MD elevator assymetry

[flyburg]

Often when taxiing out behind a MD80/82/88 I notice that one elevator is pointing up and the other one down. Then when taking the runway they align.

Can somebody tell why they do this?

Thanks

[flyburg]

Okay, I understand, but how come they align then when the aircraft takes the runway, From what you are saying I understand they would align when there's relative airflow, but I have seen them align on take off when there's no relative airflow and I don't think they are in the jet wash.

Thanks for the quick reply

[mitzy69]

MD elevators

must be that when on runway to T/O you line up into wind, and the surfaces align to airflow.
ON the B707 in a tail wind one elevator would go up and the other down onto the limit stops and would lock the control column in a mid postion, to cure go down the back and pull/push one elevator to the same positions as the other .

[ITCZ]

"Pitch control 101" for DC-9/MD-8x/Boeing 717.

Gen Y might wish to go straight to bold at the end.

Trimming is via all moving horizontal stabilizer. Actuated by ONE acme screw driven by primary AC electric motor (big) or secondary AC motor (not so big) through a planetary gearbox. Pilots actuate stab trim through 'pickle' switches on control wheel. Secondary motor is backup for primary and also used for autopilot pitch trim.

Being an all moving stabilizer, when trimmed and in un-accelerated flight (constant climb cruise or descent), MD-8x elevators 'trail', supplying no aerodynamic force. The stab does that job. Control column returns to centre when aircraft trimmed -- unlike most GA types where elevator is trimmed to provide the downforce/upforce to trim in pitch displacing the control column from central position.

WMV of MD-8x pitch trim (and failure in Alaska airlines) http://www.ntsb.gov/Events/2000/Aka2...kscrew_261.wmv

Elevators are thus used only for momentary pitch up/pitch down manouevering.

MD-8x elevators are not connected to any control cables, hydraulic servos, or other actuators -- except for one item that we will get to in a minute. Therefore an MD-8x elevator sits whereever it likes, when parked, and trails in the airstream if the pilot or autopilot is not pushing or pulling the control column.

Elevator control is effected by tabs. The captains control column runs a cable circuit to the left elevator servo tab. The FO control column runs a cable circuit to the right elevator servo tab. The Captain and FO columns are connected to each other via a torque tube with a breakout mechanism.

That provides for redundancy if there is a jam or failure in one elevator servo control circuit. Disconnect the columns, and use the 'free' column to control the aircraft in pitch.

Want to pitch up, like a rotation to takeoff? Captain pulls back on his column. Captains cable circuit pulls the left elevator servo down. Resulting aerodynamic force and moment arm to elevator hinge acts to deflect the left elevator up. Left elevator and moment arm to CofP acts to rotate aircraft nose up. Torque tube also makes the FO column pull back, duplicating servo tab and elevator deflections on right side. Aircraft pitches up.

So there is no direct connection between the pilots control column and the elevator in an MD-8x. No airflow, and the elevators sit where they like if they are experiencing different winds when parked. Gust dampers protect against damage due to abrupt changes.

Except for one thing. Being a T-tail aircraft where deep stall is a risk, the aircraft needs deep stall recovery capability. Low speed or turbulent airflow in stalled condition might make servo tab powered elevators unable to recover from a deep stall.

So Douglas/MD added an elevator augmentation system. Should either pilot push their control column full forward, this would be sensed and one hydraulic ram mounted on each elevator would force each elevator, not the tab, to the full down position for stall recovery.

It is a requirement to check 'flight controls full and free movement' before every takeoff. So, just before the cock-eyed MD-80 taxiies onto the runway, one pilot does a full and free check. When the stick is pushed full forward, left hydraulic system pressure activates the elevator augmentation and pushes both elevators full down. No more cockeyed elevators.

[flyburg]

Thanks for the explanation

[F4F]

ASFKAP I guess 411A will answer your question with undisguised pleasure... just wait a couple of minutes here

Don't know much about the good ol' Tristar, suspecting same as DC-10 = pilot muscles move cables of control column that activate servo valves of hydraulic controls...


live 2 fly 2 live

[Wodrick]

Quoting "VSB1976" in a previous thread :

Lockheed L-1011 TriStar Horizontal Stabilizer
There is something about the L-1011 horizontal stabilizer that makes it unique: In the early design stages, the airlines told Lockheed they wanted the pilots to have more power and control over the pitch system, so that in case of a jet upset, the crew could pull the aircraft out of a dive.

Lockheed designed this extra power into the tail: The L-1011 is the only commercial jet that can take off with full nose down trim. This was actually demonstrated to the airlines during the flight testing phase - with full nose down trim (which is a mis-set trim setting but has happened in the past on other aircraft and caused accidents) the pilot is still able to rotate the aircraft and climb away - it takes a lot of control wheel force to do it but it can be done - in other aircraft, the control column can be pulled full aft but with full nose down trim the nose won't come up for rotation. For those who don't know: Jet upset was an early event that happened more in the 1950's and early 1960' with the commercial jets than we see today - via rough air, the aircraft would get knocked out of it's stablized cruise condition until it was diving towards the ground out of control - the pilots would attempt to pull back on the wheel to get the nose to come up but most of the jets of that era did not have the power to overcome the excessive speed and thus they could not pull the aircraft out of the dive - when the L-1011 was being designed, the airlines wanted to make sure the L-1011 had enough pitch force to do this should it happen - to this day, a TriStar has never been lost as a result of jet upset and not being able to pull out of the resulting dive.

Now on to the technical description of the L-1011 pitch control system. The L-1011 incorporates an all-flying horizontal stabilizer to control movement in the pitch axis. This "all flying tail" is unique in the commercial aircraft industry. First of all, pitch control on most airplanes are usually controlled by elevators - on the L-1011 however, the primary part of the tail that controls pitch is the front part called the horizontal stabilizer, the leading edge of which which move up and down - the elevators are attached to the rear spar of this movable stabilizer piece - the elevators do move, yes, but not under direct pilot control. The elevators move as a direct result of stabilizer movement only via a physical mechanical link: When the stabilizer moves it's nose downward, the elevators deflect upward to increase the camber of the entire tail - this causes a downward moment and thus pulls the entire rear portion of the aircraft in a downward direction - this causes the nose of the aircraft to pitch up. When the pilot pushes the control wheel forward, this causes the stabilizer nose to move up, causing the elevators to deflect downward, increasing the camber effect to cause a lifting moment and thus pull the tail of the aircraft upward and this in turn causes the nose to pitch down.


Just to get in before the inevitable ! do you think there is a 1011 alarm in Arizona ?

[Capn Bloggs]

"Pitch control 101" for DC-9/MD-8x/Boeing 717.

Don't forget your past, ITCZ; the BAe 146/RJ has the same system re flapping elevators (although no "flying" stab).