Tham khảo tài liệu 'thiết kế kỹ thuật máy bay chiến đấu episode 1 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | ARRANGEMENT OF SURFACES 145 Cr ticxj rẠcxc-U kutwbeX Itf-t without A 4ct foj- E ịuívalc Uvt .wise tkickwess ckorel t c. CoSẶ 6 4- 3t kote tkat tke Worv oj F c is EUe structured tuickucss Fig. The effect of aerofoil sweep at the 4 chord upon fineness and hence critical Mach number. For our practical purposes leading edge and 4 chord sweep are more or less the same. Curves based upon ref. . 146 THE DESIGN OF THE AEROPLANE If we join points of equal static pressure on an aerofoil surface we obtain a pattern of isobars rather like that shown in fig. . Isobar sweep more or less follows the leading edge at mid span but it decreases towards tips and roots because of the tendency for differential pressure to leak away at the tip and because of friction and interference effects at the junction slowing the local flow. Thus thickness ratio may be increased correspondingly at roots without adversely affecting local critical Mach number which enables wing structures to be lightened and extra baggage or fuel stowed. Such thickened roots provide useful volume for landing gear units. The component of flow normal to a wing leading edge should not exceed about M for an orthodox subcritical wing section. Supercritical sections can work to higher values as we saw in the last chapter. We can calculate the required sweep angle for the leading edge from the geometrical properties of sweep . Mach number normal to leading edge M cos A 4-23a where M is the flight streamwise Mach number and equivalent streamwise thickness chord r c cos A 4-23b where t c is the structural thickness chord ratio measured normal to the wing leading edge. Thus from eq 4-23a M cos A or A sec 1 M 4-23c Comparison of backward and forward sweep Swept back wings have two disadvantages at increased angles of attack and reduced speeds Wing boundary layers tend to drift outboard assisted by the spanwise component of flow in fig. which causes them to thicken and .
