ZuluAviator994 From Australia, joined Mar 2008, 370 posts, RR: 0 Posted (4 months 1 week 5 days 13 hours ago) and read 1863 times:
Alright, another question from me...a stupid one, more than likely.
On MD-80s taxing, (all series?) the horizontal stabilizer is sometimes in a leading edge down position. Is this just the trim? And if so, why are the elevators moving in a nose up position when the entire stabilizer is? http://www.airliners.net/photo/Vikin...43&prev_id=1357033&next_id=1356947
Thanks in advance
When once you have tasted flight, you will forever walk the earth with your eyes turned skyward
Boeing767mech From United States, joined Dec 2000, 496 posts, RR: 1 Reply 1, posted (4 months 1 week 5 days 13 hours ago) and read 1847 times:
The elevators are only powered in the nose down position to push the nose over in a stall, and this is only with excessive force full forward on the yoke. All the other times the elevators are unpowered and free floating. The elevators move using reaction tabs and airflow over the tabs. So you will see the elevators in any position sitting on the ramp. I have seen the left elevator up and right elevator down.
I hope this came out right and answers your question
David
If it wasn't for maintainence, pilots wouldn't be able to get it up in the morning
PGNCS From United States, joined Apr 2007, 833 posts, RR: 1 Reply 2, posted (4 months 1 week 5 days ago) and read 1756 times:
The horizontal stabilizer is set to a specific angle before takeoff. It varies with every departure and is based on CG and takeoff flap setting. It is designed to be in trim for the engine out climb speed in the event of an engine failure during or immediately after takeoff.
The elevators on the MD-80 and DC-9 (NOT the MD-90) are normally not hydraulically powered and are free to move in the wind. They are not connected to each other, so they may be deflected in completely opposite directions. It is perfectly normal. It is a low tech system that works supremely well. The MD-80 elevators are only powered in an extreme nose down situation which would be used for breaking a stall.
If your question is based on the photo you linked to, I'd have to say that both the horizontal stab and the elevators are commanding nose-up in that picture. Should not be surprising.
As has been noted, since the elevators are unpowered at this point in the flight, and since they are very well balanced, there isn't much you can tell from their position at the moment. That is just where they ended up before this takeoff.
Depending on gross weight, CG and takeoff configuration, the horizontal stab position is quite likely just right for this takeoff. Once the plane accelerates and gets some airflow over the tail, the elevators will fly to the position commanded by the control column position-flight tab position.
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TWAL1011727 From United States, joined Mar 2006, 393 posts, RR: 0 Reply 4, posted (4 months 1 week 4 days 18 hours ago) and read 1622 times:
Quoting PGNCS (Reply 2): The MD-80 elevators are only powered in an extreme nose down situation which would be used for breaking a stall.
I think this should read : In an extreme nose high attitude (positive, nose high, pitch attitude)
it is necessary to power the elevators to give more nose down authority than the tabs can give.
PGNCS From United States, joined Apr 2007, 833 posts, RR: 1 Reply 6, posted (4 months 1 week 4 days 14 hours ago) and read 1576 times:
Quoting TWAL1011727 (Reply 4): Quoting PGNCS (Reply 2):
The MD-80 elevators are only powered in an extreme nose down situation which would be used for breaking a stall.
I think this should read : In an extreme nose high attitude (positive, nose high, pitch attitude)
it is necessary to power the elevators to give more nose down authority than the tabs can give.
No it shouldn't. What the post meant is that the elevators are used to make a nose down input far greater than normal to yield elevator augmentation. If that wasn't clear, I apologize.
A stall can occur at any attitude. The ONLY time the elevators are powered in a DC-9/MD-80 is when the pilot is commanding nearly full nose down. You are correct that the tabs cannot provide enough throw to break all stall conditions without hydraulic augmentation. While stalls are normally associated with nose high attitudes, the recovery from the stall will require forward control column input regardless of where the horizon happens to be, hence the elevator augmentation system, which is, incidentally, checked prior to every departure.
Tinpusher007 From United States, joined Feb 2005, 864 posts, RR: 5 Reply 7, posted (3 months 1 week 3 days 19 hours ago) and read 1026 times:
Quoting SlamClick (Reply 3): If your question is based on the photo you linked to, I'd have to say that both the horizontal stab and the elevators are commanding nose-up in that picture. Should not be surprising.
As has been noted, since the elevators are unpowered at this point in the flight, and since they are very well balanced, there isn't much you can tell from their position at the moment. That is just where they ended up before this takeoff.
Depending on gross weight, CG and takeoff configuration, the horizontal stab position is quite likely just right for this takeoff. Once the plane accelerates and gets some airflow over the tail, the elevators will fly to the position commanded by the control column position-flight tab position.
Ok so what occurs at rotation when you pull back on the stick? Does this not cause the elevators to move up?
"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."
PGNCS From United States, joined Apr 2007, 833 posts, RR: 1 Reply 8, posted (3 months 1 week 3 days 17 hours ago) and read 986 times:
Quoting Tinpusher007 (Reply 7): Ok so what occurs at rotation when you pull back on the stick? Does this not cause the elevators to move up?
No it does not. It causes the control tabs to move down. The aerodynamic forces that result from the control tab displacement is what makes the elevator move up. As it is moving up, the geared tabs move down to provide for lower control forces. It works great, and lasts a long time.
TinPusher007 From United States, joined Feb 2005, 864 posts, RR: 5 Reply 9, posted (3 months 1 week 3 days 3 hours ago) and read 908 times:
Quoting PGNCS (Reply 8): No it does not. It causes the control tabs to move down. The aerodynamic forces that result from the control tab displacement is what makes the elevator move up. As it is moving up, the geared tabs move down to provide for lower control forces. It works great, and lasts a long time.
Interesting...I never knew this, but was always curious myself as to why the elevators were up and down at the gate and during taxi, but seemed to come together on the takeoff run. As always, great info on the forums here...thanx guys.
"Flying isn't inherently dangerous...but very unforgiving of carelessness, incapacity or neglect."
PGNCS From United States, joined Apr 2007, 833 posts, RR: 1 Reply 10, posted (3 months 1 week 2 days 17 hours ago) and read 819 times:
Quoting TinPusher007 (Reply 9): Interesting...I never knew this, but was always curious myself as to why the elevators were up and down at the gate and during taxi, but seemed to come together on the takeoff run. As always, great info on the forums here...thanx guys.
No problem!
You have almost certainly noticed that the elevators can be split in totally opposite directions on the DC-9 and MD-80, especially during high winds. It's because the elevators are strictly aerodynamic surfaces and will move wherever the air pushes them. Of course, as the plane accelerates the relative wind is from the front so they streamline and are then controlled by the control tab. (The MD-80 also has a geared tab and an anti-float tab on each elevator as well.) If you see an MD-90 doing this, though, there is a major problem as the MD-90 elevator IS hydraulically powered (with tab backup for abnormal situations).
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