Leading Edge Slats
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Joined: Jan 2003
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From: SX in SX in UK
Leading Edge Slats
There are certain hallowed halls where the footfalls of PPL are not often heard, so forgive me for entering without knocking, but I have a question for my elders & betters!
I fly a Koliber, its the Polish-built version of the old Rallye, on steroids! It is fitted with full span retractable leading edge slats, which give excellent low-speed handling and STOL performance.
I was asked recently by a student pax what the slats did, to which I replied that they improve lift at low speeds.
But thinking about this later, it occured to me that I didn't really know how they worked mechanically, nor why they worked aerodynamically.
So if explain how I think they work, can someone come to my rescue and tell me how they really work.
At rest, the slats are held out by springs, (I think - its not in the POH).
As the airflow over the wings increases, a combination of the airflow pushing against the front of the slat and reducing pressure in the 'slat gap' (due to increasing airflow) sucking against the LE of the wing and back of the slats, closes them.
As the aircraft slows down, the strength of the springs is able to overcome the force of the airflow and the slats open. Higher pressure air from under the wing flows through the slat gap venturi, accelerates and becomes low pressure air on the top of the wing, thus increasing the lift.
The slats are kept open as the springs are able to overcome the force of the reducing airflow against them and the net 'suck' within the slat gap is also reducing as the airspeed drops.
OK, so thats Kolibears Theory of Slats - can someone now put me straight?
Excuse me now as I tiptoe out & leave you all in peace
I fly a Koliber, its the Polish-built version of the old Rallye, on steroids! It is fitted with full span retractable leading edge slats, which give excellent low-speed handling and STOL performance.
I was asked recently by a student pax what the slats did, to which I replied that they improve lift at low speeds.
But thinking about this later, it occured to me that I didn't really know how they worked mechanically, nor why they worked aerodynamically.
So if explain how I think they work, can someone come to my rescue and tell me how they really work.
At rest, the slats are held out by springs, (I think - its not in the POH).
As the airflow over the wings increases, a combination of the airflow pushing against the front of the slat and reducing pressure in the 'slat gap' (due to increasing airflow) sucking against the LE of the wing and back of the slats, closes them.
As the aircraft slows down, the strength of the springs is able to overcome the force of the airflow and the slats open. Higher pressure air from under the wing flows through the slat gap venturi, accelerates and becomes low pressure air on the top of the wing, thus increasing the lift.
The slats are kept open as the springs are able to overcome the force of the reducing airflow against them and the net 'suck' within the slat gap is also reducing as the airspeed drops.
OK, so thats Kolibears Theory of Slats - can someone now put me straight?
Excuse me now as I tiptoe out & leave you all in peace
Do a Hover - it avoids G
Joined: Oct 1999
Posts: 2,201
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From: Chichester West Sussex UK
Believe me there is nothing hallowed about these halls. So welcome.
I am very prepared to believe I am your elder – I would be amazed if I was not.
I have no idea if I am your better but on the basis of what you have written here I would doubt it. Your words seem fine to me but I have never flown an aircraft with “automatic” slats.
So give it time and I am sure somebody who has will respond. If not we shall have to dig into some books.
Cheers
John
I am very prepared to believe I am your elder – I would be amazed if I was not.
I have no idea if I am your better but on the basis of what you have written here I would doubt it. Your words seem fine to me but I have never flown an aircraft with “automatic” slats.
So give it time and I am sure somebody who has will respond. If not we shall have to dig into some books.
Cheers
John
Joined: Sep 2002
Posts: 2,188
Likes: 6
From: La Belle Province
Slats and how they work
OK, starting with how slats work aerodynamically.
First of all slats do not increase lift. In fact, if you were to deploy slats and change nothing else you would normally lose a small amount of lift.
What slats actually do is delay the onset of the stall, and that enables you to fly the aircraft at a higher angle of attack, which generates more lift. But the lift comes from the angle of attack increase.
Slats delay the stall by, in simple terms, re-energising the flow on the upper surface. A wing stalls when the adverse pressure gradient becomes too large for flow to continue to move against that gradient (which is of course on the upper surface). The flow starts to separate from the wing, since it's easier to move away from the wing than to keep fighting the pressure gradient.
The flow area which is affected first is the boundary layer, because that portion of the flow is the slowest, and has the least energy to use to overcome the gradient.
Slats (and many other stall delaying devices) help to renergise the flow by providing a 'leading edge' which is further back on the wing (so the boundary layer growth starts further back on the chord) and by providsing highly energised air moving through the slat gap, which helps to energise the boundary layer. (Boundary layer blowing works in a similar way).
As to the mechanical operation of your slats, I'd have to see the geometry. I'm surprised you say they are held out by springs (at low speed), and the airflow at higher speeds pushes them back.
A more typical installation (e.g. Me109) would be to hold the slats IN with springs, and rely on the leading edge suction which develops as you slow down to pull the slats out. But it rather depends on the wing geometry (to know how the suction behaves) and the mechanical geometry (to see how the various forces act).
Hope that helps.
First of all slats do not increase lift. In fact, if you were to deploy slats and change nothing else you would normally lose a small amount of lift.
What slats actually do is delay the onset of the stall, and that enables you to fly the aircraft at a higher angle of attack, which generates more lift. But the lift comes from the angle of attack increase.
Slats delay the stall by, in simple terms, re-energising the flow on the upper surface. A wing stalls when the adverse pressure gradient becomes too large for flow to continue to move against that gradient (which is of course on the upper surface). The flow starts to separate from the wing, since it's easier to move away from the wing than to keep fighting the pressure gradient.
The flow area which is affected first is the boundary layer, because that portion of the flow is the slowest, and has the least energy to use to overcome the gradient.
Slats (and many other stall delaying devices) help to renergise the flow by providing a 'leading edge' which is further back on the wing (so the boundary layer growth starts further back on the chord) and by providsing highly energised air moving through the slat gap, which helps to energise the boundary layer. (Boundary layer blowing works in a similar way).
As to the mechanical operation of your slats, I'd have to see the geometry. I'm surprised you say they are held out by springs (at low speed), and the airflow at higher speeds pushes them back.
A more typical installation (e.g. Me109) would be to hold the slats IN with springs, and rely on the leading edge suction which develops as you slow down to pull the slats out. But it rather depends on the wing geometry (to know how the suction behaves) and the mechanical geometry (to see how the various forces act).
Hope that helps.
Joined: Sep 2001
Posts: 764
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From: UK
The Me109 slats, at least on the "Gustav" (Me109G), had no springs. My basic understanding is that the slats extended when the stagnation point moved below their leading edge as AoA increased. In the Me109, if you had any sideslip present then one slat would extend before the other. However, no roll-off occurred which would tend to prove the above mentioned point that for a given AoA the coefficient of lift (Cl) is the same with slats in or out (but read on!). Another way of looking at what they do is that they extend the Cl vs AoA curve to a higher value but do not displace the curve left or right (whereas flaps displace the Cl vs AoA curve to the left).
I have read reports that the Me109E did drop a wing as the slats extended but that was quite a different wing to the "Gustav". Also, I seem to remember that the A4 Skyhawk had such slats, and that these could deploy asymmetrically giving a roll-off to the extent that the Blue Angels welded them fixed! Any USN A4 drivers confirm this?
I have read reports that the Me109E did drop a wing as the slats extended but that was quite a different wing to the "Gustav". Also, I seem to remember that the A4 Skyhawk had such slats, and that these could deploy asymmetrically giving a roll-off to the extent that the Blue Angels welded them fixed! Any USN A4 drivers confirm this?
Joined: Apr 2002
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From: Home
Kolibear
Assuming the Koliber wing is the same as the Rallye, the slats are not held out by springs. The slats will usually be extended after landing, but you can (gently) push them closed. If the seals on the air dampers are not in good condition (the air dampers are awkwardly located inside the wing, and prevent the slats from extending or retracting violently), the slats will move freely when you push or pull them.
The extension of the slats at high angles of attack is an aerodynamic effect. For precise details of how this works, I too need the help of an elder and better!
Assuming the Koliber wing is the same as the Rallye, the slats are not held out by springs. The slats will usually be extended after landing, but you can (gently) push them closed. If the seals on the air dampers are not in good condition (the air dampers are awkwardly located inside the wing, and prevent the slats from extending or retracting violently), the slats will move freely when you push or pull them.
The extension of the slats at high angles of attack is an aerodynamic effect. For precise details of how this works, I too need the help of an elder and better!
Joined: Jun 1997
Posts: 224
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From: auckland, new zealand
As an ex display driver on the A4 (way ex), I recall fondly (not) the assymetric slat extension we got on that aeroplane.
While the roll tendancy was easily enough controllable, there definitely was a roll tendancy. This would suggest a lift change between "in " and "out" - whether this was due to more lift on the extended side, or the lift vector moving outboard, I don't know.
I, too, would be interested to hear more on the topic from an aerody guy.
While the roll tendancy was easily enough controllable, there definitely was a roll tendancy. This would suggest a lift change between "in " and "out" - whether this was due to more lift on the extended side, or the lift vector moving outboard, I don't know.
I, too, would be interested to hear more on the topic from an aerody guy.
Joined: Mar 2003
Posts: 4
Likes: 0
From: swindon
I too have a Rallye with the Magic slats, and I can let you in on how they work: LOMCEVAK had it spot on, it's just migration of the stagnation point.
Most of you will know this,so forgive me for prattling on for a moment, but it might be new for one or two. The stagnation point is the point on a wing (section) leading edge, that marks the divide between air going over the wing, and air going under the wing. You'd expect the stagnation point to be pretty much at the front of the leading edge,but actually it's quite a bit underneath because of the upwash which goes on before the air hits the wing.
The stagnation point naturally migrates even further underneath the wing as angle of attack increases, and when it goes below the back lower edge of the slat, hey presto: out they pop! On the Rallye, you'll then immediately notice the heavy nose-down trim will disappear, along with the ugly tailplane buffet, and the craft will then zoom into the sky as smooth as silk. For a short period. Until all remaining kinetic energy has been translated to potential.
On the Rally, asymmetric deployment is avoided by interconnecting with control cables. No springs needed, just the aforementioned air dampers to stop them banging in and out.
Kolibear: Here's a useful flight test you can do. I won't bias your mind by telling you my results. Try a *slow* deceleration and see what speed the slats start extending, and finish extending. Then accelerate and see the speeds for the start and end of the closing course. Do this half a dozen times to make sure your data is good, and then repeat for a couple of weights. THEN, compare the figures to those in the flight manual. You'll find they're different, and I bet you'll find that the hysteresis in the manual (difference in opening and closing speeds to avoid them banging in and out in turbulence) doesn't exist. At least it doesn't on my French aircraft! I'd like to see if the PZL manuals are different.
Martin
Most of you will know this,so forgive me for prattling on for a moment, but it might be new for one or two. The stagnation point is the point on a wing (section) leading edge, that marks the divide between air going over the wing, and air going under the wing. You'd expect the stagnation point to be pretty much at the front of the leading edge,but actually it's quite a bit underneath because of the upwash which goes on before the air hits the wing.
The stagnation point naturally migrates even further underneath the wing as angle of attack increases, and when it goes below the back lower edge of the slat, hey presto: out they pop! On the Rallye, you'll then immediately notice the heavy nose-down trim will disappear, along with the ugly tailplane buffet, and the craft will then zoom into the sky as smooth as silk. For a short period. Until all remaining kinetic energy has been translated to potential.
On the Rally, asymmetric deployment is avoided by interconnecting with control cables. No springs needed, just the aforementioned air dampers to stop them banging in and out.
Kolibear: Here's a useful flight test you can do. I won't bias your mind by telling you my results. Try a *slow* deceleration and see what speed the slats start extending, and finish extending. Then accelerate and see the speeds for the start and end of the closing course. Do this half a dozen times to make sure your data is good, and then repeat for a couple of weights. THEN, compare the figures to those in the flight manual. You'll find they're different, and I bet you'll find that the hysteresis in the manual (difference in opening and closing speeds to avoid them banging in and out in turbulence) doesn't exist. At least it doesn't on my French aircraft! I'd like to see if the PZL manuals are different.
Martin
