Over and Under or around the sides

[flynfrog]

Thanks, flyn. Do you have any links for a little reading on the subject of blunt fronts for dummies?

Try to find a book called "The Leading Edge" BY Goro Tomoli

[greenpower dude reloaded]

Nice thought Belatti, Fastest one of these cars has ever gone is about 53mph, it had a voltage boosting(batteries run flatter much quicker) speed contoller and was back in the days of the fully enclosed car.

Tomba, on reflection I may just be able to round off the nose a bit, that it is just made of foam and bodyfiller... think we've got some more flourescent paint. It only goes as high as the black "aero tape" in the picture, I suspect that'd releave the pressure on the sides a bit, Slimjim if your out there any chance you could apply a round to that front part?

Flyn I agree with your comment, the idea is not so much to force air anywhere, the body sagged a lot and will be higher next time which should allow some air flow a bit more freely and cope with the pressure of the perhaps somewhat over the top round on the leading edge.
I've kept the underneath dead smooth, only 5 gutter bolt style flat heads stick out underneath. Ground clearance as mentioned had sagged to around 25mm(lowest point on body) a bit of work should see it nearer 60mm

Interesting you mention NACA Profiles, I looked at those but to fit them round the vehicle meant massive frontal area, probably not as bad in profile rather than plan but still big, what's your view on FA vs. shape? I think we have one of the narrowest wheels fully enclosed cars at 680mm wide.

Fairing the helmet is tricky, very easy to cause a great deal of drag but something im looking in to. Roll bar should be faired for it's next outing.

Ciro really great contribution, thanks, it's nice to see something really constructive


This is (very roughly) what i'm thinking at the moment as a starting point, but im really interested in what people think would make the ideal shape.

I can provide some basic measurements if people want to have a crack at designing and the regs are found here http://www.greenpower.co.uk/racing/rule ... us.php?a=1


Thank you soo much, all of you it's really appreciated

[flynfrog]

Nice thought Belatti, Fastest one of these cars has ever gone is about 53mph, it had a voltage boosting(batteries run flatter much quicker) speed contoller and was back in the days of the fully enclosed car.

Tomba, on reflection I may just be able to round off the nose a bit, that it is just made of foam and bodyfiller... think we've got some more flourescent paint. It only goes as high as the black "aero tape" in the picture, I suspect that'd releave the pressure on the sides a bit, Slimjim if your out there any chance you could apply a round to that front part?

Flyn I agree with your comment, the idea is not so much to force air anywhere, the body sagged a lot and will be higher next time which should allow some air flow a bit more freely and cope with the pressure of the perhaps somewhat over the top round on the leading edge.
I've kept the underneath dead smooth, only 5 gutter bolt style flat heads stick out underneath. Ground clearance as mentioned had sagged to around 25mm(lowest point on body) a bit of work should see it nearer 60mm

Interesting you mention NACA Profiles, I looked at those but to fit them round the vehicle meant massive frontal area, probably not as bad in profile rather than plan but still big, what's your view on FA vs. shape? I think we have one of the narrowest wheels fully enclosed cars at 680mm wide.

Fairing the helmet is tricky, very easy to cause a great deal of drag but something im looking in to. Roll bar should be faired for it's next outing.

Ciro really great contribution, thanks, it's nice to see something really constructive


This is (very roughly) what i'm thinking at the moment as a starting point, but im really interested in what people think would make the ideal shape.

I can provide some basic measurements if people want to have a crack at designing and the regs are found here http://www.greenpower.co.uk/racing/rule ... us.php?a=1


Thank you soo much, all of you it's really appreciated

The nice thing about the NACA profiles is there is a ton of data on them. As far as fitting a car inside of them use the profile for the majority of the car then fair in the stuff that sticks out.

Also for low drag cars such as ours a 3d air foil can help alot with cross winds and reduce wetted area.


Head faring look at the old corvettes




or even motogp




Also your model looks like a good start but you seem to have a chunky nose and alot of flats on the body. Remember with cars this slow you will see alot of side wind steering against it takes energy

Ciro another book to pick up is : Theory of wing section" Ira H Abbott and Albert E Von Doenhoff

[Ciro Pabón]

Thanks, again, flynfrog. I found a couple of things that enlightened me (or so I think). Perhaps GPDR has read it already, but I think he didn't before designing last year car (specially because of the exposed rear wheels).

Design of an aerodynamic green car:





Yes, teardrop is better. Now it finally entered my head. However, notice Morelli's car front. That's not a teardrop. Why?

Has anyone studied drops falling along a flat surface? Pictures of raindrops falling close to a tall building windowpane, anyone?

I also liked this, Hardware and software infrastructure for the optimisation of Sunswift II, but I haven't finished reading it.

My simple conclusion (has anyone mentioned this?): raise the body as much as possible, but cover the wheels. If races were in a straight line...

[greenpower dude reloaded]

Ciro so roughly along the lines of my thoughts on how i'd make my next car? This years would have been like that If it wasn't for the shortage of time. I started far far too late, mainly because my garage is freezing.


My only other concern is the car rolling and with my little sister at the wheel, I'm understandably quite cautious. I could make something super narrow but it'd have to be pretty wide.

Great PDF though, I'll try and have a proper read through when I get chance, amazing how much the Aptera looks like morellis design.

Flyn, The idea with the fat nose is that it creates a low (but acceptable) pressure area over the cockpit, which means less drag purely through there being less air, notice the low pressure leading up to the helmet in the CFD runs SlimJim kindly did for us.

I can't really curve the edges much more without increasing the Frontal Area dramatically. Will it provide that much of a benefit?

[Gecko]

There are a few principles you should observe regarding aerodynamic design.

The first one is to reduce the wetted (as in, in touch with airflow) surface. An exposed surface causes skin friction drag. Skin the mechanicals as tightly as possible, without compromising the other principles. This will also cause the frontal cross section to be as low as possible, too. Look at Newey's designs (RBR5 is excellent) for inspiration.

The airflow close to the body (in the boundary layer) is roughly split into two regions in a good design; the part at the front of the body when the flow is still accelerating (decreasing pressure), and the part at the rear of the body when the flow is decelerating (pressure recovery) towards the free stream again. The point of highest velocity roughly corresponds to the point of largest width of the body. Ideally, you want the flow to accelerate to the highest value and then decelerate again only once.

If the surface is very smooth, it helps if you can move the point of largest thickness of the body relatively far behind. Constantly decreasing pressure in the direction of the flow tends to promote laminar flow along the body, significantly reducing drag. If you, however, have a rough surface or have gaps in it, this will cause a transition from laminar to turbulent flow and reduce any such advantage.

On the other hand, you shouldn't move the maximum thickness of the body too far behind as you want the pressure recovery region to be gradual in order to prevent flow separation and too much of a turbulent boundary layer thickness. You also want the back end of the body to be at a very narrow angle so that the flow can get to the freestream value when leaving the rear part of the surface, again reducing flow separation.

An example of an axisymmetric body that best fulfills these criteria can be found here, for example:

http://designaplane.blogspot.com/2009/0 ... ciple.html

The Morelli car as shown in the photo also seems to fit the above design principles.

Also look at the shape of the natural laminar flow airfoils for inspiration.

[Gecko]

As far as your current design goes; you can see the dark blue regions at the front sides of the car. These are vortices created by the separation of the flow around the sides, which cause unnecessary drag. As I already suggested earlier, you may reduce these vortices by extending the sides of the car a bit further ahead in order to form fences, just in the region where the "blue" flow is observed. Attach extra flat panels to the side of the car that "capture" the airflow at the front of the car and push it over rather than around the sides.

If you have any time for testing, what I would also do is to attach some strings to the car in this area, then add these plates and repeat.

[greenpower dude reloaded]

Gecko, your about to read something that people never put on Forums, you are bang on that's dead right.

It is at least the sort of shape we should aim for in plan, the biggest problem is getting an adequate amount of space for the wheels to steer. But the rear could work very well with a 4 wheel design. Does that sort of profile have a specific name? it'd be nice to maybe try and work out the most suitable form.

Having a shape like that doesnt inhibit the idea of creating a low pressure area over the cockpit. either, which as you've probably geussed I feel is important.

I'm working on trying to secure some testing atm it's only a 410m tarmac oval but that's enough really i suppose, shorter the circuit more photo ops I'm quite keen to try out the fences idea. Any idea as to how they should finish? Trail off or abrupt?

Someone mentioned Micro Vortex Generators, what does everybody think about those? worth it or not?

Thanks again everyone your all really great!

[flynfrog]

=D> post geko

for some more reading try to find the GM Sun Raycer case study it should be required reading for low drag EVs

GPDR - We used vortex generators on some plane wings to prevent stalling I think at the cost of increased drag. They did help keep the flow attached longer so they may reduce drag in that sense.

If you can do some testing I would to what is called tuft testing you put yarn tufts taped allover the body of the car and take pictures this will give you a snap shot of air flow over the car.

for the wheel steering problem we have built movable fairings in the past but they were a PITA

[greenpower dude reloaded]

OK! we're now under DEF CON 1 conditions, we need to get on asap with a body for the second half of the season (commencing sept 27)

Ideally I think a simple shape along the idea of Gecko's post in plan view. Side view I'm struggling with a little bit more what kind of nose should we make?

I'm leaning back to your way of thinking Flyn RE: Helmet fairing it'd be nice to make one the drivers head could be "Cupped" by it?

I'd like a little bit more info on wing profiles does anyone know anything about the type profile Gecko posted.

This weekend I'm heading home to work on the car, I intend on at least trialing the Fences and fairing the roll bar as that's a rectangular profile at the moment. any other simple aero bits you feel should be added?

Thanks so much everyone for all your help!

[flynfrog]

OK! we're now under DEF CON 1 conditions, we need to get on asap with a body for the second half of the season (commencing sept 27)

Ideally I think a simple shape along the idea of Gecko's post in plan view. Side view I'm struggling with a little bit more what kind of nose should we make?

I'm leaning back to your way of thinking Flyn RE: Helmet fairing it'd be nice to make one the drivers head could be "Cupped" by it?

I'd like a little bit more info on wing profiles does anyone know anything about the type profile Gecko posted.

This weekend I'm heading home to work on the car, I intend on at least trialing the Fences and fairing the roll bar as that's a rectangular profile at the moment. any other simple aero bits you feel should be added?

Thanks so much everyone for all your help!

You need a shape more like the one ciro posted with the flatter bottom as you will be in ground affect. I would start with that basic shape then round as much off as possible.

for construction I would do a foam and fiber glass method like this solar car

http://www.opencortex.org/cn00000313

its light and pretty cheap and quick.
If you want to build a foam cutter let me know I can help with that too.

another thing to keep in mind with air flow keep it round where it attaches and sharp where it leave. check out the wheel wells in the lemans thread.

some cool body constriction pics for ideas.

I dont have cad installed At the moment other wise id offer to do some quick dirty modeling.

[flynfrog]

here is the wheel well I was talking about

[greenpower dude reloaded]

Right i've just spent an incredibly busy weekend working on the car mostly on strengthening key areas, but also with a keen focus on aero, set up and reducing bearing drag (told you it was a busy weekend)

Here's a run down of "improvements"

1. Fences and faired in roll bar

2. Wheel covers


3. motor mount improvement, Routed out two 500mm x 20mmx 20mm grooves, inserted ali box section and polyester resin and glass bubbled it all in place. this also increases the strength of the rear considerably.

4. reset front tracking. It was 1mm out!

5. cut down on bearing drag, wheels spin for over 3.45mins now, before it was just 45secs

6. Bolted the body on in more places in an attempt to stop it sagging like last time it's designed to have 60mm ground clearance.

7. recalibrated the shiftmap for the auto gearbox

8. strengthend the midsection with aerolam and glass bubbles

as well as plenty of other little jobs, oh and I watched a good few hours of Le Mans

so what do you all think? (bearing in mind i only had this weekend)

[Mac]

Is there a maximum ground clearance? in this case you may be better as has allready been mentiond to get the chassis as high as possible assuming cornring performance is less of an issue than energy consumption



The Eco marathon car Paccar is a good example of how high to go. At least 15cm.

If you can get hold of it, have a reed of Race Car Aerodynamics, by Joseph Katz.

For your bodywork, have you considered a light skeltal frame covered with solarfilm?

[greenpower dude reloaded]

It's something I considered but after having seen other teams do (with disasterous consequences) I decided against it, this car should have 60mm ground clearance this time, assuming the body doesn't sag again which although a long way off 150 it's still a good deal higher than most.

I have recently read the book about Pac CarII though, which was really interesting, I learnt a lot. only thing is it's a hugely different aero challenge being four wheels needs to be dynamically stable and can't be fully enclosed