PPRuNe Forums Swept wing for dummies!

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 22nd May 2012, 20:43 #1 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 Swept wing for dummies! Is there any simple explanation for the Swept Wing design, what makes it a better design. I do know the concept, of the normal vector, the effective chord, being shorter than the vector parallel to the free airstream. However I find it complicated to make a good explanation of this, without getting to technical with regards to the T/C ratio. I would just want a simple explanation that covers the concept of the swept wing design. Why does it allow for higher Mcrit.
 22nd May 2012, 22:10 #2 (permalink) Join Date: Mar 2012 Location: UK Posts: 39 I find it hard to explain it simply, maybe there is a way but I do not know it, I spent some time getting my head around the vectors and the t/c ratio and relative camber. The best way I could explain it is that on a straight wing, all the air hitting the wing is going straight, chord-wise backwards at a certain velocity. On a swept wing, part of the air is going chordwise, and part of it is going span-wise. The span-wise flow uses some of the energy that would be in the chord-wise flow on a straight wing, so if energy is removed then the chord-wise speed will decrease, so free stream mach no. must increase to get more energy to increase the chord-wise flow velocity back to sonic. I know this isn't the most correct or scientific explanation but it is one that I find simple and has enough to it to suffice.
 22nd May 2012, 22:40 #3 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 Yes, to be honest, did not exactly crack my mind, appreciate the effort. I do understand the principle and effect, but looking for a very simple way to explain it, without getting to technical. I first thought: Thickness / Chord ratios decides when sonic shockwave is formed. By decreasing the T/C, the shockwave will form late. In a straight wing with same thickness as a swept wing, the Chord in the swept wing, this will give us a smaller Thickness / Chord ratio, and hence a shockwave will form later, as the wing seems to be thinner, because of the smaller T/C ratio for the Swept wing. However the problem with this, it might bring up other questions, that will only complicate matters. Is the effective chord on the swept wing is the chord normal to the swept wing, however the air will not be flowing this way, but across the diagonal vector across the swept wing, which due it's geometry will be longer, hence an apparently longer chord, decreasing the T/C ratio. But what I fear, with this, other questions might pop up, like why does this than delay the formation of Mcrit? Because there air has to travel longer, and will slow down, so it allows the aircraft to go faster before shock-wave is formed? Hm still don't find it simple to explain without getting myself into a dump!
 22nd May 2012, 22:44 #4 (permalink) Join Date: Oct 2007 Location: fort sheridan, il Posts: 1,667 because it looks cool.
 22nd May 2012, 23:06 #5 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 Here is an explanation I find online, I love the term simple!!!# "One of the simplest and best explanations of how the swept wing works was offered by Robert T. Jones: "Suppose a cylindrical wing (constant chord, incidence, etc.) is placed in an airstream at an angle of yaw - ie., it is swept back. Now, even if the local speed of the air on the upper surface of the wing becomes supersonic, a shock wave cannot form there because it would have to be a sweptback shock - swept at the same angle as the wing - ie., it would be an oblique shock. Such an oblique shock cannot form until the velocity component normal to it becomes supersonic" Sounds to be a simple explanation!!
 24th May 2012, 11:48 #6 (permalink) Join Date: Apr 2009 Location: down south Age: 66 Posts: 12,841 The purpose of sweepback is to increase critical Mach number.
 24th May 2012, 17:12 #7 (permalink) Join Date: Nov 2011 Location: UK Posts: 51 The flow perpendicular to the leading edge is the one that matters. The speed of this flow is reduced which delays Mcrit to a higher speed. This allows a higher cruising speed by delaying the onset of drag induced shockwaves.
 24th May 2012, 17:30 #8 (permalink) Join Date: Apr 2009 Location: down south Age: 66 Posts: 12,841 wyssatt then? Does the oblique shockwave occur when the flow accelerates to supersonic speed? Last edited by Lightning Mate; 24th May 2012 at 17:33.
 26th May 2012, 13:36 #9 (permalink) Join Date: Feb 2010 Location: Planet Moo Moo Posts: 889 The purpose of sweep back is to make the Jumbo a pain in the ar\$e to land! At least IMHO.
 26th May 2012, 22:44 #10 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 LVL_CHG "The flow perpendicular to the leading edge is the one that matters. The speed of this flow is reduced which delays Mcrit to a higher speed. This allows a higher cruising speed by delaying the onset of drag induced shockwaves." But on the Swept wing, the flow parallel to the flight direction, is the one that makes the wing appear longer, and reduces the thickness/chord ratio, compared to if the wing hand been a straight wing. What this means, if I understand correct, if this specific wing had been a straight wing, with this specific dimensions, thickness and chord ratio, the effective chord would have been the one perpendicular to the leading edge. However since the wing now is swept, the effective chord is the one parallel to the fuselage/airflow, which is a longer section, chord, so this makes the wing get a lower Thickness/Chord ratio, which allows for a higher free stream speed Mcrit, before the some part of the aircraft goes sonic! Still not as simple as I would have liked it!
 27th May 2012, 00:37 #11 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 • Mainly used to increase Mcrit • Low Thickness / Chord ratio • If a wing has sweepback, the effective chord (parallel to the aeroplanes longitudinal axis) is lengthened, but the wings Thickness remains unchanged. • This reduces the Thickness / Chord ratio of the wing, which results in a higher value of Mcrit, and delays the transonic Drag rise / Shockwave formation. • The Greater the Sweep back, the higher the value of Mcrit, and the greater the reduction in drag under all transonic speeds. I found this from the Jeppesen books, now one thing that I feel is confusing and keeps getting mixed up, in many different books, or at least a bit confusing to grasp, is the constant use of the term that the Effective Chord, is the one perpendicular to the Leading Edge! Now as far as I see this is not completely right, the effective chord is the one Parallel to the aeroplane longitudinal axis! The Vector normal to the leading edge, is only to explain what would have happen if the wing would have been straight, or am I missing a point here? The Chord, perpendicular to the leading edge, can not be the effective chord of a swept wing? !!!
 27th May 2012, 15:25 #12 (permalink) Join Date: Apr 2009 Location: down south Age: 66 Posts: 12,841 ................
 28th May 2012, 07:41 #13 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 "Lightning Mate" Not to be offensive, but why even bother to reply, if you can't come with a reply that is a little more constructive!
 28th May 2012, 09:31 #14 (permalink) Join Date: Apr 2008 Location: UK Posts: 42 Truckflyer, I thought we already went through this in a thread I had recently set up? Clearly, the Jeppesen explanation makes sense through cold, hard science and mathematics. The talk of airflow perpendicular to the leading edge of a swept wing is an attempt by non-maths-savvy people to explain (badly) what Jeppesen have done so concisely.
 28th May 2012, 20:11 #15 (permalink) Join Date: Nov 2011 Location: Somewhere close to me Age: 44 Posts: 557 I have to say, of all the ATPL books/notes I have seen, which includes Oxford, Nordian, Bristol CBT /notes and Jeppesen Atlantic version, I find the Jeppesen books the best, and easiest to understand, without loads of nonsense! The Oxford books are great, but they do certain areas go to much in to minute details, the Nordian books, I prefer not to comment on, Bristol is maybe a little to minimal, but not bad, however the Jeppesen books seems to be perfect, a pity they are more or less obsolete as it seems not many are using them anymore.

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