A320 flap 2 VLS increase
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Joined: Jul 2009
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From: London
A320 flap 2 VLS increase
Does anyone know why VLS increases and then decreases.
VLS is based off config and weight.
Each stage of slat and flap should decrease VLS as you can fly at a slower speed because you have changed the config.
I'm not sure why the VLS temporarily increases before it decreases? Any ideas welcome.
VLS is based off config and weight.
Each stage of slat and flap should decrease VLS as you can fly at a slower speed because you have changed the config.
I'm not sure why the VLS temporarily increases before it decreases? Any ideas welcome.
Thread Starter
Joined: Jul 2009
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From: London
I am indeed.
When you select flap 1, VLS decreases as slats come out. It's as you select flap 2 that VLS increases and then decreases. Any other flap setting VLS just decreases (which makes sense as you are gaining more wing area and increased camber).
When you select flap 1, VLS decreases as slats come out. It's as you select flap 2 that VLS increases and then decreases. Any other flap setting VLS just decreases (which makes sense as you are gaining more wing area and increased camber).
Joined: Dec 2012
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From: Europe
It's because of the ballooning effect that is very noticeable when selecting Flap 2 - you have a momentary increase in G-load due to the ballooning, while the flaps are still extending which leads to an increased Vls
Joined: Dec 2007
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From: N/A
Depending on the config, VLS relates to a factor of the stall speed, which in itself is influenced by load factor, right?
Try it yourself when you're next flying close to green dot, if you pull a load or unload the wing, VLS reacts accordingly.
I stand to be corrected however.
Try it yourself when you're next flying close to green dot, if you pull a load or unload the wing, VLS reacts accordingly.
I stand to be corrected however.
Joined: Nov 1999
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From: UK
No, a common misconception.
Vls only changes with changes in wing config, i.e. slats, flaps and speed brakes. It does NOT change with load.
Next time you fly look closely: It is the Alpha prot strip (black and amber stripes) below the Vls strip that changes with load.
Vls only changes with changes in wing config, i.e. slats, flaps and speed brakes. It does NOT change with load.
Next time you fly look closely: It is the Alpha prot strip (black and amber stripes) below the Vls strip that changes with load.
Joined: Jun 2014
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From: Mordor
No, it doesn't. It is a common misconception. Vls is is constant for given config. It Vs1g* a constant factor (1.13-1.23, depending on flight phase).
What DOES change with g-load is Valpha_prot and Valpha_max.
What DOES change with g-load is Valpha_prot and Valpha_max.
Joined: Dec 2015
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From: France
Well, flap extension can reduce the angle of stall. Slat extension reduce it.
So if the flaps were to deploy faster, the flaps decrease of angle of attack could win against the slats increase. Until the flaps are fully deployed, and now the angle of stall will increase further than previously.
If it is not clear I will reformulate
So if the flaps were to deploy faster, the flaps decrease of angle of attack could win against the slats increase. Until the flaps are fully deployed, and now the angle of stall will increase further than previously.
If it is not clear I will reformulate
Joined: May 2002
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From: Sale
This is something that I have never gotten to the bottom of. Hopefully, somebody on this thread will come up with the answer.
Kaypam. I agree that the "Angle" of the stall changes depending on what surface moves i.e. Slats have a different effect than Flaps.
However, flap or slat extension should always result in the stall speed becoming lower. Therefore Vls (speed) should always reduce. Hence why I don't understand why Vls temporarily increases.
Kaypam. I agree that the "Angle" of the stall changes depending on what surface moves i.e. Slats have a different effect than Flaps.
However, flap or slat extension should always result in the stall speed becoming lower. Therefore Vls (speed) should always reduce. Hence why I don't understand why Vls temporarily increases.
Joined: Mar 2007
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From: Switzerland
This is pure speculation on my part.
FCOM says VLS increases with speedbrake use. Could it be that initially as the flaps come out, they produce some drag that increases VLS?
Then once they extend enough the lift provided overcomes the drag and therefore VLS decreases?
FCOM says VLS increases with speedbrake use. Could it be that initially as the flaps come out, they produce some drag that increases VLS?
Then once they extend enough the lift provided overcomes the drag and therefore VLS decreases?
Joined: Mar 2007
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From: Switzerland
Just had a read of the FCOM, below 14500 feet, the FAC uses AoA and thrust (among other parameters) to compute VLS.
When going from flaps 1 to flaps 2 both of those parameters change, the AoA will change with the ballooning and the thrust will tend to reduce. This could also be at play here.
When going from flaps 1 to flaps 2 both of those parameters change, the AoA will change with the ballooning and the thrust will tend to reduce. This could also be at play here.
Joined: Sep 2011
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From: FL390
Well, flap extension can reduce the angle of stall. Slat extension reduce it.
So if the flaps were to deploy faster, the flaps decrease of angle of attack could win against the slats increase. Until the flaps are fully deployed, and now the angle of stall will increase further than previously.
So if the flaps were to deploy faster, the flaps decrease of angle of attack could win against the slats increase. Until the flaps are fully deployed, and now the angle of stall will increase further than previously.
Joined: Jan 2014
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From: N5109.2W10.5
Hi PilotJames,
There is no explanation given in FCOM but if you look at this old oeb-819-1 it explains on page 3, how the aircraft weight is re-computed and updated for the approach.
Amongst some other conditions provided the dynamic maneuver load is <1.07 and no change in configuration then the FACs will recalculate the weight and hence V stall 1g.
I don't know how it knows there is no configuration change but if it was for say 1 second after the flap handle had been moved, and the flaps ran slowly so a recalculation was made during flaps transit then there could be an error if the maneuver load was <1.07.
Maximum error at 66 tons = 1.07*66 = 70.6 tons (4.6 tons which is about 4 kts change in VREF).
Once stable at 1g, then another recalculation to the VLS would then be correct and reflect the reduction in VLS as the flaps extended.
There is no explanation given in FCOM but if you look at this old oeb-819-1 it explains on page 3, how the aircraft weight is re-computed and updated for the approach.
Amongst some other conditions provided the dynamic maneuver load is <1.07 and no change in configuration then the FACs will recalculate the weight and hence V stall 1g.
I don't know how it knows there is no configuration change but if it was for say 1 second after the flap handle had been moved, and the flaps ran slowly so a recalculation was made during flaps transit then there could be an error if the maneuver load was <1.07.
Maximum error at 66 tons = 1.07*66 = 70.6 tons (4.6 tons which is about 4 kts change in VREF).
Once stable at 1g, then another recalculation to the VLS would then be correct and reflect the reduction in VLS as the flaps extended.
Joined: Nov 1999
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From: UK
I watched very carefully on approach yesterday, (A320), and VLS did not increase when Config 2 was selected. (It only decreased - in accordance with the change in wing shape).
Pilot James, are you sure you're talking about VLS, or do you mean V alpha prot?
Pilot James, are you sure you're talking about VLS, or do you mean V alpha prot?
Thread Starter
Joined: Jul 2009
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From: London
Hello Uplinker thank you for posting, have a look at this video (of a very nice raw data app). At 1.55 min you can see VLS increase. I have to say sometimes it doesn't happen though. It is most apparent with speed brake out.
https://www.youtube.com/watch?v=7SsECGITHCY&t=129s
Thank you goldenrivett, very interesting bulletin but it says it doesn't calculate weight when the speed brakes are out and this effect is most apparent when they are out.
Got me stumped, one of those things that you really don't need to know but it's nice to know!
https://www.youtube.com/watch?v=7SsECGITHCY&t=129s
Thank you goldenrivett, very interesting bulletin but it says it doesn't calculate weight when the speed brakes are out and this effect is most apparent when they are out.
Got me stumped, one of those things that you really don't need to know but it's nice to know!
Joined: Nov 1999
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From: UK
Well I don't know.
It only increases by about 1 or 2 knots, and only when Flap 1 and Flap 2 are selected. In each case, VLS increases very slightly when only the Slats are moving, then it reduces.
Probably what Goldenrivett said.
It only increases by about 1 or 2 knots, and only when Flap 1 and Flap 2 are selected. In each case, VLS increases very slightly when only the Slats are moving, then it reduces.
Probably what Goldenrivett said.
Thread Starter
Joined: Jul 2009
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From: London
I checked the FAC weight today and it didn't change even slightly when flap 2 selected so it's just something to do with the configuration of flap 2 when the slats come out. Doesn't happen with flap 1. But as you say triv really but I'm curious.