SQ777
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Joined: Tue May 18, 1999 2:16 pm

Why Does 747-400 Vibrates When It Takeoff

Mon Jul 05, 1999 2:31 pm

Hi,

I observe that 747-400 vibrates quite badly the moment its wheels leave the runway during takeoff. The vibration usually lasts three to five seconds. Other airplane such as 767 and 777 does not vibrate so badly. Does anybody have any explanation for this?

Regards
SQ777, Singapore
 
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VIflyer
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RE: Why Does 747-400 Vibrates When It Takeoff

Mon Jul 05, 1999 2:58 pm

It most likely sounds like it's vibration due to the spining of the wheels on the main landing gear. Most likely due to the fact that the wheels have to stop before they can be stowed. On a 767 only has 8 wheels (four to a truck) on it main landing gear and the 777 has 12 (six to a truck) vs. the 747 which has 16 (4 to a truck, 4 trucks in total). So I would expect a more noticable vibration on lift off. I'm not saying I'm right but that makes the most sense to me.
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RE: Why Does 747-400 Vibrates When It Takeoff

Mon Jul 05, 1999 6:34 pm

it's the wheels on the main gear still spinning..all 16! Before they can be stowed they must stop spinning otherwise a gyroscoping effect occures. Simply put its just the series of movements while the gear is in transit
 
cedarjet
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RE: Why Does 747-400 Vibrates When It Takeoff

Mon Jul 05, 1999 9:24 pm

All true, but I would add that the vibration from individual wheels that are out of balance (even fractionally) can often be felt much more acutely after lift-off, cos as they track along the runway, that keeps the vibration in some kind of check, but once they start spinning freely the vibration can be a lot more noticeable. Part of the retraction sequence is to brake the wheels to a halt, and often you can feel the frequency of the vibration change (reduce) as the brakes take hold. It's not dangerous, but it can give you a bit of a fright. The first time I noticed that vibration after lift-off was as a teenager, I'd just read a book about a Lufthansa 747 that crashed in 1975 after takeoff at Nairobi cos the slats weren't set (shades of the NW MD80 accident at Detroit). The plane stalled and all the passenger accounts in this book described the vibration and pre-stall buffet. I thought the same thing was happening. Nothing is worse than knowing a little bit, cos you think you know A LOT and scare yourself (and your bad-flyer Mum travelling with you). Such naivete!
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