Designing a hard-shell rooftop tent

[BigChief]

Originally Posted by freebeard

I support that choice.



Two words: Interference drag. That's sort of the opposite to what I'd hoped to convey. Have you thought about putting a propeller on the front to recharge the battery?

Let's try again. Are you familiar with the Template? Here it is implemented at a higher resolution.



This expression of the aerodynamic ideal combines a geodesic primitive with a Cartesian coordinate layout. Every edge length is a known quantity to 6 decimal places, and the layout can be based on triangles, diamonds or hexagons.

If you build that I'll owe you a six-pack of beer.

Now, as to the lever arm from the Tesla's natural roll center to the new center of pressure. Here is a post from 2009 that introduced Englar's work at Georgia Tech

http://ecomodder.com/forum/showthrea...ots-10972.html*

It's all gone behind a paywall but if you follow up you will see that he demonstrated using pumped air to increase/decrease drag, lift and ruddering. So if you truncate the rooftop 'tent' and it has kilowatts of power available you could use it to steer the top of the car relative to the bottom.

OTOH, I worry about the 8x20ft floor plan. Could you get by with 7x16ft?



I had one of these once. It was 23ft with the hitch, 7 ft wide with a 6' 2" ceiling. You're talking about perching the equivalent on top of you car. I hope you have a plan for off-street parking (a store-and-lock or something) so it doesn't have to be on top of the car all the time.

Edit: *Not the best link, it degenerates into a discussion of injecting steam into the wake. Two papers you want are

A New Aerodynamic Approach to Advanced Automobile Basic Shapes
Alberto Morelli

and

Alternative approaches to rear end drag reduction Technical Report
Torbjörn Gustavsson

I did check out the template, but it seems like it would need to be very wide across (eight or more feet) in order to have standing headroom (six or more feet). It seemed like messing with the aspect ratio severely compromises the Cd? I don't suppose you'd know where I can find a Sketchup or Autocad model of what you just linked so I can scale it to my model and see how it measures out? I can indeed fabricate something like that very precisely, because I can have the whole thing carved from styrofoam and the carbon wrapped around that; it'll conform to curves perfectly. It's certainly a lot easier than building from triangles of sheet metal, at least.

I don't know if you were joking about the propeller; only way it could work is only when the car is parked, and only if I mounted it on the hood or trunk of the car low enough that it wasn't casting a shadow on the roof (shadows drastically reduce the output of the photovoltaics). I understand it creates more drag than it outputs energy when moving, of course.

When you mention interference drag, are you saying that a wide-open pipe on the top of the car would cause amplified drag by interfering with the stream of air coming up from the windshield? I guess that makes sense. I was checking out a model of the space shuttle before and I assume that is why they use struts to give it distance from the booster rocket, so that it wouldn't mess with slipstreams:

That's a conundrum though; if I can minimize interference drag by raising the camper farther up from the car, that makes the whole thing easier to tip and of course is less practical.

Still, I don't see why on-street parking would be an issue in any case. Where I live the legal height maximum is 13 feet 6 inches, and the Tesla is 4 feet 8.5 inches. So I have almost 9 feet to play around with above it.

But yeah, two questions for you; how do I find or generate a 3D model of the Template, like you have there, and what is the make/model of that trailer? I'm assuming it's an airstream of some sort?

[BigChief]

Thought I'd post the Flow Design screenshots of my most recent mockup, a pipe with two teardrop shaped beams connecting the camper to the roof:





You can see the wake in the rear of the Tesla, the lines with a blue hue. The drag coefficient the program gives always seems to be wildly inaccurate though, even against shapes and vehicles where the real life Cd is known.

[seifrob]

Just out of curiosity: can you rotate the model 30 degrees and test it again?
With this software on hand, you should have pretty good idea whats happening in crosswinds.

[freebeard]

I did check out the template, but it seems like it would need to be very wide across (eight or more feet) in order to have standing headroom (six or more feet). It seemed like messing with the aspect ratio severely compromises the Cd? I don't suppose you'd know where I can find a Sketchup or Autocad model of what you just linked so I can scale it to my model and see how it measures out?

Thee Template is a cruel mistress. It's a half-body of revolution; one simple curve swept around an axis. One member here generated a lot of discussion about what that curve is exactly: Equation and Spreadsheet for Template.

The half-body only has headroom in the top center. The full body only has floorspace in a strip down the middle. This is why I suggested the ME-262 over the airliner nose. In fact, I've questioned the need for it to be a body of revolution.



From left to right — round, squircular and square. Later I considered lowering the equator and ditched the cartesian coordinate layout the spreadsheet necessarily imposes with the geodesic geometry. Here's a compounded curved rendition:

I don't know if you were joking about the propeller; only way it could work is only when the car is parked, and only if..

At that point I was outright mocking you.

When you mention interference drag, are you saying that a wide-open pipe on the top of the car would cause amplified drag by interfering with the stream of air coming up from the windshield? I guess that makes sense.

That makes no sense. An open-ended pipe will self-interfer. Consider seifrob's comment and then put 'vena contracta' into the search box in the upper left. An empty culvert is a long way from an inhabitable space, no?

[BigChief]

Originally Posted by seifrob

Just out of curiosity: can you rotate the model 30 degrees and test it again?
With this software on hand, you should have pretty good idea whats happening in crosswinds.

Sure thing:

[BigChief]

Originally Posted by freebeard

Thee Template is a cruel mistress. It's a half-body of revolution; one simple curve swept around an axis.

The half-body only has headroom in the top center. The full body only has floorspace in a strip down the middle. This is why I suggested the ME-262 over the airliner nose. In fact, I've questioned the need for it to be a body of revolution.

Well, again, if you have any idea where I can download a 3D model I'll be happy to try it out. I'm just an amateur modeler so I don't think I'd have much luck modeling it myself.

Originally Posted by freebeard

At that point I was outright mocking you.

I figured; just making sure. I don't know why I warrant the derision. I never said I was a super genius; I have no engineering/math background, just blessed with some time/money/insanity and really want to make this work.

Originally Posted by freebeard

That makes no sense.

Well, have a look at these then:

Here's a plain Tesla, the drag area is at the grille:



Now, if I put the culvert on top, it seems like it squished the air going over the windshield as it passes in between the car's roof and the culvert's floor, creating an additional drag area:



But, if I raise the culvert up even higher in the simulator, the drag area seems to go away:



Again, I mostly don't trust this program's accuracy as the Cd is all over the place, but that at least makes sense to me, it's why I posted the space shuttle picture earlier, I think they spaced the two structures out in the same way to reduce interference drag.

Originally Posted by freebeard

That makes no sense. An open-ended pipe will self-interfer.

But wouldn't it still be less overall drag than one of those streamlined bodies you posted on account of how little frontal area it has? That's what I'm trying to figure out. If the enclosed streamliner still is more efficient then of course I'd head back in that direction.

Originally Posted by freebeard

An empty culvert is a long way from an inhabitable space, no?

Yes, an empty culvert is not a home, but with some fold-down doors and folding furniture that could be stowed in the Tesla during highway travel, it might be a nice compromise between habitable and transportable.

[Frank Lee]

Since you're considering folding aspects of it you could just as well do a collapsible design.

[freebeard]

But wouldn't it still be less overall drag than one of those streamlined bodies you posted on account of how little frontal area it has? That's what I'm trying to figure out. If the enclosed streamliner still is more efficient then of course I'd head back in that direction.

Have you noticed that biplanes have fallen out of favor? That's because the wings in proximity interfere with each other. Their advantage was the shorter spans. A barrel shape has the problem whichever way you look at it. Back to basics: In addition to form drag you have skin drag. The barrel/culvert has twice the wetted area.

But wait, there's more.... Go back and look at the Average Drag Coeffiecient in the pictures in Permalink #32 and #35. 0.52 goes to 0.72 at 30°. In the environment an automobile operates in it's almost never 0°.

OTOH if you look at the 1.14Cd returned in #36 for the slick-top Tesla we're back to whether your Skylake i7 is up to it.

https://www.pugetsystems.com/labs/ar...-i7-6700K-641/

CPU Performance - POV-RAY / Linpack

For the last of our synthetic benchmarks we ran POV-Ray and Linpack. Both of these benchmarks are widely used in the scientific community and are usually very good at showing raw CPU performance.

Starting with POV-RAY we saw a very large 7.7% increase in performance compared to the i7 4790K. This is a significant increase in performance and is an indicator that ray tracing in general should be much better with the i7 6700K.

Linpack, however, is a bit of an aberration and to be honest we almost didn't include it because we believe there to be a problem currently between Linpack and Skylake-S. Simply put, we saw a 20% drop in performance with the i7-6700K. We are still investigating why the i7 6700K is giving such low performance (we should be seeing at least 205 GFLOPs, not 173 GFLOPs) but for now all we can say is that the i7 6700K is giving very low performance in Linpack.

So, 100 GigaFLOPs is 0.1 TeraFLOP. But notice the software must be matched to the processor architecture. It's an up-to-date platform though, Keep an eye open for CFD done in neural networks.

There's other issues, how to generate the Template in four easy steps, the longer pylons would work just as well under a half-body, etc. What is the actual design case? It sure is nice outside...

[Hersbird]

Originally Posted by BigChief

I saved up enough to either buy a really nice RV, or buy a really nice car and put a camper on top of it. I live in the city with only street parking and drive out to campsites on the weekends, so this makes more sense logistically.

I've been driving an EV for years now (Smart Fortwo with some of my own ecomods like pizza-plate hubcaps) and once you have an EV you can't go back.

So I want to make the switch to the Tesla because of all the synergy with my camper idea. My Fortwo is super fun but is essentially a highway-legal go kart, I certainly can't tow anything with it. But a Tesla is heavy, powerful, very stable, auto-leveling suspension, battery 5 times more capacity/range than mine, and all current Teslas are being produced with autonomous driving cameras installed.

Picture this: my camper like I described is 20 feet long above the Tesla, which gives it enough space for around 2400 watts of solar cells (these only add about 20 pounds of weight if you encapsulate them in the same method like they make solar race cars). At 5 hours of sunlight in the day, that's 12kwh, after the inefficiencies of charging I might gain 10kwh per day back to the 90kwh battery. That means every day I'd get back 40 miles of range, or 20 miles of range and surplus energy to heat/cool the camper and run accessories. Now imagine in 5-10 years, the Teslas log enough miles for the government to feel comfortable legalizing autonomous driving. I could hop into the drivers seat, take a nap and take up in another city.

A self-driving road-trip without ever having to fuel or charge anywhere. I could park anywhere I want or drive to the middle of nowhere and make it home for a night or a week. Now that you've asked why, I'll ask "why not"?

Because it still doesn't make sense. You have to leave the camper up there all the time or would need somewhere to park it, it might as well be a trailer all of the time. A small single axle hi-lo would be towable by the car, is insulated, has full stand up height, and not much added frontal area in the down position. It even has a bathroom. You could find a really nice one for under $10,000. Look for a 17T model.
You are way underestimating what any structure up there is going to weigh especially once you add gear. The beauty of having it all in a trailer is it is ready to go all the time, and then doesn't need unloading after the trip. Weekend trips all packed out of the trunk take forever to load, forever to set up, forever to take down, and then forever to unload back home. Got a trailer? Feel like bugging out? Gone with a stop maybe at the grocery store on the way.

[LittleBlackDuck]

Originally Posted by Hersbird

Because it still doesn't make sense. You have to leave the camper up there all the time or would need somewhere to park it, it might as well be a trailer all of the time. A small single axle hi-lo would be towable by the car, is insulated, has full stand up height, and not much added frontal area in the down position. It even has a bathroom. You could find a really nice one for under $10,000. Look for a 17T model.
You are way underestimating what any structure up there is going to weigh especially once you add gear. The beauty of having it all in a trailer is it is ready to go all the time, and then doesn't need unloading after the trip. Weekend trips all packed out of the trunk take forever to load, forever to set up, forever to take down, and then forever to unload back home. Got a trailer? Feel like bugging out? Gone with a stop maybe at the grocery store on the way.

if the time and money are not an issue then I would just drive the Tesla and enjoy it for what it is. Then, use something designed to sleep in for the rest of the equation.

Doing the exercise for the heck of it is good, but I think that reality and sanity needs to be considered too. I would not be comfortable driving within a country mile of your proposed vehicle

Simon