The results are presented in the standard graphic form (figs. 4 to 80) as coefficients corrected after the method of reference 8 to give airfoil characteristics for infinite aspect ratio and aspect ratio 6. Where more than one test has been used for the analysis, the infinite aspect ratio characteristics from the earlier test have been indicated by additional points on the figure. Table I gives the important characteristics of all the airfoils.
To view the images in full size go to the desired image and click on it. Use your browser's back button to return to the list.
4 - 5 |
6 - 7 |
8 - 9 |
10 - 11 |
12 - 13 |
14 - 15 |
16 - 17 |
18 - 19 |
20 - 21 |
22 - 23 |
24 - 25 |
26 - 27 |
28 - 29 |
30 - 31 |
32 - 33 |
34 - 35 |
36 - 37 |
38 - 39 |
40 - 41 |
42 - 43 |
44 - 45 |
46 - 47 |
48 - 49 |
50 - 51 |
52 - 53 |
54 - 55 |
56 - 57 |
58 - 59 |
60 - 61 |
62 - 63 |
64 - 65 |
66 - 67 |
68 - 69 |
70 - 71 |
72 - 73 |
74 - 75 |
76 - 77 |
78 - 79 |
80
PRECISION
A general discussion of the errors and corrections involved in airfoil testing in the variable-density tunnel is included in reference 8. In connection with this report, it was hoped that a more specific discussion of the various sources of error and separate estimates of the various errors might be given. However, after a careful study of all the measurements it became apparent that practically all the errors may be regarded as accidental; that is, of the type the magnitude of which may best be estimated from the dispersion of the results of independent repeat measurements. The major portion of these errors is caused by insufficient sensitivity of the balance and manometers, by the personal error involved in reading mean values of slightly fluctuating quantities, and by the error due to slight surface imperfections in the model. The last is perhaps the most serious source of error. The models were carefully finished before each test, but the presence of particles of hard foreign matter in the air stream tended to cause a slight pitting of the leading edge of the model during each test. This pitting was probably the major source of error in connection with the earlier tests, but it was reduced for the later tests when the necessity of a more careful inspection of each model was appreciated. After a considerable period of running the particles in the tunnel were found to become lodged, permitting this source of error to be largely eliminated during the later tests. For this report, however, the effect of the error from this source has been minimized by repeating the tests of many of the airfoils, including all of the symmetrical series originally reported in reference 2.
The magnitude of all such accidental errors was judged from the results of repeat tests of many airfoils, and from the results of approximately 25 tests of one airfoil that were made periodically throughout the investigation to check the consistency of the measurements. The accidental errors in the results presented in this report are believed to be within the limits indicated in the following table:
In addition to the consideration of the accidental errors, all measurements were carefully analyzed to consider possible sources of errors of the type that would not be apparent from the dispersion of the results of repeat tests. A rather large (approximately 1.5 percent) error of this type is present in all the airvelocity measurements resulting from a reduction in the apparent weight of the manometer liquid when the density of the air in the tunnel is raised to that corresponding to a pressure of 20 atmospheres. The effects of this error, however, are reduced by the presence of another error in the air velocity measurements due to the blocking effects of the model in the tunnel. The measured coefficients, obtained by dividing the measured forces by 1/2PV2 as well as the derived coefficients are, of course, affected by errors in the airvelocity measurement. Aside from this source of error, it is believed that only two other sources need be considered: first, the deflection of the model and supports under the air load; and second, the interference of the airfoil supports on the airfoil. The angle of attack and the moment coefficient are affected by the deflection of the airfoil and supports. The error in angle of attack, which is proportional to Cmc/4 was found to be approximately —0.1° for an airfoil
having a moment coefficient of—0.075. The error from this source in the moment coefficient is inappreciable at zero lift, but at a lift coefficient of 1 may amount to —0.001. The errors resulting from the support interference are more difficult to evaluate, but tests of airfoils with different support arrangements lead to the belief that they are within the limits indicated in the following table:
The tunnel-wall and induced-drag corrections applied to obtain the airfoil section characteristics might also be treated as sources of systematic errors. Such errors need not be considered, however, if the section characteristics are defined as the measured character-istics with certain calculated corrections applied. Errors in the tunnel-wall corrections, however, should be considered when the results from different wind tunnels are compared. For consideration of these errors, the reader is referred to references 9 and 10.
For the purpose of comparing the results from different wind tunnels and of applying these results to airplanes in Eight, it is also necessary to consider the effects of airstream turbulence. In air streams having different degrees of turbulence, the value of the Reynolds Number cannot be considered as a sufficient measure of the effective dynamic scale of the flow. The airfoil characteristics presented in this report were obtained at a value of the Reynolds Number of approximately 3,000,000, which corresponds roughly to the Reynolds Number attained in flight by a medium sized airplane flying near its stalling speed. Consideration of the effects of the turbulence present in the variable-density tunnel (see references 11 and 12) leads, however, to the belief that these results are more nearly directly applicable to the characteristics that would be obtained in flight at larger values of the Reynolds Number.
Table of Contents | Summary | Introduction | Description of Airfoils | Apparatus and Methods | Results | Discussion | Supplementary Airfoils | Conclusions | Appendix | References