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Turbulence & Seats Over The Wings  
User currently offlineTimePilot From Switzerland, joined Sep 2005, 296 posts, RR: 0
Posted (9 years 1 month 2 weeks 13 hours ago) and read 31120 times:

Is it true that you feel less turbulence when sitting over the wings? I've heard it from people, but on my last flight this weekend I didn't really notice a difference.

Last year I was a on a 767-300 (200?) sitting in row 1 and it was like buttah Guess that's why First Class and Business are up at the front, huh?  blockhead   dopey 

12 replies: All unread, jump to last
 
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 9
Reply 1, posted (9 years 1 month 2 weeks 11 hours ago) and read 31117 times:

Hello TimePilot.

Quoting TimePilot (Thread starter):
Is it true that you feel less turbulence when sitting over the wings?

I don't think that's true.  scratchchin 

If anything, I'd think vibrations from turbulence would be felt the strongest in seats that are over the wing because any turbulent air that only the wings fly through would be transferred to the fuselage in the area of the wing roots first. Then those vibrations might dissipate and be felt less as they travel throughout the fuselage & cabin.

This is just a guess on my part because I've never sat in different sections of an airliner during a turbulent flight to notice a difference. I guess a flight attendant would be able to answer this question the best.

Also, wouldn't it be true that every section of an airliner (nose, center, tail) that's flying through turbulence would experience the same vibrations (perhaps at different strengths), because turbulent forces acting apon the aircraft would be transferred throughout the whole airframe?........ since it's all one piece.

In other words, is it possible for passengers to be experiencing a ride up front that's smooth "like buttah, while passengers in the rear are being tossed around with a choppy ride? I find that hard to believe.


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User currently offlineTimePilot From Switzerland, joined Sep 2005, 296 posts, RR: 0
Reply 2, posted (9 years 1 month 2 weeks 8 hours ago) and read 31069 times:

When I did ORD > NGO this summer on an AA 777-200 ER and it was the first time I've been in 'moderate turbulence' (as the FAs put it.) It was not fun at all. When turbulence hit, I was in the bathroom in the very back. When I came out the FA said "It's bumpier in the back, you'd better go back to your seat." I also remember reading here? someone saying that 777s are bumpy in the back.

I'm always in steerage, never Business or First, so I can't tell if it's better or not. I was only once up at the front row and it was nice, but it might have just been a smooth flight.


User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 3, posted (9 years 1 month 2 weeks 7 hours ago) and read 31059 times:

Presumingly a Sudden pitch down momentarily would induce a Pitch up in the Tail section [Aft]...Compared to less Pitch in the middle,between the two ends.
Can that theory apply.
regds
MEL



Think of the brighter side!
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17110 posts, RR: 66
Reply 4, posted (9 years 1 month 1 week 6 days 22 hours ago) and read 30993 times:

I call the last few rows on large planes "the whiplash zone". You deffo feel turbulence more back there. As for over the wings, I'm not sure. In my experience those seats are about the same as others.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Reply 5, posted (9 years 1 month 1 week 6 days 12 hours ago) and read 30871 times:

The displacement of the aircraft would be the same everywhere if it were a rigid body. Since it's not, a bump would feel like a short, sharp chop near the center, but becomes a wide swing more towards the rear. If the center is where all the forces act on the plane, then the rest of the plane (longitudinally) is a cantilever behaving like a damped spring.

I personally find it easier to sleep in the rear because the sharp jerks get translated to a softer, rocking motion.

If the turbulence causes roll, then the center row is best for minimum motion. For a pitching ride, displacement is also lowest in the center.

I haven't figured out why the front of the plane, or the upper deck always attenuates the perception of turbulence.


I hope the engineers on a.net can further clarify this topic.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17110 posts, RR: 66
Reply 6, posted (9 years 1 month 1 week 6 days ago) and read 30811 times:

Quoting Comorin (Reply 5):
I haven't figured out why the front of the plane, or the upper deck always attenuates the perception of turbulence.

My theory is that vibrations travel backwards (since the plane travels forward) and amplify as they do so. So you will feel them less in the front.

But it's just a theory.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 7, posted (9 years 1 month 1 week 5 days 23 hours ago) and read 30799 times:

Up front you are nearer the wearer of the seat of the pants who is flying the plane. Obviously the pilot will iron out the bumps he/she feels when hand flying. In automatic flight the disturbances will be seen as changes read from the pitot-static system via the ADC to the autopilot. These air data sensors are also located near the nose. Either way the nose is flown smoothest and the rest of the plane follows. The tail will wag a lot more due to the see-saw effect about the centre of gravity. That's my theory anyway!


Quoting Comorin (Reply 5):
The displacement of the aircraft would be the same everywhere if it were a rigid body.

That ignores any pitching motion turbulence may induce, and there will be some. Such motion will be magnified forward and aft, compared to the centre section.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineGreasemonkey From United States of America, joined Aug 2005, 67 posts, RR: 0
Reply 8, posted (9 years 1 month 1 week 5 days 17 hours ago) and read 30751 times:

We were walking around inside a CRJ while the crew was performing an OCF and went through a batch of mild turbulence. We were investigating an airduct squeal issue that we could not duplicate on ground. Several of us noted that turbulence was most noticeable over the wings and did lessen more toward the rear and front...more so toward the front.

-GM



It's usually a good idea to know what all the buttons do...before you push them.
User currently offlineElectech6299 From United States of America, joined Aug 2005, 616 posts, RR: 3
Reply 9, posted (9 years 1 month 1 week 5 days 11 hours ago) and read 30703 times:

OK, my  twocents 

Several have touched on the ideas I learned in physics, so much of this is the previous ideas combined or re-stated, hope it helps.

Turbulence can cause many different motions on the aircraft, depending on where the aircraft strikes the air mass causing turbulence. In the case of relatively large air masses (compared to the size of the plane), in level flight the aircraft will experience a sharp increase (or decrease) in lift as the wings encounter more (or less) dense air. This is experienced like grabbing the wingtips and pulling them up or down (oversimplified, in general terms, for illustrative purposes- actually the forces act on the entire wing, but the effect is basically the same). I believe this is the most common kind of turbulence encountered, but if any more experienced (or pilots) have other views, I welcome them.

As previously stated, if the aircraft were a rigid body, the entire aircraft would pitch up and down exactly the same (ignoring pitch for now- assuming "level" flight) However, the aircraft body flexes much the same as the wings, so when the centre is almost instantly pushed up 2 feet by the wings, the wave of force takes time to travel through the body of the aircraft and changes according to (generally) two things: The flexibility of the material, and the distance from the origin. A flexible ("springy") material will absorb some of the force, and slow down the wave, whereas a rigid material will transfer the full force rapidly. Think of this like the difference between a cheerleader spinning a baton- rigid, fast, simultaneous motion- and a ribbon tied to the end of the baton. The flexible ribbon transmits the motion more slowly, so you have a visual reference of the force applied to the baton a few seconds ago. (ignore the air viscosity for now- you would see my point clearly in a vacuum, it would look a bit different on the ribbon tho-)

The second piece of this- distance from center- is also important, as Comorin noted the aircraft body becomes a lever. As the wave of force slows down moving away from the origin, it also becomes "taller". If you take a flexible rod, like a plastic yard-stick (or meter-stick) and apply an oscillating up-and-down motion of 4 inches (10cm) to the center, you realize that you can make the ends flap up and down 12" (30cm) or more. As the wave travels away from the point of force, it seems to get taller (stronger), the lever effect, right? So if the center passengers are pushed up 2 feet and weigh 100 lbs., the force above the wings is 200 ft-lbs, but at the tail end of the plane, the body flexes and the passengers are thrown up 3 feet-so if we look at another sexy 100lb passenger, she gets 300 ft-lbs of force, right?

Wrong.
This is actually a misconception-- the lever is reversed in this example. To get the ends to move with that much force, you are actually applying a greater force to the center of the stick. If each end of the stick has 5 newtons of force accellerating the flexing of the yardstick so that the tips rise, then you are applying a little more than 10 newtons of force in a rising motion to the center of the stick.

So take the example and apply a set amount of force to the center. So you raise the center of the stick over a period of 1 second with 10 Newtons of force and it moves 3 inches. (But the ends don't move just yet-) The resistance (inflexibility) of the stick limits the distance of travel at the center. But a foot further away, the wave of force acts on a section of the stick that is already moving- because of the inflexibility of the material- and so after 1.5 seconds that section of the stick is being accelerated by the peak of the wave that took time to travel to that part of the stick. It is now just under 5 Newtons of force, having lost energy to friction- but the stick moves 6 inches.

The friction acts against the wave, taking it's energy and transforming it to heat. I really don't know with an aircraft body, but I suspect by observing the effects that the effect of friction is less than the effect of the flexibility- so as the wave travels it will move the front and back of the airplane more than the center, like in the stick example, but with less force. An object with more friction (less flexibility) will absorb more of the energy and move less- like a plexiglass pane. Given a long enough (or thick enough, but that's no fun to watch) plexiglass sheet, you could hit it in the center and the wave of force would die before it reached the edges- it has smaller oscillations as it moves away from the point of impact.

So as the same wave of force travels through the stick, it accelerates the material just a little further. So by the end of the stick, the peak of the wave is 12". But with much lessforce per distance traveled. At the origin, a force of 5 newtons moved the stick 3 inches- (or the force of turbulence bounced the passengers 2 feet. Strong force acting against stationary metal and passengers and rapidly accelerating them for a short time and distance. Relatively high "G-force") By the time the force reaches the end of the stick, a somewhat slower wave with slightly less energy moves the already moving tail end to a peak of 12". (Or a slightly less powerful wave moves the tail of the aircraft three feet. Nearly the same force acting against an already moving aircraft frame, taking slightly longer to accellerate the passengers. Slightly lower "G-force".)

So the passengers of a non-moving aircraft suspended in the sky at the center of lift suddenly take a force applied at the center of lift. The passengers at the center experience a sharp force for a short time, a "jarring" motion. The passengers at the nose and tail experience a slightly less sharp force for a slightly longer time, a "rocking" motion. In a mile long aircraft, the passengers at the nose would experience a smooth flop, like a baby in the treetop. (except that the frame would fail and they'd all be falling without lift and with minimal turbulence...)

Now as to pitch, roll and yaw, as explained by Comorin earlier, those will depend on the distance from the force applied- with very little difference in perception of roll, more in pitch. As to why more turbulence in the back? Because of turbulence on the horizontal stabilizers on the tail. Hardly noticed in the front, except as change in pitch. But the back of the plane can jump 2-3 feet without the pilot's knowing the difference.

In the case of T-tails, the pitching force on the stabilizer is applied to a place actually behind the body of the airplane, so the rotational force is greater- I always wondered, but it seemed top me that the DC-9s and 727s were turbulence hogs, while the 737s and A320s just skipped on the clouds.

Sorry for the verbose response, I hope someone out there likes my prose, just blowing some steam after work....



Send not to know for whom the bell tolls...it tolls for thee
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 10, posted (9 years 1 month 1 week 4 days 22 hours ago) and read 30659 times:

I don't know about 2 cents, more like 200. In amongst the wood there are a few trees, but they are hard to see. There's a lot of confusion (force in newtons one minute, ft-lbs (??) the next). Some bogus analogy, and way too many synonyms.

If you turn you're meter stick through 90 degrees it becomes much stiffer and won't flex as much. The nose and tail will move up and down in phase with the centre. Just like a real fuselage, which acts more like a rigid body than a flexible rod.

What passengers feel is not the force applied by the turbulence, but their own intertia resisting movement resulting from it. You talk about less force hitting an already moving tail, well if the tail is moving the passengers there have already felt a force: their own inertia wanting to keep them where they were.

At the beginning you say we'll ignore pitch for now, but never come back to take it into account. The sudden change in AoA due to a gust will change wing lift but also wing pitching moment, so this can never be ignored. Pitch doesn't just come from the effect of the tail.

Quoting Electech6299 (Reply 9):
So if the center passengers are pushed up 2 feet and weigh 100 lbs., the force above the wings is 200 ft-lbs, but at the tail end of the plane, the body flexes and the passengers are thrown up 3 feet-so if we look at another sexy 100lb passenger, she gets 300 ft-lbs of force, right?

Wrong.

Newton will be spinning in his crypt. Firstly your 300 ft-lbs is in fact work, not force. Now, if the passenger at the tail moves 3 feet in almost the same time as the passenger at the centre moves 2 feet (unless we are assuming a rubber fuselage here), then the passenger at the tail has experienced greater acceleration, and so g-force (which is mass times acceleration). The only way your logic would work would be if the fuselage flexure damped the movement at the centre, and so the tail moved less than two feet. In some aircraft this kind of modal damping is done using active controls (gust alleviation).

The passengers at the centre do not feel sudden impacts either, because the wings flex too, much more than the fuselage. So the blows are softened because the wings act like a spring and damper system.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineElectech6299 From United States of America, joined Aug 2005, 616 posts, RR: 3
Reply 11, posted (9 years 1 month 1 week 2 days 12 hours ago) and read 30548 times:

Quoting Jetlagged (Reply 10):
I don't know about 2 cents, more like 200. In amongst the wood there are a few trees, but they are hard to see. There's a lot of confusion (force in newtons one minute, ft-lbs (??) the next). Some bogus analogy, and way too many synonyms.

Hmm, you sound a lot like my Composition teacher. Yeah, as I look at it again it doesn't read well. I was way too tired to be writing anything like that... I should have known, that's when I tend to use too many words. The ft-lbs error was a change I made to address a nagging devil's advocate voice, which can be (when I am awake) a very useful tool for my writing. Oops.  ashamed 

Quoting Jetlagged (Reply 10):
What passengers feel is not the force applied by the turbulence, but their own intertia resisting movement resulting from it.

Agreed, as a matter of perception...which is what this thread is about, isn't it? Good point.

Quoting Jetlagged (Reply 10):
At the beginning you say we'll ignore pitch for now, but never come back to take it into account. The sudden change in AoA due to a gust will change wing lift but also wing pitching moment, so this can never be ignored.

Yes, it looks like I gave up on that one...this is about as far as I got:

Quoting Electech6299 (Reply 9):
Now as to pitch, roll and yaw, as explained by Comorin earlier, those will depend on the distance from the force applied- with very little difference in perception of roll, more in pitch.

It just didn't work with where I was trying to go, which doesn't look like a very good destination anymore and I'm wondering why I wanted to go there in the first place. But it was at least followed by one of my (IMHO) useful points:

Quoting Electech6299 (Reply 9):
As to why more turbulence in the back? Because of turbulence on the horizontal stabilizers on the tail.

Although in terms of pitch, [b]HAWK21M[b]'s point is conceded

Quoting HAWK21M (Reply 3):
Presumingly a Sudden pitch down momentarily would induce a Pitch up in the Tail section [Aft]...Compared to less Pitch in the middle,between the two ends.
Can that theory apply.

I still believe that the fuselage absorbs some of the force of turbulence along it's length, so the turbulence felt in the nose section is dampened slightly compared to the turbulence felt adjacent to the flight control surfaces along the wing and tail. But perhaps this is an error and I am just attempting to explain my perception of turbulence which could be the result of changes of pitch- which will vary along the length of the aircraft depending on the point of rotation.

I am intrigued by the active "gust alleviation" controls you reference- can you elaborate?

Also, any other insight into the T-tails, anyone?



Send not to know for whom the bell tolls...it tolls for thee
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 12, posted (9 years 1 month 1 week 1 day 19 hours ago) and read 30510 times:

Quoting TimePilot (Thread starter):
Is it true that you feel less turbulence when sitting over the wings? I've heard it from people, but on my last flight this weekend I didn't really notice a difference.

Yeah somewhat, the difference is small. An airplane bobs and wobbles around along the fuselage (X-axis), the wing (Y-axis) and vertically through the center (Z-axis). Not by noticeably much, but it still oscillates. During turbulence, the wings are trying to lift the plane but the air is not always at the same speed, so the change in lift will bump the plane's cg up and down. Doing so causes the fuselage to flex mostly at the ends. Air is also slamming up against the sides of the plane causing a yaw flex to occur.

Not just a turbulent case, even during takeoff the tail plane will cause a positive pitch moment, rotating the plane for takeoff; doing so does flex the plane. Especially during crosswinds or the accidental microburst fly-through.

If anything, maybe sitting near the center of lift [pressure] would be a tiny bit more optimal than just anywhere over the wing, if it really mattered to you. As I understand, does the center of lift pressure move when flaps are engaged?

Quoting TimePilot (Thread starter):
Last year I was a on a 767-300 (200?) sitting in row 1 and it was like buttah Guess that's why First Class and Business are up at the front, huh?

Hold on now, that is different, those seats might have extra cushioning or even suspension.  Smile



The meaning of life is curiosity; we were put on this planet to explore opportunities.
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