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PostPosted: 28 Oct 2009, 15:03 
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Very clever Wolfman! I should use my thinktank a bit more...

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PostPosted: 04 Nov 2009, 15:00 
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Not that I want to let myself look like a major expert on the topic, but I did notice a few things...
(Just as background, I saw a fantastic show of maneuverability by a Mirage 2000 yesterday on YouTube - it doesn't have canards...)

I looked at the J-10 pictures, especially plan view (from the top). The wings are VERY basic - like most of us would've drawn it. Consider Gripen's wings, which seemingly have slats (if that's the correct word for it - like the Mirage 2000 & F1 have).
The wing design seem to be ancient.

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PostPosted: 04 Nov 2009, 16:47 
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rynopot wrote:
Not that I want to let myself look like a major expert on the topic, but I did notice a few things...
(Just as background, I saw a fantastic show of maneuverability by a Mirage 2000 yesterday on YouTube - it doesn't have canards...)

I looked at the J-10 pictures, especially plan view (from the top). The wings are VERY basic - like most of us would've drawn it. Consider Gripen's wings, which seemingly have slats (if that's the correct word for it - like the Mirage 2000 & F1 have).
The wing design seem to be ancient.


The Gripen has leading edge flaps not slats, the Mirage 2000 and F1 have both got slats and when coupled together with a delta wing, you have an unstable platform.

I'm confused are you saying the J-10 or the Gripens wings look ancient.


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PostPosted: 05 Nov 2009, 10:16 
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From the plan view of the J-10 - the deltawing doesn't have slats / flaps. or not even the jagged edge the Cheetah wing had.

Gripen's wings are DEFINITELY not ancient... [-X

EDIT: Got to correct myself, just looked at the pictures of the J-10, and it does have leading edge flaps. So I withdraw my words on the J-10 wing design being ancient.


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PostPosted: 05 Nov 2009, 10:42 
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I've always wondered why Saab opted for leading edge flaps, while the Eurofighter and the Rafale has leading edge slats. Pros, cons? Weight saving? Ease of maintenance?


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PostPosted: 05 Nov 2009, 11:00 
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Hi guys
I'm a dummy and would like to know, what is the difference between "Leading edge flaps" and "slats"? I was under the impression that it is one and the same thing.
By the way, the J10 in fact do have slats/leading edge flaps. There is one solid one on each wing. We shouldn’t be to sceptic about the newest Chinees designs as it might be to our perril...
It is interesting to note that the Gripen has an inner slat and an outer slat on each wing. These are separately controlled, as I've noticed on some of the pics in this thread. This seems to be more sophisticated for it surely needs a more complex control system. It is as if the designers were compensating in a different manner to the air flowing over the outside part of the wing than for the air over the inner side of the wing. This seems to indicate a more sophisticated control system is controlling this baby. The J10 slats are one-piece, like on most planes (ie.F1, F16, F18 and Mig. 29). I will have to check this issue on the more modern fighters like EF2000, Rafael and F22....?

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PostPosted: 05 Nov 2009, 12:45 
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boertjie wrote:
Hi guys
I'm a dummy and would like to know, what is the difference between "Leading edge flaps" and "slats"?
A leading edge flap or commonly known as a krueger or nose flap is a small auxiliary aerofoil which hinges about the leading edge.

A slat is a small auxiliary aerofoil, placed along the leading edge of the main aerofoil section, able to move forwards or backwards thus opening or closing the gap (called a slot).

Some details on the krueger flap:
Increase of maximum lift 50%, angle of aerofoil at maximum lift 25 degrees.

Some details on slats and slots:
The maximum lift coefficent of an aerofoil may be increased by as much as 60%, in addition, the air flowing through the slot causes the boundary layer to remain smooth much longer, to the extent that the stalling angle may be increased from 15 degrees up to 22 degrees.


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PostPosted: 05 Nov 2009, 13:16 
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Ok Wolfman
Am I correct in saying that the leading edge flap simply curves the leading edge more where as the slat actually curves but also moves away from the main wing and thereby create a opening that allows the air to move thru it this enhances the boundary layer over the wing.
Why does it seam as thou the flap then has a overall better effect specifically related to AoA?
I would have thought that the slat seams the better of the two seeing how it actually improves airflow....

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PostPosted: 05 Nov 2009, 14:09 
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boertjie wrote:
Ok Wolfman
Am I correct in saying that the leading edge flap simply curves the leading edge more where as the slat actually curves but also moves away from the main wing and thereby create a opening that allows the air to move thru it this enhances the boundary layer over the wing.
Why does it seam as thou the flap then has a overall better effect specifically related to AoA?

To put it in to simple words and maybe generalize, the leading edge flaps extend or retract just like a plain or cambered flap and it increases the camber of the wing which helps with lift, the slat has a fixed curvature and will either move forwards or backwards from/to the aerofoil, which in turn creates a slot which decreases the angle of attack at that part of the wing, thus delaying the onset of a stall.

Just remember the stalling angle is the angle viewed by the pilot at which the aircraft stalls.

The critical angle is the angle of attack at which airflow over the surface of the wing breaks away causing loss of lift and is therefore always constant depending on the aerofoil.

Oryx, are my facts right, its been a long time since I opened a book on aerodynamics and can you please add something to explain what advantages they have over each other.


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PostPosted: 05 Nov 2009, 15:06 
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boertjie wrote:
It is interesting to note that the Gripen has an inner slat and an outer slat on each wing. These are separately controlled,


Actually the Gripen leading edge flaps are not divided into two sections. Perhaps the dogtooth helps to create an illusion here?

The trailing edge elevons though are divided. Although I'm not sure elevons is the correct word here. The trailing edge "elevons" are not used for pitch, but for trim and roll.


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PostPosted: 05 Nov 2009, 15:41 
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Robban wrote:
The trailing edge elevons though are divided. Although I'm not sure elevons is the correct word here. The trailing edge "elevons" are not used for pitch, but for trim and roll.


Correct me if I'm wrong, but the elevons are used for pitch when something were to happen to the fly-by-wire, which would disconnect the canards so that they can move freely.


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PostPosted: 05 Nov 2009, 16:35 
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Wolfman wrote:
Robban wrote:
The trailing edge elevons though are divided. Although I'm not sure elevons is the correct word here. The trailing edge "elevons" are not used for pitch, but for trim and roll.


Correct me if I'm wrong, but the elevons are used for pitch when something were to happen to the fly-by-wire, which would disconnect the canards so that they can move freely.


That is probably correct, but during normal operations the canards provide for pitch. Although during rotation on take off the "elevons" push the tail down, while the canards lifts the nose. :)


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PostPosted: 06 Nov 2009, 09:19 
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Robban, you know way too much about Gripen... But I'm not complaining... :P


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PostPosted: 06 Nov 2009, 11:30 
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rynopot wrote:
Robban, you know way too much about Gripen... But I'm not complaining... :P


Well, I don't know about that. I'm just a happy amateur. An amateur that would like to know much MUCH more! :)

I've just been told by a very reliable source that it's not as easy as, canards provide for pitch and elevons provide for trim and roll. The control surfaces interactions are very complicated. It's difficult to explain, but to show you a very simplified explanation I've made a drawing that shows, according to my observations the general movements of the Gripens control surfaces. In flight as well as during take off and landing.

Remember these are my observations. Correct or not, I really don't know, so please don't take this drawing as fact. :)

Image


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PostPosted: 07 Nov 2009, 09:43 
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OK, this post is for people like me who are stirred into wide eyed wonderment when clever whizz-kids with big HP scientific (nogal) calculators start outsmarting each other with terms like AoA, boundary layer, viscosity, lift, thrust, Von Karman vortices, etc. And this post is in connection with flaps, slots and slats. And this stuff is just slapped together from the books on my shelf. And this is just because I would also like to know what's going on and what the whizz-kids are getting their undies in a knot about. So this is meant for guys like Pottie and me.
So why do whizz-kids stick flaps and slats and slots and stuff onto perfectly beautiful solid wings which work so well while cruising along?
Answer: Because the wing is not large enough to ensure comfortably low landing and take-off speeds; and because the wing is not large enough for serious manoeuvres in the case of the Gripen.
How large is large enough?
To provide a lift force, the wing needs to be able to deflect a stream of air; a larger stream of air deflected at a larger angle needs to be pushed harder by the wing. In turn, the air pushes harder back at the wing, providing lift, which is precisely the trick that the whizz-kids want to play on Mother Nature.
But, Mother Nature has a trick up her sleave too: to their horror, the whizz-kids discovered that if they take their cleverly conceived tiny wing and try to bend the air down far enough for their purposes, Mother Nature let go of the wing and just rushes straight over it, leaving a large wake behind it which tugs the whizz kids’ wing back instead of pushing it up. Then the wing is nothing more than a tree trunk being blown backwards in a strong gale; not very impressive huh?
Why?
Ludwig Prandtl gave us the answer in 1905. He was a German, of course. And he deserved a Nobel Prize for his insights but got none because the Nobel committee obviously think that small things, like boundary layers, amuses small minds not worthy of Nobel prizes. Ludwig said that air is almost inviscid, and not at all sticky like syrup. Obviaas! But that’s the whole point. It is almost inviscid. Almost, but not quite. Therefore it still sticks to a surface. While Pottie and me and most other people does not give this another thought, Ludwig said that because it still sticks to the surface, of a wing for instance, the air that actually touch the wing sticks to the wing, like in motionlessly attached to the wing! This happens while other air rushes over the wing at a crazy speed just a tiny distanc away from the wing. So this means that in a VERY short distance above the wing, the airspeed can differ from zero on the wing surface to Mach 0.99 just a millimetre away. Ludwig called this tiny region on the wing surface, where the air moves at such hugely different speeds from one point closer to the wing surface to tne next point further away from the wing surface, the boundary layer.
This boundary layer allows the airstream to follow the curves of the wing. Why? Because if there is no boundary layer which allows the airspeed to differ from zero on the wing surface (because of the viscosity of air) to a crazy speed just a tad away from the wing, the only other way nature can deal with its own viscosity in the presence of a wing rushing through it is to cause a jumble of mini-tornadoes (vortices) on the wing which allows the freestream rushing over the wing to slip past the wing. These vortices make it unnecessary for the airstream rushing over the wing to follow the contours of the wing.
Now, back to the boundary layer case. The craftily shaped wing curves dictate the tiny speed variations and consequently, the tiny pressure variations, above and below the wing. And if you get the shape of the wing right, the pressure below the wing is generally higher below the wing than above the wing. This pressure difference causes the pushing force of the air on the wing.
The key here is that the airstream must follow the curves of the wing. And, this can only occur if the boundary layer of the wing exists; the boundary layer being the region where the orderly increase of speed occurs from zero on the wing surface to the crazy speed of the onrushing air just a tad away from the wing surface. If the boundary layer breaks down into an incoherent jumble and tumble of vortices (like mini tornadoes mixed together), the airstream breaks away from the beautiful high-tech wing curves and cast off into its original straight path. In effect, the air now behaves as if it is flowing around a tree trunk.
(Sorry for repeating myself here, but if you are like me, you need a lot of explanations to understand nature.)
So the key here is Ludwig Prandtl’s boundary layer. No boundary layer, no lift, just huge drag like a tree trunk being blown in the wind. (If the Gripen was on fire at the time, it would be like a candle in the wind) And as the whizz-kids discovered, a boundary layer is like a woman: extremely fickle. She needs constant attention or she leaves. And that is where slats, flaps and slots come in.
For fast cruising speeds, you need a smallish wing. But for low speeds, the smallish wing needs to be angled more and more against the slower onrushing air to cause a larger deflection of the slower moving air in order to generate enough lift force to keep the Gripen in the air. This higher incidence angle, which the whizz-kids call the Angle-of-Attack (AoA), causes an ever increasing pressure recovery towards the back of the upper side of the wing. This pressure increase tend to slow down the boundary layer as it moves towards the trailing edge of the wing until it is eventually pushed off the wing. And, as explained above, to deal with the speed difference caused by the zero speed air attached to the wing surface (due to the air’s viscosity) and the feestream air rushing over the fast moving wing, nature needs either a boundary layer or a jumble of vortices on the wing. And if the boundary layer is pushed off the wing by a too steep Angle of Attack, nature causes a jumble of vortices on that part of the wing which make the wing looks like a flying tree trunk to the onrushing air. And even Pottie and I know that a tree trunk has difficulty staying in the air, let alone relying on the tree trunk to keep a whole airplane in the air. By the way, the whizz-kids’ code name for this phenomenon when the boundary layer is separated from the wing surface like this, is “flow separation.” So there we just busted another secret whizz-kid code.
So, to prevent flow separation (you hear that whizz-kids) it is important to help the boundary layer along when you want to fly slowly. Flaps, both leading edge and trailing edge flaps, make the wing look less like a flat plate and more like a smoothly curved plate. Boundary layers, like women, like smooth. When the flaps are moved slightly away from the wing to allow a slot between the flap and the wing, some of the higher pressure air below the wing slips through the slat and modifies and stabilises the boundary layer over the top of the wing to allow higher AoA’s which, in turn, allows slower speeds (or higher lift at high speeds).
And this is my 2c worth. And admittedly, I could not sit still long enough to edit this post because I need another cup of Java.

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