Does anyone know what this sound is at 12:56 on a B737-800 takeoff? The aircraft is N822SY, the airport is Denver Intl. Airport, and the runway is 34L. Thanks and have a great day!
$\begingroup$
$\endgroup$
2
-
3$\begingroup$ A time stamp and description would be helpful. $\endgroup$– Michael HallCommented Dec 29, 2021 at 16:26
-
1$\begingroup$ timestamp: 12:56 $\endgroup$– avkid34Commented Dec 29, 2021 at 17:39
Add a comment
|
1 Answer
$\begingroup$
$\endgroup$
4
It's the main gear wheels vibrating as they spin down. As the RPM of the wheel assembly drops, the wheel will pass through resonant frequencies and if there are small amounts of imbalance, the wheel will vibrate while passing through a specific speed range that is resonant with gear leg and structure, transmitting the vibrations into the fuselage and rattling interior panels.
At some point the brake system is applied automatically to stop the main wheels from spinning, but prior to that you will get those little vibrations. The nose gear also can vibrate while spinning down, and will do so until a friction brake in the gear well rubs the tires to a stop following retraction.
Airline tires are only statically balanced, a bit like motorcycle tires are, and it's not unusual for them to vibrate when suspended clear of the ground spinning at high speed, whereas when on the ground with weight on the gear, the imbalance is not noticeable because it's heavily damped by the gear and structure pushing down hard on the wheels.
-
$\begingroup$ Given that some bridges were subject to catastrophic resonance failure and modern bridge designs have shifted resonance frequencies, so it can not be excited by wind, is there a reason why it's allowed in air craft wheels? Seems like it could cause a lot of stress. Although I don't know how it compares to stress when touching down. $\endgroup$ Commented Dec 30, 2021 at 12:18
-
2$\begingroup$ @infinitezero: someone might come with a more technical answer, but from a practical point of view, millions and millions of flights without incident seem like a good proof that this is not an issue. $\endgroup$ Commented Dec 30, 2021 at 12:43
-
$\begingroup$ The level of balancing required to prevent any spindown shaking on 5 or 6 foot tire assemblies would require dynamic balancing machines that few airlines would be willing to pay for and I've never seen an airliner-size dynamic balancer machine like is used for cars. Everybody uses static balancers that hold the assembly with its axle horizontal, like motorcycle tire balancers. Spindown shaking happens on just about all airplanes when you take off, some more noticeable than others. It can be especially bad on Cessnas with the spring steel gear, since the gear acts like a harmonic amplifier. $\endgroup$– John KCommented Dec 30, 2021 at 14:03
-
5$\begingroup$ @infinitezero it's pretty hopeless to eliminate all resonances, short of filling all cavities with a heavy viscous liquid or so. The important thing to get rid of are those resonances that can actually excited without bounds. The resonances in question here are apparently not excited by wind, but only by the wheels – and those can only provide so much energy, because the more resonance there is the quicker the wheel also slows down – so you can always only get a small fraction of the wheel's rotational-kinetic energy into the resonance mode. Much less stress than during a landing. $\endgroup$ Commented Dec 30, 2021 at 16:30