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MetroMPG 11-27-2007 11:09 PM

List of aero mods you can do to your vehicle
 
Note: This thread has more or less been replaced by the master list of 60+ mods.

---

Feel free to reply with links & examples. I'll update this top post.

Note: some of these mods may make your car less safe and/or illegal (e.g. mirror/wiper deletion). Don't blame us if you do something silly and get caught or crash into a tree in the rain.

Deletions:
-----------
- Roof racks (OEM as well as aftermarket)

- mud flaps

- raised wing type rear spoilers

- Side mirror delete (MetroMPG; Peakster's video test)
(with optional inside convex mirror replacement; or for less extreme, install mirror(s) you can fold back)

- Radio antenna

- shaved excrescences (badges, door handles, rain gutters)

- windshield wiper delete (front & rear)
Replacement parts:
---------------------
- smooth wheel covers

- narrower tires

- upgrade from "bucket" style to aero headlamp assemblies

- swap to a more aerodynamic bumper cover style (Metro)

Modifications/fabrication:
----------------------------
- fabricate lexan/plastic covers over "bucket" style headlight treatments (Coyote X's Metro)

- grill block (Metro, Dodge Caravan, CRX)

- tire spats, or fully contoured deflectors

- boat-tailing behind tires (Aerocivic)

- belly pan / under tray (1st gen Prius)

- rear wheel skirts (Metro, Civic)

- front wheel skirts (Daihatsu Mira)

- if no front wheel skirts: softened/radiused aft edge of front wheel arches (see: VW 1L car, Solectria Sunrise, Honda Insight, GM EV1)

- ... and/or gap fillers to minimize the space between the exposed tire & wheel arch (see Audi A2, Toyota Prius 2 for examples of cars with mimimal tire/arch gaps)

- reduced ride height - 3 inches is often quoted as optimal (CRX)

- taped/sealed panel gaps (or siliconed & razor cut)

- partial kammback (Metro), or,

- fulll boat tail addition (Aerocivic)

- pickup trucks: partial (rear) tonneau cover at a minimum, full tonneau, or complete aero shell (boattailed cap)

- radical frontal area reduction (e.g. tandem greenhouse)

- decreased (or is it "increased" - you know what I mean) windshield angle (e.g. addition of external clear "visor"), and/or increased hood angle

- increased radii at the front-to-hood & front-to-sides of the vehicle

- front air dam (as low as the lowest underbody components, possibly lower depending on the vehicle)

Big Dave 01-20-2008 12:19 PM

Another idea. Gap seals. Outfits that make aero mods for light planes are big on gap seals. Most aircraft panels are faired or at least fishscaled, but the flaps, ailerons and vertical stabilizer have gaps. They claim increased speed at level power settings.

Those clearly do work on 170 knot (200 MPH) aircraft but they may not be as effective for 70 MPH ground vehicles unless you have egregious gaps. The cab-to-bed gap on some pickups get overly large.

JerkOfAllTrades 01-20-2008 09:07 PM

Quote:

Originally Posted by Big Dave (Post 6199)
Another idea. Gap seals. Outfits that make aero mods for light planes are big on gap seals. Most aircraft panels are faired or at least fishscaled, but the flaps, ailerons and vertical stabilizer have gaps. They claim increased speed at level power settings.

Those clearly do work on 170 knot (200 MPH) aircraft but they may not be as effective for 70 MPH ground vehicles unless you have egregious gaps. The cab-to-bed gap on some pickups get overly large.

Hmmm, some blue painters tape and some testing could prove this rather quickly. Unfortunately I'm not in a position to try this out at the moment however, I'm willing to give this a go sometime this coming summer.:cool:

I should have some repairs and a good baseline for my car by then.:p

Stan 01-23-2008 01:33 PM

I know we have good data for some of these techniques, but has anyone measured (a-b-a) the effect for each of them? I am especially interested in rear wheel covers and smooth hubcaps, as these might be something SWMBO might go for. ;)

AndrewJ 01-23-2008 04:41 PM

Stan, you just might be in luck.
I've been thinking that I should go get my testin' on tomorrow or Friday... Some good ol' A-B-A on the test hill, been thinking about throwing in a C-D-E as well.

I just might get motivated enough to do it.

MetroMPG 01-23-2008 04:56 PM

Darn you Californians and your good testing weather.

I've attempted to test rear skirts 3 times on 3 different vehicles: details here

In all cases, the tests were less than perfect, despite 2 of the 3 being A-B-A style (I concluded based on the 2 sets of A data that the drivetrain temps hadn't stablized).

Results were between 0.6% MPG improvement to 2.7%, with the largest impact seen on the Metro (which I think is due to the design & placement of the wheel arch).

Fuzzy 01-23-2008 06:57 PM

Quote:

Originally Posted by Big Dave (Post 6199)
Another idea. Gap seals. Outfits that make aero mods for light planes are big on gap seals. Most aircraft panels are faired or at least fishscaled, but the flaps, ailerons and vertical stabilizer have gaps. They claim increased speed at level power settings.

Those clearly do work on 170 knot (200 MPH) aircraft but they may not be as effective for 70 MPH ground vehicles unless you have egregious gaps. The cab-to-bed gap on some pickups get overly large.

Sailplane pilots use gap tape quite a bit too (ailerons, flaps, and wing/fuselage connections). Blue Painters tape would be okay, though you'd want to make sure that it doesn't 'sag' into the gap; and that there are no wrinkles/ripples.

brucepick 03-21-2008 07:12 PM

MetroMPG - You mentioned an air dam in the original post.

Can you (or trebuchet03 or anyone else for that matter) expand on that some, provide some detail on why it works?

I've seen some posts on non-FE sites, saying that an air dam will increase drag and reduce FE. I say that's likely true if the air dam is designed to create downforce. I'm pretty convinced of the FE value of an air dam that's designed for FE, especially if a belly pan is included to seal off the gap. But it would be good to see some more detailed analysis.

Thanks in advance.

virg 03-25-2008 12:58 PM

I wonder about a bracket up front with a ball or bulb leading the way, like that bulbuous thing on the hull of a ship that parts water. Make it a sliding-adjustable arrangement with the bulb leading the grille.

donee 03-25-2008 08:57 PM

Hi Virg,

I am no expert, but the bow bulb on a ship has to do with the interface between water and air. The bulb lifts the water ahead of the ship, which then falls away from the above water bow. Apparently its takes less work to lift the water than to push it aside (air is more compressible than water, and one is pushing against air when lifting the bow water).

On a car, or submarine, or airplane there is no such interface involved, and hence no bulbs.

mpgvalues 04-07-2008 07:51 PM

Just so you know I've been browsing through this site and I think some of things you guys try are a little loopy. Dont get me wrong, some are ingenious, practical, heck it even looks like fun. But yeah, a little odd :) Keep going I say.

Achtung 04-08-2008 10:02 PM

Quote:

Originally Posted by donee (Post 16091)
the bow bulb on a ship has to do with the interface between water and air. The bulb lifts the water ahead of the ship, which then falls away from the above water bow. Apparently its takes less work to lift the water than to push it aside (air is more compressible than water, and one is pushing against air when lifting the bow water).

I always wondered how those bulbs made it easier for the ship to push through the water... that explanation makes alot of sense! I never thought of it that way. Thanks for that! Kinda gave me a eureka moment there...

aerohead 04-09-2008 08:56 PM

bulb
 
Quote:

Originally Posted by virg (Post 16016)
I wonder about a bracket up front with a ball or bulb leading the way, like that bulbuous thing on the hull of a ship that parts water. Make it a sliding-adjustable arrangement with the bulb leading the grille.

The History Channel had a program about the Japanese Imperial Navy's flagship,Yamato.The project was very classified,and the nose of the ship was top-secret,and a hydrodynamic coup for the day,and has found its way into the U.S.Navy (USS Ronald Reagan),and Soviet Navy ships,as well as merchant vessels.According to the narrator on the TV program,the bulb created a counter bow-wake underwater,which cancelled the bowwake above waterline.Sorry! This is a little deep for me.A fellow tried this at Bonneville on a streamliner.Best I can tell,it failed to be effective.

Randy_the_Hack 04-10-2008 11:05 PM

Fellas...

It would be nice to have a sticky out here that broke things out in a quick reference manner:
  • Car mods
    • Prioritized list of aero mods
  • Truck mods
    • Prioritized list of mods

Realizing that various mods deliver different percentages of improvements per vehicle, these might be somewhat general. For example, with a truck, the likely top few mods might be a) lowering the truck, b) less aggressive tire tread and size, c) aero bed cover, etc.

So... unless there is some data out there that indicates smooth wheel covers are a huge saver, I would expect to see these generally at the lower end of the spectrum. I realize that this might be too precise, so grouping in general categories of top mods, middle mods, and everything else, might suffice.

Breaking this out by cars v/s trucks would be quite helpful...

Just my $0.02...

aerohead 04-25-2008 04:21 PM

aero mod list: benefit-breakdown
 
Quote:

Originally Posted by Randy_the_Hack (Post 18966)
Fellas...

It would be nice to have a sticky out here that broke things out in a quick reference manner:
  • Car mods
    • Prioritized list of aero mods
  • Truck mods
    • Prioritized list of mods

Realizing that various mods deliver different percentages of improvements per vehicle, these might be somewhat general. For example, with a truck, the likely top few mods might be a) lowering the truck, b) less aggressive tire tread and size, c) aero bed cover, etc.

So... unless there is some data out there that indicates smooth wheel covers are a huge saver, I would expect to see these generally at the lower end of the spectrum. I realize that this might be too precise, so grouping in general categories of top mods, middle mods, and everything else, might suffice.

Breaking this out by cars v/s trucks would be quite helpful...

Just my $0.02...

Randy,you asked for a very useful sticky.And while its a daunting task,I'm assembling whatever material I have and will post as I can.In the meantime I wanted to share info on some papers that basically answer your questions if you can access them.SAE Paper No. 649A "Aerodynamics for Body Engineers" by Kelly and Holcombe,1963,and SAE Paper No. 649B "The Body Engineer's Role in Automotive Aerodynamics" by Walter H. Korff,January 1963.Between the two papers,the authors lay out where all the energy is going,and in the second paper,especially,Korff lays out a recipe for Cd 0.21 cars (remember this is 1963!) and quantifies the difference between drag contributions for all body elements.SAE Paper No. 690189 "A Method of Estimating Drag Coefficients" by R.G.S.White,also lays out a cookbook approach to low drag cars,breaking a car's body into 9 categories.From the tables,one can deduce drag potential ( and elimination) within each of the 9 categories.Last paper is SAE No.760185 "The Optimization of Body Details-A Method for Reducing the Aerodynamic Drag of Road Vehicles" by Hucho,Janssen,and Emmelmann,all from Volkswagenwerk AG.Again,the paper as in the later book,breaks down a vehicles drag components and attaches numerical values to each and shows potential gains should each be realized.If you can get a copy of Hucho's book it will have all the data you seek.

i_am_socket 05-05-2008 10:59 AM

Perhaps some before/after thumbnails of the particular mods to get an idea of what would/wouldn't work for your vehicle could be included along with the links?

aerohead 05-05-2008 04:39 PM

before and after
 
Quote:

Originally Posted by i_am_socket (Post 23501)
Perhaps some before/after thumbnails of the particular mods to get an idea of what would/wouldn't work for your vehicle could be included along with the links?

Actually,Hucho's book has everything,and before and after for everything,with a quantitative assessment of everything done.Its quite a goldmine.

Randy_the_Hack 05-05-2008 06:59 PM

Quote:

Originally Posted by aerohead (Post 23549)
Actually,Hucho's book has everything,and before and after for everything,with a quantitative assessment of everything done.Its quite a goldmine.

Yeah but when I looked for that book, the cheapest copy I could find was something like $185, and it was in German, IIRC. Ich spreche nicht oder lese Deutschen. ;)

aerohead 05-07-2008 06:18 PM

Quote:

Originally Posted by Randy_the_Hack (Post 23573)
Yeah but when I looked for that book, the cheapest copy I could find was something like $185, and it was in German, IIRC. Ich spreche nicht oder lese Deutschen. ;)

Randy,I was surprised when I read your post,however,perhaps not as surprised as you,when you saw that price! Sounds like gouging to me.I paid $80 back in '90.I saw where members were picking them off Amazon or ebay for $100.00.If it weren't in Deutsch I'd say go for it.For what it will give you its a bargain,but not with the hassle of translation.I'm pulling some materials together for those who just don't have access but it will take more time.

i_am_socket 05-08-2008 09:49 AM

For the un-enlightened among us: what is this "Hucho's book"?

COMP 05-08-2008 09:52 AM

Quote:

Originally Posted by i_am_socket (Post 24229)
For the un-enlightened among us: what is this "Hucho's book"?

i agree,,, droping a bill on something i can't read :eek:

i_am_socket 05-08-2008 10:03 AM

Oops. I'm somehow utterly lacking the brains to think "hmmm... maybe I should search the site" this morning. LINK!

I'll be getting that second cup of coffee now... *facepalm*

aerohead 05-16-2008 03:15 PM

modifications-data-% change,or Cd change(nose-jobs)
 
Hello all. Randy_the_Hack asked about a breakdown of aero mods and their potential for savings.I've been pulling what I have together and since there is a lot of ground to cover,I'm going to post it as installments,in case the computer crashes and to respect the little time I have at the computer as of late.General note: all this data is dated now.All current production cars,pickups,vans,and SUVs should be employing some if not many aero refinements empirically proven over the last hundred years.Since carmaking is still ruled by the stylist it will be the last industry to evolve into the "form follows function" philosophy as in aircraft,ammunition,artillery,boats,canoe,kayak,pu mps,turbines,ordnance,ships,and submarines.The upshot is that ecomodders will have access to some of the existing fruit,which for their class of vehicle offers added mpg.As always,we need to be careful and mindful of the other motorists we share the road with.
I would like to have presented the material in tabular form although its beyond the scope of what I can do right now.(Nose-jobs):In 1937 Carl Breer used a "raked" front end to help his ecomoddered Chrysler Airflow to achieve Cd0.244.In 1938 the Jaray/Adler demonstrated a 15 % drag reduction with 100% grille-block.In 1962 L.W.Farington utilizes the 1976 VW "ideal" nose to help his 1956 Ford Thunderbird achieve 228-mph.In 1963 Walter Korff suggests that the optimal grille/radiator/nose/headlights/turn signals are good for a Cd0.07 improvement.In 1968 R.G.S.White allows an ideal nose to produce a 28% drag reduction.In 1974 CAR and DRIVER get 0.9mpg from airdam and 0.4 mpg from grille-block,1.4 mpg for combined,with "Crisis-Fighter-Pinto".In 1974 C and D get 1.2 mpg with 260ZG nose,and 0.2 mpg with plex headlight covers with "Crisis-Fighter-Z-Car.!975 VW Rabbit cuts Cd by 14% with Hucho's "ideal" nose.1978 Ford Capri gets a 6% drag reduction with wrap-around bumper with integral airdam and airfoil grille.1981 Pontiac Grand Prix flush headlights=1.5%,downward-sloping hood/fenders=1%,and softened hood leading edge = 1.5%.1982 Peugeot VERA cuts Cd by 0.015 with integral front airdam.1991 aero nose with headlight covers and lowered airdam add 1.526 mph to Phil Knox's CRX.1986 SAE Paper 860212: 100% grille-block cuts Cd by 0.003,airdam hurts bellypan car,airdam helps dirty-bottomed car.1986 SAE Paper 860211: 4.25-inch minimum radius needed for attached flow at leading edges,vertical or laid-back nose lowers Cd by 0.01.1986 SAE Paper 860216 (Subaru XT): front airdam cuts drag 6.9%,lowering nose by 10mm drops drag 1.7%.1997 Ford NASCAR with 1/4-inch lower front airdam beats Dale Earnhardt's Chevy.There are some other articles involving front mods however they are lumped into "kits" done for specific vehicles and no single-item breakdowns are given.I'll include this info in a future( Kits ) installment.Again,most of this info or similar will be found in Hucho's book.One member got it with inter-library loan for cost of postage.It's the Mother-lode!

aerohead 05-16-2008 03:25 PM

mods-data-Cd reduction ( hood/cowl )
 
Hello all. Just a quicky,as I found only one source for this data,and you'll find it in Hucho's. ( hood/cowl ): 1986,SAE Paper 860211, the reporter demonstrates that a maximum drag reduction of Cd 0.0475 occurs when the hood/cowl is angled up 15-degrees above the horizon.And from the previous installment,when the hoodline of the Subaru XT was dropped 10-mm,they achieved a 1.7% drag reduction.

slopemeno 05-17-2008 02:55 PM

I fly R/C sailplanes and gap seals work. Nowadays, most sailplanes are molded with a wiper/gap seal build in, but back in the day (10 years ago or so) you would cut 1" wide strips of mylar and paint them with contact cement on about 1/2 of the width. The gap seal is always attached to the furthest forward part, so if you wanted to make a seal at the gap at the front of the drivers door, it would be glued to the front fender.

aerohead 06-12-2008 05:56 PM

When I posted this originally,I forgot an important tidbit.In most fluid books of my day,the example of E.Moeller's work on the nose of the 1951 Volkswagen Van was always cited.The Volkswagen began quite literally as a "brick-on-wheels" with a drag coefficient of 0.76.With some clay,Moeller sculpted the front of the Volkswagen with generous radii,and the Cd dropped to 0.42.With the exception of the HUMVEE,there are no current production vehicles that possess square leading edges.Today,the example of the VW serves to demonstrate the significance that a little softening of leading edges has meant in the meantime.

NeilBlanchard 06-17-2008 02:58 PM

Hello,

Maybe this has already been posted?
http://www.recumbents.com/car_aerodynamics/

gary 06-26-2008 10:07 AM

hi.. new here so steep learning curve for me. I think you guys are awesome for putting into practise what others just talk about. Question.. is it possible that the reduction in drag from lowering a vehicle is coming from the resultant reduction of the gap between tyre and body rather than anything else? Hence the varying degreesof success when doing this?

MetroMPG 06-26-2008 12:47 PM

Hi gary -

I don't think there's a single reason why lowering helps (also note: it may not help every vehicle). But the reduced tire to wheel arch gap won't hurt, that's for sure. It's a good point.

Other contributing factors: a better "fineness" ratio; reduced frontal area (tires); diversion of air away from the "dirty" underside.

Welcome to EcoModder!

gary 06-27-2008 02:44 AM

thanks for the reply and welcome. Yes you are right about the other benefits and after my coffee this morning I realised just how dumb I had been. Lowering would have the same effect as putting on side skirts as well as a front air dam of the same dimensions as the drop... as well as having better cornering due to the drop in cg and there being able to maintain more momentum through and out of the corne.. hence less acceleration etc etc.

ps our gas in SA is now about $5.8 per gallon. not funny anymore

groar 10-04-2008 04:14 AM

Zig-zag tape ?

http://ecomodder.com/forum/showthrea...html#post62079

http://ecomodder.com/forum/showthrea...html#post29839

Denis.

mfor4x4 11-21-2008 10:00 PM

Front wheel skirts- why not just put a motorcycle styled fender that covers almost the entire wheel/tire. The cover could be mounted into where the hub attaches to the spindle.

I have a quick drawing of how it would mount to the spindle:http://i50.photobucket.com/albums/f3...le_shape-1.png

This motorcycle fender would have to be quite light also.

I have a question, has anyone had success with a rear body diffuser? From what I have seen, it looks like it would smooth out the air at the back of the car.

msmnick 12-04-2008 03:13 PM

Looking for materials? I found a good site for my project. I'm going to be using ABS plastic like this guy did with his Prius.
Browser Warning

Check out mcmastercarr.com Search abs plastic, go to sheet, and you can get a 12x12 square less than 10$ at 1/8in.

getnpsi 12-04-2008 03:49 PM

I added the diffuser first to the back of my car and didn't notice any smoothness change. The car did get quieter because of all the road noise in the hatchback i drive. The other aero threads state that sealing up the underside of the engine (front of car) is more important than the rear. Most rear bumpers are total parachutes so they do contribute to hurting aerodynamics, but you'll see more results with the front attack. Start with cardboard or coroplast as a stepping stone/template, to map out underside mounting points of your more high dollar materials. My 3 color aspire only deserves coroplast :)

aerohead 12-13-2008 03:31 PM

Air Density (rho) defining and using
 
Hello all.As we move into the colder weather,and later as we move back into warmer times,the density of the air will be affecting our air drag.--------- For those who'd like to contemplate the implications of changing air density,the following is offered.------------------ Air density,designated by the Greek letter rho,and used in aerodynamic drag calculations is defined as the ratio of the air's specific weight,divided by the local acceleration due to gravity (g).-------------------- A sling-psychrometer can be used to measure accurate dry and wet-bulb temperatures,and using a Psychrometric chart (available from air conditioning contractors) one can determine current specific weight of air.------------ Standard air is measured around 59-degrees F and has a specific weight of 0.07651 pounds per cubic foot.------------------- (g) as measured at sea-level,is 32.2 ft/second/second.------------------ running the numbers,for standard air, 0.07651/ 32.2 = 0.002376.------------ Rho is commonly stated at 0.00238 at standard conditions.------------ Anyone with a psychrometer,or access to local weather conditions,a chart,and a calculator can calculate current air density to determine what cold winter /or hot summer air means to there drag.

aerohead 12-13-2008 04:17 PM

Reynolds Number: origin and calculation
 
Hello all.There is oftentimes confusion over laminar flow,turbulent flow,boundary layers,separation,etc.,and all have to do with Reynolds Number effects,so I thought it would serve us to look at the Reynolds Number and have it available to all members and lurkers to use as a computational tool.--------------------------------- The number was a necessary invention,as air has mass and viscosity, and a tool was necessary for prediction of the air's effects when designing everything from a vacuum cleaner to an airplane.--------------------- Reynolds Number = force (inertia)/ force (viscous) ---------------------- The development of the formula begins with: N = [ ( velocity)x ( length)x (specific weight)]/ [ ( viscosity )x(acceleration due to gravity) ].--------------------- specific weight/ gravitational constant = rho,so formula becomes ---------- [(velocity)x(length)x(rho)]/ viscosity.-------------------- rho/viscosity is the inverse of kinematic viscosity(v),so formula can be simplified to N= [(velocity)x(length)x(inverse of kinematic viscosity)].---------------------- For" standard air"and U.S.Standard units,v= 0.0001567 seconds/ft-sq,and it's inverse is 6380.--------------------------------- So here at the end,N= velocity x length x 6380.---------------------------------------------------------------- An example for use would be my T-100 at 70-mph.Multiplying 70mph by 5280ft/mile gets me into feet,then dividing by 3,600 seconds per hour gets me into feet per second(my unit of length for calculations.------------- At 16.5 feet Length(without the boattail), and 102.667 ft/sec Velocity,multiplying by 6380 gives me a Reynolds Number of 10,807,720.------------------------------------- When comparing various shapes and their Cds as a function of Reynolds Number,one can calculate crucial data,especially if you intend to model,so that appropriate "scaling factors" can be accounted for.---------- That's it,hope it helps dispel some of the mystery of aero noodling.

NeilBlanchard 12-13-2008 08:52 PM

Hi,

Can you distill it down to an approximate percentage change in drag from say 75F down to 30F? Is it linear as the temperature drops, or is the change accelerated as the air temp drops?

aerohead 12-15-2008 02:16 PM

Is it linear?
 
Quote:

Originally Posted by NeilBlanchard (Post 78310)
Hi,

Can you distill it down to an approximate percentage change in drag from say 75F down to 30F? Is it linear as the temperature drops, or is the change accelerated as the air temp drops?

Neil,since the density is in the numerator of the force and power formula,I think the change in density affects the results arithmetically,as a percentage of change.---------------------------- The density of air at 30-degrees F,under standard conditions is 0.002520 slugs/cubic foot.Dividing by 0.00238 gives a density 5.8% higher than at 59-degrees.The drag force according to the 1/2 rho Cd A V(squared) would increase 5.8% so it looks like a clean arithmetic relationship for us.'should be same in power calculation.------------------------------ I hope to post a table for those who do not have fluids text,which will list some data in the range we might expect to encounter over the course of a year.------------------------- P.S. A lot of other variables will also be changing with cooler temps and it is customary for us to expect at least a 3-mpg loss in winter driving.

aerohead 12-15-2008 04:14 PM

Coefficient of Friction( Cf): calculation and use
 
From time to time,members have asked about automobile surface- roughness/skin-friction related issues.For those without fluid mechanics text is the following.----------------A scientific-calculator is handy for this,as the exponents are "unconventional",and also,the memory functions are a real help.--------------------------- It's necessary to first calculate the Reynolds Number for your car in order to use the formulas.This has been covered above in the aero sticky.------------------------ For Reynolds Numbers greater than 10(to the 7th power) Cf =[ 0.455/( log Nr)to the 2.58 power].------------------------------ For Reynolds Numbers less than 10(to the 7th power) Cf =[ 0.0775/ ( Nr ) to the 1/5 power].----------------------------------- EXAMPLE: My T-100 at 70-mph ( 102.667 feet per second) and length of 16.5 feet,has Reynolds Number:[( 16.5 ) x ( 102.667 ) x ( 6380 )]= 10,807,720.---------- Since this value is more than 10(7), then I use Cf= 0.455/[( log 10807720 ) to the 2.58 power]----------------------- or 0.455/[ ( 7.0337) 2.58 power]---------------------- or, 0.455/ 153.369 = 0.002966686513.----------------------- So my Cf = 0.002967(rounded).******************************** To use the friction coefficient is much like the drag force calculation,although you are concerned with the surface- area of your car instead of frontal area.--------------------- If you have a simple box-shaped vehicle like a van,you can use the height,width,and length to calculate the area.-------------------In my fluids class,the underside of the vehicle was ignored,as the flow there was unlike that in the free-stream,being dragged along the ground by the "dirty"undercarriage.---------------------------------- For passenger cars,the shapes vary so much that S.F.Hoerner recommends we simply multiply the frontal area by 10,to get an approximation of surface-area.----------------------- In that case,the formula becomes-------- Drag(friction) = [Cf x 1/2 x rho x V-squared x 10 Af ].---------------------------------- EXAMPLE: T-100 at aprox. 29.6 feet-square and 102.667 feet/sec,and standard air,--------------- Drag(f) = 0.002967 x 1/2 x 0.00238 x 102.667 (squared) x 296 feet squared = 11.017 pounds resistance.^^^^^^^^^^^^^^^^^^^^^^^ Plugging Cd 0.44(the original drag coefficient) into the drag force equation gives me 163.362-pounds resistance total for the truck.Subtracting the friction force leaves 152.345-pounds aero drag force.----------------------- The drag, do to skin-friction constitutes 6.74 % of the overall drag.-------------- If skin friction could be eliminated,at a steady 55-mph,mpg would gain 3.3%.However,do to the nature of typical surface-roughness and Reynolds Numbers effects,any additional "smoothing" to the body of a car will not reduce it's skin friction at all.It's basically a "DEAD-END" for eco-modding.--------------------------And as I mentioned in my "DIMPLING" thread,any surface roughness added near the leading-edge of a vehicle(golf-balling) will only aggravate drag and lead to lower mpg.For cars,the transition to turbulent boundary-layer occurs at around Nr=500,000 (impossible for a car to avoid at or above 20-mph).Bugs stuck to the leading-edge of an aircraft wing are enough to significantly compromise the performance of some laminar-flow wings.

aerohead 12-15-2008 05:23 PM

Coefficient of aero.drag(Cd):origin and use
 
For those without access to fluid mechanics text is the following development of the coefficient of aerodynamic drag(Cd),and it's use in the drag force and aerodynamic power calculations.This is the most important tool any aero-modder can use to predict potential performance based on theoretical changes to an automobile.---------------------------------- The aerodynamic drag coefficient (Cd) = Drag force/ dynamic pressure(q) x area(S).------------------------------ Dynamic pressure(q)=(rho) x (v)squared /2----------------------- or (1/2)x( rho )x(v)squared.------------------------- Therefore, Cd = D/[ 1/2 rho (v-squared) S].------------------------ Substituting A for S(which is used more in aeronautical work),formula becomes Cd =D/ [1/2 rho A (v-squared)].-------------------------- The Cd is calculated from measurements in windtunnels,and also determined from coastdown tests.Windtunnel results are preferred although can vary from tunnel to tunnel.+++++++++++++++++++++++++++++++++++ For examining drag, based on theoretical changes to the shape of a vehicle, drag force calculations can serve as barometer for potential results of modifications.---------------The frontal area is known,or estimated,and"standard air "values for density can be plugged into the formula along with any speed.By making the Cd the only variable,one can estimate drag reduction or gain as a function of the changing Cd.************************** EXAMPLE: The T-100 pickup. With original Cd0.44,and Af @ approx. 29.6-feet-square,and a velocity of 70-mph(102.667 ft/sec)and standard air,the drag force D=[1/2 x 0.00238 x 0.44 x 29.6 x 102.667(squared).= 163.362 pounds resistance at 70-mph.^^^^^^^^^^^^^^^^^^^^^^^^ To calculate the power requirement to overcome the aero drag force,then P = DV/(force per unit of time per power unit)------- or [163.362 pounds x 102.667 ft/sec/ 550 pound-feet per-sec per horsepower= 30.494 Horsepower,necessary to overcome aero drag at 70-mph.--------------------------If you know the mechanical efficiency of your driveline,your power requirements for rolling resistance,and the BSFC of your engine,you can predict your mpg for different values of Cd.***************************** A relationship between mpg and Cd is firmly established which offers a "back-door" in which to calculate Cd based on a vehicles "baseline" performance at a constant 55-mph driving.-------------------- If you do a reliable test of your car's mpg,at a constant 55-mpg before modifications,the following relationship may be used to estimate your new Cd.------------------ A 2% drag reduction will provide a 1% improvement in mpg,at a constant 55-mph.----------------------------------- If you were to modify your car,and after testing,realize a 5% improvement at 55-mpg,then,it follows that you've reduced your Cd by 10%.-------------- So ,for say the T-100,if I put some silly-looking device on it,and after testing,the mpg goes from 23.3mpg,to 24.465mpg,then from the formula,the Cd has dropped from 0.44,to 0.396.------------ That's all there is to it,and no expensive windtunnel needed!----------------- For extensive modifications,it is claimed that a gear change may be necessary to wring out all the MPG from drag reduction.That is a very complicated situation and for a discussion on that I would refer you to Hucho's for a full explanation.


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