Good point. It would be interesting to see wind tunnel sims of the 308 with and without the spoiler. But first things first.
I thought I would post the values that I measured now so anyone else doing the sim would have the numbers. I am going to freshen up, eat dinner etc then sit down and do the sims and post the results. Angle of the spoiler with no passengers in the car, full tanks = 10 degrees Width of spoiler 6.6 inches Length of spoiler (measured underneath) 39.5 inches Thickness given as .75 inches Note: I observed that the car's roofline is angled as well at 6 degrees towards the spoiler and this may or may not change the true angle of attack to only 4 degrees if airflow follows the roof contour as apposed to flowing straight overthe top at a tangent. So I will run the sim both ways. Food for thought. Pictures, we all love pictures: Image Unavailable, Please Login Image Unavailable, Please Login
My results: Values used in sim A (10 degree angle of attack) chord .54 ft Span 3.25 ft Area 1.7415 sqrft thickness 7.49% camber -3.00 resulted in flat upper surface using ellipse shape A of A 10 degrees Speed / Lift 50 mph 10 lbs 100 mph 26 lbs Values in sim B (4 degrees angle of attack 10-6) This was done to account for laminar flow that is coupled to the roofline. No idea if it does or not. same A of A 4 degrees Speed/ Lift 50 mph 4 lbs 100 mph 7 lbs Summary: Although crude the estimates given by the model used would seem to indicate pretty strongly that the spoiler on the 3X8s produces net positive lift, the opposite of what we expect spoiler behavior to be. However, it was noted with the smoke probe that the trailing air movement was redirected downward towards the rear deck of the car at about half the attack angle. The spoiler also caused a split in the air flow directly behind it which would indicate a slight amount of turbulance being created. My speculation is that perhaps indeed the spoiler on the 3X8s may have been added to redirect air movement over the rear deck and not obviously to create a downward force on the car. But, this comes at a slight cost of some negligable lift. All of this is purely educated guesswork and also assumes that the modeling software applies to this sort of automobile spoiler testing.
Good work! And not unexpected given BigTex's statements about the vortex. Did you have a chance to see my post #73? Some really cool wind tunnel software and 3x8s to model with it. There is a chance we can actually see the spoiler at work...
on euro Qv's the rear basket handle-spoiler-airfoil-deflector-thingy was optional. i like my euro 83 "naked". Image Unavailable, Please Login Image Unavailable, Please Login
Yes I saw that post and looked over the model but not the software. The model I don't think has enough detail in the spoiler area to be of any further significant use. The overall model may show very well the airflow effect over the engine compartment, I don't know. I personally don't want to shell out $100 for this. That is a project for another. I was really curious as to the true effect of our spoiler as a spoiler and I am satisfied with the results. Now if someone else steps up and pays the fee I would certainly sit down with it and give it a go. Thank you for bringing this modeling software to the forum it has been a fun project.
I just installed the software. I don't think it has enough resolution to determine the airflow around the spoiler, but I may shell out the hundred bucks just to see. This effort has been fun...
Great , looking forward to the posts. Good luck. It looks like this thread has a lot of interest with lots of folks looking in. I guess we have all wondered just what's really up with that thing.
Interesting work. Although there is laminar flow on the roofline, there is a dramatic gap between the roof and the spoiler. Who knows what negative pressures and vortices this sets up, but it surely has a profound effect. And unfortunately I don't think that basic CAD model has anywhere near enough accuracy to model it all properly. It might be easier to get a seasoned aerodynamics expert to give a view. Image Unavailable, Please Login
I love the topic too, but...as fascinating the analysis seems to be, isn't it a bit "overkill" related to what aerodynamic "science" was at Ferrari in 1972, when the roof spoiler was added to the 365BB for the first time? I mean, I wouldn't be surprised that the roof spoiler was added with "some degree" of downwards inclination to create the vortex that was needed over the engine louvers, and that this "some degree" has been "guesstimated"...I find it hard to believe that, at the time, the design of a roof spoiler came from scientific and precise calculation; surely there was no rocket science involved in designing that spoiler? Did they even use a wind tunnel at Ferrari at the time to design the road cars? I do not recall such things as quoted CX / Scx for the seventies Ferraris? And then, later, the roof spoiler has been simply added to the 308 just to please the eye, or to create a family resemblance with the BB?
Whether the spoiler had an intended design effect or not, it doubtless has an effect on airflow and it's interesting to work out what that is, ex post facto. Of course aerodynamics came to the fore after 1968 when the first F1 wings appeared. It then took at least a decade for things to filter down properly to production cars. The 308 has pretty good cd and even better when you add in frontal area according to this, though it seems to be educated guesswork by an amateur: http://www.mayfco.com/tbls.htm
I spent the first 10 years of my career in the filed of computational fluid dynamic. I'm what you might call an expert, (or was before I retired). My opinion is: you guys have way too much time on your hands.
I hear you, but if someone wants to look I don't mind seeing the result. I did fluid mechanics at college but never enjoyed it and was no good at it. Back then, I could just about apply Bernoulli equations in the very simplest of situations. But with the slightest complexity (body creases, gaps) things get very chaotic, very quickly.
Geeze guys, Glen don't listen to them. There are always some that preach and some that do. I think our effort has some merit. I appreciate your efforts in this amature investigation and besides it was fun. It certainly got some interest. Experts would you like to do ............
Thanks Robert, I appreciate your encouragement... Remember, Adrian Newey was an "amature" at one time!
Pininfarina styled those cars, and their wind tunnel opened in 1972. Why exactly wouldn't they use it for their prime client?
Good point. If they didn't, they were remarkably lucky at achieving such a low air resistance index for the car. air resistance index = Air resistance factor (Cd) x Front surface (A) m2 The 308 is estimated at 0.51 for the GTB and 0.52 for the GTS http://www.mayfco.com/ferrari.htm Compare to 0.61 for a 1980 911 and 1.22 for a 1990 Range Rover.
Instead of estimating, you could backtrack from the 308's top speed, engine power and the frontal area to calculate the corresponding Cd Does anyone have actual figures (e.g. not factory data) for the top speed and the engine power of a particular car? Some quoted Cd figures around 0.32 for the 308 seem a little optimistic to me... even with the use of a wind tunnel.
You could always do a coast down test. Anyone got half a day spare and some empty, flat, non-windy road nearby? http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/
Painted mine Rosso Corsa. Naturally the red makes it way more efficient at whatever it is supposed to do
