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Aircraft "Bobbing" During Taxi  
User currently offlineBoeingDrew From United States of America, joined Dec 2004, 55 posts, RR: 0
Posted (9 years 7 months 2 days 22 hours ago) and read 6224 times:

Hey All,

I don't know if anyone else has noticed this before, but I always see a slight "bobb" or "bounce" in the aircraft while it taxi's. I doubt the taxiways are that bad and the suspension on these aircraft are that bad, but obviously I have no idea why, hence me starting this thread  Big grin. Any explanation as to why this phenomenon occurs? Thanks in advance.

-Drew


You can't build a reputation upon what you are GOING to do, but rather upon what you HAVE done!
30 replies: All unread, showing first 25:
 
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 1, posted (9 years 7 months 2 days 22 hours ago) and read 6191 times:

You try pushing or pulling something heavy at rest, it will vibrate very slowly; that's what the bobbing is, that whole airplane is capable of flexing to some degree especially when sudden forces occur, like throttling up an engine after brake release or inflight turbulence. Dude, sorry I can't give a technical answer, it's like 2am here and I probably need to sleep. hehe.  Smile/happy/getting dizzy


The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineDl757md From United States of America, joined May 2004, 1562 posts, RR: 16
Reply 2, posted (9 years 7 months 2 days 22 hours ago) and read 6184 times:

Quoting BoeingDrew (reply 0):
I doubt the taxiways are that bad and the suspension on these aircraft are that bad


The combination of the the expansion joints in the taxi ways and the fact that the shock struts in landing gear are designed primarily to absorb landing forces cause the bobbing sensation you have experienced.

Dl757Md



757 Most beautiful airliner in the sky!
User currently offlineOly720man From United Kingdom, joined May 2004, 6737 posts, RR: 11
Reply 3, posted (9 years 7 months 2 days 17 hours ago) and read 5955 times:

The main undercarriage is fairly close to the aircraft centre of gravity so it does not take a lot of loading to raise/lower the nose slightly (as some cargo people have found if they don't unload the aircraft properly). The aircraft can then oscillate on the nose undercarriage, hence the bobbing.


wheat and dairy can screw up your brain
User currently offlineLoggat From United States of America, joined Feb 2000, 666 posts, RR: 0
Reply 4, posted (9 years 7 months 2 days 15 hours ago) and read 5810 times:

An airplane at rest tends to develop a bulge at the bottom of the tires when sitting for extended periods of time. During taxi out, the bulge will progressively go away as the tire heats up and the rubber changes to a more overall circular rotation. It is more prominant for people sitting over the main landing gear as this is where most of the airplane weight is centered on the ground.


There are 3 types of people in this world, those that can count, and those that can't.
User currently offlineSeptember11 From United States of America, joined May 2004, 3623 posts, RR: 21
Reply 5, posted (9 years 7 months 2 days 14 hours ago) and read 5733 times:

I like the feeling of bobbing. Talk about dramatization. Loggat's reply 4 sounds correct to me.


Airliners.net of the Future
User currently offlineFutureatp From United States of America, joined Feb 2000, 221 posts, RR: 0
Reply 6, posted (9 years 7 months 2 days 14 hours ago) and read 5703 times:

I just want to add another possibility.

All the commercial airplanes I know of in use today use a tri-cycle landing gear. I am not familiar with the physics involved, but I believe the setup can create a level of instability when in motion. I believe it all boils down to how you have just one point of contact up front vs. 2(or more) points in the rear.

Please correct if I am mistaken!

John


User currently offlineGeoffm From United Kingdom, joined Feb 2004, 2111 posts, RR: 6
Reply 7, posted (9 years 7 months 2 days 14 hours ago) and read 5613 times:

Quoting Loggat (reply 4):
bulge at the bottom of the tires when sitting for extended periods of time


You sure? So the air gets pushed out the top of the tyres and pools in the bottom of the tyre, thus raising the plane off the ground a little?

Geoff M.


User currently offlineFoxecho From United States of America, joined Dec 2000, 747 posts, RR: 17
Reply 8, posted (9 years 7 months 2 days 14 hours ago) and read 5601 times:

Just a little aside-

The possibility of a 'bulge' in tires is why when an aircraft is pushed back from the gate it is pulled FORWARD a few feet before getting pushed, therefore if a tire is bulged from sitting out there all night or whatever, the bulge is rolled out.

Andrew
JFK/MEM/MCI



..uh, we'll need that to live......
User currently offlineAIR757200 From United States of America, joined Jul 2000, 1579 posts, RR: 7
Reply 9, posted (9 years 7 months 2 days 13 hours ago) and read 5569 times:

. I doubt the taxiways are that bad

But still, taxiways aren't perfectly smooth- you'll notice cracks (usually filled), uneven, etc. that even the slightest bump/alteration will be felt.


User currently offlineCopter808 From United States of America, joined Dec 2000, 1089 posts, RR: 1
Reply 10, posted (9 years 7 months 2 days 13 hours ago) and read 5509 times:

The possibility of a 'bulge' in tires is why when an aircraft is pushed back from the gate it is pulled FORWARD a few feet before getting pushed, therefore if a tire is bulged from sitting out there all night or whatever, the bulge is rolled out.

Never heard that before... The reason that the MD-80 and DC-9 series is pulled forward, then pushed back, has to do with the nose gear/push bar configuration. If the tow bar isn't properly attached or suffers a failure, it will only come "disconnected" when the airplane is being pulled "forward". If it fails when the AC is being bushed backward, it will ride up on the rotating tire, striking the strut assembly or the airframe, which could cause considerable damage.

I don't have time to look for it, but there is a NTSB report of an AA MD-80 that landed with the nose gear stuck "up" because of a tow bar failure while being pushed backward.


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 11, posted (9 years 7 months 2 days 13 hours ago) and read 5503 times:

The Shock struts at Work.
 Smile
regds
MEL



Think of the brighter side!
User currently offlineLoggat From United States of America, joined Feb 2000, 666 posts, RR: 0
Reply 12, posted (9 years 7 months 2 days 7 hours ago) and read 5247 times:

Geoffm: It's more to do with the tactile/flexibility characteristics of the rubber than the movement of air from the top to the bottom. I'm not sure of the physics of expandability of nitrogen with reference to pressure/temps, but if you take your car for an example... it will develop a "flat spot" on the bottom if left stationary for an extended period of time. As it is made of rubber, the flat spot will rid itself in pretty quick time once rotation has started.


There are 3 types of people in this world, those that can count, and those that can't.
User currently offlinePhilsquares From , joined Dec 1969, posts, RR:
Reply 13, posted (9 years 7 months 2 days 7 hours ago) and read 5223 times:

The "bobbing" effect you're referring to is very obvious in two conditions. The first is light gross weight. I remember flying the 727 and at very light gross weights, the pogo effect was very pronounced, but at higher gross weights there was almost no bobbing.

The second condition was if there was an aft CG. The moment arm from the center of gravity will make the aircraft more susceptible to the imperfections of the taxiways.


User currently offlinePetazulu From United States of America, joined Jan 2003, 701 posts, RR: 1
Reply 14, posted (9 years 7 months 2 days 7 hours ago) and read 5155 times:

Wind can make things bob too.

User currently offlineCitjet From United States of America, joined Nov 2004, 97 posts, RR: 0
Reply 15, posted (9 years 7 months 2 days 6 hours ago) and read 5107 times:

The biggest is the flat spot that develops while at rest at the gate. Also, most of the instances where the plane is pulled forward is because the chocks are wedged in the rear part of the tire and the tug driver nudges the A/C forward a little and the chock is then dragged away.

Also understand that aircraft don't have the same style of suspension that vehicles do. It is pretty basic. You have a stiff leg of the main carriage that is locked down, with a strut that is cushioned by nitrogen and travels vertically when the plane is moving over slight undulations in the taxiway surface.


User currently offlineCkfred From United States of America, joined Apr 2001, 5238 posts, RR: 1
Reply 16, posted (9 years 7 months 2 days 6 hours ago) and read 5093 times:

If you look down at the ramp or taxiway, you can see the expansion joints. While taxiing, you can see the expansion joints going under the wing, followed by the bob or bounce as the wheels go over the joint.

It's no different than hearing car tires going over joints while traveling at high rates of speed.


User currently offlineDl757md From United States of America, joined May 2004, 1562 posts, RR: 16
Reply 17, posted (9 years 7 months 2 days 6 hours ago) and read 5074 times:

Let me expand on my previous post. Aircraft landing gear as I have stated are designed primarily for landing. That is they dampen the shock when the wheels touch the ground. They must be able to protect the airframe from damage at descent rates of 500fpm and greater. To do this the gear has a very long travel. After the strut compresses to dampen the load it will rebound to a less compressed state which varies depending on the weight of the aircraft which Philsquares summed up perfectly.
Quoting Philsquares (reply 13):
The "bobbing" effect you're referring to is very obvious in two conditions. The first is light gross weight. I remember flying the 727 and at very light gross weights, the pogo effect was very pronounced, but at higher gross weights there was almost no bobbing.
The second condition was if there was an aft CG. The moment arm from the center of gravity will make the aircraft more susceptible to the imperfections of the taxiways.

A 727 with full wing tanks and an empty center tank will have very little weight on the nose(the wheels can come off the ground).
Anyway, this rebounding is dampened very little especially at full strut extension. In order to incorporate sufficient rebound dampening to eliminate this phenomenon would add weight, complexity, and additional mtc., penalties that A/C designers don't feel are justified given the relatively short amount of time that an A/C taxis.

Quoting Foxecho (reply 8):
The possibility of a 'bulge' in tires is why when an aircraft is pushed back from the gate it is pulled FORWARD a few feet before getting pushed, therefore if a tire is bulged from sitting out there all night or whatever, the bulge is rolled out


The main reason for this is that as the aircraft sits parked with parking brakes off(they do this so the hot brakes don't warp), it settles back on the chocks(all ramps are sloped away from the terminal) wedging them under the wheels and making them impossible to remove from the wheels. Pulling the plane forward frees the chocks so that they can be removed. The bulge that you are referring to is really not there on modern airliners. Even after a plane has been in the hangar for a month there isn't a bulge. Perhaps it was a problem with DC-3 era aircraft with more balloon type tires.

Dl757Md



757 Most beautiful airliner in the sky!
User currently offlinePVD757 From United States of America, joined Aug 2003, 3413 posts, RR: 16
Reply 18, posted (9 years 7 months 2 days 5 hours ago) and read 5010 times:

Don't be surprised if the taxiways are contributing to it as well. I have seen uneven areas develop at taxiway intersections where planes turn on the pavement and "shove" the surface over time (asphalt). There are also areas where planes are waiting, holding short, to taxi onto the runway where pock marks show up on the pavement. If my light Ford focus can deform my driveway, what do you think large aircraft can do to asphalt, especially in the summer months?

User currently offlineBoeingDrew From United States of America, joined Dec 2004, 55 posts, RR: 0
Reply 19, posted (9 years 7 months 2 days 5 hours ago) and read 4959 times:

Thanks everyone! All your idea's seem very valid in my opinion. One thing I don't understand, although it may sound ignorant, is the logic this whole "bulge" theory...How can simply moving the aircraft get rid of this "bulge"? What material forms this "bulge"? Can someone try and explain this to me?  Smile

Thanks,
Drew



You can't build a reputation upon what you are GOING to do, but rather upon what you HAVE done!
User currently offlineDl757Md From United States of America, joined May 2004, 1562 posts, RR: 16
Reply 20, posted (9 years 7 months 2 days 5 hours ago) and read 4940 times:

BoeingDrew

Go out and look at your car tires. The sidewalls bulge out a little at the bottom where the tire contacts the ground. Some people seem to think that this bulge remains at the same point on the tire when it is moved after sitting for a period of time. That is as the tire rotates the bulge goes around with it. Basically there is a flat spot in the tire or it is out of round. This is largely a problem of the past. Modern tires be it aviation or automotive have largely addressed this problem. It is a non-issue especially in respect to your original post as an out of round tire will cause a rapid thumping rather than the bobbing you described.

Dl757Md



757 Most beautiful airliner in the sky!
User currently offlineWjcandee From United States of America, joined Jun 2000, 5163 posts, RR: 22
Reply 21, posted (9 years 7 months 2 days 5 hours ago) and read 4909 times:

Doesn't flexion of the wings accentuate the effect somewhat as well? Depends on the a/c, of course. Compare, say, a DC9-10 to a 737-800W with lots of weight out there at the end of the longer wing. I would think that a bounce from an uneven taxiway would cause some oscillation at the wingtips which might accentuate the effect. Or am I wrong?

--Bill


User currently offlineHa763 From United States of America, joined Jan 2003, 3660 posts, RR: 5
Reply 22, posted (9 years 7 months 2 days 3 hours ago) and read 4792 times:
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Quoting Petazulu (reply 14):
Wind can make things bob too.


Very true. I was loading the outbound docs on an empty aircraft on a windy day. Since I had some time, I took a break, sitting down in one of the seats. I could feel the aircraft swaying side to side.


User currently offlineSkydrol From Canada, joined Oct 2003, 972 posts, RR: 10
Reply 23, posted (9 years 7 months 2 days 2 hours ago) and read 4439 times:

Next time, watch carefully if one (or more) F/As exit the cockpit just prior to takeoff. There may be more to the cause of the aircraft 'bobbing' than technical science can explain! Big grin This sensation is particularly obvious on a 757 due to the length of the aircraft and height of the landing gear.  Big grin


LD4



∙ ---{--« ∙ ----{--« ∙ --{-« ∙ ---{--« ∙ --{--« ∙ --{-« ∙ ----{--« ∙
User currently offlineAogdesk From United States of America, joined Jun 2004, 935 posts, RR: 3
Reply 24, posted (9 years 7 months 2 days 2 hours ago) and read 4438 times:

The first time I taxiied a B727 it didn't bob, it bounced!!! Empty cargo bird, light on fuel, I'm surprised ground control didn't ask if we were ok. Got to the runup pad, didn't pay all that much attention to the wind direction, and proceeded to take all 3 up to takeoff power. Interesting effect when air goes sideways across the inlet, you get one hell of a compressor stall......

25 September11 : Can engines contribute to aircraft "bobbing" during taxi? .. for example 757 sitting on ground on hold, i can feel 757 "bobbing" from the engines..
26 TUNisia : I'm so glad this question was asked! Being in my car always makes me feel as if I'm on a plane while on taxi. Thud Thud Thump! I love it! Also some pa
27 Post contains images Lightsaber : Next time, watch carefully if one (or more) F/As exit the cockpit just prior to takeoff. There may be more to the cause of the aircraft 'bobbing' tha
28 Geoffm : Exactly. It's a flat, not a bulge, thanks for confirming my sanity! Geoff M.
29 Post contains images Parisien : goodness, had no idea there are so many possible explanations for the bobbing. I used to watch planes arrive at gate and each time, i could see the pl
30 Post contains images MD-90 : But should it be damped or dampened, that's what I want to know.
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