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DC9 Flaps/Slats VS. 717 Flaps/Slats

Sun May 04, 2003 2:03 pm

I have noticed on the 717, that when the Pilots Lower the Flaps/Slats, they don't make that loud "KERPLUNK" Sound. But on the DC9, They make that sound and drop suddenly. Has anyone else noticed this, and why are they different, when the wing is basically the same?
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RE: DC9 Flaps/Slats VS. 717 Flaps/Slats

Sun May 04, 2003 2:50 pm

Hmmmm....let's think about this for a little bit. I am by no means an expert on the DC-9 vs. the 717 Flaps/Slats and I have never thought about it on my 20 years on earth but here it goes.

Maybe just maybe its because the DC-9 Technology is about 40 years old and the average age of the DC-9 is older than I am? I would think that a DC-9 has a lot more wear and tear than a 717 would thus making a few creaky old parts in the Hydrallic lines. Speaking of Hydrallics I bet the 717 has a MUCH more efficent hydrallic system, thus the noise differences.
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RE: DC9 Flaps/Slats VS. 717 Flaps/Slats

Mon May 05, 2003 6:32 pm


not beeing familiar with Boeing stuff too much, I'm just making a guess.

Here are the little bits of what I have memorized on Boeing / MD. Please, do not quote me on that, and if there are any eyperts out there who know better, I'd be glad to be educated.

OK, here's my guess:

DC-9 use afaik a drooped hinge-type High lift surface support and guidance mechanism. Meaning the flap surfaces are attached to a pivot arm. And that's why they seem to drop immediately.

Boeing 707-220 uses afaik arc tracks to deploy the flaps, whereas I do not know exactly how these are driven (Pinion/Rotary Actuator/Linear Actuator/... Anyone?) These will -most likely- move the flaps differently than a drooped hinge system does, the circular path they move along is not dependant to the length of a pivot arm (=Circle radius). Such an arc track could almost have "any" shape or radius. Hence, the movement of the flaps would look different.´Sound may be different depending on what kind of power drive is used.

Boeing 727 for instance uses a track-carriage type surface guidance and support mechanism. As well, Flap surfaces do not move on a circular path. The move somewhat more linear. Again, sound depends on type of power drive.

What the 717 does, I have no clue.

Anyhow, I'd say it is more a matter of the mechanical part of the flap system architecture and design that decides about the way the flap surface movement looks to a spectator, and as well it depends on the design and architecture of the power drive system what it sounds like. I would not say it is necessarily only a matter of aircraft age (=technology level). A hydraulic piston-actuated system will always sound different from a track-carriage system using hydraulic motors.......


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RE: DC9 Flaps/Slats VS. 717 Flaps/Slats

Wed May 07, 2003 12:06 pm

The thud that you hear is coming from the slats not the flaps. The thud is actually the slat tracks bottoming out on the mechanical stops. All of the slats are connected together through cables and bolts. Thus the left side and the right side reach there collective end at the same time.......result...a loud thud. As to the difference in sound from B717 to DC-9.......I am not sure. The system is essentially the same, unless Boeing placed some sort of dampener on the control drum. It has been awhile since I had my hands on a B717 or DC-9 for that matter, so my memory is a little rusty.

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RE: DC9 Flaps/Slats VS. 717 Flaps/Slats

Thu May 08, 2003 1:02 am

Like I said, just making a guess.  Wink/being sarcastic Never heard it myself, and never had the opprtunity to be aboard a DC-9.

Interesting. I have barely no knowledge or information on 717 high lift (esp. Slat) actuation. Let me ask you some questions, if you do not mind.

-All of the slats are connected together through cables and bolts.
Are you talking about some sort of slat panel interconnection? Or are you referring to the power drive or some special means to accomplish synchronisation of the left and right slat panels? Or is the power drive accomplished shafts?

-For slat actuation, what kind of actuation mechanism is used? Rack-pinion sort of beast or more something like a linkage-system? Linear actuators or Rotaries? Pistons?

Would be glad if you could take the time to explain.



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