Aircraft "Bobbing" During Taxi

BoeingDrew · Mon Feb 21, 2005 6:10 pm

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

lehpron · Mon Feb 21, 2005 6:17 pm

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

dl757md · Mon Feb 21, 2005 6:20 pm

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

oly720man · Mon Feb 21, 2005 11:30 pm

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.

loggat · Tue Feb 22, 2005 1:23 am

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.

September11 · Tue Feb 22, 2005 1:50 am

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

futureatp · Tue Feb 22, 2005 2:00 am

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

geoffm · Tue Feb 22, 2005 2:33 am

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.

foxecho · Tue Feb 22, 2005 2:36 am

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

AIR757200 · Tue Feb 22, 2005 2:47 am

. 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.

copter808 · Tue Feb 22, 2005 3:07 am

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.

HAWK21M · Tue Feb 22, 2005 3:08 am

The Shock struts at Work.
Smile

regds
MEL

loggat · Tue Feb 22, 2005 9:04 am

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.

PhilSquares · Tue Feb 22, 2005 9:13 am

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.

petazulu · Tue Feb 22, 2005 9:31 am

Wind can make things bob too.

citjet · Tue Feb 22, 2005 9:56 am

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.

ckfred · Tue Feb 22, 2005 10:00 am

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.

dl757md · Tue Feb 22, 2005 10:10 am

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

PVD757 · Tue Feb 22, 2005 10:46 am

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?

BoeingDrew · Tue Feb 22, 2005 11:02 am

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

dl757md · Tue Feb 22, 2005 11:12 am

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

wjcandee · Tue Feb 22, 2005 11:29 am

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

ha763 · Tue Feb 22, 2005 1:02 pm

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.

Skydrol · Tue Feb 22, 2005 1:56 pm

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

aogdesk · Tue Feb 22, 2005 1:57 pm

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......

September11 · Tue Feb 22, 2005 2:12 pm

Can engines contribute to aircraft "bobbing" during taxi? .. for example 757 sitting on ground on hold, i can feel 757 "bobbing" from the engines..

TUNisia · Tue Feb 22, 2005 2:33 pm

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 parking garages (if there's a lot of people in line to leave) will give the sensation of turbulence if you're on a ramp.

Odd I know!

TUNisia

lightsaber · Tue Feb 22, 2005 5:09 pm

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. I almost fell out of my chair laughing Skydrol!

Actually, the bobbing is due to the airplane greeting all of its friends as it heads out.

Ok, long workday (almost over).

Seriously, I recall at Pratt we were having to design engine mounts to take the stress of potholes and uneven taxiways; this was learned the hard way to be the greatest vertical force upon a wing mounted engine. SFO for a long time set the design criteria.

Lightsaber

geoffm · Tue Feb 22, 2005 5:43 pm

Quoting Loggat (reply 12):
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.

Exactly. It's a flat, not a bulge, thanks for confirming my sanity!

Geoff M.

parisien · Tue Feb 22, 2005 6:40 pm

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 place bobbing up and down, more like nodding as if greeting !!!! i just thought that that was caused by the suspension of the nose undercarriage ?
never thought bulges etc; could create bobbing Wink/being sarcastic

MD-90 · Tue Feb 22, 2005 6:51 pm

But should it be damped or dampened, that's what I want to know. Big grin

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