Mounting a turn coordinator on the tail?

I've never quite bought in to the "horizontal component of lift"
explanation for what causes a plane to turn.

I started this discussion talking about a "wings level skidding turn".
There's no horizontal component of lift generated by the wings if the
wings are level. There is however a couple comprising of the
rightwards force from the rudder, and the induced leftwards force of
wind resistance acting further forward along the fuselage. It's the
couple which causes the plane to turn.

I can also generate a substantial amount of horizontal force from the
wings without the plane turning (think forward slip); it just moves
sideways through the air.

I believe what causes a plane to turn is the couple comprised of
opposing forces which aren't aligned.

For example, in a coordinated left turn, the wings generate a
horizontal force to the left. The tail generates a horizontal force
to the right. The forces aren't aligned, so a couple is generated,
and the airplane turns left.

Without a force to the right, the "horizontal component of lift" to
the left generated by the banked wings would merely cause the plane to
move left -- it wouldn't turn.

Just my $0.10 worth, and I'll happily listen to contradictory
arguments.

Tim.

On Sat, 29 Jul 2006 20:52:00 -0400, Stubby
wrote:

What causes a plane to turn is the horizontal component of the lift
vector. It certainly does not depend on the turn coordinator.
What counts is the center of gravity of the plane, not the tail.



Tim Auckland wrote:
Stubby,

Thanks, I think I've got it now.

As the turn is initiated, the tail has to go right (this is what
causes the plane to turn) and my guess is that the ball initially goes
very slightly left.

However, once a constant-radius turn has been achieved, the ball will
be on the right (outside) of the turn.

Tim.

On Sat, 29 Jul 2006 18:28:28 -0400, Stubby
wrote:

I don't think so. My understanding is the ball rests in a slightly
curved tube, arranged so that gravity tends to center it. Centrifugal
force tries to move it to the outside of the skid, up the curved tube.
Neither of these forces depends on where you mount the turn coordinator.


Tim Auckland wrote:
(Things are so slow in this group at the moment, I thought I'd post
this to get your input on something I've been mulling over...)

If you put a plane into a skidding left turn (wings level) with left
rudder, the ball on the panel goes to the right.

However, I've been trying to work out what would happen to the ball if
it was mounted on the tail.
The rudder is pushing the tail of the plane to the right, so I think
the ball would go to the left.
Can anyone confirm this?

Tim.

I started this discussion talking about a "wings level skidding turn".
There's no horizontal component of lift generated by the wings if the
wings are level. There is however a couple comprising of the
rightwards force from the rudder, and the induced leftwards force of
wind resistance acting further forward along the fuselage. It's the
couple which causes the plane to turn.

There is also the fact that the thrust vector is more aligned with the
direction of desired flight.

I believe what causes a plane to turn is the couple comprised of
opposing forces which aren't aligned.

This is always true, for any acceleration not in the direct line of
flight. There is no "one thing" which causes anything in aviation
(except at the most fundamental level, where all flight is controlled by
money). In a coordinated turn, there are several forces, as you pointed
out. However, not all turns are coordinated. What makes a car turn?
Are there analogs in aviation of these forces?

Jose
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.

On Sun, 30 Jul 2006 02:12:48 GMT, Jose
wrote:

I started this discussion talking about a "wings level skidding turn".
There's no horizontal component of lift generated by the wings if the
wings are level. There is however a couple comprising of the
rightwards force from the rudder, and the induced leftwards force of
wind resistance acting further forward along the fuselage. It's the
couple which causes the plane to turn.

There is also the fact that the thrust vector is more aligned with the
direction of desired flight.

I believe what causes a plane to turn is the couple comprised of
opposing forces which aren't aligned.

This is always true, for any acceleration not in the direct line of
flight. There is no "one thing" which causes anything in aviation
(except at the most fundamental level, where all flight is controlled by
money). In a coordinated turn, there are several forces, as you pointed
out. However, not all turns are coordinated. What makes a car turn?

Same thing that makes a plane turn: unbalanced forces not acting
through the centre of gravity.

(In the case of a car, the initial sideways forces are generated by
the front tires.)

By the way, I'm beginning to realize that it only takes one force to
turn an object, as long as that force is not acting through the centre
of gravity.
Interestingly enough, if the "vertical component of lift" were the
only sideways force acting on the plane, it would cause adverse yaw.
The wing's centre of lift is behind the plane's centre of gravity, so
a pull to the left would cause the plane to turn right.

Are there analogs in aviation of these forces?
Only when taxiing a tricycle(:-)

Jose

By the way, I'm beginning to realize that it only takes one force to
turn an object, as long as that force is not acting through the centre
of gravity.

That's not quite true (unless by "turn" you mean "yaw"). A single force
not through the CG will cause the object to rotate about the CG. To get
an airplane to actually =turn= however requires also a change in
direction, else the airplane would simply skid sideways the rest of the
flight.

Let's say you are flying straight and level, due North, and simply stomp
on the (left) rudder pedal. The first thing is that the tail will swing
to the right, because the force from the rudder is not through the CG.
But then other things happen. As the tail swings, the right wing ends
up going faster and the left wing slower through the air. So the right
wing provides more lift, and the aircraft banks to the left. Since the
airplane is still going due North, but pointing slightly west, the
airflow on the side of the plane will push the airplane somewhat to the
left, and the propeller (now pointing slightly west) will also help pull
the airplane west. The bank also introduces some net leftward force.

So, there's a lot going on. (and no, stomping on the rudder pedals isn't
usually the best way to turn).

Jose
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.

On Mon, 31 Jul 2006 18:57:30 GMT, Jose
wrote:

By the way, I'm beginning to realize that it only takes one force to
turn an object, as long as that force is not acting through the centre
of gravity.

That's not quite true (unless by "turn" you mean "yaw").
Yes, when I made the "one force" statement, I was thinking of yaw.


A single force
not through the CG will cause the object to rotate about the CG. To get
an airplane to actually =turn= however requires also a change in
direction, else the airplane would simply skid sideways the rest of the
flight.

Let's say you are flying straight and level, due North, and simply stomp
on the (left) rudder pedal. The first thing is that the tail will swing
to the right, because the force from the rudder is not through the CG.
But then other things happen. As the tail swings, the right wing ends
up going faster and the left wing slower through the air. So the right
wing provides more lift, and the aircraft banks to the left.
In my original scenario, I stated a "skidding left turn (wings
level)". I was picturing doing whatever is required with the ailerons
to keep the wings level. Come to think of it, that'll mean
down-aileron on the left side, which'll introduce more drag on the
left, which may or may not be balanced by the increased drag from the
faster moving right wing and up-aileron on the right side.....

Since the
airplane is still going due North, but pointing slightly west, the
airflow on the side of the plane will push the airplane somewhat to the
left, and the propeller (now pointing slightly west) will also help pull
the airplane west. The bank also introduces some net leftward force.

So, there's a lot going on.
I agree absolutely. This isn't a simple issue.

(and no, stomping on the rudder pedals isn't
usually the best way to turn).
but it can produce some interesting results (:-)

Tim.

Tim Auckland wrote:

The wing's centre of lift is behind the plane's centre of gravity, so

Eh? If this were true, there would be a torque acting about the pitch axis and
forcing the nose downward.

The wing's centre of lift is behind the plane's centre of gravity, so
Eh? If this were true, there would be a torque acting about the pitch axis and forcing the nose downward.

There is. That's what the tail is for - it pushes down (behind the CG)
providing the balancing torque.

Canard aircraft are different, the main wing is behind the CG, and the
canard is in front; both provide lift in the same (upward) direction.

Jose
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.

On 07/31/06 14:13, Dave Butler wrote:
Tim Auckland wrote:

The wing's centre of lift is behind the plane's centre of gravity, so

Eh? If this were true, there would be a torque acting about the pitch axis and
forcing the nose downward.

Isn't there? Isn't this what the downward force produced by the horizontal
stabilizer is trying to equalize?



--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA