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Western727
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DC-9: Slight "thud" When Slats Extend - Why?

Thu Aug 05, 2010 9:54 pm

I couldn't find the answer to this in a quick search of Tech/Ops so my advance apologies if this has been answered.

One thing I've appreciated about the DC-9 (series 20 through 50) is that the LE slats extend with a "thud" at the end. This "thud" is more noticeable during approach than prior to takeoff. I've always been a 5 year-old kid while flying on a "classic" DC-9 anticipating the "thud" while on approach.

Why did McD-D depart from this with the MD-80 and later series? One side note: the MD-80's LE slats have two extended positions, one for a lower flap setting (i.e., takeoff) and one for a greater flap setting (i.e. final approach/landing), like that of the DC-10. IIRC, the "classic" DC-9 LE slats have only one extended position.

Thanks in advance.
Jack @ AUS
 
AirframeAS
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Thu Aug 05, 2010 10:20 pm

Quoting Western727 (Thread starter):
One thing I've appreciated about the DC-9 (series 20 through 50) is that the LE slats extend with a "thud" at the end.

I don't know the answer to your question but I have to ask: Do you mean visually seeing the slats "thud" or hearing the slats "thud"?
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Western727
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Thu Aug 05, 2010 10:36 pm

Thinking about it, it's both...and feeling as well.

The MD-80/DC-10's LE slats, on the other hand, stop extending in the same way a competent car driver stops a car--with sustained, constant braking followed by a gentle taper-off at the very end to minimize the neck movements of said driver and pax.  

EDIT: grammar.

[Edited 2010-08-05 15:37:40]
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oly720man
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Thu Aug 05, 2010 10:59 pm

Quoting Western727 (Thread starter):

The reason seems to be related to the actuation

DC9 Flaps/Slats VS. 717 Flaps/Slats (by BR715-A1-30 May 4 2003 in Tech Ops)

reply 3.

Later versions probably use a different method of actuation.
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DashTrash
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 1:07 am

Quoting Western727 (Thread starter):
IIRC, the "classic" DC-9 LE slats have only one extended position.

I don't think that's correct. I want to say the slat settings are the same for all series DC-9/ MD80s. I'm sure a 9 driver will chime in on the subject.
 
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airportugal310
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 1:45 am

Quoting DashTrash (Reply 4):
I'm sure a 9 driver will chime in on the subject.

I "drive" a 9 iron, but I suppose that doesn't help out here  
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MSPNWA
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 2:41 am

I've always wondered exactly why the slats make that thud too. Good to read some answers to it. It's one of the little things that makes flying on a classic -9 unique.

Quoting oly720man (Reply 3):
reply 3.

That pretty much sums my theory. I always guessed that it was the slats hitting the stops. Amazing how easily you can hear and feel the action from the cabin.
 
B727LVR
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 3:33 am

Quoting DashTrash (Reply 4):
I don't think that's correct. I want to say the slat settings are the same for all series DC-9/ MD80s. I'm sure a 9 driver will chime in on the subject.

The DC-9 acft slats have two positions.... Retracted or Extended. They are not like the 737's. The 737's have Retracted, Extend (usually a take-off setting), and Full Extend (usually a landing setting). My names of the two extended positions might be wrong, but it does have 2 extended positions, the DC-9 only has one extended position.
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stratosphere
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 3:35 am

Quoting DashTrash (Reply 4):
don't think that's correct. I want to say the slat settings are the same for all series DC-9/ MD80s. I'm sure a 9 driver will chime in on the subject.

The MD-80 series do have have a mid and full extend on the slats. The other 9's have only extend. Here is the schedule on the MD-80's. Also the slats on the MD-80 will auto extend to the full position in the event of an approach to stall. The other DC-9 series do not have that capability.

SLATS RETRACTED / FLAPS 0
SLATS MID EXTEND/ FLAPS 0
SLATS MID EXTEND/ FLAPS 11
SLATS FULL EXTEND/ FLAPS 15
SLATS FULL EXTEND/ FLAPS 28
SLATS FULL EXTEND/ FLAPS 40

[Edited 2010-08-05 20:45:46]

[Edited 2010-08-05 20:47:01]
 
ThirtyEcho
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 4:14 am

The question seems limited to the DC-9. I have flown on every model of the DC-9, beginning with the dash 10, and there is a definite thud when the slats hit the stops prior to takeoff. You don't hear it on landing because there is an aerodynamic load on the slats while airborne that prevents the noise.
 
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Starlionblue
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 06, 2010 6:02 am

Could it just be that when they reach the end of the "run" there is a bump? Like when a marble hits the end of a marble run. They fall and then STOP. Bump.
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tinpusher007
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Sat Aug 07, 2010 8:08 pm

Quoting Starlionblue (Reply 10):
Could it just be that when they reach the end of the "run" there is a bump? Like when a marble hits the end of a marble run. They fall and then STOP. Bump.

I think you are correct. I ahve flown on many a DC-9 and I ahve noticed this as well. I think its just due to the old design of the 9 that its not as smooth as later models. It almost seems like they literally "fall" into place and make a thud when doing so!
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Western727
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Thu Aug 12, 2010 5:31 pm

Quoting Starlionblue (Reply 10):
Could it just be that when they reach the end of the "run" there is a bump? Like when a marble hits the end of a marble run. They fall and then STOP. Bump.

Right - what I meant is that I've always been intrigued that other airframes don't have this "bump".

Quoting ThirtyEcho (Reply 9):
You don't hear it on landing because there is an aerodynamic load on the slats while airborne that prevents the noise.

On the contrary...it's much more noticeable on descent than during takeoff preps. We all know the airflow changes once the slats start moving forward. This "changing airflow" sensation comes to an abrupt stop with that "bump" naturally followed immediately by the drag sounds/sensation caused by the extended slats. Very cool to watch, feel and listen to and it's something I eagerly anticipate on classic DC-9 descents.
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Tristarsteve
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Thu Aug 12, 2010 8:17 pm

Quoting oly720man (Reply 3):
The reason seems to be related to the actuation

Although I have never worked on MD aircraft, I have seen the slat actuation on a DC10, and assume a DC9 is similar.
There is a single slat actuator driving a big wheel which has cables attached to it that pull and push the slats in and out. Other aircraft have individual hydraulic actuators, or a screw jack system with a central actuator.
The cable system would mean that the slats can hit the stops much easier.

I have a vivid memory of this from a DC10 I once saw in BAH. The Engine (CF6) had failed spectacularly and bits of turbine blade had left the engine sideways, and one of them had sheared the slat cable. On the DC10 it must be 0.75 ins diameter! I could put my head up in the hole in the engine and see inside the combustor.
 
speedracer1407
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 13, 2010 3:38 am

Like pretty much everyone else here, I've been wondering the same for some time. It's been a few years, but the last (and only) few rides i had on a NW DC-9 were memorable in part because those slats not only made an audible thud, but also deployed so fast (much faster than anything else I've seen), it appeared as though they "fell" into position due to gravity. Of course, gravity probably had nothing to do with it, but still. Looked that way.

The other memorable "thud" was just before the reverse thrust powerback at MSP. I could feel each bucket reverser deploy. They weren't quite synchronized, so there was a slight, but definitely noticeable, jolt right to left as the right bucket deployed before the left.

BTW, a powerback is probably one of the rarest and coolest things you can experience in a plane, especially since I doubt they're done at all anymore.
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burnsie28
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Fri Aug 13, 2010 10:14 pm

Quoting speedracer1407 (Reply 14):
BTW, a powerback is probably one of the rarest and coolest things you can experience in a plane, especially since I doubt they're done at all anymore.

I sure do miss those self pushbacks.
 
XPjets
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RE: DC-9: Slight "thud" When Slats Extend - Why?

Mon Aug 16, 2010 1:21 pm

Quoting speedracer1407 (Reply 14):
BTW, a powerback is probably one of the rarest and coolest things you can experience in a plane, especially since I doubt they're done at all anymore.

NW stopped doing them in early 2007. I had the honor of being the marshall for one of the last ones. Nothing quite like experiencing one from the ramp!
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