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

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"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".