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Airplane turns



 
 
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  #1  
Old April 17th 04, 12:28 AM
Dave
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Default Airplane turns

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?
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  #2  
Old April 17th 04, 12:47 AM
Robert Moore
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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


  #3  
Old April 17th 04, 12:49 AM
Peter Duniho
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"Dave" wrote in message
...
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.


It depends on the airplane and the bank angle. Most airplanes have a narrow
range of bank angle at which the turn is stable, and the airplane will
continue indefinitely in the turn. At bank angles less than that, the
airplane will eventually return back to level flight. At bank angles
greater than that, the airplane will actually steepen the bank, which left
uncorrected would result in loss of control of the airplane.

The rates at which the bank changes does vary quite a bit from airplane to
airplane, as it depends on a variety of aerodynamic factors in the airplane
design. There are a few airplanes that won't return to level flight at all
without pilot input, as they've been designed as unstable airplanes (some
intentionally, some not).

Pete


  #4  
Old April 17th 04, 12:58 AM
Teacherjh
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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

--
(for Email, make the obvious changes in my address)
  #5  
Old April 17th 04, 01:29 AM
William W. Plummer
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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.



  #6  
Old April 17th 04, 02:45 AM
Teacherjh
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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)
  #7  
Old April 17th 04, 03:23 AM
Greg Esres
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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.


  #8  
Old April 17th 04, 03:59 AM
Peter Gottlieb
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"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?


  #9  
Old April 17th 04, 04:06 AM
Roger Halstead
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Default

On Fri, 16 Apr 2004 16:49:38 -0700, "Peter Duniho"
wrote:

"Dave" wrote in message
.. .
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.


It depends on the airplane and the bank angle. Most airplanes have a narrow
range of bank angle at which the turn is stable, and the airplane will
continue indefinitely in the turn. At bank angles less than that, the
airplane will eventually return back to level flight. At bank angles
greater than that, the airplane will actually steepen the bank, which left
uncorrected would result in loss of control of the airplane.

The rates at which the bank changes does vary quite a bit from airplane to
airplane, as it depends on a variety of aerodynamic factors in the airplane
design. There are a few airplanes that won't return to level flight at all
without pilot input, as they've been designed as unstable airplanes (some
intentionally, some not).


When I was down to Bonanza specific recurrency training (with
characteristics a bit different than the trainers) The one instructor
had me trim (elevator only) in steep turns as an experiment. I put it
right at 60 degrees of bank and gently eased in the trim (and power).
We were holding a tad over 120 MPH, hands off and made two complete
circuits (a 720). In two complete circles the plane had displayed no
tendency to change bank angle or altitude. The altitude varied less
than 20 feet in the check. (better than I can do without lots of
practice)

The Debonair/Bonanzas do have a lot of dihedral

I would say it's probably better behaved in a steep turn like that
than one of only 30 degrees, but OTOH it's been some times since I
took part in that experiment.

I think when I get the Deb back after annual I'm going to take it out
and spend a few hours just practicing. That and I'd like to get it
down to P-ville next month if at all possible.

What with the broken throttle cable and now being out of annual I'm
starting to suffer withdrawal.

I have a couple of guys who really want to buy it, but I keep having
thoughts about keeping it for years to come. It may have been built
in 59 but the airframe only has about 4,000 TT, or a tad less. (time
to go check the logs)

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Pete


  #10  
Old April 17th 04, 04:45 AM
Greg Esres
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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.

 




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