Diffuser Design help - Andylaurence

[andylaurence]

I thought I'd post a quick update. I've made a cardboard mock-up of the design to try under the car to check it fits. To do this, I measured the sides of the model in Google Sketchup, plotted them on paper (I only have an A4 printer) and cut them out. I re-measured and used these templates to cut out the cardboard parts that I stuck together.

The car's off for some work to be done on the chain tensioning mechanism that I'm not happy with, so I'll trial fit when it returns. Assuming it fits, I can disassemble the mock-up to use as a template for cutting the wooden formers and the hardwood skin to go over them. After that, I'll fill, primer, paint, wax, PVA and start laying up carbon. I've not done any composite work before, so this will be interesting! I plan to hang it with several bicycle spokes as the downforce won't be too great and they can take over 200kg each in tension.

I decided to give up on CFD as Caedium was just too complex to spend the time learning, despite being far less complex than OpenFOAM! It's a shame as I wanted to evaluate whether it was worth sticking with the 4.2 degree angle for the first 450mm to avoid the rear wishbones or whether accepting wishbone intrusion into the diffuser was worth it to run at closer to 10 degrees for the whole length.

In the meantime, I've been thinking of replacing my rear wing mount that's made of 6mm aluminium with carbon to save weight. This sheet weighs just 1.3kg, compared to over 5kg for the aluminium, but I'm not sure it'll take the weight of a single plane rear wing at 130mph. Anyone got any ideas?

[proutyc]

Personally I'd run between 6-9 degrees and not worriy about the suspension arms being in the tunnel, lower drag etc but in need up to max 14 (at this level you risk it detaching), attempt to have the tunnels finish cloe to the body. This should help energise and keep the airflow in the tunnels attached. my $0.02

[Smokes]

try using solidworks with flow works it probably the easiest to get cfd done in. though the accuracy isn't on par with ansys.

[andylaurence]

Solidworks appears easier, but I fear the price may be more than the cost of the diffuser I design with it!

[Caito]

andylaurence, can't you get a student version?

[andylaurence]

Last time I was in education, the year started with "19". I might struggle! I spoke to the local distributor this week and the cost for the licence was more than my whole season's budget. I've decided not to bother with CFD and will simply build it and fit it. Unfortunately, my trailer has broken its chassis, so the car is stuck at the garage. When I can find a trailer to bring it back (the new one won't arrive for 3 weeks yet) I'll trial fit the cardboard one that's been sat in the sun lounge for weeks! On the plus side, the pre-season work has lost almost 15kg since the last event, so the car is <450kg for the first time.

[proutyc]

I'm keen to see the new one.

Just curious where did most your weight reduction come from?

[andylaurence]

The car is now back in the garage, so I can get to work on test fitting the cardboard diffuser. I will post photos!

The weight loss came from a variety of sources:
5.5kg from new rear wing mount
1kg from a superfluous aluminium plate bolted under the engine
2.75kg from removing the reverse gear linkage
1kg from the existing diffuser mounts
4kg from eating less pies
0.5kg from the rear bodywork
0.3kg from a new rear towing eye

Plus a few other bits and pieces. There's scope for more weight reduction, but it's getting more tricky now to find areas to reduce weight. A big job will be re-doing the body, but it'll save a lot of weight as the existing body is very heavy. For now, the diffuser should add downforce without adding much drag (if any) and without adding weight as the existing item is extremely heavy.

[Simon42]

Back to your diffuser/rear downforce problem, I'm surprised no one mentioned big rear wing endplates. If they extend down to the diffuser, they can supercharge its effect - whatever the design. I know you have a space restriction with your trailer but such a part could be unbolted for transport. Remember that downforce is also affected by leverage, ie how far back you downforce-generating part is becomes as important (or more) as how effective the part's design is in the first place. And as a bonus, the further away your wing is from turbulence-generating parts of your car up front, the better it works.

See the Jaguar XJR-14 (and most old Le Mans cars really) or the side view of this year's McLaren MP4-27.
http://www.mulsannescorner.com/JaguarXJR-14-SMI3.jpg
http://www.mclaren.com/_includes/images ... s/4583.jpg

[andylaurence]

That's a very good point and bigger end-plates are something I've considered. However, my rear wing is 1800mm wide and the diffuser is just over half that width, so it doesn't help the diffuser to make the end-plates bigger. Of course, the reason they do it in F1 is because the rules mandate the size of the diffuser and the rear wing at similar sizes, so extending the rear wing end-plates is beneficial. My diffuser is not limited by rules, so I'd hope that I don't need artificial means of effectively making it larger.

I'm considering doing some tuft testing of a mock-up diffuser on track next month to see if I've got the basic shape about right. The idea being to use a sheet of thin flexible wood so as I can alter the profile during the day with a camera pointing in from the rear and wool tufts to visualise flow and ensure it remains attached throughout the various speed ranges and pitch.

[tomislavp4]

It will still help even if the diffuser is that much narrower than the wing, look at the Jaguar that Simon wrote about but find a picture that shows it from behind. You'll notice that the diffuser is less than 2/3 of the width of the wing.

EDIT: even better look at the Nissan P35 http://www.mulsannescorner.com/nissanp3 ... oto-9.html

[andylaurence]

Yes, bigger end plates will help irrespective of the diffuser. Probably a quick win, really!

[proutyc]

Anyone got any recommendation on endplates they wouldn't mind sharing. I have a very similiar car to Andy's and am also looking and changes to my rear wing?

Do you just make then rectangluar with longest edge vertical?

Sorry Andy for asking on your topic

[andylaurence]

About time I did another update to this. I didn't finish the diffuser before the start of the season, which was only a fortnight after my last post. At the second event, a wheel shattered and I ran a good distance on three wheels and the diffuser, so I was glad I'd not spent hours building a new one! Unfortunately, the car needed a bit of work and I missed a couple of events getting it fixed. On the plus side, I fitted new hubs and magnesium wheels, and trimmed the leading edge of the sidepods off, which got the weight down to 421kg. I also had the car properly corner-weighted and it didn't half go well at the next event.

The birth of my daughter has slowed progress somewhat, but I managed to add a flap to the rear wing and some 450mm x 300mm endplates, which have clearly moved the centre of pressure rearwards as the car now understeers in the high speed corners. I decided not to bother with the wheel covers as it's too much of a grey area!

This thread gives more detail on how my season went.

I've been wondering whether the previous diffuser design knocked up last winter was too conservative. Given that there's no regulations over the shape of the underfloor, why should I start the diffuser at the rear of the chassis? Why not start it behind the front wheels? The sidepods are not structural and there's space either side of the chassis. This brings the centre of pressure forwards and provides a much larger plan area for the tunnels, thus a little more downforce, I hope. This theory would allow a tunnel each side of 3.1m, which means that even an 8 degree angle would result in a tunnel exit of around 43cm. That's high enough to mean that the driveshafts and wishbones will have to penetrate the tunnels and also high enough that the rear bodywork (~45cm) will practically meet the trailing edge. In fact, here's a quick graphic to show just how large the tunnels would be.



The tunnels can simply replace the existing sidepods. Two pieces of flat material are all that are needed to fabricate the replacement sidepods, so it's easy to build and, if I pick the right materials, I could save a good amount of weight as the sidepods are made of beefy fibreglass and clearly overweight. In order to fit the new sidepods, I'll need to relocate the radiator, oil cooler and the exhaust. The radiator can be canted over and placed over the tunnel. This raises the centre of gravity, but hopefully not too much. The same can be done for the oil cooler on the other side of the engine bay. The exhaust is not so easy and I'll have to come up with a solution to that. The final thing to think about is that in order to get it on the trailer, I'll need to pad out the bed so the car sits a couple of inches higher and clears the front crossmember of the trailer.

So there's a weight benefit, but can I quantify the aero benefit without trying it out? I don't think I can without decent CFD and that's beyond my means. So the question is whether I fancy spending the time and effort on something that looks like it should prove beneficial. Comments appreciated!

[craigsimon]

Hi DRCorsa,

Sorry to bring this topic back from the dead and not sure if you still float around on this forum? I can't send PM's yet to get in contact directly as I haven't posted enough messages. I was wondering if you would be able to build a similar diffuser for my 7 style car. let me know if you can and an idea of pricing. my email is craig (at) thinktankcoffee (dot) com (dot) au or PM me

cheers
Craig Simon