A very basic question

"Ramapriya" wrote in message
om...
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.

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.

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.

In reality, the ailerons and rudder are BOTH very necessary to accomplish an
efficient turn. Either can be used by themselves to change aircraft
heading, but neither is very effective alone in most airplanes.

As far as the elevator and rudder having "no other secondary effect", that's
not true. Pretty much every control on an airplane has a secondary effect.
Use of rudder will induce roll, for example, while use of the elevator can
induce yaw (mostly due to propeller effects).

[...]
Is there a website you know of that can teach me such basics, without
having to bug you?

There are many. The one already provided by Stan's reply is one of my
favorites. There are also several good books on the topic, including the
FAA's own flight training manuals (available for download from their web
site somewhere, but I don't have a link handy) and a book called
"Aerodynamics for Naval Aviators".

Pete

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

"Peter Duniho" wrote in message ...

There are many. The one already provided by Stan's reply is one of my
favorites. There are also several good books on the topic, including the
FAA's own flight training manuals (available for download from their web
site somewhere, but I don't have a link handy) and a book called
"Aerodynamics for Naval Aviators".

Pete

I love jsd's site too, and have condensed all those chapters into two
Word files (in case someone is interested!). But I wish Denker had
also written the stuff for a non-aviator like me in mind. For example,
he's written loads on trim but till this day, I don't know what
exactly trimming is and how it physically works

Ramapriya

"Ramapriya" wrote in message
om...
I love jsd's site too, and have condensed all those chapters into two
Word files (in case someone is interested!). But I wish Denker had
also written the stuff for a non-aviator like me in mind. For example,
he's written loads on trim but till this day, I don't know what
exactly trimming is and how it physically works

Well, trimming isn't complicated. But there are numerous methods for
actually *accomplishing* it, so perhaps that's why Denker sort of just
assumes you're familiar with the concept and doesn't get into how it
"physically works".

That is, the basic concept is simple: "trimming" simply means to set a
particular control (the "trim"...generally you may have elevator, rudder, or
aileron trim or any combination of the three, though I don't doubt there's
at least one unusual aircraft out there that has yet another possibility I'm
not aware of) so that instead of the pilot having to hold a particular
control input, the "trim" holds it for him.

So, for elevator trim (the most common type), once the pilot has selected a
pitch attitude for a climb (for example), along with the desired power
setting (often full power), he can then set the elevator trim to hold the
elevator control input at that particular pitch attitude.

It gets complicated when you start talking about each specific trim
mechanism, since they all have subtle differences in exactly how they
accomplish that "hold the control input" action, as well as effects of power
or airspeed changes on the effect of the trim.

As an example, look at elevator trim:

Generally speaking, elevator trim can be thought of as "setting" an
airspeed, since for a given power setting, airspeed varies precisely with
pitch attitude. A further generalization is that changes in power do NOT
actually change the "trimmed airspeed". That is, for a given trim setting,
increasing power will result in the nose pitching up and decreasing power
will result in the nose pitching down, with the airspeed remaining constant
in both cases.

For some aircraft, the airspeed literally remains constant. For others, you
will actually see slight variations in airspeed. But regardless, any
variations are almost always so small that you can still think of setting
airspeed rather than a specific trim setting. Since trim doesn't really set
a pitch attitude either (except for a given power setting), it's not like
there's really a more useful paradigm to use.

Just as an example of one elevator trim mechanism: in most of the
single-engine Cessnas (and maybe their piston twins, but I don't know those
airplanes well enough to say), elevator trim is accomplished through the use
of a moveable "trim tab" mounted on the trailing edge of the elevator.
There's a wheel in the cockpit that moves this trim tab up and down. When
the trim tab moves up, it exerts a downward force on the elevator and when
the trim tab moves down, it exerts an upward force on the elevator
(aerodynamically, in the exact same way that the elevator itself changes the
horizontal stabilizer's up or down force). The trim tab is relatively
small, so that by pushing down on the elevator, the net effect is to cause
the horizontal stabilizer to create an upward force (nose down pitch), just
as if the pilot had pushed forward on the yoke to deflect the elevator
downward.

Anyway, other aviation references will provide much more detailed
information on that sort of thing. "Stick and Rudder" will give you a good
pilot's view of things, while the "Aerodynamics for Naval Aviators" delves
more deeply into the actual mechanics of flight (naturally).

Pete