Transonic Aerodynamic Testing
Aerodynamics engineers, including stability and control designers, were baffled in their attempts to get reliable wind-tunnel measurements at transonic speeds, near a Mach number of 1.0. High-speed wind tunnels suffered from the choking phenomenon, in which normal shocks originating on models under test spread across the test section as speed was increased, preventing further increases.
W. Hewitt Phillips credits Robert R. Gilruth with the invention of one method to circumvent this problem, the wing flow method. Figure 11.6 shows how small wing or complete configuation models are mounted normal to the wing upper surface of an airplane, in a
Figure 11.6 A sweptback-wing half-model mounted on the upper wing surface of a North American P-51, for wing flow testing during dives. The model is transonic, while the airplane is not. (From Phillips, Jour, off theAmer. Avia. Histor. Soc., 1992)
region where the local Mach number is much higher than the airplane’s flight speed. Phillips describes the method as follows:
A special glove is built on the wing to give a more uniform flow region. As the airplane [P-51 Mustang] goes through its dive and pullout, the model is oscillated back and forth at a frequency of about one cycle per second, to vary either the angle of attack or flap deflection. The forces on the model are continuously recorded with a strain gage balance and a recording oscillograph. The dive lasts about 30 seconds and in this period the Mach number at the model increases from about 0.7 to 1.2… .A vacuum-operated windshield wiper motor was usually used to oscillate the model (Phillips, 1992).
The wing flow method and data from small drop models were both effectively obso- leted with the invention of the porous or slotted-throat transonic wind tunnel by Ray H. Wright, of the NACA Langley laboratory, around 1948. The slotted-throat wind tunnel allows measurements to be made through a Mach number of 1.0.