DeTomaso Mailing List: June 97, Message #336
     



From: "D.Jones" <m203253@ws2200.mdc.com>

Subject: Part 2 of 2 - Aero Stuff

Date: Thu, 26 Jun 1997 15:42:11 CDT



The bodies on most production cars generate de-stabilizing lift.  Nature 

abhors a vacuum, so the air flowing over the top, under the bottom, and 

around the sides of a car will at some point (aft of the vehicle) re-join.  

Since the paths over, around, and under a car are different lengths, the 

air must flow at different speeds.  The longer the path is, the higher the 

air flow speed must be and from Bernoulli's equation, we know that higher 

speed means lower pressure.  Usually, the path over the top is longest and

the result is lift.



> Getting the air to go around the car rather than under it makes a

> huge difference.  As already mentioned, a big factor is getting

> the air coming in from the front-grill out of the engine compartment.

> In really fast door-slammer drag cars, they use solid front-ends

> (no grill openings) so air doesn't get in.  I've read in some

> of the drag mags that cars with open grills can loose control

> of the car from the air coming in the grill at 150+ mph.



This is a big stability concern.  If the cooling air cannot exit quickly 

enough, there will be a big pressure increase underneath the front of the 

car.  This is very destabilizing and at 150+ mph, can make the front end 

of the car want to fly.



> *  Does anyone know at what speeds the coefficient of drag really starts

> to become a limiting factor for the 94/95 Mustang?     (i.e. is there a

> speed at which drag starts to become exponential, thus requiring a

> significant power increase to overcome resistance, or is the drag pretty

> much linear up to top end speed?) 



As shown earlier, drag varies with the square of speed and the power

required varies with the cube of speed.  The drag coefficient is relatively 

constant for the range of speeds a typical automobile sees.  Over wider 

speed ranges (subsonic, transonic, and supersonic) this is not the case.



> *  Does anyone know what the coefficient of drag is for other performance

> cars such as the Z28, Firebird, 300ZX or whatever? 



You'll have to take these numbers with a grain of salt.  Back when I was in 

college, a friend worked at the Lockheed wind tunnel where some of the auto

manufacturers tested.  He claims the advertised numbers were often lower 

than the tested numbers.  That said, the lowest claimed numbers I have seen 

are in the 0.29 to 0.30 range.  The performance versions of cars generally 

have a higher Cd due to the added drag of wider tires and any added wings.

I think the 1980's Audi 5000 claimed a Cx of 0.30.  It's not sleek looking,

but it does have an efficient subsonic shape (smooth, rounded, and blunt).



> I'm building my own homemade wind tunnel to test out my new windsheild

> design for my '63 Falcon.  I've found a good deal on Fans from Sears

> that should give enough air flow to get meaningful results.  (no

> flames Dan) I've cleared all the parts and stuff out of the garage

> and have ripped out the backwall.  I'm working on the

> computer and getting the bugs out of the visual image processing

> system that will analyze the way the attached ribbons tussle from

> the wind.   If all goes well, it should lower my ET's by 2 tenths.



I know you're only joking, but I'll offer a tunnel tip anyway.  The proper

way to measure an automobile's drag is in a rolling mat wind tunnel.  The

rolling mat simulates the "ground effects" of the road passing under an

automobile and also takes into account the considerable aero drag generated 

by rolling tires.  So when you're shopping for fans at Sears, see if you 

can find a really big belt sander :)



> To see if your rear wing has any air flow to work with, tape several 

> pieces of yarn to your trunk lid and wing - they'll tell you what's up.  

> Check your roof while you're at it.  Then tape a bunch to a yard stick 

> and use it as a probe with the roof down - see where the air is "clean".  

> Of course, this is a 2 person effort   ;-)



Yes, when you don't have a wind tunnel at your disposal, yarn tufts are

a good way to visualize the flow field.  Tape them all over the car and

have a chase car shoot some video of your car at speed.  You can also use 

them to test the placement of boundary layer trips.



So what can you do if you don't have access to a wind tunnel?  Several 

years ago I copied down this coast down formula which can be used to test 

aerodynamic and rolling resistance drag.  I've never tried it, so caveat 

emptor.

 

                      CDHP = (WEIGHT*MPH)/(823.3*CDTIME)



 Where:



   CDTIME = coast down time in seconds

   CDHP   = coast down horsepower (i.e. the horsepower required to maintain 

            a given speed.



This formula can be used in determining the effects of changes made to a 

vehicle to alter its aerodynamic drag or rolling resistance.  As an example,

assume you have taken coast down measurements from 65 to 55 mph (under 

similar atmospheric conditions) before and after making changes to reduce 

aerodynamic drag (e.g. lowered the vehicle and added an air dam).  In the 

before case, it takes 15 seconds to coast down.  In the after case it takes 

20 seconds.  Assume the vehicle ways 3400 lbs.  Plugging this data into the 

formula yields:



   Before: CDHP = (WEIGHT*MPH)/(823.3*CDTIME)

                = (3400*60)/(823.3*15)

                = 16.51 hp



   After:  CDHP = (3400*60)/(823.3*20)

                = 12.39 hp



   Net Change: 16.51 - 12.39 = 4.12 hp @ 60 mph.



This formula indicates that the changes result in 4.12 hp less required

to maintain the vehicle at 60 mph.  Since aerodynamic drag varies with the 

square of speed, the effect will be greatly accentuated at higher speeds.

To minimize the effects of internal engine drag, coast down times for 

aerodynamics effects should be taken with the transmission in neutral.

When testing the effects of lubricants or the effects of accesory drag 

(an air conditiong compressor, for instance), leave the transmission engaged.

Coast down time should be measured on a flat, smooth, road with no wind 

or drafting, using a 2 way average, under similar atmospheric conditions.

I wouldn't put much faith in the absolute numbers provided by this formula,

but I think it might be a good tool for assessing relative changes.



Off the top of my head, I can think of several things that might be worth 

testing using these formulas:



 - lowering the car

 - adding a lip spoiler/airdam

 - raking the body with a slight nose down attitude (primarily for stability)

 - fitting a Capri hatch (supposed to be more aerodynamic than a Mustang one)

 - fitting a belly pan to the rear skirt on GT's (factory skirt looks draggy)

 - taping over door and hood seams

 - convertible top up and down

 - roll bar  

                                       Just My 2 Cents Worth,

                                                    Dan Jones

 

Eugene,



> ... so the 800 pounds of force at 180 mph drops to 200 pounds at 90 mph and 

> 50 pounds at 45 mph (not what I originally wrote, which were based on a 

> cubic function).  But this implies that even at 45 mph, the down force 

> gained by that wing seems significant.  My only questions now are generated 

> by Dan's descriptions of laminar vs turbulent flows:  Do those sport slats 

> cause enough turbulence so that they totally nullify any effects that the 

> rear wing might have?



I believe you'll find that the boundary layer is fully turbulent before it

ever gets to the sport slats.  The real question is whether or not the 

turbulent boundary layer is still attached by the time it gets back to the 

rear wing.  If the flow has not detached from the body, the wing will likely 

see clean air, since it's raised off the body enough to clear the turbulence 

of the boundary layer.  



Technically speaking, separated flow is not turbulent, even though it is

random and chaotic (and very draggy).  The laminar and turbulent concepts 

apply only to the boundary layer, which is only a few inches thick.  Beyond 

the boundary layer, flow is treated as frictionless (inviscid).  The boundary 

layer is very important since it determines skin friction drag and the 

tendency towards pressure separation (turbulent boundary layers are *less* 

likely to detach).  There is a drag increase associated with the transition

from laminar to turbulent flow but it is usually small compared to the drag 

increase associated with separated flow.  



This brings up another important aerodynamic term, the Reynolds number, which

is defined as:



                      Re_x = (Rho * V * X)/Mu

  

 where: 



   Re_x = Reynolds number at location x (a dimensionless quantity)

   Rho  = freestream air density

   V    = freestream flow velocity

   x    = distance from the leading edge

   mu   = freestream viscosity, a physical property of the gas (or liquid)

          involved, varies with temperature, at standard conditions mu is

          approximately 3.7373x10E-07 slug/(ft*sec) for air.



The location along the body at which the flow transitions from laminar to 

turbulent determines the critical Reynolds number.  Below this number, the 

flow is laminar, above it's turbulent.  Since the Reynolds number varies 

linearly with the location along the body and with velocity, the faster you 

go, the farther forward the transition point moves.  At cruising speed on a 

typical jet airliner, only a small region near the leading edge may be 

laminar.  Slow speed gliders with very slender (but still with rounded, blunt, 

leading edges) may maintain laminar flow over most of the wing surface but 

this is not the case for most practical aircraft.  Note that glider wings 

are typically designed with very short chord lengths (x distances) to help

promote laminar flow.  Laminar flow is desirable when there is no pressure 

separation. 



Automobiles operate at relatively slow speeds like gliders, but have much 

longer x distances and shapes that are less likely to maintain laminar flow.  

The bottom line is the flow is fully turbulent before it gets to the slats.



Assuming the Motor Trend article is true, the flow stays attached for the

race car without slats so the wing sees clean air and produces downforce.  

We can theorize as to whether the street car's slats disturb the flow enough 

to detach the turbulent boundary layer, but the only way to tell for sure is 

to test it.  Anyone want to volunteer to tape some tufts of yarn to a Boss 

302 wing and watch the flow patterns?  The guys at work who did it on the 

'87 LX, did it with Scotch tape and some thick yarn.



                                                    Later,

                                                    Dan Jones





Tom,



> So on most production cars - are the wings cosmetic or do they help 

> decrease lift at high speeds?  More specifically how about the Mustangs? 

> Anyone know?  At a glance I'd think they might create a LONGER path over 

> the top of the car and actually create more lift.



With the wing, the distances to be concerned with are local (i.e. over and

under the wing itself and not over the whole car).  This why the wing is

raised off the body of the car on your '93 Cobra (and my '87 GT).  What the

rear wing is attempting to do is create downforce (negative lift) on the

rear of the car, presumably to balance out a rear-biased lift tendency of 

the car without wing (assuming it's not just cosmetic).



Also, the path the air actually travels may be quite different from the 

contour of the vehicle.  For instance, a flat shape with equal distances over 

and under can produce a lot of lift.  If you don't believe me, try this 

experiment at home (just don't sue me if you do).  Step into the bed of a 

pick-up truck and lift a 4'x8' sheet of plywood over your head.  Be careful 

to hold the sheet of plywood parallel to the ground, while the driver slowly 

accelerates to 60 mph or so.  Now comes the fun part.  Grip tightly to the 

sides of the plywood and quickly tilt the leading edge upward.  What happens?  

Instant lift (and an impressive, if short lived, Peter Pan imitation). 



What you've just experienced is the influence angle-of-attack has on lift.

Take a symmetric (top-to-bottom) airfoil shape that does not produce lift

when it is aligned parallel to the air flow (i.e. is at zero angle of attack)

and point it up.  It produces lift.  Point it down and it produces downforce.

While the physical distance over the top and bottom of the plywood are the

same, the distance the airflow travels is not.  Likewise, you don't need

angle of attack or even thickness to produce lift/downforce.  A thin curved

shape like a Venetian blind slat will also produce lift.  This is an extreme

example of wing camber.



A little wing theory and several definitions are in order here.  This would

be easier to explain with illustrations, but I'll give it a shot with words.

An airfoil is the 2-dimensional cross-sectional shape obtained by the

intersection of a wing and a perpendicular plane.  The mean camber line of an

airfoil is the locus of points halfway between the upper and lower surfaces

(measured perpendicular to the mean camber line itself).  The chord of an

airfoil is the straight line connecting its leading edge to its trailing

edge.  Camber is the maximum distance between the mean camber line and the

chord line, measured perpendicular to the chord line. 



An airfoil's angle of attack is the angle between the relative wind (the 

local airflow direction) and the airfoil's chord line.  Drag is the component 

of aerodynamic force parallel to the relative wind and lift is the 

perpendicular component. 



If an airfoil is symmetric (top-to-bottom), it has no camber.  A sheet of

plywood has no camber.  A Venetian blind slat is a shape that has camber but

(practically) no thickness.  The camber, the shape of the mean camber line,

and the thickness distribution of an airfoil determine its lift and moment

characteristics.  Surface roughness also plays a roll but is usually treated

as a separate design issue. 



Because of camber, wings can have lift at zero degrees angle of attack and

because of angle of attack, wings (and sheets of plywood) with no camber can

still produce lift.  To separate these effects, aerodynamicists break an

airfoil's lift into two components: 



                       Cl = Clo + (Cla * alpha)



  where:



    Cl = coefficient of lift 

    Clo = coefficient of lift at zero angle of attack

    Cla = lift curve slope (the slope of Cl versus alpha)

    alpha = angle of attack



On low speed circuits where downforce is very important, Formula 1 race cars

will have multiple, highly cambered, wings, oriented at a relatively large

negative angle of attack.  All of this is done in an attempt to generate

downforce.  Since this approach is a relatively high drag method of

generating lift, you won't see similar set-ups on aircraft (they are not

limited by wing size rules). 



Front lip spoilers (like those on a Boss 302) produce downforce because they

are mounted at a relatively large negative angle of attack.  They have all

the aerodynamic elegance of that piece of plywood, but they work.  A pedestal

mounted, cambered, wing would be more efficient but probably wouldn't look

too good mounted on top of your hood.  Some versions of the Lamborghini

Countach have a pedestal mounted front wing on the nose. 



By the way, car spoilers really aren't spoilers at all.  On aircraft,

spoilers are devices which intentionally promote pressure separation.  They

are called spoilers because they "spoil" lift when they are deployed.  They 

are generally mounted flush on top of a wing and pop-up to reduce lift

and increase drag.  You can watch these devices at work on airliners when

they decelerate in preparation for landing. 



Rear mounted spoilers (like those on Cobra Daytonas), look more like a fixed

version of an aircraft trailing edge flap.  Trailing edge flaps generate

lift/downforce by altering the effective length of a wing and its camber line

shape. 



Since real wings exist in three dimensions, there are 3-d effects to be

concerned with.  Lateral flow along a wing is called span-wise flow and is

usually undesirable.  Guides, called fences, are often employed on aircraft

to reduce span-wise flow.  The circulation around the ends of wings is

particularly strong and creates large vortices and substantial drag.  The 

flow wants to circulate laterally from the high pressure region underneath 

the wing to the low pressure region on top of the wing (in the case of lift).

Race cars usually employ large vertical end plates on the sides of their

wings to reduce this circulation. 



Unfortunately, most of this is probably academic since I have reason to

believe the rear wings on late model Mustangs don't see all that much flow. 

A couple of guys at work (McDonnell Douglas Aerospace), tufted an '87 LX from

the center of the roof to the taillights.  They were trying to use vortex

generators to increase the flow attachment on the rear glass.  Vortex

generators are devices which are put in the flow field to intentionally

induce turbulent flow.  They are often used on aircraft to re-attach and

direct flow (especially over control surfaces).  Their vortex generators were

based on aircraft designs and they used a hang glider airspeed indicator on a 

pole to measure the boundary layer thickness across the roof.  They made the 

vortex generators two inches tall to be conservative (the boundary layer was 

approximately one inch thick and a rule of thumb is to make the generators 1.5 

time the boundary layer thickness).  They didn't see an improvement in coast 

down times, but the tufts did appear a little better behaved with the vortex 

generators.  They believe the turn at the back of the roof may be too sharp to 

permit attached flow.  Some sort of fairing might help there (or maybe a 

switch to a Capri hatch).  They also noted that much of the clean wing flow 

appeared to be coming from around the sides of the car. 



>From watching the flow patterns in the rain, one of them concluded that the 

flow over the hatch of his 1986 Camaro was still attached, though it flowed 

laterally as well as longitudinally. 



Taller wings or wings that are mounted farther aft will see cleaner air. 

Rear wings on race cars tend to be mounted high to get them out of separated 

flow and into clean flow.  The wing on the Dodge Daytona Chargers and 

Plymouth Superbirds (the ones that made them look like shopping carts) are 

examples of this.  It would be interesting to tuft the various rear wings

(LX, GT, Cobra, SVO biplane, etc.) to see if any of them are useful.



On a related note, some of the racing Shelby GT-350's had a noticeable gap

between the fastback window and the roof.  Does anyone know if this was for

aerodynamic reasons? 



I previously mentioned trying to estimate drag from coast down measurements.  

By taking measurements at several speeds, you should be able to separate the 

effects of rolling resistance (roughly proportional to speed) from 

aerodynamic drag (proportional to the square of speed).  Data scatter would 

be a real problem, but you could do a least squares curve fit.  Also, making 

runs with and without winds (at the same speeds) could be used to isolate the 

aerodynamic contribution.  Supposedly, NASA has done some work on coast down 

drag equations as part of an effort reduce drag on tractor-trailers.  They 

even put a full size tractor-trailer in the 80' x 120' Ames wind tunnel. 



I dug up some information on Mustang and Thunderbird drag coefficients from 

a couple of old magazines.  The January 1984 issue of Sports Car Graphic

claimed a Cx of 0.39 for a 1984 SVO Mustang (the model without flush

headlights) and s 1987 issue of Sports Cars of the World had these numbers: 



                 0.35 for a 1986 base Thunderbird 

                 0.38 for a 1986 Turbo Coupe Thunderbird 

                 0.34 for a 1987 base Thunderbird 

                 0.36 for a 1987 Turbo Coupe Thunderbird 



One of the guys who did the vortex generator experiment, also relayed some

information on an AIAA presentation made by Corvette engineers about the

'Vette's aerodynamics.  They claimed the 'Vette has a Cx of 0.30 and said it

was a difficult number to achieve with such wide tires.  Note that GM tests 

without mirrors, so this number may be a bit optimistic.  There was also

a drag hit with the externally mounted 3rd taillight.  GM used to test at

several tunnels, including one at Lockheed Georgia and one in Canada, but has

since built its own tunnel.  Interestingly, it does not have a rolling mat. 

The engineers admitted this is a compromise but noted they use a boundary 

layer suction device near the front tires.  This arrangement apparently yields

useful data with less scatter than a rolling mat facility. 



The 'Vette engineers also noted that more aerodynamic testing is done for

acoustic (noise) and cooling reasons than for drag reasons.  At first glance,

since it takes energy to make noise, you might think a quiet car is a slick

car.  This is not necessarily so.  Turbulent boundary layers are noisier than

laminar ones, but they often provide lower drag.  Of course, you need to

trade this off against pressure separation noise. 



                                            That's All for Now,

                                            Dan Jones



P.S. I just saw a set of 351 tunnel port heads in the SVO catalog.  Does 

anyone know if they use faired-in pushrod tubes?  I think the old 427 

tunnel ports just used cylinders to house the pushrods.  Since a cylinder 

is a relatively high drag shape, an airfoil shaped pushrod housing should 

yield a flow increase.







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