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REMOTE CONTROL By BOB YOUNG Bluff body design & automotive aerodynamics Having concluded our discussion on the aerodynamics of full size aircraft, we will now look at how those principles apply to motor vehicles. This should be of interest to the R/C model car enthusiast. As the speed of a road vehicle increases, the benefits of aerodynamic design become more and more important, particularly when combined with the spiralling costs of fuel and the fact that petroleum is a finite resource. To give an example of the savings involved, in the UK alone it has been estimated that reducing freight vehicle drag by 10% would result in a fuel saving of 300,000 tonnes per year. Keeping in mind our earlier discussion on drag, it is obvious that once again our old enemy "velocity squared" is in there stirr- ~ -==== ing up trouble as usual. The ramifications of the vZ problem when applied to the Very Fast Train (SILICON CHIP, March 1990), for example, are extensive. Not only does drag increase with vZ but lift increases also. This could result in loss of traction and possibly even contribute to the train leaving the rails under certain adverse conditions. As discussed in previous columns, fuselage lift for aircraft can be quite considerable and exactly the same principles apply to ground vehicles, which if not very carefully designed cease to be ground BASE - PRESSURf/ --l--o.76 l'\J'45• 15 L / STING ALL DIMENSIONS IN mm Fig.t(a): experimental wind tunnel arrangement for determining the drag coefficient of two discs mounted in tandem. The resulting drag is much lower (within limits) than for a single disc. 88 SILCON CHIP vehicles and become airborne. This happened to the Stanley Steamer in a very early speed run on Daytona Beach. It became airborne at 150km/h and crashed. Vehicle aerodynamics Vehicle aerodynamics involve all sorts of components. The overall flow field consists of streamlines, wakes, vortices, interference with the road surface and rotating wheels, surface pressures, and noise and rain effects. These must all be taken into account. Stability at high speed, particularly the response to side winds (yaw) and gusts can be deficient, especially in low slung streamlined sports cars. An adequate flow of cooling air is needed to remove heat from the radiator. Intakes for internal ventilation, heating or air conditioning have to be placed where the dynamic air pressure is favourable and will not result in exhaust fumes being drawn into the vehicle. A particular problem involving heavy vehicles is the generation of large sheets of spray in rain. This creates a potential danger for overtaking cars, especially in dark and foggy conditions. Reducing air resistance and adding fairings helps to minimise this problem, as well as making the vehicle more fuel efficient. All of these problems respond to the vZ law. Even the windscreen wipers get into a frenzy at high speeds and will lift off the windscreen in some vehicles. Thus, we see that the science of aerodynamics is very important to all citizens of the modern world and affects us greatly, in matters as diverse as our methods of travel, safety and pocket. \J I I 1.4 I I I I I I 0, 1 02 = , .o 1.2 02 = 70 mm I I Bluff Body design Vehicle design is generally lumped under the scientific title of "Bluff Body, Ground Proximity" aerodynamics. This is a quaint way of saying that the space inside the vehicle is more valuable than the space devoted to streamlining. To understand this, we must now move on to some of the very interesting aspects of Bluff Body aerodynamics. To begin, drag, not lift is the major item of interest in vehicle aerodynamics. Not only is lift not required, it is positively dangerous, as we have already noted. Now the measure of aerodynamic cleanliness is the "Coefficient of Drag" (Cd) and is in effect a comparison of the drag of the body under examination to the drag of a flat disc of equal cross sectional area. This flat disc is said to have a Cd of 1.00. The usual figure quoted for Cd on modern cars runs around 0.3 to 0.35, which simply means that a typical car has about one third of the drag of a flat disc with area equal to the vehicle's frontal area. Referring back to airfoil theory we can see that an airfoil can have a Cd as low as .01. Cars are a long way from this figure due to the ratio of their length to cross sectional area. A good rule of thumb for the ideal streamline shape is a ratio of 16:1. Thus, a vehicle with a frontal area of 2 square metres should be 32 metres long. Plainly, this is impossible. To overcome this problem, we have the science of "Bluff Body aerodynamics". The really interesting thing about air is that in essence it is quite capable of helping itself but is hindered by a finite response time of 335m/s (ie, the velocity of sound in air). Bluff Body aerodynamics relies heavily upon this ability of air to help streamline itself. Transonic aerodynamics is the art of dealing with air which can no longer help itself. I I I I I ,., I I I I I I 1.0 I I 0 .9 Fig.l(b): the graph plots the drag coefficient vs. gap length (L) for two discs of equal diameter. The Co is a minimum at L/D2 = 1.55. To explain, let us examine two very interesting and curious phenomena, the first being a sphere which has a minus drag coefficient. This means that it generates a small amount of thrust from its own drag. At first glance, this statement indicates that we should all be travelling in spherical cars and aircraft but the truth is that the gain is so small that any protrusions such as wheels destroy any benefits. It is also not a very practical shape to work with. It is very poor directionally and easily blown off course by cross winds, due to the lift and thrust generated on the sides. The early aerial bombs were spherical and were difficult to aim accurately as a result. The shape of the modern bomb is no accident. The second curiosity is the "two disc" pair. This arrangement is simply two discs in tandem (Fig.la) mounted in such a way that the separation between them is variable. Fig.1 b shows that the drag coefficient of the pair is related to the ratio of their diameters and the distance between them and is much lower than for a single disc. The reason for this is simply that air will form its own streamlining from eddy currents. Here then is the saving grace for the modern motor car and the core facet of Bluff Body design. The trick is to get the air to do the streamlining for you. Figs.Za & Zb illustrate this quite clearly and show the Cd for squareback and fastback vehicles. By setting up the correct conditions for eddy currents behind the car, it is possible to simulate a full streamline flow of near ideal proportions. By pumping air into this zone or bleeding air out, quite a low Cd can be achieved. Much work is being done in this area to create the ideal low drag motor vehicle. Modern designs In the early days, the designers of streamlined cars attempted to create a shape that was like a halfteardrop with a very rounded front and a gently tapered rear. However, this had to be impractically long, as noted, to give any worthwhile drag reduction and was often directionally unstable. Almost all moderately sized cars now have a distinctly short afterbody and these are characterised by definite f IXl.1! ~s:~: ,s_f~'>)Jl>> . . .,: Fig.2(a): by setting up the correct eddy current conditions behind a car, the air will form a streamlined flow and quite a low drag coefficient (Co) can be achieved. AUGUST 1990 89 o· 10° 20° 30° so• SQUAREBACK-TYPE FLOW FIELD. Fig.2(b): the drag coefficient is extremely sensitive to the angle of the back window. The lower drag of the fastback is one reason for the increasing popularity of this style of vehicle. wake flows and vortices. Two types of afterbody flows are shown in Fig.3. The recirculating bubble or spiral vortices determine the drag and stability. They also determine how much of the dirt thrown up by the rear wheels is deposited onto the rear window and body. The drag coefficient is extremely sensitive to the angle of the back window as in Fig.2b. The lower drag of the fastback is marked and accounts for this increasingly popular although rather angular shape. Air does not like abrupt changes in direction and sharp corners increase drag tremendously. The early Volkswagen Kombi of the 1960s is a good example of the advantages to be gained from wind tunnel testing. The original design had sharp front edges and wool tufts placed along the sides showed that the airflow completely broke away and was turbulent. Even quite modest rounding of the front edges and corners streamlined the airflow and reduced the drag by 40%. The important point to remember is that air is limited to a response time of 335 metres per second. This is quite slow and so we have to adopt special techniques to achieve streamlining. We do this by design90 SILICON CHIP ing our shapes so that the airflow does not have to make abrupt changes in direction. Some indication of the progress being made in drag reduction of vehicles can be gained from the Cd figures over the years. In the 1920s, average American car Cds were 0.7, falling to 0.5 by 1940. In Europe, the average Cd for 86 popular makes was as high as 0.46 even in the 1970s, the actual range of values being 0.37-0.52. Reynolds numbers Great care is needed in quoting and interpreting drag coefficients for motor cars. Tests done by General Motors on a 1/4-scale fastback gave a Cd of 0.27 at a Reynolds number of 700,000, which decreased to 0.23 at R = 2,000,000. Often, tunnel models are simple shapes that don't include all the practical details. The Reynolds number is far too complex in concept for a full explanation here but briefly, Reynolds in 1883 combined a host of factors influencing surface flow. These included form, waviness or roughness, speed of the mainstream, distance over which the flow has passed on the surface, and the ratio of density to viscosity of the fluid. He combined these into a single figure derived from the following formula: Re = Density/Viscosity x Velocity x Length. As a general rule, the higher the Reynolds number the more efficient the result. It is here that we see the problems arising in model aircraft with tapered, high aspect ratio wings such as in scale models. For any given airspeed, the Reynolds number will always be lower at the tip than at the root. When combined with the high angle of attack at landing and tip vortices, there is a great danger of the tip stalling first , causing the wing to drop and the model to fall into a spin. For this reason, washout (reduced angle of attack) at the tips is a bsolutely essential on this type of model. Conflicting requirements The skill of the vehicle designer is also shown in the way he blends the conflicting aerodynamic requirements into a working motor car. A good example of this is the Lotus Elite GT 4-seater. It is low and wide and the top of the windscreen is more than half way towards the tail. The curved wedge-like forebody was kept low by having retractable headlights and a canted engine block (the resulting aerodynamic drag power is only 30kW at 160km/h). A serious consequence of this, found in experimental models, was a large upward lift on the curved forebody which decreased adhesion of the front wheels. This would have made it dangerous in crosswinds. The cure was to fit a wide scoop under the front of the engine. This collected high speed air and passed it through a shallow, wide radiator so that the retarded air was ejected into the boundary layer on the top of the curved forebody, thereby breaking the lift suction. In this way, drag, stability, lift and cooling airflows were all combined successfully. Lift reducing traction and steering is a serious problem for vehicle designers and many solutions have been tried. The large inverted airfoil seen on some of the dirt track · racers is, to my mind, the least elegant. While it does generate a STATIC RAM 62256 4425 2764 4116-5 $19.95 $18.00 $3. 00 $1.25 41256 $5.00 4464 $6.00 2114 -2 $1.00 4164-12 x 9 SIMM Modules $12. 95 ea SPEAKERS 12 " 100 Watt Woofer 12" 30 Watt Woofer 1O" 25 Watt Woofer 8" 20 Watt Woofer $59.95 $42 .50 $25.50 $18 .50 ea ea ea ea CAPACITORS 10,000 nf 40 Volt DIODES 4002 1A $4 .50 ea $5.50/100 $65.00 ea INFARED DETECTORS Limited Quantity, 40' to 80' range, adjustable. MICROWAVE OSCILLATORS VA221 B 7.255 - 7.555 Ge V220D 6.575 - 6.875 Ge $25.00 ea NICAD BATTERIES Fig.3: the afterbody airflows for two different vehicles. The recirculating vortices for the lower model determine the drag and how much of the dirt thrown up by the wheels is deposited on the back of the vehicle. In practice, the drag coefficient is extremely sensitive to the angle of the rear window (see Fig.2b). downward force on the wheels, it creates a lot of drag and therefore is somewhat self defeating. A more elegant approach is to convert the underside of the vehicle into a venturi by fitting a fairing. This serves a twofold purpose. First, it eliminates underbody drag from the rough underside which can amount to as much as 5 o/o of the total drag. Secondly, in keeping with Bernoulli's Theorem, it generates a low pressure area under the vehicle and thus provides a downward force on the tyres. This could be an important point in R/C model cars where the light axle loadings result in very low footprint pressures. Traction in a model car is all important and much care is required in selecting the correct tyres for the conditions under which the car is being raced. Again in electric racing cars, cooling air forced over the motor batteries will help improve battery efficiency and life. 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