ATPL Principles of Flight - Class 16: Stability I.
Summary
TLDRThis video from the 'Principles of Flight' series explores the concept of aircraft stability. It explains how stability helps counter turbulence, wind, and thermals, ensuring smooth flight. The video introduces two key types of stability: static and dynamic, which can be positive, neutral, or negative. Using examples like a ball on various surfaces, it explains how stability is measured and how it affects aircraft movement. The video also covers stability along three axes—yaw, pitch, and roll—emphasizing how they relate to aircraft control and maneuverability.
Takeaways
- ✈️ Stability in aircraft is crucial for balancing maneuverability with resistance to turbulence and wind.
- ⚖️ There are two types of stability: static and dynamic, and both can be positive, neutral, or negative.
- ⚡ Positive static stability means an object tends to return to equilibrium when displaced, like a ball in a bowl.
- ⚖️ Neutral static stability occurs when an object shows no tendency to return to or move away from equilibrium.
- 🚨 Negative static stability means the object continues to move away from equilibrium when displaced, like a ball on a hill.
- 🔄 Dynamic stability refers to how an object behaves over time after the force causing displacement is removed.
- 📉 Positive dynamic stability leads to oscillations that decrease over time until the object returns to equilibrium.
- 🔁 Neutral dynamic stability leads to continuous oscillations, without the object moving closer to equilibrium.
- 🚀 Negative dynamic stability results in the object moving further away from equilibrium with each oscillation.
- 🔧 Stability in aircraft is categorized into directional (yaw), lateral (roll), and longitudinal (pitch) stability, which maintain control in different axes.
Q & A
What is the challenge when balancing stability and maneuverability in aircraft?
-The challenge is that while more stability helps keep the aircraft steady during turbulence, too much stability reduces maneuverability, making it harder to change direction. Aircraft need to balance both for safe and effective flight.
What is static stability in aviation?
-Static stability refers to the initial response of an object (or aircraft) when displaced from its equilibrium. It determines whether the object will return to its original position, remain where displaced, or continue to move away.
What is the difference between positive, neutral, and negative static stability?
-Positive static stability means the object tends to return to its equilibrium after being displaced. Neutral static stability means the object stays where it is displaced without returning or moving further away. Negative static stability means the object continues to move away from the equilibrium point after being displaced.
What is dynamic stability?
-Dynamic stability refers to how an object behaves over time after it has been displaced. It determines whether the object will return to equilibrium, remain in its displaced state, or move further away from equilibrium over time.
How do static and dynamic stability work together in aircraft?
-For an aircraft to have positive dynamic stability, it must first have positive static stability. Positive static stability ensures the initial tendency to return to equilibrium, and dynamic stability controls how the aircraft returns to equilibrium over time, either gradually or through oscillations.
What is the relationship between stability and maneuverability in aircraft design?
-The more stable an aircraft is, the harder it becomes to maneuver because greater force is required to change its direction. Conversely, less stable aircraft are more maneuverable but may struggle to maintain their course in turbulent conditions.
What is the difference between aperiodic and oscillating dynamic stability?
-Aperiodic dynamic stability means the object returns to equilibrium without oscillating, while oscillating dynamic stability means the object passes through equilibrium and gradually settles after multiple oscillations.
What happens if an object has neutral dynamic stability?
-With neutral dynamic stability, the object will continue oscillating back and forth indefinitely without getting closer to or further away from the equilibrium point.
What is the role of friction in dynamic stability?
-Friction slows down the oscillations in dynamic stability, allowing an object to gradually return to equilibrium. Without friction, the object would continue oscillating with no reduction in displacement.
How is stability described along the three axes of an aircraft?
-Stability is described as directional stability (yaw) for the normal axis, lateral stability (roll) for the lateral axis, and longitudinal stability (pitch) for the longitudinal axis.
Outlines
🛫 Stability in Aviation: Balancing Maneuverability and Turbulence
This paragraph introduces the concept of stability in aircraft, emphasizing the need for a balance between maneuverability and stability to counteract turbulence and wind. The narrator, Grant, explains that while more wheels on a bike increase stability, they also make it harder to maneuver. In contrast, aircraft require high maneuverability but also need stability to avoid being affected by external forces. The paragraph sets the stage for a deeper exploration of stability in the context of flight, including the effects of turbulence and the importance of generating forces to oppose these disturbances.
🔍 Understanding Static and Dynamic Stability
Grant delves into the two main types of stability: static and dynamic. Static stability refers to the initial response of an object to a force that displaces it from equilibrium, with examples like a ball on a curved surface. Dynamic stability, on the other hand, is the object's response over time to return to equilibrium after the initial force is removed. The paragraph explains the concepts of positive, neutral, and negative stability in both static and dynamic contexts, using the ball on different surfaces as a visual aid. The discussion highlights how these stability types are crucial for an aircraft's ability to maintain its course and respond to disturbances in the air.
📊 Stability in Three Dimensions: Axes and Control
The final paragraph discusses how the principles of stability apply to the three dimensions of an aircraft: the normal axis (yaw), the lateral axis (roll), and the longitudinal axis (pitch). It introduces the terms directional stability, lateral stability, and longitudinal stability to describe the aircraft's stability in yaw, roll, and pitch, respectively. The paragraph also touches on the importance of understanding stability moments and the perspective of the pilot when considering these moments. It sets up for the next class, where the implementation of stability concepts in aircraft design will be further explored.
Mindmap
Keywords
💡Stability
💡Static Stability
💡Dynamic Stability
💡Positive Stability
💡Neutral Stability
💡Negative Stability
💡Equilibrium
💡Longitudinal Stability
💡Lateral Stability
💡Directional Stability
Highlights
Adding more wheels to a bike makes it more stable but reduces maneuverability, while an aircraft needs both stability and maneuverability.
Aircraft experience turbulence, wind, and thermals, requiring forces to counteract them for stability.
Stability is divided into two types: static stability and dynamic stability.
Static stability refers to an object's initial response once a force displacing it from equilibrium is removed.
Dynamic stability involves how an object returns to equilibrium over time after being displaced.
Positive static stability is like a ball returning to the bottom of a curved surface (a bowl) when displaced.
Neutral static stability is when an object has no tendency to return or diverge from its equilibrium point.
Negative static stability occurs when an object moves further away from equilibrium after being displaced.
Positive dynamic stability involves an object oscillating and eventually settling at equilibrium over time.
Neutral dynamic stability occurs when an object oscillates without ever settling at equilibrium.
Negative dynamic stability is characterized by increasing oscillation over time, moving further away from equilibrium.
Aircraft stability is measured along three axes: yaw (directional), roll (lateral), and pitch (longitudinal).
More positive static stability requires more force to displace an object from its equilibrium.
Lateral stability prevents an aircraft from rolling, while longitudinal stability controls pitch, and directional stability governs yaw.
The steepness of the curve representing stability reflects how stable an aircraft is, with steeper curves indicating greater stability.
Transcripts
by adding more wheels to a bike you make
it more stable
but it makes it a bit more difficult to
maneuver
in an aircraft we need to be highly
maneuverable but we also need
a high level of stability so that we
don't get knocked around by the
turbulence and the wind
so how do we achieve this tricky balance
between the two
let's find out
[Music]
hi i'm grant and welcome to class 16 in
the principles of flight series
today we're going to be looking at
stability as we fly through the air we
experience a lot of bumps and shakes in
the form of
turbulence changes in wind and thermals
coming up off the ground for example
if we don't correct these it would throw
us off course and lead us somewhere we
don't want to be
we therefore have to generate some sort
of force that opposes these small shakes
and we do that in the form of stability
i'll be breaking stability down again
into two classes with the first part
looking at the overall concept
of stability and in the second part
we'll take a deeper dive into how we
achieve stability around the three axis
of the aircraft
so there are two main types of stability
we have
static and dynamic stability and they
both can be either
positive negative or neutral
starting with static stability this is
what we call
the initial response of an object once
the force that is displacing it out of
equilibrium
has been removed the easiest way to
think about
positive static stability is a ball on a
curved surface like a bowl
so if we move the ball away from the
equilibrium state
equilibrium stage is represented by this
dotted line here
the initial tendency of this ball will
be
back towards the equilibrium state
this has positive static stability
if we flatten out the surface into just
a plate
then we displace the ball over away from
our equilibrium point
there is no initial tendency to either
get closer
or to get further away this is neutral
static stability the final is negative
static stability or static instability
if we place the ball over here on the
top of this curved surface this hill for
instance
then the initial tendency is to continue
rolling
further away from our equilibrium point
this
is negative static stability so dynamic
stability
is a response over time for the object
to return to the equilibrium state
once that initial force that displaces
it is removed
so we'll start again with the ball in
the curved bowl-like surface
so if we let go of the ball over here
the initial tendency
is to return towards equilibrium that's
our positive
static stability and what you'll see is
it rolls down
it goes through the equilibrium point it
rolls up the other side up to about this
sort of point here
and then it will roll back and it will
roll back
and it will slowly tend towards
back to the equilibrium point if you
represent that graphically
you would have the displacement up here
it comes back
it passes through the zero point it
comes back it comes back
slowly reducing slowly reducing until it
gets back to the equilibrium point over
time
for an object to have positive dynamic
stability
it must first have positive static
stability
so this here is an example of
oscillating periodic dynamic stability
so the displacement oscillates back and
forth until
settling down on that equilibrium point
you can also have a periodic uh dynamic
stability
where there is no oscillation
essentially and the object will just
stop
at the point of equilibrium so for
instance if we put a little
plate in here the ball would then have
the initial response towards the
equilibrium state
i would roll roll roll and just stop
here
this is still positively dynamic
stability
but instead of oscillating back and
forth
it just tends back to the zero point if
we were
to remove all friction and drag forces
from our curveball
we'd we would see that same static
stability
initially but we wouldn't have the
friction slowing us down and bringing us
closer and closer back to this neutral
point
when it crosses the neutral point there
would be no friction and it would go up
to the same point and then it would
bounce back and go to the exact same
point again
so you have this oscillating back and
forth motion
so whilst we are statically stable
we are dynamically neutrally stable we
don't tend closer over time
so if you think about it graphically our
displacement starts off up here
it goes all the way down to the opposite
side and it comes all the way back
and you get this sine wave of
displacement
positive static but neutral dynamic
if we take the flat plate example we
know that this is neutrally
statically stable because if we remove
the force
it doesn't tend closer to the
equilibrium point or further away
and over time that doesn't change the
displacement remains exactly the same
so dynamically neutral instability
is just a straight line over time it
doesn't get any closer
or any further away from that initial
equilibrium point
negative dynamic stability is a tendency
to diverge
over time or dynamic instability you
could call it
so if we think about the ball in the
bowl again
you could have positive static stability
but if for some reason it gains speed
over here
i don't know why it would but just
imagine it gains speed when it goes
through this zero point
that means that it would deflect all the
way up here and then come down with more
force
and then as it passes through the zero
point it gains even more speed it would
end all the way back
up here and it just keeps getting
further and further away
as it rolls up and down this bowl so if
you think about it
in a graph you have the initial
displacement
it oscillates back and forth getting
slowly and slowly
further away as the time
increases if we think about the ball on
top of the curve again
we saw that the initial tendency was
instability negative static stability
because it tends to diverge away from
that equilibrium point
and over time it just continues to
diverge further it doesn't get any
closer back to this equilibrium point
so in terms of its dynamic stability
when you graphically represent it
it starts off by diverging and it just
continues to diverge
over time this is an example of
static and dynamic instability whereas
this is
positive static stability but negative
dynamic stability generally speaking
the more positive static stability an
aircraft has
the more force is needed to displace it
from
that equilibrium point if it's really
stable
basically it likes returning to that
equilibrium point so to get away from it
we need to apply more force an easy way
to think of the strength of stability is
think about the steepness
of the curve in the bowl so over on the
left here we have sort of a low
static stability we don't need to apply
that much force to get it over here
and it will pretend to rollback whereas
on the right here
we've got a very high statically stable
object we'll need to just apply a lot of
force to get it up
the steepness of the curve of the bowl
when you apply that to an aircraft
it means that a stable aircraft
such as on the right here will require a
large amount of force and therefore a
large amount of control
input to actually make it diverge and
maneuver it
think of a very stable tricycle is
harder to move than a normal sort of
bike
once we've managed to displace the
aircraft
if it has a high dynamic stability it
will want to remain
in this new displaced state so we've
broken out of this previous equilibrium
and we're into this new one
and any disturbances to this new
equilibrium state
will tend back towards this new
equilibrium state
this means that dynamically stable
aircraft are easier
to control and we don't need to
constantly edit the inputs as
we stop any further divergence
so if we apply those concepts of
stability to
an aircraft we can see that
we have the three dimensions to work in
essentially we've got
the normal axis the lateral axis and the
longitudinal axis
rotation around normal is yaw rotation
around longitudinal as pitch and around
lateral is roll
but in terms of stability we don't talk
about roll stability
hitch stability and your stability
because we already have
names for that sort of motion so we get
a bit confusing
so instead we call the stability in
yaw or directional stability the ability
to keep
heading in the right direction our
stability and role
we call lateral stability that's our
stability not to rotate the aircraft
and in terms of pitch our stability is
known as longitudinal stability it's the
stability
of the longitudinal axis not to go up
and down
and we'll see in the next class that you
get
essentially stability moments that you
deal with
and it's important to note that it's
positive
to the right for directional stability
and that is
from the perspective of the pilots
always so if you're heading along
and you get a corrective moment to the
right clockwise
that would be a positive corrective
moment in terms of lateral it's the same
in terms of the pilot's view if you roll
to the right you roll clockwise that's a
positive
corrective moment and in terms of pitch
you view it as positive nose up so a
nice quick
class there this is just the sort of
outline of
the concepts of stability and we're
going to break down
how the aircraft actually implements
stability concepts in the next class
so you get three types of static
stability positive neutral and negative
and then you get dynamic stabilities
within them as well
so let's first talk about positive
positive and positive
you can either have an aperiodic or
oscillating motion
a periodic is just a tendency to return
to the equilibrium point and the
oscillating passes through
the equilibrium point and bounces back
and forth until eventually settling down
on that neutral positive static and
negative sorry positive static and
neutral dynamic
is again an oscillating motion and you
have it passing through
the equilibrium point but never getting
any closer it just passes back and forth
back and forth like a pendulum
it keeps basically swinging back and
forth positive
static stability and negative dynamic
again is an oscillating motion but each
oscillation brings us further and
further
away from the equilibrium point
important to note is that
if an object is positively dynamic in
stability
it must also be positive in
static stability but not necessarily
for everything else in terms of neutral
static if you have
neutral dynamic as well that's an
aperiodic motion it doesn't repeat
and you essentially display something
and it stays displaced it doesn't get
closer over time
in terms of negative static stability if
we've got
negative dynamic again that's an
aperiodic motion and that's just a
tendency to
initially diverge and then keep
diverging over time
you can think of the levels of stability
in terms of the steepness
of these bowls something that is
not very stable will have
a low curvature to the bowl and it's
easy to get out
of this and in something with a high
level of stability
the sides of the bowl are a lot steeper
and it requires a lot more force to
diverge from that equilibrium
and then when we apply stability to our
aircraft axes we've got
stability in pitch known as longitudinal
stability
stability in row is known as lateral
stability
and stability in yaw is known as
directional stability
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