QUOTE (Sam @ 05/12/2010, 9:47)
2/ The animations are not correct on the trailing edges. For example when I move the elevator, only the little flaps are moving, and not the whole elevator.
I'm guessing I'm doing something pretty stupid. I thought I knew a lot about the Maddog, but obviously something is not quite right!
Nope; nothing stupid, Sam. The tail surfaces are doing what they're supposed to. The '80 - along with hundreds of other commercial jet aircraft - uses "flying control surfaces".
Someone like Paul Edwards or Jared - feel free to correct me here, please! (I was an avionics guy in my previous life).
(Also - Wims - none of this requires hydraulics, except when the aircraft is in a deep stall.)
1). At the trailing edges of the each elevator there are three tabs - each tab being linked to its counterpart tab on the other half of the stabilizer.
2). The whole tailplane (horizontal stabilizer - 'stab' to be short) is laterally hinged at the rear spar, with the nose of the stab being captured and moved up or down by a long jackscrew running vertically up through the leading edge of the vertical stabilizer.
The electrically-driven jackscrew sets the general longitudinal trim for the aircraft i.e. it changes the angle that the stabilizer presents to the longitudinal axis of the fuselage: this is what moves when you set the take-off trim on the ground. If you have, for e.g. a nose-heavy aircraft, we may want, for e.g. 7.5 degrees nose-up: we roll the trim-wheel backward to the degree setting we need - our 7.5 degrees. The electric motor at the top of the vertical stab rotates the jackscrew - and the leading edge of the stab is moved downward until it meets that desired setting. A negative incidence on the horizontal stab will result in a nose up trim - i.e. a tail that "pulls down" - which is what we need to balance or lift the extra weight in the nose.
Likewise - for a tail-heavy aircraft we will roll the trim wheel forward, 2, 3, 5 degrees, etc., - the jackscrew will rotate in the opposite direction, driving the nose of the stab upward. A positive incidence on the horizontal stab will result in a nose down trim - i.e. a tail that "pulls up" - which is what we need to balance out the lack of weight in the nose.
Setting the longitudinal trim has the effect of "presetting" the load which the pilot will feel as the aircraft gains speed. As it becomes airborne the trim should not present any strain to the PF; however as the speed builds up the lift will increase, and the nose will rise, causing the pilot to push somewhat more forward on the yoke. As this is happening he will be rolling the trim wheel forward to counter that pitch increase. Changes in airspeed are why we need to continually trim: or purchase autopilots!
The inboard tabs actually "fly" the two separate elevators. The control column is connected via cables to the inboard tabs, called 'Control' tabs by Douglas, such that when the PF pulls back on the column - the tab deflects downward, causing air pressure to lift the trailing edge of the elevator upward, causing an "up elevator" effect to occur. Tail goes down: nose goes up.
There is also a separate system, whereby – if the yoke is pushed forward enough that the tab moves upward without corresponding down elevator, both elevators are pushed down via two hydraulic rams - as in a case where a stall is imminent.
This last note is somewhat taken from a post by "F4E" in the PPRune forums. The outboard tabs are anti-float tabs, and are also geared to the horizontal stabilizer movement: they prevent 'down float' of the elevators when the horizontal stabilizer position is greater than 10 degrees nose up. i.e. in high nose up stab trim settings, the nose of the stab is pointing downward, and blanks the airflow over the underside of the elevator, causing the elevator to 'float' downward. The anti-float tab prevents this by changing the local aerodynamic pressure, causing it to fly the elevator to a neutral position relative to horizontal stab. The other four tabs are already attempting to fly the elevator up, but the lack of pressure under the elevator requires an extra tab pulled in to reinforce their effort.
.. and that's the tale of the MD-80 tail!
Re the first question:-
Read through this, and see if you're doing something different:-http://www.flythemaddog.com/forum/index.ph...ost&id=4016
... and, as an afterthought - here's a fearsome accident that came about as a result of a jackscrew maintenance problem:-