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Dougal

Question About Flight Dynamics

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Hello folksI'm about to choose a new corporate jet for my FS hanger - probably from Eaglesoft.When i was looking at the various models, I noticed there's some huge design differences between some of them. The most obvious being those wonderfull swept back wings of the Cessna Citation X, compared say, to the straight wings of the other Cessna jets.Can anyone explain the merits of those swept back wings. It's surely not just to look good, as most airliners have them too?Just interested I guess and wondered why....Thanks

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Hello folksI'm about to choose a new corporate jet for my FS hanger - probably from Eaglesoft.When i was looking at the various models, I noticed there's some huge design differences between some of them. The most obvious being those wonderfull swept back wings of the Cessna Citation X, compared say, to the straight wings of the other Cessna jets.Can anyone explain the merits of those swept back wings. It's surely not just to look good, as most airliners have them too?Just interested I guess and wondered why....Thanks
This one is interesting.http://www.century-of-flight.net/Aviation%...47%20Bomber.htmI googled "swept wings"L.Adamson

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When i was looking at the various models, I noticed there's some huge design differences between some of them. The most obvious being those wonderfull swept back wings of the Cessna Citation X, compared say, to the straight wings of the other Cessna jets.Can anyone explain the merits of those swept back wings. It's surely not just to look good, as most airliners have them too?
Aircafts have swept wings to go faster.As you maybe know, the speed of the air increases on the upper surface of the wing. When the aircraft speed reaches a certain Mach number (called Critical Mach Number), on some points of the wing the air reaches supersonic speed, and shockwaves form. This is bad for various reasons, for example drag rises and control surfaces may lose effectiveness.When the wing is swept, it only "sees" the normal velocity component of the incoming air, so the effective air speed "seen" by the wing is reduced. So swept wings delay the onset of shockwaves and increase the Critical Mach Number, allowing the aircraft to go faster.The main disadvantages of swept wings are:.unfavorable behaviour near and after stall: wing tips tend to stall first, producing an early loss of aileron authority and a dangerous pitch-up tendency;.flatter lift slope: significant nose up attitude required during landing;.strong lateral stability: in some cases this may produce undesirable dutch-roll characteristics.Marco

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As far as corporate jets are concerned, there are a few reasons why they most often have swept wings:The first reason is the one that people have already explained and which most are familiar with; i.e. a swept wing delays the onset of the shockwaves that would otherwise make high speed air penetration too difficult by having the shockwave jam the ailerons etc.Next, and one which a lot of people don't often think about; sweeping the wing shifts the weight backwards, so the centre of lift is further back. This makes the aircraft slightly less aerodynamically stable, but also makes the thing more fuel efficient because the rear stabiliser doesn't have to do as much work on a wing with a more rearward centre of lift, thus there is less aerodynamic drag and big fuel savings. The Cessna Citation, especially when it has winglets installed, is one of the most fuel-efficient aircraft there is.It's not generally realised by many that this was the main reason the WW2 Messerschmitt 262 jet-powered interceptor had swept wings. Designer Wili Messerschmitt knew that the wing sweep on the 262 was not enough to make it faster at high thrust, and that was never its intended purpose. In fact the 262 design originally had straight wings, but when the proposed BMW-designed engines it was to use were not able to be developed in time for it, it ended up with much heavier replacement Junkers Jumo engines, which prompted the change in the design of the wings to include a slight sweep in order to get the weight back a bit, to compensate for the heavier engines and move the centre of lift back a bit.In addition to the aerodynamic reasons for sweeping the wings, weight balance is of course also one of the reasons why airliners with podded engines under the wings such as the A340, 707, 747 etc have a swept wing. You may have seen pictures of old jets stored in boneyards in the deserts of the US with their engines replaced by blocks of concrete slung under the wings to prevent them tipping onto their tails when the engines were removed.Another thing to consider with corporate jets and swept wings is the practical application of speed in their typical flight regime. Most corporate jets, including the Lear, Citation, Jetstar etc, cruise at or above 41,000 feet, which is up where the big airliners occasionally get in their cruise phase, but often above most traffic for the kind of flight routings corporate jets take. This has led to concerns over the slower speed of the Cessna Citation Mustang, which as you probably know, has a straight wing. The Citation Mustang can land at just 90 knots, which is good because it is designed to be a baby corporate jet which a typical piston engined Cessna driver can transition to easily and fly solo in (which incidentally, is why it also has no overhead panel). The problem is that it has a cruise speed of around Mach .65/340knots, and that means it could potentially clog up the airways up at 41,000 feet when flying slower than everything else up there.Lastly, and by no means least, corporate jets are status symbols, even the diminutive unswept Citation Mustang costs over 2.5 million, and for that kind of money, they are meant to look 'sexy'. To Joe Blow and his buddies who don't know much about aeroplanes, a plane with swept wings is more flashy, sexy and impressive. Al

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Next, and one which a lot of people don't often think about; sweeping the wing shifts the weight backwards, so the centre of lift is further back. This makes the aircraft slightly less aerodynamically stable, but also makes the thing more fuel efficient because the rear stabiliser doesn't have to do as much work on a wing with a more rearward centre of lift, thus there is less aerodynamic drag and big fuel savings. The Cessna Citation, especially when it has winglets installed, is one of the most fuel-efficient aircraft there is.It's not generally realised by many that this was the main reason the WW2 Messerschmitt 262 jet-powered interceptor had swept wings. Designer Wili Messerschmitt knew that the wing sweep on the 262 was not enough to make it faster at high thrust, and that was never its intended purpose. In fact the 262 design originally had straight wings, but when the proposed BMW-designed engines it was to use were not able to be developed in time for it, it ended up with much heavier replacement Junkers Jumo engines, which prompted the change in the design of the wings to include a slight sweep in order to get the weight back a bit, to compensate for the heavier engines and move the centre of lift back a bit.
Me 262 was an exception, wing swept is never driven by weight shift considerations in modern designs. Aircraft stability/trim drag are not a factor in the design phase in deciding the wing sweep. Also note that sweeping the wing back and therefore shifting rearward the c.g. makes (all the other conditions being the same) the aircraft MORE stable (increasing the trim drag) and not less.Marco

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A swept wing also introduces a phenomena called Dutch Roll. Not good and can lead to loss of control without yaw dampers. The 727 was especially suceptable to this. Yaw dampers were always 'on' in this aircraft. All training material and the POH I have for this aircraft mandate it.

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Me 262 was an exception, wing swept is never driven by weight shift considerations in modern designs. Aircraft stability/trim drag are not a factor in the design phase in deciding the wing sweep. Also note that sweeping the wing back and therefore shifting rearward the c.g. makes (all the other conditions being the same) the aircraft MORE stable (increasing the trim drag) and not less.Marco
I suspect that USAF Captain Ray Popson would disagree with you, sadly he was killed when the Bell X-5 he was flying could not recover from a spin when the wings on his X-5 were swept back 60 degrees, thus making the aircraft too unstable to effect a recovery. The X-5 incidentally, was directly based on the concepts used in the construction of the Messerschmitt P1101 - another wartime design with swept wings - the wings of which could be adjusted between 30 and 45 degrees of sweepback, so the Me262 was most certainly not an exception in terms of wing sweep for a variety of reasons. In modern times we have seen the F-111, Panavia Tornado and B-1 use the concept too. Their sweep settings are not always based on the speed they fly at, but often determined by the loads slung under the wings.It's not true to say that weight considerations are never a factor in wing sweep design considerations. Taken to extremes, it is known that sweeping a wing forward, rather than aft, will result in considerable advantages, notably favorable stall behavior and improved stability at transonic speeds, but it is not done currently on anything other than experimental aircraft because of the other design difficulties it creates, notably the strength of the materials required and the distribution of the weight to a point well forward of what one normally finds, which precludes the inclusion of outer wing fuel cells amongst other things. Technology demonstrators such as the Grumman X-29 and the Sukhoi Su-47 are two modern examples of the concept, although interestingly, we are now seeing the use of the advanced composite materials used for these experimental craft in the construction of modern airliners, notably the B-787 and the A-350, so we may one day see an airliner or corporate jet with forward-swept wings.In more practical terms, altering the rear sweep across all, or some of the wingspan can alter the way in which a rear-swept wing will stall and at what point on the wing will stall first. This means that altering the sweep at the extremities of the wing can be used as an alternative to tip washout. The resulting alteration in structural needs based on the twist, or lack of twist in a washout, or even a varied sweep angle, can either create, or solve weight distribution difficulties based on which way the aircraft designers choose to go. Their choices will affect what can be done internally with the wing based on the aerodynamic load that part of the wing has to bear, i.e can you put a fuel tank in there or not. In short, the design of a wing, and how it is swept very often has weight distribution as a consideration in its creation.Anyway, sort of back on the original topic of business jets, which do people think is better, the Wilco Citation or the Eaglesoft one?Al

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Hi Al,first of all, for clarification, there's a lapse in my previous post, where it read "shifting rearward the c.g." it should have read instead "shifting rearward the center of lift", of course.Now, let me quote your first post. You said:

Next, and one which a lot of people don't often think about; sweeping the wing shifts the weight backwards, so the centre of lift is further back.
Your phrase here is not very clear. It seems to imply that the backward weight shift causes a backward shift of the centre of lift. There's obviously no causal relations between the two. It is true, however, that in a variable sweep aircraft, the sweeping of the wing (if not accompanied by a forward movement of wing roots) causes a rearward shift of the centre of lift. In this situation the centre of lift moves _rearward_ in relation to the center of gravity.This makes the aircraft _more_ longitudinally stable in _normal flight regimes_ and not, as you said, less stable.
I suspect that USAF Captain Ray Popson would disagree with you, sadly he was killed when the Bell X-5 he was flying could not recover from a spin when the wings on his X-5 were swept back 60 degrees, thus making the aircraft too unstable to effect a recovery.
Yes, as I said in my first post, sweep wings may render the aircraft less stable in _near stall, stall and post stall regimes_.Your first post and the Cessna Citation example, seemed instead to imply that aircraft designers choose to sweep the wing back in order to make the aircraft a little less stable and therefore less draggy. That is not correct.
In more practical terms, altering the rear sweep across all, or some of the wingspan can alter the way in which a rear-swept wing will stall and at what point on the wing will stall first. This means that altering the sweep at the extremities of the wing can be used as an alternative to tip washout. The resulting alteration in structural needs based on the twist, or lack of twist in a washout, or even a varied sweep angle, can either create, or solve weight distribution difficulties based on which way the aircraft designers choose to go. Their choices will affect what can be done internally with the wing based on the aerodynamic load that part of the wing has to bear, i.e can you put a fuel tank in there or not. In short, the design of a wing, and how it is swept very often has weight distribution as a consideration in its creation.
Absolutely agree 100% on all here. Weight distribution is always a consideration, for every part of the aircraft that is being designed. What I meant was that designers do not sweep back the wings to decrease the aircraft longitudinal stability and therefore trim drag, simply because sweeping it back the opposite effect would be obtained.Marco

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