On the asymmetrical deployment of slats, or when a sim has so much love to details

[sergio_]

I've always been a sucker for aerodynamics. Or that's what I used to think until I actually discovered the slats of the 109. I mean, really has this thing existed since this early, and I never heard of it? Oh man, the simplicity of the concept and the build. 

For those of you who don't know, these are surfaces at the leading edge of the wing that provide better lift at the cost of a bit of track. In the 109, these deploy and retract automatically by the airflow. No need for mechanical devices or pilot interaction.

 

Now, what would happen if you blow air at a slower speed to one wing than to the other? Would this slat deploy while the other remains? 
Turns out the test is easy: take a 109 and get slow enough for the slats to barely want to deploy. Then perform a turn. The inner wing will go slower than the outer one, same as the inner wheels of a car in a curve will roll slower than the outer one. As expected, the inner slat will deploy before the outer one. If you hold on at the right speed, you can fly with only one slat deployed.

But that's not all. Slats don't really deploy at a certain speed, but at a certain angle of attack (being the speed one of the factors that affect it). So, if you are in the 'edge' speed where the slats are just about to deploy, you can easily deploy and retract them at will by banking. When banking you change the relative angle of attack for both wings, increasing the AoA of the descending wing while decreasing it on the raising one. Awesome.

 

Congrats to the dev team, and thank you indeed for covering this. I mean, each plane is a model that behaves close to reality, but a simulation can only get close to an extent, so I didn't think this would happen the first time I attempted it (more than a year ago actually, yet I never found the need to comment in the forum).

 

I'm including some links to pictures below to illustrate, for each 109, turning left and right.

 

Now, the behavior is so consistent (I invite you to try by yourself) that I was almost about to report a bug in two of the models:

- F4 seems to have the opposite behavior, deploying the faster wing. I don't know if it is aerodynamically different enough from the F2.

- G4 seems to always want to deploy the right slat first, no matter the flight conditions (and I tested everything that came to mind). 

Again, it is so consistent with all the models and with these two exceptions that I do think there might be something wrong with them. But I well may be me as well. I don't know how many variables are involved. I'd like to test my luck in case some dev sees this post and wants to point out something.

Please don't take this as a complaint! The sole fact that I can show these 'weird (?)' behavior is because there is so much detail in this simulation, and it should only be praised.

 

Spoiler

 

E7: https://gyazo.com/ab127c948d49eb376245f9ca1cf2ebfc

E7: https://gyazo.com/5d78155b8bf7d40201f3354957274d91

F2: https://gyazo.com/a579804c89fb7860e835727efc2cfdf3

F2: https://gyazo.com/64c16369c3d48289ed67101c05268886

F4:   https://gyazo.com/8c488c9dcd92aa66dac0f7d224a92edc <- wrong?

F4:   https://gyazo.com/903eaec782d37c2097b5350a0d3ab9a1 <- wrong?

G2: https://gyazo.com/4620b3dd9234b3d2e289f6a33db4b62c

G2: https://gyazo.com/9d6fe661c07db8e910ff47dbe3aecf22

G4:   https://gyazo.com/e3ebdda4c90f75b9bdb96af690f861af <- wrong?

G4: https://gyazo.com/741a9d4ed8d71755eba3c3363cfae30a

G14: https://gyazo.com/2790c3efd8c67deb17f1e43a7c82cb39

G14: https://gyazo.com/ebfe30e980dcb48af401ea153bcb67dc

K4: https://gyazo.com/c4b38d3a37298313bf045d530498653f

K4: https://gyazo.com/d3309445ceea74dc59279c01bb73e0f5

 

 

Bonus track: yes, the Me262 indeed behaves as expected as well. And it is a pleasure to see, because this bird has not one but three independent slats in each wing. You can almost feel the air flow by looking at the slats deploying soft and slow, each one at their own pace.

Edited June 9, 2020 by sergio_
Grammar

[Raven109]

I haven't tried this, but could the outliers happen when the plane is not flying coordinated, i.e ball not centered?

[sergio_]

4 minutes ago, Raven109 said:

I haven't tried this, but could the outliers happen when the plane is not flying coordinated, i.e ball not centered?

I honestly didn't pay that much attention to the ball while I tried to keep my eyes on the slats. Yet I tried to force those outliers to behave 'as expected' that it included both pedal work, different banking angles and re-loading the aircraft. They insist on behaving like that. 
But of course some testing from someone else would be more than welcome. It's one of the most intense tests I've done, but far from what I'd call 'scientific' ?

[Bremspropeller]

On 6/9/2020 at 10:36 PM, sergio_ said:

Now, what would happen if you blow air at a slower speed to one wing than to the other? Would this slat deploy while the other remains? 

 

The deployment actually depends on the position of the stagnation-point.

If AoA goes up, the stagnation-point moves to the lower surface and the slat gets sucked out (or not pushed in anymore). It's not so much dependant on airpeed per-sé.

The slat with the higher local AoA should deploy first (that is the inside slat in a turn).

 

AoA and airspeed normally correlate, though, unless you're going uncoordinated.

[unreasonable]

In the cockpit for a very quick test I found it hard to see which slat comes out first, but if you get them both out and then slowly ease off the turn, you can see which one goes back in first with no problem.

 

Just tried F4 and G4 - in both cases the lower, inner wing kept it's slat out after the upper, higher wing slat had shut.  Not exactly what you were testing, but nothing obviously wrong here.

 

I agree BTW that this sort of detail - even if done by sleight of hand - is very cool. 

Edited June 14, 2020 by unreasonable

[sergio_]

Thank you guys!
Still, despite you didn't find the abnormality, I can still reproduce this at will, that's why I went ahead and reported it, just in case it is a bug.

If it turns out not to be such thing, well... that'll be great!

 

Edit: I'd like this post not to be a bug report itself, but the praise I wants to be

 

Edited June 16, 2020 by sergio_

[69th_chuter]

I just took the G4 for a spin before commenting to a) make sure my memory was correct and b) an update didn't change things.

 

As noted, the slat deployment is dependent on AoA and it should be possible to get one to deploy (or retract) before the other by slipping or skidding.  In game this doesn't work well; although one will deploy or retract first it isn't directly controllable by yaw angle (i.e. making the left side or the right side deploy by skidding one way or the other).  One more important thing is that it doesn't alter lift at all (though in game it likely allows increased angle and reduced speed).  In the game the slats simply pop in or out without any apparent effect if in steady flight.  Lift is a function of camber (among other things) and the slat drops the leading edge increasing the wing's camber increasing lift requiring quick corrective aileron to prevent roll if the other slat isn't deployed.  This is the problem some pilots were referring to when they talked about "aileron snatch".  While dogfighting at low speeds and/or high angles and using all controls, including rudder, to draw a bead on the target the uncoordinated flight condition (and possibly wake turbulence as well according to Brown) could deploy one slat requiring immediate aileron correction to maintain the sight picture (Brown said in his tests he found it impossible to keep the target in sight).