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COVIN TECHNICAL ARTICLE “AERODYNAMICS & COOLING” Part 1 - 2004 By Darren Parker: Clubs Technical Advisor As most of you probably know by now my Covin uses the RS Turbo engine. For those who didn’t know, you do now. From the day I built my Covin and it first hit the road I have never been satisfied with the cooling side of things and in the quest to remove almost 1000’C of heat from the turbo, charge air via intercooler 150’C and engine coolant heat 110’C, I have fitted a new design of cooling system approximately every six months for the last few years. My latest cooling system modification in September 2004 was to be the one that really worked. However, for all this new cooling system has improved things, it still requires the fans to run constantly and as speed increases so does the temperature, so during my 200 mile journey home from this years Castle Donnington show with ambient temperature 28’C, the engine was running hot and my brain ticking over wondering what else could be wrong, I decided enough was enough and the time had come to look elsewhere for the cause of this problem, that’s when I came up with a theory that needed further investigation. After all you can’t fit 5 different cooling systems and not see significant changes to engine operating temperature. Before you start wondering what this has to do with “aerodynamics” read on, and remember this all started from a theory….. THE THEORY Most car designs have had thousands of pounds spent on them and many hours in the wind tunnel while the engineers refined and altered, tested and assessed to achieve their goals. But contrary to popular belief, many of those hours weren’t used to create a body shape that looked good. Instead the engineers were spending the time optimising the cooling system airflows - making sure that plenty of air reached the radiator(s) (engine) and could then leave without obstruction as well as ensuring the car was stable at speeds with good all round balance of aerodynamics. The Covin however is very different. This is a basic Porsche 911 Turbo body shape stuck onto a purpose made chassis. So my theory is this. The 911 shape should have the same aerodynamic properties that a real 911 has regarding downforce and upper body aerodynamics etc, but what about what’s going on under the car? Could my cooling problems all be due to pressures, or rather an equal pressure? Confused? I know I was. The results of many hours of research has shown me that this subject is both very complex and highly detailed so I’ve tried to put it down in a much more basic form as I’m sure non of us are Aerodynamic specialists and if one of you just so happens to be in this category then why didn’t you notice this problem years ago and tell us about it? It would have saved me so much trouble? Our Covins fall into one group, “the unknown”. There has been no wind tunnel testing for aerodynamics carried out ever, so all we can base things on are the facts that the shape is 911 and er the shape is 911 … So that covers that bit. The 911 relies on enough downforce/high pressure air travelling down through the fin and out through the rear underside in order to cool the engine but it also uses front mounted oil coolers and as the Porsche or VW air cooled engine is not the same beast as the water cooled unit due to the forced air ducted cooling effect created by the various tin ware round the engine, what would Porsche do if they fitted water cooled engines? Well they have, and the amount of air ducts, radiators and undertrays prove one thing to me. The original design was not to good for a water cooled engine. Adding further problems to this point is that our Covins use their own chassis which is wide open on the underside both front & rear and it’s this that has caught my attention. With the Covin underside being so wide open at the front & rear I believe that this is causing excessive air turbulence under the car and it’s this turbulent air that is causing high pressure to be created, remember the downforce mentioned earlier? The 911 relies on enough downforce/high pressure air travelling through the fin and out through the underside in order to cool the engine but if the underside has also developed high pressure then there can only be one result. The faster you go the higher both pressures become and the less airflow will travel across the engine resulting in….HIGH OPERATING TEMPERATURE. Remember air can only move if there is a pressure difference, the greater the pressure difference the more air will flow. the EVIDENCE & the technical part Drag and Lift Drag
Drag is proportional to the drag coefficient, frontal area and the square of vehicle speed. You can see a car travelling at 120 mph has to fight with 4 times the drag of a car travelling at 60 mph. You can also see the influence of drag to top speed. If we need to raise the top speed of Ferrari Testarossa from 180 mph to 200 mph like Lamborghini Diablo, without altering its shape, we need to raise its power from 390 hp to 535 hp. If we would rather spend time and money in wind tunnel research, decreasing its Cd from 0.36 to 0.29 can do the same thing. Fastback
Lift Another important aerodynamic factor is Lift. Since air flow above the car travels longer distance than air flow underneath the car, the former is faster than the latter. According to Bernoulli’s Principle, the speed difference will generate a net negative pressure acted on the upper surface, which we call "Lift".
Fastbacks (Covins) are particularly bad in this aspect, because they have a very big surface area in contact with air flow. It seems that good drag and good lift are mutually exclusive; you can't have both of them. However, as I did more research on aerodynamics, I found there are some solutions to achieve both of them.... Aerodynamic Aid Wing (rear spoiler) In the early 60s, Ferrari's engineers discovered that by adding an air foil (we simply call "Wing") to the rear end, lift can be dramatically reduced and even generate downforce. At the same time, drag is only slightly increased.
The wing has the effect of directing the majority of air flow to leave the roof straight without going to the back, this reduces lift. If we increase the wing angle, a hundred kilograms of downforce may even be available. Covins (911’s) take full advantage of this downforce and use it to good effect to direct high pressure air into the engine bay and out through the underside. But only if the underside (lift) pressure is lower. In this situation you will achieve good downforce from the rear wing but it will be counteracted by the high pressure air turbulence (lift) from the underside resulting in lack of air flow and reduced downforce effect, the result being hotter operating temperatures and poor stability at speed. underside air flow Air flow underneath the car is always undesirable. There are many components, such as engine, gearbox, drive shafts, differential, steering, sub frames etc, exposed in the bottom of the car. They will not only increase the surface area but also obstruct the air flow. This not only causes turbulence which increases drag, but also slows down the air flow thus increasing lift. (Remember Bernoulli’s Principle?). The Covin has massive underbody components that are all exposed to underside air flow which greatly increases this effect and adds to the underside pressure build up. So what can we do? Smooth Undertray & CONCLUSION The solution is a simple one, in order to reduce the underside air turbulence we need to reduce the total surface area of the underside of the Covin, we could achieve this by rebuilding our Covins to the size of a go cart, but we don’t really want to do that do we? The other option is to copy designs from other high performance cars that have had extensive wind tunnel testing carried out such as Ferrari and our old favourite Porsche. What they use is an “undertray” and what this does is creates a flat surface area under the car that has less surface area than the upper body surface area thus reducing lift significantly, increasing downforce and at the same time creating a lower underbody pressure (less lift) resulting in increase of airflow across the engine, but it doesn’t stop there. This is only one of the advantages.
Ground Effect
In the 70s, Collin Chapman (again) invented a completely new concept to provide downforce without altering drag - Ground Effect. He incorporated an air channel into the bottom of his Lotus 72 racer. The channel is relatively narrow in front and expands towards the tail. Since the bottom is nearly touching the ground, the combination of channel and ground forms virtually a closed tunnel. When the car is running, air enters the tunnel in the nose and then expands linearly towards the tail. Apparently, air pressure is reducing towards the tail so that downforce will be generated. Ground Effect is so superior to wing that it was soon banned in Formula One. Ground effect is not too suitable for road cars. It requires the bottom to be very close to the ground to form a closed tunnel. Road cars should have much higher ground clearance and this greatly reduces the effectiveness of Ground Effect but the principle is there and some ground effect will be created by fitting a good undertray to our Covins.
The picture above clearly shows the effect of air flow over the 911 shape with high pressure air in “blue” and low pressure air in “red”. The “blue” high pressure air is what gives us the downforce, stability and air flow over the engine on our Covin, but what you can’t see is the added underbody air pressure, turbulence and “lift” that is also doing the same thing and possibly on a greater scale, thus a cancelling out situation is occurring where both downforce and lift are fighting each other for supremacy and neither is winning. So by fitting a simple undertray to our Covins we can replicate some of this “Ground Effect” that will not only reduce turbulence, pressure build up and lift from the underside but at the same time because of this lower pressure from the underside it will increase the downforce pressure from above and with that comes better driving stability and it should solve high operating temperatures at speed due to better air flow across the engine (high pressure above & low pressure below) all at the same time. Now that can’t be a bad modification can it? For those of you who still might need convincing, here’s a picture of a Porsche 964 front undertray. Now ask yourselves why would Porsche go to the expense of testing, manufacturing and fitting such a large undertray to the front of their cars? The answer is simply to reduce underbody pressure (lift). Have you ever wondered if a real Porsche handles like a Covin? The answer is most certainly not and if you have a look at the underside of your Covin you’ll see the reason why, just look at that vast open space with all those components there just waiting to cause massive amounts of underbody lift at speed like an aeroplane on take off. No wonder we all suffer from tram lining and vague steering when we’re driving on the motorway or country lanes. Porsche 964 Front Undertray
final words There remains only one further point to make. This theory is new and it now requires putting into practice. I will be carrying out this undertray modification in the very near future during the winter months. I will need to remove some of my Covins underbody parts like heater hoses etc so it will have to be taken off the road while I carry out the work. So in order to add this article to the latest club magazine issue I will have to do it in 2 parts. So apologies for this and please be patient and wait for the test results and full details of making your own front and rear undertray for your own Covin in the next magazine issue. I’m convinced that there will be a very interesting outcome that might just transform our Covin driving experience…… Part 2: Aerodynamics & Cooling: Test Results & Construction to follow in next magazine issue 21 & included on web site Regards Darren Parker…. Club Technical Advisor
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