Leading edge slats - the in's and out's....
12-31-2012, 03:36 AM
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Leading edge slats - the in's and out's....
Hi guys,
I'll be starting my large SM F-86 Build shortly and I was wondering about the leading edge slats on this model. Whist I definitely want to make these operational on my F-86, I have no idea how, when or why to use them CORRECTLY during flight.
Any of you guys with experience of MODEL leading edge slats, please respond and allow me to learn everything I need to know about these. How much do I need and when should I use them? How much servo power will I need - the SM F-86 utilises two servo's per wing.
Thanks for sharing your experience.
Cheers,
Jan
I'll be starting my large SM F-86 Build shortly and I was wondering about the leading edge slats on this model. Whist I definitely want to make these operational on my F-86, I have no idea how, when or why to use them CORRECTLY during flight.
Any of you guys with experience of MODEL leading edge slats, please respond and allow me to learn everything I need to know about these. How much do I need and when should I use them? How much servo power will I need - the SM F-86 utilises two servo's per wing.
Thanks for sharing your experience.
Cheers,
Jan
12-31-2012, 05:33 AM
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RE: Leading edge slats - the in's and out's....
Well I can explain what they do and when to use them. However I have only experienced models with built in slots up to now, but my FEJ 86 has them and this will be flown one the weather improves. I am told that they are well worth having.
Basically a slat ensures that the airflow over the top of the wing stay attached to the wing for longer, and at higher angles of attack. There fore the slat delays the onset of the stall allowing rotation somewhat earlier, and landings to be slower without the risk of departure. So basically the are used on take off and landing.
In fact the F86 had the original Handley-Page slat arrangement. They ran on on tracks using rollers. They actually fell out into the deployed position because they were heavy. Once the aircraft had taken off air pressure would push them shut. The pilot had no control over them.
John
Basically a slat ensures that the airflow over the top of the wing stay attached to the wing for longer, and at higher angles of attack. There fore the slat delays the onset of the stall allowing rotation somewhat earlier, and landings to be slower without the risk of departure. So basically the are used on take off and landing.
In fact the F86 had the original Handley-Page slat arrangement. They ran on on tracks using rollers. They actually fell out into the deployed position because they were heavy. Once the aircraft had taken off air pressure would push them shut. The pilot had no control over them.
John
12-31-2012, 05:52 AM
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RE: Leading edge slats - the in's and out's....
I had leading edge slats on my FC F86 Sabre. They need fairly low power servos to deploy them and in the FC model had 2 servos per wing. From memory they were JR341 servos. The servos were arranged so the servo arm was almost in line with the track arm when fully deployed. The slats are only used for low speed manoeuvres like landing when the flaps are deployed. On mine we had 50% flaps and slats for takeoff and 100% flaps and slats for landing. I have since had others say they should only be used 100% deployed with 100% flaps. They make a huge difference and well worth using.
John
John
12-31-2012, 06:57 AM
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RE: Leading edge slats - the in's and out's....
I have working slats on my Fibreclassics F-86 (bought from JGWright in post above!). To expand on what JohnMac has said-
The slot formed by the opening of the slat does not increase the lift at any angle of attack up to the normal stall point, so for the same AoA you get the same lift. What it does is push the stall angle up to a higher AoA which means you can raise the nose to greater angles and thus get more lift. You get no benefit from the slot unless you are willing to pull AoA higher than the normal stall angle. If you just pull to the normal attitude that you would without having slats, the slot formed by the slat makes zero difference. So it will only be capable of having any effect if you are willing to pull the nose higher than you normally would.
The slat itself, if it moves forward which it does on the F-86, increases the wing area which reduces the stall speed though the difference in our scale is going to be very small, so few mph that you are unlikely to be aware of it. Change in speed is the inverse of the square root of the change of area so the sqrt of say a 5% increase in area on a stall speed of say 35mph isn't enough to notice (less than 1mph!). The slat also moves the area of the wing forward thus moving the centre of lift forward a little bit, helping with nose up trim at low speeds.
However, you can’t just open up a slot and expect it to work. For it to actually increase the stall angle by a useful amount and not just add drag, the shape of the slot and the depth have to be very carefully designed and wind-tunnel tested, otherwise it has only a small effect. Full-size put a lot of work into testing it and getting it to actually do something worthwhile.
That also tells you that varying the gap such as having separate take-off setting and landing settings is pointless. Assuming that you have a correctly designed and proven slot shape and gap, why prevent it from working at its optimum by changing the gap and removing the insurance of the extra AoA that is available?
As JohnMac said, the F-86 had no pilot control for the slats. Air pressure from forward speed closed them. The speed at which the slats start to close after take-off and will be fully open as it slows down to land, is well above the take-off and approach speed. So it took off and accelerated away for some time with slats fully open, and they were fully open again before landing. Therefore if you want scale realism, set your slats program for fully open or fully closed, and open them for take-off and landing. The full-size had a speed range during which the slats were driven shut or could open up due to the varying air pressure but it was above initial climb or approach speeds and was a function of the forces involved rather than any need for a variable setting. I did have the reference with the fully open and fully closed speeds but can’t find it at the moment.
On my Fibreclassics F-86 I have the slats driven by the flap switch, so that flaps up = slats closed, flaps at take-off or landing = slats fully open.
The slot formed by the opening of the slat does not increase the lift at any angle of attack up to the normal stall point, so for the same AoA you get the same lift. What it does is push the stall angle up to a higher AoA which means you can raise the nose to greater angles and thus get more lift. You get no benefit from the slot unless you are willing to pull AoA higher than the normal stall angle. If you just pull to the normal attitude that you would without having slats, the slot formed by the slat makes zero difference. So it will only be capable of having any effect if you are willing to pull the nose higher than you normally would.
The slat itself, if it moves forward which it does on the F-86, increases the wing area which reduces the stall speed though the difference in our scale is going to be very small, so few mph that you are unlikely to be aware of it. Change in speed is the inverse of the square root of the change of area so the sqrt of say a 5% increase in area on a stall speed of say 35mph isn't enough to notice (less than 1mph!). The slat also moves the area of the wing forward thus moving the centre of lift forward a little bit, helping with nose up trim at low speeds.
However, you can’t just open up a slot and expect it to work. For it to actually increase the stall angle by a useful amount and not just add drag, the shape of the slot and the depth have to be very carefully designed and wind-tunnel tested, otherwise it has only a small effect. Full-size put a lot of work into testing it and getting it to actually do something worthwhile.
That also tells you that varying the gap such as having separate take-off setting and landing settings is pointless. Assuming that you have a correctly designed and proven slot shape and gap, why prevent it from working at its optimum by changing the gap and removing the insurance of the extra AoA that is available?
As JohnMac said, the F-86 had no pilot control for the slats. Air pressure from forward speed closed them. The speed at which the slats start to close after take-off and will be fully open as it slows down to land, is well above the take-off and approach speed. So it took off and accelerated away for some time with slats fully open, and they were fully open again before landing. Therefore if you want scale realism, set your slats program for fully open or fully closed, and open them for take-off and landing. The full-size had a speed range during which the slats were driven shut or could open up due to the varying air pressure but it was above initial climb or approach speeds and was a function of the forces involved rather than any need for a variable setting. I did have the reference with the fully open and fully closed speeds but can’t find it at the moment.
On my Fibreclassics F-86 I have the slats driven by the flap switch, so that flaps up = slats closed, flaps at take-off or landing = slats fully open.
