Tall Paul

From some tests we did on the Tristar, Feb 1980.... We had sold British West Indies Airlines a couple of planes... without finding out one of their stops was at an airport the normal Tristar wouldn't be able to get out of safely with an engine failure, so a larger rudder was designed and built.. more chord.
We tufted both sides of the aft fuselage and ran a series of engine failure tests at Edwards.
Here's two images scanned from the negatives.. one with the rudder faired, showing normal flow, and the other with the rudder hard-over, showing the way the tufts react to the airflow. We used parachute cord, with the stiff center removed for tufting. The weaved outer shell of the cord was floppy yet wouldn't disappear as rapidly as wool, at the speeds we were testing.
For the car, wool should be fine.
Make sure the color is contrasting to the background.
We tufted an engine pylon also..
And when testing a glove fairing for the wing engine pylons, got this unusual puddle of air, which told us the fairing wasn't of much use.

allanflowers

Here ya go. Just cover the horizontal "stab" with solarcells.
Allan

Tall Paul

A couple of "scene-setting" images for the fin tuft testing..

Ben Lanterman

I believe I saw that "wheel" motorcycle on a TV show in which it went unstable and did an interesting flip and slide. I believe it's why he went to the horizontal tial for stability.

The designs on the web site of last years race are interesting. Thinking about the speeds and looking at the heights above the road and shapes involved there can't be a whole lot of ground effect. The canopy on some sure looked draggy. You need something that could pick up the bow wave of the car that is following the racers! I would imagine there is a minimum distance to keep that from happening.

I also bet they are hot to drive, no airconditioning and a black upper surface busy absorbing energy, yeech.

But looking at the winners I don't see much room for improvement in aerodynamics unless you could bury the driver inside and have him see and steer by a small video camera in the nose and use a liquid crystal screen in the cab for viewing.. That would give you an unbroken top. Certainly lower drag. There isn't much need to look around and make hard maneuvers, etc. Of course laying down would have a tendency to put him asleep. Might need big speakers yelling wake up.

I assume it must carry a person but how about remote control steering. He could just go along for the ride as cargo while someone else in the chase car did the steering - again no canopy. Of course you would have to severly drug him to get him in the machine. Maybe have some dvd's to watch on the way, a game or two to play.

Hatty

Pretty good ideas, but the rules prevent all of that stuff. But you are correct when you way that there isn't much improvement to aerodynamics that can be done. There has been a big discussion about making the cars more passenger car-like. This would increase the drag, but give students more things to think about, while making the cars more practical. I would like to see it go that way. But it is very interesting to see how the cars have changed from, say, the 1990s.

But, make sure you check out the route that the cars will follow this summer. If it is close to where you live, come watch. You can even come to one of the media stops and say hey to my team. I will be there. Solar racing it the greatest thing, behind RC of course .

Ben Lanterman

Paul, doesn't it boggle the mind when the airplane goes one way and the tufts go the other!! That shot of the wing is priceless, no one would have predicted it, and the ones that said they did are lying :-) We tufted our 13 percent scale slow speed model of the F-15 but luckily nothing strange showed up. When we were doing a supercirtical wing on our F-14 entry (lost that one) our 5% model was too small for tufts but we used the oil drop technique to check for low transonic effects. It was fun, each run required many dozens of little drops to be put on the wing before each wing.

I will look for the race this year, I saw that they came through St. Louis last time but missed the whole show.

Tall Paul

The results of the pylon glove tufting were controversial, so we did a couple of times. From the Lear it looked like this...

allanflowers

I can see from the photo that the airflow is not so great in that one area. On the other hand, the tuffs are only indicative of the boundary layer flow and don't necessarily mean that the general flow is all that bad. Sure it was worth working with to make better but you should see the tuff studies on the hood of a 77 Buick. One would be delighted to have flow like that on your wing.
Getting back to the subject of this solar powered car, I am afraid that the impression is being created that the tuff studies, if these students do them, will be so easily interpreted. For instance, if the tuffs show that the flow on their side is flowing over the edge to the top, is that so bad? And what do they do about them?
IF they can adjust the angle at which the vehicle runs, that is definately worth trying - and probably a reasonable way to improve the cd. However, most people would THEN come up with the idea to fence the edge, to prevent the flow going over it. And it is very unlikely that this approach would do anything but increase the drag, even though it would straighten out the tuffs.
Allan

Jimmbbo

Yep, it's that simple- as BMatthews said, use contrasting color yarn taped in a regualar pattern.

If the tufts show an area of disturbed flow, you can make and install fairings from heavy paper and tape to smooth the flow. For larger regions of disrupted flow, more significant modifications may be required...

Cheers!

Jim

old git

I was with Lola Cars and Ford Advanced Vehicles circa 1964-65 working on fabrication of the GT 40. The body design was done in a wind tunnel in Germany. It took 18 of us to move the clay plug from the transporter into the workshop of Specialised Mouldings in Crystal Palace. Little was known about aerodynamics in the race team headed up by John Wyer (ex Aston Martin). I was asked to make a spoiler for the rear body by folding a 4' length of aluminium sheet across the edge of the bench. This was fixed to the body with self tapping screws. At around the same time Jim Hall was reported to be using a huge cooling fan at the rear of the Chapparal. The real reason was said to be to produce negative pressure on the underpan. It was also said to lift detritus off the race circuit and project it into the eyes of the competition.

The GT 40 was not an immediate success, overweight and underpowered but there was a steep learning curve. As a lowly mechanic/fabricator when there was discussion about where to put a cooling duct for the brakes I was shouted down when I suggested that there was positive pressure under the car. Another anecdote comes to mind, we had Booked Monza for a week to test the two cars. One lost the rear quarter when the rotoflex coupling failed and the drive shaft flailed around destroying whatever it met. The other car was driven by Bob Bondurant and he said "As I arrived at the braking point the throttle would not come up, so I stamped on the pedal and $%£&*, I got more. He had walked the circuit so he picked a place where the trees were smallest 3"-4" dia to GO OFF. The result looked like an air crash. 200yds of wrecked greenery and flattened trees with a huge abount of fibreglass shards everywhere. We got one car working from parts of the wreck and used the de-cornered car. The car set off on another test lap without the rear bonnet/engine cover fixed. The rear end took off into the air and reached about 100' altitude. This was repaired and the car carried on testing. We used 2" scotch tape all over the car inside and out and were called "The Black Tape Special" both within the team and by some of the other teams.

rmh

When racing was really fun---
The roundy round stock car guys were still running " stock bodies " but the thinkers were playing with angles of attack, and taping things as they could without getting flagged by the tech boys .
Some of the streamlined bodies were worse than the squared ones - they actually lifted the rear of the car --
Ther was a good ol boy by the name of Smokey Yunik who kept showing the high paid engineers the short way home .
He was considered a renegade but his ideas really worked -till the tech boys figured them out.

Hatty

Actually, we can adjust the AOA, to a certain extent. The ride height of our car is adjustable, so, if needed, we can change the front and rear in the opposite directions to improve the drag. And if necessary, we can rip off parts of the shell and reattach them to change the angle. Lets hope it doesn't come to that.

Ben Lanterman

It would certainly be worth while then to do the coast test noted earlier in the thread. Adjust the angle until the drag is minimized. At that point you might not have any bad flow.

Salto

Hatty,

I've only just come across this thread, but I'll add my bit anyway. I have some experience in this field, having designed and built the fourth placed entry in the innaugural World Solar Challenge across Australia way back in 1987. I've been involved in other solar car projects since then.

My first comment in relation to your question about rounding off the rear corners is that there should be absolutely minimal additional surface area over and above the area of the solar cells (plus the necessary equivalent area under the car). In other words the car should be no wider than the panel, so there should be no area that you need to taper. The shape in side view should come to a point at the rear (as you have done), but ideally the shape in front view should also come close to a point at the "tips", therefore adding no extra wetted area.

At the speeds these vehicles travel the trade off between aerodynamic drag reduction and added complexity and weight, comes down strongly in favour of improving aero drag. Since the vehicle shape should be close to an ideal low drag body, the flow will stay attached to the surface until some point close to the back, the further back the better. With mostly attached flow like this, the drag of the whole vehicle will almost entirely come from skin friction. So, keeping the surface area of the whole vehicle down is a top priority. Maintaining laminar flow as far back from the leading edge as possible is also vital.

As someone has already stated here, your vehicle body should not be generating any lift, so vortex control should be irrelevant. Up high in clear air the ideal shape for lowest drag would be based on a symmetrical airfoil of some sort. Wind tunnel tests have shown that as a body approaches the ground the ideal shape morphs to a cambered airfoil. When this cambered shape is set at the correct angle of attack, there will be no lift, and hense little or no tip vortices.

On the question of wheel fairings, you should definitely add fairings around all protruberances such as wheels, driver bubble, lights, drive chains etc. These get complicated around the steered wheels, but if properly done, should produce lower drag.

A really good reference book when building these vehicles is Speed of Light by Roche, Schinckel, Story, Humphries, and Guelden, published by the University of New South Wales. It is a technical record of the 1996 World Solar Challenge, and also has general instructional chapters on the technical aspects of solar vehicle design.

Hope this helps,
Graham.

Ben Lanterman

Interesting reading Graham, is there drag to be won back by going to wheel fairings that steer with the wheels than having a fixed fairing that allows steering to be achieved or is there a minimal steering command used that is just enough to keep it going straight down a straight road?

Basically you are saying you need a knife edge all round the circumference of the body which sounds OK, have there been studies done that determine when the flow goes turbulent down the length of the body. Whether a classic airfoil shape is better than a equal camber front and back or is completely flat across the top the best?

And where do you put the airconditioner when dashing across Australia?

interesting stuff!!

Salto

Ben,

Most possible wheel fairing configurations have been tried, from fully faired spats mounted on sliding turntables on the belly pan that steer with the wheel, to spats wide enough to accomodate a restricted steering lock, to no spats at all - just discs added to the spoked wheels. You only need a small steering angle to travel down the road, but the vehicle also needs to be maneouvred through intersections, around obstacles, and on and off the road at the start and end of the day's racing.

Of course, at zero yaw angle a narrow spat will be best, but in the Australian race where significant cross winds can be expected you want to choose an airfoil that will maintain low drag over a wide angle of attack. So generally an airfoil wider than the wheel will be used.

Another consideration is how close to take the spat to the road. If mounted from the body there must be enough ground clearance to allow for suspension travel and pot holes, but then again the bottom of the wheel is travelling at low or zero airspeed so it doesn't really need to be faired at all. So low mounted bodies with only the lower part of the wheels exposed can probably get away with no fairings, but these bodies will be too close to the ground to achieve lowest drag, so this is not a good layout. A more ideal high mounted body will expose all or most of the wheel so fairings will be needed. Its all one of those deliciously intertwined engineering compromise problems.

Yes, I believe that the ideal shape should be thin and rounded along it's sides. This layout minimises any area of high pressure on the sides relative to that above or below the "wing", therefore minimising any flow circulation around the tips.

With careful attention to airfoil choice and surface finish, and sensible location of wheels and canopy etc. there is good prospect of maintaining laminar flow to maybe 50% chord, at least on the top surface. So the NACA 6 series forms are a good starting point. Flat topped is definitely not good. Some additional surface area needs to be added at the front because the cells typically won't conform to the sharper leading radius. The section also needs to be thick enough to accomodate the driver and all the mechanical and electrical components.

In Australia we started off giving the drivers ice cubes rolled up in towels, but these were not popular as they only increased the humid clammy feeling of being locked in a small plastic dome at 50 deg C. So basically, we just told them to shut up and drive.

Graham.

rmh

Cool em with em dry ice and tell em to hold their breath
(engineer's approach)

Ben Lanterman

Graham - totally interesting.

And I can see why Dick isn't going to be invited to the party. Of course the heat generated by the body would vaporize the dry ice rapidly and if shot out the back would increase the forward speed. Have a truck runing behind with a really long tube sticking forward and dumping in "cooling, wink wink" dry ice in a hole in the top of the cockpit. Get all kinds of thrust.

Feed them really gassy foods and carry a lighter. More than one way to skin a cat.

Reminds me, an episode of Malcom in the Middle had the older brother sitting in a rolling chair with a CO2 fire extinguisher pointing away from him and letting it shoot. Lots of smoke and it propelled him across the room. fun stuff.

Hatty

I appreciate your response. To be honest, I was not the one who designed the shell. They put the tapers on the shell before I joined the team, so I had the idea that that was the way it was suppose to be the whole time. But now that you have said something, I should have thought for myself and axed the tapers. You are very correct when you say that a zero-lift body shouldn't have and vortices. But I know that if I would have changed the design and gotten rid of the tapers, I would have faced a lot of opposition, so it probably wouldn't have happened anyways. Oh well. Thank you for your suggestions. It is always great to hear form another solar raycing alum.

rmh

so -drummed out of the corps?
I guess that's the price we pay for forward thinking-------------
Reminds me ot the time our new Lance Corporal was caught with an ostrich--
But that's another story----

Ben Lanterman

Please let us see how the finished car looks. To say one thing again for emphasis be sure to do the tests for minimum drag pitch settings. That is something that would be hard to do with just theory.

Dick, I am speechless with laughter, a mind that can concieve of that, I just ................ whow.

Jimmbbo

Ah yes.... the Marines also have their own definition of small farm animal....


Interesting reading, Graham - I did some design work on a man powered airplane for the Kramer Prize eventually won by Paul McReady and we faced the same pilot heating issues, except that since he was powering the airplane (putting out about .5HP) we had to accomodate his wishes for a means to cool the powerplant...

Cheers!

Jim

allanflowers

Keepin' it cool?
Obviously a silvered canopy with minimum see-through area. No point in looking at the sun. The lightest most reflective color on the body would be good but the photocells cover most of it anyway.
Then there is the heat generated by the system, which the driver won't want to have inside with him.
I remember reading that the oilcooler on the P-51 could actually generate a little thrust due to heat expansion of the air going through it. Would something like that work to vent the heat from the motor/speed control and batteries yet actually produce a little thrust? Probably such a system would just make more drag.
Allan

Salto

Here's a bit more information.
A typical panel on a solar race car is 8 sq m. In full sun the solar flux is going to be about 1kw/sq.m. so that's about 8kw hitting the panel. Since the cells are very carefully designed to absorb as much of the sunlight as possible (texturing, anti-reflective surface coatings, reflective backings, etc.), and only about 20% of the energy will be converted to electrical energy, you can see that the panel has to loose several kw of power in heat. Some goes from the top surface, but a good proportion goes into heating up the inside of the car.

The DC-DC converters, and motor and controller will be very efficient (maybe 80-90% system efficiency) so heat loss from these items won't be as much of a problem as from the panel.

Any system of venting cooling air through the vehicle interior represents energy lost from the airstream so is to be avoided at all costs. For this reason, cooling is usually kept to the minimum required to maintain reliable operation of the equipment.

Graham.

Hatty,
How's the project going? Any pics of wool tufting you care to share with us?

Ben Lanterman

Design the top of the car as a sandwich of cellsouterskin-openspace-innerskin. Take all of that energy wasted and put into the air flowing into a full width but not too high inlet, thru a properly designed ducting (the open space) and ala P-51 get a little drag reduction as it shoots out the back through a suitable full width nozzle to get thrust.

That's a lot of power to use.