Mounting a turn coordinator on the tail?

On Sun, 30 Jul 2006 23:36:47 GMT, Judah wrote:


More importantly, if you put the TC in the tail, how much rudder pressure
would the pilot have to apply in order to turn the tail quickly enough around
to see it?

There's a Tony Hancock radio sketch where he gets ejected from a
plane, lands on the tail, lassoos the controls, and steers it home.


I believe cats and dogs have the ability to achieve the necessary speeds,
almost even to the point that some may catch their tails in their mouths as
they look to focus on their balls. But I don't think airplanes are quite
limber enough for this capability.

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

On Mon, 31 Jul 2006 08:06:07 -0400, Ron Natalie
wrote:

While the horizontal component of lift is what pulls
you to the interior of the turn, the tail is VERY important to
actually "turn" the aircraft direction
----------------------------------------
so that the horizontal
component continually gets pointed to the center of the turn.
----------------------------------------
Now that's a VERY interesting way of thinking about it.

What got me thinking about all this was the observation that I found
it difficult to keep the ball exactly centered in a 50-degree bank, so
I started thinking about whether it's even possible to do this in a
60-degree bank. In theory, in a 60-degree bank, the angle of attack,
controlled by the elevator, has to be such that 2g of lift is
generated. However, in a steep bank, the rate of turn is mostly
controlled by the elevator. The rudder's forces are mostly acting
vertically, so it has a large effect on whether the nose is pointing
up or down.
If you keep the nose roughly horizontal with the rudder, and 2g of
lift with the elevator, you've no controls left to affect the rate of
turn.
Do the forces in this case work out such that the ball is centered?

Any areobatic piliots out there? Is the ball typically centered in a
60-degree bank?

Tim.

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.

Ron Natalie wrote:
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.

...opinion deleted...

While the horizontal component of lift is what pulls
you to the interior of the turn, the tail is VERY important to
actually "turn" the aircraft direction so that the horizontal
component continually gets pointed to the center of the turn.

The elevator/rudder mechanism is for applying the torques to the plane
so it rolls and yaws. Also, as I remember from the first day of
physics class, a physical body behaves as a point mass at the center of
gravity with 3 translational forces and 3 rotational torques that can be
applied to it.

The horizontal component of lift behaves like a string tied to a rock
being swung around. The string does indeed apply force to the center of
gravity of the rock and "points" to the center of the turn. If you put
a paint spot on the rock and want to make the spot always face you, the
rock will have to yaw at the same rate as you are rotating it around
you; consequently something like a rudder will be needed.

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

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.

Stubby wrote:


The horizontal component of lift behaves like a string tied to a rock
being swung around.

No, it doesn't. The lift vector points in a direction (roughly)
perpendicular to the wing. Nothing causes it to point to the
a "center" other than the other aerodynamic surfaces .