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Traditionally viscous effects have been taken into account in a global sense by considering the results of model tests on standard aerofoil forms and then plotting faired trends. The viscous behavior of water, however, provides a generally small but, nevertheless, significant force on the propeller blade sections and, as such, needs to be taken into account in calculation methods.
![lifting a load at angle lifting a load at angle](http://www.absoluterescue.com/wp-content/uploads/2016/01/Sling-Tension-Load-Angles.png)
Carlton FREng, in Marine Propellers and Propulsion (Third Edition), 2012 7.8 Boundary Layer Growth Over an AerofoilĬlassical theoretical methods of the type outlined in this chapter very largely ignore the viscous nature of water by introducing the inviscid assumption early in their development. As a consequence, most trailing edge flap systems on jet transports are of the double-slotted type. Though the maximum lift coefficient may be increased further by using triple-slotted flaps, such as those employed on the Boeing 737 shown in Figures 5.32 and 5.33, and on the Boeing 747, experience has shown that the added weight and complexity of such flaps are not completely cost-effective in airline operations. The increase in maximum lift coefficient for that airfoil is shown as a function of flap deflection angle in Figure 5.31. Note that the travel of the flap is such that the projected elemental wing area dS is not materially changed.
![lifting a load at angle lifting a load at angle](https://velascoindonesia.com/wp-content/uploads/2020/06/Lifting-Angle.jpg)
In the fully deployed position, Figure 5.30(b), the two slots separated by a small airfoil element are readily seen. In the stowed position, Figure 5.30(a), the airfoil shape is suitable for efficient high-speed flight. The mechanism for driving the flap is not shown but the travel of the individual elements should be clear. The practical embodiment of such a flap on an airfoil is presented, for example, in Abbott and Von Doenhoff (1959) and is shown in Figure 5.30.
![lifting a load at angle lifting a load at angle](https://image.slidesharecdn.com/liftingprocedure-110517104150-phpapp01/95/lifting-procedure-22-728.jpg)
A schematic diagram of a double-slotted flap was shown in Figure 5.27(d).
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