Airplane turns

Dave wrote
I'm curious as to how an airplane behaves during turns. It is my
understanding that once you put the plane into a turn and return the
yoke to a level position, the plane will stay in a turn but will
straighten itself out slowly. Does the rate at which the airplane
straightens out vary by model? For instance, a Cessna 152 vs. a
Boeing 747? Does it vary among jetliners, i.e. a 757 vs. a 777?

From the FAA Flight Training Handbook:

TURNS
A turn is a basic flight maneuver used to change or return to a
desired heading. It involves close coordination of all three flight
controls: aileron, rudder, and elevator. Since turns are a part
of most other flight maneuvers, it is important to thoroughly
understand the factors involved.

For purposes of this discussion, turns are divided into three
classes: shallow turns, medium turns, and steep turns.

• Shallow turns are those in which the bank (less than approximately
20°) is so shallow that the inherent lateral stability of the airplane
is acting to level the wings unless some aileron is applied to maintain
the bank.

• Medium turns are those resulting from a degree of bank (approximately
20° to 45°) at which the airplane remains at a constant bank.

• Steep turns are those resulting from a degree of bank (45° or more)
at which the “overbanking tendency” of an airplane overcomes stability,
and the bank increases unless aileron is applied to prevent it.

Bob Moore
Flight Instructor

To add to this answer (about how an airplane behavs during turns), most
airplanes are designed with dihedral. This means that the wings point up a
little bit. If you looked at a plane from in front of the nose, the wings will
form a slight V shape. This makes the "lift" that each wing produces point a
little bit inward, towards the center of the plane, rather than straight up.
Now, if the plane goes into a shallow bank, the wing that is lowered will
become more level, and the lift will point straight up, while the wing that is
raised will become more tilted, and the lift will point more towards the center
of the plane. More of the lift on this "tilted" wing is "wasted" (in the sense
of not holding the airplane up). So, since the other wing exerts more upwards
force, it causes the plane to return to level flight. This is one of the
things that makes an airplane inherently stable in flight.

Separate from this, when an airplane is banked in coordinated flight and
turning, the outer wing (which is the one that is raised to bank the plane to
make the turn) is actually travelling faster than the inner wing. It has to,
because that wing is further from the center of the turn. (to see this,
imagine the plane turning so sharply that it's just about pivoting on one
wingtip) The faster wing will produce more lift, and cause the airplane to
bank in the direction of the turn. This is called "overbanking tendency".

So, there are two opposite tendencies. Dihedral is more important with shallow
banks (and gentle turns), and the overbanking tendency is more important with
steeper turns and banks. Somewhere in the middle, they cancel out.

Jose

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"Teacherjh" wrote in message
...
To add to this answer (about how an airplane behavs during turns), most
airplanes are designed with dihedral. This means that the wings point up
a
little bit. If you looked at a plane from in front of the nose, the wings
will
form a slight V shape. This makes the "lift" that each wing produces
point a
little bit inward, towards the center of the plane, rather than straight
up.
Now, if the plane goes into a shallow bank, the wing that is lowered will
become more level, and the lift will point straight up, while the wing
that is
raised will become more tilted, and the lift will point more towards the
center
of the plane. More of the lift on this "tilted" wing is "wasted" (in the
sense
of not holding the airplane up). So, since the other wing exerts more
upwards
force, it causes the plane to return to level flight. This is one of the
things that makes an airplane inherently stable in flight.

Separate from this, when an airplane is banked in coordinated flight and
turning, the outer wing (which is the one that is raised to bank the plane
to
make the turn) is actually travelling faster than the inner wing. It has
to,
because that wing is further from the center of the turn. (to see this,
imagine the plane turning so sharply that it's just about pivoting on one
wingtip) The faster wing will produce more lift, and cause the airplane
to
bank in the direction of the turn. This is called "overbanking tendency".

So, there are two opposite tendencies. Dihedral is more important with
shallow
banks (and gentle turns), and the overbanking tendency is more important
with
steeper turns and banks. Somewhere in the middle, they cancel out.

Looking at the nose of the plane, we see the V-shape of the wings, the
dihedral angle. Lift is always perpendicular to the wings. So in flight,
the two lift vectors tilt in and "cross" over the plane itself. In level
flight the horizontal components of the two lift vectors are equal and
opposite. Thus, they cancel and the plane flies straight. Now, if you
bank the plane so that one wing is horizontal, that wing will have no
horizonal lift vector component. But, the other wing will have double. The
result is a big, net horizontal force on the plane. This forces the plane
to the center of the turn the way the force in a string swinging a rock
keeps the rock in a circle. That's what makes a plane able to go in a
circle, not the rudder.

Now, if you
bank the plane so that one wing is horizontal, that wing will have no
horizonal lift vector component. But, the other wing will have double. The
result is a big, net horizontal force on the plane. This forces the plane
to the center of the turn the way the force in a string swinging a rock
keeps the rock in a circle. That's what makes a plane able to go in a
circle, not the rudder.


Not quite. This does not change the direction of the nose, which is essential
for turning. By itself, what you post would lead to a slip. The plane would
travel in a straight line, at some angle to the nose.

Jose

--
(for Email, make the obvious changes in my address)

"Teacherjh" wrote in message
...

Now, if you
bank the plane so that one wing is horizontal, that wing will have no
horizonal lift vector component. But, the other wing will have double.
The
result is a big, net horizontal force on the plane. This forces the plane
to the center of the turn the way the force in a string swinging a rock
keeps the rock in a circle. That's what makes a plane able to go in a
circle, not the rudder.


Not quite. This does not change the direction of the nose, which is
essential
for turning. By itself, what you post would lead to a slip. The plane
would
travel in a straight line, at some angle to the nose.


I guess this confuses me a bit. Maybe I'm tired and my brain confuses
easily.

Isn't it the case, in an earth-centric reference frame, that an object
moving in a straight line, when subjected to a horizontal force
perpendicular to that motion, will move in a circle?

when subjected to a horizontal force perpendicular to that motion,
will move in a circle?

In a sideslip, lift is perpendicular to the longitudinal axis of the
aircaft, not the direction of motion.

Isn't it the case, in an earth-centric reference frame, that an object
moving in a straight line, when subjected to a horizontal force
perpendicular to that motion, will move in a circle?


If the force is in fact perpendicular to the MOTION (the course) and not to the
HEADING. In the case of simply banking, the nose does not move (so the heading
remains the same) while the course changes (due to the force you are talking
about). But so long as the nose does not change direction, the force will not
change direction either and the plane will simply continue in a straight line,
in a slip.

(Actually, if you simply bank the plane (with alerons), the nose will tend to
move in the =opposite= direction, because of the drag induced by the alerons.
If your plane uses spoilers, the reverse is true)

Jose

--
(for Email, make the obvious changes in my address)

"Teacherjh" wrote in message
...

Isn't it the case, in an earth-centric reference frame, that an object
moving in a straight line, when subjected to a horizontal force
perpendicular to that motion, will move in a circle?


If the force is in fact perpendicular to the MOTION (the course) and not
to the
HEADING. In the case of simply banking, the nose does not move (so the
heading
remains the same) while the course changes (due to the force you are
talking
about). But so long as the nose does not change direction, the force will
not
change direction either and the plane will simply continue in a straight
line,
in a slip.


I'm having a hard time wrapping my brain around this. Maybe the different
reference frames are confusing me.

Let's say I put the plane into a 30 degree bank with ailerons yet maintain
neutral rudder. To simplify things let's say I do this instantaneously.
Right at that moment, the nose is pointing in the original heading. But
what happens as this fairly large (earth referenced) horizontal lift force
acts on the plane; clearly it moves it sideways. But what happens to the
heading, that is messing me up. If the force continues, either it results
in an acceleration which causes increasing velocity or it reaches a terminal
velocity where the force is balanced by drag. Does the slip result in such
large drag so quickly that the net result is similar to a crosswind, or does
the plane act in the medium it is flying in (of course ignoring the earth
below) and have the nose indeed change heading because the plane is now
"climbing" sideways? I know from experience I need a lot of *opposite*
rudder to counteract a 30 degree bank and keep the nose on the same heading
(as in crosswind landings).

Now, if the plane goes into a shallow bank, the wing that is lowered
will become more level,


This is NOT how dihedral works.

Dihedral depends on sideslip. No sideslip, no stabilizing force.

Now, if the plane goes into a shallow bank, the wing that is lowered
will become more level,

This is NOT how dihedral works.
Dihedral depends on sideslip. No sideslip, no stabilizing force.


Huh? I don't believe that is true, but explan further.

Jose

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(for Email, make the obvious changes in my address)