ATPL Principles of Flight - Class 17: Stability II.
Summary
TLDRIn this educational video, Grant explores the principles of aircraft stability, focusing on the three axes: longitudinal, lateral, and directional. He explains how disturbances are corrected through the aircraft's design, such as the tail plane's role in pitch stability and the fin's influence on yaw. The video delves into static and dynamic stability, illustrating how these factors affect flight and can lead to issues like pilot-induced oscillations and Dutch roll. Grant also touches on how stability can vary with speed, center of gravity, and tail plane area, providing a comprehensive look at the aerodynamic forces that keep planes stable in the sky.
Takeaways
- π« Static longitudinal stability is crucial for an aircraft's initial response to pitch disturbances, with the tailplane playing a key role in creating a corrective nose-up pitching moment.
- βοΈ The restoring moment's strength in longitudinal stability is influenced by the length of the balance arm, which can be optimized by positioning the center of gravity further forward.
- π The coefficient of moment is a useful tool for assessing an aircraft's longitudinal stability, with positive values indicating nose-up and negative values indicating nose-down tendencies.
- π A graph of the coefficient of pitching moment versus angle of attack helps visualize an aircraft's static stability, with different line slopes indicating varying stability levels.
- π Dynamic stability in pitch can manifest as short period oscillations, which are rapid and can be dangerous, or as long period oscillations known as phugoid oscillations, which are slower and easier to correct.
- π Pilot-induced oscillation occurs when a pilot's corrective actions are out of sync with short period oscillations, exacerbating the problem.
- π§ Directional stability, or yaw stability, is maintained by the vertical stabilizer and is essential for consistent heading, with the force and distance involved in the yawing moment being critical.
- π The coefficient of yawing moment is simplified for understanding by considering dynamic pressure, fin area, and wingspan, and is represented graphically against the angle of sideslip.
- π€ΈββοΈ Lateral static stability ensures an aircraft returns to level flight after a roll disturbance, with the lift difference between the wings creating a corrective rolling moment.
- π Roll and yaw are interconnected; uncoordinated rolling motions can lead to slipping or skidding, affecting the side slip angle and overall stability.
- π Dutch roll and spiral instability are stability issues that arise from imbalances between directional and lateral stability, leading to wobbling or spiraling flight paths.
Q & A
What is static longitudinal stability in aviation?
-Static longitudinal stability refers to the initial behavior of an aircraft in response to disturbances in pitch. It ensures that any disturbances cause a pitching moment that corrects the problem, such as using the tail plane to create a nose-up pitching moment to counteract a nose-down disturbance.
How does the position of the center of gravity affect stability?
-A further forward center of gravity increases the balance arm, which in turn strengthens the restoring moments, enhancing both longitudinal and directional stability.
What is the role of the tail plane in maintaining stability?
-The tail plane creates a downforce that counteracts disturbances causing a nose-down pitching moment. It helps restore the aircraft to a stable pitch attitude.
How does airspeed affect the corrective moment in an aircraft?
-The faster the aircraft travels, the stronger the corrective moment will be, as it is dependent on the dynamic pressure (related to the square of the airspeed).
What is the significance of the coefficient of moment in stability analysis?
-The coefficient of moment is a useful tool in stability analysis as it represents the moment per unit dynamic pressure, per unit area, and per unit mean aerodynamic chord, simplifying the evaluation of an aircraft's stability characteristics.
What are the two types of dynamic stability in terms of pitch?
-The two types of dynamic stability in terms of pitch are short period oscillation, which is a quick fluttering motion, and long oscillation or fugoid oscillation, which occurs over a few minutes and involves larger changes in altitude and airspeed.
What is pilot-induced oscillation and how does it relate to short period oscillation?
-Pilot-induced oscillation is a phenomenon where the pilot's attempts to correct for short period oscillations can inadvertently worsen the situation, causing the oscillations to grow more severe due to being out of sync with the quick changes in pitch.
How does directional stability differ from lateral stability?
-Directional stability, which involves the yawing motion, is the initial tendency of an aircraft to return to its original heading after a disturbance. Lateral stability, on the other hand, is the tendency of an aircraft to return to a wings-level state after a disturbance in roll.
What is the role of the fin in directional stability?
-The fin, or vertical stabilizer, provides the corrective yawing moment necessary for directional stability. It responds to the angle of side slip, creating a force that rotates the aircraft around its center of gravity to realign with the airflow.
How can the strength of the corrective moment in yaw be increased?
-The strength of the corrective moment in yaw can be increased by having a forward center of gravity, increasing airspeed, enlarging the fin area, or adding dorsal and ventral fins to increase the overall surface area contributing to the yawing moment.
What is the relationship between roll and yaw in terms of aircraft stability?
-Roll and yaw are linked in aircraft stability. An uncoordinated roll (without rudder input) can lead to a yaw due to the difference in lift on each wing, and a yaw can lead to a roll if not countered by coordinated control inputs.
What is Dutch roll and how does it relate to lateral and directional stability?
-Dutch roll is a wobbling motion that occurs when there is strong lateral stability but weak directional stability. It involves a combination of roll and yaw oscillations, and can be mitigated with better coordination between the rudder and ailerons or through the use of a yaw damper.
Outlines
π« Understanding Aircraft Stability
This paragraph introduces the concept of stability around the three principal axes of an aircraft: longitudinal, lateral, and directional. It explains how changes in stability are achieved and what factors influence it. The discussion focuses on static longitudinal stability, which is the initial response of an aircraft to disturbances in pitch. The example of a jet airliner is used to illustrate how weight and lift act through different centers to create a nose-down pitching moment, which is corrected by the tailplane. The paragraph also discusses how disturbances are countered by changes in the angle of attack at the tailplane and how the strength of the restoring moment is influenced by factors like the distance from the center of gravity, speed, and tailplane area. The concept of the coefficient of moment is introduced as a tool to represent and analyze moments in flight dynamics.
π Dynamics of Stability and Oscillations
The second paragraph delves into dynamic stability, specifically in terms of pitch. It differentiates between short period oscillations, which are quick and can be dangerous due to the stress they put on the aircraft's structure, and long period oscillations, known as phugoid oscillations, which occur over a longer time and are easier for pilots to correct. The paragraph also touches on pilot-induced oscillations, where attempts to correct short period oscillations can inadvertently exacerbate the problem. The discussion then moves to directional stability, explaining how the vertical stabilizer, or fin, provides the corrective yawing moment. Factors that influence the strength of this moment, such as the position of the center of gravity, speed, and fin area, are also covered. The concept of the coefficient of yawing moment is introduced to simplify the analysis of yawing moments.
π Lateral Stability and Its Impact on Flight
Paragraph three discusses lateral static stability, which is crucial for an aircraft to return to level flight after a disturbance. The paragraph explains how an aircraft without positive lateral stability could recover slowly or even invert after a wing drop. It describes the phenomenon of slipping or skidding during uncoordinated turns, where the aircraft's motion causes the air to hit at an angle, known as the sideslip angle. The paragraph further explains how the difference in lift between the shielded and exposed wings due to a sideslip angle leads to a rolling motion. The coefficient of lateral stability is introduced as a way to quantify this stability, with positive static stability being represented by a downward sloping line on a graph, indicating a corrective moment that opposes the sideslip.
βοΈ Stability Interactions and Flight Control
The fourth paragraph explores the interactions between directional and lateral stability and how they can lead to different flight behaviors. It describes spiral instability, which occurs when an aircraft has strong directional stability but weak lateral stability, causing a continuous rolling motion that can lead to a spiral dive. Conversely, when an aircraft has strong lateral stability but weak directional stability, it can lead to a phenomenon known as Dutch roll, characterized by a wobbling motion. The paragraph emphasizes the importance of rudder coordination with ailerons to counteract these instabilities. It also introduces yaw dampers, automatic systems that can correct yaw-induced oscillations more effectively than human pilots. The summary concludes by reiterating the importance of understanding and managing stability in all three axes for safe and controlled flight.
π Graphs and Stability in Flight
The final paragraph provides a graphical representation of stability, focusing on how different stability characteristics manifest on graphs. It explains how positive and negative static stability are depicted in relation to pitch, yaw, and roll. The paragraph uses the example of a positively statically stable aircraft, where an increase in pitch results in an increase in the corrective moment that opposes the disturbance. It also discusses the relationship between directional and lateral stability, and how an imbalance can lead to spiral instability or Dutch roll. The importance of understanding these graphical representations for pilots and aircraft designers is highlighted, as it aids in predicting and controlling an aircraft's response to disturbances.
Mindmap
Keywords
π‘Stability
π‘Longitudinal Stability
π‘Center of Gravity
π‘Angle of Attack
π‘Dynamic Stability
π‘Coefficient of Moment
π‘Directional Stability
π‘Lateral Stability
π‘Dutch Roll
π‘Yaw Damper
Highlights
Stability around all three aircraft axes is crucial for flight control.
Longitudinal stability is the aircraft's initial response to pitch disturbances.
The tail plane's downforce counteracts disturbances causing nose-down pitching moments.
A longer balance arm, achieved by a forward center of gravity, enhances restoring moments.
Airspeed and tail plane area influence the strength of corrective moments.
Coefficient of moment is a useful metric for assessing longitudinal stability.
A graph of coefficient of pitching moment against angle of attack visualizes aircraft stability.
Positive static stability is indicated by a downward sloping line on the stability graph.
Dynamic stability in pitch includes short period oscillations that can be dangerous due to high stress on aircraft.
Pilot-induced oscillation occurs when corrective actions exacerbate short period oscillations.
Long oscillations, or fugoid oscillations, are easier for pilots to correct due to their slower nature.
Directional stability is crucial for maintaining consistent flight heading.
The vertical stabilizer, or fin, provides the corrective yawing moment for directional stability.
Increasing the fin's surface area with dorsal or ventral fins can enhance directional stability.
Lateral static stability ensures the aircraft returns to level flight after a roll disturbance.
Uncoordinated rolling motions without rudder input can lead to skidding or slipping.
Lateral stability is influenced by the wing's area and wing span, affecting the correctional moment.
A yaw damper is an automatic system that corrects Dutch roll by opposing induced yaw.
Strong stability in one axis and weak in another can lead to spiral instability or Dutch roll.
Transcripts
stability around all three of the
aircraft axes is important but how do we
change the level of stability and what
factors influence the stability in all
three of those axes
let's find out
[Music]
hi i'm grant and welcome to class 17 in
the principles of flight series today
we're going to be expanding on the
concepts we learned in the previous
class and in this second part we'll take
a bit of a deeper dive into how to
achieve
longitudinal stability lateral stability
and directional stability
static longitudinal stability is the
initial behavior of an aircraft in
response to disturbances in pitch
what this means in practice is any
disturbances must cause a pitching
moment that corrects the problem
so let's take a look at a normal jet
airline
we know weight acts through the centre
of gravity and the lift acts through the
centre of pressure which is often
located behind the center of gravity
this causes
a nose down pitching moment
which we then correct with the tail
plane creating downforce which
counteracts and creates a nose up
pitching moment this is something we've
established a few times now
so if we were to experience any
disturbance that was to cause a further
nose up pitching moment the angle of
attack at the tail plane would increase
the airflow is still coming in straight
but the whole aircraft is pitched up
this increase in angle of attack causes
a reduction in the strength of the
downforce or even a lifting force
that will counteract this disturbance
and create an opposite
moment
that opposes this disturbance the same
is true if we have a nose down
disturbance if something is to cause a
nose down pitching moment the angle of
attack would become negative and create
more of a down force that would
counteract that
input that made us pitch the nose down
so because it's a moment it's dependent
on the force and the distance we can say
then that the restoring moment the
strength of the tail to create restoring
moments
will be better with a longer balance arm
times the distance
and a longer balance arm comes from
having a further forward center of
gravity
we can also say
that the faster we travel
if our v squared value goes up
the stronger the corrective moment will
be
and also the larger area of the tail
plane the stronger the corrective moment
will be a useful tool when talking about
moments is to use a number that
represents the moment
this is what happens in mass and balance
for instance when you're looking at load
sheets you get given an index which
essentially represents a moment
depending on where the items where the
cargo is loaded
this index that you get given is
essentially a coefficient of a moment
and in coefficient of moment positive in
relation to pitch
refers to nose up
and negative refers to a nose down
the coefficient
of moment for longitudinal stability
will be dependent on the strength of the
moment that's created obviously that's
the actual
value in terms of newton meters and what
we do is we then find out the value per
unit dynamic pressure per unit area and
per unit to mean aerodynamic cord
and that gives us
a nice easy to work with value
if we look at the coefficient of
pitching moment on a graph we can
visualize the stability of the aircraft
a lot better so this is a plot of the
coefficient of
moment for
sorry the coefficient of pitching moment
against the angle of attack which
represents the overall pitch of the
aircraft this first line here represents
positive
static stability the point where it
crosses the line is our equilibrium
point
any decrease in the angle of attack
means
an increase in the coefficient of moment
and it will move us back towards that
zero point so if you think about we
pitch down
the strength of the corrective moment
increases in terms of its positivity
which is its nose up characteristic
so we decrease the pitch the strength of
the moment
returns to a
nose up position and it drives us back
towards this equilibrium point if the
line was completely flat across the
axis of the angle of attack then that
would be neutrally stable and if the
line slopes up like this that means
we're longitudinally instable
because if you think about it if we
increased our pitch the strength of the
moment would actually be positive and
that would make the problem worse it
would mean that we
increase increase in pitch and keep
increasing in pitch
all of these lines actually curve off at
the end think about the neutral one as
well there
because the aircraft will eventually
stall there's also the case where
you get a range of levels of stability
depending on the angle of attack that
you're at and the line would be some
sort of curve shape
and depending on the angle of attack
detail can either help or hinder you in
terms of that corrected moment normally
uh statically stable
aircraft will also be dynamically stable
and this means that it tends to
overshoot a little bit and it does that
oscillating stability that we looked at
in the previous class and there can be
two broad types of this dynamic
stability in terms of pitch you get what
we call short period oscillation and it
only lasts a few seconds and it
basically causes very little change to
our altitude or airspeed because it just
happens so quickly it's a it's a quick
like fluttering motion these however can
be quite dangerous and cause a lot of
stress on the aircraft because these
quick changes in pitch
cause increases in the load factor and
the structures of the wing are pressured
you know one way then the next
and so on
and due to the short nature of these
oscillations by trying to correct the
problem the pilot's actually trying to
correct the problem you can often end up
out of sync because it's just happening
so fast
and then you make the problem worse and
it grows and it grows and these changes
in pitch get even more severe
and the only and easiest solution to do
this is just like let go of the controls
and hopefully the thing will eventually
settle down
this phenomenon where the pilot actually
adds to the problem of short
period oscillation is known as pilot
induced oscillation
so you can think of it as
it goes up comes down it goes up
and that's a very short period of a
waveform
the other form
is a long oscillation
again you would think about it
graphically as a very spread out wave
long oscillation is also known as a
fugoid oscillation and it happens over
the course of a few minutes so as these
new changes in pitch are experienced for
a longer time
it means that the aircraft has time to
experience the effects of this larger
angle of attack for instance
and that means that it starts to climb
and then when it gets to the
point when the pitch drops
it experiences a descent so you get
these large changes
in altitude and air speed with a fuvoid
oscillation that you don't get with the
short period of oscillation
however because it takes
place over a few minutes the pilots have
loads of time to correct and it's
actually quite easy to correct them and
stop them static stability in terms of
yaw or directional stability is the
initial tendency to return to the
direction that we pointed in before the
disturbance
so directional stability is key because
it means we can fly a heading
consistently and remain pointed in the
correct direction
the stability comes from the vertical
stabilizer known as the fin
and is a symmetrical airfoil mounted
vertically on the tail
it provides this corrective yawing
moment
uh when the air hits at an angle for
instance in this case it's from the left
it will be at a certain side slip angle
we give it this
beta symbol here
to the left is considered negative and
to the right is considered positive so
this
airflow coming in from the left hand
side will create an angle of attack to
the symmetrical airfoil and create a
force
in this direction
the force causes us to rotate around the
center of gravity and it'll cause a
rotation this way
and that will realign the aircraft with
the airflow because this is a moment
much like the
hitching moment
it is dependent on the force times the
distance so we can say that with a
forward center of gravity like within
pitch
strength
of the corrective moment in terms of yaw
will be greater
and it's also dependent on the force
which is a half ruby squared scl so
again we can make some same assumptions
where the faster we go and the larger
area
again will increase the strength of its
corrected moment
and our coefficient of lift will go up
according to the
angle of attack
to the
fin which in this case is the
angle of side slip so the larger the
angle of side slip
also the stronger the corrected moment
another way to increase the strength of
this corrective moment is to increase
the surface area
of the fin
and you can do this by adding in
dorsal fins or ventral fins
a dorsal fin goes above
on the upper surface of the
fuselage
and a ventral fin goes beneath on the
underside of the fuselage and they're
essentially
fins that aren't located at the tail
they're located just in front of the
tail and they increase the overall area
and help with this corrective moment so
as with the pitching moment we can
simplify the yawing moment into an
easier to understand unit
which is the coefficient of yaw moment
n standing for moment around the normal
axis and coefficient around the normal
axis
as you can see it is
dependent on the dynamic pressure the
area of the fin and also the wing span
and if we represent it graphically
with the axis
along from left right here being
representative of the slides of angle
and
this being representative of the
coefficient of moment to have positive
static stability we would have a line
like this
so if we have an angle of side slit to
the right a positive
angle of side slip
we would also have
a positive corrective moment positive in
this case
being clockwise
and that would pull us into the
direction of the side slip
conversely
a line like this
would be
negative in terms of its directional
stability if we have an angle coming in
from the right
our corrective moment is negative
which in this case is anti-clockwise
so the wind's coming in from this
direction
and then we
align the nose
more to the left which increases the
slide's lip angle and it makes it worse
and worse
and neutral again
is just this flat along
line
lateral static stability is a tendency
to return to a wing's level state after
a disturbance in role
if an aircraft does not have positive
lateral static stability it will recover
slowly from any wing drops
or could continue to roll if a wing
drops and could lead to the plane
becoming inverted so any uncoordinated
rolling motion that's so a rule where
the rudder is not also used this leads
to either skidding or slipping
in or out of the turn
and that means that the air will hit the
aircraft at an angle
it's been quite hard to draw as you
might be able to tell but essentially
you would have the aircraft rolling it
would start to slip into the turn
and as it slips into the turn the air
starts to hit it from this angle and if
you view it from above you have some of
the air hitting
at this angle
which
coincidentally is also a sideslip angle
lateral static stability is a tendency
to return to a wings level state after
any disturbance and roll
if an aircraft does not have positive
lateral static stability it will recover
very slowly from any wind drop or lead
to further wind drop and eventually the
inversion of the aircraft so basically
any uncoordinated rolling motion where
we don't use the rudder causes either a
slip or a skid in or out of the turn
and
this causes the air to hit the aircraft
at an angle
known
also as the side slip angle
if you think about an aircraft flying
along and then it rolls
some of the air now hits it from this
side and then when we correct it
and look at it from above we see that
there is this implicit side slip angle
which is what i've tried to draw here
because this side slip angle comes from
one side of the aircraft it means that
the other wing
is slightly shielded
by the fuselage
and because it is slightly shielded that
means it produces a lower amount of lift
and the side that's fully exposed still
creates this full large amount of lift
so you start to turn and side slip and
then you get more lift from the wing
that's fully exposed and it should try
and correct you again we can equate this
moment to a coefficient
we use cl for lateral but we already
have cl for coefficient of lifts we give
it a little dash
and then l standing for the moment in
the lateral
stability as you can see you can it's
dependent on the dynamic pressure and
therefore the speed also the area of the
wing and the wings span if you think
about the area of the wing and the wing
span they're
inherently important to this
correctional moment so the value of this
moment is positive
when it turns us clockwise
and negative when it turns us
anti-clockwise so on a graph positive
static stability looks like this this
downward sloping line any
size slip angle from the right positive
means that we have a negative
correctional moment in terms of lateral
or cl dash which is anti-clockwise so we
have a side slit from the right which
means our right wing is going down
then we have this negative corrective
moment which picks that bit wing right
back up a neutral line again
would be neutral stability and a line
like this would be negative stability if
we have side slip from the right
that means that our corrective moment is
positive
and that means that we actually roll
further in to the turn so roll and yaw
are always linked when we roll the
aircraft we also yaw unless we fight
that moment with the rudder and
coordinate the turn
likewise if we yaw the aircraft the
faster moving wind will produce more
lift and eventually lead to a rolling
motion unless it's stopped
in terms of stability some interesting
things happen when we have strong
stability in one axis and not in the
other
so when we have strong directional
stability but our lateral stability
enroll is weak then we have what we call
spiral instability
so basically
due to strong levels of directional
stability
the aircraft in this case will produce
force the right and yaw
sorry force the left and yaw to the
right
this is the directional stability
that leads
to the outside wing traveling faster
through the air
which means
the speed goes up that means it produces
more lift
you have a higher amount of lift on this
wing
a lower amount of lift on that wing and
that is what has happened in this
picture is caused the rolling motion
so this rolling motion will continue and
the size slip angle will continue to
increase which then feeds back into the
start and we correct more which leads to
faster traveling when more roll more
side slip and the pitcher quickly runs
away and it leads to this spiraling
motion so the opposite case of that is
when we've got strong lateral stability
but weak directional stability
then we have a tendency for something
known as dutch roll
so when we experience
a disturbance in roll that causes our
side slip angle
to
increase
that means the wing that's exposed to
this angle produces more lift and the
one that's not has the shielded area and
that produces less lift that's what we
just learned about in terms of lateral
stability
when the lift goes up
it also means that our drag goes up
our induced drag goes up because of this
increase in induced drag it means that
the wing that is exposed to this air the
fully exposed wing experiences more drag
and we will therefore yaw towards that
wing
so it's the wing that is going up the
way
and you end up with the sort of wobbling
motion
so
you roll
it corrects
and when it's correcting it yaws towards
it you end up with this sort of wibbling
wobbling motion
it's quite hard to draw graphically and
even my hand motions are definitely not
sufficient so i'll link a video down
below with a good explanation of dutch
roll with somebody who can actually do
animations
and there should be one in there for
spinning as well
so these problems of dutch roll and
spinning
can be helped with better coordination
between the rudder
and the
ailerons or the
controls causing the roll
quite often the coordination needs to be
quite fast
and probably faster than humans can
manage
so what you can use is something called
a yaw damper
a yaw damper is essentially a auto
correcting system
so when the yaw is experiencing dutch
roll there's an automatic input
in the opposite direction to fight
against the induced yaw and dampen down
this wobbling motion so in summary then
in terms of our longitudinal stability
the tail plane
creates a angle of attack
that will correct any disturbances in
pitch if it's positively statically
stable
represented on a graph it would be the
downward sloping line
we get a decrease in pitch that means
that we get an increase in our moment
and increasing our moment in this case
is nose up
so the nose down come over here as a
nose up corrected moment
opposite case for negative we get an
increase in pitch
we also get an increase in our
corrective moment that is positive and
that leads the problem to going further
and further away dynamic stability in
pitch is normally in an oscillating
fashion that can either be short period
or long period known as fuboid
in a short period the pitch changes
occur very quickly over a short period
of time
and that means it's a high stress on the
aircraft and you can
try and correct it completely out of
sync and make the problem worse known as
pilot induced oscillation
the other version the fugoid is a much
slower process
but it does mean that you're
experiencing these changes in pitch for
a long time and you feel the effects you
feel the climb the reduction in speed
the descent and increase in speed but
because they take place over a few
minutes it's relatively easy to correct
them
in terms of our directional stability
our stability and yaw
again an angle is created between the
airflow and the
fin
in this case it's coming from the right
if we were statically stable it would
produce a force off to the left
and that would create a yaw to the right
and turn us into this airflow
graphically represented you have a
positive line sloping up like this
that means that we have a positive side
slip angle slide slip from the right
then we get a positive yawing motion
which is what we've just seen here
the opposite case would be a sloping
down line
and that would mean that if we had a
positive angle here we'd actually have
the force coming off in the direction to
the right here and that would lead us
further and further away and increase
the size of the bangle more the lateral
stability in roll
is due to this effect of turning around
the corner and if we're uncoordinated we
slip into the turn and that means the
air hits us at an angle which is a size
loop angle
this means that some of the wing becomes
shielded by the fuselage
and that means that a lesser amount of
lift is produced on this side than on
this side
this is obviously the case of a
positively statically stable aircraft in
terms of the lateral stability
and this basically means that the more
lift is produced on the wing that is
on the inside of the turn and that
corrects
the turn for us it resolves this moment
graphically
the positive is the line that's
descending
we have a size of angle from the right
that means our corrective moment is
negative anti-clockwise sides to the
right means we're just are turning
banking to the right
and the negative corrective moment
correct us out of that
and the opposite case for negative
we would bank
and the corrective moment would be
clockwise and it would continue to turn
us
so lateral and directional stability are
linked
and if we have more directional than
lateral we end up with spiral
instability and if we are more lateral
and less directional we end up with a
tendency for dutch roll which is this
wobbling sort of motion as we fly
through the air
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