The corrections found were applied to the readings obtained with the model at various angles of attack and at various pressures. The velocity of the air flow is approximately the same in all tests, with an average value of about 76 feet per second.
RESULTS OF THE TESTS
The results of the tests are given in the tables and diagrams of this report. Table XXXVl gives the exact shape of the wing sections tested, the ordinates being obtained by measuring the finished model. It is worth mentioning in this connection that in several cases no strict specification of the section exists, as, for instance, several tables of ordinates for the R. A. 35 published at different times differ from each other. In such cases the ordinates in general
use have been adopted.
Tables I to XXXV give the lift coefficient, drag coefficient, and some of them the moment coefficient for a series of angles of attack, each table for an approximately constant dynamic pressure. The Lift coefficient Cl, and drag coefficient Cd are obtained by dividing the measured lift L and the corrected drag D by the wing area S and by the dynamic pressure q.
where
The velocity is always about 76 feet per second. The moment coefficient & is obtained from the moment measured with respect to the point on the chord one-quarter chord length from the leading edge, by dividing it by the wing area, by the chord c and by the dynamic pressure q,
C, is positive if it tends to increase the angle of attack; that is, to lift the leading edge up and turn the trailing edge down. The average Reynolds N-umber for each test is also inserted. The Reynolds Number is computed with reference to the chord as characteristic length of the model.
The results are illustrated by diagrams in the form of the so-called polar curves. There are two diagrams for each section. The one refers to the test at about 20-atmosphere tank pressure, which can be considered as equivalent to a full-size test for an airplane of moderate size. This diagram contains a profile of the section. The Lift coefficient is plotted vertically, and against
it to the right are plotted the induced drag coefficient (giving a parabola), the observed drag coefficient (giving the polar curve proper), and the negative moment coefficient.
In a second diagram al1 polar curves of one section observed at different pressures, and hence at different Reynolds Numbers, are drawn side by side. The parabola of induced drag is inserted again.
It appears that the polar curves obtained at different Reynolds Numbers differ appreciably from each other, particularly with the thicker sections. The drag coefficient has generally the tendency to decrease with increase of Reynolds Number, while the lift is not very much affected except in the neighborhood of its maximum.
Table XXXVII has been prepared by Lieut. Walter S. Diehl to supply a condensed summary of the test data in the form commonly used for study of comparative tests. It should be noted that these data are not comparative with tests conducted on other models in other wind tunnels unless appropriate corrections are applied.
CONCLUSIONS
While only conclusions of the most general nature can be safely drawn from these tests, there are certain outstanding features, as follows:
(I) At any given lift coefficient the drag coefficient has a tendency to decrease as the Reynolds Number is increased.
(2) The greater the value of minimum drag coefficient at one atmosphere the greater the decrease in minimum drag with increase in Reynolds Number. This is true whether the decrease be taken as an absolute value or as a ratio. In this connection Lieut. Falter S. Diehl points out that the absolute decrease in minimum drag coefficient in passing from 1 to 20 atmospheres for the seven airfoils of this report varies approximately as the cube of the minimum drag coefficient at I atmosphere. This relation is probably accidental and is merely cited to show the general nature of the variation.
(3) Except in the neighborhood of the maximum, the lift coefficient does not appear to be influenced by change in Reynolds Number.
(4) The maximum lift coefficient is very much affected by change in Reynolds Number. The effect on maximum lift coefficient is quite erratic, and in passing from 1 atmosphere to 20 atmospheres varies from an increase of 14 per cent to a decrease of 23.4 per cent.
Since there is no known reason for doubting that the variations obtained in airfoil characteristics at the higher pressures are a direct effect of the variation in Reynolds Number, it is believed that the aero dynamical properties obtained from tests made in the variable density wind tunnel at full scale values of the Reynolds Number are more reliable and more directly applicable to design than similar data obtained in the ordinary atmospheric wind tunnels at the usual ton variables of Reynolds Number. This conclusion is supported by the fact that at a pressure of 1 atmosphere the variable density wind tunnel gives results which are in very good agreement with those obtained in the conventional atmospheric wind tunnels.