What a Drag!

[Tony C]

I saw an old thread which referred to the possibilities of making the Se7en more aerodynamically efficient and though this might help pass some time during these long cold winter evenings.

 

The formula for Parasite drag (Form [ pressure] Drag plus Skin Friction Drag plus Interference Drag) is:-

 

Half the air density multiplied by Velocity squared multiplied the Drag coefficient multiplied by the frontal area.

 

You must use SI units (kg/cubic metre for density, metres per second for Velocity and square metres for frontal area). The resulting Parasite Drag will be in Newtons. Don't despair, it is quite straightforward really!!

 

If we consider Sea Level, the air density is 1.225 kg/cubic metre on a "Standard" Day, so half of that makes it 0.6125.

 

50 mph is 22 m/s - near enough

100 mph is 44 m/s - near enough

But don't forget the velocity has to be squared.

 

The Drag Coefficient (Cd) of a flat plate is 1.2, a sphere of the same area is 0.6 and a streamlined shape of the same area is 0.06. The Se7en body alone is quite streamlined, but I think that because of the openings - cooling air intake, cockpit, gaps in the front suspension and all the openings underneath it might be better to use 0.6 for the Cd as a rough guess (WAG).

 

Frontal area is roughly the area of a frame through which the car could roll - I need an estimate here because I haven't got my car yet!!!

From the dimensions in the catalogue, I estimate 1.6 square metres.

 

All that gives: 0.6125 x 22 x 22 x 0.6 x 1.6 = 285 Newtons

Divide that by 9.81 gives kg (not really the 'correct' thing, but who cares!): 29 kg at 50 mph

However, because of the speed squared function, that will be 116 kg at 100 mph.

 

Frontal area can be reduced by, for example, replacing the standard windscreen with a wind deflector, but what you have gained on this particular 'swing' will probably be lost in the increase on the 'roundabout' of increased Form [pressure] Drag and Interference Drag from the big hole filled by a big body, of the cockpit.

 

With a 140 bhp K Series engine, six speed gearbox, 205/50 15 tyres and 3.62 Diff', changing up at 7000 rpm in each gear gives approximately: 291 kg Thrust at 50 mph and 188 kg Thrust at 100 mph.

At 50 mph only roughly (291 - 29) 262 kg is left to accelerate the car, but at 100 mph only (188 - 116) 72 kg Thrust remains to accelerate the car.

 

Obviously, any reduction in Parasite drag will significantly improve performance.

 

Regards

Tony

 

[c21vhpd]

FWIW, There was an article called 'The Raw and the Cooked' (which seems to have vanished, but google has a cached copy here) comparing the 7 and the 21 before going onto compare the elise and the 21. It quotes the Cd of a 7 at 0.76 and that of a 21 at 0.42.

 

That's why I almost crashed into the back of HOOPY in Finland - he'd given up accelerating and I still had my foot flat to the floor. Oops

 

Honda Passion Orange, 640kg , and proud of it 😬

 

Edited by - C21VHPD on 14 Nov 2002 22:45:15

[Julian Thompson]

What difference would aerofoil wishbones make?

 

😬

[Tony C]

About 10 percent of very little.

Cover-in the cockpit - now you're talking.

TFC

[ChrisG]

Taking the front shocks inboard would also help a fair bit, I think there was some fairly advanced testing of this in some mag or by a manufacturer (cant remember!), that said that taking the front shocks out of the airstream was the single biggest thing you could do to the 7 shape without changine the body iteslf. Anyone know who did this testing, Sylva maybe seeing they have inboard shocks on the Striker?

 

Chris

[scooby dooby doo]

which is why i use an aeroscreen and have a tonneau covering the passenger side.

 

Empircal evidence shows the aeroscreen is a BIG improvment over the screen - in line with the maths i did a while back which reconked it was worth 20 bhp at 100 mph.

 

The coefficients I've heard are 0.62 for cycle wings, 0.67 for swept.

 

You could work it out more accurately by seeing what the terminal velocity of a car is, what the power output at that speed is (ie you need to know the rpm and curve shape and transmission losses) and then work back to CdA.

 

And yes - Craig got VERY close to me at about 100 mph *eek*

 

HOOPY 500 kg R706KGU

[Trevor Phillips]

Would underfloor panelling help? Has anyone done any testing on the 7 for ground effects? In racing you usually want to increase down-force and I remember that some W********s ran front and rear wings in the old 750MC Supersports Championship.

 

Racing pics and items for sale here

[scooby dooby doo]

the main aero problem is that you get loads of lift at the front (ISTR 60kg at 100mph). Anyway, its funny beating 911s, Elises etc using 50s aerodynamics 😬

 

HOOPY 500 kg R706KGU

[Laurence Wilson]

Kumschick - the swiss distributor for Sevens - used some limited underpanelling on their turbo xe competition model. There are some pics here.

 

Peardrop

[V7 SLR]

Empircal evidence shows the aeroscreen is a BIG improvment over the screen - in line with the maths i did a while back which reconked it was worth 20 bhp at 100 mph.

He he, I prefer to add another 20bhp.!! 😳 😬 😬

 

Worcs L7 club joint AO.//Membership No. 4379//Azure Blue SLR No. 0077//Se7ens List Tours

 

[scooby dooby doo]

I've removed the screen and will add 50bhp this winter 😬

 

HOOPY 500 kg R706KGU

[Alex Birtwisle]

There is an article on the Curborough website in which a chap takes his 7 to the wind tunnel at MIRA.

 

Click here to read it.

 

Makes for interesting reading.

 

Cheers

Alex B

 

S713UMY

1.8K Viper Blue

[Tony C]

The Thrust figures I quoted are very accurate. When I mentioned 'roughly' I was refering to the approximate, but not too bad Drag figures.

 

As we all know, acceleration is proportional to Thrust (Force) and inversely proportional to mass.

 

I worked out the thrust figures using a rolling road trace; what is required is the hp at the wheels. I understand the rolling road supplies wheel torque and all other values are calculated from that - can anyone confirm my assumption.

 

The formula for Thrust is Wheel hp x 550 ft/sec divided by vlocity (ft/sec).

 

Regards

TFC

[Joe 90]

Terminal velocity is one of the best ways to estimate drag, remembering to subtract about 1 bhp for every 10 mph which is used to overcome rolling resistance. The figures I have seen quoted give a Cd of 0.7, which is pretty appalling. Removing the windscreen clearly reduces drag because it reduces the frontal area, but does it improve the Cd?

 

99,000 miles so far

[scooby dooby doo]

We don't really care about the Cd itself do we? Only CdA has any real relevance. so even if going aero increaes Cd, it drops CdA so we're happy.

 

or am I missing something 🤔

 

HOOPY 500 kg R706KGU

[Peter Carmichael]

Tony,

 

[here we go again mode]

You are on the wrong track by looking at "at the wheel" rolling road figures as these have lost a lot of power to the tyre being deformed over a small radius roller as opposed to driving on a flat road.

[/here we go again mode]

 

Hoopy, you are not missing something.

 

Cd is an arbitrary constant. The Cd describes the drag of a shape. The shape is scaled according to the size of a characteristic area. It is just convention that says that frontal area is used.

 

If you change to aeroscreens, you have a different shape. If you use frontal area as the characteristic area, then you also have a different frontal area. The Cd will be higher, the drag will be lower.

 

Consider a Seven and an accurate 1:10 scale model of a seven. The 1:10 relates to linear proportions, so in area terms the ratio is 1:100. i.e. the drag force on the model at 100 mph will be one hundredth of the drag force on the full size car.

 

If you then wanted to hold some aspect of design equal and vary others and accurately report the impact on drag there is nothing that says you have to stick to using Cd WRT frontal area.

 

For example:

You might choose the area, for instance determined by frontal oblong bounded by the front tyres to get a clear indication of how good a drag shape you can build on top of a standard Seven chassis.

 

Suppose the Cd WRT frontal area (1.6m^2) of a windscreened Seven is 0.76.

13 inch wheels with 21 (533mm) inch ACB10s on widetrack of 1.336m = 0.713m^2

Cd WRT to front track oblong is 0.76*1.6/.713 = 1.71

 

When we take the aeroscreen off, the drag at 100mph the drag drops by 34kg force - this happens to relate to 20bhp (20 * 750 /100 *3.6/1.609/9.81 = 34)

Drag force before was (100*1.609/3.6)^2 x 0.713 x 1.71

Drag force after is (100*1.609/3.6)^2 x 0.713 x Cd

 

Therefore the new Cd = 1.71 - 34*9.81/(100*1.609/3.6)^2/0.713 = 1.48

 

Now. If the reduction in windscreen also accounted for 0.5m^2, then the equivalent frontal area Cd would be: 1.48*0.713/1.1 = 0.956

 

This is all a sleight of hand related to Cd and CdA, and one would hope that conventions are merely applied to try and be helpful. Car manufacturers jump up and down about the Cd they have designed in order to tout the pretty streamlined shape they have produced, but if the cars keep getting bigger then the overall drag (and hence top speed and fuel economy) will be worse.

[scooby dooby doo]

This is all a sleight of hand related to Cd and CdA, and one would hope that conventions are merely applied to try and be helpful. Car manufacturers jump up and down about the Cd they have designed in order to tout the pretty streamlined shape they have produced, but if the cars keep getting bigger then the overall drag (and hence top speed and fuel economy) will be worse.

that i understand and was my initial viewpoint. I think I see what you're saying with the calculations but don't see how/why its relevant. Let me go home, have some tea, and give it another read

 

Can I just say "We only care about Cd WRT frontal area" instead of CdA then 🤔 That's equivalent to saying "We only care about drag force at 60 mph". Both give me a number that scales with the performance characteristic I'm interested in, the former is easy to calculate stuff from, the latter is an actual effect.

 

Hmmm..... getting hungry now - must go home ❗

 

HOOPY 500 kg R706KGU

[Tony C]

Peter,

Bli'me!

Please understand, I'm not disputing anything you say; you seem to have fact and figures to back-up you calculations.

 

Bearing in mind that the rolling resistance on the rolling road might be slightly different to the rolling resistance obtained on the road - my estimation is that it can't be significantly different. What other value can be used to give a reasonable estimate of Thrust Available?

 

Maybe you can e-mail me the data you have on Drag and Cd of a car. I've been using the basic information relating to aircraft - as I consider it to apply to a car.

Where do you get the 34kg force reduction from changing the standard screen for aeroscreens?

 

My understanding is that if you consider the Se7en car, it will have a Cd of, I am now aware, of 0.7. and an estimated frontal area of 1.6 square metres. If the standard screen is exchanged for an aeroscreen, the frontal area is reduced, but the Cd will probably increase because of increased Pressure Drag due to the more open cockpit.

 

I guess the only way is to put a Se7en in a wind tunnel on balances so that Drag (Thrust Required) can be measured with various configurations.

 

BUT, I "think" that using my "back of the envelope" figures could be used for comparison purposes, rather than empirical values.

 

Good stuff, eh?

 

Best Regards

Tony

[Julian Thompson]

Tony

 

My knowledge is limited, but I would imagine that your windscreen acts like an air dam: you have high pressure in front of it pressing down on the scuttle, but lift behind it created by the pressure void which in my opinion will be worse than with an aeroscreen, where the same situation exists but with much reduced vigour. Obviously this situation changes rather once you pop the roof on or run Hoopy's tonneau

 

(which of course you shouldn't because then you'd have to wear goggles and then you'd look like Toad of Toad Hall.)

 

Which is not good.

 

😬

😬

😬

[scooby dooby doo]

My silly winter hat and night time oakleys make be look more like Ali G...

 

HOOPY 500 kg R706KGU

[Julian Thompson]

[Peter Carmichael]

Hoopy,

 

The reason you didn't understand what I was going on about was that I was agreeing with you. You aren't used to it. I think you are exactly right to only give a damn about CdA.

 

I swamped my salient point in equations and blather: "Cd is arbitrary and determined by the arbitrarily chosen area you relate it to". "Cd on its own is a useless quantity". "To be useful, the Cd must be linked to the convention of which area it relates to."

 

From my example, the following two statements are true and non-contradictory (if facile):

 

"The Cd of the caterham Seven is 0.76"

"The Cd of the caterham Seven is 1.71"

[Peter Carmichael]

If you said:

 

"The SAE Cd of the Caterham 7 is 0.76" then that would be a different matter.

[scooby dooby doo]

ah ❗ that's what confused me. You seemed to agree and then wonder off and disagree...

 

Hoopy, you are not missing something.

hmmm... and aha ❗ Me missing the word "not" in the above didn't help either

 

As a further example supporting our case - a std windscreened 1.6 K SS is meant to do about 120 mph with 135 bhp. This is awful close to what a normal family car will do with similar power despite it having approx half the quoted Cd of the 7. ie the frontal area must be double (ish).

 

Anyway - what does the (SAE ) Cd and frontal area for an aeroscreened, tonneaued 7 come out at roughly 🤔 Compared to the same seven with a screen and doors 🤔

 

 

 

 

HOOPY 500 kg R706KGU

[Tony C]

Peter,

Sorry to keep going on about this, but there are a couple of things I do not understand, which maybe you would be willing to explain.

 

I instruct Principles of Flight to Airline Transport standard and the Parasite drag formula we use is:

 

Parasite Drag equals half the air density, multiplied by Velocity squared, multiplied the Drag Coefficient (Cd), multiplied by frontal area.

 

I am not aware of any other formula for Parasite Drag.

 

Parasite Drag is made up of:

Skin Friction - hardly of significance in this case,

Form (pressure) Drag - which is the difference in pressure between the leading edge (front) [higher than atmospheric] and the trailing edge [pretty much atmospheric], and

Interference Drag - made up of the interference between the boundary layers of the components where they attach to one another.

 

Drag coefficient is the ratio of Dynamic Pressure (half the air density multiplied by the velocity squared) to "Drag Pressure" (Pressure differential between front and back.

 

For example: a sphere of any cross sectional area will have a Drag coefficient (Cd) of 0.6

 

Can you tell me what CdA is?

How can the aerodynamics of a car differ from those of an aeroplane?

 

Scale effect? In this case it has no significance - in my opinion.

 

Very confused of Aylesbury,

Respect,

TFC