VLS speed increase in turn ?

[WhiteKnight]

Hello,

why does the vls speed(airbus) rises in a turn.

According FCOM VLS=1,28VS1g in clean configuration.

Isn`t VS1g speed a constant value for a given wing?? What do I miss?

Thanks for any comments!

[Mad (Flt) Scientist]

I'll take a guess that the system calculating Vls is using the current AoA as a means to determine the current effective weight of the aircraft.

For example, suppose my stall speed is at 10 alpha and zero lift is at 0 alpha (arbitrary numbers, I know they are wrong).

In S&L flight, I may be flying at 6 alpha. So my system decides I'm flying at a CL equal to 6/10ths of stall CL. That means, assuming a linear CL-alpha curve, I'm 1/square root(0.6) above stall speed. So I can use my current airspeed and divide by that stall speed factor, to get my predicted stall speed. Then I can multiply that predicted Vs by 1.28 to get Vls.

Now, enter a turn at the same airspeed. Because I'm at 'g', I'm now at 7 alpha. So the system now sees that I'm at 1/sqrt(0.7) times stall speed, which means, since my actual speed is the same, my predicted stall speed is higher. So my Vls also increases.

Basically, the Vls value isn't 'g'=compensated.

(Based on the 'FAA green line' logic in our own a/c, which behaves exactly like this)

[Field In Sight]

Vls doesn't change. It only changes with change of config which includes activation of spoilers.

However, alpha max or (Vsw in alternate/direct law) does change when the aircraft is loaded/unloaded.

You can see it at high gross weights in a turn, the Vls bar gets shorter, but the top of the Vls bar doesn't move.

FIS.

[PantLoad]

Vls = 1.28 VS 1g

This is the basic formula....stall speed increases in a bank (greater
than one G).

So, let's ask the question, 'Why does Vls increase with speedbrake?'
(Isn't Vls = 1.28 VS 1g ?)

Same answer......

Mad (Flt) Scientist tells it in a proper way. Look at the
AoA. What is it doing in a level turn? What is it doing
with speedbrake extension?

Fly safe,

PantLoad

[hetfield]

A300 (310?) does the opposite.

All maneuver speeds DECREASE in a turn.
Just a design flaw....

[BlackandBrown]

You're confused because you are making things up. Vls in clean config is 1.28Vs not Vs1. It is in the fundamental stalling lesson in PPL that with bank angle or weight increase the stall speed increases.


For your reference I have provided all copied and pasted links which I hope help you understand.


From a320 FCOM 3.4.10:

VLS : Lowest Selectable speed.
Represented by the top of an amber strip along the airspeed scale on the PFD.
Computed by the FAC based on aerodynamic data, corresponds to 1.13 VS during takeoff or following a touch and go.
Becomes 1.23 VS after retraction of one step of flaps.
Becomes 1.28 VS when in clean configuration.
Note : If in CONF 0 VLS were 1.23 VS (instead of 1.28 VS), the alpha protection strip would hit the VLS strip on the PFD.
Above 20000 feet, VLS is corrected for Mach effect to maintain a 0.2g buffet margin.
In addition, VLS increases with speed brakes extension.

The table wont properly copy but read across Bank Angle/Load Factor/Stall speed increase factor.

30° gives 1,15G which gives 1,07
45° gives 1,41G which gives 1,19
60° gives 2,0G which gives 1,41
75,5° gives 4,0G which gives 2

Turning

As can be seen in the table above, when the aircraft banks 30° the stall speed increases with 7% due to the fact that aircraft weight in a level turn increases by 15% caused by the increased load factor. Although load factor increases, the stalling angle of attack will remain the same.
Image shows clearly that in a 45° banked turn the amount of generated lift is much more than the vertical component which keeps the aircraft from descending in a level turn.

The equation to calculate this is:

Vs new = Vs old weight x √(new weight / old weight).

All taken from:

Stall Speed Factors, Weight, Load Factor, Turning, Altitude, Power, Flaps, Wing Contamination, Airplanes - EAI (http://www.experimentalaircraft.info/flight-planning/aircraft-stall-speed-1.php)

[Microburst2002]

Valpha max or VSW have to be corrected, of course.

VLS does not. It represents a margin above VS1g, so it doesn't matter the load factor that you actually have at a given instant.

VLS is the speed below which you don't want to fly because you will have reduced margin, if you do. VLS doesn't change with load factor, just the same as V2 or VREF do not change with load factor.

[DJ77]

Extract from the A330 FCOM 3.04.10 P1

" The FCOM uses VS for VS1g. "

I doubt A320 uses a different definition so Microburst appears to be right.

My 2 cents.

DJ.

[qwertyuiop]

BlackandBrown,

Forget all your fancy formula. On the Airbus the indicated Vls does not move with bank angle or loading, it does with speed brakes.

End of. You are WRONG!!!

[Field In Sight]

A picture (or video) is worth a thousand words.

Note the change in alpha max and alpha prot as the aircraft is loaded/unloaded and banked.

Also note that Vls doesn't change from 119kts at anypoint as the config doesn't change.

‪Airbus A320 TOGA takeoff, cockpit (PFD) View‬‏ - YouTube

[BlackandBrown]

Fair enough, thanks. So Vls doesn't rise in a turn. The posters question has been answered one way or another! If I'd read the lines above Vls in FCOM I'd have seen but to be fair if they mean Vs1g then they should write Vs1g not Vs given Vs has a defined meaning.

Thanks again for taking the time to teach.

[Mad (Flt) Scientist]

OK, so if the indication doesn't change there's two ways to implement that.

One is the same kind of AOA based indication as I discussed up there ^^^^ somewhere, but "g-compensated" i.e. using the measured normal acceleration and using that to adjust for the effect of increased load factor on AOA.

The other is to have it as a lookup function based on weight or whatever, and have it be nothing at all to do with AOA.

I suspect the former.

Thinking about it, the behaviour I described (of a moving indication) was accepted by FAA but never implemented for JAA, perhaps because they wanted something more like the VLS behaviour shown. We looked at g-compensation but it wasn't required so wasn't implemented.

[PantLoad]

Watched the video....that was about an eight or nine degree
banked turn. No significant increase in stall speed.

Sorry, the video fails to prove the point.

But, it's a nice video, anyway.


Fly safe,

PantLoad

[Field In Sight]

Just look during a SID turn the next time you fly.

Simples.

[Microburst2002]

Mad scientist

It wouldn't make any sense that VLS was load factor sensitive. It would be as weird as if V2 was.

When you fly at VLS you know that you have a margin (arbitrarily determined) that should allow you "reasonable" manoeuverability without stalling. In the airbus PFDs, if you fly at VLS and pull up and bank steeply you will see Valpha prot and alpha max increasing towards a steady VLS, thereby indicating the effect of the load factor.

Otherwise VLS would increase above actual airspeed and that would be annoying because VLS is supposed to be a safe enough target airspeed.

[Mad (Flt) Scientist]

Mad scientist

It wouldn't make any sense that VLS was load factor sensitive. It would be as weird as if V2 was.

When you fly at VLS you know that you have a margin (arbitrarily determined) that should allow you "reasonable" manoeuverability without stalling. In the airbus PFDs, if you fly at VLS and pull up and bank steeply you will see Valpha prot and alpha max increasing towards a steady VLS, thereby indicating the effect of the load factor.

Otherwise VLS would increase above actual airspeed and that would be annoying because VLS is supposed to be a safe enough target airspeed.

I agree that a display that does that isn't very useful as a target speed while manoeuvring. BUT

I know for an absolute fact that the "green line" displayed on CRJs for FAA aircraft, which is intended to show a nominal Vref type speed, and thus is very like this VLS speed, does precisely what the OP suggests VLS does on the display, and for the reasons i gave in the second post.

As I said, we considered a g-compensated display, such that it did not move during manoeuvring, but this was not required of us by the FAA, so what ended up implemented was the simple version. When the display was considered as an option for non-FAA aircraft, other authorities required that it be g compensated, but there was no requirement for the indication either, so it simply was omitted.

[PantLoad]

Sorry, I was wrong....

Called my buddy who is in the training department at my old airline.
You're correct....the Vls does not move. The stall bar does move,
however. But, the Vls does not.

Ha....I learned something.

Thanks,




Fly Safe,


PantLoad

[Microburst2002]

Actually, I prefer it not to move with g loads. For me, VLS is a target airspeed, weight sensitive, which gives me minimum V2 and VREF. I don`t want V2 and VREF to change, I just want to fly those speeds, knowing that I will never stall as long as I don`t pull too hard or bank too steeply. Vaprot and Vamax will take care of me if I incur in too high a load factor.

I don`t know how the CRJ works, and I am sure that in that airplane it makes sense that it is load factor sensitive.