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speedracer1407
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DC-9 Control Tabs

Thu Jun 15, 2006 4:50 pm

I had the, er, privelage/punishment of riding in the way-back of a NW DC-9 recently. My GOD what an astonishing noise. Anyway, I knew that DC-9s and MD-80s had control tab elevators, but I was surprised to see tabs on the ailerons as well. Learning this and watching them in action brought to mind a slew of questions:

Do all DC-9 family planes have 'tab ailerons? Even the MD-90?

How in the world do those tabs actaully do anything? Seems to me like a little tab on the end of a big aileron (or elevator) would simply not have the aerodynamic authority to overcome the enormous drag that happens when that big aileron is pushed against high speed air flowing over the wing. obviously, it works. But I'm curious as to how. Then again, I still can't figure out how a britta water filter jug fills past the bottom of the filter aperatus.

Are the tabs hydrolically powered? Does the tab's presumably reduced hydrolic assistance (if any) result in an easier-to-control plane in the event of hydrolic failure?

Roll control didn't seem to be augmented by any spoiler deployment, despite a rather gusty landing in the -9. Is this true of the whole family?

Pilots: Do control tab....control surfaces have a different feel than fully hydrolic ones? Is there more control wheel action needed to effect a similar control surface movement? Is there a delay before aerodynamic forces react to the tab, then the aileron, then the plane's pitch/roll?

Should GM just give up and switch to producing rental cars only?

O
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
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Starlionblue
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RE: DC-9 Control Tabs

Thu Jun 15, 2006 8:24 pm

First of all, "hydraulic" is the kosher spelling.

Quoting Speedracer1407 (Thread starter):
Do all DC-9 family planes have 'tab ailerons? Even the MD-90?

AFAIK yes.

Quoting Speedracer1407 (Thread starter):
How in the world do those tabs actaully do anything? Seems to me like a little tab on the end of a big aileron (or elevator) would simply not have the aerodynamic authority to overcome the enormous drag that happens when that big aileron is pushed against high speed air flowing over the wing. obviously, it works. But I'm curious as to how. Then again, I still can't figure out how a britta water filter jug fills past the bottom of the filter aperatus.

But they do work. The tab pushes the aileron. The aileron pushes the wing.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
avioniker
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RE: DC-9 Control Tabs

Thu Jun 15, 2006 11:08 pm

One of the foremost reasons for the "smooth" ride in the DC-9 family (including the MD's) is the flight controls are flown rather than driven by hydraulics.
The pilot effectively can't overdrive the control surface.
If you'll notice, on takeoff the nose comes up smoothly, when it's ready, and not abruptly like in Boeing and Airbus. That's because the elevators don't have the needed authority until there's sufficient airspeed.
The only hydraulic power to the primary flight controls is to the elevators in the event of an imminent stall. They are driven down to get the nose down when there's insufficient airflow to move the elevator with the tab.
It's one more differance in the design philosophy between the major manufacturers.
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
SlamClick
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RE: DC-9 Control Tabs

Thu Jun 15, 2006 11:22 pm

It could also be mentioned that the 'baby nine' the DC-9-10 series flies like a fighter. It is one sweet handling airplane.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
speedracer1407
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RE: DC-9 Control Tabs

Fri Jun 16, 2006 5:43 pm

Quoting Starlionblue (Reply 1):
First of all, "hydraulic" is the kosher spelling.

Damn spell checker. I swear, i fixed that in an attempt to edit my horrible spelling habits. Must have clicked something wrong.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
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HAWK21M
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RE: DC-9 Control Tabs

Fri Jun 16, 2006 9:49 pm

Quoting Speedracer1407 (Thread starter):
How in the world do those tabs actaully do anything? Seems to me like a little tab on the end of a big aileron (or elevator) would simply not have the aerodynamic authority to overcome the enormous drag that happens when that big aileron is pushed against high speed air flowing over the wing.

On the B737 The Tabs normally attach to the Aileron via Balance Panels that use Aerodynamic loads to assist Surface Mvmt.

Quoting Speedracer1407 (Thread starter):
Are the tabs hydrolically powered

Tabs are normally not Hydraulic operated.
Not sure about the DC9.Presume would be similiar.

regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
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Starlionblue
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RE: DC-9 Control Tabs

Fri Jun 16, 2006 10:02 pm

Quoting Speedracer1407 (Reply 4):
Quoting Starlionblue (Reply 1):
First of all, "hydraulic" is the kosher spelling.

Damn spell checker. I swear, i fixed that in an attempt to edit my horrible spelling habits. Must have clicked something wrong.

No worries. Common mistake. And better than the old A.nut classic "breaks" instead of "brakes".

English is the trickiest language to spell of those I speak. Take "laugh". How the heck does the spelling relate to the pronunciation? And what about "its" (posessive). Why is there an exception that removes the apostrophe in that case?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
HotelEchoFox
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RE: DC-9 Control Tabs

Fri Jun 16, 2006 10:28 pm

It's not an exception, Starlionblue. Think of 'his' and 'hers'. No apostrophe. The word "it's" is reserved for the contraction: "it + is". Right? Right!

BTW, what other languages do you speak?

HEF
 
HotelEchoFox
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RE: DC-9 Control Tabs

Fri Jun 16, 2006 10:32 pm

Sorry! I'm a confessed language nut... Couldn't help myself. Thanks for indulging my OCD!  Smile

HEF
 
ftrguy
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RE: DC-9 Control Tabs

Fri Jun 16, 2006 10:37 pm

The 707 has a very similar system. The only hydraulics for the control surfaces on the 707 is a rudder assist. I hear its like flying a Mack Truck.
 
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Starlionblue
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RE: DC-9 Control Tabs

Sat Jun 17, 2006 12:06 am

Quoting HotelEchoFox (Reply 7):
It's not an exception, Starlionblue. Think of 'his' and 'hers'. No apostrophe. The word "it's" is reserved for the contraction: "it + is". Right? Right!

BTW, what other languages do you speak?

Aaaah, I see. But that's still different from "John's car" or "the car's engine" for possessive.

To tell you the truth, I have no real difficulty with English spelling apart from the occasional screw-up. I think 30 years of experience has just made it instinctual.

I speak Swedish, Italian and French. I also dabble in Spanish and German (not fluent, but understand slow speakers). My wife and I are trying to pass on our skills to our daughter. I speak Italian to her, my wife speaks Swedish, the nanny speaks Spanish and we live in the US.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
SlamClick
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RE: DC-9 Control Tabs

Sat Jun 17, 2006 12:36 am

Quoting Starlionblue (Reply 10):
Aaaah, I see. But that's still different from "John's car" or "the car's engine" for possessive.

The apostrophe still stands in place of missing letters like it does for 'it is.'

'John's car' probably came from some archaic, awkward construction like 'John, his car' or something. Anyway one never, ever, ever uses an apostrophe to form any plural. It always stands for missing letters or for a space closed up.

Back to the DC-9 its control tab[no ']s.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
miamiair
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RE: DC-9 Control Tabs

Sat Jun 17, 2006 12:45 am

Toss in the fact that the MD-80's have an extra tab on the elevator. It is an anti-float tab.
Molon Labe - Proud member of SMASH
 
JBirchall
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RE: DC-9 Control Tabs

Sat Jun 17, 2006 3:17 am

Re. crazy English spellings. Try this one out:

'A rough-coated, dough-faced thoughtful ploughman strode through the streets of Scarborough and after falling into his slough he coughed and hiccoughed.'

It contains all 9 different pronunciations of the '..ough' vowel sound. Nothing to do with DC-9s or MD-80s I agree, but illuminating nonetheless! Any Americans care to give it a try and report back?
 
speedracer1407
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RE: DC-9 Control Tabs

Sat Jun 17, 2006 7:10 pm

Quoting HAWK21M (Reply 5):
On the B737 The Tabs normally attach to the Aileron via Balance Panels that use Aerodynamic loads to assist Surface Mvmt.

737s have control tabs too? Ok, mabye I should do some research, or at the very least, start sitting in the back of these planes. I feel like I'd have noticed control tabs on the ailerons of, say a 737-300, which I've flown on recently. But i learn new stuff around here all the time. But i SO like getting off the plane first.

O
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.
 
NKP S2
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RE: DC-9 Control Tabs

Sat Jun 17, 2006 10:39 pm

Quoting Speedracer1407 (Reply 14):
737s have control tabs too? Ok, mabye I should do some research, or at the very least, start sitting in the back of these planes. I feel like I'd have noticed control tabs on the ailerons of, say a 737-300, which I've flown on recently. But i learn new stuff around here all the time. But i SO like getting off the plane first

Those aren't control tabs on the 737, they're balance tabs. They do not "fly" the surface, but assist in the movement of it. They are linked by rods and move in the opposite direction of the surface...in proportion to the movement of the surface.

Oh, and yes, the DC9/MD80 does utilize the spoilers to assist the ailerons on the roll axis.
 
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HAWK21M
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RE: DC-9 Control Tabs

Sun Jun 18, 2006 12:04 am

Quoting NKP S2 (Reply 15):
Those aren't control tabs on the 737, they're balance tabs. They do not "fly" the surface, but assist in the movement of it. They are linked by rods and move in the opposite direction of the surface...in proportion to the movement of the surface.

 bigthumbsup 
true.
The Balance Tab Attaches to the Aft Spar of the Aileron by a Balance Panel.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
miamiair
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RE: DC-9 Control Tabs

Mon Jun 19, 2006 8:57 am

Quoting HAWK21M (Reply 16):
The Balance Tab Attaches to the Aft Spar of the Aileron by a Balance Panel.

Makes no sense. The tab is actuated by a rod. And depending on the weight of the tab, you have to add balance weights to the nose of the aileron.
Molon Labe - Proud member of SMASH
 
n8076u
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RE: DC-9 Control Tabs

Mon Jun 19, 2006 11:37 am

I have no personal experience with the dc-9, but perhaps I can clear up some confusion on the 737.

On a 737, each aileron has a balance tab. The tab always moves in the opposite direction of the aileron itself. The forward edge of the ailerons are connected to balance panels, which counterweight them. The aileron trim adjustments occur at the aileron centering and feel mechanism that is in the wheelwell (by altering where the mechanical center is), and not at the tabs. One reason for those tabs on the 737 is so that if hydraulics are lost, and the pilot must manually control the ailerons (manual reversion), those tabs will assist in moving the main surface, and make that task far less difficult.

Chris
Don't blame me, I don't work here...
 
SlamClick
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 12:12 am

Quoting N8076U (Reply 18):
if hydraulics are lost, and the pilot must manually control the ailerons (manual reversion), those tabs will assist in moving the main surface, and make that task far less difficult.

Okay correct me if I'm wrong here, but it is my understanding that this system will make it possible and not merely less difficult.

It is my understanding of the rigging of the system that the cables from the yoke, which position the selector of the control valve on the aileron hydraulic actuator go past that connection and to the tab itself. Under normal operation, that is, with hydaulic pressure you don't even know or care that the cables are also pulling the tabs in the proper direction because that action is made irrelevant by the hydraulic actuator physically moving the aileron.

So if hydaulic pressure is lost, you continue to operate the yoke as usual (except stiffer) the cables still position the selector valve at the actuators but there is no response because there is no pressure. But the cables are still routed beyond the hydraulic unit to the tab and now your pulling the cable by turning the yoke pulls the tab up or down as needed. The tab goes one way, flying the aileron the other way which makes the wing fly up or down as desired.

The difference between this and another inference I could take from your post is that it might be possible to believe that the control cables physically move the aileron itself. They don't, they move the tabs which fly the aileron.

Again, this is my understanding of how the manual reversion works. I'd certainly welcome correction if I have it wrong.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
NKP S2
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 2:06 am

8076U is technically correct.

Manual reversion ( on the 737 ) is actually strong-arming the aileron surface itself, with the balance tab assisting in an aerodynamic fashion. In manual reversion, the actuators ( actually actuators and a "walking beam" type assembly ) will simply be just another "rod" in an input from the yoke to the "Christmas tree" ( Don't ask why it was coined, it doesn't look much like one, it's just an assembly with the most vague pyramidic sillouette ) which is the shaft-pully/centering cam assy that connect to the ailerons via cables. I say this in a broad sense due to the fact that though the pilot/copilot yokes are connected via bus cables, the copilot input shaft actually connects to the Christmas tree via a rod, the tree being attached to the actuators; The pilot side shaft attaches to the actuators via rods right to their input pilot valves.

In normal ( hydraulically powered ) ops the actuators appear to float as input from either side will impart either input directly to the pilot valve, or the actuators themselves ( actually both ) but it matters not since either input will cause pilot valve movement relative to the actuator itself. The actuators/beams will move till the pilot valve nulls the input by centering the pilot valve.

All that is happening on manual ops is that the dead actuators turn into links and are along for the ride. There will be some lost motion due to pilot valve displacement on the actuator(s) though. It takes a fair bit of force to move them manually on the ground, so I would imagine the use of the balance tab is crucial to prevent the ailerons feeling like thy are incased in concrete inflight.

I'll admit my explanation may seem confusing, but there's a lot happening down there. and many links in the system. If you see it happen ( in both normal and manual ), it all makes sense, but even then, the brain/eye have to do a fair bit of reverse engineering to make it clear(er).

[Edited 2006-06-19 19:09:08]

[Edited 2006-06-19 19:12:53]
 
n8076u
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 3:20 am

"Slamclick", I am not sure if you are referring to the 737, of which I was referring, or to the DC-9, but you may be right regarding the DC-9, as I don't know.

I didn't design the 737, so I can only assume that without the tabs, it would just be a lot more difficult to move the ailerons, but it may well be impossible.

"NKP S2"'s explanation sounds pretty good to me, but I will elaborate on a couple points using layman's terms. Keep in mind, this is specific to the 737 ONLY, as not all aircraft are designed the same way, although the 727 was very similar, from what I remember. Douglas did things differently, so this will not apply to them. Since I have never worked on a DC-9, I will leave any explanation of that aircraft's systems to those more experienced than myself.

The 737's aileron's tabs are not connected to any cables at all. Just to the aileron itself, and a control rod, which is connected to a rigid point on the wing on one end, and to the tab on the other. The tab is "along for the ride" and has no outside influence from anything other than the wing and the aileron. All it does is pivot in the opposite direction of the aileron.

The cables from the captain and first officer yokes go to the wheelwell, where they connect to the monkeymotion that "NKP S2" spoke of (consisting of an input shaft, centering and feel mechanism, output shaft, as well as the hydraulic actuators, autopilot actuator, various links, trim motor, lots of stuff there). Different cables go from the output shaft quadrant out to the ailerons themselves, and the "input" and "output" are connected, albeit in a roundabout way, through that complicated mechanism that is in the wheelwell. So in effect, if you turn the control wheel, the ailerons themselves do move, with or without hydraulics. The tabs then move, because the ailerons are moving.

As a backup in case the aileron system jams, the first officer's control cables will still actuate the flight spoilers to allow for roll control of the aircraft.

Chris
Don't blame me, I don't work here...
 
SlamClick
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 3:55 am

Quoting N8076U (Reply 21):
"Slamclick", I am not sure if you are referring to the 737, of which I was referring, or to the DC-9, but you may be right regarding the DC-9, as I don't know.

I didn't design the 737, so I can only assume that without the tabs, it would just be a lot more difficult to move the ailerons, but it may well be impossible.

Sorry. Yes I was referring to the 737 and not the Doug. And I was also referring to cables coming [indirectly] from the yoke. I'm aware of the aileron/spoiler mixers etc. in the wheelwell. I was leaving them out of it as not being part of my discussion.

So my questions would turn on whether these cables coming out into the wing actually move the aileron itself when there is no hydraulic pressure or whether they move tabs which in turn move the ailerons.

Your description suggests the latter.
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NKP S2
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 6:06 am

So my questions would turn on whether these cables coming out into the wing actually move the aileron itself when there is no hydraulic pressure or whether they move tabs which in turn move the ailerons

It's the former.

The cables going from the feel/centering unit/pully assy that go out to the ailerons move the ailerons in both hydraulic and manual reversion mode. ( 737 )
 
n8076u
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 6:59 am

The cables that go out to the ailerons directly actuate the ailerons, through a bellcrank assembly. The tabs are merely along for the ride, and only react to the aileron movement. If you cut the cables, and moved the ailerons by hand, the tabs would still move normally.

If a 737 is sitting on the tarmac, with no electrical or hydraulic power whatsoever, if you moved the control wheel, you would see the ailerons themselves move.
Don't blame me, I don't work here...
 
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HAWK21M
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RE: DC-9 Control Tabs

Tue Jun 20, 2006 5:51 pm

Quoting Miamiair (Reply 17):
Quoting HAWK21M (Reply 16):
The Balance Tab Attaches to the Aft Spar of the Aileron by a Balance Panel.

Makes no sense. The tab is actuated by a rod. And depending on the weight of the tab, you have to add balance weights to the nose of the aileron.

My Mistake.Guess wrote in a hurry.
I was referring to the Balance panel attacted to the Aft wing Spar & the front of the Aileron.
regds
MEL
I may not win often, but I damn well never lose!!! ;)
 
Corsair2
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RE: DC-9 Control Tabs

Thu Jun 22, 2006 2:48 am

The DC-9 aileron control system does not use any hydraulics, the system is controlled only using the control tab. The control wheel in the cockpit will move the control tabs to effectively "backdrive" the aileron surface (using the air loads) in the opposite direction. In the event of a jam in the cockpit controls and underfloor aileron control cable & pulleys, the two control columns can be disconnected and control each control tab independently. A separate "bus" cable links the two ailerons together and allows equal and opposite movement. If the aileron surface itself were to jam, then one would be out of luck.

The elevator control system is very similar, but uses hydraulics for an elevator boost system in the event of excess angle of attack.
"We have clearance Clarence. Roger, Roger. What's our vector Victor?"
 
sideflare75
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RE: DC-9 Control Tabs

Fri Jun 23, 2006 12:47 pm

Quoting Avioniker (Reply 2):
The only hydraulic power to the primary flight controls is to the elevators

The rudder also can be powered by hydraulics or controlled by the control tab on the DC-9/MD-80.
 
DH106
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RE: DC-9 Control Tabs

Fri Jun 23, 2006 5:24 pm

So, to summarise, the 737 (in manual mode) uses the tabs to aerodynamically assist the pilots direct inputs to the ailerons.
In DC-9, the pilot's inputs (always manual) feed direct to the tabs, which being a much smaller surface are easier to move. The deflected tab aerodynamically moves the aileron, which has no direct connection to the pilot.
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