication effort is huge. Reshaping of an existing car body can be done, and has been done by custom car folks for years. But it is expensive. The use of a portion of an existing platform and cutting away other portions, to be replaced with fiberglas, would be a fairly cost effective way to at least get some answers as to what works as an effective compromise.

It seems to me that the major questions are these: 1. do you want to design something conceptually with a clean slate and no compromises, or, 2. are you interested in modding something existing for a reasonable amount of money? (In the latter case, how much money do you want to spend?)

It seems to me that if number 1 is your objective, then the discussion can continue as to what functional shapes would be the "best" compromise for a 2 person, or 4 person, street legal auto. But note that VW has probably already done this work with their L1, I think that was the designation. And the Aptera is also a solution to this question. Both cars would probably be outside mainstream taste as "attractive" car shapes, though I like them both.

If conceptual only is the objective, then I suspect that lots of advanced aerodynamic modeling is in store. Some expensive design software would probably be required to have any confidence in the test shapes.

I'm just kinda thinking out loud and don't really understand exactly how you would proceed, but its interesting to contemplate.

I want to sum up where I am heading with this and what I am after.
We can all agree that until some magic free horsepower fuel of the future comes along getting better mileage with the technology we have is the only option and is also a compromise.
Design and fabrication is what I have done for a living for many years. In the past I have focused on performance not mileage and built some pretty nasty cars. My latest build which is pretty much waiting for me to bolt it together is a 1963 MG midget that I auto-x. I got the chassis out of the blackberries at a friends house for $200, and spent a year building it up. I replaced all the MG running gear with a myriad of components, the rear end is a 87 RX-7 IRS narrowed 14" with posi, attached to coilovers which pick up a custom shock tower in the trunk. The front also has RX-7 hubs, spindles, and brakes with 92 prelude upper A-arms and aftermarket coilovers tied into another custom built shock tower. I stripped the body, cut out the stock fenders and rivited/moulded on fiberglass wide body fenders, which allowed fitting the car with 13x8 slicks. Originally I fitted it with a stock 12A rotary engine, it was very quick as it was under 1300#s. The rotary was a pig though and in the last few years parts have gotten harder to get and more expensive.....so I decided to swap....shoehorn..... in a japanese market nissan SR20DET. It has been quite a task. I had to notch and re-enforce the frame to clear the clutch slave, and purchased an aftermarket manifold so I could remount the turbo higher to clear the footwell. I did all my mods and changes, test fit everything, took it apart and fully welded and painted everything, and then our auto-x club lost it's places to race so it's just been sitting.
I really enjoy driving the car and thought "why do I need anything more than this to go to work and back?"
Being such a small light car should also yield good mileage.
I started considering building another one using D-mod prep parts which are more aero and not widebody, but that still left me with a convertable roadster type car which isn't very practical in this wet "evergreen" state.
My fascination with aero shapes goes way back, as a kid I was always amazed that I could take the lamborghini body and put it on 4 different slot car chassis's and that would always be the fastest one..
I am designing this one from the ground up built around the components I wish to use, because I don't have to compromise using an existing cheap steel chassis and put up with a production car that had to be designed to fit and perform for the general public.
Look at the front and rear view of the prius. The sides are pretty flat and there is not alot of curves on the sides. They focus on splitting the air over and under the vehicle, and I believe that often the curves or rounded shapes that deflect air to the side may cause more tubulance and drag, in the same way airplane wings have the turned up tips because swirls of turbulance off of the straight cut ends made the last few feet of wing in-effective.
I don't mind sitting on basically a padded floor leaned back at 20* if that's what it takes to get good mpg and as a bonus also enjoy a low CG for handling purposes.
As far as the aero design is considered, it may come down to just building it and go drive it. The aluminum nose shape can always be played with if the car has too much or not enough downforce.
I am still contemplating using the upper firewall dash and A-pillars from a Civic to ease the whole controls situation, but then that means a bolted steel/alum connection which may be more pain than it's worth.
Overall I'm simply looking to combine a few things together...
The sleekness and aero of a sports car/exotic
The efficiency and practicality of the lean burn VTEC engines
Lightweight materials and components(just get away from damn iron and steel)
A transmission that doesn't spin the engine at 3500 rpm on the freeway

Can a homemade car be street legal? What about kit cars?

You should consider buying an old 1.6L Miata and getting 30mpg. They usually don't leak

I already have a 1993 Civic Sedan with a B20B swap that gets 33mpg, and it's alot quicker than a miata too. I'm disapointed actually I thought the Miata would do better than that for mpg..

A homemade vehicle can be licensed in our state, I can chose 2 ways: taking the receipts for all parts materials etc and being assigned a vin for a custom built vehicle, or use parts from a donor vehicle, which will get scrapped, and then becomes the identity of the modified vehicle. I'm leaning towards the custom built licensing because then any exhaust I make would be the original and therefore not subject to our states bogus modified exhaust laws which they removed the sound limit and left it to the officers discretion(basically if your exhaust is shiny and aftermarket here's a ticket loud or quiet).
Building my own I would be subject only to the federal standards of 95 decibels or less.

People on Miata.net say they get 30mpg with normal driving with the 1.6L. The 1.8 is worse. I get about 26mpg. Main problem with the Miata is the low gearing.


With your home built car, I hope you would use a catalytic converter, otherwise pollution will be much much worse. I believe a small 2 stroke scooter emits something like 10x the pollutants other than CO2 as a car.

Interesting recommended studies for your build
 

It is very good to learn that you are not only a grease-under-the finger-nails builder/fabricator but also a racer with a growing eco-heart. Your SCCA, Solo Series & MG club racing experience is also a great insight to your true car passion. Your thread general discussion may need to extend onto a larger scope for not only for an aero/ FE engine package but also one with great handling & performance dynamics. I suggest adding to your research. TTW - Motorized
I've have been studying TTW for some time because of it's hybrid driving dynamics & important smaller aero front profile. A racer enthusiast's joy of great driving experience with a great FE capabilities can use this as a great inspiration for the build.
http://www.youtube.com/watch?v=6cQETVQOYag
http://www.youtube.com/watch?v=7ITwD...eature=related
Crash tested too.
http://www.youtube.com/watch?v=S3ywadBp3wc

I will be using a cat and all emissions with the honda engine, they don't like running without them and also experience far longer warm up times. The stock cat flows pretty well, and I haven't noticed any gains in horsepower that were worth not having the cat. Typically you gain a little top end but at a loss of bottom/midrange power, it's something I only do for the track to keep the heat out of the engine when you're at WOT so much.

The Miata's big problems are the truly awful gearing—the high ratios way too short and the low ratios are too tall to just swap the rear end out. If I do an aero Miata project (it's 2 designs for a Miata, a tandem 2-seat Impreza IF I can make it look marginally attractive, and a cessna-bodied 3-wheeler I'm deciding between), I'm swapping the transmission with a 2nd generation RX-7. I believe you can swap the bellhousing and bolt it in, but I need to look into it better. From there, I'd be covering the undercarriage, building an external hood vent for the radiator, extending the rear bodywork into a Kamm/boattail type thing, and either building an aerodynamic hardtop or covering the passenger side of the cockpit over, removing the windshield, and installing a sliding teardop aircraft canopy over the driver's side. I'm just reluctant to only have a single seat vehicle.