ATPL Principles of Flight - Class 16: Stability I.

Summary

This 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

00:00

🛫 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.

05:00

🔍 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.

10:02

📊 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.

Keywords

💡Stability

Stability refers to an object's tendency to return to its equilibrium or original state after being displaced. In the video, stability is discussed as essential in aviation to ensure smooth flight despite turbulence. There are different types of stability (static and dynamic) that affect how an aircraft behaves when external forces are applied, such as wind or turbulence.

💡Static Stability

Static stability refers to the immediate response of an object after being displaced from equilibrium. It can be positive, neutral, or negative. In aviation, static stability helps an aircraft maintain its course when minor disturbances occur, like slight shifts in wind or turbulence. The video uses the example of a ball on different surfaces to explain this concept.

💡Dynamic Stability

Dynamic stability describes how an object behaves over time after it has been displaced from equilibrium. It can also be positive, neutral, or negative, and determines whether an aircraft oscillates back to equilibrium or diverges further from it. The video uses the example of a ball in a bowl, showing how positive dynamic stability brings the ball back to rest after oscillating around the equilibrium.

💡Positive Stability

Positive stability occurs when an object, after being displaced, tends to return to its original position. In the context of the video, an aircraft with positive stability will return to its steady state after encountering turbulence or wind shifts, making the flight smoother and easier to control. Positive stability can apply to both static and dynamic contexts.

💡Neutral Stability

Neutral stability is when an object, after being displaced, neither returns to its original position nor moves further away. In the video, an aircraft with neutral static stability would maintain its new position after displacement without trying to return to its original state, meaning it doesn’t automatically correct itself but stays where it was displaced.

💡Negative Stability

Negative stability refers to the tendency of an object to continue moving further away from its original position after being displaced. The video describes negative static stability with a ball rolling off a hill—once displaced, the ball continues to move away. In aviation, an aircraft with negative stability would become harder to control as it diverges further from its original flight path.

💡Equilibrium

Equilibrium is the state where an object is at rest or balanced, with no net forces acting on it. In the video, equilibrium is the natural state of the aircraft that stability seeks to maintain. If the aircraft is displaced by turbulence, stability determines whether it returns to equilibrium, stays in its new position, or continues to move away.

💡Longitudinal Stability

Longitudinal stability refers to an aircraft's ability to maintain its pitch angle (nose up or down) and resist deviations in the vertical direction. This concept is crucial for the safety and control of the aircraft during flight. In the video, longitudinal stability is essential to prevent undesired climbs or descents, ensuring smooth flight in changing conditions.

💡Lateral Stability

Lateral stability is the ability of an aircraft to resist rolling movements and maintain its orientation along the lateral axis. The video explains that lateral stability helps an aircraft resist side-to-side rolling, which is critical for maintaining a straight flight path. Proper lateral stability reduces the need for constant corrections by the pilot.

💡Directional Stability

Directional stability refers to an aircraft's ability to maintain its heading or direction and resist unwanted yawing movements (rotation around the vertical axis). The video highlights that directional stability helps the aircraft stay on course, reducing the pilot’s need to continually adjust the aircraft’s heading during flight.

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