Race car aerodynamic help?

[Bennett_Racing]

Originally Posted by aerohead

Luke,apology for late response and thanks for the data.Since many dragsters have so much power that they might go through 32-partial clutch engagements before actually reaching lock-up close to the finish line,to avoid wheelspin.----------- If that were the case for your car,then,perhaps you might have some power available for some additional mass,in the form of aero add-ons.------------------- Or,with substitution of carbon-fiber epoxy,you could add structure and simultaneously LOSE weight.------ If your clutch is fully engaged by the finish line,and you are not experiencing wheelspin,is the car still accelerating strongly? or is the power and gearing playing out?---------------------- With respect to the time/speed curve,some drag racers believe that they're at speed so briefly,that power-to-weight is the most important parameter and to hell with aerodynamics.-------------------------------------- I have very little data regarding dragsters and what I have is from the work of Don and Gerald Arivett who were developing a low-drag AA Fuel Dragster,fully-enclosed,with Cd0.20 ( Cd 0.18 @ 12-degrees yaw ).This was 1987,and at that time,no complete windtunnel tests had ever been performed on a dragster.--------------------------- They looked at Indy cars,since they were also open-chassis,so to speak,and the best of those cars had Cd 0.69,and they believed the best funnycar had Cd 0.60.---------------- The Arivett Bros. believed that their car,in 250-mph territory,would free up over one thousand horsepower.------------------------- So it looks like drag reduction does pay.--------------------------------------- Looking at Relentless,my thought is that (1) the integral nose/wing combination lowers the efficiency of the wing.My intuition tells me that a smaller front wing,moved forward,would provide as much downforce with less drag penalty,operating in "cleaner air". (2) The abrupt chopping-off at the back of the nose section is accompanied with complete flow separation and turbulent wake trailing back to the engine.(3) The engine itself poses a "torture-chamber" to the air,both in it's leading aspect and flow separation behind it.(4) The firewall of the altered's body is so sharp-edged,along with the windscreen frame,that I'm fairly confident that you've got complete flow separation at that point,with little recovery if any,which may be why you had to raise the drag chutes up so their droque-chutes could catch some "clean" air.(5) The rear fenders are sharp-edged enough that flow is torn up there,although it may not be a problem due to the separated flow ahead of them.(6) As mentioned by others,the tubing and padding for the roll-cage could be streamlined like aero struts on aircraft,along with the support structure for the rear wing.(7) The Goodyears,as exposed wheels,have been windtunnel-tested and themselves have a drag coefficient of 0.57.------------------------------------------- If any of these areas can be faired in,or enclosed without violating class restrictions,especially the rear wheels I think it would pay you dividends.The entire wheelie bar,if enclosed like a boat-tail,tapering to 50% of the cross-sectional area of the body ( like drag-bikes )with some fairings in front of the wrinkle-walls would dramatically reduce drag,but also might violate rulebook.(8) My final thought for now,is the rear wing.And you may have fully investigated this already but back in the 1980s,Joe Amato ran an Eldon Rasmussen "high-wing" spoiler which is credited for Joe's victory and 260+ mph runs.The wing was small,placed very high,and behind the car,operating in very clean air,with max downforce,minimum drag,and more "leverage".If Relentless is stable now,your starting from a good place.------------------------------ If you can get that windtunnel model going you could do a lot of "safe" shakedown runs without risk to your dad.I don't know if you'd have access to the MIRA facility.There is a Donald Campbell tunnel at London's Imperial College and at Southampton.Williams Grand Prix Engineering has it's own tunnel.If these can't help,perhaps another university with Mechanical Engineering Dept.and windtunnel might take your car on as a undergraduate or graduate project if you threw a few hundred Pounds their way.Keep us posted,and we'll keep thinking about Relentless.Race safe,Phil.

Phil,

Sorry for delay in replying…thanks for the post, made good reading!

One thing regarding our clutch, it does not use timers or anything, it’s a 2 plate pedal clutch from Crower. Due to the nature of the fuel and the blower we use, it will pull hard till about the 1/8th the the rest is down to taking fuel away to keep the AFR good I mean it still pulls but not a strongly. To give an idea, it will do 175+mph at the 1/8th but will only gain another 40ish mph to the finish line.

(1) I have thought about adding a slim wing to the front of the car, infront of the bodywork to smooth the whole front,

(2) What about making some angles pieces that sit to help with the bulkhead?

(3) I think for now with this body we will have to make the best of it….

(4) Good point, how can we help this?

(5) We need a wing tunnel I think

(6) We will definetly be making the wing struts from areo tube this year I hope.

(7) Would a canard style wing in front of the rear slicks help this???

(8) I remember the wing, everyone laughed till he went 260mph!

Thanks for the info I will try these and try to get the car in there.

Also thanks for the post, appreciate it
Luke

[MechEngVT]

Forgive my bit of drag racing ignorance specific to classes/rules etc, but I did notice you were working on your wheelie bars to keep your wheelstands under control and that doesn't seem like something that would be as beneficial as other courses of action.

I used to work with a guy who was deeply involved in drag racing a similar class in southern California back in the 60s and he was heavily into the unintuitive tricks to tune the cars. He did a lot of work on rear suspension and engine placement optimization as well as clutch slippage and blower speed modification.

They were running mechanical FI at the time and stopped rejetting fuel for the conditions and started changing blower speed to achieve AFR with a fixed fuel flow to improve consistency. If you have AFR problems try adding more blower rather than pulling out fuel. This will mean you're always putting out about the same horsepower and will reduce the variability of your suspension/clutch setups.

You're better off setting up your rear suspension so that the instant center is vertically higher as this would improve squat resistance, changing the force transfer from chassis to tire to plant the wheel more so than rotate the chassis. Ideally the car will accelerate without changing attitude, but if your move your rear instant center too high your rear end will lift up and the front will dive which is about as bad as doing wheelstands.

Another option is to dial in more clutch slippage off the line to keep your front end down. If you're bogging in the last 1/8 you might need more clutch slippage there, or your geared for a higher trap speed than you're currently achieving. Maybe this won't be an issue if you can solve the problem that's forcing you to pull out fuel past the 1/8 mark.

Another thing that may help is varying your valve clearance. If you're getting tire spin at launch you can run a step or two tighter valve clearance which will increase valve overlap and lift (reducing low-RPM torque and improving high-RPM power). The opposite is also true; launch bog can be helped by running a step or two looser valve clearance to improve low RPM torque at the expense of high rpm power by loss of lift and loss of overlap.

[Bennett_Racing]

MechEngVT,

Thanks for the reply, we still run mechanical FI but this cars has no suspension. We have to slip the clutch to a degree but this car is 2500 horsepower + so it harder to control.

The reason for pulling fuel is due to the design of a roots type blower. Its an ineffeciant blower in its design (made better now as we use a hi helix, which means it has a 120 degree rotor twist) But in essence the faster you spin these blowers the faster they become ineffiecent on the track and will then actually heat the air.

We spin ours fairly slowly, 1/8th marker was just to give an idea, this car is an animal and will pull hard probably until the 1000foot marker but we have yet to do a full pass with this combo. Initial testing though is so promising, we did a soft half pass and it still ran a 7.60 at 133mph

[DonR]

From looking at the pictures, I think you could blend the body lines of the gas tank back to the tub with the body material of your choice. Obviously leaving a hole for the motor & what ever accessories need cooling.

Rear fender flairs. You could move the front edge of the flairs forward allowing you to reduce the abrupt angle of the flair. I think moon covers are out of the question, but if you make your outer beadlock ring "full disk" as possible (current F1 cars as an example) it should help aerodynamically. if you have the resources you could screw on a carbon fibre plate to the beadlock ring to reduce rotating mass.

Don

[aerohead]

Originally Posted by Bennett_Racing

Phil,

Sorry for delay in replying…thanks for the post, made good reading!

One thing regarding our clutch, it does not use timers or anything, it’s a 2 plate pedal clutch from Crower. Due to the nature of the fuel and the blower we use, it will pull hard till about the 1/8th the the rest is down to taking fuel away to keep the AFR good I mean it still pulls but not a strongly. To give an idea, it will do 175+mph at the 1/8th but will only gain another 40ish mph to the finish line.

(1) I have thought about adding a slim wing to the front of the car, infront of the bodywork to smooth the whole front,

(2) What about making some angles pieces that sit to help with the bulkhead?

(3) I think for now with this body we will have to make the best of it….

(4) Good point, how can we help this?

(5) We need a wing tunnel I think

(6) We will definetly be making the wing struts from areo tube this year I hope.

(7) Would a canard style wing in front of the rear slicks help this???

(8) I remember the wing, everyone laughed till he went 260mph!

Thanks for the info I will try these and try to get the car in there.

Also thanks for the post, appreciate it
Luke

Luke,just now got your post and looked at some of the other comments.Looks like your getting some good feedback on the project.With respect to your latest comments (1) yes,I think a smaller canard at the very front would pay.(2) for the nose piece,a curved,tapering tail ( fish/birds) would be best,although most difficult to fabricate.Tapered panels,with say,no more than 12-degrees inward taper would help manage that wake.(3) even a flat,engine-height, vertical panel,spanning from behind the engine to the firewall would help minimize circulation (drag) there.(4) anything would help that sharp edge at the firewall and windscreen frame.A simple 45-degree chamfer would help direct air around,although of course,a radius would be the best( as large as you could get away with),allowing the air to flow around those corners without separating.(5) If you had a university in the area with a small windtunnel,a couple hundred Pounds might get the students to do a study of the car.My friend Kenny Lyons paid $300 US to Texas Tech University Aero Lab back in the early 90s,for a work up on his Becker/Lyon motorcycle streamliner he runs at Bonneville.Really useful data! (6) the aero-strut material is a sure-thing! Not a lot of frontal area involved but it's amazing what that teardrop profile does to drag reduction.(7) I think something like a deflector would be best for the back tires.Something that could steer the air around and over.It may also violate the rules.And something I would bounce off my local Goodyear Race jobber,is that if you shield the tires,do you run the risk of over-heating them and inviting a high-speed blow-out? Dragsters and funny cars can have shielded tires,but they may be a different compound than yours.You don't want to fix one problem,and then create another unawares!--------------- Finally,I had a question about the air/fuel ratio during the second 1/8th mile.Could the air around the blower be so turbulent right now,that the airstream isn't carrying some of that heat-of-compression away,leading to the richer mixture? Is it possible,that if air was purposely directed down the sidewalls of the supercharger,that at over 175 mph,she'd run a bit cooler,and you could "keep your foot in it"? ------------------- Let us know how things fall together and best to you with the project,Phil.

[aerohead]

Luke,I've been thinking about the richening problem on the race car and wanted to toss out the following.---------------------- I found four articles from HOT ROD Magazine dealing with mixture issues which went in a direction I would not have thought.------------------ The racers were experiencing water pump cavitation which led to uneven combustion chamber temperatures,which played hell with mixtures.--------------- Their solution was to use old-tech cast-iron impellers,with more efficient hydrodynamic vanes instead of stamped-steel impellers.----------- Sometimes they used smaller diameter pumps to reduce angular velocities at the vane tips,and lastly,they might under-drive the pump a bit to keep speeds below what would cause cavitation.------------------- The modern electrically-driven pumps may do away with these issues,don't know.----------------------------------------------------------------- The other thing I noticed is that the flat-faced driven pulley of the blower,and brackets behind it,near the bug-catcher,look like they could actually interfere with airflow into the throttles,and lastly,the throttles on some bug-catchers have virtually no radii at all ahead of the openings,just a sharp-edged corner.----------------------------------------- All elementary fluid dynamics textbooks would say this was a no-no,and lead to what they call a "vena-contrata" entry loss,where the air cannot flow anywhere near the walls of the opening due to flow separation at the inlet,effectively squeezing down the flow at a point where you need it the most.--------------------- If you have this,not only will you have an immediate flow restriction,you also raise the power necessary to impel the air into the blower which will add heat to the compressor,expanding the charge,and requiring a leaning of the fuel supply to maintain stoiciometrics.------------------------------------------------------- I can't tell as much as I'd like from the photos of the car and your hardware may be better evolved than others,but if the problem with the mixture persists you might consider a second look there at the inlet.Best to you,Phil.

[Bennett_Racing]

Back from our first event of the year, and good news is that we won.

Also we can a new personal best, a 6.6352 but most importantly at only 181mph

This would of been a low 6.5 or high 6.4 but as this was only our 2nd run and were still testing, came off of it early and still ran the 6.63

We have also added a gurney flap to the rear wing and also to the rear wing spill plates too. Another addition is some 1" angle alloy to the sides of the body to get it lower to the ground and added strength

Any more tips and hints please help us go faster!

Thanks

[aerohead]

Originally Posted by Bennett_Racing

Back from our first event of the year, and good news is that we won.

Also we can a new personal best, a 6.6352 but most importantly at only 181mph

This would of been a low 6.5 or high 6.4 but as this was only our 2nd run and were still testing, came off of it early and still ran the 6.63

We have also added a gurney flap to the rear wing and also to the rear wing spill plates too. Another addition is some 1" angle alloy to the sides of the body to get it lower to the ground and added strength

Any more tips and hints please help us go faster!

Thanks

Luke,congratulations on the win! I posted a few more things,but the stuff on the supercharger may not mean anything as you mentioned you were switching to a centrifugal style from the Roots type.I have been looking at the composite bug-catcher with three throttles,like Kenny Bernstein and others have run.It looks like it is better streamlined,and also projects forward into less turbulent air,which might be torn up by the engine/pulley/belt/etc..

[Bennett_Racing]

Thanks! we were very happy.

Well the injector that we run was designed and built by us, we took an 80's designed injector and brought it right upto date, it is working extremely well so far, few bits we have tried inside which will help power too.

[Bennett_Racing]

Bit of an update.

We won our 2nd meeting of the year and have now also ran another PB of 6.4261 at 191mph

We were getting to approx 195mph and the rear slicks were rubbing on the inner arch so we need to raise the body up some more.

I have bought a nice wing that I intend to add to the very front of the car at a shallow angle to help the whole front end to be more efficient. But we will still keep the canards on there too. So that the smaller narrow wing helps to get the air over them.

Also stability of the car has always been very good and has not changed since we changed the body from the 2007 design.

Any more suggestions are very welcome.

Thanks
Luke