A Q on horizontal turns

[Ramapriya]

During a cruise, just pressing on one of the rudders should make an
aircraft turn horizontally in that direction, when no ailerons are
used. Reducing power in the opposite engine should logically accelerate
such a turn.

Makes me wonder why then was it is that, in the crash at the end of
2001 just outside of NY, the A300's rear bulkhead came loose when the
pilot used the rudder to correct a yaw induced by probably a wake
vortex ?

Ramapriya

[Peter Duniho]

"Ramapriya" > wrote in message
oups.com...
> During a cruise, just pressing on one of the rudders should make an
> aircraft turn horizontally in that direction, when no ailerons are
> used. Reducing power in the opposite engine should logically accelerate
> such a turn.

With respect to rudder input: rudder is used to coordinate a turn. You can
turn a plane with rudder alone, but not nearly as efficiently or quickly as
when you bank with aileron and use the rudder to keep the turn coordinated.
Rudder control input creates a force that is then transmitted to the
airframe, which then reacts according to various other forces at play, along
with the inertia of the airframe, to create a change in yaw (one hopes).

> Makes me wonder why then was it is that, in the crash at the end of
> 2001 just outside of NY, the A300's rear bulkhead came loose when the
> pilot used the rudder to correct a yaw induced by probably a wake
> vortex ?

The accident you're talking about, the vertical stabilizer itself came off.
It came off because the aerodynamic force applied to it exceeded the
strength of the structure. That force exceeded the strength because the
rudder was fully deflected back and forth multiple times, to the point where
the yaw angle was so great that an opposite rudder deflection exerted too
much force on the structure.

The accident has little to do with how one turns an airplane, whether or not
you accomplish the turn solely with the rudder.

If you are asking "why did the rudder input simply not just turn the
airplane?" the answer is simple: the first rudder input(s) did. But at some
point, the yaw angle was so great and/or the rate of yaw was so great in one
direction, that a full opposite rudder deflection created so much force
relative to the inertia of the airplane, that the vertical stabilizer
separated, rather than being able to transmit that force to the rest of the
airframe (and thus causing the commanded yaw).

Same thing could potentially happen in a boat, by the way. Any aerodynamic
or hydrodynamic control surface, whether it's the rudder for an airplane, a
boat, or something else, has to create a force that is then applied to the
vehicle structure. Even if the vehicle had almost no inertia (i.e. mass),
there would be a force large enough to break the structure (though the
control surface may or may not be capable of creating such a large force).
As the vehicle's inertia goes up (and in an airliner, you can see inertia is
quite large indeed), the force itself required to break the structure
attaching the control surface to the vehicle actually goes down.

In the case of the Airbus accident, the control surface was capable, under
the right circumstances, of creating a force greater than the strenth of the
attachment structure, and because of the inertia of the airplane (again,
because of just the right circumstance) that force could not be relieved
before the structure broke.

If the vehicle structure cannot change its momentum quickly enough to
relieve an applied force that is greater than the structural strength before
it breaks the structure, the structure will break. Simple as that.

As for why the aircraft was not designed so that the structure was not
strong enough to withstand the strongest force the control surface could
create, that's a reasonable question. However, the general answer is that
aircraft design is a compromise. More than in any other kind of vehicle,
weight is a serious issue, and if a structure can be made weaker (and thus
lighter) without creating what someone feels is a serious safety issue, it
will be. In this particular accident, I'm sure that the question of who
knew that the airplane shouldn't have been flown the way it was flown, and
whether they told the right people, will be something that the lawyers argue
about for years to come. But the fact will always remain, if you make an
airplane strong enough that it simply cannot break under any circumstances,
it may be too heavy to get off the ground.

Pete

[Colin W Kingsbury]

"Ramapriya" > wrote in message
oups.com...
> During a cruise, just pressing on one of the rudders should make an
> aircraft turn horizontally in that direction, when no ailerons are
> used. Reducing power in the opposite engine should logically accelerate
> such a turn.

Langewiesche's book "Stick and Rudder" goes into a lot of detail on this.
The basic idea is that what makes a plane turn is a combination of banking
which induces a horizontal lift vector in the direction of the turn and the
rudder inducing a weathervaning effect into the relative wind. It's actually
a rather complicated thing. Butthe easiest way to think about it is, "the
rudder does not make the plane turn." It does, but not the way most people
think. In fact, you'll hear a lot of CFIs here argue that many private
pilots trained these days don't really understand the rudder or use it
properly. Langewiesche himself argued that the rudder was a design artifact
that would be unnecessary on a properly-designed airplane. If this
discussion is at all interesting to you, I recommend buying and reading the
book. It is written for the novice and is still in print.

-cwk.