T Tail question

Something I saw raised a question in my mind: An MD-80's T-tail had
one half pitched up and the other half pitched about neutral. My
question is why any T-tail aircraft would allopw differential motion
of the two halfs of the horizontal stab? It seems to me that the
rolling couple such a position would impose in flight would act as a
tremendous shearing force on the fin.

I'm no kind of structural or aero engineer but does any military
aircraft with a T tail support differential movement of the two halves
of the horizontal stabilizer?

"Paul Austin"

wrote:Something I saw raised a question in my mind: An MD-80's T-tail had
one half pitched up and the other half pitched about neutral.

This is a situation that is specific to the DC-9/MD-80 series airplanes. These
aircraft have elevators that are not connected to the control yoke in the
cockpit. Rather, the yoke drives the trailing edge tabs only, and thus the
elevators follow the "trim force" from the trailing edge tabs. Pitch trim, in
the usual sense, is attained by a trimable horizontal stabilizer. During
pretakeoff checks, the pilots will push forward on the control yoke to activate
the "stall recovery" (for lack of a better word at this moment) actuator; this
actuator will force the elevators down in unison, to produce a nose down
control input should a "stall recovery" be commanded by the aircrew. Just prior
to takeoff, the elevators appear in "harmony" to be connected together.

Therefore, seeing a split elevator on DC-9/MD-80 aircraft, while on the ground,
is normal, though visually disconcerting.

VL

After an exhausting session with Victoria's Secret Police, "Paul
Austin" blurted out:

Something I saw raised a question in my mind: An MD-80's T-tail had
one half pitched up and the other half pitched about neutral. My
question is why any T-tail aircraft would allopw differential motion
of the two halfs of the horizontal stab? It seems to me that the
rolling couple such a position would impose in flight would act as a
tremendous shearing force on the fin.

I've jettisoned all my DC-9 and MD-80 manuals, but look at this
link... http://142.26.194.131/aerodynamics1/controls/Page2.html

Scroll down to the part about Tab Controls, that's how DC-9s' and
MD-80s' elevators work. That "split" elevator thing you saw goes away.
It always makes me pause and think, "WTF were those engineers
thinking?"

Now the Boeing 757...there's a tits machine!

Juvat

(MLenoch) wrote:

"Paul Austin"

wrote:Something I saw raised a question in my mind: An MD-80's T-tail had
one half pitched up and the other half pitched about neutral.

This is a situation that is specific to the DC-9/MD-80 series airplanes. These
aircraft have elevators that are not connected to the control yoke in the
cockpit. Rather, the yoke drives the trailing edge tabs only, and thus the
elevators follow the "trim force" from the trailing edge tabs. Pitch trim, in
the usual sense, is attained by a trimable horizontal stabilizer. During
pretakeoff checks, the pilots will push forward on the control yoke to activate
the "stall recovery" (for lack of a better word at this moment) actuator; this
actuator will force the elevators down in unison, to produce a nose down
control input should a "stall recovery" be commanded by the aircrew. Just prior
to takeoff, the elevators appear in "harmony" to be connected together.

Therefore, seeing a split elevator on DC-9/MD-80 aircraft, while on the ground,
is normal, though visually disconcerting.

VL

That system was also used on all flight controls on the Canadair
Argus...most disconcerting to 'other type' crews (especially
civil airliners) following an Argus to see BOTH ailerons
deflected up, and then the elevators moving independently of each
other and the ailerons hanging down when the gust locks are
selected off.

I've heard literally dozens of comments like... "..ahh 739 from
Speedbird 665, be aware that your ailerons BOTH appear to be
deflected UP"..."Thank you Speedbird, that's normal for this
type".
--

-Gord.

"MLenoch" wrote
"Paul Austin"

wrote:Something I saw raised a question in my mind: An MD-80's
T-tail had
one half pitched up and the other half pitched about neutral.

This is a situation that is specific to the DC-9/MD-80 series
airplanes. These
aircraft have elevators that are not connected to the control yoke
in the
cockpit. Rather, the yoke drives the trailing edge tabs only, and
thus the
elevators follow the "trim force" from the trailing edge tabs.
Pitch trim, in
the usual sense, is attained by a trimable horizontal stabilizer.
During
pretakeoff checks, the pilots will push forward on the control yoke
to activate
the "stall recovery" (for lack of a better word at this moment)
actuator; this
actuator will force the elevators down in unison, to produce a nose
down
control input should a "stall recovery" be commanded by the aircrew.
Just prior
to takeoff, the elevators appear in "harmony" to be connected
together.

Therefore, seeing a split elevator on DC-9/MD-80 aircraft, while on
the ground,
is normal, though visually disconcerting.

Thanks for the explanation.

The Argus could turn on a dime, too, Gord. What a ride, especially with those
four big bangers at full throttle!
Joe F

I've heard literally dozens of comments like... "..ahh 739 from
Speedbird 665, be aware that your ailerons BOTH appear to be
deflected UP"..."Thank you Speedbird, that's normal for this
type".

MLenoch wrote in message
...

Therefore, seeing a split elevator on DC-9/MD-80 aircraft, while on
the ground,
is normal, though visually disconcerting.

VL

I worked my way through college refueling airliners and this is correct.
The DC9/MD-80 series often show "split" tail feathers while on the
ground.

--

Scott
--------
"the Arabs should remember that they invaded and occupied important
parts of Europe hundreds of years before the Crusades wars. "
Zuheir Abdallah-columnist for the London-based Arabic daily Al-Hayat
http://www.memri.org/bin/latestnews.cgi?ID=SD55103

Isn't this the Douglas type that has a screwjack milling out the teeth in
the elevator actuator nut so the crew eventually loses all pitch control as
happened off the California coast?

WDA

end


"Paul Austin" wrote in message
...
Something I saw raised a question in my mind: An MD-80's T-tail had
one half pitched up and the other half pitched about neutral. My
question is why any T-tail aircraft would allopw differential motion
of the two halfs of the horizontal stab? It seems to me that the
rolling couple such a position would impose in flight would act as a
tremendous shearing force on the fin.

I'm no kind of structural or aero engineer but does any military
aircraft with a T tail support differential movement of the two halves
of the horizontal stabilizer?