.04 Gas Turbine Engineering Handbook
where的1is a function of the staggerangle， maximum thickness， and the position of maximum thickness as seen in Figure 7-27. The second term of the equation should only be used for camber angles 0 .e. . The third term must be used only when the mach number is between 0.75 .M .1.3.
The use of NACA cascade data for calculating the exit air angle is also widely used. Mellor has replotted some of the low-speed NACA 65 series cascade data in convenient graphs of inlet air angle against exit air angle for blade sections of given lift and solidity set at various staggers. Figure 7-2 shows the NACA 65 series of airfoils.
The 65 series blades are specified by an airfoil notation similar to 65-(1)
10. This specification means that an airfoil has the profile shape 65 with a camber line corresponding to a lift coefficient (CL)二 1. and approximate thickness of 10 of the chord length. The relationship between the camber angle and the lift coefficient for the 65 series blades is shown in Figure 7-2..
The low-speed cascade data have been replotted by Mellor in the form of graphs of α2 against α1 for blade sections of given camber and space-chord ratio but set at varying stagger寸， and tested at varying incidence (i二 α1 -.1) or angle of attack (α1 -寸) as seen in Figure 7-30. The rangeon each block of results is indicated with heavy black lines， which show the attack angle at which the drag coefficient increases by 50 over the mean unstalled drag coefficient.
NACA has given ""design points"" for each cascade tested. Each design point is chosen on the basis of the smoothest pressure distribution observed on the blade surfaces: if the pressure distribution is smooth at one particularincidence at low speed， it is probable that the section will operate efficiently at a higher Mach number at the same incidence， and that this same incidence should be selected as a design point.
Although such a definition appears somewhat arbitrary atfirst， the plots of such design points against solidity and camber give consistent curves.These design points are replotted in Figure7-31， showing the angle of attack (α1 -寸) plotted against space-chord ratio for different cambers. The design attack angle of a cascade of given space-chord ratio and camber is independ-ent of stagger.
If the designer has complete freedom to choose space-chord ratio， camber，andstagger， then a ""design point"" choice may be made by trial and errorfrom the plots of Figure 7-30 and 7-31. For example， if an outlet angle ( α2)of 15 is required from an inlet angle of35， a reference to the curves of thefigures will show that a space-chord ratio of1.0， camber1.2， and stagger 23 will give a cascade operating at its design point. There are a limited variety ofcascades of different space-chord ratios， but one cascade that will operate at
.06 Gas Turbine Engineering Handbook
Figure 7-28. The .ACA 65 series of cascade airfoils.
Figure 7-.0.The .ACA 65 series cascade data. (Courtesy of G..ellor， .assa-chusetts Institute ofTechnology， Gas Turbine Laboratory .ublication.)
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Figure 7-.1. Design angles of attack(α. -寸) for .ACA 65 series.
""design point""at the specified air angles. For example， if the space-chordratio were required to be 1.0 in the previous example， then the only cascadethat will produce design point operation is that of camber1.2， stagger 23.
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