Morelli Shape

[NeilBlanchard]

The Schlörwagen (aka Göttinger Ei) has obvious flaws around the lower/rear as shown in the wind tunnel picture I found on the German aeronautics site a while back: the air lifts up starting just ahead of the rear wheels and it lifts up a lot behind the rear wheels. The trailing edge needs to be widened and flattened (into a "beaver tail), not unlike the Morelli/Aptera does. Also wheel strakes would help a lot.

Don't forget it has the Mercedes 170H chassis with it's rear 4-cylinder engine -- this makes cooling it quite difficult -- and it needs a lot of cooling like all ICE's. Make the Schlörwagen an EV and you probably can claim back much of the .061 drag, and with the outside aero tweaks, it might even go below the 0.13 of the early model.

The early Mercedes B/B model beats the Schlörwagen model by about 0.035 (in an already stellar realm) and you don't think that is amazing? This is about a 27% reduction -- from amazing down to stellar, in my opinion.

Can you point to *any* four wheeled street car with a Cd within even 50% of this?

The Edison2 VLC has an "old" method Cd of 0.145, which is ~52% higher drag than the 0.095 that this Mercedes B/B model achieved!

Obviously, the Cd gets worse as you make it "real", but if we can see even some of the benefits of such a stellar starting point (as opposed to even the 0.13 of the Schlörwagen!), then it is worth trying.

We can't forget the most real example of ultra-low drag: the early Aptera "Zen" -- with a CFD drag of 0.11, it falls between the Schlörwagen and the Mercedes B/B model. And it seats 2 people. The Mercedes B/B is 4 (5 if you make it an EV and set it up like the CarBŒN) -- and the Schlörwagen seated up to 7 people!

Which brings us to the next challenge: keeping the covered front wheels without the huge increase of frontal area that that hampers the Schlörwagen design. I think I have the best answer in the articulated hinged panels on the CarBŒN. With the Morelli based solutions, you are forced to add the outboard wheels and the required structural supports and the *interaction* that these will necessarily have with the main chassis.

The Aptera "Zen" is the best example of how this can work -- it more than doubled the 0.05 Morelli shape up to 0.11. The Mercedes B/B model takes the 0.04 fish and the 0.06 computer version, up to 0.095. So, unless there is something I am missing, I stand by my assertion that the Mercedes B/B model is the best starting point for the lowest possible drag car. The fact that it does this while seating 2X as many people puts it into the stratosphere...

And if you require 4-5 seats, it is even better, since it beats the (unimproved) Schlörwagen, as well.

[3-Wheeler]

Originally Posted by NeilBlanchard

....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....

Hi Neil,

Can you elaborate on the flip at the back? I'm curious.

Thanks, Jim.

[Jim Bullis]

Robert Smalls,

The height necessary to avoid ground effect that Morelli showed for a teardrop with a flattened bottom was about half its width. As I recall his data was scaled to automobile sizes and speeds, but I am away from home where that paper exists so I can not check that; and again from recollection, Morelli was a little short on detail such that some confusion remained. However, his published test curve showed that when the vehicle was half the width the ground effect had mostly tapered off. Here we were seeing a free flow Cd of .07. The flattened body is not so much flattened that the body height differs a lot from its width.

It looks like the solar cars would meet a criterion for road clearance that is based on body height, and this makes sense since airflow that would otherwise pass through the body region has to be split between top and bottom. The wing model is relevant here, and there are known wing shapes where airflow underneath is quite straight. So perhaps they could be somewhat lower and still qualify as free flow vehicles.

But I am not too interested in solar cars, being more practical by nature. So for bodies having roughly the same width and height, I reason from comparing flow patterns that would occur for a true body of revolution (BOR) that the height needed might be somewhat less than that needed for the flattened BOR of Morelli, since the impact of the ground would be reduced at points further out laterally.

It would be extremely useful to have real Computational Fluid Dynamics (CFD) modeling capability on this project. I pursued this with financial limitations, and started with a "cloud" service called CHAM or Fonix or something like that. They offered a free month trial, which I signed up for, and proceeded to validate the model for an airship body. For coarse mesh sizes, the model would run but the Cd values were way too high. As I reduced mesh sizes and flow sizes the model became more and more unstable and took huge amounts of computer time. In the end, I think I brought the servers to their knees, and the company was not even interested in selling me their service after that trial. Ultimately, the best I could do was a Cd of .12 which was a run that only worked once. That was with substantial separation where Cd should have been about .04. (The flow field looked steady at the outer edges, but even that was not satisfactorily tested.) Clearly, this would not help much with the kind of fine optimization that would be great to have. I did get some feeling for flow effects for the airship form relative to a boundary. I would not say this makes me an expert. I might search again for CFD services, since that effort was some time ago. Does anyone have suggestions? (Though Miastrada Company is a large organization -joke- it does not have a CFD analysis division.)

Given your background I imagine you perceive that this is not a fully resolved issue, and yes, I see this as a development program whereby an optimum shape would be determined before anything like production would begin. However, I feel fairly comfortable with a four foot diameter body and a two foot clearance for starters. If possible in building a full scale model, I will make provision for adjusting strut height so that some hard data can be obtained. But ultimately it is necessary to actually build and test, especially in this field, so I am trying to accomplish this.

The real hard facts are the airship test data itself, including angled measurements and measurements width fins of different sizes attached. (See Freeman 1934 from the NASA reports server or the miastrada.com site. ) This is a complicated read, but reading this, and other reports of that era that considered attached gondolas and so on, gives me some confidence that this can be made to work.

[NeilBlanchard]

Hi Jim,

The bit at the very back of the roof that curves up a little:

[Jim Bullis]

Neil,

You mention the CFD drag for the early Aptera. Is that published somewhere?

But did you hear yourself? That was for a double wide seating arrangement, and as I recall Steve Fambro said a long time ago that he considered double wide seating an absolute requirement.

So imagine perhaps, that we get rid of the double wide seating idea, and consider how the road effect will be changed with the body of revolution, rouded underbody instead of the flattened one that double wide seating imposes.

[Jim Bullis]

Neil

From your boxfish flow picture, it looks like the aft flow is not at all laminar, and over a big volume at that, so it is hard to understand how they do as well as they claim. In fact, it looks like a bluff body which is the industry assumed standard.

I wonder if the PR people that wrote the reports were really the ones running the wind tunnel. Oh well, forever the skeptic is me.

[Frank Lee]

Unless the Miastrada is burdened with one heavy-*** undercarriage, it will tip over before it gets out of the neighborhood, rendering aerodynamics moot.

[Jim Bullis]

Frank Lee,

Right you are, except the burden is not as bad as you seem to think. If the articulated form is used, the wheel system has a very strong stabilizing action, but the detail of that is a little difficult to see from the first picture. If we just use low weight to handle stabilization and use a four wheel system, that will work also, though in that case the wheel base will need to be widened some.

[RobertSmalls]

Originally Posted by 3-Wheeler

Hi Neil,

Can you elaborate on the flip at the back? I'm curious.

Thanks, Jim.

Jim, the 2010 Prius has a trailing edge that may be worth emulating.

[Patrick]

Originally Posted by NeilBlanchard

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

Laminar flow and boundary layer control.