High speed buffeting

[bob m]

Can anyone comment on the flight condition in turbulence known as high speed buffeting, how dangerous is it, how often it happens, what action flt crews take.

Thanks,
Bob

[Gabriel]

Originally Posted by bob m

Can anyone comment on the flight condition in turbulence known as high speed buffeting, how dangerous is it, how often it happens, what action flt crews take.

Thanks,
Bob

High speed buffeting has nothing to do with turbulence.
High speed buffeting happens when shockwaves start to form on the wing, those shockwaves disturb the airflow, and the disturbed airflow disturbs the shockwave, making a feedback loop that causes that the pressure distribution over the wing changes cyclically creating a buffeting.

The buffeting is not dangerous in itself. Rather, it is a symptom that the plane is flying too fast, which can be a dangerous condition.
As the airflow over the wing approaches the speed of sound, the center of lift moves aft (from a theoretical 25% of the chord in well subsonic speeds to 50% of the chord in supersonic speeds). This shift of the lift makes a nose-down pitching moment that can put the airplane in an unrecoverable dive. There is a good margin between the point where the high-speed buffet starts to where the planes gets into an unrecoverable sonic dive.

As there is a high-speed buffeting, there is also a low-speed buffeting that precedes the aerodynamic stall. The airplane's operative envelope is within those types of buffeting.

[snydersnapshots]

Originally Posted by Gabriel

High speed buffeting has nothing to do with turbulence.
High speed buffeting happens when shockwaves start to form on the wing, those shockwaves disturb the airflow, and the disturbed airflow disturbs the shockwave, making a feedback loop that causes that the pressure distribution over the wing changes cyclically creating a buffeting.

The buffeting is not dangerous in itself. Rather, it is a symptom that the plane is flying too fast, which can be a dangerous condition.
As the airflow over the wing approaches the speed of sound, the center of lift moves aft (from a theoretical 25% of the chord in well subsonic speeds to 50% of the chord in supersonic speeds). This shift of the lift makes a nose-down pitching moment that can put the airplane in an unrecoverable dive. There is a good margin between the point where the high-speed buffet starts to where the planes gets into an unrecoverable sonic dive.

As there is a high-speed buffeting, there is also a low-speed buffeting that precedes the aerodynamic stall. The airplane's operative envelope is within those types of buffeting.

Well-said Gabriel. The only thing I might add is that the higher an airplane flies, the closer the "buffet margin" gets. Going high enough to where they're just a couple of knots apart is referred to as the "coffin corner."

[Gandalf123]

What action can the crews take?

I would say, reduce power, and perhaps bring the nose up a bit to help slow down, depending on the severity.

Near stall buffering... That is when that dreadful sounding stall warning conmes on... Nose down and full power and make it snappy or the plane will fall out of the sky like a grand piano test flown off the the Empire state building

[bob m]

Some of the best answers I've read in a while, thanks.

Bob

[3WE]

Originally Posted by Gandalf123

What action can the crews take?

I would say, reduce power, and perhaps bring the nose up a bit to help slow down, depending on the severity.

Near stall buffering... That is when that dreadful sounding stall warning conmes on... Nose down and full power and make it snappy or the plane will fall out of the sky like a grand piano test flown off the the Empire state building

Wrong, you will break up and die regardless!!!!!

(actually, to be serious for a rare moment- that's a texbook overzeaolous stall recovery, very likely to result in an overspeed, where you....

...might just break up and die.

[Highkeas]

Originally Posted by Gandalf123

.............. Nose down and full power .............

I worked on stall recovery systems for several aircraft and as far as I recall the stall recovery procedures were that the pilot apply nose down input and reduce power.

[MCM]

Here we go...

[snydersnapshots]

Originally Posted by 3WE

Wrong, you will break up and die regardless!!!!!

(actually, to be serious for a rare moment- that's a texbook overzeaolous stall recovery, very likely to result in an overspeed, where you....

...might just break up and die.

This brings to mind a few quotes from days gone by:

"We've all gotta die sometime."

"If you're gonna die, make it spectacular."

and my favoirite:

"Death is a small price to pay for looking s%*t-hot!"

[Gandalf123]

Originally Posted by 3WE

Wrong, you will break up and die regardless!!!!!

(actually, to be serious for a rare moment- that's a texbook overzeaolous stall recovery, very likely to result in an overspeed, where you....

...might just break up and die.

Come on guys..... You should have learned this in flight school.

You are in a stall most likely because you have an excessive AOA and/or, not enough power. You MUST reduce AOA and increase power. YOU NEED MORE AIRSPEED NOW OR DIE!!!!! Once you have recovered from the stall, you aren’t going to sit there like a dildo, leave the engine at full power and fly the plane into oblivion. You will reduce to climb power or cruise power or whatever, and return to normal flight.

Originally Posted by Highkeas

I worked on stall recovery systems for several aircraft and as far as I recall the stall recovery procedures were that the pilot apply nose down input and reduce power.

Sure, if you have enough altitude to play with. On takeoff and landing though, you don't have a hell of a lot of altitude to play with. Would you want to bring the nose down and reduce power when you barely have enough airspeed to stay airborn and there are only a few hundred feet between yourself and certain death?

Remember, that when you are close to stall, your aircraft has already started to descend because you don't have enough airflow over the wings to maintain altitude. Now you bring the nose down to decrease drag, but you also reduce power? You will now descend faster with the decreased AOA and by reducing power, you are reducing your rate of acceleration at the same time and increasing the time it will take to regain a safe air speed and recover if you had enough power to begin with.

[Gabriel]

MCM, it wasn't my intention this time, but what can I do with these kind of, eerrrr, "information?"

Originally Posted by Gandalf123

You are in a stall most likely because you have an excessive AOA and/or, not enough power.

That's why gliders are always stalled.
You can be stalled or not stalled with any amount of power.
You ARE stalled if beyond one specific AoA and you ARE NOT stalled if below that AoA.

Remember, that when you are close to stall, your aircraft has already started to descend because you don't have enough airflow over the wings to maintain altitude.

How wrong. A plane close to the stall can fly straight and level and even climb.

If you had enough power and enough controllability, you could fly and climb even with the plane fully stalled.

It is a matter of power required and power available. If there's available more that required, you can always climb (assuming you can control the plane) no matter stalled or not.

[Gandalf123]

Well Gabriel, that disagrees with over 100 hours of my flying experience in the Cessna 172.

[Gabriel]

Which part?
Tell this guy that he could not have climbed close to the stall:
http://www.atsb.gov.au/media/3906100/vi ... 11-165.wmv

[Gandalf123]

Originally Posted by Gabriel

Which part?
Tell this guy that he could not have climbed close to the stall:
http://www.atsb.gov.au/media/3906100/vi ... 11-165.wmv

That glider looked like it had more than enough airspeed to me. Nothing in that video indicated to me that it was anywhere near stall speed. Gliders usually rely on updraughts to keep them airborne. Glider pilots know to fly close to mountains or near clouds, places they know they will find updraughts to keep them in the air without power (Because they don't have any), otherwise they will simply glide down to the ground (Provided they maintain a safe airspeed). You should also know that it takes a powered aircraft to get them into the air You should know that powered aircraft can't rely on updraughts like gliders to to stay in the air. They are too heavy.

Look very carefully at what you are saying here, Gabriel. You have surprised me tonight/morning/evening, what ever the time is where you are. You usually post such great information about aerodynamics/systems and so on. You really seem to know your stuff. But now, a reply like this from you??? I am shocked.

Tell me, was that glider that you just showed me, in your opinion stalling? He seemed to have plenty of airspeed to me.

Close to stall (stall warning going off) means that there is flutter over the wings due to lack of airspeed, which causes lack of lift, weather it be a glider weighing a few hundred kilograms, or an A380 weighing 560 tons.

Now you come and tell us that Gliders always fly in a stall? How the hell? If there is no air flow over the wings, it is stalled. It can't fly. Without air flow over the wings, even a powerless, very very light aircraft like gliders are going to fall out of the sky.

To maintain lift, you need air flow over the wings. The slower you fly, the more AOA you need. The faster you fly, the less AOA you need. The more AOA you have, the more drag it creates, there fore the more power you need to maintain air flow over the wings to keep the aircraft straight and level.

Would you disagree on this?

[mawheatley]

Am I missing something here? Where was the glider?

[Deadstick]

Um, what did I miss? The video Gabriel referenced didn't have a glider in it. And Gabriel, was your comment about gliders always being stalled some kind of humor?

[Gabriel]

Originally Posted by Deadstick

Gabriel, was your comment about gliders always being stalled some kind of humor?

Yes. A kind sometimes called irony.

If, as claimed, not enough power could cause a stall, then gliders, that have no power, would be always stalled.

The no-no smily should have been a clue.

[Gabriel]

Originally Posted by mawheatley

Am I missing something here? Where was the glider?

Nowhere. The intruder and the victim were both C-172.

[Gabriel]

Originally Posted by Gandalf123

Look very carefully at what you are saying here, Gabriel. You have surprised me tonight/morning/evening, what ever the time is where you are. You usually post such great information about aerodynamics/systems and so on. You really seem to know your stuff. But now, a reply like this from you??? I am shocked.

You are mixing too many concepts: Stall, AoA, drag, climb, power, speed...

It would be too long to sort it out, plus I have already explained it several times in these threads, plus (because of the above) people like MCM become impatient when we start discussing it yet again and complain with things like "Here we go again..."

But let me show you 4 questions:

Can a plane be flying pretty fast, with max power, and stall?
Can a plane be equally fast, again with max power, and not stall?
Can a plane be flying too slow (even below the "official" stall speed), with no power, and stall?
Can a plane be flying equally slow, again with no power, and not stall?

The answer to all of them is "YES!"
And I could have combined them with "when the plane is climbing" and "when the plane is descending" and "when the nose is high" and "when the nose is low" too, and the answer to all 16 combinations would still have been "YES!"

So ok, power, attitude, climb rate, speed, bank angle, load factor, weight all have to do with stall. And as 3WE likes to say, flying too slow, aiming the nose too high or pulling too many Gs are good ways to stall a plane.
But all these factors combine in many different things ways and, in the end, regarding stall, are reduced to only one:

If the plane stalls it exceeded the critical AoA.
If the plane doesn't stall it doesn't exceed the critical AoA.
Stall is a matter of AoA ONLY. When you try to do some things (like pulling up too hard or climb beyond the plane's performance) you increase the AoA and can stall.

There is a thread with the title Stall is this aviation safety forum. I invite you to continue the discussion there or via PM.

[Deadstick]

Originally Posted by Gabriel

Yes. A kind sometimes called irony.

If, as claimed, not enough power could cause a stall, then gliders, that have no power, would be always stalled.

The no-no smily should have been a clue.

Ah, I figured as much but I can be a little dense sometimes.