Food for Flying Thoughts

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Theories on the Aerodynamics of Rally Cars

Several hundred tons of laden Boeing 747 need only be propelled at about 170 mph to become airborne. Lighter aircraft, even those with the modest power of a four-cylinder piston engine, can take off at a quarter of that speed. Small wonder, then, that modern competition cars are subject not only to the normal forces associated with movement whilst in surface contact, but with aerodynamic forces too.

Cars were not meant to fly, but when their makers learned enough about wind resistance they were made more streamlined in order to pass through the air (but still in ground contact) with the minimum absorbtion of power by this resistance, and consequently more quickly. This was very simple technology, and it was not until much later, when cars were considerably faster, that streamlining alone was found to be no longer enough.

Hitherto, the contact between tyres and road surface had been enough to provide all the control a driver required. But at higher speed, and with constant changes of ground pressure as cars became successively lighter and heavier on their suspensions, something else was required to ensure that control was maintained during moments of low ground pressure and of total loss of contact, such as when cresting a brow: not a lifting device such as the wing of an aircraft, but something to keep the car on the ground as much as possible, with that vital pressure maintained between tyres and road surface.

Simple streamlining was about to progress into more sophisticated aerodynamics and, just as they had when seeking improved designs of engines, frames and other components, racing car manufacturers looked to the aviation industry for guidance.

Various forms of aerofoil were designed, tested, improved, rejected and even banned, and similar contrivances soon found their way to the bodies of rally cars which, although they may not achieve the speeds of racing cars, are driven far closer to hazardous obstacles and — more important—are in the air more often and for longer periods.

When a rally car leaves the ground it loses all propulsion and continues its forward motion only by virtue of its momentum. It follows, therefore, that the longer it can be kept on the ground, the faster it can be driven. On the other hand, many of the crests encountered on special stages are so severe that to negotiate them with wheels on the ground would mean slowing down so much that the advantage of maintaining ground contact would be completely outweighed. Thus it has to be accepted that a rally car will become airborne, and that any aerodynamic devices should be designed not to prevent that, but to keep the car completely stable whilst it is in the air, and as it lands; in other words, in a safe, controlled attitude throughout its “flight” until it recovers full ground adhesion and stability after its subsequent landing.

An aircraft’s attitude can be changed by varying the angles of what are called control surfaces, ailerons, elevator and rudder. However, it also has lifting surfaces, its wings, and — just as a car’s steering is ineffective unless it has front wheels — without these there can be no significant aileron effect. However, in the case of high speed movement by a vehicle not intended to fly, and which does not have lifting surfaces, ailerons could indeed have an appreciable effect in preventing sideways roll whilst airborne.

What is more, it could be further controlled by devices similar to a rudder and an elevator to ensure that it remains in the correct attitude and lands properly on its wheels, which are kept in line with the direction of travel. Together, the rudder and elevator would act like the feathers which keep a dart in the true flight intended by the player, all the way to the board.

But a dart cannot be controlled after being thrown, whereas a pilot does have in-flight control of his aircraft. A driver, on the other hand, although sitting in his car, has no means of controlling it once it has left the ground. Just as the dart’s movement through the air depends entirely on its attitude the moment it leaves the player’s hand, so the car’s progress in flight is determined by what it is doing at the precise moment it takes off from a crest. Whether something can be done about this is a matter for engineers, but we feel that aeronautical principles offer rallying plenty of food for thought and, by taking a few more lessons from aviation, rally engineers might succeed in making their cars more controllable whilst airborne, and thereby less prone to those fearful accidents which happen when cars gyrate after a lump, land awkwardly and roll end over end several times.

A car is controllable in one axis, and then only when in ground contact. An aircraft has three-axis control, and if this versatility were introduced to a rally car, without increasing the driver’s work load to an impossible level, an immense safety feature would result — although we daresay that many would consider it more a means of increasing speed.

It has long been a popular — and to some extent justifiable — thought that because acceleration lifts the nose and braking dips it, adding power just before taking off from a crest will tend to keep the nose up, whilst braking before take off could bring the nose dangerously low whilst airborne. A car’s attitude whilst in the air can also be influenced by the torque effect of its engine, according to the direction of crankshaft rotation and whether the engine is mounted transversely or longitudinally, and this is another factor to bear in mind when applying throttle. The torque effect of a transverse engine is in the pitching plane, whilst that of a longitudinal engine is in the rolling plane. An MG Metro, therefore, would tend to roll in flight if revved, whereas a Peugeot 205 would tend to pitch.

Let’s look at the three axes of aircraft control. and consider how each may be incorporated In a rally car.

1. Vertical Axis (YAW)

Conventionally, this is the only directional control axis provided by car manufacturers, via the steering wheel to the front wheels, and only effective when friction exists between the front tyres and the ground. Unconventional vertical axis control can be obtained by causing loss of adhesion through driving techniques. eg handbrake turns. Neither of these employs any aerodynamic forces.

An aircraft uses its rudder to provide vertical axis control, and although the idea of fitting such a device to a car might seem absurd, it could prove to be a useful steering aid when airborne, or when the steered wheels have lost their grip. A pilot uses two pedals to control his rudder, but a car driver’s feet are already well occupied, so perhaps the steering wheel itself could be coupled to the rudder, the coupling becoming engaged automatically only when sensors detect suspension at maximum travel (with wheels “hanging”) due to loss of ground contact.

The presence of such a tail fin during normal, fast travel may be undesirable, and may inhibit conventional steering, so a means would be needed to render a rudder, and its accompanying fin, inoperative when not required, possibly by retraction activated automatically, manually or both.

2. Transverse Axis (Pitch)

An aircraft will raise or lower its nose when its elevator, positioned in the horizontal sections of the tailplane, is moved up or down by the rearward or forward movement of the control column by the pilot. A car has no such control axis, but what a boon it would be for a driver who, in the middle of a jump, finds his car’s nose dipping dangerously.

Many rally cars now have rear horizontal surfaces to provide aerodynamic stability, but these are only movable by adjustment, and not by the driver whilst in motion. If such a device were to be placed so that airflow would be over both its upper and its lower surfaces, and its trailrng edge were made to hinge upwards and downwards, than an elevator would be created. If this were then linked to a control within reach of the driver, he would be able to move the elevator and produce a nose-up or nose-down attitude as required.

If the steering wheel were made to move forwards and backwards, the driver’s means of pitch control could be the same as that of a pilot, and by easing the wheel back towards his body he could raise the elevator and check any dangerous dipping of the nose whilst flying over a crest. If he eased the wheel forwards he could check any lifting of the nose, which can be equally dangerous at times.

During normal travel, the presence of such a horizontal surface with its trailing edge elevator would be no more an embarrassment than the present fixed surfaces, for the elevator could be positioned neutrally whenever required. Fore and aft wheel movement would have to be damped, of course, so that it would not flop around freely in the driver’s hands.

Alternatively, an elevator could be fitted to the front of a car as in an aircraft of the canard design, but in this position it would be very vulnerable indeed and there would be a much higher risk of its being damaged or even destroyed.

3. Longitudinal Axis (Roll)

A car taking off from a crest when not lined up properly with its direction of travel, ie: when slightly sideways, can very easily twist in flight about its longitudinal axis. Aviators call this the rolling axis, but it should not be confused with the rallying interpretation of the term. Generally, however, a driver knows in advance from his notes that he is likely to jump sideways and prepares for it. What he cannot prepare for is something like a dislodged stone being struck by one of his wheels, throwing one side of the car further into the air than the other.

This would generate a mid-air twisting movement which, if not checked, could result in the car landing very awkwardly indeed, perhaps not even on Its wheels but on its side.

Although a highly dangerous manoeuvre for a car, such twisting is normal for an aeroplane. Indeed, its turns are made by banking which is induced by lowering one wing and raising the other. The trailing edges of the wings are fitted with hinged devices called ailerons which are moved in opposition to each other, ie: when the left aileron is raised, the right aileron is lowered, this producing a bank to the left.

Just as the ailerons can be made to induce bank, they can also be made to prevent it, and if similar devices could be fitted effectively to a car, then that dangerous in-flight twisting could be checked or even prevented.

Simple ailerons could be made by fitting a horizontal surface to the left side of the car and another in the corresponding position on the right, both exposed completely to the airflow produced by forward movement. It would be undesirable to have them extend outwards beyond the bodyline of the car, so perhaps the left and right roof edges would be the best places. These ailerons should be hinged at their forward edges, and linked in such a way that when the left one is raised, the right one is lowered.

Then we have to think of how they can be controlled by the driver, and again the pilot’s means of aircraft control comes to mind. An aircraft’s ailerons can be moved either by turning (as a steering wheel) the hand grips of the control column, or by moving a stick left or right. The former would conflict with the normal movement of a steering wheel, so perhaps it should be made to move from side to side.

If all these additional control axes were provided in a rally car to give the driver control when jumping, his work load would be increased tremendously, and he would be over confident indeed who would declare approval and acceptance before trying it. However, the thought of reducing from its present high level the risk involved in jumping is very likely to persuade him to give it a try, even though he would probably be faced with a steering wheel which not only turns, but moves bodily fore and aft and from left to right.

The bodywork of a rally car being driven at high speed along narrow tracks between solid obstacles is always vulnerable, and experienced rally people may be no more than amused at these theories, and write them off as impracticable. Aeroydnamicists may be equally amused, and perhaps even offended by our over-simplification and by our less than precise treatment of their exact science. However our intention was to present no more than a hypothesis, one which we trust will have achieved its purpose — to provide food for thought — G.P.