DIY Aerodynamic Improvements

[thehelix112]

Guys,

 

I've been reading a little about aerodynamics lately and was thinking about how one would go about practically measuring drag and lift.

 

I was thinking: for a given AFR ratio, at a given speed, and controlling everything else as much as possible (gear ratio, tyre, tyre pressure, weather), could you

get a measure of the energy required from the injector pulse width required?

 

That would give you something of a measurement of how much power/energy is required to move the car at a certain speed.

 

For lift (as in negative), I was thinking you could just have a linear pot (which I hopefully have access to) between the suspension members to measure the height of the car. Controlling the bit of ground you went over and plotting the values against one another should give an indication any additional compression of the springs caused by downforce. Knowing the spring rate then lets you calculate the effective downforce at either end quite readily.

 

Any thoughts?

 

Dave

[OlderThanMe]

I have thought something similar for the suspension height (can't think of a word) tester? evaluater?. Since I know my spring rate within 2% of the stated value all I would need to know is how far the spring is compressed. You could use a device that would work similar to a gas tank sender out of a first gen...Resistance based.

[thehelix112]

Page 51: http://www.motec.com.au/pdf/MoTeC_Catalog.pdf

Is what I'll be using I hope.

 

Dave

[TheNeedForZ]

Power = Force X Velocity

 

If vehicle velocity is constant and instantaneous power is known(from fuel consumption) then yes Force can be calculated but that force will be the sum of air drag + all frictional losses.

[thehelix112]

Quite right. And assuming I can control all other frictional losses (from 1st post: "gear ratio, tyre, tyre pressure, weather") I should be able to measure the difference in power required to maintain a set velocity, and from this deduce the difference in forces.

 

Would you agree? Others have mentioned that the trick will be in resolution/repeatability, and I quite agree. But we'll see. I'm just wondering if I'm on the wrong track entirely, and hunting for others who have tried this before.

 

Dave

[TheNeedForZ]

assuming I can control all other frictional losses (from 1st post: "gear ratio, tyre, tyre pressure, weather") I should be able to measure the difference in power required to maintain a set velocity, and from this deduce the difference in forces.

 

I doubt frictional losses can be controlled to be a constant since rpm changes with speed, and frictional losses may not be linearly proportional to rpm. So there is no way to really isolate internal frictional losses even if you carry out all your experiments in one gear.

 

However there is a way around this internal frictional thing. If you go to a dyno or jack up your car so the rear wheels do not touch ground and "run" the car at say 50mph in 3rd gear, and the horsepower needed is "A". Then "A" means horsepower needed to overcome internal frictioal losses when car is operated at given condition(50mph,3rd gear)

 

Now you actually run your car on the road at 50mph in 3rd gear, and the horsepower needed is "B" then "B" equals to "A" plus "horsepower loss in air drag"

 

horsepower loss in air drag at 50mph(gear becomes irrelevent now) = B - A,

call it C.

 

Then use P = FV , F = P/V = C/V

 

drag at 50mph = "power loss in air drag at 50mph" divided by "speed 50mph". As long as you use corresponding units, they'll cancel out and give you a number CLOSE to actual air drag.

 

Why is the number close? because if you do the test on the dyno, the friction includes dyno and rear wheels/tire but excludes front wheel/tire friction. If you do it by jacking up the rear wheel, then friction includes rear wheel but excludes rear tire-to-ground friction, and front wheel/tire friction.

[RedFive]

Oh god. Just do some CFD.

[tube80z]

I've been reading a little about aerodynamics lately and was thinking about how one would go about practically measuring drag and lift.

 

For drag I think a simple coast down test would be sufficient. SAE has some info about these tests.

 

Your idea for measuing lift and the distribution should work fine. It's commonly used to create aeromaps for cars. You will need to keep accurate records of the atmospheric conditions, properly setup the car, and make specific changes for this to work.

 

You will need a datalogger but you don't necessarily need linear pots. I've had some success using plain old rotary pots and some linkage from RC cars. Good penny-giles pots will set you back about $250 a piece. If you have access to a motec system it will surely work. But you can do this with just about any data system and some low cost sensors if you use a little ingenuity.

 

Cary

[TheNeedForZ]

Oh god. Just do some CFD.

 

The method I described is actually very simple and works in all situations, no matter what aero mods has been added to the car. I dunno what CFD is but apparently you want people to ask so I humbly ask you : what is CFD?

[Wheeler]

CFD = Computational Fluid Dynamics

 

Prescot AZ = Home of Emery Riddle (sp?) Aerospace Engineering school

 

Redfive = Proud Student of Emery Riddle (is my guess)

 

A good CFD program would run upwards of 5000.00 so thats not an option for us (except Redfive).... May be he would do it for us for extra credit?

 

On your other thought, about the linear pots for recording ride height. I think you would see too much noise (small bumps in the road) and the change in ride height due to areodynamic effects would be small in proportion the suspension due to bumps in the road. I think you would end up with a real mess of data that would be hard to make any real conclusions.

[Guest ON3GO]

i know this is a little off subject but ive been toying with this idea.

 

i see alot of the "lift" happening from the air hitting the firewire (which the air is coming from the mouth of the car) and then going down under the car where the tranny tunnel is. thats just the way i see it.

 

what if i vented the vent end of the hood, like a slot all along the back of the hood with say some mesh grille to make it look nice.

and around the engine have like a "ramp" of some sort pushing the air towards the back of the hood area where the vent is.

 

this is such a ghetto picture drawing but you sorta get the idea,

 

[OlderThanMe]

Great minds think alike...Just how to make it look good rather than having a motor coming out of a mountain of aluminium will be difficult...

[veritech-z]

I wonder how much of a gap between the hood and the cowl you would need to move that much air? It would probably work better with some kind of restricted radiator entry...Also, there isn't too much in the way of space to either side of a standard L series to really do something like that once you take the exhaust into account....would it need to ramp all the way to the top, or would just a smooth transition to vertical down near the bottom do the job, kind of like using a splitter on your air dam, but with a curve where it meets the firewall? I've got some ideas, but I can't host any images from work...

[goodoldjam]

Ive been thinking about it also and i was thinking just redirect it right behind the radiator. Theres a picture of a japanese car somewhere on these forums that vents the air through the hood , but i was thinking maybe venting from the side of the car.

 

[OlderThanMe]

the only thing about diverting it out the side is you don't bvenefit from downforce. If you had it on top of the fender that would be better IMO. I have been thinking about having a venting system diverting air from the front mouth into the fender and having an air tunnel along the top of teh spacious fender to house an oil cooler or even mount 2 small radiators, 1 on each side of the car, under the fenders and sealing the most of the front of the car off.

[JMortensen]

You can't vent the hood at the back. Well you can but what you'll get is air coming in the vent, not going out the vent. The air that goes over the hood is low pressure air, good to vent to. Once the air hits the windshield it packs up and becomes high pressure. Whereever you put a vent in this area, the air will go INTO it, and not come out of it. This is why there is a fresh air vent in the cowl on most cars and we have cowl INduction hoods, not cowl EXduction hoods. What you'd probably accomplish with this is to slow the air coming through the radiator at speed. Either the pressure of the air entering the radiator or the pressure at the cowl would be greater, and whichever turned out to be the winner would force air in their direction. So if at speed the pressure at the radiator was greater and you had sealed the area between the core support and the air dam then the air might still flow in the right direction, although slower than if the "vent" in the back weren't there. If the pressure at the cowl was greater, then it would force air backwards through the radiator. This is assuming a perfect seal around the engine, which realistically wouldn't happen.

 

If you want to vent the engine compartment, do it in the middle of the hood towards the rear, like where Nissan put the vents on the 280 hood. You could use the same deflector idea and shove the air up and out of the middle of the hood.

 

Better yet is Terry's tunnel for radiator air to go through and out the top of the hood, but that's also pretty extreme from a fabrication standpoint.

 

Goodoldjam's idea is interesting, but I don't think you'd want to vent where his diagram shows because that's basically right in front of the front wheels. Maybe further back and out a fender vent with a lip in front of it to create a little suction at speed. Still out the hood is best because it would give you more downforce.

[Guest 73Turbo240z]

Depending on your Z's livelyhood factor for daily driving, the solution could be simple... cut the firewall.

 

The cowl area is a recess trough basically to house a wiper motor and some linkages. If you don't need your wiper, open up the whole firewall there. If you want to keep the wipers, open up the firewall on either side of the motor.

 

Then modify the cowl panel, opening it up from the slots it has, to a metal frame w/ metal mesh center, trim the openings to make it look more stock, and you how have a giant CFM ramp.

 

Although the one thing i'd wonder about is how the high pressure ridge would be affected at the base of the windshield if you then were dumping large CFM high velocity air upward there.

 

**edit**

 

jmortensen hit on what i was talking about... so yea... scratch that idea...

[Guest 73Turbo240z]

heres another idea, though the logistics of doing this on an L block could get messy since in theory it can only work really well on one side of the hood due to engine layout... reverse cowling...

 

 

if your car is airdam equipped, fab a plate that ties the upper lip of your airdam to the lowest point of your radiator, that way your force loading the air. then have the reverse cowl behind that, it wouldn't remove 100% of the air, but it would bleed some of it off.

[OlderThanMe]

haha...with carbs I may have enough room to do that around the distributor...one reverse cowl on each side. I am planning on doing that airdam seal this weekend.

[veritech-z]

How about diverting it around the engine like goodoldjam suggested, but instead of doing it in front of the tires, do it after into all that empty space behind the wheel. Then you could seal that up and make a tunnel through the door structure like some cars do for the ac vents to get more airflow to the backseat, but instead of that you could dump it out the back somewhere? Or if you had one of those cool body kits like the red Monza Mystery Z we were all commenting on in the non-tech forum you could spit it out the back of the front fenders....