bio15
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Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 12:16 am

Hi everyone, I'm back from a 3 month trip. I got the chance to fly several times, and noticed something that I never payed close attention to. When the aircraft used the speed brakes on descent they generated a rumble. I could tell they had just been deployed just by the shaky rumble they generated on the entire aircraft. It's similar to that sound you hear when the gear comes down. My question is, why does this happen?, Has anyone else noticed this? Thanks in advance.


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-Alfredo
 
mjzair
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 12:25 am

What you are hearing is the sound of the disturbed airflow around the wing...
 
modesto2
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 1:41 am

I've noticed this many times...as you said...when speedbrakes and gear are extended. As Mjzair said, extending speedbrakes introduces disturbances to the airflow. Similarly, I'm sure you've noticed the disappearance of the sound as the speedbrakes are stowed.
 
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Aaron747
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 1:42 am

Welcome to the exciting world of drag.  Big grin
If you need someone to blame / throw a rock in the air / you'll hit someone guilty
 
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c172akula
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 2:04 am

"Welcome to the exciting world of drag."

Indeed! I'll take one serving of Induced and a side dish of Parasitic!  Smile

STOP THE RUMBLE! (stows speedbrakes)
 
aa61hvy
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 2:23 am

It's kind of like a plane "down shifting"  Wink/being sarcastic  Laugh out loud
Go big or go home
 
bio15
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 4:12 am

Thanks guys. I had the idea it was something like that. They must slow the aircraft down real quick since they interrupt a lot of airflow. They must kill an important amount of lift also. When the pilots deploy them, is there a lot of sinking of the aircraft?

-Alfredo
 
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Aaron747
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 5:32 am

Deploying speedbrakes often allows an aicraft to descend at a higher rate without inducing increased airspeed. You'll often notice the nose angle hardly changes at all - that's because trim is taking care of it all Big grin
If you need someone to blame / throw a rock in the air / you'll hit someone guilty
 
rendezvous
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 4:57 pm

The speed brake deployment in my photo up the top was pretty loud, but shortly after that they took them up to what I'm assuming is flight detent. Now let me tell you, that was a sound for someone who's like bass!

Something I saw a few months back I never thought I'd see - Virgin Blue 737 with speed brakes deployed along with flap and a 30* bank turn.
 
Mr.BA
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 8:28 pm

"Something I saw a few months back I never thought I'd see - Virgin Blue 737 with speed brakes deployed along with flap and a 30* bank turn."

I think with Boeing spoilers aren't supposed to be deployed with flaps extended no matter at what setting. I think what you say is the spoilerons (which are also the spoilers) coming up as a certain amount of aileron is imput to turn the aircraft. I think you were sitting at the inside of the turn?

Cheers!
Boeing747 万岁!
 
Rick767
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RE: Rumble When Speed Brakes Are Deployed

Thu Aug 14, 2003 8:51 pm

The speedbrakes / spoilers issue crops up on here every so often and the search function might help you learn a bit more about their use.

They serve two basic functions in flight, either:

1) To increase the rate of descent whilst maintaining the same IAS
2) To reduce the IAS while maintaining a fixed rate of descent

We can extend the speedbrakes with flaps extended on the 757/767, up to Flap 20. The drag associated with Flap 25/30 settings combined with speedbrake deployment would be very high and the descent rate would therefore be excessive, and the vibration is an issue.

Below 800ft Boeing suggest no spoiler extension whatsoever (with associated warning light) but most airlines round this up to 1,000ft AGL (which is also the common "must be fully configured for landing" point).

On many aircraft there is a "flight detent" on the speedbrakes (a position beyond which the speedbrakes can only extend to when the aircraft is on the ground). On the 757 there are six spoiler panels on each wing, and panels 4 and 9 (inboard spoilers on the outer set) only extend on the ground. On the 767 all panels are used both on the ground and in flight. The speedbrakes extend to a greater maximum angle on the ground than in flight.

Similarly some of the spoilers operate with the ailerons to assist in roll control.

On the ground the speedbrakes are used to "dump" the lift generated by the wings and thereby placing almost all the weight of the aircraft onto the main gear, which reduces landing runway required by increasing friction and allowing the wheels to grip the runway surface better and therefore allowing a higher braking level to be used.

It is a common misconception that the spoilers actually produce aerodynamic drag to slow down the aircraft on the ground. Whilst they clearly will do this, the effect is very minimal in comparison to the primary function described above.

Similarly in flight the actual parasite drag they induce is smaller in comparison to the lift destruction, which is the primary purpose.

Hope this rather long explanation helped you understand a bit more about these things!
I used to love the smell of Jet-A in the morning...
 
bio15
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RE: Rumble When Speed Brakes Are Deployed

Fri Aug 15, 2003 1:27 am

Yes Rick, thanks for that one. I had the misconception of the spoilers's purpose on touchdown. I thought the slowing down was accounted on the most part for the drag generated by the spoilers, and also, the lift dump required you talked about, but in equal amounts. It's easy to have that misconception since the spoilers are huge when deployed and block a large amout of air, or at least that's what I see through the window. Thanks again

-Alfredo
 
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Aaron747
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RE: Rumble When Speed Brakes Are Deployed

Fri Aug 15, 2003 2:01 am

If you look out the window and see deployment in a turn - don't be surprised -that's just roll assist at work.

One thing I love about coming home to Oakland is that prevailing weather usually has us on the visual to runway 29 - it's virtually a guarantee the crew will be pushing 250-240 KIAS and use the speedbrakes to slow down before the last turn, like in this pic I took:


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ANA777Master
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RE: Rumble When Speed Brakes Are Deployed

Fri Aug 15, 2003 8:46 am

Its the wind against the metal of the speedbrakes or flaps. The metal itself reverberates and creates the sound.
 
musang
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RE: Rumble When Speed Brakes Are Deployed

Wed Aug 20, 2003 12:16 am

Are you sure?

I would suggest the vast majority of the disturbance is from the turbulent air aft of the spoilers, impinging on the spoiler itself and the wing behind. Certainly the spoilers themselves are vibrating, but only transmit part of that to the airframe

Regards - Musang

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