Blade Vortex Interaction
As concerns a detailed analysis of the rotor wake, corresponding to that outlined in Chapter 2 for the hover, the complication introduced by forward flight comes down to the fact that at a given radial position, the blade incidence, and hence the circulation, varies widely around the azimuth. Each change of circulation results in a counter vortex being shed into the wake and since the change is a circumferential one, the vortex line in this case lies in the spanwise direction. This system of ‘shed’ vortices is now additional to the ‘trailing’ vortex system arising, as in hover, from the spanwise variations in circulation.
Undeterred by such multiplicity of complication, the modern computer, guided by skilled workers among whom pioneers include Miller, Piziali and Landgrebe, is still capable of providing solutions and indeed building year upon year. The power of the modern computer is releasing rotor analysis from restrictions which were necessary in the past to achieve a realistic result. An example from Landgrebe’s calculations shows in Figure 5.14 a theoretical wake boundary at low advance ratio, compared with experiment by smoke visualization and also with the Glauert momentum theory solution. The numerical solution and the experimental evidence agree well; momentum theory gives a much less accurate picture. A feature to note is that the boundary at the front of the disc lies close to the disc. This is illustrated in Figure 1.22c. At a higher advance ratio, more representative of forward flight, this feature and the general sweeping back of the wake would be much more marked.
This brief reference to what is a large subject in itself will suffice for the purposes of the present book. Extended descriptions can be found in the standard textbooks.
Figure 5.14 Wake boundaries at low advance ratio (after Landgrebe)