How it Works: Aerodynamics

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How it Works: Aerodynamics


Economy, Speed, Minimal Power



As early as the 1920s, the advantages of proper streamlining were very obvious. Throughout automotive history some companies managed to prove their point conclusively. Proof of how important aerodynamics are, and always have been, was seen in the Citroen DS, which was able to do 100 mph from only 73 bhp and would cruise all day at very high speeds while using only small quantities of fuel.

Of the more sporting cars in automotive history, the Lotus Elan, Porsche 356 / 911 and Jaguar E-type demonstrated that, with modest power, it was possible to volume-produce high performers that took full advantage of their engines. Aerodynamic efficiency in a car offers very solid advantages right along the line, but the most important is that if the shape is flowing through the air easily, it obviously requires less propulsive effort for a given speed. Thus, a higher top gear ratio can be employed for faster and more economical cruising.

Aerodynamic Side Effects



But there are some side effects, too. A car spearing through the air rather than battering its way is going to make greater demands on the braking system, since it will have far less rolling resistance. The desirability of streamlining is further enhanced by the fact that a target performance can be achieved with an engine in a lower state of tune than would be necessary with an un-aerodynamic approach. Hence, greater mechanical reliability on a long term basis and less maintenance. But before the problem could be seriously considered, engineers needed to learn that air was the reluctant host to objects man tried to thrust through it. The early engineers knew that air was far more friendly to aerodynamic shapes adopted by planes and gliders which rode on its hospitable substance.

Around a car, however, designers were to discover that the air became a series of changing pressure areas as the shape rose and fell from the nost to the tail. At the front, where the car thrust its nose into the air, there was a substantial high pressure zone from which the radiator cooling air was taken. The Citroen DS, Elan and E-type all had comparatively small, unobtrusive intakes placed to take full advantage of the high pressure area in which they were located. The gaping mouth of most cars from the 1950s, when aerodynamics really started to become a science, was governed by stylistic whims rather than practicalities. That resulted in an intake of excessive size to say nothing of the turbulence-inducing hardware surrounding it - chrome bling that looked great but did little aerodynamically.

Reducing The Air Intake / Grille



Experiments during the late 1950s proved that by closing off the intake with a sheet of metal or wood it was possible to reduce the drag by as much as 15 percent - without doing anything about the frontal obstructions. The problem was that, even if the projecting sector of the car was streamlined and the air intake was the correct size, there was still the problem of getting air out from the engine compartment where, with the aid of the fan and all the obstructions it found under the bonnet, it would become wildly turbulent. In a perfect world, aerodynamicists would exit the air out the back of the car from the long tunnel which would also serve to stabilise the air again. Strangely, very few websites mention this as one of the advantages of rear engined cars - although the principal was obviously never lost on Porsche.

On a front engined car, most air is emptied out underneath the car, where its volume and turbulence do a great deal in destroying the vehicle's passage. It was a relatively cheap and simple matter to shroud the engine so that the air passing through the radiator never had to tangle with the engine - however not much was done in this regard until the late 1990s. Then, having got control of the flow it was fairly easy to route it out under the car in such a way as to minimise disturbances. However, reintroducing the waste air back into the flow was undesirable from all angles, so quite obviously the smaller the amount of air the better.

Why Spats Were Not Merely Cosmetic



The beginning of the cure was in reducing the size of the intake and at the same time taking advantage of the natural pressure system at the front. Citroen had shown where the payoff was. It raised the maximum speed of the DS from 93 mph to 100 mph just by cleaning up an already good shape and without increasing the 75 bhp (net) of the engine. And the overall frontal area was very substantial at 24 sq ft. The road wheels themselves cause aerodynamic chaos, representing as much as 35 percent of the total drag. As they rotate, the wheels churn up the air wildly within the arches and then spew the eddies out along the body sides.

Wheel arch spats will do a lot to lessen the effect, but they they became unpopular with stylists in the 1970s. The last real effort in this regard goes to the Honda Insight Series 1 - so from an aerodynamic point of view, it is a shame that this design highlight did not once again prove popular. Regardless of spats or not, no car can purport to be streamlined unless it has a full and effective undertray. In many cases the undertray is of more importance than the topside, since at high speeds the pressure the waves can extend down far enough to touch the road surface and break up, causing an increase in drag.

The Optimum Ratio



The air must be able to pass right under the car and come out the back cleanly, particularly where it joins up with the flow coming off the topside. If it does not then the resulting eddies may well ruin the good work of the topside shape. With cunning design, the undertray can be made to deflect the flow from around the wheels, which makes it represent extremely good value on this score alone. Surprisingly the classical teardrop profile is not the ideal for a car. Besides, it's highly impractical anyway. To achieve the optimum ratio between profile depth and overall length it would be necessary to have a tail some 2.5 metres longer than the normal overall length, because the teardrop would tend to create a strong downwash effect at the back of the car and this would lead to fairly strong turbulence and therefore drag. It could also lead to a high pressure area building up under the tail at certain speeds which could contribute to lifting the back wheels off the road.

Professor Wunibald Kamm



Skin friction plays such a major part in streamlining that if the designer does make a long tapering tail they may well find that the increased skin friction has completely destroyed the advantages they was seeking. All this leads to the point that Professor Kamm and his bob-tail approach to streamlining is probably the best compromise. Kamm's idea was that you taper the back of the car as much as possible to avoid turbulence, and then cut it off abruptly. In this way the "chopped" or flat area should be quite small and therefore the wild air zone will only be the same size. Kamm knew his stuff, for in the 'thirties he built a number of cars - really only variations of existing models - which had about half the drag of cars that were built some half a century later. And even by today's standards these cars look like slightly rounded boxes on wheels.

Of course, it is no earthly good going to a lot of trouble to give a car the Professor Kamm tail if the air passing over the car is turbulated before it gets to the back. Ridges and rain gutters at the top of the windscreen, as found on cars up to the 1990s, can break up the air. And so can the windscreen itself if it is raked at an unsatisfactory angle. And even things you may have done yourself can prove a real problem to aerodynamics. Re-positioned your front number plates so they hang below the bumper? This seemingly innocuous modification can account for 20 percent of the total drag in a reasonably well-shaped car. An external sun visor, and accessory wind deflector for the driver's window and a couple of outside mirrors can run up an additional 25 percent.

The Car Does Run Better When It Is Clean!



Remember the protective rubber strips that adorned the side of cars? Long gone from current day models, manufacturers knew that by cleaning up the side panels and filling in recesses and such was a worthwhile modification that could easily be done in production, just as could smoothing out door handles and getting the side windows as far outboard as possible. Hoods over headlights and various dummy grilles and humps seriously detracted from the car's overall operating efficiency. If you are serious about streamlining you should also ensure that his car is clean and polished, as a build up of mud and dirt will interfere with the boundary layer of air and cause turbulence at high cruising speeds.

The Neutral Steer Line



Apart from the problems of getting the air to pass over the car smoothly, there is also the stability factor in which aerodynamics play a vital role. It is fairly easy to see that air creates different pressures around different parts of the car and that these pressures, if they are great enough, can affect that car's ability to remain directionally stable. Cars have a force called the neutral steer line which runs along the longitudinal axis and would be dead centre if the axles were equally loaded and suspension-less. As loading changes, though, so does the neutral steer line. It is constantly varying as suspension angles change with roll on corners. If understeer is present, the neutral steer line will slope back at the top, which tends to produce more instability.

Since it is not always possible to have the car heading directly into or out of the wind, there will often be a side force acting on the vehicle. On a car with a highly streamlined shape, the side thrust is generally more pronounced because the centre of pressure (the most important aerodynamic source in this context) may be as much as a car's length in front of the vehicle. As an oversimplification of the problem, it can be said that an aerodynamically efficient car will oversteer in the straight, sometimes strongly. There are two solutions, though. Firstly, the designer can shift the centre of pressure further back or he can move the neutral steer line forward until it is ahead of the centre of gravity.

Of the two, moving the centre of pressure back is the best solution, providing it can be done without seriously harming the streamlining. Take three quite different cars - all with understeer - as examples with which to work. The first is an immediate postwar box-on-wheels. Its neutral steer line will be between the centre of pressure and the centre of gravity. Consequently, the car will tend to oversteer when hit by a crosswind, although not strongly. That's because the neutral steer line, the centre of pressure and the centre of gravity are all close together, so that the car's inherent understeering characteristics will automatically make good the correction.
1939 BMW 328 Kamm Racing Saloon
The aerodynamic shape of the 1939 BMW 328 Kamm Racing Saloon.
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