440
P. Liu
necessary to study the lightweight and efficient propeller system with low
forward ratio, low Re, and low Ma. At first glance, such problems need
to be studied in low (variable) density wind tunnels, but since the Ma is
low in incompressible flow and only Re is similar, it is worth discussing
whether such wind tunnels are still needed.
Fig. 6.70 Stratospheric airship
Fig. 6.71 Curve of wind speed and height