Wish you all very, very happy New Year. We are on the tarmac of flight laboratory airship,

and you could see that I am surrounded by so many aircraft, a similar aircraft you

might have seen when you did the performance course, but it’s a customary that I introduce

all the aircraft to you because I do not know really how many students have done this course,

first course, which is the prerequisite for second course that is on stability and control.

So, let me explain you whatever aircraft we have with the specific impetus on the components,

which would be used for, very, very useful for stability and control understanding. This is

Cessna 206 airplane. Here you could see the wing is again a high wing configuration and

these are the struts to support the wing and in performance course you know what is this

propeller is meant for, the propeller has a unique role to give power, extract power from the brake,

but in performance also we decided that we understood that we were precise that

wing is meant for giving lift, it has nothing to do with stability because you assumed that

in performance analysis that aircraft is most statically and dynamically stable, okay.

So, in performance course, we are more bothered about what is the lift generated by the wing,

what is the drag generated by all the components lift by all the components and we primarily aiming

towards an L by D which is maximum right. Now but in this course, we will be talking

about stability and control. So, our focus will suddenly go towards this portion, which is called

horizontal tail. This is complete horizontal tail and this horizontal tail will be seen and we will

understand now that this is primary responsible to give the longitudinal stability okay.

And we could see the part of this horizontal tail can move up and down and this is called elevator.

It also has a trimmer which goes up and down in the opposite direction, and we will discuss about

what is the trim tabs we will be doing. Most important thing to understand is,

this total area both side together is responsible to give both static and dynamic stability in the

longitudinal plane and part of it which is called elevator is responsible to give the longitudinal

control that is if I want to move the airplane up and down, I will be using this elevator okay.

Similarly, it has a trim tab, you see here I do not know whether it’s visible to you or not,

a small portion which is required to use at the end so that pilot can fly hands off,

we will talk about that okay. Similarly, if you see here, there is a vertical tail which

gives you stability in terms of directional and lateral motion, a part of this is called rudder,

which can be moved either way to turn the airplane this way or that way. So, that is a rudder

control. So, we have now elevator control, rudder control, and now we will go for the aileron.

So, what is the role of elevator? If I want to pitch like up and down like this I will be using

elevator, so it is a elevator control. If we want to move the airplane, yaw in this direction then

this is the rudder okay, so rudder control. If we want to roll the airplane or bank the airplane

like this, then this control is called aileron. So, if I move it down like this you could see as

it goes forward, force will be acting upward and it will give a, left wing will go down. Similarly,

if I put it like this other thing it will bank like this, so this is called aileron, there will

be another pair of aileron that side. So, you can move it together you can move it differentially,

so this is called aileron or aileron control. So, we have now three primary control, one is

elevator for longitudinal control, rudder for directional control and aileron for roll control

or lateral control. These are the primary three controls surfaces we will be using and when you

defined everything about stability, we will also try to see, how this control forces or

moment generated gives the response vis-à-vis the stability characteristics of the airplane, okay.

That will be the total part of this one of the module of this course, and I will be flying this

Cessna 206 with my pilot and also give you live demonstration of what is the meaning of static

stability or dynamic stability in flight. This is our Hansa 3 aircraft, I will be walking

down to Hansa 3 aircraft, which is our nation’s pride, one of the finest light weight aircraft.

You could see that if I compare Cessna 206 and Hansa 3 you could see this wing is a low wing a

high wing configuration. So, from stability control point of view, this thing will have

implication, that is why I am stressing you please keep back of your mind Cessna 206, which I showed

you just now as a high wing configuration, and this is a low wing configuration, okay.

Similarly, we have this is Sinus manufactured by Pipistrel, this is called motor glider.

Motor glider because conventionally gliders were launched by cable winch combination, right, it

is like flying a kite, but nowadays we find the gliders, gliders means high L/D ratio,

right so that they can loiter for longer duration, but need to take off. So, instead of cable what

has been done, a small engine has been put. So, that is used for takeoff, once you go to

a particular altitude, you switch on the engine, there are gliders where the whole

propeller will go inside, so that you have very good L/D configuration, and you fly and

then when you find that it has come down to a lower altitude, you will start the engine and

go to another altitude and glide. So, you have a very large endurance from that perspective.

This glider if you see also has high wing configuration from stability and control

point of view, you please note down that there is a there must be some requirement to have high wing

consideration, sometime low wing consideration and primarily from stability and control point of view

sometime it could be because of maintenance point of view also. So, we will be talking

about those things in detail, okay. And since I am talking about the gliders,

I have talked about Hansa 3, the speeds are less than around less than 0.3 Mach. You could see

this is these are the aileron which by now you know and these are used to give a bank

to the airplane and airplane can be rotated like this through this aileron, and this is

also one of the control, like we have elevator, we have rudder and we have aileron, okay.

These are extremely important to understand because please understand in this course, we are

not going to talk in terms of performance, we are going to talk in terms of stability and control.

So, we will be really bother about how much moment each component gives what center of gravity and

whether that contributes to static stability or not, that is very important right. For example,

if we see this airplane, right and if you assume that let’s for a fictitious way, we assume that

CG somewhere here let’s say here okay. Let’s say hypothetically. So, if this wing

sees some angle of attack, its lift force I can represent at the quarter chord point, so this

force will give a nose up moment, okay. As the angle of attack will increase this will give nose

up moment that means with the increase of angle of attack, this wing will contribute towards the

de-stability. It will further take the aircraft up. But if you see the tail since CG is there,

if there is the change in angle of attack, then this will have a force in this direction,

that will give a nose down moment. So, this will try to reduce the angle of attack if there is at

all given through a distance, right. So, this we will be talking it has a some

sort of stabilizing effect, all those things we will be discussing and in general we find that

any component which is located ahead of center of gravity will give destabilizing effect, any

component, which is behind center of gravity will give stabilizing effect. So, this is in general we

need to understand in an airplane and then we will be talking in detail and formulate it and try to

see how can I understand this characteristics and configure an airplane with adequate, well planned

and designed stability characteristics right, that is the basic purpose of this course.

So, we are talking about low subsonic or low speed aircraft. Now in this course as

far as stability and control are concerned, we will not be restricting ourselves to low

speeds, we will also go for high speeds. So, let me introduce one of our, another pride aircraft,

I have the model for that aircraft and you all must be knowing this is Tejas, right! this is

delta wing configuration. This is one of our pride, Tejas subsonic aircraft, and because it

has delta wing, lot of sweep, okay. And you could see control surfaces are laid

in a different than, different manner than what is in a conventional aircraft and in performance when

you’re talking about sweep, we are talking in terms of critical Mach number primary to reduce

the drag, however once the sweep for such high speed airplane is there, we need to discuss about

what is the effect of speed on the stability, what is the effect of variation of parameters

with speed in terms of dynamic stability, then what is the sweep is going to do as far as

stability and control consideration are there. So, that is why this Tejas, which I am purposely

showing you, I always love to show our own aircraft, unfortunately out of so many aircraft,

only two are our own aircraft, I would like this sort of a course should held by youngsters that

after 10, 15 years this tarmac is full of aircraft designed and manufactured by India only, okay.

With that aspirations, with that wishes in the New Year, I again say Happy New Year to all of you and

let’s start learning this course with a clear cut ambition in next 10, 15 years, this tarmac will be

full of aircraft manufactured, built, designed by we, we the Indians. Thank you very much.

We are back again and Kanpur is a – it’s basically a pleasant part of the winter session,

you will find during this time people are wearing fancy dresses, warm clothes, lots of fog around,

lots of disruption in flight, but there is tacit freshness among all the individual. With

that freshness, we will start this course title Flight Stability and Control. And I will underline

two words, one is stability and control. In my first course on airplane performance,

at the end we have discussed about these two terms, stability and control. In this course,

will start from there but now we will be unfolding few salient things in an exhaustive manner to

understand the true meaning of stability and control and how a designer will ensure the

stability and control aspects of airplane. So, that finally the pilot will fly, he is flying

at ease do not forget whatever you design with the flight vehicle, if it is a man flight vehicle we

need to be bothered about the human being, the pilot, the passengers, if we are very,

very careful about their comfort, if they are not comfortable then the vehicle is not

well designed. To ensure that we take care of these issues, we need to know more detail about

stability and control. Now the question come, how I am going to take this course forward?

See in introduction to airplane performance, we were talking about mostly response of the airplane

in terms of translatory motion. So, you assume that yes we apply Newtonians law that is force

equal to M into acceleration of central mass. So, we simplify the whole description to a point mass

approximation or point mass model, but in this course since we are talking about stability and

control, we will be talking about not only how the airplane is going in a translatory motion.

We also like to talk about is rotational motion right, and the moment something rotation comes,

your point mass approximation or point mass simplification will not work,

because you know in rotation, it is not the total mass, it is the mass distribution or we say it is

the moment of inertia that plays a role, okay. At the same time, we will be also bothered about

what is the moment coming on the airplane at a given flight conditions. Since these things

we have touched upon in last lecture, so you will find that few of the last lectures will be

repeated, so that there is a continuity. Why that is important please understand after

all we have learned by now some important phases of airplane

motion and this part was taxing and takeoff. This was climb, this was cruise or it could be

accelerated flight, this is loiter, here it is landing. The question we ask to our self is if

I want to climb, I need to generate particular amount of lift and once I generate particular

amount of lift how do I generate that, that is what are the control surface I deflect and that

part is taken care in the topic of control. And the next question comes to our mind, if I

deflect the control, let’s say elevator whether the airplane is going to be stable or not or to

be more precise, more correct, how this control deflection will generate response to an aircraft

which is stable. Same control input will have a different type of response if the aircraft is not

stable right. So, we need to build a relationship between stability and control in a very tacit

manner and we should understand exactly what it means, what this relation is going to give us.

For example, so far we have assumed that for a cruise, we have to give some elevator deflection,

but question is if I give a elevator deflection does the airplane immediately comes to the angle

of attack or there are transience, and if you want to really design an airplane better you should

know clearly about the transient response. So, that is the part of a stability which is

we referred to as dynamic stability, okay. So, in stability we have static and dynamic,

both, okay. And by control here we will be primarily meaning this elevator control,

it could be aileron, it could be rudder, there could be canard like this. So, we will

try to understand each of this and try to see how stability and control are connected, how they are

connected and why we are interested in how they are connected, because finally we know through

their relationship what is the handling qualities of the airplane, because we are more bothered

about the pilot and the passenger, okay. So, that will be the whole direction road map

for this course and we will be taking back to our earlier lectures may be around eight to 10 where

we will start the introduction the way we used in that time, so that there is continuity, okay.

I will use Mann Ki Baat too frequently in the first 15 lectures,

so as to make ensure that we are seamlessly going into direction where the continuity is there,

okay. Let us go back to this term stability and we agreed that we will try to study it through

static and dynamic stability, okay. What is static stability?

You recall, you are clear by now through our first course that every system, which is in equilibrium

or any system which is in equilibrium and if it is disturbed above the equilibrium

and if it has initial tendency to come back to that equilibrium we say the body is in static

equilibrium or the body is statically stable, we will say the body is statically stable that

is precisely more correct, appropriate. Just to recall remember, we have given this example.

I draw this line, this is a ball. Now what happens, it’s must, excite your mind to go

back to your class 10th, 11th where these diagrams were frequently used, let’s say this body is in

equilibrium, this dotted line show the equilibrium position. What is the meaning of the equilibrium,

the net force and its moments are zero, right. The similar case if I come through aircraft….

…one of the equilibrium state is for the airplane is in cruise okay, where lift = weight,

thrust = drag, net force is 0, net moment is 0, so cruise is also in equilibrium state. Now what

is the question you are going to address, you are going to address whether this body

at this equilibrium is statically stable or not, meaning thereby if I come again from here to here,

if I have the same question. Yes this body is in equilibrium here

because net force is 0, whether this body is in body statically stable or not, how do

I check? I say I give a small disturbance. The moment I disturb it from the equilibrium state,

you see there is a component of weight will be there which would try to take it back to the

equilibrium state. So, it has initial tendency to take the body toward the equilibrium state,

the catch word is initial tendency. So, we say this body is indeed in statically stable mode

or statically stable condition. In contrast if you say this body here,

if I displace it by small amount now the component of weight will take it away from the equilibrium

state, so we say, it does not have any initial tendency to come back to equilibrium. So,

this is statically unstable, this is clear. Similarly with this understanding if I try

to know whether this gentleman or friend aircraft is statically stable or not,

what we have to do, we have to first see what is the equilibrium state because static stability,

we are talking with reference to the equilibrium, a disturbance about the equilibrium.

So, first we find out equilibrium state, here I know the cruise where lift = weight,

thrust = drag and moments are automatically balanced. This is typically a equilibrium

state and now what I, if I want to check whether this is static stability or not satisfying static

stability condition or not what I have to do, I have to give it some disturbance, as Delta Alpha,

angle of attack, will be an upward gust coming like this and if it immediately generates a

moment nose down that is to nullify… …that Delta Alpha because initially the

equilibrium state was let’s say Alpha 2 degree and because of the disturbance let’s say angle

of attack became 3 degrees, so static stability demands that it should automatically generate a

nose down moment, so that it has initial tendency to come back to 2 degree of Alpha, is it clear?

Okay. If it has, it is statically stable, if it is does not have, it is statically unstable. At

this point please understand it is not that we cannot control a statically unstable airplane.

The catch statement is unstable does not mean uncontrollable, you can do an experiment,

you can take a stick on your finger. Imagine a stick is there that’s statically unstable,

but you can control it, okay. In fact your most of our fighter airplane they are marginally

stable or marginally unstable in terms of static stability sense. That is why they are

highly maneuverable, okay. Once you do that static stability we will be spending lot of

time on dynamic stability, that is okay, fine. We understand that suppose I take a mass spring

system right and assume that this whole mass spring system is isolated from the environment,

and the spring is linear, there is no air, completely vacuum you know very well at some point

this is the equilibrium. That is here if I draw the free body diagram I will find MG acting here

and here the force because of extension case and they are balanced so it is at equilibrium.

Now if I want to check whether this body is statically stable or not, what I have to do,

let’s say I stretch it, I stretch it here to some length and then release it. So,

what will happen the moment I release it, because there will be a force Kx acting opposite so it

will start moving towards the equilibrium, it will overshoot. Again as it overshoots, the force

will be acting downward, so it will again take it like this. So, it will go on oscillating like this

and since there are no air no medium a spring is linear it will keep on oscillating like this.

So, do you call it statically stable or not, let us check from the equilibrium,

where I deflected the body, it has tendency to come back to the equilibrium, so it has initial

tendency to come back to equilibrium but because of inertia it goes up and again as it goes away

from the equilibrium, it has the tendency to come back to this, so it goes on doing like this.

So, it does have static stability but does that mean it has dynamic stability,

if you want to know what is dynamic stability, we have to put another condition there,

not only the initial tendency to come back to equilibrium but also its amplitude should

decay in finite time that is it should go on doing like this and finally it should come

here to equilibrium. So, this will not happen here, this is the case where it is statically

stable but dynamically not stable. Now think of a situation, if I put some

water or oil in to this, what will happen? It will have this and amplitude will come down,

so there is a damping. And for dynamic stability not only the initial tendency, it should also

its amplitude also should decay in finite time, so there will be concept of stiffness,

there will be a concept of damping when you are talking about dynamic stability, right.

And this damping decides the passenger comfort or the handling qualities of the pilot. So,

we see how they get connected, okay. So, we will be talking more and more on the dynamic stability

in this course. Two part of static stability I have covered in the last course.

Because there will be there will be many students who will be joining these course who may not have

the first course exposure, even if they have it may be one time so it’s my duty to see that we

revisit again the important point and we will make you prepare ready to cruise or accelerate into the

stability aspects including dynamic stability aspects. This is one thing. Second would be the

response. That is if we see, let me erase this part. Suppose this is the airplane and this is

the tail and this is the elevator, the wing and there is the rudder and there is an engine.

Let’s say it’s flying at some speed. We have seen whether it is statically stable or not,

dynamically stable or not once we have done that, then we will try to know if I give unit

deflection of elevator, how the angle of attack is building, right. Say, whether it is building

something like this or building something like this or building something like this.

So, in this 3 diagram, I have shown that they are all finally coming to some angle, let’s say Alpha*

but these 3 graphs also tells you that transience of going to that Alpha star is different. So,

we have to clearly see, how do I calculate Alpha* that the response for a deflection of

elevator in time domain, right, with time how it is happening. So, this is also extremely important

topic that we will be discussing in this course. So, you could see that initial few eight to 10

lectures will be exactly what we have done. In our last course, in fact some same video clips

will be shown to you. I will be coming again after two or three lectures, as a Mann Ki

Baat session and try to discuss few things more because there will be many students who are who

might not done airplane performance, even if they have done they have missed few concepts.

But once we build a solid base, then we go systematically into static stability, what are the

control required, dynamic stability part, response and how do I design the handling qualities. So,

this will cover the whole course, which is it is a huge course, please understand. It will require

little bit of effort from your side, we have to sit with pen and pencil and solve some equations

which are again are not more than a first-degree equation may be some time, I will be using Laplace

transform which I will explain, don't get panic about all these names. I will ensure and take

guarantee that it will be as simple as you are a student of class 12th or the most first year.

So, with that positive note we will take up this course, I hope you will enjoy and this time for

a change you will be allowed to interact with me directly on question sessions through my another

email ID I will be creating or I would like to interact with you directly along with the TA’S

right, so that helps sometimes there is a lag which I learned from last course. Thank you.