STYLING AND AERODYNAMICS

BY V. M. EXN E R

CHRYSLER CORPORATION

For presentation at the DETROIT SECTION, S.A.E. GREENBRIER MEETING White Sulphur Springs, West Virginia September 14, 1957

SOCIETY OF AUTOMOTIVE ENGINEERS, 100 FARNSWORTH AVE. DETROIT, S, MICHIGAN


This paper will discuss wind resistance and stability in cross winds as they relate generally to automobile styling and performance. In particular, I should like to explain how aerodynamic considerations influenced design of our current production cars and how the DART experimental car shape evolved from further application of these air flow factors.

Any vehicle moving at speeds more than 30 miles an hour must contend with problems created by the air through which it is traveling. Two aerodynamic effects are involved -which are pertinent to this discussion. One is the resistance of the air to the passage of the vehicle which is known as aerodynamic drag. The other is the effect cross winds have on the stability of the vehicle's movement its ability to travel straight along an intended path.

Since the function of an automobile is to move through the air, the sensible thing to do is to make this passage as easy as possible. This even be- comes logical and desirable when it is considered that the less air a car disturbs, the smoother and quieter will be its movement which adds to passenger comfort. Also, less air resistance means a reduction in required force which conserves energy, thereby economizing on fuel. In addition, good stability in cross winds eliminates some of the need for steering corrections by the operator. This reduces driver fatigue, thus helping improve conditions for safer motoring.

But what is sensible and logical sometimes becomes less desirable when other human factors and motivations are involved. This condition is partly brought about by tradition what we are accustomed to and by unexpected and largely unexplainable changes in public tastes. For example, what was accepted as stylish and the "last word" in automobile performance in 1900 looks quaint and in adequate in 1957; just as today's vastly improved motor cars will be old-fashioned in the year 2000.

To better understand the part that popular acceptance plays in automobile designing, it might be well to briefly consider the early American vehicles and realize how their influence has affected modern cars.

Figure 1 (Shown in PDF)

Like present-day automobile manufacturers, the early stage coach builders took great care to analyze the uses and abuses to which their vehicles would be subjected. These vehicles (Figure 1) were used to transport a number of people and their baggage over the rough paths which spanned a vast and growing nation. As a result, the stage coaches were sturdily built with special attention given to passenger comfort and large luggage space. Abundant motive power was also a prime factor in the early horse-drawn vehicles. The long distance to be covered, the difficult terrain, and the necessity of accomplishing the voyage with- in the shortest possible time required the use of several horses. These teams of horses supplied a reserve of power beyond what was really needed to pull the heavy coaches.

When American cars first appeared on the scene, much of the difficult traveling conditions experienced by stagecoaches still prevailed and adequate motive power was needed. While tremendous improvement of our highway system has been made, the long distances to be traveled still require plenty of sturdy, lasting power. Therefore, over a period of many years, the American motorist has come to expect comparatively large and roomy vehicles as well as more than adequate power to propel them. Specifically, he wants a powerful, four wheel, six passenger car with side by side seating, front and rear. There is some indication many American motorists now are considering four passenger cars with the same seating arrangement.

All of this has had a very definite influence on American motor vehicles in so far as chassis design and body styling are concerned. The wide body needed to seat three people abreast obviously creates wind resistance problems. Some other pre determined factors which add to these problems include the necessity of providing for a front air intake in order to cool the engine, thereby creating wind drag; designing windshields and other glass areas so as to reduce wind resistance, yet providing good visibility; and covering wheel housings for better aerodynamics, thereby making it more difficult to cool the brakes. Because of the complex interplay of so many interrelated parts in an automobile, the solution of one problem may create several new ones. Perhaps the importance of automobile aerodynamics may be better appreciated when it is realized that more of the net engine power is required to overcome aerodynamic losses than all other losses combined at speeds over 50 miles an hour (Figure 2). So, unlike the airplane designer who has sleek aero dynamic shapes to work with, the automobile designer is given a bulky package which he tries to

streamline while maintaining the over-all shape people presently associate with automobiles.

THE IMPORTANCE Of CAR AERODYNAMICS

EFFECT OF AIR DRAG ON POWER REQUIREMENT A MAJOR FACTOR IN ANY CAR SPEED M P H

Figure 2 (Shown in PDF)

The motor car of today, pictured beside the gasoline buggy of 1897, reveals an amazing metamorphosis (Figure 3 shown in PDF). Aside from the fact that each has four wheels, an engine, and a seat, there is no resemblance between the two. Yet, the latter obviously could not have been possible without the former. Strangely enough, the major evolutionary styling processes between these two vehicles have been few in number.

Manufacturers of the early gasoline carriage gave no thought to the effects of drag and cross winds on a motor car's movement. They were not greatly concerned with the external design or style of the vehicle. They worried only about its ability to get the driver and his passengers from one place to another under its own power. Thus, the carriage body sufficed.

From 1910 to 1930, manufacturers gave serious attention not only to mechanical innovation and improvements, but, also, in an ever increasing degree, to styling. The element of style was beginning to emerge as a potential sales factor. During this era, the classic principles of proportion, decorum and symmetry were regarded as the cardinal virtues of motor car styling (Figure 4 Shown in PDF). However, little thought was given to streamlining the body so as to reduce wind resistance and increase stability.

1924 CHRYSLER
Figure 4 (Shown in PDF)

In the 30'9, automobile styling shifted emphasis from lines to surfaces and highlights. Pseudo streamlining gradually began to replace symmetry. Slopping windshields and fender "skirts "were seen for the first time, heralding more ambitious and scientific efforts at streamlining. Centers of gravity were lowered, permitting more streamlined shapes and improving appearance as well as safety through better road-holding ability.

Progress was slow in the 40's and early 50's because streamlining became largely a styling device connected only by chance to function. For the most part, style was dictated by what looked good--in terms of popular conceptions of automobile stream- lining--rather than by scientific considerations of wind resistance and vehicle stability problems. As a matter of fact, the 1934 Chrysler Airflow (Figure 5), whose shape resulted in part from a whole new concept in passenger weight distribution, had a lower wind resistance than many of the 1940 and early 1950 cars which were styled purposely for their so called streamline appearance.

The important point, however, is that in the 1940's the public began to accept streamlined automobiles and to appreciate the improvements in safety, com- fort, economy and appearance that result when wind resistance is decreased and stability improved.

It seemed to us at Chrysler that the next logical step was to put scientific fact behind streamlining; to accurately measure wind resistance and to adapt the most aerodynamic shapes to automobile design. We hoped to create a functional and central styling theme which would lend itself to various adaptations, like the wide number of arrangements that can be made on a basic melody,

Accordingly, two major research projects were carried on simultaneously some 4,000 miles apart.

In Detroit, we knew, of course, that the teardrop shape (Figure 6) has the least wind resistance.

Figure 6 (Shown in PDF)

But, we are not concerned about a vehicle that moves up in the air. Ours must travel along the ground. When applying this shape to ground movement it is necessary to flatten the tear drop at the bottom (Figure 7 Shown in PDF) to provide a level floor for passengers and to accommodate components of the
Figure 7

vehicle. It is also necessary to provide a compartment for a front-mounted engine because of the weight distribution problems peculiar to this country's automobiles. Thus, our tear drop now has grown a snout to house the engine and a hump to enlarge the central passenger compartment (Figure 8). The rear end is blunted to keep the
Figure 8

length within reasonable bounds. From the wind resistance standpoint, this shape is good. However, its stability in cross winds is poor. The only way to stabilize the car without destroying its good wind resistance properties is to use fins. This results in a dart or wedge silhouette. This shape (Figure 9 Shown in PDF) is used for aircraft, racing boats and cars and missiles for the same reasons. Airplane and boat designers evolved this shape from information obtained in wind tunnel research.

(Figure 9 Shown in PDF)

Since the dart or wedge shape was so common to airplanes and boats, it was familiar to the public. So, this contemporary shape was adapted to our 1955 Chrysler Corporation cars. We refined and, in our opinion, greatly accented this basic shape in 1956 and 1957.

Tests on these dart, or wedge, shaped cars were conducted in the University of Detroit's wind tunnel. In some of these tests, a specially constructed 3/8 scale plastic model car weighing 500 pounds was used (Figure 10). The model car's wheels rotated. Its miniature cooling system included an operating fan. The undercarriage was duplicated in minute detail so that the scale model was a faithful replica of our proposed 1957 production model.

The scale model, equipped with detachable fins of various sizes for comparison purposes, was sup- ported in the wind tunnel by a shaft connected to the tunnel's balance frame. By means of mechanical and electrical devices, it was possible to measure controlled amounts of wind forces and compute their effect on the car. A turntable placed the model in any desired position in the wind tunnel in which velocities up to 160 miles an hour could be generated. In addition to measuring effects of wind force on the car, other tests included the compilation of wind pressure data from 100 tiny holes set in all surfaces of the car and the use of explain as simply as I can how this vehicle stability is brought about. For aerodynamic reasons, most of the force of a side wind acts on the front portion of a finless car tending to make it veer from its course (Figure 12). When fins are added, a larger surface is presented at the rear of the car directing the side wind so that its force is better balanced about the car's center of gravity which acts as the pivot point. Thus, the addition of rear fins equalizes the forces about the pivot resulting in less tendency of the wind to turn the car off its course (Figure 13). By aerodynamic design, then, the side wind itself is made to compensate for its own ill effects. This relieves the driver of some of the steering effort required to keep the car within its highway lane when gusty conditions prevail.

THE IMPORTANCE OF CAR AERODYNAMICS

Figure13 (Shown in PDF)

While this research activity was being carried on in Detroit, we selected Carrozzeria Ghia, of Turin, Italy one of Europe's greatest designers and one of the best body builders in the world to create a car around the aerodynamic facts revealed in wind tunnel tests. Chrysler specified the basic dimensions of the car and certain styling features. It was to be a full size, four-passenger sedan, with a 129-inch wheelbase, 80 inches wide and 54 inches high. We also supplied advanced engineering innovations for the chassis, engine and other parts of the car. Except for these considerations, the shape was to be determined solely by the form which showed the least possible air disturbance in aerodynamic research.

Under the direction of Dr. Giovanni Savannuzzi, designer of the wind tunnel at the University of Turin and chief engineer of the Ghia Body Company, a one-fifth size plastic model was covered with horizontal and vertical lines, similar to those on a piece of graph paper. This model was then placed in the wind tunnel.

(Figure 14 Shown in PDF)

Drops of ink were placed at various points on the body surface. Winds were then developed, up to speeds as high as 200 miles an hour. The ink blots resulting from this wind velocity traced the path and force of the air over the model's surface Analysis of these measurements led to an aerodynamically-styled body around which the air streamed smoothly. That is how the Dart was created!

(Figure 15 Shown in PDF)

The Dart is one of the most nearly perfect aerodynamic passenger car designs in the world today (Figure 16). Its trim appearance should not lead anyone to think it is a small car. It is built on a 129-inch wheelbase, the same as the Imperial. It has full-wrap, rubber-mounted bumpers. These provide complete protection around the car body, yet blend into the car so aerodynamically they seem a part of the car body. Brake cooling is improved through finned wheel covers which induce air flow over the brake drum surfaces at a rate of 80 cubic feet per minute at 40 miles an hour.

(Figure 16 Shown in PDF)

Fins are an essential and integral part of the Dart's shape The wind tunnel tests in Italy confirmed our Detroit wind tunnel studies that the tail fins are functional units, minimizing wind wander of the vehicle at normal driving speeds.

(Figure 17 Shown in PDF)

We do not consider the Dart a show car. It is literally a laboratory on wheels. It has undergone rigorous tests in competition with other cars at Chrysler Corporation's Engineering Proving Grounds at Chelsea, Michigan.

In some of these tests, our object was to compare the wind resistances of the Dart shape with those of other auto body types. We attempted to minimize the effects of those chassis components which were not comparable. Thus, propeller shafts were disconnected and tires were inflated to 60 pounds. Other factors, like the inertia of the wheels, were taken into account so that the calculations would be as accurate as possible. In some of the tests, the cars were towed to speeds up to 100 miles an hour and then cut loose. Their rates of deceleration were measured. From these figures, our engineers could determine the force required to propel the cars at specified speeds. It was found that in competition with other cars, the Dart required less force to combat wind resistance. Translated into terms of how aerodynamic styling affects performance, that means improved acceleration and reduced fuel consumption. It is especially significant, I think, that the improvement in fuel economy extends through the normal driving range speeds, as well as at high speeds.

Our styling approach, then, was based on sound aerodynamic principles which were verified by wind tunnel and actual performance tests. We are continuing these tests to learn ways of further improving appearance; to obtain more fuel economy and better performance from a given size engine by reducing the force necessary to penetrate the air; and to reduce driver fatigue, thereby increasing safety, by controlling wind wander and improving stability through more functional design.

The work already done confirms the value of using as many aerodynamic facts as possible as guides to contemporary styling concepts. Here is another important tool which stylists and engineers can use to create better performing and better appearing cars. However, it is hoped that no one will get the impression from what precedes that the Dart is a preview of what our cars are going to look like in the future. Its shape suggests one way of reducing wind resistance. Our task is to design an automobile whose appearance will be admired by the public and whose performance will be aided by its shape. After all, the true measure of a product's design worth is public acceptance. Perhaps the Dart will help us a little in this respect.

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