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Tag777

Doubt about La-5 slats operation

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Hi all.

Only recently have I noticed that La-5 slats open in circumstances where, at least as far as I know, they shouldn't. According to what is known about the operation of automatic slats, they work with a spring system, keeping them folded in their cavities due to the air pressure on them. When the air pressure decreases (what happens when the plane lowers its speed and especially at the point near the stall) the springs automatically deploy them. This should not happen at high speeds where air pressure would have to keep slats in their cavities. Of course it also depends on the angle of attack of the wing, but for example when starting a turn with the plane at 400 km / h or more the slats open. I don't know if it's just my impression but it seems to me that it shouldn't happen in those conditions. In Bf 109 that have a similar system that does not happen when turning at high speed. In the case of La-5 when they deploy at high speeds they work as aerodinamic brakes. At least that is what I noted. I can be mistaken.

Below is a link to an article explaining Bf-109 slats operation but I think is similar in the case of La-5.

Regards,

http://109lair.hobbyvista.com/techref/systems/control/slats/slats.htm

 

Edited by Tag777

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The 109s having heavier elevator forces can't achieve the neccesary AoA to cause an opening in the slats at high speeds most of the time. However if you adjust the stabilizer to have higher pull up tendency you can manage to turn hard enough (increase in aoa) to open the slats at high speeds.

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I guess the solution for increasing the energy conservation in turns for the La-5 is doing more gentle turns and not pulling as hard.

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18 hours ago, Tag777 said:

According to what is known about the operation of automatic slats, they work with a spring system, keeping them folded in their cavities due to the air pressure on them.

 

They don't work with a spring system. Simplified, they work on air pressure alone. This means at low angles of attack the air pushes them in, at high angle of attack the air in fact sucks them out. The change from pressure to suction at the leading edge of the wing is primarily dependent on angle of attack and basically independent of speed. So technically there's nothing wrong with slats deploying at high speeds.

 

Also, at high angles of attack, they don't really add to drag, at all speeds they mostly add to lift by delaying the stalling point and permitting higher angle of attack without stall. In most cases, however, slats are used over the outbord part of the wing, where they prevent stalling of the outer wing section while having the inner section stall, so that a stall is more gentle (less sudden wing drop) and ailerons are still working at high angles of attack (great thing for safe landings).

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On 8/28/2019 at 9:13 PM, Tag777 said:

Hi all.

Only recently have I noticed that La-5 slats open in circumstances where, at least as far as I know, they shouldn't. According to what is known about the operation of automatic slats, they work with a spring system, keeping them folded in their cavities due to the air pressure on them. When the air pressure decreases (what happens when the plane lowers its speed and especially at the point near the stall) the springs automatically deploy them. This should not happen at high speeds where air pressure would have to keep slats in their cavities. Of course it also depends on the angle of attack of the wing, but for example when starting a turn with the plane at 400 km / h or more the slats open. I don't know if it's just my impression but it seems to me that it shouldn't happen in those conditions. In Bf 109 that have a similar system that does not happen when turning at high speed. In the case of La-5 when they deploy at high speeds they work as aerodinamic brakes. At least that is what I noted. I can be mistaken.

Below is a link to an article explaining Bf-109 slats operation but I think is similar in the case of La-5.

Regards,

http://109lair.hobbyvista.com/techref/systems/control/slats/slats.htm

 

 

On 8/29/2019 at 4:11 PM, JtD said:

 

They don't work with a spring system. Simplified, they work on air pressure alone. This means at low angles of attack the air pushes them in, at high angle of attack the air in fact sucks them out. The change from pressure to suction at the leading edge of the wing is primarily dependent on angle of attack and basically independent of speed. So technically there's nothing wrong with slats deploying at high speeds.

 

Also, at high angles of attack, they don't really add to drag, at all speeds they mostly add to lift by delaying the stalling point and permitting higher angle of attack without stall. In most cases, however, slats are used over the outbord part of the wing, where they prevent stalling of the outer wing section while having the inner section stall, so that a stall is more gentle (less sudden wing drop) and ailerons are still working at high angles of attack (great thing for safe landings).

 

Flaps aren't sucked out by air pressure, just as dust isn't sucked in by a vacuum cleaner. A vacuum (an absence of matter) cannot exert force on an object.

 

With vacuum cleaners, air that is getting pushed by its surrounding air molecules into the nozzle area is dragging the dust with it.

 

With automatic, spring-loaded slats, there's a balance system more or less comparing the pressure differential between the slat's frontal area and the wing's bottom area. If that difference reaches a specific point, the slats deploy automatically.

 

TL;DR: Aerodynamically, whether slats like the ones found in WW2 planes deploy isn't dependent on speed, but angle of attack.

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There are no springs in the 109's (or 262's, or La-5's) automatic LE slats, they work purely via airpressure, deploying as the stagnation point moves lower down the LE  as AoA increases, and the slats are pushed out. In short they work at any speed so long as a certain AoA can be attained.

 

On piston engined aircraft slats were usually situated on the outboard section of the wing as the inboard section was already energized during powered flight (prop wash) and as such would stall much later than the outboard section. Mounting slats on the outboard section thus allowed for an overall signficant increase in the lift the wing could provide during powered flight, as well as making the stall during unpowered flight extremely tame with little to no wing drop.

 

 

Edited by Panthera

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On 9/7/2019 at 2:29 PM, Panthera said:

There are no springs in the 109's (or 262's, or La-5's) automatic LE slats, they work purely via airpressure, deploying as the stagnation point moves lower down the LE  as AoA increases, and the slats are pushed out. In short they work at any speed so long as a certain AoA can be attained.

 

On piston engined aircraft slats were usually situated on the outboard section of the wing as the inboard section was already energized during powered flight (prop wash) and as such would stall much later than the outboard section. Mounting slats on the outboard section thus allowed for an overall signficant increase in the lift the wing could provide during powered flight, as well as making the stall during unpowered flight extremely tame with little to no wing drop.

 

 

As I wrote above, I can be mistaken. But, as the AoA is the main factor that controls the deployment of slats, wings geometry could have influence also? Because with the same AoA the La-5's slats are deployed and the 109's slats are not.

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7 minutes ago, Tag777 said:

As I wrote above, I can be mistaken. But, as the AoA is the main factor that controls the deployment of slats, wings geometry could have influence also? Because with the same AoA the La-5's slats are deployed and the 109's slats are not.

How can you tell the AOA is the same between the two aircraft? 

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You can design slats to practically open at any angle of attack. Yes, La and Bf109 can be different.

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