2H4
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Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 5:59 am




I ran across this photo:


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Photo © Brett B. Despain





...and the photographer mentions that the aft end of the engine pylons have movable control surfaces to aid in pitch control. Specifically, they provide additional nose-down pitch authority to prevent stalls from occuring.

Here are some close-ups:







This is the first I've heard of movable control surfaces on engine pylons. Further research revealed a previous thread on the topic.

Does anyone know if any other aircraft utilize such a system, or is it unique to the MD-90?


Thanks!




2H4


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aeroweanie
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 8:17 am

Its unique to the MD-90...
 
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HAWK21M
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 12:59 pm

Amazing.Things one learns of Everyday  bigthumbsup 
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bohica
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 1:14 pm

Very interesting.
(filler)
 
bri2k1
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 1:42 pm

You may or may not know, the MD-80, which flies using small servo tabs that deflect the larger control surfaces, has a built-in stall aversion system, as well. The elevator is the only primary control surface (I believe) which has the ability to be powered to the nose down position without requiring any airflow. The T-tail design begs for some stall recovery method, and on the -80, it's accomplished with a powered H-stab. On the -90, it appears to use engine thrust and smaller control surfaces to get out of the "deep stall."
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AR1300
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 2:03 pm

Uhhh???I feel really ignorant now, I didn't get a darn word  crazy 

Again, please???

Thx, Mike
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bri2k1
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 2:16 pm

I'm not sure what you're asking!

A small plane might use a series of cables and linkage to move the control surfaces (ailerons, elevators, rudder, or some combination of those) to change the airplane's pitch, roll, and yaw.

A larger plane might be "fly-by-wire," where the control inputs are translated into electronic commands, carried out by hydraulic or electric actuators that move the control surfaces.

The MD-80 is somewhat unique in that instead of the above methods, it takes advantage of the air flying over the control surfaces to deflect them. Control inputs just move small "servo tabs" electrically. These tabs deflect into the airstream, forcing the control surface in the direction commanded.

However, the T-tail design can result in a "deep stall" where airflow to the elevator is blocked by the wings, making a nose-down pitch change (to recover from a stall) impossible. So, the MD-80 has the ability to electrically move the elevator to the nose-down position when the control wheel is pushed fully forward to the stops. This allows breaking the stall even when minimal airflow is present at the elevator.

The MD-90 accomplishes the same stall recovery by using the movable mini-elevators behind the engines.

I hope this answers the question I think you were asking!
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 3:33 pm

Quoting Bri2k1 (Reply 6):
The MD-80 is somewhat unique in that instead of the above methods, it takes advantage of the air flying over the control surfaces to deflect them. Control inputs just move small "servo tabs" electrically. These tabs deflect into the airstream, forcing the control surface in the direction commanded.

Commonly referred to as McDonnell-Douglas' "cable-and-pulley" airplane, the MD's control inputs primarily move cables connected to small "control tabs" on the larger control surfaces. Moving these control tabs changes the local aerodynamics such that the pilot is "flying" the control surface in order to control the plane. Boeing calls it "manual reversion" (a backup mode of operating). MD drivers simply are flying in manual reversion mode all the time.

Quoting Bri2k1 (Reply 6):
However, the T-tail design can result in a "deep stall" where airflow to the elevator is blocked by the wings, making a nose-down pitch change (to recover from a stall) impossible. So, the MD-80 has the ability to electrically move the elevator to the nose-down position when the control wheel is pushed fully forward to the stops. This allows breaking the stall even when minimal airflow is present at the elevator.

Hydraulically powered (not electrically) when yoke is pushed near full forward position. In fact, it is the control tab position that actually commands the hydraulics to activate (yoke-to-cables-to-elevator control tab-extreme movement-mechanically activating elevator hydraulic actuators-moving entire elevator as a unit).

Quoting Bri2k1 (Reply 6):
The MD-90 accomplishes the same stall recovery by using the movable mini-elevators behind the engines.

The MD90's Pylon Flaps are in addition to everything the MD80 utilizes (i.e. they do not replace MD80 flight controls).
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vikkyvik
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 5:30 pm

Question:

If there's not enough airflow over the h-stab to enable aerodynamic control surface deflection, but you deflect the control surface hydraulically, how does this really affect the deep stall situation? The minimal airflow would need to create sufficient up-force on the h-stab. It just seems surprising that there's not enough airflow to deflect a control surface, but there's enough that once said control surface is deflected, the resulting force will pitch the nose down.

Maybe I'm missing something.

Thanks guys.

~Vik
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atlamt
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RE: Movable Control Surfaces On MD-90 Engine Pylon

Mon Oct 17, 2005 10:52 pm

Quoting Vikkyvik (Reply 8):



Quoting Vikkyvik (Reply 8):
Question:

If there's not enough airflow over the h-stab to enable aerodynamic control surface deflection, but you deflect the control surface hydraulically, how does this really affect the deep stall situation? The minimal airflow would need to create sufficient up-force on the h-stab. It just seems surprising that there's not enough airflow to deflect a control surface, but there's enough that once said control surface is deflected, the resulting force will pitch the nose down.

Basically in a deep stall the airflow over the elevator is reduced and or blocked due to the angle of attack, leaving you unable to control the elevator with the servo tabs. The servo tabs need air flowing over them to work. The hydraulic assist will help you get the nose down and air flowing over the horiz. stab. so the servo tabs once again work.
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon Oct 17, 2005 11:10 pm

Quoting Bri2k1 (Reply 6):

The MD-80 is somewhat unique in that instead of the above methods, it takes advantage of the air flying over the control surfaces to deflect them. Control inputs just move small "servo tabs" electrically.

The BAC One-Eleven had that system way back in 1962 but it was deleted and PFC's installed after a couple of accidents (in test flying) where it was found that the sluggish response of the elevators at low speed was a contributery factor.

Are you sure the Servo Tabs are electric? The One-Eleven's were cable/rod driven by the control column
 
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Oct 18, 2005 12:30 am

Quoting Vikkyvik (Reply 8):
If there's not enough airflow over the h-stab to enable aerodynamic control surface deflection, but you deflect the control surface hydraulically, how does this really affect the deep stall situation? The minimal airflow would need to create sufficient up-force on the h-stab. It just seems surprising that there's not enough airflow to deflect a control surface, but there's enough that once said control surface is deflected, the resulting force will pitch the nose down.

The pilot moves the (relatively small) control tab.... the control tab changes the local aerodynamics such that there will be enough local aerodynamic force for the free-floating control surface to move... the change in control surface position changes aerodynamics that pitch (roll or yaw) the entire plane. Hence the phrase: "the pilot flys the control surface." At slow speeds there may not be enough airflow over the control tab to create enough local aerodynamic force to move the control surface, but due to the much larger size if the control surface, if it were to move it could still create enough aerodynamic force to change the pitch (roll or yaw) of the entire plane.
To provide for that possibility, MD designed the MD80 elevator with a hydraulic assist system at large nose-down control column inputs. With the yoke pushed near its forward limit, the control tab is deflected near its limit and it mechanically activates the elevator hydraulic system. The hydraulic actuators directly move the elevators in the nose-down direction and will maintain that pressure until the yoke (and therefore control tab) position is reduced. All mechanical, nothing electrical.
The MD90 is another stretching of the DC9 airframe and one of the methods MD incorporated to assist in recovery from potential stall was to add the pylon flaps (they only work for nose-down commanded movements). In all my time flying the -90 I never once felt them so I have no idea how effective they may/may not be. They were just..... there.  Wink
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vikkyvik
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Oct 18, 2005 6:55 am

Gotcha, thanks AAR90. That is definitely a cool form of control surface deflection. Actually, maybe that explains why on a recent takeoff on an AA MD-80, the ailerons were deflected on both sides until we were moving along at a good pace on the takeoff roll. I was wondering about that.

~Vik
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474218
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Oct 18, 2005 11:11 am

What moves this "control surface" on the aft end of the pylon?

Does it have its own hydraulic actuator?

How is it connected to the yokes, cables?

Does it only move up from the faired position?

You would think with the boundary layer as thick as it is on the aft portion of an aircraft these two small surfaces would have almost no effect.
 
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HAWK21M
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Oct 18, 2005 4:46 pm


Any Sketch on the Details of the Duct on the Undersurface illustrated above.
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Oct 18, 2005 9:57 pm

Quoting 474218 (Reply 13):
Does it only move up from the faired position?

My guess is that it would only move down. Moving up would not force the nose down.
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Jan 10, 2006 2:28 am

The tab system is alsoused on the BAe-146/ARJ series, I saw on another thread.
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nonfirm
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue Jan 10, 2006 7:54 pm

When we were getting ready to replace the MD-80 with the MD-90 we asked about the tab on the pylon and we were told that they were placed there due to the increased size of the pylon and would help with stall recovery and they would deflect on their own when needed but since we never received the a/c I do not know if this is correct.
 
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Wed Jan 11, 2006 2:12 pm

Quoting Nonfirm (Reply 17):
When we were getting ready to replace the MD-80 with the MD-90



Quoting Nonfirm (Reply 17):
since we never received the a/c

What replaced your MD80s.
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Wed Jan 11, 2006 2:31 pm

Quoting HAWK21M (Reply 18):
What replaced your MD80s.
regds
MEL

we replaced the MD-90 order with the 737-400 this was back in 1993 to 1994 time frame.
 
YYZYYT
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Sat Jan 14, 2006 1:41 am

Quoting AAR90 (Reply 11):
The pilot moves the (relatively small) control tab.... the control tab changes the local aerodynamics such that there will be enough local aerodynamic force for the free-floating control surface to move... the change in control surface position changes aerodynamics that pitch (roll or yaw) the entire plane. Hence the phrase: "the pilot flys the control surface." At slow speeds there may not be enough airflow over the control tab to create enough local aerodynamic force to move the control surface, but due to the much larger size if the control surface, if it were to move it could still create enough aerodynamic force to change the pitch (roll or yaw) of the entire plane.

Well, that sounds ingenious, but I'm still usure of one thing:

Let us take an elevator for example. If the pilot needs the elevator up*, a control tab will open to change the aerodynamics to raise the elevator... for example a small flap deflecting down to create pressure that "flies" the elvator up. But when the elevator starts to rise, would not the pressure from the airflow stop the movement? Isn't the whole point of control surfaces to create pressure by changing airflow? In other words, how does the control tab overcome the greater resistance of the (presumably) larger control surface? Or am I missing something?

Anyone who works on MD products / has an aeronautical engineering degree care to help?

YYZYYT

* or does he/she press the button and wait for the doors to open?
 
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Sat Jan 14, 2006 10:14 am

YYZYYT,
I don't have a real simple/neat way of describing it. Here's one fellow's attempt:
http://pilotfriend.com/flight_traini.../aerodynamics/primary_controls.htm

Tab Control Systems
In the early days of large aircraft many designers avoided the need to provide boosted controls, as described above, by using tab activated controls. The DC-9 for example uses tab actuated controls. In a tab controlled system the pilot moves only a small actuating tab on the larger control surface. The force generated by the tab then moves the main control. This is of course the same way trim tabs work. Therefore, you can think of this system as being like trim tabs if they were connected to the control wheel instead of a separate control wheel. Note that in a tab controlled system there is no direct connection between the control column and the control surface.


For me (a non-engineer) I think of it simply as force and leverage. The control TAB has leverage on the control surface (hinged to it). Move the control TAB and you push/pull on the moveable control surface (elevator in this case). Since the elevator is moveable, the control tab has no leverage to move the entire airframe. OTOH, the moveable control surface is hinged to the airframe and has leverage over it. Move the control surface and the airframe will move. It is simply a matter of how you make the control surface move.... direct cable/pully connection, hydraulic power, or control TAB, etc.
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2H4
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Thu Apr 29, 2010 4:15 pm

Quoting AAR90 (Reply 7):
Hydraulically powered (not electrically) when yoke is pushed near full forward position.

Ok, apologies for resurrecting such an old thread, but might any of you have the means to acquire photos of the pylon control surfaces in the deflected position?
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Thu Apr 29, 2010 4:39 pm

Quoting YYZYYT (Reply 20):
In other words, how does the control tab overcome the greater resistance of the (presumably) larger control surface? Or am I missing something?

The tab is on the trailing edge of the surface & this gives it a much larger moment arm about the surface hinge than the surface itself. The parent surface may also be aerodynamically 'balanced' and be relatively force neutral to the airflow by having it's hinge point placed some distance behind it's leading edge. These together allow the relatively small tab to 'fly' the elevator.
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PGNCS
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Thu Apr 29, 2010 6:47 pm

Quoting Bri2k1 (Reply 4):
You may or may not know, the MD-80, which flies using small servo tabs that deflect the larger control surfaces, has a built-in stall aversion system, as well. The elevator is the only primary control surface (I believe) which has the ability to be powered to the nose down position without requiring any airflow. The T-tail design begs for some stall recovery method, and on the -80, it's accomplished with a powered H-stab. On the -90, it appears to use engine thrust and smaller control surfaces to get out of the "deep stall."

All DC-9's and MD-80/90 have unpowered ailerons (hydraulically powered spoilers assist in roll control), a hydraulically powered rudder (aerodynamic tabs for backup), and DC-9/MD-80's have tab-controlled elevators with hydraulic assist in the nose down direction (to help with stall recovery). The MD-90 has a full time hydraulically-powered elevator, with aerodynamic tabs as a backup. All of these aircraft have an electrically powered trimmable horizantal stabilizer which is used by pilots and by the autopilot to keep the aircraft in trim. The MD-90 also has "pylon flaps," control surfaces that deflect downward, which are hydraulically powered, on the aft of the engine pylon.

Quoting Bri2k1 (Reply 6):
The MD-80 is somewhat unique in that instead of the above methods, it takes advantage of the air flying over the control surfaces to deflect them. Control inputs just move small "servo tabs" electrically. These tabs deflect into the airstream, forcing the control surface in the direction commanded.

The tabs have no electrical actuators; they are manually connected to the control columns via cables and pulleys.

Quoting Bri2k1 (Reply 6):
However, the T-tail design can result in a "deep stall" where airflow to the elevator is blocked by the wings, making a nose-down pitch change (to recover from a stall) impossible. So, the MD-80 has the ability to electrically move the elevator to the nose-down position when the control wheel is pushed fully forward to the stops. This allows breaking the stall even when minimal airflow is present at the elevator.

The MD-80 as well as the rest of the family ALWAYS has the ability to move the elevator, but never electrically. You CAN move the trimmable horizontal stabilizer electrically with the trim switches (and the autopilot can too), but this is unrelated to stall recovery. There is a stick pusher system installed as part of the extensive stall warning protections to move the yoke forward as a stall prevention feature, but the yoke is what still moves the elevators.

Quoting AAR90 (Reply 7):
Hydraulically powered (not electrically) when yoke is pushed near full forward position. In fact, it is the control tab position that actually commands the hydraulics to activate (yoke-to-cables-to-elevator control tab-extreme movement-mechanically activating elevator hydraulic actuators-moving entire elevator as a unit).

   Exactly. Very concisely said, AAR.

Quoting AAR90 (Reply 7):
The MD90's Pylon Flaps are in addition to everything the MD80 utilizes (i.e. they do not replace MD80 flight controls).

  

Quoting VC-10 (Reply 10):
Are you sure the Servo Tabs are electric?

Good observation VC-10, they are not.

Quoting AAR90 (Reply 11):
The pilot moves the (relatively small) control tab.... the control tab changes the local aerodynamics such that there will be enough local aerodynamic force for the free-floating control surface to move... the change in control surface position changes aerodynamics that pitch (roll or yaw) the entire plane. Hence the phrase: "the pilot flys the control surface." At slow speeds there may not be enough airflow over the control tab to create enough local aerodynamic force to move the control surface, but due to the much larger size if the control surface, if it were to move it could still create enough aerodynamic force to change the pitch (roll or yaw) of the entire plane.
To provide for that possibility, MD designed the MD80 elevator with a hydraulic assist system at large nose-down control column inputs. With the yoke pushed near its forward limit, the control tab is deflected near its limit and it mechanically activates the elevator hydraulic system. The hydraulic actuators directly move the elevators in the nose-down direction and will maintain that pressure until the yoke (and therefore control tab) position is reduced. All mechanical, nothing electrical.

   Brilliantly said. There's nothing more to add.

Quoting AAR90 (Reply 11):
In all my time flying the -90 I never once felt them so I have no idea how effective they may/may not be. They were just..... there.

   Neither have I and I never want to feel them, either!  
Quoting Vikkyvik (Reply 12):
Actually, maybe that explains why on a recent takeoff on an AA MD-80, the ailerons were deflected on both sides until we were moving along at a good pace on the takeoff roll. I was wondering about that.

The ailerons can be at any position during ground operations; they will be opposite each other as they are bussed together, but wind can push them to any position; they will quickly streamline with airflow over the wing during takeoff roll. Likewise the elevators can be in any position on an MD-80 or DC-9 and they are NOT bussed together, so they can be deflected in opposite positions (this won't be a factor on the MD-90); not to fear they will streamline quickly on the takeoff roll as well.

Quoting 474218 (Reply 13):
What moves this "control surface" on the aft end of the pylon?

Does it have its own hydraulic actuator?

Yes it does.

Quoting 474218 (Reply 13):
Does it only move up from the faired position?



It only moves down from the faired position in an effort to lower the nose (to break a stall).
 
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Fri Apr 30, 2010 6:51 pm

Quoting PGNCS (Reply 24):
The MD-90 has a full time hydraulically-powered elevator, with aerodynamic tabs as a backup.

I've told this story before but I have a somewhat interesting story about those hydraulically powered elevators. Years ago I was lubing the horizontal on an MD-90. In the middle of doing this one of my fellow techs decided to turn on the hydraulic pumps. On most aircraft when you do this the hydraulically powered control surfaces will slowly move to the faired position. Not so with the elevators on the 90, they would slam into place. Fortunately I had previous experience with the 90 so I knew as soon as I heard the pumps to get out of the way. Needless to say I had some choice for my co-worker.
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wn700driver
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Sat May 01, 2010 6:04 am

>Quoting Vikkyvik (Reply 12):
Gotcha, thanks AAR90. That is definitely a cool form of control surface deflection. Actually, maybe that explains why on a recent takeoff on an AA MD-80, the ailerons were deflected on both sides until we were moving along at a good pace on the takeoff roll. I was wondering about that

I noticed that on a DL machine as well. Knowing what I do about pulley controlled, it was no cause for concern, but I thought it was neat watching them bus up as we began to roll. What was funny was seeing them spring back to a "left roll" position almost as soon as we vacated the runway on arrival.
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PGNCS
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Sun May 02, 2010 8:38 pm

Quoting LMP737 (Reply 25):
Quoting PGNCS (Reply 24):
The MD-90 has a full time hydraulically-powered elevator, with aerodynamic tabs as a backup.

I've told this story before but I have a somewhat interesting story about those hydraulically powered elevators. Years ago I was lubing the horizontal on an MD-90. In the middle of doing this one of my fellow techs decided to turn on the hydraulic pumps. On most aircraft when you do this the hydraulically powered control surfaces will slowly move to the faired position. Not so with the elevators on the 90, they would slam into place. Fortunately I had previous experience with the 90 so I knew as soon as I heard the pumps to get out of the way. Needless to say I had some choice for my co-worker.

Glad you stayed out of the way! The yoke jumps powerfully in the cockpit too when the pumps are turned on, and the rudder pedals can kick quite a bit when the right system is pressurized. Some Boeings behave similarly. It's interesting to me that some aircraft contol surfaces move gradually to faired, while some jump when the hydraulics are pressurized. I guess I thought they would all move pretty briskly to the commanded position when the hydraulics were turned on, but I don't see it from the outside as much as you. I do recall the L-1011 ailerons briskly moving to the neutral position on the ground when pressure is turned on. Thanks for your insight!  
 
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon May 03, 2010 4:26 pm

Quoting AAR90 (Reply 7):
Boeing calls it "manual reversion" (a backup mode of operating). MD drivers simply are flying in manual reversion mode all the time.

I hope not!

Manual reversion in a Boeing (I speak for the 737 Classic) is when all hydraulics to the flight controls are lost and is a major inconvenience. The checklist is over 4 pages long. Imagine driving your car with the power steering failed - its like that in all axes! Except yaw, as the rudder's not contollable without hydraulics.

Regards - musang

[Edited 2010-05-03 09:33:42]
 
PGNCS
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Mon May 03, 2010 5:34 pm

Quoting musang (Reply 28):
Quoting AAR90 (Reply 7):
Boeing calls it "manual reversion" (a backup mode of operating). MD drivers simply are flying in manual reversion mode all the time.

I hope not!

Manual reversion in a Boeing (I speak for the 737 Classic) is when all hydraulics to the flight controls are lost and is a major inconvenience. The checklist is over 4 pages long. Imagine driving your car with the power steering failed - its like that in all axes! Except yaw, as the rudder's not contollable without hydraulics.

What AAR90 is getting at is that the aerodynamic tabs control the ailerons and elevators on all DC-9 and MD-80's (though MD-90's have powered elevators) at all times. The rudder is normally powered by the right hydraulic system, and if that fails it too reverts to being tab-driven. In other words (excepting the MD-90 elevator) pitch and roll in the DC-9 and MD-80 family is always simply cables and pulleys and requires no hydraulic power of any sort (although there is roll augmentation built into the spoilers which are hydraulically powered like most other aircraft).

While I understand the point AAR90 is making, please understand that the DC-9 family was designed to fly that way from the outset and has very nicely balanced controls, although more yoke deflection is sometimes needed compared with some other aircraft. I happen to like the control feel of the DC-9/MD-80 (and actually prefer it to the MD-90), though there are plenty of people who prefer the Boeing.

In the Boeings I have flown with manual reversion capability (B-727 and B-737) the aircraft are designed to have full-time hydraulically-powered control surfaces, and when you have hydraulics they fly just fine. With a loss of hydraulics, however, both are absolutely brutal to try to fly, and it's a simulator exercise that really gets your attention. The controls are VERY heavy and a lot of muscle is needed; I prefer the Douglas approach which results is a nicely harmonized aircraft that is easily controllable no matter what your hydraulic status is. Do note, though, that the DC-9 family has no alternate means of extending the flaps and slats without hydraulics available, so you will end up with a clean, very fast-moving machine on final approach with a total hydraulic failure.
 
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue May 04, 2010 2:06 am

Quoting musang (Reply 28):
its like that in all axes! Except yaw, as the rudder's not contollable without hydraulics.

How does it go about spoilers? And since rudder is inop, would it not end up, if spoilers were too inop, in a slip-like situation? (Sorry, I learnt this stuff in Czech, cant tell in English) - basically ball full "down" side and plane barely turning?
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue May 04, 2010 7:30 pm

Hello PGNCS - agreed. I was just pointing out that "manual reversion" is a term referring to the total hydraulic loss scenario so isn't really appropriate for DC-9s.

Hi Fabo - the spoilers are not powered in a no-hydraulics situation. The rudder simply trails straight. Roll control is purely with ailerons, its stays pretty well co-ordinated, and its hard work because there is no self-centering of the ailerons. You have to return the control wheel to neutral manually.

I've only done it in the simulator. I have often wondered what the real thing is like!

Regards - musang
 
AAR90
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue May 04, 2010 9:18 pm

Quoting musang (Reply 31):
Hello PGNCS - agreed. I was just pointing out that "manual reversion" is a term referring to the total hydraulic loss scenario so isn't really appropriate for DC-9s.

Not an official term used in MD aircraft; however, it provides an apt description of the way the DC9 series operates.

I had a little trouble understanding the MD80 flight controls until the ground school instructor said: you were a 727 FE weren't you? You remember "manual reversion"? Well the DC9 is essentially operating in manual reversion all the time.

Okay, now I get it (light bulb goes on over my head). The DC9 flight controls are totally manual and since they were designed that way, their "balance" is actually quite nice. Slightly slower response to control inputs than a fully hydraulic system, but not bad and... very reliable.

Quoting musang (Reply 31):
I have often wondered what the real thing is like!

Lots of pilots complain about the DC9/MD80/MD90 but I actually enjoyed flying the plane (especially the MD90). The only real "issues" I encountered were the tendancy for wing buffet at/near .80m and the fact that AA was flying the planes on "long" routes --regularly encountered weight limitations.
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musang
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue May 04, 2010 10:09 pm

Indeed.

"Reversion" implies that it has reverted from something else, i.e. from hydraulic drive in the Boeing case, to unassisted cable drive.

I agree the servo tab system works nicely. I used to fly 146/RJ-100s. If you punched the centre of the control wheel in flight , the whole aircraft would gently occillate in pitch for a second or so at about 5 hz.

Regards - musang
 
2H4
Topic Author
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Tue May 04, 2010 10:15 pm

Quoting musang (Reply 33):
If you punched the centre of the control wheel in flight

Stressful job?  
Intentionally Left Blank
 
musang
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Wed May 05, 2010 10:57 am

Right!

I guess someone discovered it by accident and word got around.

Actually the more I think about it, there was a mass damper weight in the first officer's elevator control circuit, specifically for smoothing out sudden shocks in turbulence. The sudden impulse to the column no doubt excited this weight and it simply did its job.

Regards - musang
 
PGNCS
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Wed May 05, 2010 4:55 pm

Quoting musang (Reply 31):
Hello PGNCS - agreed. I was just pointing out that "manual reversion" is a term referring to the total hydraulic loss scenario so isn't really appropriate for DC-9s.

I agree with you. I was trying to explain that AAR90 was getting at the conceptual way the system functions, not that someting is wrong with the DC-9 series flight control architecture.

Quoting AAR90 (Reply 32):
Not an official term used in MD aircraft; however, it provides an apt description of the way the DC9 series operates.

Spot on.

Quoting AAR90 (Reply 32):
The DC9 flight controls are totally manual and since they were designed that way, their "balance" is actually quite nice. Slightly slower response to control inputs than a fully hydraulic system, but not bad and... very reliable.

I agree with you completely. It is nicely balenced in all axes, although it requires more input than some other aircraft, especially in roll before you get into the aileron-spoiler mixing regime. Great, very reliable design, though and nice to know that with a loss of hydraulics that controlling the plane is not among your problems!

Quoting AAR90 (Reply 32):
Quoting musang (Reply 31):
I have often wondered what the real thing is like!

Lots of pilots complain about the DC9/MD80/MD90 but I actually enjoyed flying the plane (especially the MD90). The only real "issues" I encountered were the tendancy for wing buffet at/near .80m and the fact that AA was flying the planes on "long" routes --regularly encountered weight limitations.

Some people complain, but a lot of the complainers grew up with Boeing and think that their way is the only correct way, and/or they haven't actually flown the aircraft (much anyway) themselves. The DC-9 is actually better at .8 Mach than the MD-80/90 for buffet, but like you say, it's really only a factor when you are heavy and near the max altitude for your weight. I like the MD-90 too, and love the power and super-quiet cockpit, but I don't think the pitch feedback is as good with the powered elevator as on the earlier tab-elevator planes, but that's a personal preference. Still a great plane in my book.

Quoting musang (Reply 33):
"Reversion" implies that it has reverted from something else, i.e. from hydraulic drive in the Boeing case, to unassisted cable drive.

Great point, and that was why I was trying to explain that AAR90 was discussing a conceptual method of thinking about the system, not implying that something was broken or faulty in the system.

[Edited 2010-05-05 09:56:55]
 
LMP737
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RE: Movable Control Surfaces On MD-90 Engine Pylons

Fri May 07, 2010 2:48 am

Quoting PGNCS (Reply 27):
Glad you stayed out of the way! The yoke jumps powerfully in the cockpit too when the pumps are turned on, and the rudder pedals can kick quite a bit when the right system is pressurized. Some Boeings behave similarly. It's interesting to me that some aircraft contol surfaces move gradually to faired, while some jump when the hydraulics are pressurized. I guess I thought they would all move pretty briskly to the commanded position when the hydraulics were turned on, but I don't see it from the outside as much as you. I do recall the L-1011 ailerons briskly moving to the neutral position on the ground when pressure is turned on. Thanks for your insight!


So was I, while the days off would have been nice I'd rather skip the head injury/truama.  

[Edited 2010-05-06 19:51:10]
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