Coordinated turns without rudder, and autopilots

Ron Natalie writes:

Get in most any aircraft. Take it to a normal cruise airspeed.
Put your feet flat on the floor and roll her into a standard rate
turn. I can almost guarantee the ball will be centered. It
would be a highly inefficient and impractical design if it
doesn't fly coordinated without rudder impact in that
regime.

Then why would you ever need the rudder in a turn?

Ron Natalie wrote in news:465c02e7$0$30412
:

Dan wrote:
\

So where is this perfect airplane? I don't know about you, but I
need
the rudder pedals to fly the aircraft.

Get in most any aircraft. Take it to a normal cruise airspeed.
Put your feet flat on the floor and roll her into a standard rate
turn. I can almost guarantee the ball will be centered.

No, it won't, actually. The tendency towards adverse yaw is amelierated
in most modern light airplanes, but it's still there. It's just that
most modern airplanes ahve been dumbed down to allow almost co-ordinated
flight without rudder usage, but there isn't a conventional airplane
flying without CAR that is totally co-rdinated in when roll is
introduced, though many come close.
That includes airliners which we fly with feet on the floor most of the
time. Some, on approach, have some sort of third axis feature, but most
are two axis most of the time (exceptying FBW busses and the 777) The
yaw damper does a lot towards co-ordinating, but if you introduce a high
rate of roll, they'll yaw, at least momentarily, the worng direction.


Bertie

Andrew Sarangan wrote:
On May 28, 11:59 am, Luke Skywalker wrote:
On May 28, 8:11 am, Ron Natalie wrote:





Dan wrote:
On May 27, 5:44 pm, Mxsmanic wrote:
How do autopilots make coordinated turns even when they cannot control the
rudder?
If they don't control the rudder, they do not make coordinated turns!
--Dan
Boy we have the blind leading the blind here.
The whole point of that big vertical slab of metal sticking out of the
ass-end of your airplane is to provide a natural tendency for the
aircraft to fly coordinated. The pedals are just there for the
outlying conditions (low speed, high AOA for example) and fine
adjustment.
Ron...

oh my goodness...get some time with a good book on the subject and
then a CFI.

Robert- Hide quoted text -



Ron is correct. The vertical fin makes the airplane weather-vane into
the wind, and that's what co-ordination is all about. The rudder is
there only to help the vertical stab do this job.

A perfect airplane will not need rudder.

Not true. The vertical fin can only provide a weather-vane affect when
a slip or skid has been induced. In coordinated flight there is no slip
or skid and hence the fin provides no lateral force. When you begin a
turn, most airplanes will induce adverse yaw and the rudder can counter
than before a skid occurs. The fixed fin can only act once an
uncoordinated condition has been induced. Sure, it does mitigate the
skid or slip, but it absolutely can't prevent it as it can't provide a
force until uncoordinated flight is already established. The rudder can
do this and is why it is included.

The rudder isn't there to help the vertical stab do its job, it is there
to do a job that the vertical stab can't do.


Matt

Matt Whiting wrote:


Not true. The vertical fin can only provide a weather-vane affect when
a slip or skid has been induced.

You have no clue what you are talking about. The skid and slip are the
result of the airplane NOT weather vaning into the wind. There are
a number of reasons for this. The primary one in turns is the "adverse
yaw" due to the differing drag caused by the displaced ailerons. Many
designs do a lot of things to mitigate this. Still it takes a lot of
aileron displacement to overcome the natural desire for the airplane
to track into the wind (due to the vertical stab).

In coordinated flight there is no slip
or skid and hence the fin provides no lateral force.

This is the definition of coordinated flight, not cause and affect.

The rudder isn't there to help the vertical stab do its job, it is there
to do a job that the vertical stab can't do.

Sorry. The incorrect. You need the vertical stab to even fly
coordinated when you are not turning. If it is two small the
airplane will tend to yaw on it's own (the more bulbous your
fuselage, the more this is a probelm...there was a design Piper
tried that used an almost helicopter like bubble on the front...
without the slab sides to help the vertical stab, the plane
just would as well fly slipping as nromal).

The vertical stab is nearly always set up to get the aircraft
to fly coordinated in normal cruise level flight. It is frequently
slightly offset to correct for other aerodynamic unbalances.

The rudder is just at trim to handle other flight regimes.
It's mostly there for the high AOA regimes of Take-off and
landing.

Ron Natalie wrote:
Matt Whiting wrote:


Not true. The vertical fin can only provide a weather-vane affect
when a slip or skid has been induced.

You have no clue what you are talking about. The skid and slip are the
result of the airplane NOT weather vaning into the wind. There are
a number of reasons for this. The primary one in turns is the "adverse
yaw" due to the differing drag caused by the displaced ailerons. Many
designs do a lot of things to mitigate this. Still it takes a lot of
aileron displacement to overcome the natural desire for the airplane
to track into the wind (due to the vertical stab).

In coordinated flight there is no slip or skid and hence the fin
provides no lateral force.

This is the definition of coordinated flight, not cause and affect.

The rudder isn't there to help the vertical stab do its job, it is
there to do a job that the vertical stab can't do.

Sorry. The incorrect. You need the vertical stab to even fly
coordinated when you are not turning. If it is two small the
airplane will tend to yaw on it's own (the more bulbous your
fuselage, the more this is a probelm...there was a design Piper
tried that used an almost helicopter like bubble on the front...
without the slab sides to help the vertical stab, the plane
just would as well fly slipping as nromal).

The vertical stab is nearly always set up to get the aircraft
to fly coordinated in normal cruise level flight. It is frequently
slightly offset to correct for other aerodynamic unbalances.

The rudder is just at trim to handle other flight regimes.
It's mostly there for the high AOA regimes of Take-off and
landing.

I don't know where you got your engineering degree, but you better
demand a refund. A vertical stabilizer does not provide any lateral
force unless there is some degree of slip or skid. In coordinated
flight, it is just along for the ride. Many airplanes will oscillate
slight in the yaw axis for this reason. It takes a very large vertical
stab to keep the excursions small enough to not be detectable,
especially in a longer fuselage airplane. The rudder can provide a side
force in anticipation of a slip or skid and thus maintain coordinated
flight and never allow the slip or skid to develop in the first place.

Matt

Matt Whiting wrote:

A vertical stabilizer does not provide any lateral
force unless there is some degree of slip or skid.

Precisely! Now you are beginning to understand. As soon
as uncoordinated flight occurs (skid or slip) the vertical stab
deflects the aircraft back into the coordinated flight.

That's it's job! It is the primary job to provide the primary
aerodynamic forces to keep the airplane coordinated.

In coordinated flight, it is just along for the ride.

Yep, and as soon as something deflects the aircraft from coordinate
flight, it generates a force to correct it.

Many airplanes will oscillate
slight in the yaw axis for this reason.

And they oscillate in pitch, and they oscillate in roll. This is
one of the fundamental modes of stability.

It takes a very large vertical
stab to keep the excursions small enough to not be detectable,
especially in a longer fuselage airplane. The rudder can provide a side
force in anticipation of a slip or skid and thus maintain coordinated
flight and never allow the slip or skid to develop in the first place.

Are you trying to tell me that you sit there and tweak the rudders
during flight continually to damp yaw oscillations? Don't think
anybody finds that fun. The few airplanes where it is a persistant
problem have autopilots that do that, but for most it's unnecessary
in normal flight regimes.

Ron Natalie wrote in
:

Matt Whiting wrote:

A vertical stabilizer does not provide any lateral
force unless there is some degree of slip or skid.

Precisely! Now you are beginning to understand. As soon
as uncoordinated flight occurs (skid or slip) the vertical stab
deflects the aircraft back into the coordinated flight.


No, it doesn't.

It can help guide it back towards coordinated flight but it won't center
it there.

Bertie

Ron Natalie wrote:
Matt Whiting wrote:

A vertical stabilizer does not provide any lateral force unless there
is some degree of slip or skid.

Precisely! Now you are beginning to understand. As soon
as uncoordinated flight occurs (skid or slip) the vertical stab
deflects the aircraft back into the coordinated flight.

It does sounds like you are beginning to understand! With use of the
rudder, you can enter and exit a turn and maintain coordination at all
times. Without rudder and depending on the fin alone, you will be in
and out of coordinated flight as you enter and exit the turn. The
degree of departure from coordinated flight maybe be large or small
depending on the specific airplane and the rate at which you enter and
halt the turn, but without using the rudder, you will NOT be coordinated
at all times.



That's it's job! It is the primary job to provide the primary
aerodynamic forces to keep the airplane coordinated.

In coordinated flight, it is just along for the ride.

Yep, and as soon as something deflects the aircraft from coordinate
flight, it generates a force to correct it.

Yes, which is precisely what I said at the start. You have to enter
uncoordinated flight for the fin to work. With proper rudder use, you
can remain coordinated at all times (if you are good enough).


Many airplanes will oscillate slight in the yaw axis for this reason.

And they oscillate in pitch, and they oscillate in roll. This is
one of the fundamental modes of stability.

Yes, and that is one reason you have control surfaces, to stop these
oscillations or prevent them.


It takes a very large vertical stab to keep the excursions small
enough to not be detectable, especially in a longer fuselage
airplane. The rudder can provide a side force in anticipation of a
slip or skid and thus maintain coordinated flight and never allow the
slip or skid to develop in the first place.

Are you trying to tell me that you sit there and tweak the rudders
during flight continually to damp yaw oscillations? Don't think
anybody finds that fun. The few airplanes where it is a persistant
problem have autopilots that do that, but for most it's unnecessary
in normal flight regimes.

No, that is what yaw dampers are for. But, yes, I use the rudder and
ailerons and elevator to damp oscillations caused by turbulence when
they exceed a certain threshold. No sense in waiting through several
oscillations when you have the controls to stop it now.

On May 29, 7:16 am, Matt Whiting wrote:
Ron Natalie wrote:
Matt Whiting wrote:

Not true. The vertical fin can only provide a weather-vane affect
when a slip or skid has been induced.

You have no clue what you are talking about. The skid and slip are the
result of the airplane NOT weather vaning into the wind. There are
a number of reasons for this. The primary one in turns is the "adverse
yaw" due to the differing drag caused by the displaced ailerons. Many
designs do a lot of things to mitigate this. Still it takes a lot of
aileron displacement to overcome the natural desire for the airplane
to track into the wind (due to the vertical stab).

In coordinated flight there is no slip or skid and hence the fin
provides no lateral force.

This is the definition of coordinated flight, not cause and affect.

The rudder isn't there to help the vertical stab do its job, it is
there to do a job that the vertical stab can't do.

Sorry. The incorrect. You need the vertical stab to even fly
coordinated when you are not turning. If it is two small the
airplane will tend to yaw on it's own (the more bulbous your
fuselage, the more this is a probelm...there was a design Piper
tried that used an almost helicopter like bubble on the front...
without the slab sides to help the vertical stab, the plane
just would as well fly slipping as nromal).

The vertical stab is nearly always set up to get the aircraft
to fly coordinated in normal cruise level flight. It is frequently
slightly offset to correct for other aerodynamic unbalances.

The rudder is just at trim to handle other flight regimes.
It's mostly there for the high AOA regimes of Take-off and
landing.

I don't know where you got your engineering degree, but you better
demand a refund. A vertical stabilizer does not provide any lateral
force unless there is some degree of slip or skid. In coordinated
flight, it is just along for the ride. Many airplanes will oscillate
slight in the yaw axis for this reason. It takes a very large vertical
stab to keep the excursions small enough to not be detectable,
especially in a longer fuselage airplane. The rudder can provide a side
force in anticipation of a slip or skid and thus maintain coordinated
flight and never allow the slip or skid to develop in the first place.

Matt- Hide quoted text -


You are assuming that the primary role of the rudder is to fly co-
ordinated. I would argue the opposite. The primary role of the rudder
is to fly un-coordinated, such as in a cross-wind landing, forward
slip, spin etc.. An airplane that always flies perfectly co-ordinated
(with or without rudder) would be of little use.

Nevertheless, I don't believe your analysis is correct, even from an
engineering control system point of view. The vertical stab and yaw
can be thought of as a closed loop system. Yaw is the error signal.
The vertical stab creates a lateral force that minimizes the error
signal by providing a negative feedback. One could argue that a
vertical stab serves no purpose if there is no yaw. But no airplane
flies perfectly co-ordinated. They continuously slip and skid as they
fly, and it is the vertical stab that kicks in the feedback to
stabilize the system. Since the effect of the vertical stab is highly
dependent on the airspeed, at lower airspeed one would need a bigger
vertical stab. In other words, you would need an adaptive feedback.
Since it is clearly not practical to enlarge the vertical stab during
flight, the next best thing you can do is to rotate it, and this what
the rudder does. Simply put, a rudder provides the means to enhance
the effect of the vertical stab during flight.

The original statement that the rudder simply assists the vertical
stab at the outlying regions is correct. If a vertical stab could be
designed such that its effectiveness is independent of airspeed, then
a rudder won't be necessary to fly co-ordinated. But for reasons I
stated earlier, such an airplane would still not be very useful.

Andrew Sarangan wrote:
On May 29, 7:16 am, Matt Whiting wrote:
Ron Natalie wrote:
Matt Whiting wrote:
Not true. The vertical fin can only provide a weather-vane affect
when a slip or skid has been induced.
You have no clue what you are talking about. The skid and slip are the
result of the airplane NOT weather vaning into the wind. There are
a number of reasons for this. The primary one in turns is the "adverse
yaw" due to the differing drag caused by the displaced ailerons. Many
designs do a lot of things to mitigate this. Still it takes a lot of
aileron displacement to overcome the natural desire for the airplane
to track into the wind (due to the vertical stab).
In coordinated flight there is no slip or skid and hence the fin
provides no lateral force.
This is the definition of coordinated flight, not cause and affect.
The rudder isn't there to help the vertical stab do its job, it is
there to do a job that the vertical stab can't do.
Sorry. The incorrect. You need the vertical stab to even fly
coordinated when you are not turning. If it is two small the
airplane will tend to yaw on it's own (the more bulbous your
fuselage, the more this is a probelm...there was a design Piper
tried that used an almost helicopter like bubble on the front...
without the slab sides to help the vertical stab, the plane
just would as well fly slipping as nromal).
The vertical stab is nearly always set up to get the aircraft
to fly coordinated in normal cruise level flight. It is frequently
slightly offset to correct for other aerodynamic unbalances.
The rudder is just at trim to handle other flight regimes.
It's mostly there for the high AOA regimes of Take-off and
landing.
I don't know where you got your engineering degree, but you better
demand a refund. A vertical stabilizer does not provide any lateral
force unless there is some degree of slip or skid. In coordinated
flight, it is just along for the ride. Many airplanes will oscillate
slight in the yaw axis for this reason. It takes a very large vertical
stab to keep the excursions small enough to not be detectable,
especially in a longer fuselage airplane. The rudder can provide a side
force in anticipation of a slip or skid and thus maintain coordinated
flight and never allow the slip or skid to develop in the first place.

Matt- Hide quoted text -


You are assuming that the primary role of the rudder is to fly co-
ordinated. I would argue the opposite. The primary role of the rudder
is to fly un-coordinated, such as in a cross-wind landing, forward
slip, spin etc.. An airplane that always flies perfectly co-ordinated
(with or without rudder) would be of little use.

I'm not assuming that at all nor did I ever say that. The rudder has
many roles. Coordinating turns is just one of the roles. My point was
simply that the vertical stabilizer alone will not provide a coordinated
turn with an airplane whose ailerons generate any adverse yaw at all.


Nevertheless, I don't believe your analysis is correct, even from an
engineering control system point of view. The vertical stab and yaw
can be thought of as a closed loop system. Yaw is the error signal.
The vertical stab creates a lateral force that minimizes the error
signal by providing a negative feedback. One could argue that a
vertical stab serves no purpose if there is no yaw. But no airplane
flies perfectly co-ordinated. They continuously slip and skid as they
fly, and it is the vertical stab that kicks in the feedback to
stabilize the system. Since the effect of the vertical stab is highly
dependent on the airspeed, at lower airspeed one would need a bigger
vertical stab. In other words, you would need an adaptive feedback.
Since it is clearly not practical to enlarge the vertical stab during
flight, the next best thing you can do is to rotate it, and this what
the rudder does. Simply put, a rudder provides the means to enhance
the effect of the vertical stab during flight.

That is precisely what I said. As a controls engineer, I'm quite
familiar with the operation of control systems. My point was that you
must have an error (yaw) in order for the fin to provide any stabilizing
force. This means that you have to enter uncoordinated flight before it
does anything. The rudder can be used similar to feed-forward control
or model predictive control. When a turn is planned, the rudder can be
applied in coordination with the ailerons to exactly offset the adverse
yaw force and maintain coordination throughout the turn entry. With a
fin alone, the airplane will be uncoordinated during the turn entry and
will only enter coordinated flight again once the transient has been
damped. That is the entire point and is in contrast with Ron's earlier
comment about a rudder not being needed to provide coordinated flight
and only being needed for "outlying" conditions. I guess if you
consider turning the airplane to be an outlying condition, then Ron is
correct.


The original statement that the rudder simply assists the vertical
stab at the outlying regions is correct. If a vertical stab could be
designed such that its effectiveness is independent of airspeed, then
a rudder won't be necessary to fly co-ordinated. But for reasons I
stated earlier, such an airplane would still not be very useful.

No, that statement is not correct. The rudder doesn't just assist the
vertical stab, it does things it can't do. The stab can't prevent
unbalanced forces from the ailerons from causing adverse yaw. It will
provide a restoring force once the adverse yaw exists, but it won't
return the airplane to coordinated flight until the unbalanced aileron
forces cease. The rudder CAN prevent adverse yaw by countering the
unbalanced aileron forces BEFORE coordinated flight has departed.

This is a simple concept. Is it really that hard to understand?

It is like the difference between controlling your speed manually in
hilly country vs. using cruise control. If I want to invest the
concentration, I can hold speed much more precisely on hills than can my
car's cruise control. The reason is that I can anticipate the hill and
start feeding in throttle before the car slows down. The cruise
control, OTOH, is like the vertical stab and can't do diddly until after
the car has already begun to slow down as it needs an error signal to
work with. I don't need an error signal and can thus control the speed
more tightly. Same when going over the crest of the hill. I can
anticipate this and back off the throttle before the car beings to gain
speed. Most cruise controls will overshoot at least 2-3 MPH going over
a hill as they need an error in order to start responding and the
natural lag in the system will cause overshoot.


Matt