TECH ARTICLE: aerodynamics info

[trinydex]

want to know how aerodynamics helps and which kits you should buy?

http://e30m3performance.com/myths/splitter/splitter.htm

http://e30m3performance.com/myths/splitter/splitter2.gif

ok so here we see what the typical air flow looks like at the front of a car and where the subsequent high pressure zones would be.

note that it is nice to source brake ducting air from these high pressure areas.

http://e30m3performance.com/myths/splitter/venturi.gif

now notice here that not only is it important to catch air above a splitter to create downforce but also to expedite air underneath in order to maintain uniform flow and what not.

notice at the back end of the vehicle, the bird tailing of the undercariage air flow, this must rejoin with the overcar air flow and as such you see many times that race cars curve their rear paneling upwards.

i had this really great paper before and it was about two stanford engineers and their experimentation with two dimensional diffusers and their subsequent critical values of flow seperation and what i will refer to as reverse flow turbulence. they did this by setting up clear plastic ramps over which they flowed water. they would make clear walls that would make a channel then seperate the walls to create a diffusion in the flow.

what a rear underbody diffuser does is it prevents flow seperation. what the engineers found was that at certain critical fluid speeds the diffusing flow would curl. you can imagine this,

if a stream of water is in a channel shaped like this:

@@|......|
@@|......|
^@/........\
@ /...........\

and the water is flowing down then it will diffuse to fill up the "triangle" part of the channel. if hte triangle area is too wide... the water will not be touching walls anymore and at the edges of hte flow there will be flow seperation where the flow gets very turbulent and disruptive, curling back on itself and making things very ununiform and slowing the overall flow down.

picture a garden hose, when under high pressure the water that comes out fans, and the water on the outter most parts is going LOTS slower than the middle stream.

so what did they do? they put slats in. the slats prevent the outter edge fluid from curling on itself... it will curl into the wall and then keep going.

of course this looks different in a flat flow than it does in 3d. the horizontal component from curling on itself and the verticle component either hits the ground or curls up to rejoin with the overcar fluid.

the gurney flap

http://www.allamericanracers.com/gurney_flap.html

the gurney is used to reduce flow seperation at a certain critical air speed over a wing. hte top and bottom flow must rejoin in order to minimize drag.


isn't another theory to keep as much air out from underneath as possible? using dams, or is that a rules play?
mmm you definitely want to keep as much air away from underneath the car as possible. some rules don't allow you to but in a rules barred vehicle you could.

that said you'd have to keep hte air out of ALL of the vehicle. keeping air out in front and letting it come in the sides isn't that helpful.

i hate the fact that i can't find pictures to describe what i want to convey. i can draw the pictures but the lack of a permanent type of hosting is really annoying.

anyway onto discussing the double wing, double-decker wing, slotted wing etc. everyone has seen this on the back of some civic that didn't need it. many have laughed at them. the fact is this is one of the best ways to gain more downforce without drag inducing flow seperation.

what happens is the first wing seperates the flow top and bottom, when these two rejoin you can increase the angle of attack as the rejoining flows are faster than the original flow. this then feeds the second wing which seperates those flows and in that way the tail flow is sufficiently fast to not seperate and this combination creates more downforce.

http://public.cranfield.ac.uk/me/me028/rubini/thesis/2004/T.Newton2004.pdf

start looking at diagrams after page 75. if you choose to read this thing you'll be better versed than me.

http://www.dcetech.com/nitish/reports/formula1.htm

take note of the picture of the see through wing.

next up is naca ducts.


Just thought I'd add something about the splitter and gurney flap (sort of).

These two you can get from Mitsu in Japan:

http://i47.photobucket.com/albums/f172/x838nwy/front.jpg
^this is kind of a front lip, but is recessed in slightly (by about 1" from the very front edge). Extends downwards by about 1.5-2". Does not go as far down around the centre. There's a metal backing to it, but the lowe half is rubber.

http://i47.photobucket.com/albums/f172/x838nwy/rear.jpg
^difficult to see from here, but it's a sort of mini-gurney flap. It's section is like a capital 'B' and is about 3/4 the widht of your finger in height. Sticks on the rear edge of your rear wing.

I actually got these two for a rather special price from Japan. I had doubts that they would work, but after installation, my top speed is certainly harder to reach, which at least means that they're having _some_ effect, even if detrimental!! But seriousl, the car feels more stable at silly speeds, it's not very pronounced, but it is detectable. Obviously these aren't really meant to make night and day difference, but I can assure you that even with these very small changes, there is a difference. I don't really know g numbers with/without, but I think the front has more effect though...

http://forums.evolutionm.net/showthread.php?t=167941


I went down to join Chronohunter at the track as he and a bunch of the guys from the CSU engineering school played with suspension and aero setup on his car.

I was hoping to get some good chase video but unfortunately my brother corded the rear tires of the Caterham without telling me and then decided to take all the video equipment to the UK (including my lipstick track camera) for his vacation. :mad:

This means that I was only able to get some digital pics.Â* Either way it was a good amount of fun and as always it is a blast to be taken around with Paul.Â* The suspension is really well setup on his car and he really has mastered the Evo around this track {thumbup}

http://forums.evolutionm.net/attachment.php?attachmentid=78140&stc=1

Cool Front Splitter
http://forums.evolutionm.net/attachment.php?attachmentid=78141&stc=1

Strong Enough? --Â* Only weighs 2 pounds
http://forums.evolutionm.net/attachment.php?attachmentid=78142&stc=1

Paul was about 1 second per lap faster than that Speed Challenge SRT-4
http://forums.evolutionm.net/attachment.php?attachmentid=78143&stc=1

My car next to Paul's for comparison
http://forums.evolutionm.net/attachment.php?attachmentid=78144&stc=1

Are these calipers discolored?
http://forums.evolutionm.net/attachment.php?attachmentid=78145

Check out the rear wing...
http://forums.evolutionm.net/attachment.php?attachmentid=78146


Paul, a prima donna?Â* He seemed to be doing just fine yesterday with what appeared to be 10 CSU students waiting on him hand and footÂ* ;)



The splitter was something the CSU boys fabbed up for him.Â* It brought the front lip down about 4 inches, was made out of carbon fiber, honeycomb center, carbon kevlar underside and included 2 nice intakes for brake ducting.Â* It weighed about 2 pounds and bolted securely to the radiator mount. {thumbup}Â* {thumbup}

The shiny part you are referring to is my brake ducting intake.Â* Not as efficient as Paul's but does the job :)

http://forums.evolutionm.net/attachment.php?attachmentid=78154&stc=1



What is the do-hickey on the rear wing?



it's an additional wing element with vertical supports all made of carbon fiber. it bolts on and off weighs nothing and is very effective while adding minimal drag (compared to a typical Gurney/wicker).


Note to self: bring more grapes next time... ;)

It was a good time, track was crowded but Paul was able to put down a 1:13.78.Â* We brought setup scales, alignment equipment, tire pyro's, and a small crew to make the most of the day.Â* The aero bits shaved nearly two seconds from the previous day's times.Â* His EVO was noticeably faster than everything else there: vipers, vettes, caged M3's, and every imaginable 911 including a RUF.Â* The only faster cars were two formula spec racers and CSR mazda.Â*

Paul definitely waits 3 seconds after seeing God, check out those brake rotors!Â* :eek:

http://img.photobucket.com/albums/v198/djazair/IMG_7601-b.jpg
http://img.photobucket.com/albums/v198/djazair/IMG_7659-b.jpg
http://img.photobucket.com/albums/v198/djazair/IMG_7694-b.jpg

[rammsteinmatt]

street cars generally do not lend to significant advantages due to aerodynamic effects. if you want my honest opinion, dont get a body kit for aerodynamic effects, because the companies dont test them, and if they did they would all display big fat 0's.

aerodynamics are for those that need the last few seconds off their lap times. this is because to get any car to have radical improvements due to aerodynamics, it becomes so impractical that any street driving is nearly impossible. granted, ferraris and the exige have compromised both, but even in the case of the enzo, it makes what? 500kg at 180mph? and it also has 2 inches of ground clearance and a nose raising feature to get into nearly every driveway. i would hardly call that practical.

the people that really need the aerodynamic benefits will seek out specialists to custom make components for very specific applications (to the point where components varry from track to track). but for 99.999% of evo drivers, the benefits (or lack thereof) will never be experienced.

for example: kent jordan and robi's cars would take advantage of aerodynamic parts because they are purpose built race cars, and the cost of having a specialist develop components can be justified. now take anyone else here that tracks their car: gt40, blaze, c-spec, etc... i would say that they wouldnt justify the cost of a custom aerodynamic analysis for weekend tracking. but that would be a guess. because first, its really expensive, second the car would have to be trailored in (or bring lots of spare parts to the track with you for the drive home)

dont get me wrong. aerodynamics play a very important part in cars, such that i would like to transition into that field for a living. but you have to know what tool to use and when.

many people PM me and ask about splitters (aparently thats the most widely used aero piece out there). they ask what it does, what angle should it be put at, etc... of course i answer their questions, but i do add that either way (short of screwing up and having the airstream rip the bumper off) there generally wont be a noticible difference, and that it would be more for looks. heres the deal: underbody aerodynamics are very powerful tools (just look at the enzo, no wing at all, its all underbody magic). however, they are quite unpractical for any street car. mainly because to be effective they have to be really close to the grolund. basically they use pressure changes (through area changes) to suck the car to the ground. how effective is a vacuum when its a large distance from an object? not very good. same deal here. also a completely smooth underbody is essential (or you'll be wasting your time). this presents lots of problems, namely heat and trapping it between the underbody panel and the car's unibody. yes i made a rear diffuser for my car, vanes and all. and guess what, on my home driveway the vanes scrape if i back straight out. definately not practical.

davy, great information in that link.
anyone happen to know what CSU school they are referring to on the evom link?

since it seems that we finally might have an aero thread stickied, i'll work over the next couple days to post a bunch of stuff (of course if workload lends to that). i honestly think between me and davy (trinydex) we can over educate everyone that is willing to listen.

[rammsteinmatt]

guess i'll start now:

this is a direct copy/paste from a rear diffuser technical report that i wrote concerning a rear diffuser that i designed, thats why it refers to "my design" alot. just bear with it

Rear Bumper Aerodynamics, Concerning the Implementation of a Rear Diffuser,
Lancer Evolution 8

Identification of Problem
On the factory Lancer Evolution 8 rear bumper, there exists an aerodynamic problem. As air travels under the car it increases above atmospheric pressure. This increase in atmospheric pressure coupled with the decrease below atmospheric pressure of the airflow over the car, leads to a lift force produced on the car. Additionally there is a pressure differential created at the rear of the car from two different air pressure flows coming together. This pressure differential creates pressure drag on the car as a whole. The general shape of the car lends to a large bubble of low-pressure air that is constantly behind the rear of the car when moving. This low pressure behind the car “sucks” the car back while driving similar to how a vacuum uses low pressure to suck items off the floor. This is also another form of pressure drag, which inhibits foreword movement to an extent. The most commonly identified source of drag on a car is the “parachute effect” caused by the rear bumper. The rear bumper sticks down into the natural underbody airflow of the car and due to its shape, that very much like a parachute, it naturally catches air from the air stream, and “parachutes” thereby creating drag.

Identification of a Solution
The solution to these drag problems can be solved or reduced, however. The way to reduce the parachuting effect of the rear bumper would be to place a flat plate under the rear bumper that covers the lip of the rear bumper and through the smooth transition of the plate from behind the axle, to block air from going over the top of the diffuser, to the rear bumper, will create less possibility for the air to get “trapped” on some underbody pieces and create drag as a result.
The low-pressure bubble at the rear of the car cannot be completely eliminated without the use of some radical mechanical devices, but it however can be reduced. The way to eliminate this is to move air into the space, as the air from any other part of the car is at a higher pressure than in the low-pressure bubble. To “blow” air into this area can be accomplished through two different approaches. One approach would be to put an airfoil hanging at the rear bumper that has a steep angle of attack to move air into this area, where the other would be to add fins to the rear diffuser main plate and use the vortices created to move some air into this low-pressure area. The airfoil would move much more air into the low-pressure area, and reduce much more drag than the fins would; however the airfoil produces some ill side effects. The airfoil, since at a very steep angle of attack, would create drag, as drag is a by-product of lift. Additionally since the airfoil would need to be located under the farthest rear part of the car, it would succumb to constant dragging on the ground as the car is driven on inclines, unless the car is sufficiently raised (which would in turn allow more air under the car, which would create more lift and more drag on the car as a whole). Therefore fins will be used because they create very little drag, help to straighten underbody airflow, and can be used to decrease the low-pressure area behind the car.
The pressure differential that is created by airflow over the car and under the car cannot ever be completely eliminated, as air will always be able to get under the car. However you can reduce the amount of air that gets under the car through various other ways, namely lowering the car and the body’s proximity to the ground. The pressure differential that comes back together at the rear of the car creates drag. However the rear diffuser can help that. If the diffuser is placed at a negative angle of attack, so that is still does not catch air from the underbody air stream, one can effectively increase the area under the rear bumper per unit length from the beginning of the diffuser. Through the continuity equation we find that an increase in area leads to a decrease in air stream velocity, and Bernoulli states that an increase in velocity leads to a decrease in underbody airflow pressure. Since the underbody airflow pressure is the highest air pressure of the airstreams around the car, lowering it will decrease the pressure differential, as the two pressures will be closer to one another when they meet at the rear of the car.

Rear Diffuser, and its Implementation of the Solution
The rear diffuser is based off a flat plate with minor modifications to its shape and to accommodate the features of the Lancer Evolution 8, such as the axle and suspension, and the exhaust system. The diffuser uses this flat plate to reduce the drag created by the rear bumper’s “parachute”, and to gradually decrease the air pressure through increasing the area, so as to reduce drag created by the pressure differential at the rear of the car. The implementation of fins on the design will reduce the low-pressure area behind the car while maintaining a very low contribution to total drag on its own. The fins also help to straighten the airflow as it comes across the diffuser, as the air becomes very turbulent as is passes over and under all the irregularities under the car. The “box” design around the exhaust system both serves to straighten air flow with the use of its sides, but also uses the bottom plate of the box as an inverted airfoil to create down force mechanically rather than through pressure like the rest of the diffuser does. The box design was created in the size that it is to accommodate the stock and the different aftermarket exhaust systems available for the Lancer Evolution 8.

Data and Graphs
All runs done in the east and west direction, to account for wind and elevation change, with all windows up

Without Diffuser
Speed During Test [MPH] Drag Force [lbf]
45-40 92.13
65-60 132.22
75-70 204.46

With Diffuser
Speed During Test [MPH] Drag Force [lbf]
45-40 106.85
65-60 141.02
75-70 214.61



With Modified Diffuser (box reduced vertically by 2”, box angle of attack reduced to 6°)
Speed During Test [MPH] Drag Force [lbf]
45-40 95.69
65-60 140.86
75-70 158.88


Data Analysis
Although the diffuser contains all the aforementioned drag-reducing elements, the data shows that the diffuser increased drag. This is due to the fact that the diffuser is creating down force. As stated earlier, drag is a component of down force, since the diffuser is reducing drag from the parts on the factory version of the car, yet showing a net increase in drag, one is lead to believe that down force is causing the calculated drag created by the diffuser, since the drag creating elements on the car have been severely reduced through the implementation of the rear diffuser.

Error Analysis
In the manner that the tests for drag calculation were ran, this involved using the car’s speedometer for a speed change reference and a stopwatch. The error would be involved with the human error associated with starting and stopping a stopwatch and ability to react to a condition. It was calculated that the human’s ability to react was less than one quarter of one second. Since the time values vary, the error accounts for between 0.5% and 7.5%, on average about 4% or between 4 and 8 pounds-force of drag. Additionally, the tests were carried out on different days, therefore one must account for atmospheric changes. It was however calculated that temperature and density changes would account for a difference of less than 1 pound-force of drag between the calculated values and expected values, so those calculations were omitted from the drag calculations for simplicity.

[rammsteinmatt]

i know someone will ask how i measured drag, so here it is:

determining drag is relatively simple. drag is a force. newton says F=ma. i can find the mass of my car (with driver and timer) in kg, because metric is easier to work with. i can also find the acceleration (or rather deceleration of my car due to air friction)
how to do that-pick a target speed/range. i did 40-45, 60-65, 70-75 for my tests and documentation. let's make this simple and say we're going to do it for 40-45.
find a road, straight, flat, no traffic whatsoever (because we'll be speeding, slowing, and they will disturb the air - think about how they draft in nascar) go up to 50, put into neutral, the car will nodoubt start to slow have the timer start at 45mph and time to 40 mph (please dont try to do this yourself, you're supposed to be driving). now that we have a delta v (change in velocity) convert to m/s units. and we have a time. well a velocity per time is an acceleration. and ta-da! we have a mass and an acceleration, we can multiply and get a force (in newtons), convert to lbf (pounds). and we have pounds of drag.

this isn't however the whole story. there is also friction in the bearings of the wheels that is contributing to this drag, and the rolling resistance of the tires which also contributes as well. therfore the total drag is equal to:

A + Bv + Cv^2

because things like rolling resistance (moment of inertia stuff) is constant regardless of speed
things like bearking friction vary with speed
and things like aero drag vary with the square of speed (as senn here: D = 1/2(rho)v^2 * Sref)

so solve for A, B, C. and you could theoretically calculate the approx. drag at any speed. or figure out how much aero drag contributes to total drag.

so you need to test stock vs. diffuser. you need to do it at different speeds (and you can make a graph showing trends) and you need to do it both directions on that street you picked out (to account for any windage or slope), and you need to do it abunch of times which will hopefully average and counteract any error you created.


if you dont like this way, because its inprescise, you can get an accelerometer, and that will calculate your acceleration as well. but i dont have one, so this will have to do. [additionally an accelerometer calculates acceleration the same way we do using F=ma and a=v/t, it just calculates it alot more times (instantaneous) than we can do]

[rammsteinmatt]

i dont have any good pictures uploaded. (well i did on the old socal software)

but here was the second mounted design
http://img.photobucket.com/albums/v476/rammsteinmatt/CIMG2077.jpg

the evolution of the [mounted] designs:
1. outside of bumper (like pictured above) with big box (hung down low to accomidate the real large canister mufflers) - outside, large box design
2. cut box, moved it up 1.5 inches - outside, small box design
3. put inside bumper for a cleaner look - inside, small box
4. re-angled the box for lower angle of attack (less downforce) - inside, modded box design


the diffuser is made in 1 rectangular piece with a box shape, and angle brackets laid into the CF. epoxied, then the diffuser cures, and then is removed from the mold (either clear is added or not). the diffuser is then cut by hand to fit a template. it is ready to go at this stage. its mounted (either inside or outside - owner's preference). box is changed to owner's specs (either leave it alone, raise it, change the angle of attack, cut out a section to accomidate the exhaust, etc...). fins are attached (either the large 4.5" or smaller 2.5") and thats it.

if it is chosen that the box be cut, then the diffuser becomes 3 pieces, with an easy to remove box section that makes it very convenient to change the axle back exhaust. IMO 3 pieces is far easier than the one large piece to take off. when its 1 piece it takes close to 1 hr to remove, and when its 3 pieces it takes probably only 30 mins max.


with all that said. a person looking for noticible performance gains IMO shouldnt get a diffuser of this design (basically anything that fits a stock rear bumper). simply because there is not enough room to varry the pressure significantly. look at the size of the venturi tunnels (the diffuser) on an enzo or F430. theres where they need to be at. and notice how the body is designed around them? kinda difficult to retrofit an existing car to that, but it can be done.

[rammsteinmatt]

questions on rear diffusers?

moving on to vortex generators

without the VG's (bad)
http://img.photobucket.com/albums/v476/rammsteinmatt/evowithoutVG.jpg
with VG's (good)
http://img.photobucket.com/albums/v476/rammsteinmatt/evowithVG.jpg

the red ares shows high pressure air, the blue shows low pressure air, and the orange shows approximate air stream over vehicle

sorry for the poor quality, but im doing what i can with what i got

especially note the differences in the airflow (orange) over the rear window, and its proximity to the rear window, which can be characterized by the size of the low pressure bubble (flow seperation) on the back window

so what does this show. if you concentrate on the VG section you will notice a difference. one (the one whith the huge VG) lets more orange in closer to the car's back window.

this means 2 things.
1. there is more airflow to the rear wing (if you have one)
2. the low pressure bubble behind the rear window is reduced.

low pressure bubbles (or seperated flow). in short, they are bad for aerodynamics.
think of this: how does a vacuum cleaner work? well it sucks air into it. how does it do that? by pumping air out thereby creating a low pressure, and the sucking is the air rushing in to equalize the pressures (its really that simple )
so effectively low pressure bubbles "suck" cars back (this is one of the reasons for rear diffusers and vortex generators, among other things)

how the vg reduces low pressure bubbles. the vg is exactly what it is named for. it creates vorticies. if you dont know a vortex is basically a "whirlpool" you might see in a draining bathub/sink, or a river. but this vortex is made in the air (because air is a fluid, just like water is a fluid) the vortex, if you notice, has a big side, and a small side. at the tip of the vg is the small side. as distance from the vg increases, the size increases. what this does, is infringes on the low pressure bubble, and "mixes" higher pressure air, with the top layer of the low pressure bubble, effectively moving the pressure gradient closer to the car than a non-VGed evo

so what the VG does, it reduces drag on the vehicle [by reducing the low pressure on the rear window] and it increases downforce [by allowing more air to pass over the rear wing's airfoil]
however the vg's also do create their own drag, because the frontal area technically would create a high pressure region, but the benefit outweighs the cost in this case


here is mitsu's official technical write-up on the subject (and they have real CFD pictures!!!!)
http://www.mitsubishi-motors.com/cor...004/16E_03.pdf

now, i know you are thinking "well matt, what about the STi's new contraption above the rear window?"

well, that also aims to reduce the low pressure bubble (or tries to keep the flow from separating). but it does it in a little different of a way. see the vg uses a vortex to mix air of different pressures. the STi more forces air back onto the window by airflow directing. there are good and bad sides to this approach.

1. more air can be directed to the back window, therefore much less low pressure bubble = good
2. that wing, in that steep of an angle of attack creates FAT drag. think of it, drag is calculated by dynamic pressure, drag coefficient, and the reference area. what is the refernce area (the area seen by the free stream, namely the area when viewed parallel to the ground) between the vg's and the STi contraption.

well we have little spikes that have maybe 1 in^2 of area per fin * so many fins. the STi has the effective height of the airfoil times by the length.

time for a rough calculation........

say we have 8 fins and the antenna nub (im pretty sure we have 8, if we dont the math is simple enough that you could figure it, and the results will be so different it wont make a real difference anyway)
ok, so 8 fins * 1in^2 + say the nub is 3 in^2 = 8+3 = 11 in^2 of area.

STi -
we'll be generous and say the effective height is only 1/2" (but im pretty sure its more than that) and whats the width? an evo is 40", so we have 40" * .5" = 20 in^2

well 20 is alot bigger than 11, and we were being generous. the way the drag calculation is setup, the reference area is a multiplier, so we multiply out the dynamic pressure and the Cd, then times by the reference area. well we can see the evo's reference area is almost half that of the STi's, so what would be the quick assumption? that the evo's vg creates half the drag of the STi's fin thing. except they dont have the same drag coefficient. the drag coefficent is basically just another multiplier that shows the dragginess of some object, the closer to 1.0 you are the more draggy you are. so what has more dragginess, a flat sheet, or a fin sticking up? you guessed it the fin has a lower Cd.

so im not saying STi's are stupid for running their version of the flow seperation preventer, they are just taking a different approach to the same problem , after all they can reduce their low pressure bubble much more than we can (they just create drag while removing it)

so thats pretty much it, as far as VG's go. questions? post 'em up.
im sure there are some other things of note in the mitsu tech report, but they would have nearly negligable effects (like they discuss why they chose the fin shape as opposed to the square that you see on most airplanes)

[rammsteinmatt]

drag increases exponentially with respect to velocity. namely, as velocity increases, drag increases parabolically

but if you REALLY wanted to figure out how much faster you could go with a vg'd RS, that is entirely possible (but it would be somewhat lengthy, and furthermore very difficult to type coherently on an online forum (but i'll try, using excel function commands - stay tuned) )

D = Cd*1/2(rho)v^2*Sref. note the v^2 term. thats the real pain in the ass as far as driving fast goes

lets take a stab at this (quickly at the elementary level):
assume:
elevation 1000ft STP, v=60mph
and the Sref i calculated experimentally with my car
evo's stock top speed (without vg) is 160mph at which point the drag is too great and the engine cannot overpower it

D = (.36) * (2.31e-3 [slugs/ft^3] ) (235 [ft/s] )^2 * (20 ft^2)
= 918.5 [lbf] drag (actually you can calculate this from the car's horsepower as well, but thats another day)
918.5 = (.36-.004) * (2.31e-3) (v)^2 * (20)
v = sqrt (918.5 / (.356*.0462))
v = sqrt (55845.4)
v = 236.3 [ft/s]
v = 161 mph

therefore a 1 mph increase on top speed. which isnt that interesting, but what is interesting is the drag at 160 mph. 920 lbs of drag!!!!!!

see how much fun all these theoretical differences are, and the hard calculations to find out that delta v is only 1 mph


found this, kinda explains the whole thing: http://www.edmunds.com/advice/specialreports/articles/106954/article.html

[rammsteinmatt]

"so all this drag force this, downforce that. all i want to know is how much faster can i go, and how this effects the effective poweroutput of my car." well, here you go:

since we know that the unit of a horsepower (butchered) is: (lb*ft/s)/550

to calculate the power required, we take: P = D * V. or, power required is drag times velocity

therefore, if we take a force, a speed, and divide them by 550 from our car, we would get a horsepower. a force = drag, a velocity = speed, and 550 = 550 (lest you were confused =P).


using numbers that i gathered experimentally, we find:

velocity = 72.5 mph = 106.33 ft/s
drag = 158.9 lb

such that, power required at 72.5 mph is: (158.9 * 106.33) / 550 = 30.72 horsepower

of course drag is a function of V^2, so the drag, and therefore Preq increases exponentially. naturally at some point your Preq will be greater than your power available (Pavail), and that defines your top speed (discounting things like governors, road conditions, fuel, weight, gusts, etc)

since drag is a second power term (varies by V^2), you need 3 points to find the coefficients of the quadratic eqn. there are also factors that contribute to drag that are a function of V (road-tire resistance, friction in bearings, etc) and also some that are constant regardless of speed (rolling inertia of tires, etc)

so as we can see driving about at speed requires horsepower to remain at a constant speed, then the remaining power is available to accelerate the car (at which point the drag increases and the remaining power becomes less)


[really hope i got this next part right, as davy should be an uber expert on this]

you might be confusing work and power. work = force * displacement. power is work or energy per time (see below)

the equations to find power are as follows:
p = w/t (work per time)
p = e/t (energy per time)
p = dw/dt (change in work with respect to time, or the deravitive of the work function at a time t this will give the instantaneous power at the time t)
p = de/dt (change in energy wrt time, or the deravitive fo the energy fcn at time t, instanteous power)
p = (f*d)/t (expanded form of p = w/t
p = f*v

all give units of: force*displacement per time ( (f*d)/t )

if we do it my way p = f*v we have the force (drag) that we gathered experimentally (or i suppose you could calculate it given the proper information - dont worry i already looked for it and cannot find it, namely the frontal area needed. but the Cd ~ .36 ). and lastly, we need the speed. look at the speedometer (or GPS receiver)

so hopefully you now understand that it can be figured many different ways, but i picked the most simplist. hey, engineers are lazy right?

most importantly, make sure your units are proper. namely the speedometer shows speed in mph or kmh. to get units of horsepower the speed has to be in ft/s. there are many online unit converts to convert units if you dont want to do unit dimensional analysis. i cannot stress this point enough. so many things have been lost (crashed) because unit mistakes, it is rediculous

cheers, matt

[rammsteinmatt]

linky not work matt..... i'll keep reading O0
http://www.edmunds.com/advice/specia...4/article.html
PS. i'll delete my post later on to keep the thread clean


noted and fixed. it didnt like me copying and pasteing from another post, as it included the ellipses :bang:

[Terry S]

That's a massive difference between the diffuser and no diffuser. Have you done repeats of the test with your own car? Or perhaps with different cars?

Terry S

[trinydex]

couple things to mention. splitters are not all about downforce. splitters keep air coming through the top of the air dam and provide a high pressure zone for ducting and fascia to feed off of.

this is important for people that have big intercoolers and can't run an undertray. this is also important for people who want to feed their brake ducting or oil cooler ducting.

[trinydex]

here is link to vg and other aero related pdfs from mitsu

http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/2004.html

here is a diagram of vortexes generated to serve a similar purpose but in a different geometry

http://www.mulsannescorner.com/vortexlift.html

[rammsteinmatt]

That's a massive difference between the diffuser and no diffuser. Have you done repeats of the test with your own car? Or perhaps with different cars?

Terry S


those numbers were found experimentally with my car. 4 times in each direction to correct for elevation windage, and minor timing errors. my car was completely stock with only a shortty antenna, the greatest efforts were put forth to try and keep all variables constant except the design of the diffuser for an accurate representation.

if the car was lowered the numbers would probably be greater in difference though

the numbers are seemingly large, but you have to remember that the car weighs over 3200lbs, has 1+ people in it, and other misc. cargo. but yes, you can change aerodynamics numbers rather greatly. you should see the tests i did with the windows down. IIRC the drag numbers were ~1.75 times that of the stock car

[silex]

If that's true with the windows down, I can see a bigger market for vent visors than there actually is.

[trinydex]

vent visors create drag in and of themselves though. the difference is that the amount of drag they produce is constant. you don't have your windows constantly down and the amount of drag that vents MAINtain when your windows are slightly down is merely giving you that little opening at ALmost no cost.

but then the point becomes that they're a drag at every other time.

[Blak94GSX]

From the crayon drawings by Rammsteinmatt you'd think he was a rocket scientist or something... :p

[earlyapex]

I'm sorry, did you say crayon or cave drawings?

[JDMC-WestEVO]

great post should sticky

[rammsteinmatt]

fyi

sport compact car, october 2006 has a great article on aerodynamics

[silvery_eagle]

fyi

sport compact car, october 2006 has a great article on aerodynamics

thanks for letting us know... i'm gonna go get one this weekend!

[trinydex]

A couple years ago in the Sport Compact Car's Ultimate Sport Car Contest, the late Paul Mumford won the competition in a dodge viper. The brakes on the viper being it's weakest ]http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTI3MTQwMTZzNDEzZGZkMzF5NTQx.jpg[/img]

http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTI3MTQwMjZzNDEzZGZkMzF5NTQx.jpg

http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTI3MTQwMzZzNDEzZGZkMzF5NTQx.jpg

http://www.we-todd-did-racing.com/wetoddimage.wtdr/wMTI3MTQwNDZzNDEzZGZkMzF5NTQx.jpg

CMP is terrible on brakes, and I hope this may help a bit! Any input would be cool! :D

[trinydex]

it'll help, we call them "kickouts" BTW. Used to help brake cooling but primarily to increase front downforce by making the splitter more effective.

If you bolted a splitter to your lip that terminated with the outside edge of the kickouts you would greatly increase front downforce {thumbup}

[trinydex]

http://www.diynetwork.com/diy/ab_parts_accessories/article/0,2021,DIY_13690_4918834,00.html

matt... they did it already, there goes our idea

[trinydex]

No, not quiiiite correct. NASA's Glenn Research Center has a good website on introductory aerodynamics with a special section devoted to dispelling that myth, as well as some others, about what actually generates lift.

This is the page dispelling that particular incorrect theory: http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

The page with the correct theory: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

The simplest and most accurate way to think of it is that the airflow is turned. Because of the net initial vorticity created by the turning, this theory was effectively proven correct when Ludwig Prandtl was able to photograph the counter-rotating shed vortex, described here: http://www.grc.nasa.gov/WWW/K-12/airplane/shed.html

The aerodynamics index for the GRC has a lot of good pages, if anyone is interested in reading: http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

-Adrian

[rammsteinmatt]

http://www.diynetwork.com/diy/ab_parts_accessories/article/0,2021,DIY_13690_4918834,00.html

matt... they did it already, there goes our idea


wonder how much all the computers and servos weight though?(didnt notice if it said it or not in the article) unless it was a dedicated race car, i dont see the effects greatly outweighing the weight penalties

also this one looks like you adjust it how you want once, kinda like a high speed/low speed setting like you get with the veyron. it isnt the autonomous version that we were theorizing where it would vary downforce depending on inputs from steering, throttle, brake, and speed. more of like a SAYC of aerodynamics.

but its cool that someone did it, they just need to take it to the next level now

[trinydex]

hey matt you wanna explain the turning of flow and how that accounts for some parts of the velocity increase over the wing?

[rammsteinmatt]

hey matt you wanna explain the turning of flow and how that accounts for some parts of the velocity increase over the wing?


turning of flow?

do you mean the streamlines over an airfoil shape, or the flow sliding parallel to the wing along its surface?

[trinydex]

mmm read the article from nasa linked above... i'm not fully understanding it

[rammsteinmatt]

ok i read through it all. i like how in the first article they speak very elementary about lift and in the next sentance they are talking about counter rotating vorticies to prove conservation......so much for baby steps.

if you go to that first link and click on the link "turning" they explain how lift is created there, and (i think) answer your question about velocities.

so F=ma. a = dv/dt. v is a magnitude and direction. now as air blows it travels in parallel lines (lets say its obstruction free, like in an empty wind tunnel) the wind travels with perfect streamlines (if you put in a smoke generator to "see" the airflow). when you put an airfoil in the freestream, the air of course has to go around the airfoil. it cannot go through it, best i can figure :)

this is where the definitions from physics come in. as the streamlines change direction to go over the arifoil, they do just that - change direction. since velocity is a scalar and direction, they are technically chaning velocity, because they are chaning direction. but thats the pedantic (asshole) answer, thought i would say that for fun
the airfoil shape, having a bulbous nose, then a tapering back creates a high pressure at the front, then a lower pressure at the back (all of course on one top/bottom face of the airfoil, the pressure differential between the top and the bottom is different, and not what im referring to here). as we know, air always tries to equalize pressure to be in equilibrium. the air will "rush" from the high pressure to low pressure trying to equalize, this is a speed increase, not just a velocity increase.

so there is a direction and speed change. since a = dv/dt, there is no doubt a change in a, over the airfoil shape, and with a given mass of air (or rather a mass flow - or speed and density - multiplied by time). given F = ma, we have an airfoil that creates a force, by flowing a mass, m, of air over a shape that gives said mass an acceleration, a. airfoils have that sheap because the happen to create their pressure differentials more efficient than other shapes, premember there is a pressure drag created by flow seperation. a down slope will always try to seperate unless the slope is really gradual, or you re-energize the air by using a VG or something

does that help any?

as for the vorticies and counter rotating to suffice conservation laws. thats not really important to us, also because i dont fully understand what they are talking about. basically they're saying that flow over the airfoil creates a vortex when the different pressures meet at the trailing edge. with conservation laws, there cannot be a vortex rotating one direction without one somewhere else rotating the opposite way to "cancel" it out. they say that this canceling vortex is somewhere downstream and eventually dies (or rather decays) because air has some viscosity, i suppose its an exponential decay so it never really reaches zero (as there is an asymptote) but can be taken as zero after some period of time. this is why there is a break between airplanes taking off at an airport (well one of the reasons). the wings create huge vorticies at the wing tips, and if a 747 takes off and immediately a cessna followed it, the cessna would be flipped. ATC allows time for the vorticies to die down before sending the next plane down the runway

[trinydex]

actually... that didn't help any. i understood that stuff.

i even understand the conservation as the flow around a wing "binds" a vortex inside the wing or rather around it, such that a torque is produced and this changes the overall angular momentum inside the fluid. basically if you expand the reference frame to include the "entire" fluid... you'll see that unless there is another vortex of equal and opposite orientation and magnitude.

this is actually just a proof, it doesn't actually say much or explain much about the functionality happening inside the fluid.

what i was hoping you could explain was the whole discrepency in the "equal transit" model. i realize thatÂ* the fluids are not meeting at the end of the aerofoil. i also realize that one side may indeed flow significantly more fluid from some point a in teh front to some point b in the back. but what is then accounting for the change discrepency?

is it the high pressure zone motivating air at the front of the aerofoil and the scooting it along towards the rear at a faster than 'predicted' (by equal transit).

but that doesn't account for a flat plate spoiler which indeed does still turn flow (albeit at the cost of drag as you already mentioned).

so i guess i'm coming to the conclusion that turning flow will always create a high pressure zone and a low pressure zone... firstly by normal forces... secondly by distance differences and then thirdly by creating some sort of negative fluid zone whereby fluids are "sucked" into.

now the quesiton is am i missing anything else?......

http://www.av8n.com/fly/vortex.htm

this link seems to have a very good illustration of all three of the phenomena i just described.

http://www.onemetre.net/Design/Downwash/LiftLine/Liftline.htm

here is a page explicitely showing you how the bound vortex works.

but i found an even better example in my head. remember the last time you were at a pool. if you put your hand in the pool at a diatonal angle with respect to you looking down into the pool (the pool then becomes a two dimensional fluid) you pull your hand through the water and you'll see the water CIRCULATES around your hand and you SEE the shed (start) vortex peel off your hand and spin down in the water!!!!

now the question becomes... do you produce lift because: the bound vortex works against the "beneath" flow and expedites the "above" flow? i think that's a part of it.

and also... when you have staged wings... are you trying to produce an expedited under flow for all the stages or are you preserving the bound vortex by creating multiple smaller bound vortices?

[trinydex]

Got the Feb issue of Racecar Engineering last night and there was a very interesting column comparing various diffuser angles.

Caveats: flat-bottom 350Z in a non-moving-floor wind tunnel
Results:
Increasing the diffuser angle did little to affect drag but it did significantly change the aero balance of the car. 10deg was max overall downforce, 12deg was max forward shift of aero balance. In other words, as the diffuser angle was increased the front downforce increased faster than the rear downforce. It peaked at 12deg and then all downforce started to drop significantly (flow separation).

What does this mean to us? Adding a proper diffuser could reduce lift in the front AND the rear. It works because the diffuser allows more airflow under the car. Assuming the nose of the car is closest to the ground, this region forms a "throat" whose static pressure decreases as total airflow under the car increases. Voila, reduced front lift.