The G-Series Wind Tunnel Test Report
As the 1960's came to a close, the Ford and Chrysler camps were regularly battling for supremacy on the nation's stock car tracks. In the Summer of 1970, the NASCAR rules for the 1971 season were announced, abruptly signaling the end of the Aero Wars. Bill France, head of NASCAR, felt it was time to stem the tide of the exotic factory racing efforts. With a stroke of his rule making pen, France effectively legislated the present (and future) Ford and Chrysler aero warriors out of existence.
Because fielding winning teams in NASCAR was no small task, the factories maintained substantial research and development (R&D) departments, working a year or more in advance on major racing projects. France's drastic re-direction of the NASCAR rule book for 1971 not only made worthless thousands of hours of R&D by Chrysler, but may have tipped the scales in favor of Chrysler's eventual decision to officially withdrawal from the sport.
Among the many R&D casualties of France's poison pen were the results of a series of wind tunnel tests conducted in early 1970 by Chrysler's Research Office. These test results were presented in a report published in October of 1970, entitled Results of the Wind Tunnel Tests of the "G" Series Dodge and Plymouth, B-Bodies for the Development of Grand National Race Cars. This document offers a rare glimpse into what Chrysler might have put on the tracks in 1971, had Big Bill France exercised a bit more restraint. Portions of this 406 page report are showcased here.
The Report As Presented Here
To fully appreciate the wind tunnel study, the reader is required to possess much more than a passing familiarity with aerodynamic practices and concepts. Fortunately, the report's authors presented a significant amount of information, including summary and conclusions, in relatively non-technical language. In an effort to make this discussion much less lengthy and dry, as well as more comprehensible to the aerodynamics novice, the most intensely technical portions of the report (including almost 200 pages of graphs that only an aerodynamic engineer could appreciate) were omitted. Verbatim excerpts from the report are presented here in black text.
All photos in the study, including those not accessible by hyperlinks in the verbatim excerpts, are presented via links in the table found below. A complete listing of all figures in thumbnail format (model photos, graphs, etc.), is also available. Although most of the photos appearing in the wind tunnel test report were grouped in sets of two (facilitating illustration of common features and/or configurations), they were always assigned just one figure number. To avoid confusion and additional complexity in preparing this page, that format was adopted here also.
The author of this page has provided explanations for a number of terms which may be unfamiliar to the reader. These definitions are meant to present no more than brief, cursory discussions of certain concepts, so that "aerodynamically challenged" readers might find it a bit easier to wade through the more technical portions of the report.
And because the study included well over 350 individual black and white photographs, drawings, graphs and tables, the verbatim text found below is hyperlink intensive, almost to the point of being overwhelming. This of course was not the intent, but the wind tunnel study is information dense, and lends itself particularly well to the hypertext format.
Hyperlink Subject Matter
108 individual photographs were included in the report to illustrate and demonstrate various features and car configurations. In some cases, hyperlinks in the report's text point to photos that (1) only partially illustrate the features which are the subject of the text where the link appears, and/or (2) include features not germain to the discussion at hand. For example, the influence of various rear deck spoilers on Plymouth vehicle aerodynamics is discussed, but the only photos included in the test report were of rear deck spoilers mounted on the Dodge model. Since the report did not specify otherwise, it is assumed that both Dodge and Plymouth spoilers were essentially identical, and as such the Dodge photos were used to illustrate what the Plymouth rear spoiler configuration might look like. In any event, where photos and the text with which they are associated don't seem to quite be in sync, the photos presented were judged to be of some value in illustrating a portion of a feature, or combination of features, not exclusively pictured in the wind tunnel test study.
Drawings & Graphs:
These links are specifically associated with the word "Figure" followed by a number or number/lower case letter combination such as "Figure 177a". The figures are primarily drawings of various noses and wings tested (Figures 4 through 19) or graphical presentations of data - lots of data (Figures 167a through 181c). These links were included in the text in the interest of completeness, and will almost certainly be of much less interest than the photos (especially from Figure 167a on). Some of the figures were also rather difficult to read in the original copy of the report, and are no better here.
The Best Shapes For 1971?
After all of the 600 or so runs in the wind tunnel were said and done, what combination of features would Chrysler have fielded on its race cars in 1971, assuming the results of the wind tunnel test report were heeded? Given the conclusions in the report, the most likely basic configurations are shown below. Unfortunately, no single photo within the report captures any of the following sets of configurations on one vehicle model at one time.
Dodge Optimum Configuration
Plymouth Optimum Configurations
The wind tunnel test results pointed to three basic combinations that would produce optimum performance on the Plymouth.
From a practical standpoint however, a huge number of other factors would have probably come into play during the final design decision making process on the 1971 race cars. No doubt some sort of compromise between test results, real-world styling considerations and production costs would have resulted in racers somewhat different than that demanded by a strict following of the G-Series wind tunnel test report results.
Frank Moriarty, in his excellent book Supercars - The Story of the Dodge Charger Daytona and Plymouth Superbird, included a brief discussion of the G-Series wind tunnel tests. He was able to collect a number of facts that could not be culled simply from studying the report itself, such as the cost of the testing ($70,000+), and although no mention was made of it in the report, that a tri-winged configuration (referred to as "The Red Baron") was also tested! Moriarty's book also contains a number of model photos taken during testing - apparently the color versions of a few that appear in the report itself.
RESULTS OF THE WIND TUNNEL TESTS OF THE
"G" SERIES DODGE AND PLYMOUTH, B-BODIES FOR
THE DEVELOPMENT OF GRAND NATIONAL RACE CARS
R. G. Lajoie
G. F. Romberg
October - 1970
Wind tunnel tests were conducted during the months of January and March of 1970, in the Wichita State University 7' x 10' wind tunnel. Three-eighths (3/8) scale models of the 1971 Dodge Charger and Plymouth Road Runner cars were the test subjects. The initial test period was used to develop aerodynamic front end extensions, front undernose spoilers, backlights and various other items to improve the vehicle aerodynamics. The second test phase was utilized to develop rear stabilizer systems, a greenhouse reduced to minimum size, additional nose shapes and other subtle configuration changes.
This report presents all the data obtained from these tests and the results of the analysis. Data for the various parameters are compared with each other as well as with the "F" Series Dodge Charger Daytona data. The best aerodynamic configurations are pointed out for each body and greenhouse.
The objectives of the wind tunnel study were:
The wind tunnel tests were performed in two phases. Phase I testing started January 5, 1970 and continued to February 6, 1970. Phase II testing began March 2, 1970 and ended March 26, 1970. The 3/8 scale, fiberglass models were pitched through body rake angles of 0 to -3 degrees and yawed from -3 to +9 degrees. All testing was done at a freestream dynamic pressure of 40 psf (125 MPH).
The results of the "G" Series race car aerodynamics program are comprehensively compared to the "E" and "F" Series aerodynamic results.
The basic "G" and "H" Series B-body external shapes are identical with some aerodynamically insignificant exceptions. Therefore, all "G" Series, B-body test results and analysis also apply to "H" Series, B-body.
The tests utilized two 3/8 scale "G" Series models: A Dodge Charger, GW-23, and a Plymouth Road Runner, GR-23. The models were of laminated fiberglass construction which were formed in molds taken from 3/8 scale clay models.
The models were constructed to allow flexibility of test parameters. They consisted basically of three parts: Main Body, Underbody and Wheels.
The body was a high fidelity 3/8 scale aerodynamic reproduction of the actual car, but had limited detail in the grille and tail light areas. The front end of the body shell was made separable at two locations to accommodate several basic front end configurations. Further modifications were made on the Dodge model for the Phase II tests. The roof was made removable at the "belt line". Reinforcement and attachments were provided on the body to accept a clay plug for the minimum greenhouse investigation. In addition, side glass, on the standard greenhouse, was also made removable to determine window out effect on car performance.
The engine compartment was constructed to retain the same volumetric displacement as the actual race cars, thus simulating similar cooling air mass flow characteristics. However, due to design problems, the model's firewall was located aft of that on the full size car. Ducting was installed from the air inlet area to the forward face of the radiator for the 9 and 18-inch noses and aero front end simulating actual race car conditions. Ducting on the standard and minimum change nose started at the 45 inch line. All the inlet flow was ducted through the radiator. The duct has variable geometry to accommodate relative attitude changes between the body and underbody when the model was raked. A radiator was utilized that had similar pressure drop characteristics across the core as that on the full size race car. Limited detail wood engines were utilized to simulate the racing hemi and its volumetric displacement in the engine compartment. A scale fan was mounted in the wood block utilizing a bearing which allowed it to "free wheel" during the tests. Attempts were made to simulate engine compartment air venting by providing proper cut-outs throughout the engine compartment. However, only marginal geometric simulation could be accomplished.
Two underbodies were fabricated, one for each of the basic car models. The construction was also laminated fiberglass. Proper geometric simulation of the race car underbody was a basic requirement for aerodynamic similitude. Particular attention was directed toward exact simulation and location of the "K" member and other parts essential to the aerodynamics of the underbody. The underbody was also cut and hinged at the 12-inch side view line to provide body to underbody attitude simulation.
A set of scaled 9-inch race wheels were constructed for each model. They were made of plastic with balsa wood cores to reduce weight. The front wheels were mounted to brackets attached to an inner steel frame. The brackets were slotted allowing vertical height adjustment. At the rear, the wheels were also attached to brackets on the inner frame. An axle was utilized with a simulated differential and affixed to the wheels. Slots were provided in the wheel brackets to permit the body to move vertically relative to the front wheels and rear wheel-axle combination. All wheels were rigidly attached to the wind tunnel balance.
Car Attitude Adjustment
A steel inner frame was constructed to implement the rake angle (body attitude) changes required in the tests. The frame consisted of two interconnected longitudinal steel angles. The forward part of the frame was attached to the front part of the underbody, forward of the hinge line. The aft section was fixed to the aft portion of the body. Two bolts were attached, in a vertical plane, to plates that were mounted on the body in the dutchman area. These bolts screwed into nuts welded to the fixed inner frame. Access to the bolt heads was provided by body shell clearance holes located in the dutchman. These bolts were the mechanism to rake the body relative to the ground. The aft portion of the underbody was bolted securely to the body and the forward part of the underbody was secured to the frame. The body attitude change was achieved by turning the bolts, located at the dutchman. This allowed the body and aft section of the underbody to move in the pitch plane relative to the wheels and inner steel frame. The front portion of the underbody, forward of the hinge line, remained a constant distance above the ground (minimum race clearance of 6.5" full scale) for all body attitudes.
Spoilers were fabricated of sheetmetal and fastened to the various front ends with screws and tape. In all configurations, Position "A" was located at the aft edge of the air inlet opening.
Phase I Tests:
Both the Dodge and the Plymouth models were constructed to accept various front ends. The basic models were cut at the 45-inch side view line and at the cowl area. This provided the flexibility of investigating nose shape changes as well as complete front end modifications, i.e., the aero front end.
Phase II Tests:
Pedestals were constructed for each model to accept the various vertical-horizontal stabilizers systems. This made it possible to adapt the same sets of stabilizers on each model.
The greenhouse on the Dodge model was removable. This feature provided the flexibility of installing various greenhouse configurations and allowed minimum tunnel "down" time. A clay greenhouse "buck" was utilized for the various major roof changes investigated.
Phase I Model Description
Four front end shapes (from the 45" S. V. line fwd.) and one complete front end configuration change, from the cowl forward, was investigated for both the Dodge and Plymouth. The front ends tested were the standard, minimum change, 9-inch extension, 18-inch extension as well as a completely modified aerodynamic front end (Figures 4 and 5). In addition, a 12-inch nose was also tested on the Plymouth model only. The various noses and front ends were developed for their optimum performance, utilizing various combinations of front undernose spoilers (Figures 6, 7, 8, 9, 10, 11 and 12) and air inlet size and locations. The standard and aerodynamic front ends were constructed of fiberglass and attached by means of steel bulkheads and bolts. All the other noses were made of clay to allow for ease of modification.
Various backlights were investigated on both the Dodge and Plymouth models. In addition to the standard backlights, clay was used to define and test minimum change, semi-fastbacks, and full fastback backlights. Figure 13 presents the three backlight modifications tested on the Dodge model. Tests of the Plymouth model, however, were limited to only two backlight modifications, a semi-fastback and a rev. 1 semi-fastback, as shown in Figure 4.
Phase II Model Description
The Dodge model was reworked for the Phase II tests, with a removable fiberglass greenhouse. A clay roof "buck" was provided for studies dealing with a minimum envelope greenhouse (Figure 15). In addition, the standard greenhouse included removable sideglass to investigate a side window out configuration. Two Daytona type noses were evaluated. The tests include a 13.3-inch and an 18-inch nose extension. The effect of aerodynamic fairings on the underbody was evaluated by fitting the Plymouth model with belly pans. Three belly pans were tested; (1) from the "K" member to the hinge point on the model, (2) the second was from the hinge point on the model to the forward part of the rear wheel axle, (3) the final configuration was from the "K" member to the aft part of the rear axle.
Three rear deck stabilizer systems were evaluated utilizing both models. A 10-inch chord (full scale) single and bi-wing configuration (Figure 16) and a 19-inch chord (full scale) system was used (Figure 17).
The horizontal stabilizer section was a Clark-Y airfoil section. Section characteristics for both the 10- and 19-inch chord stabilizers are shown in Figure 18. An N.A.C.A. 0012 airfoil section was used for the vertical stabilizers, and the section characteristics are presented in Figure 19.
SUMMARY OF RESULTS
The "E" Series Daytona 500 is, generally, a better aerodynamic configuration than either "G" Series configurations. However, the "E" Series car front lift is significantly higher (450 pounds) and the directional stability is lower (center of pressure is 2 inches further forward) than the "G" Series car.
Phase I - Nose Study:
Development tests were conducted for the minimum change, 9-inch, 18-inch and aero front end in addition to the basic car. The model with the 18-inch nose or the aero front end, without front undernose spoilers, were found to have the best aerodynamic characteristics as shown in Figures 169a, 169b and 169c. The 18-inch nose configuration has the lowest axial force of all the nose configurations but has a high down force at the front wheels that is undesirable. On the other hand, the aero front end has good front and rear wheel lift characteristics but the axial force is slightly greater than the 18-inch nose data. Lateral stability of the aero front end is better than that of the 18-inch nose, CP is about 4.6-inches further aft. From an overall aerodynamic standpoint, the aero front end without a front spoiler is the best configuration.
Phase II - Nose Effect Summary - Minimum Greenhouse:
The 18-inch nose and aero front end were found to be the best configurations by the Phase I tests and were, therefore, the only Phase I noses tested in Phase II. Two new noses were evaluated during the Phase II tests, the 13-inch and 18-inch Daytona noses. A nose evaluation was made with the minimum and standard greenhouses as shown in Figures 170a, 170b, 171a and 171b. Once again the 18-inch nose configuration, in this case, with a front undernose spoiler, exhibited the lowest axial force. The 18-inch Daytona nose was next best for low axial forces.
Lift characteristics of the 18-inch nose configuration are not desirable, i.e., there is a large down force at the front wheels and high lift at the rear wheels. Lift characteristics of the vehicle with the 18-inch Daytona nose are more desirable than that attained by the 18-inch nose configuration.
Front Spoiler Effects:
The effect that front undernose spoilers have on the vehicle aerodynamic forces have been evaluated for various spoiler positions, nose, backlight and greenhouse configurations as shown in Figures 172, 173a, 173b, 173c, 174a, 174b, 174c, 175a and 175b. Most of the nose configurations, except the minimum change and 13-inch Daytona, indicate the axial force reduction due to the spoiler is greatest at zero degrees and diminish when the rake angle is increased from zero to -3 degrees.
Variation of the spoiler axial location affects the front and rear wheel lift primarily. The forward most position (A) yields the greatest reduction of front wheel lift and increase of rear wheel lift. Moving the spoiler aft from position A to position B causes almost a 50% decrease of the front and rear wheel lift increments with virtually no change in axial force. An evaluation was made of the effect the backlight configuration has on spoiler effectiveness. The results of the evaluation for the 18-inch nose and aero front end configurations are presented in Figures 174a, 174b and 174c. These data indicates the spoiler effectiveness is independent of the backlight configuration with the exception of B=0 degrees. At B=0 degrees, the minimum, semi and full fastbacks exhibit the higher spoiler effectiveness.
The greenhouse effect on spoiler effectiveness was also evaluated (Figures 175a and 175b). The axial force increment is most sensitive to the greenhouse configuration. Max. CX reduction is achieved with the minimum greenhouse and minimum CX with the standard. The only change of front wheel lift occurs for the minimum greenhouse. Rear wheel lift slope relative to rake angle changes with the greenhouse configuration.
Comparison of the Best "G" Series Race Cars with the Daytona:
The best "G" Series Dodge race cars are compared with the "F" Series Daytona in Figures 176, 177a, 177b and 177c. Consideration was made for the standard and minimum greenhouses plus side glass removal on the minimum greenhouse. From an overall aerodynamic standpoint, the 18-inch nose with spoiler and minimum greenhouse (side glass out) configuration is best. If the minimum greenhouse configuration cannot be considered, then the aero front end on the standard greenhouse with semi-fastback (side glass out) is the best configuration. It is interesting to note that the "G" Series car indicates an additional axial force margin over the "F" Series Daytona when the side glass is removed. This is attributed to the fact that the Daytona side glass is closer to being flush than the "G" Series side glass, therefore, removing the side glass on the Daytona increases the axial force much more than it does on the "G" Series cars.
Comparison of Standard "G" Series Race Car with "E" Series Race:
The "G" Series Plymouth with either a standard or minimum change front end and front undernose spoiler have axial forces greater than the "E" Series race car without an undernose spoiler as shown in Figures 178a and 178b. Front lift of the "G" Series cars is about 350 pounds lower than the "E" Series car and considered an improvement. The "E" Series front wheel lift is high because of the absence of an undernose spoiler. The rear lift of the "G" Series cars is 300 pounds greater than the "E" Series which results in a degradation of the handling characteristics.
Phase I - Nose Study:
Five nose configurations were tested on the Plymouth during the Phase I test, the minimum, 9-inch, 12-inch, 18-inch and aero front end. The 12-inch nose with the undernose inlet has the best overall aerodynamic characteristics of all the noses tested on the Plymouth (Figures 179a, 179b and 179c). Rear wheel lift is greater than desired but this can be over come with a rear deck spoiler. All the noses tested reduced axial force below the standard car values.
Addition of a front undernose spoiler affects both axial force and lift as shown in Figure 180. Axial force coefficient can be reduced as much as .046 by adding a spoiler to the 18-inch nose at zero rake angle. For this configuration and set-up, the front lift is reduced 650 pounds and the rear lift increased 370 pounds. Spoilers are most effective when the body is at zero rake. As rake angle is increased, the spoiler has less of an effect on the aerodynamic forces. The "F" Series Daytona spoiler is more effective for axial force reduction than any of the "G" Series spoilers but the lift changes are within the same order of magnitude.
Comparison of Best "G" Series Plymouths with the Daytona:
Two configurations emerge from the configurations tested which are better than the others; the aero front end and 12-inch nose both having 45 square inch air inlets, flush glass and semi-fastbacks. Data for these configurations are compared with the Daytona data in Figures 181a, 181b and 181c. Data points are not shown for the aero front end configuration because the measured data had to be adjusted for an air inlet area change. The 12-inch nose exhibits the lowest axial forces, but the aero front end has slightly better lift characteristics. Neither of these "G" Series configurations is as good as the Dodge Charger Daytona.
The following conclusions are made from the analysis of the "G" Series Dodge and Plymouth B-body test data presented in this report.