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Fly by wire in conjunction with relaxed stability lets you maintain aircraft control at higher AoA, thus reducing approach speed. Now, if you know why the Viggen has canards, perhaps you can tell me how they relate, in function, to the LERX of the F-18 ? Perhaps you can even tell me why the Swedes choose a delta wing, with its inherent drawbacks for a short field fighter ?

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Fly by wire in conjunction with relaxed stability lets you maintain aircraft control at higher AoA, thus reducing approach speed. Now, if you know why the Viggen has canards, perhaps you can tell me how they relate, in function, to the LERX of the F-18 ? Perhaps you can even tell me why the Swedes choose a delta wing, with its inherent drawbacks for a short field fighter ?

 

Delta wings are known for being a good choice for high transonic and supersonic flight, especially at low altitudes. It's a wing that fits a low altitude high speed strike role very well.

 

The hornet's LERX are vortex generators that significantly delay boundary layer separation over the interior portion of the wing at very high angles of attack. They also help with airflow past the vertical stabilizers at extreme AoA, which helps retain yaw and roll authority.

 

The canards of the Viggen move the center of lift significantly forward on the aircraft, providing more stability and controllability in the pitch axis than a standard delta configuration with an aft balanced center of lift would have. By providing lift near the nose they also help improve stall behavior, but they're not really designed for increasing extreme AoA pitch control to the extent that the all moving canards of some Sukhois do.

 

The Viggen's delta wing is geared to the main mission, the canards and the reversers are there to mitigate the drawbacks of the delta wing.

 

Edit: It's a rather clever set of choices. SAAB got the high speed and low speed performance characteristics that they wanted with features that are mechanically simpler, lighter weight, and cheaper than the swing wings that other designers went with during that era when looking for planes with good handling across wide ranges of speed.

 

 

It might be worth noting that while FBW systems and relaxed stability are geared in part toward gaining low speed control at high AoA, this mostly in the context of air combat. For the most part this isn't that useful for landing because extreme high alpha creates problems with visibility and tail strikes. It's nice to have for a panicky wave off, but if you're near max AoA on approach you're really hoping that the plane starts to pull up before the back end of the plane starts making horrible noises and throwing sparks as the runway starts grinding it off.


Edited by esb77
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I am not an expert on the topic of canards, but from a more engineering related forum i have followed a discussion on the Rafale vs Eurofighter. the jist of it was that the Rafale's canard is geared towards delaying flow separation at low speed, high AoA, and the Eurofighter being geared towards agility at high speed.

This all has to do with how the canard interacts with the airflow over the wing. I myself have assumed the Viggen is more related to the Rafale as they both are required by design to land on a short runway/carrier. So, if the Rafale does it, why not the Viggen ?

And also, the Viggen still has positive stability, so how can you say that the lift center is moved forward ?

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but I'd like to know more about the studies on Soviet conventional strike capabilities which led to the rather tight Tornado program requirements regarding the short take-off and landing distances (wrong forum, I know).

 

 

There was a lot of concern within the RAF in the 1960 and into the 70`s about the viability of conventional runways and the ability of the enemy to put them out of action, in fact the main role of the Tornado was anti runway with jp233. This was re-enforced by the experience of the Six day war.

 

 

You have to remember the mentality of the time was that within a couple of days of a conflict starting on the central front, the main runways and the ancillary buildings of most major airbases would be destroyed or rendered unusable by area denial munitions. They were expecting the only facilities that would survive would be the harden aircraft shelters and short lengths of taxi ways. The fact that the Tornados short range placed their Airbase within strike range of most of the Soviet tactical airforces reinforced this concern

 

 

The Tornado specification took a lot from the original TSR2 spec i.e. the ability to operate from austere bases with short runways and limited ground support. The use of the thrust reverser, at the time, made perfect sense in this overall context.

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And also, the Viggen still has positive stability, so how can you say that the lift center is moved forward ?

 

This means that both the canards and the main wing produce lift.

A conventional tail design, the horizontal stabilizer needs to produce a downforce, to counteract the pitch up moment of the main wing. This downforce acts in the same direction as weight and needs to be counteracted by increasing lift. Increasing lift means increasing induced drag.

A canard can be a more efficient design, compared to conventional tail designs.

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Thanks Goblin, learned something new today. Back to Rafale vs Eurofighter The main difference between the two is that the Rafale has its canards in the same plane as the wing, thus delaying flow separation for the inner part of the wing, much like a LERX. My private theory has been that the Viggen does the same thing when it lowers its canard flaps. But this also increases lift, so perhaps this is different ?

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You are correct, it is not as black and white as Im making it out to be. However for all practical purposes, at the landing speeds and considering the airflow through the engine at those speeds, compared to the engine at 0 knots, and considering the relative decreasing speed of the aircraft, the difference in it's effectivness is almost negligible.

 

Um you clearly didn't read the papers I linked ìt is more than just engine thrust that changes with airspeed.

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Delta wings are known for being a good choice for high transonic and supersonic flight, especially at low altitudes. It's a wing that fits a low altitude high speed strike role very well.

 

The hornet's LERX are vortex generators that significantly delay boundary layer separation over the interior portion of the wing at very high angles of attack. They also help with airflow past the vertical stabilizers at extreme AoA, which helps retain yaw and roll authority.

 

The canards of the Viggen move the center of lift significantly forward on the aircraft, providing more stability and controllability in the pitch axis than a standard delta configuration with an aft balanced center of lift would have. By providing lift near the nose they also help improve stall behavior, but they're not really designed for increasing extreme AoA pitch control to the extent that the all moving canards of some Sukhois do.

 

The Viggen's delta wing is geared to the main mission, the canards and the reversers are there to mitigate the drawbacks of the delta wing.

 

Edit: It's a rather clever set of choices. SAAB got the high speed and low speed performance characteristics that they wanted with features that are mechanically simpler, lighter weight, and cheaper than the swing wings that other designers went with during that era when looking for planes with good handling across wide ranges of speed.

 

 

It might be worth noting that while FBW systems and relaxed stability are geared in part toward gaining low speed control at high AoA, this mostly in the context of air combat. For the most part this isn't that useful for landing because extreme high alpha creates problems with visibility and tail strikes. It's nice to have for a panicky wave off, but if you're near max AoA on approach you're really hoping that the plane starts to pull up before the back end of the plane starts making horrible noises and throwing sparks as the runway starts grinding it off.

 

Yes, i have always admired the SAAB designs, they are marvels of design and economy. buy a foreign engine. Mate it to the the most affordable supersonic wing, and then add the extras you need to meet your design goals.

Not the best fighters in the world, but perhaps the most intelligent way to obtain capability for a small independent country.

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Try deploying a drag chute on a road strip in the forests of Sweden. This is a design criteria for the Viggen.

The ones i have seen all had trees standing right next to the road. If you don't get it that thrust reversers are a huge asset and safety in this scenario, then you just don't get it.

 

Not a problem.

The drag chutes ain't larger than the aircraft wingspan, so you will not get tangled to the trees. And the engine thrust will keep it rear even in the wing (that even trees side of the road will block).

 

And the usual emergency landing roads are open and wide, you are not going to put a aircraft with 11m wing span to land on a road that has open width just less than 7m.

 

720985974_swedishroadplan1.PNG.ec5360e4261db4b11c684c04e0093cd0.PNG

 

And you want to be on the safe side anyways where the roads are very open, but the roadbases are on the sideroads with narrow one-way width.

 

(that curve has the same opening as the straight where Gripen is landing

 

The drag chute is not a problem really. It just might not be so effective in the ice roads as reverse thrust but if you will land in case of emergency.

 

So I think that the Viggen reverse thrust isn't making any better of the Viggen wings to cut those roadside trees down?


Edited by Fri13

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Yes, i have always admired the SAAB designs, they are marvels of design and economy. buy a foreign engine. Mate it to the the most affordable supersonic wing, and then add the extras you need to meet your design goals.

Not the best fighters in the world, but perhaps the most intelligent way to obtain capability for a small independent country.

 

But not as intelligent as buying a proven design off the peg. Let's face it, the SAAB aircraft program was just a political gesture to ensure people were employed in Sweden. The fact that not a single export was made of any of the SAAB designs says it all. Nice designs, but economical?

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So here, courtesy of the illustration powers of KSP, is a brief lesson on center of lift.

 

Every lifting surface has a center of lift. This is a theoretical tool that allows you to simplify calculations by finding the point where a single large force produces the same behavior as evaluating all of the small amounts of lift provided by each infinitesimal area of the wing.

 

No one wants to do 200 trillion calculations by slide rule to figure out what a wing is going to do.

 

For weird situations, like around the AoA where separations occur, this doesn't work quite as simply, which is where the power of modern computers and CFD comes in, because then you can do calculations for each little bit of the surface.

 

 

Anyhow for a delta wing the forces diagram looks about like this:

The blue arrow is the lift vector through the center of lift, the yellow ball is the center of mass, and the elevon is highlighted in green.

 

https://www.dropbox.com/s/s2k3nnktakrq3f4/VigFigOne.jpg?dl=0

 

The problem here is that the downward force of the mass is far from the center of lift, and is pulling the nose down quite a lot. So the elevon has to produce a lot of down force to counteract that torque. It makes trimming things nicely a pain, and the elevon is using a lot of it's control authority just to keep the nose from sinking.

 

 

Now add in a set of canards, fairly large fixed leading portion with a flap, just as in the Viggen. Also give the canards a few degrees of positive AoA.

 

https://www.dropbox.com/s/5ceic1cs3lv1hhn/VigFigTwo.jpg?dl=0

 

Notice that the blue center of lift now moves almost exactly to the center of mass. Because the lift and mass are basically at the same point, now the elevon can use all of the control force it generates to maneuver the plane, because it's not wasting a lot force to keep the nose up. This also reduces your trim drag for level flight because the elevon can be in a neutral or near neutral position.

 

 

The AoA of the canard contributes a lot to the forward movement of the center of lift. In the above linked diagram, the few degrees of AoA I put in accounts for almost half of the center of lift movement.

 

The reason for this is that the AoA of the canards tilts their lift vector toward the plane's tail, so more of the canards' lift vector is aligned in a direction that counteracts the nose down moment of the delta wing. Here's a very exaggerated example

 

https://www.dropbox.com/s/wbf670yxv35gpvj/VigFigFour.jpg?dl=0

 

 

Finally here's a diagram of canards by themselves, showing just the canards' center of lift in a Viggen-like configuration.

 

https://www.dropbox.com/s/e9mexl6tzvnh5mb/VigFigThree.jpg?dl=0

 

It's possible that the canards on the Viggen do help prevent separation a bit, sort of like a leading edge slot, but they're very big and far enough ahead of the wing so that most of their effect is likely just from moving the center of lift to be closer to the center of mass.

 

Anyhow, I hope the linked images help explain how canards help move center of lift around.

 

 

Disclaimer, KSP uses a simplified aero model, and as such is not a good aircraft design tool. However, with the large catalog of parts and Center of Gravity, Thrust vector, and Center of Lift tools it's really great for throwing together a diagram for certain aspects of basic aerodynamic principles.

 

Now if you excuse me, I think I'm going to go put landing gear on my diagram and see how it flies. :)

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Not a problem.

The drag chutes ain't larger than the aircraft wingspan, so you will not get tangled to the trees. And the engine thrust will keep it rear even in the wing (that even trees side of the road will block).

 

And the usual emergency landing roads are open and wide, you are not going to put a aircraft with 11m wing span to land on a road that has open width just less than 7m.

 

[ATTACH]160357[/ATTACH]

 

And you want to be on the safe side anyways where the roads are very open, but the roadbases are on the sideroads with narrow one-way width.

 

(that curve has the same opening as the straight where Gripen is landing

 

The drag chute is not a problem really. It just might not be so effective in the ice roads as reverse thrust but if you will land in case of emergency.

 

So I think that the Viggen reverse thrust isn't making any better of the Viggen wings to cut those roadside trees down?

The Viggen was made to operate from road bases. not in case of an emergency but full time. When you have thrust reversers you can use them to stop at slow speed on ice, you can park the plane between trees on either side, and then reverse out again. You can not do a go around after the chute has been deployed/failed. Add all the little advantages together and you get an aircraft that can operate more efficiently away from heavy infrastructure than any contemporary design.

Otherwise, my post was a bit crass, for that i am sorry.

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But not as intelligent as buying a proven design off the peg. Let's face it, the SAAB aircraft program was just a political gesture to ensure people were employed in Sweden. The fact that not a single export was made of any of the SAAB designs says it all. Nice designs, but economical?

 

Economical yes, if you allow for the fact that independence is worth paying for in the eyes of the Swedes.

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But not as intelligent as buying a proven design off the peg. Let's face it, the SAAB aircraft program was just a political gesture to ensure people were employed in Sweden. The fact that not a single export was made of any of the SAAB designs says it all. Nice designs, but economical?

 

The fact that no such design existed at the time might have influenced the decision somewhat.

 

You might want to do some fact checking about SAAB exports though.

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Thanks esb77,

 

from how i understand you: As lift increases the separation of centre of mass and lift causes the nose to pitch down. From before i know that deltas have poor sustained turn rates, you need a greater angle of attack to do the same work. Is this because the elevons has to counteract a aft lift centre as well as the force to turn ?

 

IE, the aft centre act as a leverage on the weight centre counter to your control input ?


Edited by pegon

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This thread is full of bad posts. Please stop making bad posts about the Viggen and make good posts instead.

 

A few points of order:

 

It's possible that the canards on the Viggen do help prevent separation a bit, sort of like a leading edge slot, but they're very big and far enough ahead of the wing so that most of their effect is likely just from moving the center of lift to be closer to the center of mass.
Please refer to the fpl 37 aerodynamics compendium, page 14 and onwards, for an extensive discussion of the aerodynamic effects of the a/c 37 canards.

 

And yes, those F-16 can also land on a 800 meter strip without problem, with a chute of course.

And the Gripen can do it without a chute. Both are several tons lighter than the Viggen, though. Would you like some graphs? Probably not, but here you go anyway:

 

hXLm1AW.png

0AUS1Nx.png

 

Seriously, you guys. Stop. The claim that the F-16 can land on an 800m strip with a chute is true as far as I know, but it's also completely nonsensical in this context and shows you haven't even understood what the design specifications for the Viggen called for. The Viggen wasn't designed to be able to land on 800 meter strips. It was designed to live on 800 meter strips. That means, an 800 meter strip is what you have, and all you have. The aircraft has to be able to make do with the 800 meter strips in all possible operational conditions and even with heavy loads. On the AJ 37 you can do the 15.5° no-flare approach with thrust reverser pre-selected up to an aircraft weight of 14 tons (80% fuel with no external load) and still have good margins on an 800 meter strip even on a slippery runway.

 

Saab didn't put the thrust reverser on there because they thought it looked cool, they put it on there because at the time it was the only way to meet the landing roll requirement on an aircraft this heavy (other than a carrier-style hook, but that requires infrastructure so it's even worse than the chute).

 

But not as intelligent as buying a proven design off the peg. Let's face it, the SAAB aircraft program was just a political gesture to ensure people were employed in Sweden. The fact that not a single export was made of any of the SAAB designs says it all. Nice designs, but economical?

Not sure if you're trolling or if you're genuinely this confused.


Edited by renhanxue
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The Viggen was made to operate from road bases. not in case of an emergency but full time. When you have thrust reversers you can use them to stop at slow speed on ice, you can park the plane between trees on either side, and then reverse out again. You can not do a go around after the chute has been deployed/failed. Add all the little advantages together and you get an aircraft that can operate more efficiently away from heavy infrastructure than any contemporary design.

Otherwise, my post was a bit crass, for that i am sorry.

Well, I meant with emergency as a war time, not the peace time other than training. As common roads doesn't like the heat and wearing caused by the aircrafts.

 

And sure you can go easier on ice, but as viggen doesn't have a nails in tires it is as well in risk for sideslips and such if speeding too much.

So basically same limits as others, just capability to slow down other means at slow speeds.

 

And yes, the capability to taxi to short shelter side of the road and then just reverse away by own power is good. But I take more likely even a common tractor to push aircraft out if the shelter has the service gear and small crew.

 

It is fancy and useful feature, not worse than a chute. And that is important thing.

 

An trained crew and field operation base turns aircrafts back to combat quickly, not because how those brake or turn, but how quickly you can re-arm and refuel (can you refuel while engine running etc?) And that is where a Gripen and Viggen etc has put effort on.

 

Like consider a Su-25A... Hate when you need to wait engines spool down before rearm and refuel. With Viggen it is just nice to land, turn around (roads offer enough space to do it normally) and then rearm and refuel and take off.

 

With a Su-25 even I will go to look something else to do meanwhile...

 

The reverse thrust is fancy and useful feature, but I wouldn't take it as so superior to landing shortening.

As there are other ways to do same like a same method as carriers catches aircrafts with cables if the aircraft supports it. And instead running out from deck, on road you have longer distance to smoothly slow down with them. And it as well puts less wear to cables as does to aircraft hook. This if really wanted ie stop in 200-300m distance even.

 

The chute benefit is as well that in a inverted flat spin, you can always open the chute and use it once to drag you pointing down :-D

With reverse thrust you can't even do that, even if you could reverse in flight ;-)

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This thread is full of bad posts. Please stop making bad posts about the Viggen and make good posts instead.

 

A few points of order:

 

Please refer to the fpl 37 aerodynamics compendium, page 14 and onwards, for an extensive discussion of the aerodynamic effects of the a/c 37 canards.

 

 

And the Gripen can do it without a chute. Both are several tons lighter than the Viggen, though. Would you like some graphs? Probably not, but here you go anyway:

 

hXLm1AW.png

0AUS1Nx.png

 

Seriously, you guys. Stop. The claim that the F-16 can land on an 800m strip with a chute is true as far as I know, but it's also completely nonsensical in this context and shows you haven't even understood what the design specifications for the Viggen called for. The Viggen wasn't designed to be able to land on 800 meter strips. It was designed to live on 800 meter strips. That means, an 800 meter strip is what you have, and all you have. The aircraft has to be able to make do with the 800 meter strips in all possible operational conditions and even with heavy loads. On the AJ 37 you can do the 15.5° no-flare approach with thrust reverser pre-selected up to an aircraft weight of 14 tons (80% fuel with no external load) and still have good margins on an 800 meter strip even on a slippery runway.

 

Saab didn't put the thrust reverser on there because they thought it looked cool, they put it on there because at the time it was the only way to meet the landing roll requirement on an aircraft this heavy (other than a carrier-style hook, but that requires infrastructure so it's even worse than the chute).

 

 

Not sure if you're trolling or if you're genuinely this confused.

Yes, swedish made it as requirement as first thing in war would be that airfields are bombed and nuked.

 

And so did soviets have a same thing. Requirements to land on short sand roads even and take off from such. Why very sturdy undercarriage and all protection for FOD and boosters to get aircraft up from a short road no matter what weather, from south to north, west to east in their country. And that country is world largest one, so you need to be able operate anywhere at any time.

 

This is reason as well why F/A-18 was chosen by Finland as they can do as well short landings from roadbases. Experience from the viggen put it there as well very likely.

 

British designed the Harrier, just so the same thing could be done.

 

Everyone in europe wanted and needed a short landings. Americans didn't as they wanted long distance fighter to fight over sea. A F-14 and F-15 designed just for that worst case scenario. Other just scrapped the aircraft hook operations.

 

 

Viggen is just so nice and clever aircraft with special features.

And that puts me to wait a Gripen module to be released in 30-40 years...

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"blablabla whole post"

 

Sweet way to bitch slap some sense into a thread. Couldn't +rep you any more but kudos at least.

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Please refer to the fpl 37 aerodynamics compendium, page 14 and onwards, for an extensive discussion of the aerodynamic effects of the a/c 37 canards.

 

 

It doesn't really get interesting until around page 18, where it starts going into depth about vortex lift effects, but from there until the references at the end it is a wonderful read. If you don't mind Swedish grammar, or more specifically the grammar of Swedish engineers.

 

Thank you for the reference.

 

 

I'm slightly jealous of virvel as a word root compared to whirl or vortex, now. Are aerodynamic texts supposed to have linguistic side effects?

Callsign "Auger". It could mean to predict the future or a tool for boring large holes.

 

I combine the two by predictably boring large holes in the ground with my plane.

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The F-16 does not have a delta wing. However, it does have fly by wire. If you don't know know how this relates to landing speed and distance, what point is there to argue with you ?

 

 

This thread is full of bad posts. Please stop making bad posts about the Viggen and make good posts instead.

 

(...)

 

And the Gripen can do it without a chute. Both are several tons lighter than the Viggen, though. Would you like some graphs? Probably not, but here you go anyway:

 

 

Seriously, you guys. Stop. The claim that the F-16 can land on an 800m strip with a chute is true as far as I know, but it's also completely nonsensical in this context and shows you haven't even understood what the design specifications for the Viggen called for.

 

Jeeez, I think what people here should stop is thinking it's some sort of my plane vs. your plane contest and ease down with emotions. I linked to interesting topic about braking chutes hinting that they also allow F-16 to land on a short strips never claiming that it makes them equal to Viggen capabilities. And voilla' being jumped on like I'm bashing your favourite toy or something.

 

Discuss the topic. Stop coming here with a mindset that this is an argument. Or is that impossible in the internet?

 

And thanks for the graphs. No sarcasm.

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It might be worth noting that while FBW systems and relaxed stability are geared in part toward gaining low speed control at high AoA, this mostly in the context of air combat. For the most part this isn't that useful for landing because extreme high alpha creates problems with visibility and tail strikes. It's nice to have for a panicky wave off, but if you're near max AoA on approach you're really hoping that the plane starts to pull up before the back end of the plane starts making horrible noises and throwing sparks as the runway starts grinding it off.

 

Speed has a vertical as well as a horizontal component. Thus, when you increase the vertical speed, you can decrease it in the horizontal. This you can do without raising the nose aggressively, loosing neither visibility or fearing tailstrikes. It is my understanding that both the LERX (on F-18 ) and canards work in the same way by allowing for a high sink rate on landing. Can anyone confirm this ?


Edited by pegon
added "F-18"

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My private theory has been that the Viggen does the same thing when it lowers its canard flaps. But this also increases lift, so perhaps this is different ?

 

That's correct.

The Viggen canards work in a manner like big vortex generators that supply high energy air to the main wing, in high AoA flight. LERX work in a similar manner.

LERX are essentially low aspect wings, meaning their wingspan is short in relation to the chord. Think stubby wings. A glider has high aspect wings, as a comparison.

Low aspect wings does not produce that much lift but has the advantage of having a high maximum AoA, meaning they will stall at a higher AoA. This means the LERX will continue to supply high energy vortices for the main wing that will help it delay the flow separation associated with a stall.

In a nutshell, anyway ;)

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