Complete induced-velocity curve
It is of interest to know how the induced velocity varies through all the phases of axial flight. For the vortex-ring and turbulent-wake states, where momentum theory fails, information has been obtained from measurements in flight, supported by wind tunnel tests (Gustafson (1945), Gessow (1948), Brotherhood (1949), Castles and Gray (1951) and others). Obviously the making of flight tests (measuring essentially the rate of descent and control angles) is both difficult and hazardous, especially where the vortex-ring state is prominent, and not surprisingly the results show some variation: nevertheless the main trend has been ascertained and what is effectively a universal induced-velocity curve can be defined. This is shown in Fig. 2.10, using the simple momentum-theory results of Equations (2.10) and (2.13) in the regions to which they apply. We see that moving from hover into descent the induced velocity increases more rapidly than momentum theory would indicate. The value rises, in the vortex-ring state, to about twice the hover value, then falls steeply to about the hover value at entry to the windmill-brake state.
It is of interest to know how the induced velocity varies through all the phases of axial flight. For the vortex-ring and turbulent-wake states, where momentum theory fails, information has been obtained from measurements in flight, supported by wind tunnel tests (Gustafson (1945), Gessow (1948), Brotherhood (1949), Castles and Gray (1951) and others). Obviously the making of flight tests (measuring essentially the rate of descent and control angles) is both difficult and hazardous, especially where the vortex-ring state is prominent, and not surprisingly the results show some variation: nevertheless the main trend has been ascertained and what is effectively a universal induced-velocity curve can be defined. This is shown in Fig. 2.10, using the simple momentum-theory results of Equations (2.10) and (2.13) in the regions to which they apply. We see that moving from hover into descent the induced velocity increases more rapidly than momentum theory would indicate. The value rises, in the vortex-ring state, to about twice the hover value, then falls steeply to about the hover value at entry to the windmill-brake state.
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