ATPL Principles of Flight - Class 17: Stability II.

Summary

In 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

• 🛫 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.
• ⚖️ 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.
• 📈 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.
• 📊 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.
• 🔁 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.
• 🔄 Pilot-induced oscillation occurs when a pilot's corrective actions are out of sync with short period oscillations, exacerbating the problem.
• 🧭 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.
• 📉 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.
• 🤸‍♂️ 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.
• 🔗 Roll and yaw are interconnected; uncoordinated rolling motions can lead to slipping or skidding, affecting the side slip angle and overall stability.
• 🔄 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

00:00

🛫 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.

05:02

🔁 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.

10:04

🌀 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.

15:04

⚙️ 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.

20:07

📉 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.

Keywords

💡Stability

Stability in the context of the video refers to the aircraft's ability to maintain its equilibrium and return to a steady state after encountering disturbances. It is crucial for the aircraft's safety and control. The video discusses three types of stability: longitudinal, lateral, and directional, each related to different axes of the aircraft. For example, the script mentions that 'stability around all three of the aircraft axes is important', highlighting the multifaceted nature of stability in flight dynamics.

💡Longitudinal Stability

Longitudinal stability is the aircraft's tendency to return to its original pitch attitude after a disturbance. It is associated with the aircraft's nose-up or nose-down behavior. The video explains that static longitudinal stability is the initial response to disturbances in pitch, using the example of a jet airliner where the center of pressure and center of gravity relationship creates a moment that is corrected by the tailplane.

💡Center of Gravity

The center of gravity is the point around which an aircraft's weight is distributed. It plays a significant role in determining the aircraft's stability. The video mentions that 'weight acts through the center of gravity', and its position relative to the center of pressure influences the aircraft's pitching moment, which is crucial for maintaining stability.

💡Angle of Attack

The angle of attack is the angle between the oncoming airflow and the wing's chord line. It is a critical parameter in determining the lift and drag forces acting on the aircraft. The script explains how changes in the angle of attack at the tailplane can correct disturbances, such as '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'.

💡Dynamic Stability

Dynamic stability refers to the aircraft's behavior over time in response to disturbances. It can be short period or long period, affecting how the aircraft oscillates after being disturbed. The video distinguishes between short period oscillations, which are quick and can lead to pilot-induced oscillations, and long period oscillations, known as phugoid oscillations, which occur over a longer time frame and are easier for pilots to correct.

💡Coefficient of Moment

The coefficient of moment is a dimensionless number that represents the moment generated in response to a force, normalized by dynamic pressure, area, and mean aerodynamic chord. It is used to quantify the stability characteristics of an aircraft. The video describes how a positive coefficient of moment in relation to pitch refers to nose-up, and how it is graphically represented to visualize the aircraft's stability.

💡Directional Stability

Directional stability is the aircraft's ability to maintain its heading and align itself with the relative wind after a disturbance. The video explains that it comes from the vertical stabilizer, or fin, which provides a corrective yawing moment when the aircraft is subjected to a sideslip angle. The script mentions that 'directional stability is key because it means we can fly a heading consistently and remain pointed in the correct direction'.

💡Lateral Stability

Lateral stability is the aircraft's tendency to return to a wings-level flight after a disturbance in roll. The video discusses how positive lateral stability helps the aircraft recover from a wing drop and how the lack of it can lead to uncoordinated rolling motions, such as slipping or skidding. It is illustrated through the concept of the aircraft rolling and the resulting side slip angle, which influences the lift on each wing.

💡Dutch Roll

Dutch roll is an oscillatory instability that involves a combination of roll and yaw motions. It is characterized by a wobbling motion of the aircraft. The video describes it as occurring when there is strong lateral stability but weak directional stability, leading to a coupling of roll and yaw motions that can be challenging to control. The script mentions the use of yaw dampers to counteract this phenomenon.

💡Yaw Damper

A yaw damper is an automatic flight control system that helps to counteract unwanted yawing motions, such as those experienced during Dutch roll. The video explains that it provides an automatic input in the opposite direction to the yaw to dampen the wobbling motion, making it easier for pilots to maintain control. It is an example of how technology can assist in managing aircraft stability.

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