About cockpit shaking，is it a bug?

[HiWeee]

Hi all,I have a question which is always bother me for a long time. When the aircraft is close to the stall state, the cockpit should start shake,  such as Mig-21, Mig-15, but in Mig-19 it's strangely calm. Is it normal?or is it a bug for FM?

[MAXsenna]

Wouldn't call it a "bug". Not implemented maybe. Can't remember if my FFB stick shakes.

Sent from my MAR-LX1A using Tapatalk

[rossmum]

Not all aircraft experience prestall buffet at all speeds, so it is entirely possible it's intentional. At very low speeds in particular, some aircraft will experience almost none, and some will experience it long before an imminent stall at higher speeds.

[Dragon1-1]

Also, some wing types, particularly supercritical wings, have a nasty stall behavior where it basically comes out of nowhere. When an aircraft equipped with such a wing stalls, it just drops like a rock, with little to no buffet. Often, stick shakers are installed in such aircraft to compensate, not sure if that's the case here. 

[Bremspropeller]

On 6/10/2022 at 4:00 PM, Dragon1-1 said:

Also, some wing types, particularly supercritical wings, have a nasty stall behavior where it basically comes out of nowhere.

You mean like the AF447 A330, pulling around 45° AoA for the better part of two minutes and still retaining limited roll-control via the ailerons, before pancaking into the ocean? Every Cessna out there has a nastier stall than that. I guess old tales die hard.

The MiG-19 with it's what - 60° LE sweep - should have a wild vortex dancing on it's wing and hence buffet your fillings out.

[Dragon1-1]

5 minutes ago, Bremspropeller said:

You mean like the AF447 A330, pulling around 45° AoA for the better part of two minutes and still retaining limited roll-control via the ailerons, before pancaking into the ocean? Every Cessna out there has a nastier stall than that. I guess old tales die hard.

FYI: a stall doesn't have anything to do with loss of control, it is a loss of lift. Indeed, a stall in which you also lose control is called a deep stall, and is a significantly worse condition, not present on most aircraft. Supecritical wings don't buffet before stall, and this is not just some old tale, but a well documented aerodynamic phenomenon. Indeed, if there was any buffet on AF447, it might have brought some sense into the copilot, who sure didn't act like they were stalling. That is what makes this kind of behavior nasty, not any loss of control. In a Cessna there will be all sorts of shaking preceding the stall that will make it abundantly clear to the pilot what's going on. An aircraft with a supercritical wing usually drops like a rock instead, which is why they're typically equipped with stick shakers, stick pushers and other such devices to compensate.

A MiG-19 might have a highly swept wing, but a supercritical airfoil will exhibit little if any buffeting. Pre-stall buffeting has nothing to do with vortices, and everything to do with airflow separating unevenly from part of the wing. As I said, a supercritical wing stalls all at once. There's no uneven separation, one moment you have lift, the next you don't.

[Bremspropeller]

11 minutes ago, Dragon1-1 said:

FYI: a stall doesn't have anything to do with loss of control, it is a loss of lift. Indeed, a stall in which you also lose control is called a deep stall, and is a significantly worse condition, not present on most aircraft.

A stall will always come with a change of lift-(pressure)-distribution around the wing and hence a change in pitching moment and hence with some sort of stall-break. Most favourably, that break happens cleanly ahead, but on many occasions (minor sideslip, manufacturing impurities, minor alterations to the surface, e.g. bugs, dents, scratches) one wing will stall first. That's going to introduce you to a new view of the world. A whole windscreen of it.

You CAN "falling leaf" in a semi-stalled configuration with a high sinkrate and going straight ahead (and down), but that's not a full stall.

A deep stall is a stall where you're locked in high alpha due to a pitch-moment reversal. It's common for T-tails, but other aircraft (e.g. the Viper) can end up there, too. You're not really losing control, you're most likely just running out of airspace for recovery.

18 minutes ago, Dragon1-1 said:

Supecritical wings don't buffet before stall, and this is not just some old tale, but a well documented aerodynamic phenomenon.

No. There's different kinds of buffet. One is the buffet created by vortices impinging onto the wing. That's your common fighter buffeting.

The other, mostly associated with pre-stall flight, buffeting is detached flow hitting the stabilizer. And yes, supercritical wings exhibit this behaviour, too.

21 minutes ago, Dragon1-1 said:

Indeed, if there was any buffet on AF447, it might have brought some sense into the copilot, who sure didn't act like they were stalling. That is what makes this kind of behavior nasty, not any loss of control.

I suggest you read the accident report and ctrl+F for "buffet".

https://bea.aero/docspa/2009/f-cp090601.en/pdf/f-cp090601.en.pdf

Also, this Airbus Training Video disagrees woith your presumption.

Pre-stall buffet in an A310 (also supercritical wing):

 

34 minutes ago, Dragon1-1 said:

In a Cessna there will be all sorts of shaking preceding the stall that will make it abundantly clear to the pilot what's going on.

Not really. The buffet is pretty subtle actually. It's more the continuos amount of force, uncomfortable pitch (doing a power on stall) and the stall-horn blaring in your ear significantly above stall-speed. And yes, she'll drop a wing on you if you're aggressive enough.

36 minutes ago, Dragon1-1 said:

An aircraft with a supercritical wing usually drops like a rock instead, which is why they're typically equipped with stick shakers, stick pushers and other such devices to compensate.

None of the latter on a 'bus. In ALT2 or Direct, you can just haul her into a stall in wild abandon. The Cavalry Charge might disturb you a bit. And the buffet, shaking your fillings out (see above). Stick Shakers are mostly a Boeing thing nowadays. But they're steampunk anyway. Stick pushers were a thing with t-tails, even though (especially with the Brits) their mandate wasn't always necessary.

40 minutes ago, Dragon1-1 said:

A MiG-19 might have a highly swept wing, but a supercritical airfoil will exhibit little if any buffeting.

Myth busted. The MiG-19 also doesn't have a supercritical airfoil.

40 minutes ago, Dragon1-1 said:

Pre-stall buffeting has nothing to do with vortices, and everything to do with airflow separating unevenly from part of the wing.

Flow separation is nothing but vortices.

41 minutes ago, Dragon1-1 said:

As I said, a supercritical wing stalls all at once. 

Nope.

41 minutes ago, Dragon1-1 said:

There's no uneven separation, one moment you have lift, the next you don't.

Negative.

[kablamoman]

3 hours ago, Dragon1-1 said:

FYI: a stall doesn't have anything to do with loss of control, it is a loss of lift.

Hey friend, not to be super pedantic, but I would argue that it's exactly about loss of control.

Indeed, the lift coefficient generated by the wing is at its highest at the critical angle, and still very high beyond, so it's not really about the loss of lift. The important change is that the curve starts trending downward beyond that point and this leads to the loss of the vertical and roll damping that are normally an intrinsic part of the plane's design.

Normally, if the plane is disturbed vertically or in roll and one wing or both move downward, the increase in the amount of relative air coming from below, increases the angle of attack on the wing surface and it self-corrects. But beyond the stall the opposite happens, and there is a runaway effect where the wing or indeed the entire plane just sinks faster and faster when the AoA is increased. This is the important change in behavior that happens beyond the stall that defines the region of flight, and it has very much to do with control.

Edited July 15, 2022 by kablamoman

[Koty]

20 minutes ago, kablamoman said:

Hey friend, not to be super pedantic, but I would argue that it's exactly about loss of control.

If you want to be pedantic, it really is just the loss of lift.

20 minutes ago, kablamoman said:

The important change is that the curve starts trending downward beyond that point

= loss of lift.

You will usually start losing control before the wing is fully stalled. Some wings are designed in a way that allows part of the wing to stall earlier to warn you of incomming stall before your controls get into the stall-disturbed air as your wingtips inevitably stall. A really good example of this can be found with the Yak-28:

the wing has a constant chord on the part outboard of the engines, between the engine and fuselage the chord lenghtens, decreasing the relative thickness and increasing leading edge sweep. The leading edge in front of the ailerons also has an extension which curves down (similar method was used on Convair deltas, however there it was primarily to reduce drag). This ensures you will feel the onset of stall long before your ailerons are stalled, retaining control.

EDIT: On a side note, this reminds me how much I'd love to see the Yak in DCS

Edited July 15, 2022 by Koty

[kablamoman]

3 hours ago, Koty said:

= loss of lift.

I understand why you might focus on that, but the point I was making is that what's special about the stalled regime and what makes it such an undesirable place to be is not that you suddenly lose lift (you don't), or even that you gradually lose lift (you don't, necessarily) -- it's that the relationship between angle of attack and lift is reversed and this leads directly to profound controllability issues  -- runaway acceleration -- whether it be associated with the inability to maintain altitude, the inability to maintain wings level or both. It's the loss of vertical and roll damping that's the big problem, not so much the loss of lift (as the wing is still close to generating its max coefficient of lift).

It's all about control, which is why you see some of those features you alluded to like changing wing geometry (eg. washout), vortex generators, and stall strips. They're all meant to keep critical control surfaces out of the regime entirely and help maintain roll damping even beyond the overall wing stall (loss of vertical damping). I'd say a stick pusher, whether automated or human, is the ultimate version of these things, which works by restoring the vertical damping by putting us back on the normal side of the lift curve, and also has the added benefit of restoring normal control response. If that damping and control response isn't restored we've still got lift -- it's just that everything is reversed: Trying to raise a wing with normal aileron input only drops it, which in turn makes it drop faster still, and pulling back on the control column only makes you sink faster, which in turn makes you sink faster still.

Edited July 15, 2022 by kablamoman

[kablamoman]

And sorry just to steer things back on topic:

A deep stall is when the pitch control surfaces (eg. elevator, stabilator) -- the primary means of recovery from a stalled condition -- are shadowed by and rendered ineffective by turbulent air kicked off the more forward parts of the airframe in a stall, making recovery difficult to impossible.

An aircraft's buffeting behavior is entirely specific to its airframe, and can have causes that are manifold. Types of buffeting (felt throughout the entire airframe vs. through the control column because certain control surfaces are being hit by turbulent air) can also be varied.

It's hard to say exactly what kind of buffeting will occur or be felt in the lead up to stall by just wing shape alone, as it can vary based on many factors. You could have plenty of buffeting (warning) or you may have none (sudden). This would be best answered by the developers and should be based on actual accounts and official reports.

[Tiger-II]

Interesting discussion.

AF447 - the official report is a whitewash to bury the fact the F/O royally screwed up and to hide deficient training and systems for post-stall recovery. It was a crash that should have never happened.

Aircraft stall characteristics can be anything from not-a-damn-thing, to shaking so violently you can't see what you're doing anymore. I don't actually know what would be correct for the MiG-19. From what I do remember reading, it gives a little bit of warning before the departure, and we have that in the sim.

We also get a little bit of Mach buffet around Mach 1.0. Nice touch, and I don't remember that being present the last time I flew it.

Edited August 8, 2022 by Tiger-II

[dresoccer4]

My cockpit is shaking at cruise speed and altitude (32,000' @ 350kt ias). This happen to anyone else?

UPDATE: discovered this happens only in transonic region

Edited August 12, 2022 by dresoccer4

[Bremspropeller]

On 8/8/2022 at 4:14 AM, Tiger-II said:

AF447 - the official report is a whitewash to bury the fact the F/O royally screwed up and to hide deficient training and systems for post-stall recovery. It was a crash that should have never happened.

Most crashes should never have happened. The was no post-stall, they ran out of time and pancaked in at high aoa.

There were a lot of factors, but this is not the place for that discussion. I can't see any whitewashing.

On 8/8/2022 at 4:14 AM, Tiger-II said:

Aircraft stall characteristics can be anything from not-a-damn-thing, to shaking so violently you can't see what you're doing anymore. I don't actually know what would be correct for the MiG-19. From what I do remember reading, it gives a little bit of warning before the departure, and we have that in the sim.

Being a 60° swept wing, it should have a good buffet at aoa.