Tham khảo tài liệu 'thiết kế kỹ thuật máy bay chiến đấu episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | DRAG FLAPS AND WAKES 195 Fig. showed that the trailing vortices move inwards behind the wing during readjustment of the air after being displaced radially by the passing volume of the aircraft. Circulation around the bound and trailing vortices causes both of the trailing vortices to be washed downwards through an amount d at a distance behind the wing as measured along the line of flight. Reason concludes that the wake starts at the trailing edge of the wing. Lifting line theory assumes that it starts at the bound vortex located more or less at the 4 chord of the wing. Either way the difference between each view is small while the sheet of shed vortices curves downwards with the concave surface upwards - its radius of curvature increasing downstream as the sheet flattens out when seen in elevation. The wake can be assumed to be nearly flat in a spanwise direction across the middle third of the wing. Wake displacement d is usually measured in terms of wing semispan 5 orb 2 as d b. This is a function of distance I downstream so that we may also use the measure l b along the line of flight to define the position of the wake in chords behind the wing. As we also need Cl to complete the picture it may be introduced as follows wake displacement factor d sCL l s or 2d bCL 2l b 5-17 Wake factor curves are given by different sources those of the Royal Aeronautical Society in the ESDU data sheets Appendix A Item as ratios of the semispan s. Hoerner on the other hand uses wingspan b for his ratios ref. but this does not alter the shape of the curves only the numbers for the ratios by a factor of 2 on each side . We assume that as wings canards and tails are usually located between 2 and 4 chords apart the following equations may be used for determining wake characteristics. Further because we work in multiples of wing chord we shall transform eq 5-17 into d c and l c by means of eq 2-4 wherecis the SMC of the wing such that 5 can be written A 2 c. Fig.
