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aerodynamic drag / air resistance on a full caged Locost
alfas - 16/6/14 at 06:20 PM

my locost has a rather high full cage, no windscreen only a small wind deflector and front winglets, 205/60r14 semiīs, 3.9 english diff:



it originally came with a mildly up-graded 1300x/flow..now converted to a mildly tuned 1600 x/flow

with both engines the car gets really tired with a speed higher than 70miles...like somebody trys to stop you.

do you think this rollcage is responsible for that, in terms of aerodynamic drag?

pewe - 16/6/14 at 06:28 PM

Given most Se7ens have the aerodynamics of a brick I think the roll-cage is pretty far down the scale.
Probably simply lack of power to overcome the overall drag?
No doubt one of the techies will be along soon to give you chapter and verse.....
Cheers, Pewe10

wylliezx9r - 16/6/14 at 06:32 PM

Have you got a rear diffuser ? Otherwise the rear tub acts like a parachute.

bi22le - 16/6/14 at 06:44 PM

Good air flow from the engine bay and half tonneau will make big differences.

Sam_68 - 16/6/14 at 07:08 PM

AFAIK, nobody has put a Locost in a wind tunnel, but a Westfield has a Cd of 0.64 and a Caterham's is 0.70.

The rollcage certainly won't be helping, but neither will it be contributing a disproportionate amount of drag on its own; when your base drag coefficient is that bad, the effect of an extra bit of scaffolding tube is probably neither here nor there.

As a very rough guide, based upon the Westfield's CdA, aero drag, rolling resistance and transmission loss combined will be absorbing about 30bhp at 70mph, but the drag curve is rising steeply... by the time you're doing 100mph, that figure will have risen to around 75bhp.

Given that a mildly tuned 1600 Crossflow might only be producing 80-90bhp at the flywheel, you can understand why it feels as though it's running into an aerodynamic brick wall as you start to go much above 70mph, and is flat out at not much over the ton.

alfas - 16/6/14 at 07:53 PM

to reply to some questions:

i have no diffusor installed. underneath the propshaft tunnel is closed..enginebay and rear axle open from bottom.
no side doors, no passenger side cover.


is there a suplier for a diffusor which could be made to fit( with only slight modifications) to a book locost (ron champion)?
and where could i purchase a half tonneau, maybe also half doors (softbits never replied to my requests, aerodynamix half doors are too small)?

Sam_68 - 16/6/14 at 08:18 PM

Diffusers don't need to be anything sophisticated - a folded sheet of aluminium will do the job (have a look under the back of an S1 Elise if you want proof!) - so why not just make your own?

Don't expect it to make a dramatic difference, though. You might get a 2 or 3 mph on your top speed, if you're lucky. And don't expect it to actually act as a diffuser and generate any noticeable downforce - the speeds are too low, the ground clearance too high, and the airflow too dirty on a 'Seven' for that; it'll just clean up the underbody airflow enough to reduce the drag a bit, that's all. Simply panelling in under the rear axle will be almost as good.

The best you can hope for with any 'Seven' type is to go from absolutely diabolical to merely dreadful aerodynamics.

What you really want is one of these:




[Edited on 16/6/14 by Sam_68]

Texan - 16/6/14 at 08:39 PM

quote:
Originally posted by wylliezx9r
Have you got a rear diffuser ? Otherwise the rear tub acts like a parachute.


Could you elaborate a little?

Do you mean the open area behind the driver catches air and acts like a parachute and therefore a cover over that would help?

wylliezx9r - 16/6/14 at 08:47 PM

quote:
Originally posted by Texan
quote:
Originally posted by wylliezx9r
Have you got a rear diffuser ? Otherwise the rear tub acts like a parachute.


Could you elaborate a little?

Do you mean the open area behind the driver catches air and acts like a parachute and therefore a cover over that would help?


Yes exactly that. If no cover/rear diffuser is fitted, air will enter under the rear and hit the back panel like an air brake. My car felt completely different after fitting a diffuser especially 70 mph plus, it's like taking the handbrake off in a way. The car now feels much more stable too.

hth

daniel mason - 16/6/14 at 08:48 PM

The air flowing under the car hits the rear tub if the floor is not flat acting like a parachute but it won't make massive difference. I would still do it, and under the engine bay,and possibly some vents in the side panels

Steve ads - 16/6/14 at 09:02 PM

http://m.youtube.com/watch?v=UPgu7Vylq44 why stop at a rear defuser lol

balidey - 16/6/14 at 09:13 PM

Been a long time since I studied it, but the frontal area is responsible for (going from memory now) approx 75% of all aerodynamic gain.
So will a rear diffuser help reduce drag? Yes.
But aero aids on the front will help more.

But all this is rather relative. As said before, most open wheel / open top cars have the aerodynamics of a brick, so all you will end up with is a slightly aerodynamic brick.

austin man - 16/6/14 at 09:36 PM

Also consider what differential you are running as this can and will have a dramatic effecr to top end power

Texan - 17/6/14 at 12:10 AM

Ahhhh, now I see. i wasn't thinking of an unfloored rear section and I didn't think of the floor as a diffuser.

onenastyviper - 17/6/14 at 07:05 AM

quote:
Originally posted by balidey
Been a long time since I studied it, but the frontal area is responsible for (going from memory now) approx 75% of all aerodynamic gain.
So will a rear diffuser help reduce drag? Yes.
But aero aids on the front will help more.

But all this is rather relative. As said before, most open wheel / open top cars have the aerodynamics of a brick, so all you will end up with is a slightly aerodynamic brick.


Yes but don't forget that how the flow is returned to the open stream is also important as well.

Sam_68 - 17/6/14 at 06:22 PM

quote:
Originally posted by balidey...the frontal area is responsible for approx 75% of all aerodynamic gain.


Do you mean for a typical car?

Surely, in terms of general principles it depends on the bluffness of the shape: very bluff shapes, the majority of the drag is pressure drag (ie. frontal area), whereas for very slender shapes, the majority of the drag tends to be viscous (skin friction)?

alfas - 17/6/14 at 08:21 PM

quote:
Originally posted by austin man
Also consider what differential you are running as this can and will have a dramatic effecr to top end power


see my first posting --> all info there


205/60r13 tyres, 3.89 diff, 4 speed box

so if i interprete all the above posts correctly:

non of the aerodynamic things mentioned for rear or underbody, will have a severe effect on top-speed, max. gain would be around 5mph or less.

interestingwise i have driven similar powered westfields and sylvas WITHOUT any aerodynamic modifications..and all those cars never had this problem...with or without windscreen....but standard rollover bar fitted.

that was the reason i was thinking about the rollcage on my locost acting like a "brake"

[Edited on 17/6/14 by alfas]

eddie99 - 17/6/14 at 08:39 PM

Having done a lot of aero research (CFD and wind tunnels) and investigation/proving out on tracks, the cage adds drag but not significantly. Best improvements you can do on a seven are at the front.

mcerd1 - 18/6/14 at 09:03 AM

quote:
Originally posted by eddie99
Having done a lot of aero research (CFD and wind tunnels) and investigation/proving out on tracks, the cage adds drag but not significantly. Best improvements you can do on a seven are at the front.

Eddie, how much difference do your oval wishbones actually make ? (or is that a trade secret )

smart51 - 18/6/14 at 09:55 AM

a 15mm oval tube has mabe as low as half the drag of a 25mm round tube. 60% because it is thinner plus a bit more becuase it is cross section is long and thin rather than round.

I've done a quick calculation and an MSA compliant roll hoop, with stays and a diagonal, made of 50mm tube uses 31 BHP to overcome drag at 100 MPH. (28 BHP for 45mm tube). If you were to wrap the tube in something aero profiled, you could reduce that drag from 31 BHP to 3 BHP.

Similarly, a set of round tubed wishbones is going to cost you about 3.5 BHP at 100 MPH. Aero profiles will save you around 3 BHP. Oval tube may save 0.5 BHP per wishbone.

alfas - 18/6/14 at 10:31 AM

quote:
Originally posted by smart51
....

I've done a quick calculation and an MSA compliant roll hoop, with stays and a diagonal, made of 50mm tube uses 31 BHP to overcome drag at 100 MPH. (28 BHP for 45mm tube). If you were to wrap the tube in something aero profiled, you could reduce that drag from 31 BHP to 3 BHP.
......


does this mean: my estimation with the roll hoop being responsible for my problem is correct?

(by the way: my engine has "estimated" 90BHP only)



[Edited on 18/6/14 by alfas]

smart51 - 18/6/14 at 10:42 AM

My calcs are for the roll hoop on its own. Obviously it will be attached to the rest of the car on the road and that will have some influence.

A general rule when sorting aerodynamics is to fix the front first, then the middle, then the back. Once you have disrupted the air flow, it hardly matters what you do. As far as a car with no windscreen is concerned, the roll hoop is in effect part of the front of the car. At least the bit that's not near your face.

You could try sellotaping a V shaped piece of card to the back of your roll hoop to see if it makes any difference. Sure, it won't be in any way optimised, but it could make some improvement. Then you'll know if the hoop is a significant factor.

alfas - 18/6/14 at 11:16 AM

do you see a possibilty to make a small sketch where this v-shaped cardboard should be fixed?

[Edited on 18/6/14 by alfas]

bi22le - 18/6/14 at 01:31 PM

I assume on the trailing edge creating a tear drop. The tail that you have just added will tidy the air up after the tube

smart51 - 18/6/14 at 09:46 PM

quote:
Originally posted by alfas
do you see a possibilty to make a small sketch where this v-shaped cardboard should be fixed?

[Edited on 18/6/14 by alfas]


See the pic on this page. The tail points backwards.

http://nptel.ac.in/courses/Webcourse-contents/IIT-KANPUR/FLUID-MECHANICS/lecture-31/31-2_control.htm

alfas - 28/6/14 at 01:29 PM

what i also recognized, compared to all other speedster-screen seven-ish cars iīve ever driven:

even iīm only 1.70m tall, i have a lot of wind pressure on my breast, also turbulences on my head which pull on my helmet.

the kirkey-seat is mounted as low as possible (no runners, fixed position, in contact with floor), has nearly zero padding.

compared to the other sevenīs i have driven or still own the it looks like the general Locost seat position in realtion to the scuttle is rather high. is that correct?

[Edited on 28/6/14 by alfas]

alfas - 31/7/14 at 08:45 PM

a bit of an update:

for my next race (last weekend) i fully closed / paneled the area around the rear axle...and i removed the front wings (as they have been unstable /wobbling at speed)...far better...

after qualifying, i changed from my 205/60r13 semi slicks to slicks...the slicks had been slightly lower in profile....and here we go:

due to the slightly reduced drive ratio (lower tyre profile) the car was more lively even at higher speed!!

and i won the 2nd race!!! (ok...not race...regularity test....but every body is driving "regular fast"