PPRuNe Forums > Flight Deck Forums > Tech Log > swept wings PDA View Full Version : swept wings clear_right12th Apr 2004, 18:03Hi Could someone explain to me how a swept wing will increase the critical mach number. Cheers Empty Cruise12th Apr 2004, 18:11When increasing sweep angle for same wing profile, the effective chord of the wing increases in length. Increased chord for same thickness = decreased camber (not sure this is correct technical term here:confused: ), decreased camber = increased mach no. Hope this is close enough for government work. Brgds, Empty. Genghis the Engineer12th Apr 2004, 20:15Not how I was taught it, although the argument about thin aerofoils is true - it doesn't explain why high speed aircraft don't simply have deep chord, thin stubby wings (apart from the horrendous tip losses). The critical Mach number is not the absolute Mach number, it is the local Mach No, perpendicular to the leading edge of the wing. So, if you resolve the triangle of velocities, for the "ambient" Mach number to be that parallel with the wing centreline, then we get a lower apparent Mach number perpendicular to the wing leading edge. (It's a bit like calculating crosswind component). So, the greater the sweep, the lower the perpendicular Mach number for the same flightpath Mach number. G BEagle12th Apr 2004, 20:31Yes, Ghengis - I too was given that load of hor\$eshit! Empty cruise's explanation is far more plausible - and remember the effect on apparent Reynold's No as the effective chord length increases at high sweep angles. Genghis the Engineer12th Apr 2004, 20:54Ah, but I was given that load of horse dung by a professor of aerodynamics, who got it from a book by another professor of aerodynamics. Another such prof (mine is long retired) gives a good explanation at http://142.26.194.131/aerodynamics1/High-Speed/Page2e.html which covers both reasons. G BEagle12th Apr 2004, 22:17"When the wings are swept back the airflow is accelerated less as it flows over the wing. Only the component of the airflow perpendicular to the wing is actually accelerated at all. Therefore, a swept wing will have a proportionally higher critical mach number." BOLLEAUX! Genghis the Engineer13th Apr 2004, 07:11That is a rather poor piece of wording I'll grant you, but the general sense of the piece is relatively sound, as well as being rather more accessible than the traditional textbooks like Houghton. G Empty Cruise13th Apr 2004, 15:38Genghis, You are absolutely right - it is a more grapically correct representation to use the airflow vector resolution. I tried to give a "laymans" explaination of the problem :oh: - should know that it would cause notin' but trouble :} High-speed aircraft have used short/stubby, deep chord wings, X3 and X15 being some of the more extreme examples, the F104 one of the more conventional. These aircraft used it instead of large sweep angles to achieve a high Mcrit. But of course, the low aspect ratio made for lousy low-speed handling, so not an option on transports. Btw, your explaination also demonstrates how big an increse in sweep angle it takes to bring by a small increase in Mcrit. Brgds, Empty Specnut72714th Apr 2004, 09:11K.W. I haven't done any reading on this, but here's a question which sprang to mind. Taking your 'increasing chord' idea a bit further, it would appear that a delta wing would achieve this better than a swept wing. I assume there are some other factors which make swept wings a better option for high subsonic speeds. Can you explain please. Thanks in advance, Spec. FE Hoppy14th Apr 2004, 17:20K.W Wasn't this called the fineness ratio or the thickness ratio? The other way to reduce Mcrit is to use a supercritical section. This has a larger radius leeding edge and quite flat top surface the underside being under cambered towards the trailing edge. This allows quite a thick section to be used. Prof2MDA15th Apr 2004, 07:54Any object moving through a fluid sends a wave in front of it that effects the flow of the air as the object approaches. Those waves move out radially, so the portion of wing that is upwind essentially transmits a wave that influences the air that is in front of the portions that are more downwind of it. So, essentially, sweeping a wing transmits a signal to the air that the wing is approaching, where otherwise it is taken more by "surprise". Specnut72719th Apr 2004, 06:51Thanks K.W. That's a great explanation. Despite it's drawbacks, there still aren't many aircraft around as pretty as Concorde. Too bad they're all destined to be static in museums. Spec.