Airplane turns

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)

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

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

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

First, understand that for an airplane to right itself from a bank,
there must be a moment that rotates the aircraft around the
longitudinal axis. Therefore, the total lift on one wing must be
greater than the other. This extra force, times the moment arm,
creates the necessary moment.

The orientation of the wing lift to gravity is irrelevant.

When the aircraft sideslips in one direction or the other, there is a
component of the relative wind that moves parallel to the lateral axis
of the airplane. Due to dihedral, parallel to the lateral axis is not
parallel to the wing. When you resolve the vectors, you can see that
the sideslip creates a component of the relative wind perpendicular to
the surface of the wing, which increases its angle of attack. The
reverse happens on the other wing.

Most aerodynamics books contain detailed vector diagrams of this. The
FAA's material get this wrong, as does most aviation training
material. The point is, all dihedral effects depend on sideslip. In
theory, an aircraft in a coordinated turn should experience no
righting tendencies.

Exactly right, dihedral has no effect in a coordinated turn.

On 2004-04-17 10:37:18 -0400, Greg Esres said:

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

First, understand that for an airplane to right itself from a bank,
there must be a moment that rotates the aircraft around the
longitudinal axis. Therefore, the total lift on one wing must be
greater than the other. This extra force, times the moment arm,
creates the necessary moment.

The orientation of the wing lift to gravity is irrelevant.

When the aircraft sideslips in one direction or the other, there is a
component of the relative wind that moves parallel to the lateral axis
of the airplane. Due to dihedral, parallel to the lateral axis is not
parallel to the wing. When you resolve the vectors, you can see that
the sideslip creates a component of the relative wind perpendicular to
the surface of the wing, which increases its angle of attack. The
reverse happens on the other wing.

Most aerodynamics books contain detailed vector diagrams of this. The
FAA's material get this wrong, as does most aviation training
material. The point is, all dihedral effects depend on sideslip. In
theory, an aircraft in a coordinated turn should experience no
righting tendencies.

First, understand that for an airplane to right itself from a bank,
there must be a moment that rotates the aircraft around the
longitudinal axis. Therefore, the total lift on one wing must be
greater than the other. This extra force, times the moment arm,
creates the necessary moment.
The orientation of the wing lift to gravity is irrelevant.


Gee, now that you mention it, you're right. gobsmacked


When the aircraft sideslips in one direction or the other, there is a
component of the relative wind that moves parallel to the lateral axis
of the airplane. Due to dihedral, parallel to the lateral axis is not
parallel to the wing. When you resolve the vectors, you can see that
the sideslip creates a component of the relative wind perpendicular to
the surface of the wing, which increases its angle of attack. The
reverse happens on the other wing.


So the high wing is sort of "blown back down" by the sideways component of the
relative wind. Makes sense to me. (the raised wing has a lower angle of
attack, the lowered wing has a higher angle of attack).

Never thought of it all the way through that way. Thanks.

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).

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.


The heading does not change. The force is pushing the aircraft sideways.

Well, ok, the vertical tail will encounter some (sideways) resistance, causing
the plane to weathervane a bit into the wind and changing the heading too, but
that effect is small for small banks.


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.


The latter, ultimately. And yes, the plane is then "climbing" sideways. Part
of the reason for the heading change you would fight is the tail, which
produces down lift, keeping the nose up against gravity. When banked, this
results in a turn. But then less is available to keep the nose up, which is
why you apply back pressure on the elevators.

So all these things are interdependent, and become more so as the amount of
bank is increased.

Taken to the extreme (an aerobatics pilot might chime in here), if the wings
are vertical (a knife edge) the only thing keeping the airplane up is the
(once) vertical stabilizer and the fuselage. The (once) horizontal tail will
want to keep the nose "up", which is sideways, and the plane will want to turn
(do a loop horizontally). Ignoring other effects, of course.

Jose

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

Peter Gottlieb wrote:

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.

In my aircraft, the nose would swing in the opposite direction to the bank. The plane
would continue to fly the original heading. Sideways.

George Patterson
This marriage is off to a shaky start. The groom just asked the band to
play "Your cheatin' heart", and the bride just requested "Don't come home
a'drinkin' with lovin' on your mind".

On Sat, 17 Apr 2004 at 14:53:33 in message
, Teacherjh
wrote:
So the high wing is sort of "blown back down" by the sideways component of the
relative wind. Makes sense to me. (the raised wing has a lower angle of
attack, the lowered wing has a higher angle of attack).

Don't think too much in terms of raising and lowering as it is a
sideslip effect and can occur at any attitude. The sideslip can be
generated by the rudder. This is why it is surprisingly easy to fly an
RC model with rudder and elevators only. Plenty of dihedral and a
powerful rudder and the slip generated by the rudder will create a roll
and you fly around almost forgetting you have no ailerons. There are
some things you cannot do tidily though. Not easy to do an axial role
for example.
--
David CL Francis