Morelli Shape

[RobertSmalls]

Originally Posted by Rokeby

Something like the VW L1 concept vehicle perhaps:

It's an excellent design. Its very low seating position allows a low roofline for smaller frontal area, and more importantly a low center of mass which allows narrow track width. The driver's pelvis ought to be 8" off the ground for good aerodynamics.

The VW L1 has a good but not great drag coefficient of 0.195, but due to its tiny frontal area, its 0.2m² of drag area is far better than that of most aerodynamic concept cars.

Would raising the body away from the ground and moving the wheels farther apart reduce CdA or increase it? Well, the VW L1 is slightly more aerodynamic than the Aptera, so I'd say it depends on your implementation.

[Jim Bullis]

Robert Smalls,

Maybe so, unless that is all they can afford, and maybe they might want to pay the mortgage. But it is fine for starters to sell into the second vehicle market, which is plenty big for me.

[NeilBlanchard]

Well, maybe we are talking past each other? I am wondering how any vehicle can actually be built and be usable on the road with a Cd less that 0.1?

No reasonably practical road vehicle has ever been built with less than a Cd of 0.1 except the early Bionic/Boxfish model. The next lowest that we know of is the Aptera "Zen" at 0.11 with it's nearly pure Morelli and wheels on struts.

How can a rolling, practical car be built to beat lower this drag?

What I mean is, while the blue clay model may have had a Cd of 0.095, I disbelieve that its basic body shape would make for lower drag than that of the other cars I mentioned. A direct translation from blue clay model to real, drivable car (which M-B didn't do) would add considerable drag and would give it an impressive but not unprecedented Cd, and a rather ordinary CdA.

Matt, other than some additional ground clearance to clear normal road bumps and an air intake to cool the passengers, the design would seem to be able to at least match the Aptera "Zen", in my opinion. They do say that the shape is totally counterintuitive -- nature "figured out" something that we have not yet. The very low stagnate point and the wedged hood and hard chined front fenders are probably key. They match the function of that funky nose on the real fish, and they "work with" the ground surface.

I plan on integrating these into my CarBŒN design, as well as the more curved sides and roof; with the little "flip" at the back. Mercedes left most of these off of the final Bionic car, and they added a diesel which needs lots of cooling; and so their Cd slipped quite a bit. An EV has a minute need for cooling, so I think that much of the Cd could be "kept" in a real car.

The shape is certainly practical for lots of seats.

It's ironic that Jim is saying we should be able to build a real car with 0.06-0.08 Cd, and Matt is saying that 0.1-0.11 is way too much to expect...

We all want to have that ultimate low drag, low weight, uber-high efficiency car, I think, and I think that all of us are willing to accept what the best design requires within reason. (No maglev systems!) I think a monolithic design like the Boxfish model has the best chance of hitting these needs. I think that with a shape with outboard wheels has it's own more complicated challenges.

[KamperBob]

My motorcycle Cd is probably a lot worse than my truck [but] Af (frontal area) and Wt (total weight) are so much less that is goes about 5X the distance per gallon of gas. Shape is important, but just one factor to overall performance.

Cheers
KB

[Jim Bullis]

RobertSmalls,

The Aptera fixes the wheel well problem but is not high enough off the ground to get much of a free flow aero condition. Thus they have to depend on the Morelli camber effect to keep the Cd down. And this means the body is not very efficient for carrying things.

[Jim Bullis]

Neil and gang,

Everything said here about what can or can not be done with aerodynamics, where that is based on the overwhelming experience base of the automotive world, is mostly irrelevant.

Clear the air! Figuratively and in actual practice. Just because the fashion is established should not force us to always think about keeping vehicles close to the ground.

The great breakthrough for me that the Morelli paper provided was data on how high a vehicle needed to be to function as if it was in free flow conditions. Insight into this was provided by the way he ran his wind tunnel experiments. I never was all that interested in the shape that I call the Morelli shape since I came to the game thinking free flow as a starting point.

Since it is hard to get high enough to replicate the kind of flow that is expected in wind tunnels, it is necessary to live with some imperfection, and adjust somewhat with that in mind.

Aerohead seems to see that it would not be all that hard to make a four wheeled vehicle with an elevated body. We know from the dynamics shown by the Tango in the Xprize tests that a fairly tall and narrow vehicle can be stabilized quite well, just by gravity effects with a weight distribution that can be readily accomplished with batteries and no engine.

[RobertSmalls]

Jim,

How high does a vehicle need to be to mostly avoid aerodynamic ground effects, and does this height depend on Re? Do most solar cars meet the criteria?

Originally Posted by NeilBlanchard

No reasonably practical road vehicle has ever been built with less than a Cd of 0.1 except the early Bionic/Boxfish model.

That's the problem right there. The Boxfish model has never been made in to a car, and if it were, it would have a Cd MUCH higher than 0.095. It's just a Kammback, tapered in plan as well as in profile. In that regard, it's a lot like like the long-tailed EV1, or a truncated, oversized tadpole trike, and you would expect a drag coefficient similar to those examples.

[NeilBlanchard]

The early Aptera was very high up -- and they would not be stable enough for real driving conditions. They lowered them for a very good reason -- not flipping over.

I think the Mercedes Bionic/Boxfish model could be made into a working car an keep the Cd down at a record low level, for a practical, road going car; if it was an EV. If it has an ICE drive, or even a hybrid drive, it jumps to more or less where the final Bionic car was. Which says something that it matched the EV1, which has a fraction of the cooling needs that the diesel powered Bionic has.

I think the Mercedes B/B design is something very special (unique?) -- it doesn't look like any other uber-low drag car, and it has a far more practical, usable, human-friendly interior shape, and even though it is "just" a Kamm back, it beats all comers, so far.

Nothing ordinary or conventional about it.

[KamperBob]

If you guys are not familiar with the All American Soap Box Derby it's worth looking. Too bad I didn't have digital camera in 1978, a wind tunnel, or otherwise ability to calculate Cd. I was only 12 and my racer is long gone now but pix of others should abound on the web. Ignore junior division cars with upright but forward leaning driver. Focus on senior division with reclined driver. Only helmet protrudes from above. Construction is lathe/glass with rounded bottom and very little floorboard left to interact with the ground. Axle trees stick out like wings or fins but the dimensions are very small on a relative scale. Nose rounded in 3D. Tail tapers to a vertical line like a fish. H:W aspect ratios probably in the 4-5 range. For this discussion they are real ground vehicles with very little ground effect I believe. It sure would be neat to know what Cd these sleek buggers achieved... (sigh)

Cheers
KB

[RobertSmalls]

Originally Posted by NeilBlanchard

I think the Mercedes B/B design is something very special (unique?) -- it doesn't look like any other uber-low drag car, and it has a far more practical, usable, human-friendly interior shape, and even though it is "just" a Kamm back, it beats all comers, so far.

Nothing ordinary or conventional about it.

I am a mechanical engineer, and that is nonsense. The Boxfish model does not use any magical or arcane methods of reducing drag. It is a design that could be arrived at by squaring off and truncating an "egg-shaped" HPV or an airship, or by tucking in the rear wheels of a Prius (and adding a grille and cowl block and deleting the mirrors, good for at least -0.06). Neither the truncation nor squaring reduces drag; they are compromises that make the vehicle more practical, but less aerodynamic. A vehicle based on the blue clay model would have drag that is very impressive, but far from unprecedented.

It's not a bad design if you need a tall vehicle, and it would be more aerodynamic than anything you can buy from a major car maker, but that's not saying much.

Here's an example of a vehicle that gained +0.061 Cd between clay model and real vehicle, and whose entry at Wikipedia I have repeatedly corrected: http://ecomodder.com/forum/showthrea...6-a-13198.html