Wings/aerofoils Are they really needed? and if so when?

[modnrod]

Time for my dumb question of the week. 

I know those small aerofoil lips over the top of the back window do wonders for stability at speed and I've heard even lowering the Cd, but just by their very design don't they also rob valuable cooling air from the engine compartment vents below it?

[spanners]

the yellow car shows exactly my point, it has this that and the other, but it needs to come down 2"..

[Peter Shattock]

As requested a golf ball floor on new a new VW.

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Interesting little vortex type deflector at the back of the floor too. If its intended to prevent the need for an enclosed floor to the rear it could be good for us as it would mean the area at the back of the car could remain open so as not to cause any potential cooling drams by extending the flat floor to the rear of the car. That said I'm not at all sure that is what its for but rest assured it will not have been done for fun!

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As ever more questions than answers.....

Peter

[Bernard Newbury]

This is interesting Pete, as when I raced a boat (quite a few years ago) there was a theory that the golf ball effect on the underside of the hull would create pockets of air that would lift the boat thus less drag.  I did try it once on a jet ski but I could not feel a difference.  VW might have made the floor like this for strength and these shapes could be cheaper to press, tooling wise.

[richie]

As requested a golf ball floor on new a new VW.

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Interesting little vortex type deflector at the back of the floor too. If its intended to prevent the need for an enclosed floor to the rear it could be good for us as it would mean the area at the back of the car could remain open so as not to cause any potential cooling drams by extending the flat floor to the rear of the car. That said I'm not at all sure that is what its for but rest assured it will not have been done for fun!

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As ever more questions than answers.....

Peter

So did you reach any conclusions from the wool testing at EBI ? or was that just to keep the car warm


cheers Richie

[Peter Shattock]

Yes! The wool tufts were to long.

Round 2 at Bugjam all being well.

Assuming I have anything worth sharing I'll post it here.

That said it looks like the top of the rear window rubber is the end of the attached flow over the roof and the rear window air flow remained disrupted down the rear window. Hopefully I'll be able to confirm in a couple of weeks.

Peter

[BeetleBug]

That said it looks like the top of the rear window rubber is the end of the attached flow over the roof and the rear window air flow remained disrupted down the rear window. Hopefully I'll be able to confirm in a couple of weeks.

Peter

That is absolutely correct and if you search you will find wind tunnel pictures proving it.

-BB-

[autohausdolby]

This is interesting Pete, as when I raced a boat (quite a few years ago) there was a theory that the golf ball effect on the underside of the hull would create pockets of air that would lift the boat thus less drag.  I did try it once on a jet ski but I could not feel a difference.  VW might have made the floor like this for strength and these shapes could be cheaper to press, tooling wise.

Pointless fact - the guy that runs our local trailer hire place (BDS trailers) told me he used to race power boats with you Mr Newbury

[richie]

Check out the bottom rear of the front wings in this video, an interesting approach

<a href="http://www.youtube.com/v/_Vo-ikdgQGQ" target="_blank">http://www.youtube.com/v/_Vo-ikdgQGQ</a>

[Andy Sykes]

I like his dash it would be interesting to find out how much the air is opening the vents

Cheers Andy

[Peter Shattock]

Interesting flaps on the back of the wing. There is clearly air wanting to escape from under the wheel arch which makes good sense given all the air rushing under the front of the car assuming there is no floor and wheel arch covers to reduce the amount of air getting in there. Where this turbulent air goes once it exits the flap and what effect positive or negative it has on the remainder of the cars aero who knows.
I didn't get round to checking this on my car last time out but one other thing I've been thinking about is the shape of the front and rear wings, as in principal they are not dissimilar to the whole car wing shape. I was going to do some wool tuft testing on the wings to see where the air detaches. Assuming this is correct we could expect each wing to create lift given the high speed low pressure air over the top of the wing and what is at best atmospheric if not high pressure under the wing. Clearly the area this pressure differential is acting on is far less than the roof but none the less it must be significant as the LMP cars all have wing top vents these days and the regulators have sought to control their size and shape in the classes where they are permitted. Again there is the issue of where the turbulent air then goes once it exits the wing top and what effect that then has on the lift and drag.
If you look at the wheel arches of a lot of the touring cars and rally cars they have big cutaway sections behind the wheel arch extensions used to cover the increase in track. This is perhaps similar also to our wings which extend from the main body line.  They again clearly have seen some benefit as the regs control their shape location and size which supports the need for the flaps and holes on the wing backs
For what its worth I think the first thing to do is reduce the amount of air getting under the car (lower the car with the added benifit of reducing the c of g) to reduce the problem at its source. Then if you can stomach the compromise of the look, fit an air dam. What ever you decide to do above then enclose the area's in front and behind the floor pan linking them with either the air dam / front valance / front wing lips and a diffuser at the rear. This I think would help reduce the effect of the turbulent air (less drag) and tidy and speed up the air under the car. Assuming this can be done to a point where it creates some negative pressure with the now high speed air which is acting over a large area (the whole of the underside of the car) could create some significant negative lift with little additional or potentially reduced drag.
Wouldn't it be nice to keep the same look externally and reduce drag and lift in the process! Certainly would be a lot better than having a big strap on on the back of the car (if it works).
All very well in theory but need less to say in practice it will be a bit more difficult as although we do in a standard car have a good start with the floor pan the rest is a bit of a mess. That with the need to get the exhaust and the hot engine air out from the back of the car (blown diffuser?), assuming its not a drag only car, sump extensions (unless you are dry sump) air in to coolers etc makes it all a bit of a challenge in practice when you start really looking at it. That said it would be an obvious thing to try and include an oil catch tray with a diffuser at the back of the car.
To end my ramblings on a positive callook note a nose down rake on the car is a good thing in fact that is understating it as at speed a nose up stance could be very dangerous so it does need to be kept this way when the car is under load too.
So there it is callook is safe!
Peter

[Andy Sykes]

Few ideas I've had no idea if they will work and nothing to compare to, stripped the guts from the heater chanels to vent from front to rear to feed the turbos





Thinking this when fitted correctly will release preaur from the top of the wings

[Andy Sweeney]

Just thought I'd share my thoughts on aero as my flat floor, tube frame build continues...



I'll be running a Wikk style front end with splitter and a roof mounted lip spoiler to aide cleaner separation in preparation for the air to meet my home made wing.



A chance meeting with an aerodynamicist with access to CFD used by Airbus found him offering to help design a wing for me based on the parameters I gave and my choice to stick to a single element for ease of construction and setup simplicity. In clean air at 150mph it is calculated to produce 236 kg of downforce at 13 degrees AOA with soft stall occurring at 16 degrees.

Armed with the profile, and in the absence of an autoclave, this is what I made...

Given that the wing needs to support the weight of three adults + safety margin, I set about designing something I hope will be strong enough...

I started to make the the outer end spars which will be the template formers for the hot wire cut foam core and ultimately the end fence mounts.

To ensure a uniform airfoil section it is essential that these two templates are the same. In this vein, I cut out the paper template supplied by Mr CFD and copied this onto some 64th marine ply which I then transferred onto my 6mm billet plate. I've got a bigger off cut for the main centre spars.



The trailing edge extends a good 2" beyond the pointy end of the templates, but this will just be a sandwich where the upper and lower skin meet and continue up and back towards a Gurney flap.

Once I'd rough cut these to shape, I clamped, then drilled and tapped them together so they could be cut out and finished 'as one' and hence be exactly the same shape. The tapped holes will be where lightening holes will go further down the line.



I then used my flat bed belt sander to get the profile almost perfect, but then finished of by hand with progressively finer files, and finaly emery cloth on various blocks.





Pretty pleased with how they've come out as now I know I can do it, it'll save me the expense having to get them machined or laser cut, and what I save on the swings, I can spend on the motor...



Next job was to cut the centre spars which also double as the main mounting spars, hence the extended lower edges.





Again, these were bolted up to the end spars to ensure they are all perfectly the same profile.

To join the four spars together and give strength to the wing, they are all joined with four aluminium tubes of varying sizes.



I contemplated how I was going to fold the aluminium skin round the leading edge of the wing in a uniform and even manner without getting witness marks, kinks or creases around the spars given that the foam centre is likely to give a little during the skin forming process.

To overcome this I decided it would be a good idea to have a mandrel at the leading edge in the form of a tube running along the length of the nose to bend the skin round, so I cut an appropriately sized recess in the leading edge...



To hold the front tube like so...

[Andy Sweeney]

Then it was a case of cutting the strengthening tube holes in the spars...



And then some strategically placed lightening holes, leaving me with these.



Which fit together like this...





Quite please with how it looks so far as it's already pretty rigid even in it's skeleton form and very light indeed. The body of the wing will be cut with a hot wire and made from blue high density styrofoam, then skinned and bonded in aluminium skin with the tubes running through the foam like bones.

Firstly, apologies for the poor quality pictures - I dropped my regular camera one time too many ans it's now officially dead, so until such time as a replacement is found, I'm stuck with a crappy phone camera.

Anyway, I 'borrowed' some 50mm thick styrofoam from Dad, which not being quite thick enough will need to be laminated, which isn't a bad thing as it allows me to solve a problem - more of this in a moment.



Luckily it was 610mm wide, which as my spar is 300mm, allowed me to cut it down the middle to laminate leaving me with a 305mm block.



Now what of this problem? I pondered how to cut perfectly straight, true and accurate holes down the length of the foam for the three aluminium tubes to go through without cutting through to the edge of either the spar or the foam which would compromise the strength of the structure.

I then came up with a cunning plan whilst staring at the laminated halves. Firstly, I pinned the spars to the end of the laminate where it will sit for the cutting, strategically placing the three tube holes over the join in the foam.





I Then marked down the length of one of the halves where the tubes will sit.

[Andy Sweeney]

Next, I clamped a long steel rule to the foam and modified my soldering gun by adding a depth gauge and used the tip of the soldering gun to carve a little hole all the way down the length of one of the foam halves which will allow me to feed the hot wire through the foam and use the holes in the spar as the hot wire template to cut perfect fitting holes for the tubes.





It worked really well and left me with a perfect little slot to feed the hot wire through when the two halves are bonded together.



I then smeared the halves with polyurethane adhesive and let them bond under the weight of a couple of old car batteries and a tool box.



You can see in that picture that I have done this process twice - one piece for the longer centre section and the shorter piece for the outer ends which will be cut in half when finished.

I also got my 2 x 1 sheet of aluminium sheet for the outer skin.



Next job is to mount the spars on the ends of the
foam block laminates and hot wire the foam to shape.

I carried on making the rear wing, concentrating on cutting the foam centre sections to make sure everything is going to fit together with precision before I kin it in aluminium.

Pleased with how its turning out so far, just the main centre piece to cut now.

[Andy Sweeney]

The wing mounting spars needed some clevis' machining - here's what happened next. A solidworks drawing was made...



And also a stainless clevis pin, as shown in green, here...



A program for the CNC lathe using gibbscam after the solidworks rendering was complete.

...then chucked up the aluminum in the lathe.



The green button left the machine do it's thing.

Here's the final pass "in action".



Done!



I need four of them...



Next, the part was turned around and chucked it up in the lathe.

Then drilled and tapped to 8mm x 1.25 thread x 20mm as seen below.



Repeat three more times...



Everything on the lathe is finished.

On to the mill...

A fixture was made out of Delrin to hold the part in the vice.



To machine the slot in the aluminum....

Placed the aluminum in the fixture and mounted it onto the vice on the mill.

Here we are after the slot has been cut.

[Andy Sweeney]

Next, a 1/2" end mill machines the flat portions on either side of the slot.



Then turned the part 90º to machine the holes for the clevis pin.



After the holes were milled, the bottom portion of the blind hole was tapped with an 8mm x 1.25 thread tap.



Here's the finished part along with the stainless steel clevis pin. No pictures of these being made, sorry.



Close up of the shouldered pin mounted in the clevis...



The final product!

[Andy Sweeney]

A wise man once said, "I'd rather be born lucky than good", a theory I've subscribed to many times in my life.... this isn't one of them.

Still, I'm soldiering on as best as I can despite my ability bankruptcy status.

Since the last time, I had wrestled the alligator by hand and skinned the wing in 20swg aluminium sheet. I'm sure if I'd had a roller, the skin would have followed the contour of my foam core beautifully with silky flowing lines.... however, I am pleased to report than when forming the skin by hand, it follows the form of the core a bit like a child's first attempt at clay modelling.

Needless to say, a 'waffer thin mint' of filler was required after I'd spent two and a half ice ages with my dollies and hammers getting it to resemble something like the air foil section hidden beneath it's 'house of mirrors' surface.

Here we are in the final stages of prep where I've blocked it to death and am working the final contour of the upper concave surface.





And after I'd just applied a final very light skim of surface stopper before I etch prime then 2K seal and prime it.



I'm having to go to these lengths as my good painter friend, Bruce from WRP Paint and Restoration who will be doing all the body and paint on the car deserves better than to be handed a 'creation' to work with that is too far below spec. I want him to have at least something workable as he applies his show car utter perfection.

Thankfully, the underside with it's continuous convex surface will require somewhat less prep - thank goodness.



The mounting lugs work well...



Fitting perfectly onto the spar mounting lugs...

[Andy Sweeney]

Finally got the underside of the wing in primer. Might not look much, but part of my soul is lost forever in those uniform curves.





Just in case I forget to mention it at a later date, I really hate doing body work prep. Just sayin'

At this point, and after consultation with my CDF bod, I decided to add a Gurney flap to the wing for reasons I'll not bore you with now, but it required me to split open the rear of the wing and add the flap at the heavily researched size...

The adhesive set on the Gurney flap, so I started to re-do the body work on the wing where I split the rear skin join to fit it. I'm very please with how it's turned out.





Still lots of prep to be done, but it's close.

Meanwhile, I've been itching to cut the wing end plates. You'll have to trust me on the shape and wait to see what they look like on the wing when mounted on the car - all will become clear.

I transferred my design pattern onto a 500mm square of 1.5mm aluminium plate



Then carefully cut them out with a jigsaw



The final position is yet to be decided, but this will give flavour...

[Andy Sweeney]

Since I inserted my Gurney flap, I've had to re-do some of the body work on the wing so I blocked it all back, and am guide coating and block sanding it all again to get it perfect. Also finally got my new camera with a wider angle lens I wanted to try out.
*
Just a few shots of how it looks as of yesterday...

[Andy Sweeney]

Despite a streaming cold, I did a bit more on the rear wing. Further research and after consultation with my CFD guy, it was considered advantageous to add a central wing fence on the high pressure upper side of the wing.

First I took a deep breath and took a cutting wheel to the top surface of the wing to give the fence a slot to fit in for strength and support.







A trial and error, cut and trim session followed leaving me with a correct cardboard template...







Which was then transferred to 1.5mm thick aluminium sheet - the same as the main end fences.





Leaving me with this...







It's now resting whilst the adhesive bonds before final finishing. The profile of the trailing end will match up with the shape and angles of the main end fences so it looks 'correct' when viewed from the ends.

Another small step.

[Andy Sweeney]

All that was left to do was mount the rear wing end fences.

First I had to make sure that when the top edge of the end fence is parallel to the reference plain (floor) of the car, the wing would be at my desired angle of attack which already takes into account a small amount of rake and also the angle of the air being presented to the wing from the roof, based on VW's own wind tunnel data.





Then I strategically drilled and tapped the end spars to mount the fences.





And hey presto, other than rounding off the end fence corners, it's done

[Andy Sweeney]

All that was left to do was mount the rear wing end fences.

First I had to make sure that when the top edge of the end fence is parallel to the reference plain (floor) of the car, the wing would be at my desired angle of attack which already takes into account a small amount of rake and also the angle of the air being presented to the wing from the roof, based on VW's own wind tunnel data.





Then I strategically drilled and tapped the end spars to mount the fences.





And hey presto, other than rounding off the end fence corners, it's done

[Andy Sweeney]

Tonight I decided to make a start on the rear roof spoiler which is designed to provide a clean and defined airflow separation point above the rear window before natural turbulence occurs and to provide the rear wing with cleaner, more effective and less turbulent flow.

First, I cut to aluminium templates to the required shape. The two holes are to fix the template to the blue foam core. Notice the slight under curve on the bottom surface to match the longitudinal radius of the Beetle roof.





I then selected a chunk of blue foam to cut the spoiler out of using my home made hot wire foam cutter.







Here is my home made cutter, fabricated from some stainless rod, bowed to keep the Ni-Cr resistance wire under tension. It's nothing fancy but it does the job admirably.









To provide the correct amount of current (heat) through the resistance wire, I hook it up to a 12V car battery which works out perfect for the length of wire I'm using.





Using the aluminium templates at each end, it's simply a case of carefully cutting the foam to shape...





Once you know the correct speed, heat and gauge wire to use, this technique car leave very neat accurate profiles.

[Andy Sweeney]

Due to the length of the foam available and the capacity of my wire cutter, I had to make the spoiler in two halves, which isn't a problem, as joining the foam is very easy. It's also preferred to keep the lengths shorter if possible as the longer the profile cut, the less accurate the shape in the middle as the wire drags behind during the cut. Here's the two pieces end to end which is the correct width between the gutters on a Beetle roof above the rear window.







So.... why the slot cut down the rear edge?

Obviously there is quire a tight lateral radius in a beetle roof and the spoiler will need bending to the right shape. The blue foam is surprisingly strong and whilst flexible enough to follow the contour, I'd not be happy trying to 'adhere' it to the roof under permenant tension to straighten out.

So I thought the best thing to do would be to give it a back bone. After some experimentation, I discovered that the best material that would bend and more importantly stay bent afterwards giving the spoiler the correct shape and a bit of strength was 10mm diameter copper micro bore tubing, which even if I say so myself, works really well.











That is only roughly formed to shape at the moment, but it'll be very easy to get it perfect once I've got a roof to use as an accurate template. Once bent to perfect shape, I'll coat the entire spoiler is an epoxy glass cloth to provide a stable and tough substrate for finishing. Also, I can use the rear off cut from the tube slot cut in half to plug the gap behind the tube to seal it in position.

Still plenty to do, but it's well on it's way. Not bad for an evening's work.

[Andy Sweeney]

With the car being over 150 miles away, it was difficult to guess the correct curve of a Beetle roof to set the exact profile of the rear spoiler so I can skin it in epoxy GRP.

To this end I made a visit to my good friend Mr Smull on the basis that he'd let me meddle with the roof of his Beetle as long as I meddled with the set up of his engine whilst I was there. Done deal.

Firstly, I measured and marked with a small section of masking tape where the top of the rear window on an Oval beetle is which can be seen near the top of the rear window in the first picture. Then it was a case of examining the flow data from Volkswagens wind tunnel research which shows that the top of the rear window is where the air flow begins to separate from the roof causing turbulence and drag, so just above this point was chosen to position the roof spoiler. This is a little below the level that the rear wing will sit to ensure the rear wing is fed with the best possible, least turbulent air flow.

The positions were marked with tape and the copper tube 'bone' of the spoiler was carefully bent to match the profile of the roof.










Once happy, I inserted the tube into the spoiler and offered it up to the roof.













It needs final trimming and a load of work to skin then finish it, but as a concept I'm really pleased with the results.

[Andy Sweeney]

During a recent visit to Rob's to see the car, I made a profile template of the roof from blue styrofoam to take home with me...





As Smull's Mexis is still an hours drive away from me, it made sense to make my own roof 'buck' to work from, so using the blue styrofoam template, I cut two matching templates out of 18mm MDF and boxed these up so I had my very own replica roof section to work from...





Which allowed me to screw down the spoiler into position so it adopts the perfect (or near as matters) curvature of the roof...







Which when at the correct angle will give me something like this for the air flow to deal with, leading nicely onto the main rear wing...





I'll tidy the ends up with small and neat end fences - nothing too hideous. Now it's fixed in the correct shape, I'll apply several thin laminates of glass cloth and resin to not only form a tough skin to apply a final profile and finish to, but will set and hold the spoiler in the correct curved shape so that when I release the screws at the ends, it won't just spring back straight again. I can live with a little flex as it will be tiger seal/bonded into position on the roof, but I didn't want too much excessive pull at each end that might risk the bond failing under vibration or aerodynamic load.

I'll skin it up with 3 layers of 6 oz cloth and resin and see how rigid it is by slowly releasing the screws - if it straightens too much, I'll add another layer or two until it doesn't.

[Peter Shattock]

Andy,

That looks like a lot of work! Hats of to you for making the wing from scratch though and nice fabrication work too. Hope it does what it was designed to when you get it on the car.

Do you have something planned at the front of the car to balance it over all?
 
Peter

[Andy Sweeney]

Andy,

That looks like a lot of work! Hats of to you for making the wing from scratch though and nice fabrication work too. Hope it does what it was designed to when you get it on the car.

Do you have something planned at the front of the car to balance it over all?
 
Peter

Hi Peter,

The main purpose of the wing is to improve high end stability at the top end, which along with the IRS 4 bar should keep things pointing the right way,

Up front I'll be running the nose as low as possible with a large splitter to help balance the aero load as well as wheel arch lips and high pressure vents etc to try and reduce drag.

Like everything though, we'll just have to see.

I'll probably just stuff it into the barrier on the first run though.

[spanners]

https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-frc3/p480x480/971260_267693070035364_1515860515_n.jpg
Ive gone a different route with my circuit car, here in the winter o/h,  its working towards a diffuser, but its developing to that stage from just creating all the flat bottom area i can squeeze in,  its mainly to clean up the underside air flow drag wise, all the lumps and bumps have gone, ie, jack points, lower rear shock mounts etc,the exhaust is high mounted anyway for track use and kerb hoping, the diffusor will extend into the rear bumper,  i also run various fairings on the front axle to clean up the torsion arms and the lower t/tube and shock mounts, the front ARB is high mounted between the axle tubes for the same reason, this car is pure bug, with no Porsche parts or double wishbone conversions in sight, its doing ok as it is, thats the fun of it,  the big problem with a circuit race bug is the front, the more power it can put down, the harder it is to get the thing to turn, the back is easy to get working.
the problem with added aero, the more so if it really does work and you can make it effective, is if the dependency on it is removed suddenly for any reason, cresting a rise, or a big slide for example, you can then fly, this is the reason Sports cars/ LMP etc, have a raised splitter section to maintain airflow at all times, rather than a sudden 'underflow' that will lift the car as we saw with Peter Dumbreck's Mercedes at LeMans 10 or more years ago. Subtle does it....