A very basic question

On Sat, 6 Nov 2004 10:41:12 -0800, "Peter Duniho"
wrote:

"Ramapriya" wrote in message
. com...
Unlike the elevators and rudder that change an aircraft's pitch and
yaw with no other secondary effect, why does the banking of wings by
the use of ailerons not just roll an aircraft but also produces a turn
(yaw)?

The simple answer is that, theoretically, the ailerons act exactly as you
would think. That is, a turn is not caused by a change in bank.

In level flight, the wings are generating 1g of lift, equivalent to
the weight of the aircraft and all occupants inside. If this lift
vector is rotated by the ailerons then it will point in the direction
of the rotation, and therefore force the aircraft to change its
direction of flight, and therefore to turn.

And there will a corresponding loss of lift against gravity; all
simply calculated by geometric functions of sine and cosine. So the
aircraft will begin to descend, as it turns.


A more complicated answer is that since the "center of lift" is ahead of the
"center of gravity", having the lift vector tilted to one side or the other
by bank does pull the nose of the airplane around a bit, inducing a turn.

If the center of lift actually was ahead of the center of gravity then
the aircraft would loop nose-up, so it isn't. They are aligned. But
it is the acceleration in the direction of the rotated lift vector
which changes the direction of the airflow around the aircraft. So
the airfoils at the tail force the airplanes nose to point into the
direction of the changing wind.

This also changes the direction of the lift vector to the new
location, which is actually the same location, and it is known as the
center point of the circle the airplane is drawing out in 3-d space.
The circle is actually the bottom of a cone, with the cone drawn by
the lift vector of the aircraft. The tighter the turn then the
flatter the cone. If there is no turn then the cone is not a cone but
a flat plane instead.

In other words, the aircraft in a turn is flying in a circle, instead
of just accelerating sideways and retaining its former forward
velocity, which it does not do. The changing wind over the airfoils
rotate the aircraft into flying into a circle.


An even more complicated answer points out that the ailerons themselves
create increased drag on the raised wing and reduced drag on the lowered
wing, which creates a yaw opposite in direction to the intended turn.

More or less. A lowered aileron has the increased drag, while a
raised aileron has less drag. This will pull the nose around opposite
from the direction of expected bank.
Adverse yaw is the ailerons acting in place of the rudder, and it
prevents the aircraft from lining perfectly into the wind.

But once the aircraft is banked then the aircraft will turn. The
aircraft turns because it is banked.

A banked aircraft will not turn if, and only if, the wing is not
generating lift. A wing will not generate lift if its angle of attack
is so controlled by the horizontal stabilizer.

One other note, the aircraft will lose lift and so descend as it banks
into a turn. But as it descends, the wings will regain upward airflow
and restore the lift lossed. This stops the downward acceleration,
with the airplane having reached its terminal velocity. But the lift,
and the loads on the wing, have increased just from the aircraft going
into a bank; even if adjustments have not been made for level flight.

(I think this is ~correct. Pretty sure.)

--Mike

"Mike Rhodes" wrote in message
...
[...signed...]
(I think this is ~correct. Pretty sure.)


You ought to *know* before you post, I guess.

With respect to your specific comments:

The simple answer is that, theoretically, the ailerons act exactly as you
would think. That is, a turn is not caused by a change in bank.

In level flight, the wings are generating 1g of lift, equivalent to
the weight of the aircraft and all occupants inside. If this lift
vector is rotated by the ailerons then it will point in the direction
of the rotation, and therefore force the aircraft to change its
direction of flight, and therefore to turn.

Wrong. In the theoretical case I describe (which isn't the reality case
anyway), banking would simply cause the airplane to sideslip sideways,
without any turn occurring.

The "1g of lift" stuff is irrelevant, except inasmuch as there IS lift (a
force) that is redirected sideways.

A more complicated answer is that since the "center of lift" is ahead of
the "center of gravity", having the lift vector tilted to one side or
the other
by bank does pull the nose of the airplane around a bit, inducing a turn.

If the center of lift actually was ahead of the center of gravity then
the aircraft would loop nose-up, so it isn't. They are aligned.

Wrong, again. The center of lift is actually behind the center of gravity
(I screwed up in my original post). The horizontal stabilizer balances out
the difference in force to prevent the nose from dropping as a result of the
difference.

To revist my original post, the correct statement would have been "since the
'center of lift' is behind the 'center of gravity', having the lift vector
tilted to one side or the other by bank does pull the nose of the airplane
around a bit, inducing a turn *opposite to that intended*."

I apologize for resulting confusion, but the fact remains that your
statement is entirely incorrect.

[...]
An even more complicated answer points out that the ailerons themselves
create increased drag on the raised wing and reduced drag on the lowered
wing, which creates a yaw opposite in direction to the intended turn.

More or less. A lowered aileron has the increased drag, while a
raised aileron has less drag. This will pull the nose around opposite
from the direction of expected bank.

Heh...one of the few things you get right, and it's exactly what I wrote.

Adverse yaw is the ailerons acting in place of the rudder, and it
prevents the aircraft from lining perfectly into the wind.

"In place of"? Uh, okay...I guess you could say it that way.

But once the aircraft is banked then the aircraft will turn. The
aircraft turns because it is banked.

No, it does not. Any turn as a result of bank is actually due to other
design features of the airplane, such as dihedral and a vertical stabilizer.

Pete

On Sat, 06 Nov 2004 22:22:22 GMT, Mike Rhodes
wrote:


A banked aircraft will not turn if, and only if, the wing is not
generating lift. A wing will not generate lift if its angle of attack
is so controlled by the horizontal stabilizer.

I was not quite right with the "if and only if". Of course the rudder
can also stop the turn, as in a side-slip. And the side-slip Peter
mentioned is what pushes the nose around in the turn by its push on
vertical stabilizer. I did not point directly at the vert stabilizer
as Peter did in his reply.

Because the banked aircraft is aligned less with gravity, it would
then want to accelerate 'up', as 'up' is relative to the aircraft.
But this would immediately change the angle-of-attack of the both the
wing and the horizontal stab. So the wing loses some lift, while the
horizontal stab increases its already downward push. This would tend
to push the nose 'up', and restore the angle of attack of the wing.

The turn is a relatively slow process (the pilot has time to make
adjustments), and maybe the mechanics are not so simple as I think my
post implied.

--Mike

On Mon, 08 Nov 2004 19:30:24 GMT, Mike Rhodes
wrote:

On Sat, 06 Nov 2004 22:22:22 GMT, Mike Rhodes
wrote:



Because the banked aircraft is aligned less with gravity, it would
then want to accelerate 'up', as 'up' is relative to the aircraft.
But this would immediately change the angle-of-attack of the both the
wing and the horizontal stab. So the wing loses some lift, while the
horizontal stab increases its already downward push. This would tend
to push the nose 'up', and restore the angle of attack of the wing.


Oops. I got this wrong. If both wing and horizontal stab are pushed
down then the net effect is no change in angle of attack.

--Mike