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Tham khảo tài liệu 'thiết kế kỹ thuật máy bay chiến đấu episode 2 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 420 THE DESIGN OF THE AEROPLANE to start with a stick-fixed static margin of 0.02 to 0.04 SMC in the aft CG cruise. From there we can then build up the remaining margins using table 11-5. Table 11-5 Practical values of static and manoeuvre margins Margin Stick-fixed Stick-free Static Kn 0.02 to 0.04 but could be 0 for a hot ship K n Kn 0.03 but with most light aeroplanes K n Kn. Manoeuvre Hm K 0.03 0.06 for most light aeroplanes H m Kn 0.02 to 0.03 0.08 to 0.25 i.e. a ratio around 1 to 3 between aft and forward CG limits If we compare the values given in the table with eqs 11-25 11-27 11-40 and 11-43 while using eq 11-48 to calculate the lift slope ratio aja it is a straightforward matter to transpose the required increments for tail volume coefficient Vx. However we need to know something about control surface characteristics and a brief introduction is given in chapter 12. Trim Assuming that Kn has the right value the next step is to check that the aeroplane can be trimmed in two critical conditions At the stall with the wing working at Cunax flap down and the CG on the forward limit. At the low Cl and aft CG case. Power and ground effects must be allowed for using eq 7-30 and fig. 6.10. The checks amount to ensuring that the net moment about the CG is zero in each case i.e. Cmcg Clih - ho Cm k - tịsVsCl 0 from eq 11-8 Some of these terms need attention Cmoc . Cmò. the pitching moment coefficient at zero lift which is mostly due to the wing-plus-fuselage. The wing contribution is obtained from sources like ref. 3.6. The fuselage contribution can be estimated using results for similar shapes wind tunnel results are most useful . The nose produces the most powerful effects. Light aeroplane fuselages produce Cmo values which vary between 0.03 and -0.03 based upon wing area. The width of the fuselage in terms of the wing span is important the greater the ratio the more powerful the moment. Large broad canopies produce a LONGITUDINAL STABILITY 421 nose-up pitching .
