Flaps

[jojo]

Yep I meant "retracted".

 

Reliability isn't the cause to retract SLATS.

 

Otherwise nobody would use SLATS for landing, and outside of Mirage 2000, from airliners to fighters almost all planes use LE devices for landing.

 

Your thinking is biased...

[vparez]

Ok, look, it is really simple: I just do not agree that deployed slats reduce drag (i.e. that retracting them increases drag). I could be wrong, but I don't think so. If I am wrong, I have no problem of admitting that, but I can't simply accept this claim without explanation. So, if you can explain how slats decrease drag I would be very grateful. :)

[Frederf]

With landing retracted, the "Becs" start to extend at 4° AoA, and are fully extended at 10° AoA.

 

My mistake I was reading the French manual and was reading the "Bebattements" figure from Becs internes/externes of 17.5° and 30°. That appears to be their movement range not when they are active.

 

From what I have been told, on M-2000 the "Becs" mainly reduce drag. So extending them for landing would lead to too flat approach flight path.

 

OK so the leading edge devices work too well for landing. The poorer aerodynamics without becs is then desirable for attitude and engine setting.

 

Errr... no. No slot between the slats and the wing on the M2000.

 

Looking into it not all slats have a slot. There are slats which are a continuous surface. Mirage's are definitely slats despite their lack of slots.

 

I wonder if Dassault had it to do over again they wouldn't lock the becs in landing config but instead schedule the becs later (say 16-22 degrees AOA) so it wouldn't change normal approach but would add more approach stall safety margin.

 

Comparing slats in-out drag we probably want to compare regimes of equal lift. Perhaps simply sticking out the slats increases drag but may lower the drag at any particular lift. Flow separation causes a heck of a lot of drag so preventing that can more than offset the drag of the device.

[jojo]

Mirage's are definitely slats despite their lack of slots.

 

In export Mirage 2000 and Mirage 2000-5 cockpits, written in english, the switch is labelled "SLATS" instead of "BECS" :thumbup:

[jojo]

Slats operate rather differently from flaps in that they have little effect on the lift at a given angle of attack. Rather, they extend the range of angles over which the flow remains attached. This is shown in figure 9.

 

http://adg.stanford.edu/aa241/highlift/highliftintro.html

 

Looking at figure 9 you will notice that, without FLAPS, for a given AoA, the CL is actually slightly decreased by SLATS.

But SLATS reduce drag because the airflow is detached from the wing further down the chord, which allow you to pull more AoA and then finally increase lift.

[mvsgas]

Rafale, Gripen, Typhoon, J-10, J-20...all fly by wire delta fighters.

Agree, but non are tailless deltas or like Azrayen more eloquently put it "pure deltas"

 

You're right, SLATS increase instability. But FBW can handle it very smoothly.

Also agree, but my point was it would make since for the slats to be retracted with landing gear out to slow down the aircraft pitch rates. This together with what you posted about using wing drag to allow a higher engine RPM on approach, I think, helps explain in more detail the design of the slats movement.

 

Well, there is also the Rafale and Typhoon. Granted they're not pure deltas, but delta-canards.

I was talking mainly about "pure deltas", but I could not think of that term, thanks...I did forget the HAL Tejas.

Edited October 4, 2016 by mvsgas

[jojo]

I really don't think the pitch rate with SLATS is an issue since air refueling is also done with SLATS extended

 

 

For the record, it seems that Rafale also land with SLATS retracted, but the canards planes allow to counter the pitch down moment of the elevons deflected down to increase lift. So the Rafale can make it's finale approach at 16 degrees AoA/ 120kt

Edited October 4, 2016 by jojo

[Azrayen]

Ok, look, it is really simple: I just do not agree that deployed slats reduce drag (i.e. that retracting them increases drag). I could be wrong, but I don't think so. If I am wrong, I have no problem of admitting that, but I can't simply accept this claim without explanation. So, if you can explain how slats decrease drag I would be very grateful. :)

 

I understand your issue. I'm sorry I'm not able to provide an explanation (again, not an aerodynamician); I can just tell you that what you think doesn't match with what I read/was being told... etc. from good RL sources, writing/talking about this aircraft and not in general :)

[zaelu]

Ok, look, it is really simple: I just do not agree that deployed slats reduce drag (i.e. that retracting them increases drag). I could be wrong, but I don't think so. If I am wrong, I have no problem of admitting that, but I can't simply accept this claim without explanation. So, if you can explain how slats decrease drag I would be very grateful. :)

 

 

Maybe they do this in certain conditions. Low Speed and High AoA. As the AoA increases the wing will have an increased drag... lets call it value A. At a certain point the flow of air over the wing gets disrupted because of AoA (it doesn't flow over the wing normally but tends to leave the wing earlier).

 

Now with the Slats deployed the air comes under them and it's forced/pushed against the top of the wing making it stick longer on the upper surface. Because of this the flow is less disturbed thus drag is less then A, aka reduced.

 

It doesn't matter the A value is a high value. Simply with Slats out it gets diminished. With slats out at high speed and low AoA I gues the "hole" in the wing creates a disturbance that makes the air flow unregulated and thus creating drag bigger than the one of the clean wing... let's call it value B

 

This might be why it looks counter intuitively that the slats reduce drag. They normally would increase drag but when going slow and with high AoA they behave in a different manner.

 

edit

 

I found this:

 

min 8:30

 

and a good article that quotes same video

 

http://www.boldmethod.com/learn-to-fly/aircraft-systems/heres-how-leading-edge-slats-get-you-off-the-ground/

Edited October 4, 2016 by zaelu

[vparez]

http://adg.stanford.edu/aa241/highlift/highliftintro.html

 

Looking at figure 9 you will notice that, without FLAPS, for a given AoA, the CL is actually slightly decreased by SLATS.

But SLATS reduce drag because the airflow is detached from the wing further down the chord, which allow you to pull more AoA and then finally increase lift.

 

Ok, I understand now what is the problem. Sure, if there is flow separation over the wing, SLATS will reduce total drag. So, I agree with what you and Azrayen said, I just needed this additional piece of info (that we were talking about separated flow). And yes, at typical AOA of a final approach there is bound to be separation, so slats out would reduce drag. So, ok, I don't have a problem anymore with this explanation of why SLATS are in with L/G out; thanks for a nice and enlightening discussion!

 

Here is a picture I was trying to find yesterday, it is very telling:

 

 

Of course here we can only compare FLAPS vs FLAPS+SLATS but the principle is the same and it is regarding CL vs CD. For a CL=2.4 drag is much lower with SLATS (blue line) than without (red dash line). This is only true as the AOA increases enough to cause separation.

We can clearly see that when AOA approaches 0, the SLAT config has more drag than the FLAPS or CLEAN configuration.

[vparez]

You are comparing apples and oranges here as the Mirage 2000 has sealed slats which work in a different way than slotted slats as they can't use the accelerated flow of slotted slats to keep the air attached to the wing at a higher AoA.

 

Still, the camber of the sealed slats have an effect on the separation. The Mirage wing near the root has a very large cord/thickness ratio, so with slats extended, i guess it can actually be approximated to a flat plate with a small camber near the LE. In the NASA video that Zaelu posted you can see that this camber is better with respect to separation than a flat plate without it. So although there is no ventilation, the camber and increased cord length do play a big part.

[jojo]

Ok, I understand now what is the problem. Sure, if there is flow separation over the wing, SLATS will reduce total drag. So, I agree with what you and Azrayen said, I just needed this additional piece of info (that we were talking about separated flow).

 

We were talking about SLATS effect in landing configuration.

Provided that delta wing are known to land with higher than usual AoA (14° for M-2000), I didn't expect someone to assume laminar flow in these conditions.

 

Still, the camber of the sealed slats have an effect on the separation. The Mirage wing near the root has a very large cord/thickness ratio, so with slats extended, i guess it can actually be approximated to a flat plate with a small camber near the LE. In the NASA video that Zaelu posted you can see that this camber is better with respect to separation than a flat plate without it. So although there is no ventilation, the camber and increased cord length do play a big part.

 

At wing root the flow is energized by the vortex caused by the "streaks" on air intake to improve lift at high AoA.

 

 

Also when the SLATS are extended, inner and outboard SLATS aren't aligned anymore, creating a "dogtooth"

 

 

Close up view on SLATS

Edited October 4, 2016 by jojo

[vparez]

We were talking about SLATS effect in landing configuration.

Provided that delta wing are known to land with higher than usual AoA (14° for M-2000), I didn't expect someone to assume laminar flow in these conditions.

 

Flow separation can occur in laminar as well as turbulent flow, that's not the issue. The issue is that nobody was mentioning flow separation.

 

Gears come down way (and slats go in) before touchdown and, correct me if I am wrong, you don't fly the whole final at 14deg AOA. So I was first thinking about the approach, before I would get into a discussion about a more extreme attitude near touchdown. If I reflect on the discussion, I should have realized that even at smaller AOA there would be some separated flow towards the TE and the slats would indeed make a difference.

 

It is important to be clear when you discuss like this through messages; see how Azrayen thought I said I had jet fighter experience because I was not clear enough :D

[jojo]

Flow separation can occur in laminar as well as turbulent flow, that's not the issue. The issue is that nobody was mentioning flow separation.

 

Gears come down way (and slats go in) before touchdown and, correct me if I am wrong, you don't fly the whole final at 14deg AOA. So I was first thinking about the approach, before I would get into a discussion about a more extreme attitude near touchdown. If I reflect on the discussion, I should have realized that even at smaller AOA there would be some separated flow towards the TE and the slats would indeed make a difference.

 

It is important to be clear when you discuss like this through messages; see how Azrayen thought I said I had jet fighter experience because I was not clear enough :D

 

Well, stop playing with words.

At 14° AoA you have turbulent flow.

And Dassault used several tips to delay/ control flow separation

- streaks on air intake

- SLATS

- leading edge dogtooth on SLATS.

 

For a classical "break approach", you check your final approach speed by taking 14° AoA level at the end of downwind, keep the speed during final turn, and fly again 14° AoA during final approach >]-O-[<

 

Depending on conditions, pilots may take some margin for flare by flying a tad faster at 13°: AoA > < at the top of ] [

[vparez]

Well, stop playing with words.

At 14° AoA you have turbulent flow.

And Dassault used several tips to delay/ control flow separation

- streaks on air intake

- SLATS

- leading edge dogtooth on SLATS.

 

For a classical "break approach", you check your final approach speed by taking 14° AoA level at the end of downwind, keep the speed during final turn, and fly again 14° AoA during final approach >]-O-[<

 

Depending on conditions, pilots may take some margin for flare by flying a tad faster at 13°: AoA > < at the top of ] [

 

Ok buddy, I am going to leave it at this. I am not playing with words nor trying to troll you or anything like that. However, be careful when you say a thing like "at 14deg AOA you have turbulent flow". In fluid mechanics this doesn't mean much.

 

As always, thanks for a nice discussion. I didn't know that jets were landing with higher RPM than needed, so I am happy to have learned this.