NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 'J'J 



angle of incidence of 9 degrees, however, this stalhng tendency dis- 

 appears. This is a good feature, as it shows that the wing should 

 not get into a bad stalling position when side slipping. The dihedral 

 angle seems in this respect superior to the swept back wing, which 

 has a stalling tendency at all incidences. 



The curves for rolling moments, L, show for all incidences a roll- 

 ing moment which increases rapidly with angle of yaw and with the 

 incidence. This moment is a natural banking moment, and hence is 

 one which is favorable for lateral stability. Comparing the rolling 

 moments curve at 6 degrees incidence, for a normal wing, with the 

 dihedral curve, it is seen that the magnitude is from two to three times 

 greater in the case of the dihedral. Comparing this same dihedral 

 rolling moment curve with those for the swept back wings (fig. 

 29), it is seen that the dihedral gives a rolling moment of magnitude 

 about equal to that obtained with 20 degrees swept back wings, 

 except at a large angle of yaw. 



As it is much more difificult structurally to build a 20-degree swept 

 back wing than a dihedral, and as the latter is as effective, it seems 

 that the dihedral is of more value for purposes of lateral stability. 



It is of interest to note in this connection that Professor Langley's 

 " aerodrome " of 1902, as well as his previous power-driven models, 

 were given dihedral angle wings inclined upwards by about 6 degrees. 



X. CRITICAL SPEEDS FOR ELAT DISKS IN A 

 NORMAL WIND 



By J. C. HUNSAKER 

 I. Prefatory Note on Normal Elow Past a Circular Disk 



By E. R. WILSON, Professor of Mathematics, Massachusetts Institute 

 OF Technology 



Theoretical hydrodynamics cannot yet give a very satisfactory 

 quantitative account of Mr. Hunsaker's experiments to which this 

 note is attached. The classical theory treats two cases which are 

 qualitatively somewhat like nature : Eirst, symmetric continuous 

 flow; second, asymmetric discontinuous flow. In both cases the flow 

 is irrotational and the fluid incompressible. In the first case, owing 

 to the symmetry of the lines of force, there is theoretically no result- 

 ant pressure to urge the disk down stream. Such a condition may 

 possibly approximate to nature in cases of slow flow under high 

 pressures. In the second case, as the method of solution depends on 



