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PilotJames
31st Jan 2017, 20:36
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.

Chris Scott
31st Jan 2017, 20:46
Quote:
"Does anyone know why VLS increases and then decreases."

Are you referring to a temporary increase during transit from Flaps 1 (i.e., slats only) to Flaps 2?

PilotJames
31st Jan 2017, 20:58
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).

Flo121142
31st Jan 2017, 22:11
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

Sidestick_n_Rudder
31st Jan 2017, 22:58
Not true. Vls is not dependent on load factor!

student88
31st Jan 2017, 23:18
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.

Uplinker
31st Jan 2017, 23:23
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.

Sidestick_n_Rudder
31st Jan 2017, 23:25
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.

KayPam
31st Jan 2017, 23:53
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

Field In Sight
1st Feb 2017, 22:42
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.

Polorutz
1st Feb 2017, 23:18
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?

Polorutz
1st Feb 2017, 23:29
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.

Fursty Ferret
3rd Feb 2017, 09:30
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.

I'm with KayPam on this (because I came to the same conclusion a few years ago). The effect is particularly noticeable if you have full speedbrake deployed when going from 1 to 2.

PilotJames
3rd Feb 2017, 13:53
My only issue with that theory is that when flap 2 is selected slats come out before the flaps.

vilas
3rd Feb 2017, 14:11
Yes! Flaps come out last to position 2.

Goldenrivett
4th Feb 2017, 11:10
Hi PilotJames,

There is no explanation given in FCOM but if you look at this old oeb-819-1 (http://www.blackholes.org.uk/PP/oeb-819-1.pdf) 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.

Uplinker
4th Feb 2017, 11:16
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?

PilotJames
4th Feb 2017, 13:37
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!

Uplinker
5th Feb 2017, 11:15
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.

PilotJames
5th Feb 2017, 19:07
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.

KayPam
6th Feb 2017, 01:46
If you'd like, I could ask the question to the very guy responsible for characteristic speed computation on Airbus SA.

Uplinker
6th Feb 2017, 09:22
A321 yesterday. There is definitely no VLS increase when Config 1 and Config 2 go out; it just drops away as you would expect.

On the youtube clip of the raw data approach you posted James, I think the speed brake is out - VLS is right up against the speed bug, so I think this brief few knots increase only occurs if the speed brake is deployed AND when only the slats are moving out.

vilas
6th Feb 2017, 09:49
There are a few threads that start on a wrong assumption and members enjoy giving free reign to their imagination. It never ends because nobody can produce an official answer nor anybody is ever sure that it really happens.

Uplinker
6th Feb 2017, 10:58
I hope that wasn't a dig at me vilas?

I have reported what I have seen in actual A320 and A321s when configuring for actual approaches.

I have watched the youtube clip James posted and have offered my observations about what I saw there and how I know the slats and flaps deploy on the A320 family.

KayPam has offered to find out the real reason, which seems reasonable.

PilotJames
6th Feb 2017, 21:05
Thank you for your observations Uplinker. Yes it's very obvious when the speedbrake is out!

KayPam
9th Feb 2017, 17:08
It's actually pretty simple and should occur in all configurations (most easily seen when flaps are extending)
When flaps lever is in intermediate position, coefficients for the computation of VLS change (increase) due to A/C realizing it is not in a stable phase of flight

PilotJames
9th Feb 2017, 18:44
Thank you Kaypam

vilas
10th Feb 2017, 16:13
KayPam
Do you have a reference or you are just guessing? Because flap lever is not active in intermediate position?

KayPam
10th Feb 2017, 16:22
My previous message on this topic was guestimating.

For the above, I do have a very precise reference but if I told you I would have to kill you thereafter :p
The reason I was given is because the design office considered the A/C to not be in a stable phase of flight when flap lever is moved.