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.