Flat or Teardrop?
 

I have been planning an EV build for some time, but recently have been talking to an engineer about the possibility of building scratch built a Solar/EV as opposed to adapting an ICE vehicle.

The two options are a custom built spaceframe or welding a spaceframe to a VW chassis.

The VW option means I only need to comply with 1971 Regs (which in Australia is a godsend).

The car will have a reasonable battery pack and the solar panels will be more for range extension and charging when stationary, but I can easily fit 1.2kw worth of 24V panels on the area I'm thinking about.

The car will be a 2 seater (side by side) with aeroscreens and aero fairings behind the heads of the occupants (much easier than a sealed cabin which requires lots of heavy stuff to go with it.

The passenger opening will normally be sealed.

Aerodynamics is at the forefront of my mind, and I was wondering if It would be better to have a flat wing profile with a fairly horizontal top (like most EV racers) or have a Teardrop as per the 'ideal' profile and send all of the air over the car.

http://www.wired.com/images_blogs/au.../solar_car.jpg

I'm worried that if I go for a slim wing profile, it may mean having wheel fairings that can be damaged and a chassis that would act like a diffuser.

Having the ideal teardrop shape is appealing but the curved 3D aspect required to make an efficient one was daunting me a bit as I have never used composites.

I got thinking of alternatives and recently spotted this -

http://i.ebayimg.com/00/s/MTIwMFgxNj...TQNgX/$_57.JPG

If I cut the bottom off the truck scoop off just under the scallop and used it as the front body work and then extended the body back as per the template (with the solar panels on the 'tail' would it be close enough to the template ideal?

It would make life a bit easier.

I'm sure aerohead will be along to weigh in on this in short order; this is right up his street.

For my own part, I want to comment on the human factors.

The wing profile would mean you could cover the car, as time and money permit, with solar panels. It would become a de facto perpetual motion machine. You wouldn't need to plug it in very often. If your daily commute is fairly short, you might never plug it in except under unusual circumstances.

The wing profile places several demands on the user however. Seating would necessarily be cramped and likely relatively unadjustable. Getting in and out would be radically different from a conventional car and possibly a lot more time consuming. That may not sound like a big deal but just try spending an extra ten seconds - it doesn't sound like much, does it - struggling with a canopy in a pouring rain and a ten-year-old Falcon starts to look pretty good.

The wing profile doesn't rise into the viewing plane of most drivers. For lots of drivers if you aren't tall, you aren't there. Similarly if you aren't wide you also aren't there, to which many motorcyclists can painfully attest. People are creatures of habit, and are in the habit of looking for things that are more or less car shaped while driving. Things shaped like cars are to be avoided while driving, things that are shaped like pedestrians are to be avoided. Things outside of those two very basic silhouettes take just a tad more time to process, and in that time accidents happen. You might build a perfectly lovely wing shaped car based on completely conventional VW underpinnings, how could anyone ever fail to recognize that it's a car, of course it's a car I can see that standing here looking at it, but can you expect every Tom Dick and Sheila affixing her lashes on her morning coffee run to understand that in the quarter-second's attention she lavishes upon you and your not-conventional ride? Maybe not.

The teardrop is already more or less car shaped. Cars have been evolving toward the teardrop ideal for the last 20 years, some more quickly than others but it's definitely happening. The Prius is probably the purest representation of this form and has been on the roads now for a decade, no driver looks at Prius and fails to see a car. They might fail to notice it, but if they notice it they don't see something that isn't a car.

The teardrop gives you more height. If you're as tall as the Falcon in the next lane over, the inattentive Bruce in the gigantic GMC trying to change lanes will see you there and probably not merge all over you. Probably. Your roofline is just visible beyond the top of his fender, so maybe he has an inkling not to try the change just yet.

The teardrop gives you radically more volume within its shape. Volume = seats, space for heads not encased within nacelle canopies, more seats in the back, and doors shaped more or less like all the other doors on the road. That right there is great, it means you have a wide world of door hardware to choose from, hardware that's already been engineered to be sturdy and reliable and all you have to do is find the right stuff whose geometry is close to what you're building, and you're good to go.

Lots of studies and talk on that platform done in this forum over the years.

I suggest you do a forum search.

It's one thing to just think about doing something, and another to actually do it.

The simple approach is often best if you don't make to make a lifetime long project out of it.

Do you want to be working on this project a year from now?

Do you still want to be working on this project five years from now?

How about 10-years from now?

solar EV
 

I'd recommend that you look at as many World Solar Challenge competitor cars as you can stand,and see if something strikes you and looks like you could mimic the fabrication.They're all going to be low drag,whether wing,or half-body in origin.
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If you were going to try to use the VW chassis,you might be able to graft your sectioned truck fairing for the nose.It would save a lot in fabrication time.
This 2012 Bochum University SolarWorld GT is a nice and easy to live with 2-door coupe.You just walk up to it,open the door,get in and go;without requiring a 'crew' to get in and out.It's Cd 0.137.pretty sweet.
http://i1271.photobucket.com/albums/...2/HPIM1705.jpg
If you extend and soften the rear contour you could see Cd 0.10,like the 2013 Cambridge University's Eco Racer,CUER
http://i1271.photobucket.com/albums/...ohead2/602.jpg
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For 'wing' type side-by-sides,you should find a bunch of examples from past WSC competitions.They would be easier to fabricate and they'd have less frontal area.
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Plex or Lexan bubble windscreens may not pass legal requirement.You need to know.
This 'VORTEX',from the 1980s used flat laminated safety glass for its windscreen.It might be a 'default' design if push came to shove.
http://i1271.photobucket.com/albums/...ad2/06-249.jpg
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Fun project!!!!!!!!!!!!!!!!!

If this car was available for sale, I would buy it in a heartbeat.

I agree, and I had almost forgotten about this car. It is better in several ways than the EvE car.

Do you have a 1971 donor car? Or is there a cut-off date in the regulations?

If you're doing it for the VIN number, it would basically be a one-off kit car. Else you could just use the VW front axle and make a reverse tricycle.

What are the dimensions of you intended solar panels? Are they flexible?

The one thing people forget when positing low-drag car bodies is stability. The classic half-teardrop is a stability nightmare as soon as the air flow deviates from straight ahead. Add a vertical fin along the medial line in back and the nightmare's averted.

The issue is torque around a vertical axis, yaw in aeronautics. When the center of pressure is forward of the center of mass, crosswinds turn the car, and the farther it turns, the greater the torque. The vertical fin has little effect when airflow is parallel to it, but acts like a parachute in back in crosswinds, countering the torque on the front of the car, and keeping you pointed in the same direction, albeit perhaps a few feet downwind.

The wing-based design gains stability from the slab sides, while CUER and Bochum U are not suitable for road use, based on the views shown (could gain stability from features not visible in the photos).

Just another aspect to consider...
Frank

stability
 

In his book,'The Leading Edge,' Goro Tamai commented on how stable these shapes were.
With simple attention to body inclination,zero lift was very easy to achieve.The streamline body of revolution in free flight is incapable of generating lift.
The teams had no difficulty in crosswind or gust conditions,simply by incorporating a spine at the roof ridge.
Whereas upper fins had been the historical palliative for center-of-pressure,center-of-gravity issues,Professor Alberto Morelli achieved balanced yaw response by incorporating rear wheel fairing/fins to keep the CP aft of the CG,killing two birds with one stone.
Here you can see how the belly of the SolarWorld GT is acting as an inverted wing section with the aft-wheel structures acting as capping plates,with a degree of dirty transom for weather-vaning.
http://i1271.photobucket.com/albums/...2/HPIM1716.jpg
It's my opinion,that at 'normal' vehicle weights,and 'normal' vehicle velocities,that these forms are perfectly fine.Transonic speeds would be something different.Even the Lamborghini Aventador gets spooky up there.

If I can keep the floorpan then yes, it becomes just a modified car as opposed to an individually constructed vehicles. An ICV must meet many modern Australian Design Regulations which makes the whole process a lot more difficult.

The solar panels I was hoping to use are semi flexible. They are very lightweight and aluminum backed and will follow soft curves fine and I was going to use them AS the top bodywork. They are 2020mm long by 990mm wide. I was going to have 2 side by side in front of the driver compartment and 2 behind for a total of 24V @ 1200W.

I think a 2D wing shape might be the easiest to implement as it will leave me with a flatish top.

I've overlayed the shape on a VW chassis and it should work OK.

If I use the VW floorpan, I could get away with a shape like below and still cover all the hard points with 13inch wheels.

It would use 4 large semi flexible solar panels. The front 2 panels would tilt forwards to allow fairly easy entry and exit and to allow access to the front storage area. The battery pack would be behind the front seats and the motor would sit in the cavity where the VW gearbox would normally reside driving a diff from an E30 BMW or similar in direct drive.

The dimensions would allow sufficient overhang at the sides to allow the front wheels to turn full lock and not foul on the sides. I would run a 'straight ahead' fairings on what was visible of all 4 wheels.

It would be quite large in plan at 1980mm wide and 4900mm long but the frontal area should be pretty small and the 2D nature of the design should make it relatively easy to manufacture. I can further reduce the weight of the donor floor pan by replacing the steel floor with aluminium panels.

http://rccv.info/evplansmall.jpg

These are the solar panels I wish to use.

They are 24V and 300W each.

They weigh 9 Kg each and are 2020 x 990 x 2.5mm.

I could fit additional panels on the sides but their value would be minimal unless the sides could flip out when parked.


http://i.ebayimg.com/00/s/MTYwMFg4Nz...nRg~~60_57.JPG

shape like below
 

Way fun project!
Consider a couple of guide-ons or camera system for the front corners,as your going to have difficulty discerning where the front of the car is from your seated position.
'don't want to crunch that beauty!

solar 'power'
 

Here is a cleanup of the drag/power table for the 1996 HONDA Dream WSC winner.
The blue area is Rolling-Resistance
The yellow,aerodynamic
At 99.419 mph,the car requires only 5.862 kW (7.861 hp)
If you give it the powertrain mechanical efficiency of a 1962 pickup truck and the BSFC of a Chevy V-8,your looking at 188 mpg @ 99 mph.
This is the game you're entering into.:D
http://i1271.photobucket.com/albums/...ntitled2-5.jpg

The plan appears logical, except for the 8 solar cells to either side of the passenger compartment. Do they come as small subsets of the four big ones?

You know what it reminds me of? Joseph Ganz's Mayfly:

http://ecomodder.com/forum/member-fr...ure6297-g9.jpg

and that Bugatti Stratos concept:

http://ecomodder.com/forum/member-fr...x384-19946.jpg

Here's what the Stratos concept did to deal with the crosswind problems you get with those slab sides
http://ecomodder.com/forum/member-fr...-concept-0.jpg

The best way to approximate a compound curve with a rectangle would be to leave the central diamond flat and roll each of the corners down. That would introduce gaps in the middle, front and back, and either side of the doors. But it would help crosswind performance. In the real world it's always a crosswind.

Edit: Think about fineness ratios. The headrest fairings and wheel spats should be 10:1 but the main body can be 2.5-4:1. I estimate yours as about 10:1 and Ganz's as 4:1. Maybe I should be taking twice the height—aerohead may have an opinion.

But consider a stretched version of the Ganz profile. The front panels could roll over the front wheels and drop down to lower the beltline, and then the rear panels roll over the rear wheels to provide a bluff body for the turbulence from the open cockpit to reattach.

What is your measured wheelbase. It looks to be a foot or more long. A 165/50R-15 tire has a 2 3/8" sidewall, and so a 20 1/2" diameter on your 15" rims. Else Rabbit 13" rims might clear the brake parts.

I'm a little restricted in overall shape by how far the solar panels can curve.

That shape is probably getting close to the curve limit.

I legally need the front to be section to be above 400mm so I can fit the headlights and Number Plate in the front curve. The front will be a tight radius curve of lexan. I can't really fit the headlights anywhere else with the solar panels.

The little solar panels are just to fill in the gap. I'm sure I can find something in a similar size or even just make my own.

I was planning on running 13 inch wheels with a 135/80R13 tyre which is why the wheelbase looks long. It's pretty much to scale with that tyre.

http://rccv.info/evplansmall.jpghttp://ecomodder.com/forum/member-fr...858-images.jpg

I'm looking at these with a 4" brass caliper held up to my screen. YMMV The Beetle has a wheelbase measured at 2 1/4" and a cowl height of 3/4" — 9/3. Your drawing measures 4 1/4" and 1" — 17/4. If that's a small-diameter wheel, then it's an even tinier mannequin (blue dot to top of head = wheel diameter suggests 5' figure). I know it's a lower seating position, but still...

If you move the 0° camber point forward, there would be more curvature in the front facia and less needed in the solar panels. Are the windscreens transparent over the solar panels? You could have a legal-height light in the front of each one and the melt-the-roadsign-reflectors lights low on the front or they could pop up out of the headrest fairing.

I'm curious about transmission you mentioned. My thought was to get a small enough motor that you could take the gearset and nosecone off the transaxle and stuff it inside the case.

Thanks for the feedback.

I think cooling of the motor would be a problem if it was enclosed, but it might save weight. I really need a small lightweight alloy diff...maybe a Subaru rear diff or similar.

The pan I'm thinking of using is a Type 3 with trailing arm rear end.

I'm not sure if the wheelbase is different but I got it as close as I could using the rulers on paintshop...:cool:

Yeah, the windscreens I'm thinking of are clear plastic off a motorbike.

Yeah, the shape may change a bit depending on seating position and legalities.

I'm just trying to get a plan together so I can talk to the engineer before I start so I don't end up backed into a an expensive corner down the track.

You have an engineer? Do you know your auto history? Here's the Bugatti 'Tank', the Type 32

http://upload.wikimedia.org/wikipedi...pe_32_Tank.jpg
http://gomotors.net/photos/9a/26/car...nk_f0689.gif?i

Notice the plan taper ahead of and behind the axle lines. It wasn't enough to make it a successful design. It came in third in one race. Have you thought about considering other solar panel options? The long narrow strips on that solar racer look more managable. Personally, I'd prefer 4" hexagons that snap together with conductors in the edges.

A Type III pan, eh? The front footwells aren't so cramped, but do you know about the differences in the front suspension? The framehead comes out in four fingers to hold a stamped axle beam.

http://www.vwtype3.org/owners.manual...aynes.susp.gif

The best thing about the Type III is the front and rear suspensions are both on rubber isolated subframes, so they ride real nice. And you can adjust the torsion bars front and rear with the stock parts.

What do you plan for between the flooprpan and the solar panels?

In Australia, modifications have to be approved by a Government Approved Engineer to ensure the car is safe and complies with all the regulations before it can be registered and driven on the road. If you can present a plan before you start, the engineer can look it over and tell you what needs to be changed so you don't end up with an expensive boat anchor at the end.

The 'tank' has much more frontal area than I'm hoping for but I will try and taper the edges to prevent votice formation, but it's a compromise between having maximum solar area and aero. If I can keep the frontal area really small, the slab sides hopefully wont be too much of an issue.

With 13 inch wheels and a small tyre, the suspension beam will be the highest chassis hard point on the car.

There will be a spaceframe chassis between the pan and the panels that will also house the batteries and other associated electronics. I'll probably use a chromoly steel to keep the weight down. I may even get it made from alloy tube and bolt the spaceframe to the pan. I'll calculate the weight difference and go from there.

I want to house all the EV drive stuff behind the front seats to keep cable runs short and to keep the weight balanced and low.

I tried calculating a few other solar panel options but none could provide the same output in the available space. I may end up using some smaller panels or single cells on the sides, but I'm not sure how much benefit they will provide (although the sun is rarely directly overhead)...

If you take the shape of the Bugatti, only above that swage line 10" above the ground, and the diameter of the wheel w/o the tire, it's a pretty close match to your plan—given that the wheelbase is 18" less and the track is 10" less than the VW.

The plan taper is less than what would be helpful; but the real problem is the 90° edge between the top and sides. Theory wants a minimum radius 4% of the gross width. With your 86" panels, that adds 7".

http://ecomodder.com/forum/member-fr...chnical-10.jpg

Yeah, that spaceframe is kind of what I'm doing, but extended out beyond the wheels obviously.

I do intend to have the edges rounded, but to keep it simple I was going to get an exhaust shop to bend some round pipe to the correct shape and notch it in to the top edge of the spaceframe so it's level with the rest of the frame on the top and sides.

I was going to use 3 inch pipe, so that would be the edge curve. I could use 4 inch I suppose.