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copter808
Topic Author
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Joined: Sat Dec 30, 2000 1:14 pm

MD-80 Elevator

Mon Jun 07, 2004 3:24 am

On the MD-80, when parked on the ground, I frequently notice that one elevator may be "up" while the other one is "down". Do they function like an aileron in flight, or is this just something on the ground?
 
dl757md
Posts: 1483
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RE: MD-80 Elevator

Mon Jun 07, 2004 4:39 am

When an AC is parked on the ground with the hydraulic systems off the elevators will move in response to wind forces acting upon it. Depending on the wind direction these forces can cause opposite elevators to move in different directions. The elevators on many GA aircraft are mechanically linked, so they only move in unison. On the Md-80 as well as most transport AC the elevators are not mechanically linked, they are held in position by hydraulic actuators. The Md-80 elevators have control tabs that create an aerodynamic deflection causing the elevators to move up or down. The control tabs are mechanically linked to the control column (and therefor cannot split) but the elevators themselves are not. On the ground with no hydraulics the position of the elevators has no relation to the control tab position and hence are able to move independantly of the control tabs and each other.
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SlamClick
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RE: MD-80 Elevator

Mon Jun 07, 2004 11:46 pm

What Dl757md says is exactly right and it has led to some funny moments. We've had panicky lightplane pilots board and come up to the cockpit to warn us that our elevator is broken - they'd seen it from the terminal windows. Also happens on ground control frequency from time to time. I have to admit it does look kind of strange.

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AAR90
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RE: MD-80 Elevator

Tue Jun 08, 2004 5:12 am

When an AC is parked on the ground with the hydraulic systems off the elevators will move in response to wind forces acting upon it.

Even with aircraft hydraulic systems powered, the only time the MD80 elevator is actually powered is when the control column is pushed near its maximum forward movement (full nose-down). Only then does the elevator receive any hydraulic assist (boost). At all other times each half of the elevator is free-floating with control tabs linked by cable to the control columns "flying" the elevator to desired position(s). You will regularly see the split elevator condition on taxiing MD80s.
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Corsair2
Posts: 243
Joined: Thu Jan 25, 2001 11:02 pm

RE: MD-80 Elevator

Tue Jun 22, 2004 11:07 pm

The ailerons are split on the MD-80 as well and the surface flown through the balance tabs.
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miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: MD-80 Elevator

Fri Jul 16, 2004 8:32 pm

I'll throw my two cents in...Most control surfaces are balanced in the nose heavy condition. That's why you'll usually see DC-9 and MD-80 elevators in the nose up position. If these things were to be balanced in a tail heavy condition, you'll get flutter, and that will not make for happy flying.
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L-188
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RE: MD-80 Elevator

Fri Jul 16, 2004 8:35 pm

BTW...The Hindenburg had a simular system for it's control surfaces.....Servo tabs causing control surface movement.
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miamiair
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RE: MD-80 Elevator

Fri Jul 16, 2004 8:44 pm

THey are not mechanically linked, so the wind might blow one up...When the relative wind begins to pass over them (take-off roll) they fair into their normal position and then the control column inputs actually drive the control tabs, which in turn "fly" the elevator itself to the position desired.
Molon Labe - Proud member of SMASH
 
AmericanAirFan
Posts: 387
Joined: Mon Jun 28, 2004 10:22 am

RE: MD-80 Elevator

Sat Jul 17, 2004 1:41 am

So it'd look something like this?



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miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: MD-80 Elevator

Sat Jul 17, 2004 2:20 am

Exactly! The elevators will move independantly, but the tabs are linked together.
Molon Labe - Proud member of SMASH
 
JetMechMD80
Posts: 370
Joined: Fri Jul 09, 2004 2:27 pm

RE: MD-80 Elevator

Thu Jul 22, 2004 3:27 pm

The Elevators on the MD-80/DC-9 are not hydraulically powered, they are moved by the tabs.
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Super Em
Posts: 427
Joined: Sat Nov 25, 2000 7:55 am

RE: MD-80 Elevator

Fri Jul 23, 2004 1:33 pm

I've noticed that while taxiing, most T-Tail aircraft will have the elevators in the down position. Once the takeoff roll begins, they go back to neutral. Any reason for this?
 
miamiair
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

RE: MD-80 Elevator

Fri Jul 23, 2004 7:32 pm

Relative wind. Keep in mind that the elevators are balanced. As soon as the air begins to flow over the surface the surfaces are moved to their neutral positions, unless some control input is applied. If they didn't return to their neutral position during the take off roll, you would have a premature rotation which brings in a whole horde of other issues.
Molon Labe - Proud member of SMASH
 
Dalmd88
Posts: 3176
Joined: Fri Jul 28, 2000 3:19 am

RE: MD-80 Elevator

Sat Jul 24, 2004 7:34 pm

Keep in mind the wind is moving them to neutral. From the pilot's point of view they were always in the neutral position. Without any air moving across the surface the tabs can't effect the elevator's position.
 
n685fe
Posts: 337
Joined: Fri Dec 12, 2003 7:01 am

RE: MD-80 Elevator

Sun Jul 25, 2004 12:56 pm

AAR90

Even with aircraft hydraulic systems powered, the only time the MD80 elevator is actually powered is when the control column is pushed near its maximum forward movement (full nose-down). Only then does the elevator receive any hydraulic assist (boost). At all other times each half of the elevator is free-floating with control tabs linked by cable to the control columns "flying" the elevator to desired position(s). You will regularly see the split elevator condition on taxiing MD80s.

Correct. Hyd. pressure is applied to the elevator when a commanded nose down greater then 10 degrees is placed. It then allows a manifold valve to open and apply hyd. pressure to the elevator, when pressure exceeds 750psi in the line you get an "Elevator Power On" annunciator light on the overhead panel.



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