md80 split horizontal stabilizer

[trot]

hi,

after an unsuccessful research regarding the topic, i was wondering if anybody could briefly explain the neccessity and benefits of the split stabilizer on dc-9 and md-80s.
why do they work as one in flight, but on the ground they split if deflected by winds.

thanks trot

[Black_Dawn]

Hi trot,

the MD80's elevators are not interconnected and they are servo-tab controlled.

There is no purpose the way they split on the ground; it's only due to the peculiarity of the design of the hydraulically powered augmentor system that provides elevator control for additional nose down capability in extreme, high angle of attack flight conditions.

B_D

[Old Smokey]

Independant Elevators Yes! Split Stabilizers, ABSOLUTELY NOT!!!

Everything that Black_Dawn says is spot on. In windy conditions, the only time that you can reliably expect the elevators to be faired (in the down position) is when the pilot does the 'push' test immediately prior to Takeoff.

Douglas was way ahead of everyone else.

Regards,

Old Smokey

[trot]

thanks for your replies,
but still not all clear.
may be i need to back up a few steps.
i assume that the stabilizer is electrical controled via a motor and a long torque tube. since you mentioned the elevator is moved solely by servo tabs, i guess that cables run from the control column to a pcu which then moves the servo tab. are there 2 pcu's, one for each elevator side? and is it, that because they are servo tab controlled, that the elevators just hang there on their hinges while on the ground, and can be moved by wind if it's strong enough?

cheers

[Old Smokey]

trot,

The stabilizer is controlled electrically, and operated by a primary and a secondary electric motor (115VAC) via a huge screw jack. No manual reversion.

The Elevator control tabs are entirely mechanical, operated by fly by wire, that is, a great big wire from the pilot's control columns direct to the control tab (the best type of fly by wire). No PCUs needed. And yes, the elevators (on the ground) move at the whim of the wind, but the control tabs are in a position appropriate to the control column position. No manual reversion needed, it's entirely manual. (So are the ailerons, and the rudder is not so bad at all in the manual mode).

How's that for 25 years memory of engineering systems? Now, if you really want to test me, ummmm, the unfeathering pressure for the DC3 (R1830) was, and still is 650 PSI.

Regards,

Old Smokey

[Capt Claret]

The elevator and stabiliser are two different control surfaces, and should not be confused with one another.

[trot]

thanks folks,

problem solved!!!

[F4F]

Yeah, the ol'beast has wind powered primary flight controls (but for the rudder, hydraulic in normal mode, but falls back to control tab in case of P loss or by a cockpit commanded lever).
"Black_Dawn", the elevator is not servo-tab controlled. The servo-tab (called geared tab on the MD's elevator) is only installed to relieve some of the required force applied to the control column. The elevator is controlled (well more or less, again dependent on the movement of the air around the craft) by the control tab, logical eh?
Even more interesting is the Anti-float tab that is deflected mechanically once the stabilizer reaches 10-12.5° ANU trim and serves also to assit the deflection of the elevator as it starts getting blanked off by the LE of the stabilizer...

Well, I guess this shows once more how the american industry manages over the years to transform a perfectly good (albeit too small) aeroplane such as the DC-9 series into an MD having probably one of the most appaling rate of roll and pitch of all flying craft :{

Two nice things about the MD: Quitest in the pointed end. And what a beautiful shape :ok:

[CV880]

The Boeing 707 and Convair 880 elevators were also tab operated (no PCU's) and could exhibit the same split on the ground dependant on wind effect as the DC9/MD80. The DC8 had the same basic control tab set up as the DC9 except that the elevator themselves were mechanically linked together and so moved in unison. The reason for linking them together on the DC8 was, as far as I could see, to provide a central point to install the old fashioned gust lock system Douglas had utilised.

[Shore Guy]

Ah, the C880/990 series aircraft.........what an incredible airplane for its time....

Great article on them in Airliners magazine some time back....

To my knowledge, article not available on the web (copied and send them to a couple of friends).

:) :) :)

[Black_Dawn]

F4F

you've got a point!

I think is just the way you name things ; for me, the control tab is synonymous of servo-tab and this is why I called it that way.

But to stay in the MD way, you are correct :D , also in my manual I found what you said.

Greetings

B_D

[CV880]

Shore Guy
Didn?t have a lot to do with the 990 but Swissair used to fly them into our airport in the 70?s and I turned around many flights. Also got to look at some a US charter company sent some over for major overhauls etc (ex American Airlines aircraft I think). I did spend 6-7 years maintaining CV880-22M?s (with leading edge slats) and thought it a great little ship with some innovative systems and a strong structure. The engine (basically a civilian J79) was also interesting compared to the agricultural JT3 in the B707/DC8.
Black Dawn
You are correct. The aircraft manufacturers seem to invent their own names for things just to confuse us.
A Douglas CONTROL tab and a Convair FLIGHT tab, to use the manufacturers names, are both servo tabs being directly connected to the control column, rudder pedals or control wheel. I can?t recall what Boeing called the servo tabs on the 707.
A Douglas GEARED tab as fitted to the elevators was, I think more correctly called a balance tab and was linked to the rear spar of the horizontal stabilizer. Movement of the elevator away from neutral resulted in the tab deflecting in the opposite direction to assist the elevator movement reducing pilot forces. The DC8, and as far as I am aware, the DC9/MD80, had a control (servo) tab and a geared (balance) tab on each elevator.