View Full Version : turbulence

phase slag
27th Apr 2002, 00:08
When flying on the big jets I'm always a passenger and spend a lot of my time looking out of the window. Not being the best passenger in the world I am always concerned by the way wing tips flex and engines rattle around in their mountings during take off and turbulence.

I'm well aware they are designed to do this but how much stress can they take? Do large planes ever fall apart due to excessive vibration?

I would appreciate any views from people with a bit of a technical background.

27th Apr 2002, 01:44
While at engineering school, our Dean told the story of how he would not sit by the window on 727s because Reisebeck designed the wing, he got a ‘C’ in structures, and it scared the Dean to see the wing flexing. Otherwise they are designed to flex quite a bit, the slang term is ‘soft ride’.

27th Apr 2002, 10:27
Maybe you should take a trip to Boeing in Seattle. They do wing testing. They bend the wings alot further than they go in flight!!

27th Apr 2002, 16:40
I remember hearin a story very long time ago (early 80s)about a new FO who looked out at the F28's wing during a wild ride through a nastey Top End CB and remarked at its lack of flex.

Captain said "hey mate dont you know the Irish built that wing?"

Apparentley the FO broke out in a very panicky sweat! :D

27th Apr 2002, 18:34
Succinctly: Flexing stops it breaking. :p

28th Apr 2002, 16:08
Exactley Tinny. Proven by simple experiment:

Get a wooden matchstick and apply a force of 2 kgs. Itll bend a tiny bit then snap.

Get a rubber eraser of equal dimensions to the matchstick and apply the same 2kg force to it. Itll bend. Apply 3, 4, 5 kgs - Itll STILL bend and not break. Might even bend right around 180 deg!


28th Apr 2002, 18:39
Aviation history is filled with examples of why flexible structures are best.

Some (quite a lot actually) years ago the Royal Navy used RR Nimbus engines in their Wasps. The combustion chambers were found to be flexing, so RR added strengthening webs to them. The webs stopped the flexing, but the stresses caused the webs to crack. The cracks ran into the chamber walls and pieces started to fall out. A small problem had been turned into a much bigger one. A cleverly designed modification was then added to reduce the size of the webs, so that the chambers could flex a little bit without cracking.

I would be more concerned flying in an aircraft in which the wings didn't flex. Brittle failures give no warning, just drop that glass your holding and you will see what I mean.

phase slag
28th Apr 2002, 22:21
What about wig mounted engines? Surely they must be bolted to the wing structure and excessive vibration must place great strain on the wing/engine coupling.

29th Apr 2002, 06:41
Actually, the Discovery channel Wings program showed the test to destruction of the 74 during orginal certification tests. The wing was bent upwards 28 feet before finally breaking! Also, if you want to see a really good "flapping wing" display, try the Galaxy C-5 with the wing damper system selected off!;)

Mike Southern
29th Apr 2002, 14:16
It's not really how much it bends (that's just a function of the stiffness and the applied load), it's really all about whether you go beyond the elastic limit of the material. In the elastic range it is simply a function of load (stress) and movement (strain), double the load and the strain should also double..... (simple stuff really, actually it gets a bit more complicated if you start looking into fatigue, but that's the next grade....!)

Metals (and lots of other materials) have a degree of elasticity and you can stretch and bend them in a purely reversible manner up to a certain, point. Once you exceed the elastic limit that's when the trouble starts.... Then you start getting non-linear response and things start to fail, e.g. double the stress and, oops, the wing fell off!

Still that's what materials engineers get paid to calculate and, let's face it, most of the time they get it right.

29th Apr 2002, 23:01
Engines hanging from a wing are actually a good thing sometimes. The lifting force will try to bend the wing upwards, but the weight from the engines opposes this, causing the bending moment in the wing to be less.

Another good analogy is a tree. Trees bend in the wind, if they wouldn't do this, they would break a lot more often then they do now!

As for how much stress you can apply to the wing? In tests they apply loads to wings that make the tips move up over 20 feet I think, and the wing will still not break. There are some impressive videos showing these tests, you might come across them on the Discovery Channel.

30th Apr 2002, 07:39
Another little-known gem is that a wing full of fuel is stronger than an empty one. With wings that have integral fuel tanks, the fuel itself takes up part of the stress loads because the fuel is an embedded part of the wing structure overall.

30th Apr 2002, 09:46
Have to admit, I know buggger all about wing flexing, so I'm not in a position to answer phase slag's question.

While all answers here are interesting, and I'm sure that they are technically accurate, maybe the subject could do with a bit more sensitivity? Phase slag describes himself as "not the best passenger in the world", and I don't think stories about how the dean of an engineering school wouldn't sit near the window of a 727 are going to do much to make his travelling experience more pleasant.

Phase slag, it seems that the message from this thread is that the wings would have to bend an extraordinary amount before there's any danger of them breaking. In fact, the flexing is actually beneficial, as bendy objects are less likely to snap.

Personally, I think flying is one of the most amazing experiences, and the view from up there can't be beaten. Try to relax, enjoy the view, and don't let the wing flexing worry you! :)


30th Apr 2002, 20:34
Sorry for being insensitive, I don’t work in the marketing department. I do think it is an amusing story. My first post was apparently not read through by some respondents. I mentioned wing flex and soft ride, and that the wings were designed to do so. A previous thread (Reporting Points – The seat belt sign is on) about a similar issue discussed ‘telling it like it is’, so I did and am berated. Well this is a technical forum and I prefer straightforward talk. If you have an amusing anecdote, such as the Irish wings, even more fun. Tact is certainly desirable, but is not a requirement for posting on this forum. This is the technical forum, not the spin-doctors forum.

To answer the question, "Do large planes ever fall apart due to excessive vibration?", at the additional risk of being insensitive.

3 aircraft mishaps come to mind as the result of turbulence. One in Anchorage, a 747 dropped an engine during climb out as it hit a mountain wave (pods not designed for lateral shear loads). The others are Braniff 250 a BAC 1-11, and BOAC 911 a B-707-436, both of them structurally failed in flight due to turbulence. There are probably other cases of structural failure due to turbulence; there certainly are many cases of jet upset due to turbulence, NWA 705, a B-720 being one.

phase slag
30th Apr 2002, 23:02
thanks for the concern however, I'm military aircrew and thrive off bad taste banter. All the reasurance in the world wouldn't prevent me feeling any better about wobbly wings and rattling engines but these posts give me something scientific to think about next time while I wait for the G&T to kick in.

I watch helicopter blades do very unnatural things on a daily basis and don't bat an eyelid - a strange thing the human mind.

1st May 2002, 06:56
Actually "wobbly wings and rattling engines" together can be a very bad thing. Two Lockheed Electras were lost around 1960 when a weakened prop support permitted the props to enter "Whirl mode", where the prop disk precesses or wobbles.

The frequency of the wobble induced wing flutter, tearing the wings from the aircraft in flight, with all passengers and crew lost. (Braniff's N9705C on Sep 29, 1959, and Northwest's N121US on March 17, 1960)

The problem was fixed at Lockheed's expense under the Lockheed Electra Achievement Program (LEAP), however the type never really recovered from the bad publicity, and the entry of the new domestic jets. Lockheed didn't recieve any further orders for the type, despite it being a fast and economical design.

1st May 2002, 09:26
Checks I heard the problem was one of harmonics in certain unique situations and the problem was fully solved by tilting the Allesons up by 3 deg. For a long time the Vne of the Electra was only 227 knots I was told.

1st May 2002, 11:07
Ok, I admit it, I was not in a particularly good mood yesterday when I posted to this thread, and my post was a little out of character. %MAC - I agree, your story is pretty amusing, and I did chuckle - just wasn't sure if it was right for the audience. But, as you've pointed out, this is a technical forum - and it seems that phase slag isn't bothered by the nature of the replies anyway, so no harm done.

I apologise if my post caused any offense - but I don't think it did, so that's ok! :)


1st May 2002, 17:14
Sorry to be so acerbic/ caustic in my reply there Triple F, guess I also had a similarly poor day :)

Whirl Mode
The BE-1900 had a problem with broken or cracked engine mounts before Service Bulletin 2255 mandated stronger trusses and increased inspection period. Seems that whirl mode flutter may have been the cause of the cracks. The accident offshore Block Island, (29 December, 1991) may have been due to engine separation as the result of wmf. That seems to be ALPAs position, the NTSB maintains it was pilot error, loss of aircraft control. Though the finding was pilot error, the manufacturer issued SB 2255 shortly after the accident.

Whirl mode is present in most rotating bodies, such as a ceiling fan that not only rotates, but if not rigid will wobble or precess.

2nd May 2002, 23:12
Another related questions then. Although the wings are flexible and thus can withstand turbulence, what about how the wings are joined to the fuselage?

Young Paul
3rd May 2002, 00:10
I still like Slasher's first reply about wood and rubber and the QED at the end.

I knew there was a reason that so many early aeroplanes were built out of rubber.

3rd May 2002, 00:14
Phase slag

Funny you should mention helicopter blades. I remember seeing a documentary showing some film some while ago taken with with a rotor head mounted video camera.

Now that was scary!

In a similar vein to Apex45's question I am always fascinated to think of the loads transmitted through the engine mountings of a 777 in an engine failure on take-off situation at max. weights or are they machined out of rubber too?:D :D :D :D :D

3rd May 2002, 04:10
Hmm, after a bit or research, Slash!

As the Electra became operational, they found some excessive vibration in the centre cabin, and the outboard props were more highly stressed than the inboard ones - this was the reason the outboard nacelles' incidence was raised 3° upwards.

The Vno was reduced from 324 kt to 275 kt, and then a week later to 225 kt (with a Vne of 245 kt) after the second accident I mentioned above, and stayed that way until the LEAP was completed.

3rd May 2002, 08:32
Considering the bending relief of aircraft wings, it is interesting to note the maximum designed movement (full up to full down) of the wing tip in normal service is....
DC-6/DC-7, 12 feet
L1649A, 14 feet
B707-320, 22 feet
Beyond this amount, skin deformation occurs.

6th May 2002, 01:58
Just a short thought. Not only is it important to not exceed the elastic limit of the material, but it is also V important not to stress the material to its proof stressor beyond. In other words, the material has been bent, but by such a small degree that it may go un-noticed. 1-2% proof stress can cause critical crystal/grain distlocations in some materials which could decrease the ultimate strength of the material by 25-40%. This eats into the Factor of Saftey and even into the normal operating load range, with associated brown pants!

I am sorry if this was boring or incorrect.

6th May 2002, 05:33
Yeh thanks Checks. Be interesting to see what John T says on the subject.