This article was first published in 2000.
United States Patent No 5,863,090 was awarded on January 26, 1999 to Robert J. Englar of the Georgia Tech Research Corporation (Atlanta, GA). The patent covers a novel way of improving aerodynamic downforce by blowing air across aerodynamic surfaces. The air supply can come from an engine driven compressor, a turbo - or even the engine's exhaust!
High performance sports or racing cars present special control and handling problems to designers and drivers. Often, these types of cars travel at such high speeds, and are so aerodynamic in design, that the body of the car generates an upward lift force. The lift force generated may be so strong as to affect the handling of the car.
In an effort to remedy such dangerous handling problems, and to improve cornering performance, it is common to see very large horizontal wings on high performance sports cars or racing cars. These horizontal wings are often three-dimensional aerofoils, which have an inverted camber such that as air passes over them, a downforce (or negative lift) is imparted on the vehicle. This downforce can overcome the undesired lift force generated by the vehicle's body and improve the vehicle's handling characteristics.
At one time, it was common to also shape the bottom body panels of the vehicle in order to create additional negative down load. This was used to either improve the effect of the inverted wings or to replace the wings. However, current racing rules prohibit this type of design in many racing categories. Therefore, many racing cars are restricted to using size-limited inverted aerofoils on the front and rear of the car (and no curvature of the lower body) in order to generate the necessary downforce. Although these inverted aerofoils do generate negative lift, which increases with the square of the vehicle's speed, such aerofoils have a number of inherent drawbacks. These drawbacks include mechanical complexity, weight, increased drag, and a lack of on-the-fly adjustment.
Therefore there exists a need for a light-weight, simple, non-moving, dynamically adjustable system for producing variable downforces of much higher magnitudes, with or without drag generation, on a high performance ground vehicle or racing car.
The Nuts and Bolts
This invention covers an airflow control device for a high performance ground vehicle that comprises a source of air (or other gas) under pressure, and a mechanism for regulating and controlling the flow of air/gas from the source. Such a source of air can be compressed air from onboard turbochargers, superchargers, or compressors powered by the engine. The source of air can also be actual engine exhaust gases, routed from the exhaust manifold. If available, heater or air conditioning sources may prove suitable to supply the blowing requirements of this invention.
The regulating mechanism can comprise a valve. The valve should preferably be manually controlled by a driver or, alternatively, a computer receiving data from various sensors (such as accelerometers or pressure transducers) can control the valve.
The compressed air is routed from its source, through the regulating mechanism, to a horizontal aerodynamic surface attached to the car. This aerodynamic surface preferably has a rounded trailing edge. The compressed air is discharged from the aerodynamic surface through a slot in an outer surface of the aerodynamic surface. Preferably, the slot is on an under-side of the rear portion of the aerodynamic surface. The ejected air then attaches itself to the curved aerodynamic surface and sweeps around the rounded trailing edge. External flow across the lower aerodynamic surface is then entrained, and a greater negative lift force (downforce) is generated.
The system can work whether the aerodynamic surface is placed at the rear or the front of the vehicle, or on the underside of the body itself. This system will also work if an aerodynamic surface is placed at both the rear and at the front of the vehicle. Additionally, the system can be improved by placing a plenum inside the aerodynamic surface for collecting the compressed air, permitting an even discharge of air out of the opening in the aerodynamic surface.
The aerodynamic surface can be composed of pneumatic (blown) aerofoils of many different shapes. For example, standard aerofoil shapes modified with rounded back surfaces and blowing slot/slots can be employed, or the aerofoil can be shaped as merely a circular cylinder with a tangential blowing slot. The air can even be caused to discharge from both the upper and lower surfaces of the aerodynamic surface. Or, if desired, more than one aerodynamic surface can be used at either the front or back portions of the ground vehicle.
The effect of an inverted aerofoil aerodynamic surface can be enhanced by a blowing system incorporated on the actual underbody of the vehicle. In this approach, there is preferably a source of air under pressure as well as a means for regulating the flow of the air from its source. There should be an appropriately sized and spaced slot along the bottom surface of the ground vehicle, and a rounded surface to the rear of this slot. Through this slot, pressurized air will flow, attach to the vehicle, and follow the curved rear edge of the vehicle itself. The airflow from the body of the vehicle will interact with the horizontal aerodynamic surface, causing an even greater downforce to be imparted upon the ground vehicle. If desired, the slot can extend around the sides of the ground vehicle in order to add stability and control to the vehicle.
A high performance racing vehicle normally has a front horizontal aerodynamic surface (11) as well as a rear horizontal aerodynamic surface (12). The front horizontal aerodynamic surface is usually positioned close to the ground, while on the other hand, the rear horizontal aerodynamic surface is normally positioned much higher off the ground.
The arrows in this diagram and in the one above depict the airflow about a typical high performance racing car during forward movement.
This diagram shows the side view of an inverted pneumatic wing assembly, more typical of a front wing assembly. This assembly comprises a substantially horizontal aerodynamic surface (17) with an inverted aerofoil section. The aerodynamic surface (17) is bounded by an end plate (18) at each end. Typically, these end plates can serve two purposes. Often, the end plates support the assembly and attach it to the ground vehicle, and the end plates also improve the negative lift distribution of the aerodynamic surface. Therefore, even if the end plates do not support the aerodynamic surface, the benefits of better lift distribution and lift improvement is such that end plates are still normally used. Such a configuration is common among high performance racing ground vehicles.
In this invention, the aerodynamic surface (17) preferably comprises a rounded trailing edge (19). Inside the aerodynamic surface is located a plenum (21) that holds an air supply able to be ejected through a slot (22) in the lower rear surface of the aerodynamic surface. Although use of a plenum is not necessary to this invention, the plenum evenly distributes airflow along the length of the slot. Preferably, this slot ejects air tangentially to the local surface, transverse to the direction of vehicle motion, and this slot should span the entire width of the aerodynamic surface. However, any shaped opening, or series of openings, would work with the present invention.
A source of compressed air is connected to the plenum by a pipe (102). The source of compressed air can be compressed air from onboard turbochargers, superchargers, compressors powered by the engine, or even actual engine exhaust gasses routed from the exhaust manifold. This air source should preferably have its flow regulated by a flow regulation device, such as a valve (94). A computerised control system (96) actuated by sensors (such as accelerometers or pressure transducers) can operate the flow regulation device, controlling flow out of the slot. On the other hand, a driver can manually actuate the valve to control the flow rate of air out of the slot.
As pressurised air is emptied into the plenum, the air then flows, under higher pressure, through the slot. Due to low external static pressure, it then curves around the rounded trailing edge (19) of the aerodynamic surface. The action of this flow rounding the trailing edge of the aerodynamic surface entrains the free air flowing over the aerodynamic surface so that the circulation around the aerodynamic surface is greatly increased. This results an increase in the force which the aerodynamic surface develops. Because the aerodynamic surface typically has an inverted aerofoil section, the force imparted is downward, as depicted by arrow 23.
Note that the functioning of this invention does not depend on the existence of the end plates - a designer of the ground vehicle may not wish to pay the associated drag penalty of end-plates. In this case, any suitable support means can be used for the wing assembly. For example, a rear aerodynamic surface can be attached to the ground vehicle by a strut at a centre point of the aerodynamic surface.
Furthermore, a rounded trailing edge with a substantially uniform radius, as shown, is not the only trailing edge configuration that will work with this invention. So long as the trailing edge of the aerodynamic surface is at least mildly rounded, the flow coming from the slot will attach to the wing surface due to reduced static pressure. Even if the trailing edge of this rounded surface is sharp, decreased drag can result from blowing air out of the slot. In addition, the aerodynamic surface can have an aerofoil section of any camber. In fact, a symmetrical aerofoil, with no camber at all, can be used with great effectiveness.
This diagram shows a cross section of an aerofoil which is simply a circular cylinder. This aerofoil section also comprises a rounded trailing edge (27) and a slot (28) on the lower surface of the aerofoil section. A plenum (29) is housed in the interior of the circular cylinder aerofoil section. A circular cylinder section functions in the same manner as the negatively cambered aerofoil section of the aerodynamic surface (17) described above. However, this circular cylinder aerofoil section produces a much higher coefficient of lift (downforce) when air is blown from the slot 28 and, therefore, a corresponding increase in the downforce on the vehicle results. Although not shown with end plates, the circular cylinder aerofoil of this diagram can be used with end plates.
It is not necessary to this invention that only one slot on an under-surface of the aerodynamic surface be employed. As shown in this diagram, a first slot (32) and a second slot (33) may be formed in the upper and lower surfaces of the aerodynamic surface (34). The first slot permits a flow of air by means of a first plenum (35) distributing the air from a source of pressurised air. The second slot also permits a flow of air, but this flow is distributed by a second plenum (36). The flow of air into each plenum should preferably be separately controlled. This configuration of aerodynamic surface would likely use an un-cambered aerofoil.
This diagram shows the dual slotted aerodynamic surface described above supported by end plates. These end plates are more typical of an assembly for the rear of a ground vehicle where the end plates are used to support the aerodynamic surface.
In a further improvement to this invention, this diagram depicts the under-surface of a ground vehicle which contains a slot (67) along the under-surface of the ground vehicle and near its trailing edge (68). This slot near the rear surface of the ground vehicle ejects air to the under surface of the vehicle which will entrain the flow along the under-surface of the vehicle. Preferably, the trailing edge of the ground vehicle under-surface is rounded or curved upward. The blowing system depicted in this diagram comprises the same elements as the blowing system for the pneumatic wing assembly shown earlier.
This approach is directly applicable to improve the download produced by the body of a Formula-One or an Indy-type racing vehicle. The slot (67) can wrap around and extend all the way to the side of the ground racing vehicle. This essentially converts the entire body of the ground racing vehicle into a type of flat inverted aerofoil, but the blowing on the curved edges entrains the flow field beneath the car, creating inverted circulation, negative lift, and downforce on the vehicle. This is enabled by the surface's round trailing edge and blowing slot, since a conventional flat aerofoil will produce virtually no lift at zero angle of attack. This can also be used to control back dynamic pitching by rapid adjustment of the blowing along the rounded trailing edge.
In all of the forms of the present invention, blowing at pressures of slightly less than 14 psi can produce rapid, and substantial, force changes occurring at the speed of sound.