Lotus 7/Locost Aero Rebody

[JackMcCornack]

Thanks guys, I appreciate it. I have a tendency to go "Nahh, I already thought of that," and ask follow-up questions that are so tightly focused as to border on the argumentative, but it's part of the process and I value your comments. So when I ask, for example "Does that make sense?" it's because I want to know, it's not a rhetorical question.

From Christ:

> Is that thing a Go-Kart?

That's 'cause I modeled it with a high end modeler (Rhino 4.0) and rendered it with a low end renderer (Rhino 4.0). Seriously, if I wanted to spend four days on detail work, give it a scaled background (a showroom with other cars in it, or the pits at Talladega) and render it with Flamingo it could look like a real car. Everything rendered with the 'stock' Rhino renderer comes out looking either like a fake whatever-it-is, or a real a bathtub toy. But for data to drive a CNC machine, Rhino can't be beat.

> It doesn't look road legal at all...

Everything that shows meets the Federal equipment specs. There were some design compromises for light height.

> With all sincerity, I just can't imagine keeping attached flow over that rear slope, and those sharp corners appear like nightmares to a 10 year old.

About 1930 somebody verified that large radii aren't necessary to keep airflow attached. Previous to that, every car-of-the-future had a bulbous nose and front edge radii approaching half the width of the car, but it turned out to be a waste of space forward of the front axle. From nose to front wheel the edges are well rounder than Hucho's non-dimensional radius of r/b = .045 (page 41 Aerodynamics of Road vehicles, which I reference a lot in this forum since most folks here are familiar with it and have confidence in it).

However, the radius of edges parallel to airflow (I hope! Though all I'm sure of is they're parallel to the direction of travel) are r/b = .18, roughly a third what they should be if they were perpendicular. My assumption is, since air going over those edges so obliquely (if at all) the "effective radius" is very high. Does that make sense?

But looking over my model again, there's one way those sharp corners would be a nightmare to a 10 year old--if I hit him in the shins in the school crosswalk.

From Bicycle Bob:

> I'd definitely narrow the front end, so that air would not be spilling from the front to the side...

I think you're right. Models and tufts, that's what I need. There are conflicts there between aero (I think the body should be the width of the tires before it gets to the front of the tires, to reduce the tire's drag and separation behind the tire, But for gentlest slope over the nose for a given body length, the body height shouldn't be the height of the tires until it gets to the middle of the tires) and styling (how round can it be and still claim to be an updated/inspired by/version of the Type 32?) and cost (the more slab sided the cheaper, which was probably what drove Bugatti--his later tanks were more organic) but from reading these comments I'll redraw it with a narrower and less pointy front end. Ain't CAD wonderful?

> ...and get the front and rear stagnation points closer to level.

Huh? I'm not following you. Level side-to-side? Level front-to-rear? Should F and R stagnation points be the same height?

> There's not enough air feeding to the bottom of the tail to maintain attached flow at that angle.

Any shallower and the tail lights get too low for the law. Besides, Hucho says four degrees so that's what I did. It will have a full bellypan which should help a bit. I don't think it's that bad; maybe it's an optical illusion, caused by the sides tapering in 15 degrees.

> The cockpit area looks ripe to perform mischeif, too.

As a roadster it'll have a tonneau and a fairing behind the driver's head (with a roll bar inside). It's a pity the road rules prohibit Lexan canopies...I'll have to look at how that single seat Metro was done (second post on this thread--MetroMPG, do you have a link to that?).

As a coupe, well, I'll have questions for y'all in the Spring. My concept isn't fully developed yet.

> A classic small flat racing screen would leave a big wake.

True indeed, and I doubt I'll use one. I know in some states a windshield is required, not in Oregon though so I may just dress like a biker for weather and eye protection.

Here's another view of the same draft as the blue on, just not rendered, from more to the side. The tail doesn't look so extreme when viewed from the side. Any other angles anybody'd like to see?



PS--samandw wrote:

> I wonder if the aft treatment on the Fiat Turbina would be something to look at.

To look at, and indeed to drool over. Gorgeous and efficient. But probably 20 times as expensive as my budget ($2k for the body) allows.

[Bicycle Bob]

Ahh, I'd assumed from a single view that it was slab sided all the way. 3-views clear up a lot of uncertainty.
One cheap way to find the natural streamlines is to carve a shape from soap or wax, and put ink dots on it that will be smeared by moving fluid. You can take it for a ride on your car hood, or put it in a flowing water stream.
As a first guess, I'd use the same shape you have from the center of the front wheels in side view, in plan view as well, to knock off the current corners. That should get you close enough. There will be some variation with crosswinds anyway, or there'd be no need for those radiused corners. It is fine for the air to fall off a "too sharp" edge at extremes, because you don't want that much side force to build up anyway. The curve gives a gentle stall, so there's no twitch, just better handling. Easy, na?

The side-marker/turn signal lights should solve any perceived apparent width problem with the headlights.

I recommend using Colin Chapman's "something for nothing" design, the venturi-bottom as on the Lotus 79 http://3lib.ukonline.co.uk/lotus/story.html You can start with a symmetrical wing design in Rhino, and squash the bottom against the ground so that the air goes across the top and bottom surfaces at the same speed. Therefore, there is no lift. The bottom still suffers from the wheels, which have to accommodated as well as possible.

[Christ]

Hey, what about moving the front wheels inboard to make OK spacing for a front wheel skirt?

Random suggestions box, please.

[basjoos]

Originally Posted by JackMcCornack

> Make the radiator inlet driver adjustable to the current heat load as you are driving. On my car I can adjust it from 0 square inches to 50 square inches depending <snip>

Good idea. I'm wondering how big the maximum opening needs to be to cool my 32 hp diesel. Basjoos, do you ever drive with the full 50 inch opening, and if so, how high a horsepower load do you think you're feeding it then? I doubt you're at full throttle much, and brief bursts can be carried by thermal inertia, but do you ever ask for more than 32 hp for extended time?

The only time I use the maximum opening is when doing the full throttle climb up into the NC mountains at the NC/SC state line on a hot summer day. Also when in low speed stop and go summer traffic, mainly to keep the fan from kicking on as often as those speeds.

[JackMcCornack]

Thanks guys. Basjoos, you're making me quite brave about a small radiator opening--I have less than half your horsepower, I doubt I'll get burned (or boiled) with half as big a radiator opening. Bicycle Bob, that's an interesting thought--I understand how ground effect cars make downforce, but don't see how (or even if) they reduce drag...I'll look into that. And Christ, that would clearly reduce drag (I think so, but I don't always think clearly...like does the rear wheel 'draft' the front wheel in the traditional layout?) but it would be a major undertaking and would reduce roll resistance. So I'm not going to do that on the first pass, but I won't throw away the parts I cut out in case I want to make front skirts later--if I get, say, 98 mpg with wheels exposed and skirting the front wheels will reduce drag by 3%, it would be worth the trouble to get the car into the three digit club.

[Bicycle Bob]

We are not aiming for downforce, just balancing the upforce, so there is no net induced drag. By using the space under the car for more airflow, the effective frontal area is reduced, and less air has to go over the top, doing the lifting-wing thing there. John Cobb's Railton is a good example of the profile I have in mind, although it could have the parallel center section as drawn. People are worried about the effect of ram air being compressed under the nose, but the venturi effect makes up for it. A rounded nose lets the stagnation point move up or down a bit, so you don't have to select and maintain the ideal splitter height. It also saves a bit on skin friction and weight.

[Christ]

Jack -

Just a crooked thought from my tilted brain. If you work it out, sweet. If not, O Well!

[jamesqf]

Originally Posted by JackMcCornack

Thanks guys. Basjoos, you're making me quite brave about a small radiator opening--I have less than half your horsepower, I doubt I'll get burned (or boiled) with half as big a radiator opening.

Just remember that some of us will be doing multi-thousand foot climbs up 7% or better grades in midsummer :-)

[JackMcCornack]

Originally Posted by Bicycle Bob

Ahh, I'd assumed from a single view that it was slab sided all the way. 3-views clear up a lot of uncertainty.
<snip>
As a first guess, I'd use the same shape you have from the center of the front wheels in side view, in plan view as well, to knock off the current corners. That should get you close enough.
<snip>
I recommend using Colin Chapman's "something for nothing" design, the venturi-bottom as on the Lotus 79 Classic Team Lotus You can start with a symmetrical wing design in Rhino, and squash the bottom against the ground so that the air goes across the top and bottom surfaces at the same speed.

Done, done (see pix below), and thinking about it. I guess next on the keyboard is to round the nose more in side view to move the stagnation point up, and try to predict its influence on airflow.

I'm not fully convinced about the benefits of increasing the height of the stagnation point relative to the belly pan. The Schlör "Pillbug" (being discussed on Neil Blanchard's <Scion xA aero mods> topic) had a higher stagnation point than my blue body and it had a smooth flat belly (as does my car) but IIRC they spent time testing the lowest drag height and found that the higher they went, the less body drag they had...but the higher they went the more tire was exposed, so they stopped at the crossover point between the tire/body drag curves. I don't think increasing airspeed under the body reduces drag, though for track cars that have minimal ground clearance at the longitudinal edges of the belly, venturis provide more downforce for less drag than a wing would (dimensions as defined by the race organizers).

[Bicycle Bob]

Looking good!
To get around the trade-off between wheel exposure and the official, hoped-for stagnation point, we need separate fairings ahead of the wheels. I hope that is not getting too far from the original Tank with its simple construction.

You might want to try large-diameter plastic sewer pipe to form top-side edge. It can be hacked and heat-formed over the wheels, too.

The Daihatsu UFE III has a nice front end compromise.