Why Some Airplane Have Swept Back Wings & Some Have Straight Wings?
Why Some Airplane Have Swept Back Wings & Some Have Straight Wings?
Do you ever think why the some airplane have swept back wings and some have straight wings. Modern plane mostly have swept back wing but why? Some of the aeronautics enthusiast’s questions about the question why does some airplane have straight wings and some have swept wings but the answer may astonish you. We all know that airplanes fly at the velocities which are comparable to the speed of the sound and in this following cases may happen. Say ‘V’ is speed of the airplane and ‘A’ is the speed of sound then, if V<A – the airplane is flying at the velocity less than the speed of the sound then in this case flight will be subsonic flight, similarly if V=A – the flight will be sonic flight, if V>A – the flight will be supersonic. A special case also exist i.e. it may that happens that if airplane is flying at velocity which is equal to 5 times the speed of the sound or V>5A, in this case the flight will be called as hypersonic.
Now, from your higher school physics you should aware of the Mach number which is defined as the ratio of speed of airplane to speed of the sound (in aeronautics definition may vary slightly). And as per definition in our case, it will be V/A which we designate as M, so all of the above cases becomes M<1 (subsonic), M=1 (Sonic), M>1 (Supersonic) and M>5 (hypersonic). I think you are familiar with this term if you ever seen documentary about airplanes on National Geographic channel. Let go back to the question.
During the flight phase, gas/air expands on the top surface of the wing near leading edge and the velocity increase rapidly and hence Mach number. There are/may be some point on the wings where local Mach number may be greater than free stream Mach number. Local Mach number means ratio of velocity of air/gas to speed of the sound at that point (on the surface). The situation gives rise to the new term called critical mach number and it defined as the free stream mach number at which firs sonic flow is obtained somewhere on the wing surface.
Now if free stream Mach number is increased slightly above the critical Mach number, a bubble of supersonic will occur surrounding the minimum pressure point on the wing. Again if free stream Mach number is increased further, a sudden rise in drag will appear and at this point shockwaves will appear which is not desired and the situation gives rise to a new term called Drag Divergence Mach Number. By the definition drag divergence Mach number is the free stream mach numbers at which drag begins to increase rapidly and it lies somewhere between critical Mach number and unity Mach number.
Now we are in position to answer our question. Almost all the modern high speed aircraft have swept back wing because it is desirable to increase critical Mach number. Let see how?
If you take a straight wing and swept back wing (say swept the wing at 30 deg.) with critical Mach number 0.7 then the swept wing will “see” only the component of flow normal to the leading edge of the wing. It means aerodynamic properties of local section of swept wings are governed mainly by flow normal to leading edge, the critical Mach number for swept number itself would be high as 0.7/(Cos 300) = 0.808 and by this way large increase in delay would be increased and in case of straight wings the leading edge will see the normal component equal to 0.7 which is lower than 0.808 and as per our above discussion straight wing will see shockwave earlier than swept wing and this is why we sweep the wings back. At this point you make ask question why we can’t swept wings in forward direction and the answer is forward wings have some structural limitations due to which we cannot swept wings forward.
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