Departure tendencies

I understand that modeling anything post-stall in simulators is probably an incredible feat, but either way, I think this is worth asking.

If I pull right through critical AOA in a wings-level, coordinated condition, one wing ALWAYS seems to stall first and roll the jet around. I understand that there's a chaotic element to the way a wing stalls... meaning even a tiny difference in wind over either wing will result in a larger discrepancy as countless random changes are made to the airflow as it separates, and that can lead to a asymmetric lift situation. Regardless, it still feels like the tendency to "spin" is pretty extreme... in a well coordinated/balanced lift situation, shouldn't I be able to pull into an accelerated stall and have the airplane mush out?

Just looking for discussion.

Regards,

Beaker

How far back are you deflecting the stick before this happens, how pronounced is the roll angle, and what (if any) amount are you yawing?

I get similar results, but usually there's either pronounced roll or yaw.

Probably comes down to wing design (I know you have to work some designs damn hard to get them to spin). The Goshawk from this footage (which I believe you have already seen) is pretty scary:

Headspace -

Depends on airspeed obviously. I can operate the aircraft happily and most situations, and fly it around the edges of the envelope. I have a full center-stick built that allows me to really "feel" where I am with a large travel. Obviously, if I want to avoid departure, I play the travel range accordingly. All I was getting at was, if I pull like hell while keeping the aircraft coordinated, and accidentally stall the wing, I don't think it should always go to an aysmmetric lift scenario. I've never once just "mushed out," which I would imagine is possible for a relatively docile straight-wing design like the Hog.

Hassata - Definitely seen that video! (Horrifying!) However, in that case, he was at a very slow airspeed, and most likely over-controlling with the ailerons, which is a recipe for a spin situation. If I get a wing drop in the Hog at that speed, I assume it's my own dumb fault. :D I'm just referring to the upper edges of the envelope.

If you take a look at the G-meter when you are pulling the stick you must notice drop of G-force. And this is your first bell... in RL you never pull the stick further. There is no departure yet if you do not try to pull harder.

If you stall wingtips there are no miracle to prevent from wing autorotation

All in all, spin, departures now are not finally tuned, so this tendencies to wing drop can be more or less... it will depend on feedback from A-10 drivers we are waiting now.

Awesome! Thanks Yo-Yo. Of course, if I don't want to let the jet depart, I don't pull further than the 1deg AOA tone, or that loss of load. I was just wondering if the amount of spin tendency I was noticing in testing was correct.

To hear that you are still tuning it based on real Hog driver feedback is excellent news! Thanks a bunch.

Good thread Beaker - I've been wondering the same about this aspect of the flight model. In the videos / live demo I've seen - the mush is very visible after they snap the nose up from their guns pass. I can get the pull up looking close to the real thing in the sim, but it seems to drop a wing a bit quicker/at a lower AOA than the real jet - it's hard to tell though.

Yo-Yo, that's great to hear you're getting feedback from pilots!

Not just limited to the F4. That's how we teach stall recovery to new pilots, to use rudder. If your wing is stalled and you lower the aileron on that wing you further increase the angle of attack on that portion of the wing, causing a deeper stall.

A bit of trivia: For the early F4s at low speed, if the pilot tried to correct a wing drop with ailerons, that wing would stall. The pilot had to correct with rudder to get back to straight and level.

Using rudder to prevent stall doesn't make any sense. It might actually result in a spin. Neutralize all the controls if a stall is impending. Only when a wing has a tendency to drop should you apply rudder cautiously. Decrease pitch, increase power in a coordinated stall.

Your method might work in F4 (haven't tested it) but that says something about how the FM is coded, not how it works in real life (or properly coded FM).

I use the rudder to prevent a spin when performing power-on stalls in light aircraft. I'd imagine the principles are the same in larger more powerful aircraft.

You get a wing low at the break and you will spin, and with your ailerons being all but ineffective, you just use some rudder to help slide your ass end around to keep the wings level.

Not in F4 - in AN F-4, as in, in a Phantom.

I think the confusion comes from "stall" and "incipient spin"....

I think the confusion comes from "stall" and "incipient spin"....

Hehe...yeah, jamming the rudder at that point would increase the pucker factor exponentially. :eek:

Using rudder to prevent stall doesn't make any sense.

No it doesn't, but that's not what we're saying. You use rudder during stall recovery to initially correct for uncoordinated stall. If your left wing is low, don't use right aileron, use right rudder. Deimus has it.

Entering a perfectly coordinated stall should result in both wings stalling symmetrically and auto-rotation should not occur. Some wing designs are more forgiving than others (i.e. the precision required is lower and some stall from the root, while others the tip), but all things considered this should hold true.

While WWII fighters with lots of torque, p-factor, and slip-stream issues had poor power-on, low speed handling qualities often spun (similar to the way the jet spins in the sim right now), a jet fighter doesn't suffer from the same serious turning tendencies in a low speed, power on situation as a prop aircraft. I would think it should be harder to create an uncoordinated condition at low airspeeds.

Cross controls - Departure

...or lack theroff..

1. Thrust at flight idle from 140 Knots

2. Full left rudder

3. Right Aileron until aircraft roll is 0 (Almost full right stick)

The speed decays until 35 knots - No stall, no departure, -1300 (ish) fpm.

Stays at 35 knots with steady descent.

This behavior seems very strange....

The main reason that causes wing autorotation is the negative derivative of lift vs AoA function after the critical AoA. Any (Any!) fluctuation of wing AoA due to very tiny pilot's input, turbulence, self turbulence of the airflow leads to the positive feedback loop.

 

For example, right wing increases its AoA, lift decreases, wing goes down increasing local AoA, lift decreases...

If derivative are positive, the feedback is negative and any roll movement is dampened.

Wingtips are more sensitive because the bank moments is higher due to the longer arm and AoA change caused by roll is higher due to the same reason.

 

That's why if the root is in post-stall region while the wing tips are still not stalled they damp weaker roll tendencies of stalled root parts.

That said, does the A-10C stall at the root or the tip first? It has a pretty normal dihedral, rectangular planform, and thick NACA671X airfoil which would tend to indicate wing moments/forces like a light GA aircraft.

For example, right wing increases its AoA, lift decreases, wing goes down increasing local AoA, lift decreases...

If derivative are positive, the feedback is negative and any roll movement is dampened.

Wingtips are more sensitive because the bank moments is higher due to the longer arm and AoA change caused by roll is higher due to the same reason.

 

That's why if the root is in post-stall region while the wing tips are still not stalled they damp weaker roll tendencies of stalled root parts.

This is how it works for real, on wings where the stall starts at the root and then extend to the rest of the wing, so you get that mushy feeling and soonafter, the feeling that the floor got kicked out from under you when the stall hits (i.e. a very sudden drop in loading). Too bad you can't replicate that in the game but the g-meter sure shows it.

But I am curious, does the engine divide the wing into finite elements like in X-Plane? Or does it work differently?

That said, does the A-10C stall at the root or the tip first? It has a pretty normal dihedral, rectangular planform, and thick NACA671X airfoil which would tend to indicate wing moments/forces like a light GA aircraft.

Given that the A-10 has slats near the wing roots, I would guess it's designed for stability and thus has the former characteristic, but it would be interesting to know for sure. [Edit: n/m, was answered]

Slats are implemented mostly to reduce airflow disturbancy at engines inlets at high AoA.

I assumed this was because of where the engines are positioned (almost analogous to the deep stall issue with T-Tail configurations) where the engines are in the path of chaotic airflow after the wings are stalled. The -1 has charts on where airflow will disturb the engines based on AoA and Mach number. Is engine airflow disturbance modeled in DCS?