Mark,

I believe you are incorrect re: the dive planes only creating DF via impulse. I suspect there is a small amount of force, but it is likely that anything meaningful would rip the canards/planes from their mounting. Again, according to a friend that is a lead aero person for a very leading edge OEM, they work to extract air from under the car (primarily the pressurized wheel wells). And, as Scott points out, work to create vortices down the side of the car to seal the car and prevent unwanted airflow to enter the underbody from the slide and slow the airflow under the car.

We do not run tunnels - just the very long diffuser (it starts ahead of the rear axel). The diffuser does create drag reduction by extending the smooth(er) airflow further behind the car (as drag is created by the air attempting to "grab" the surface are at the rear of the car. However, it also should create downforce (although I cannot measure to prove) - at both the front and rear of the car - but requires airflow to work. When we ran a flat splitter the splitter would "seal" to the track when the car dove under braking. When we opened the splitter in the center to feed the diffuser (on the guidance of our aero guy) it seemed to work better overall and improved most under braking. The underbody shape of the opening also works to help locate the forward center of low pressure.

All of this is just design, build, and field test as we don't have tunnel data, but as you point out the car is working well so ....

Cheers,

Its only 40 hr tow to Canada. Rolling road , its in the future plans, only $5-10 mill. Then the cost of using the tunnel goes UP. Plus set up time is increased.Right now for the average racer, relative aero numbers and changes can be measured.

All fascinating stuff, but as i reported to Scot in the other thread, the vortex's can be predicted based on the air flow speed and angle. their size is relatively small as you can see from windtunnel smoke tests, so im not convinced that you can gain that much of the underbody sealing, as you might predict.

however, is working and there maybe things im not taking into account. like the splitter keeping flow from going under the car as it rolls off the nose, maybe these side diveplanes do the same thing for the air rolling off the front high pressure sides, and tosses it into a vortex , mid fender hieight, so it cant reach the bottom entrance of the car behind the wheel, or even mid front fender. Also, the high pressure in the wheel wells is the air feed from on coming brake ducts it vents to the wheel area after cooling the brakes, with stagnation and then exits the wheels. the outside flow generally is the same pressure as the bottom flow. your car is lower than mine, but ive measured ambient the fenders and rockers as well as under the car and i have a large splitter, 3" ABOVE the ground.

the diffuser if starting at the rear axile is genius to reduce drag as the underbody air meets the exiting over body air. its not going to produce downforce and not slowing the air down that much unless it really diverges.........and if it does the pressure will actually go up producing lift under the rear of the car where it diverges. however, you have a wing to counteract that
interesting that the cut out in the splitter helped over all. i would say, most of the nose air in the center, if released would raise ,not lower pressure under the car. but maybe if you ahave such an efficient wing and diffuser, you needed to release some of the front downforce.
its not like we are guessing, but its calculated guesses with experimentation. all things of interest to me. thanks for sharing.

I'd agree on the idea that dive planes / Canards are essentially used for trim rather than producers of massive amounts of downforce. Even though you'd be forgiven for thinking they would actually create quite a lot of downforce based on the old 'stick your hand out the window at speed' as we did when we were younger. You could feel a lot of pressure on the hand if you held it flat karate style and twisted the angle from the wrist.

However, having discussed this with a few professional Aerodynamicists they suggested we remove the large dive planes from our setup. Together with some other changes we saw significant improvements in downforce. Unbelievably we were actually getting lift with large splitter and dive planes. Large dual element rear wing was clearly working also.

Reading this talk about 'opening' up the front splitter to allow the rear diffuser to work properly made me wonder if there were any diffusers in the splitter? Rather than dropping the rocker panels and creating up a tunnel of sorts, we have opted to try and release the underbody pressure by venting out of the hood, fenders, front wheel wells and doors. Also out via the rocker panels which we actually raised as well. We can't run a flat bottom and therefore felt that a rear diffuser could be counterproductive. Our plans for next year include a new splitter and rocker panels to increase this front venting / downforce.

First pic you can see the dual large dive planes. 2nd pic you can see the front actually lifting down the main straight. Far from ideal! 3rd pic shows the dive planes gone. Wider splitter with two smaller dive planes and end fence. 4th pic shows some of the diffusers under the splitter. New design forthcoming.
With the change in the front end aero we picked up about 20kmh through the fastest corner.

Patrick, your car is a technological, thing of beauty and its pretty good looking for being half a 928! ;)

i agree with your reasoning and the fact that the dive planes, intuitively, seem like they do a lot, but with 30 sq-in of surface area, how much can .35psi give as downforce per side . its about 10lbs x .7 if they are at a 45 degree angle, so thats 7lbs per side, x 2 for two sides of the car, or a total of 14lbs of downforce up front... hey, that is actually a lot , but not enough to make a massively noticeable change drag or handling wise.. but all the little things add up now, i dont believe that there is much interaction of the lower side flow, but that's something i would need to see more information on.
However, your hood venting, is something i do know works and you have designed that well on your racer. Love the fact that you can see the different attitudes of the car based on balance or unbalance coming down the track at 150mph+! :)

Interesting paper. In the Aerodynamic Improvement Test which configuration did you feel was the best compromise? If you had to select one and run it for a season which one of the eight would you pick and why? I have my own ideas but I'm curious what you and the aero team chose. Do you think the drag reduction of the canards was primarily due to them now covering up the previously exposed tires or the potential for the shape to induce vortices? Might be hard to say since the wheels weren't spinning and the floor was fixed during the test.. How many hours to complete the Aero portion of the 944 test? Cool stuff.

Mark,

I don't agree with you, at all. The vortices created by the dive plans do work as do rockers that reduce the area at the side of the car for air to get under. You don't have to have lower than ambient pressure for it to make a difference as you trying to keep the pressure for becoming higher than ambient pressure. Anything you can to keep from running under the car from the front or sides is a good thing. Why do you thing splitters are so effective?

It's ok to disagree, but it is mostly theory and wonder if we see eye to eye on the overall downforce effect upf front in the car.

Actually down force is all about differential pressure. if you are not lowering the pressure underneath, it stays ambient. there is nothing to increase pressure under the car unless the air hitting the nose of the car, gets diverted under the car. it's not accelerated, its compressed so the pressure can rise under the car. this relates to the splitter this way: the splitter takes the air that normally would be rolling off the nose and going under the car , to the sides OR into the radiator vent or nose opening where the air can be diverted to the hood vents . i think we agree here. keeping flow from going under the car is the object and goal. in this case of the hood vent, the air flow is added to over car flow and turns what normally is very low pressure, to 0 pressure or maybe even slightly higher pressure than ambient because you are feeding the air to the air going over the car.. thus raising its pressure. this very small change of pressure over the car, is over a wide surface area so you get downforce. in some cases this results in just lift reduction and other times it results in actual net downforce increase

the dive planes are very small .. usually only about 30sq-in and if the max pressure they can generate is .35psi or .08 psi, thats only a few lbs of down force... it all adds up, but its not much . the Vortexes are small and dissipate quickly as seen by much higher lift and pressure differential of the wings and winglet (end plates at the rear of the car). i dont see, or am convinced the vortecies are large and powerful enough to create a curtain to keep air from going under the car. if it can that would be interesting to hear how (or see with flow from the windtunnel)

the rocker panels that drop down low, on most cars that are not "tunnel " cars, are more for preserving ambient pressure when the car is yaw . straight ahead, the pressure under the car is near ambient, and so are the sides of the car. in turns however, the on coming air can enter from the front corners of the car . like taking turn with your window down and air blows in your face. (but not when going straight... same thing)

Couple points to add:
- Creating an opening in the splitter or apron in the front can help "feed" the underbody aero and diffuser, if designed properly to handle the flow volume. This depends on a number of factors, and no, I am not an expert, but big ones are flat bottom and diffuser are two obvious ones. Openings in the front been used on many race-dedicated cars in the past. Under car aero isn't only about eliminating the air under the car. It's about controlling its flow, which can mean feeding extra air.
- The above is completely contingent on having a flat underbody (or undertray). If the under of the car is not flat, a diffuser won't work well, and air will be slowed down as it gets "stuck" by the protrusions under the car. This turbulence will cause pressure to rise under the car. So without a flat bottom and or diffuser, you are better off with a low splitter or apron on the front of the car to stop the air from getting under there in the first place.

Scott

my view is that they are independent. sure , the diffuser will work better with more flow to it, but more molecules of air under the car creates lift, especially with the relationship of under air pressure compared to over car pressure. feeding more air to the top of the car and removeing air from going under the car, creates more downforce. underbody characteristics is mainly about drag reduction, and diffusers are also about drag reduction as the over and under flows meet. one of the reasons that they open up the underbody flow, is to as you say , improve diffuser fuction which reduces drag... also, for a car that might be runing a track with a lot of undulations, too much splitter downforce creation, can be a problem when it is removed if you are leaning on it being there. we have all seen the flipping ALMS cars. i think the tunnels give more consistent downforce characteristics . and lets face it , many open the spliter up to underbody tunnels which create lower pressure zones for downforce too.

there are a lot of abstract concepts going on here. basic wing design and function is so much simplier for aero with airplanes. cars are so complex.. that's what makes it so interesting to talk about and research and test.

Mark,

When considering how a diffuser works to create downforce you may need to think a little differently - and at a molecular level. A diffuser creates dowforce, or localized pressure reductions, by accelerating the air under the car across inflection points in the underbody. It does this by the air expanding in the diffuser physically attracting, and therefore accelerating, the air molecules at the front of the car across the most constrained forward location (hopefully near the front axels) and at the point of transition from the underbody to the diffuser.

This assumes that you do not have much flow into the diffuser from the sides - which is very hard to do on a relatively high GT car and why the diffuser is much less effective than on a very low prototype or formula car. There are several good articles by Simon McBeath in Racecar Engineering re: this principal.

While the DF from the diffuser is not as great as folks seem to think, it is there and essentially free - with the added benefit of drag reduction.

Cheers,

Ok, and excuse me to others here, if this question gets into the weeds, but how does the air accelerate into the defuser, if by definition , its a divergent flow device designed to slow flow and increase pressure, or return pressure to the flow it is trying to meet? generally, the flow, if not accelerated is at near ambient pressure as flow under the car starts with what it feeds it.... under the front splitter. (just as the air that is fed out of a hood vent, doesnt accelerate teh flow, it just adds to the molecules in the flow with no change in speed and thus increases its pressure to create downforce)

If i hear you correctly, you are saying that the the diffuser slowing the flow down at the rear, and increasing pressure, can effect and "attract" and create accelerated flow, and i dont believe in theory that is possible. the diffuser is going to slow flow and raise pressure at the rear of the car, all for the purpose of cleaning up the air and returning the air to the same speed as the air traveling over the car to reduce drag.

you also can compare the air flow to a venturi, if there are underbody venturi tunnles that accelerate the air.. this drops the pressure by speeding up the air due to the path restricting .. instead of the molecules compressing, they speed up and the pressure goes down. at the divergent area (the diffuser) the air then slows down, pressure returns to ambient as it exits the diffuser. at ths point in the air flow, the ambient pressure is the same as what is on top of the car, but the wing now provides the downforce. without a venturi tunnel , there is no reason or "attraction" for the air to speed up on the exit and create a lower pressure area.. quite the opposite.
when you say that the diffuser is "creating a acceleration of the air and rise of pressure," the opposite would happen. speed goes up, pressure goes down. it wouldnt attract flow, it would flow back to the lower pressure. air moves to differential pressure high to low, not the other way around... supersonic flow is quite the opposite though.

what am i missing here or what principle is present to create downforce with the diffuser alone ?

think of a wing on a plane... in flight ambient pressure is underneath the wing, but the top of the with the curve forces the air to make a longer distance in the same time. it speeds up on the front of the wing and slows back down on the back of the wing. cars put an "anti wing structure" on the back of this wing structure we call a car, which is the car's shape, and that is a "downforce wing" . then, in front, they put splitters or hood vents so that air is routed to the sides or to the high speed , low air pressure air area on top of the wing to neutralize the pressure and kill the lift .

the thing with ride hight and side skirts, is that you are trying to create a venturi under the car, front to back. the inlet is what rolls off the nose and goes under the car, as well as on coming air, and the exit is the release of the accelerated air. the problem is , the sides of the car are HUGE leaks for air to enter the bottom of the car. the venturi is very difficult to create and the diffuser just acts as the rear of the venturi to return the flow back to ambient. the downforce, if any can be created, is under the body of the car, not at the nose and not at the rear divergent areas of the diffuser.

EDIT:ive added some diagrams of the venturi effect.

mostly good stuff, but a little inaccurate with the diffuser. the rear divergent area of the diffuser is said to speed up the air in the "venuri" however, venturis do not work that way. the diffusers only job is to efficiently match the flow rate and directions of the air it is trying to meet. the angles and air tracts are there to make the transition as smooth as possible.. it cannot increase underbody flow and reduce pressures. Unless i dont remember this stuff correctly, i think the website's info is a little off in this area. what do you think?

I think it is a waste of time engaging with you.....