good morning so we are discussing about aircraft
design and we have seen in the last lecture
ah two components of an aircraft and if i
draw an aircraft why not this is the horizontal
tail and this the elevator there is a vertical
tail there is a radder and importantly there
is a wing and there will be flap there could
be engine and of course we will have landing
gear these are the primary components we have
right we also know this aircraft is primarily
designed to carry passenger or a cargo from
one point to another in a civil sense right
so we have to also bother about what is there
inside this fuse large right and also we know
that somebody will be flying it so there is
a pilot there may be more than one pilot they
look crew members to say all those things
are to be integrated in the design the requirement
is to be integrated in the design many times
this requirements are conflicting so as we
develop we will see how to handle these issues
as i have told you earlier this course will
focus primarily on the conceptual configuration
design with more stress on the aerodynamic
aspects right
once i say aerodynamic aspects let us go back
to our first course where we talked about
airplane performance and we knew that if a
flat plate is moved with a velocity v then
it will have a reaction experienced and one
of the component of reaction is called lift
another component is called drag right as
a designer i need lift because i need to lift
the weight as a designer i want drag to be
as low as possible because this drag will
try to reduce the speed because after all
how i am getting the speed is through an engine
which is producing a thrust a force so if
i want this drag to be handled with at is
i will always prefer the drag to be as low
as possible but in totality most of the time
we look for a configuration where l by d is
maximum
most of the time whenever i think of lift
and from aircraft we know that the major responsibility
of the wings is to take care of the lift right
and from here also we know if there is a angle
between velocity vector and the plate that
will be responsible for generating the lift
at a given speed given altitude etcetera we
know right and whenever you say the aircraft
should produce lift equal to weight when i
am flying at a cruise like this that is if
i recall your performance lecture you have
list to popular equation lift equal to weight
and thrust equal to drag if i see here first
equation lift equal to weight how a designer
will see this condition lift equal to weight
why it is important question i need to have
an airplane i need to have an wing because
we have agreed that lift is primarily responsible
for generating lift so i need to have a wing
especially designed so that it can generate
lift at different altitudes a different speed
as per our requirements here also the designer
when i see lift is equal to weight so if i
can somehow minimize this weight without compromising
the volume because after all we have to carry
cargo passenger etcetera and if somehow i
can reduce the weight reduce the weight means
what it is the aircraft empty weight somehow
if i reduce this it will help me my conditions
will be lesser taxed
so if i now try to see little more through
this equation what do i see as a designer
when i write lift equal to weight i have weight
and lift these two aspects one is primarily
aerodynamic driven which is the lift this
is aerodynamic driven and this weight is basically
material when payload etcetera driven right
why weight is important we will find aircrafts
are categorized by their weight class right
so below seven hundred k g below fifteen hundred
k g like that one characterization will be
there so weight does play an important role
so that there is a need to characterize an
airplane one way through weight why weight
is important let us see this if i use thrust
equal to drag and lift equal to weight i know
that thrust is required it will be w by l
by d or w by c l by c d so we could easily
see that thrust required for a cruise flight
this is cruise is directly proportional to
the weight we are carrying and inversely proportional
to c l by c d so here a and smart aerodynamic
person or smart aerodynamic approach we will
try to ensure the c l by c d is maximum an
a person an expert who are on the material
side they will try to ensure that the weight
is low
however the primary objective of carrying
capacity are carrying pay loaders etcetera
then the strength also comes very very important
you need to have particular material a particular
composition particular combination of material
to ensure that it is enough strengthened to
counter the stresses develop when the airplane
moves in medium which is air in this case
so you could see that this is very very important
if i want to ask myself how much thrust is
required for a particular mission requirement
lets say you want to design an aircraft there
will be some mission requirement we will talk
[spleas/explicitly] explicitly on mission
requirement
but to start with what could be mission requirement
that you say ok my airplane should be able
to climb up to altitude of ten kilometer eleven
kilometer my aircraft will be capable of cruising
at that altitude with hundred fifty meter
per second hundred meter per second and if
it is high speed is three hundred meter per
second even then it can go up to one point
five to depending upon what where you are
designing but if you recall i have mention
the initially we will be talking about low
speed aircraft so i need to know this weight
because that will tell me what is the thrust
required and i can now think of what sort
of the engine i will be picking up right also
remember i have told the engine part will
be almost like that taking things on the shelves
right standalone engines reli[able]- where
i am going to design engine in this course
at all
so weight is that way extremely important
and second part as a designer you need to
also look here when i say lift equal to weight
lift i know half rho v square s c l equal
to weight right rho is what rho is whatever
altitude you are cruising or taking off exist
of course for cruising i am talking about
c l is the lift coefficient and of course
w is the weight 
and if you see here the speed required to
maintain lift equal to weight will be given
as two w by s by rho c l so from this cruise
condition we find that whatever altitude i
am flying i need to have enough engine thrust
or power so that i can attain this much of
v and that v is proportional to w by s it
is inversely proportional to density inversely
proportional to c l or c l to the power half
or to the power half in particular and if
i write it here also to the power half
so if i want to cruise at a speed which is
lower so that my engine requirements are less
what option i have got then i must if i want
to really see that v cruise which ensure lift
equal to weight if i want it lower that means
i have to ensure that w by s is also low c
l is higher and rho is also higher it's directly
from here as for as rho is concerned that
is not in our hand you get depending upon
which altitude you are flying rho in change
right now there are two things one is w by
s another is c l these are two important parameter
which we need to understand at the designer
and one is w by s which is primarily the weight
by wing area and this is primarily aerodynamic
and this w by s is known as wing loading where
that lift equal to weight and half rho v square
s c l equal to weight right ah v equal um
to two w by s by rho c l under root this is
the speed required to maintain lift equal
to weight at an altitude rho which is flying
at a given c l
but when i talk about wing loading please
understand this this wing loading has a ah
clear cut ah ah the wing loading need to be
clearly understood because there is a generally
confusion i have seen among the student when
i say w by s is the wing loading 
i am assuming lift equal to weight right but
you can see that airplane can also flies at
the lift equal to n w having a load factor
of n in that case wing loading or v required
to fly will be two n w by s rho c l and you
could see that if i write this as two w by
s star by rho c l what i am seeing w by star
is nothing but n w weight that is the wing
loading has the actually increased right one
is way for a load factor n
so whatever wing loading we talk about we
are clear back of our mind that we are talking
about lift equal to weight that means if you
want to if designing an airplane for a mission
and you have chosen w by s some number you
should not expect that number is good enough
to do a flight where the wing loading required
is w by s star which is nothing but n w by
s so as if the weight has increased by n times
right you could easily understand if i go
on increasing the wing loading meaning thereby
what does it mean if i go on increasing the
wing loading that means i am relatively i
am reducing the wing area
for example if i have a mass or weight w i
flatten it and lets say w remain same and
area is s one second case same thing but i
flatten it further and that time the area
is s two the first case it is w by s one second
case it is w by s two the wing loading w by
s two is less than wing loading w by s one
the moment ah wing loading is less for the
second case you could see that speed required
to maintain level flight is also less the
wing loading place or the important role in
deciding the speed for a given c l right we
further see what this wing loading
actually wings an in designer you are aware
of v stall we have talked about v stall as
the root two w by s rho c l max you also know
c l max of other two cruises this is c l versus
alpha this is plot it like this some where
there is a alpha stall and this gentleman
is c l max typically the c l max value for
the average side if i say or a conventional
wing without any flaps and all ah it's not
a bad idea to take that value as one point
two ok now see here what is v stall v stall
is the minimum speed required to maintain
lift equal to weight right
so lets have an idea if w s is increased what
will happen v stall will go on increasing
similarly if rho goes on decreasing that is
i am taking off in kanpur then taking off
in lay ah ladakh v stall also will increase
in lay ladakh for same w by s or c l max that
is for same w by s and for same c l max if
we want to fly with a minimum speed lift equal
to weight that will be higher compared to
in delhi or kanpur because rho is ah will
ah less in lay ladakh area
let us see lets have a num field for number
let us see w by s is ten k g per meter square
please understand this w by s you need to
be consistent should be newton per meter square
right because lift equal to weight so this
gentleman is newton so w also has to be newton
right but i am writing w by s as ten k g per
meter square and and if rho if i take it is
one k g per meter square meter q and lets
a c l max i am taking here again one one point
zero just to make like simpler what will be
the v stall v stall will be two into w by
s that is ten this is k g per meter squares
we have to multiply by nine point eight
so for the newton and rho is one and c l also
i have taken one so this will be around the
ah lets say roughly i take this as ten two
into ten into ten so i am approximately nine
point eight equal to ten just for calculation
purpose divided it by one so this will be
ten ah root two this is ten root two and root
two is one point four one four so this is
roughly fourteen point four roughly let say
meter per second and what is this fourteen
meter per second as the designer how do you
visualize what is fourteen meter per second
because mostly like what the filling in terms
of kilometer per hour fourteen point four
means roughly fourteen point four into three
point five so multiply this will give you
kilometer per hour roughly right
so around fifty kilometer per hour right that
is the speed now see if i increase wing loading
in this case wing loading first two as ten
k g per meter square now suppose i raise this
wing loading to forty k g per meter square
this forty k g per meter square is the wing
loading typically for motor glider right we
have shown you the first lecture the sinus
nine one two this is typically the wing loading
if i assume other things that ok other things
are good enough in approximation then i will
get v stall as under root let me write it
here
so what do i am saying is w by s is forty
k g per meter square so we will be v stall
will be two w by s is forty again into nine
point eight to convert a newton rho i have
taken one and c l i have taken one please
understand c l typically is one point two
maximum you get for v stall all i need to
take c l max but we are taking it one and
this also now comes out to how much
twenty eight
how much
twenty eight
twenty eight meter per second which is equal
to this multiplied by three point five nine
eight k m per hour so ninety eight kilo meter
per hour you understand if you drive a motorcycle
is almost hundred kilo meters per hours a
huge number right we will see what happens
is w by s become hundred which is typically
passenger small business aircraft order will
be around hundred small two seater three seater
if w by s equal to hundred k g per meter square
if what happens to v stall again keeping density
and c l whatever we are using earlier this
will be two into hundred into nine point eight
by one into one this will be how much you
know just check
forty four point three per second
forty four point three meter per second that
will be how much one fifty five kilometer
per hour so you could easily see that as i
am increasing wing loading from ten to hundred
the stall speed is changing from around fourteen
to to forty four or forty five meters per
second most of our airplane cessna saratoga
all this thing there will be v stall will
be around this right this number has a meaning
we will understand if i now go to typical
aero model for aero model w by s is ah ah
other of one k g per meter squares smaller
one i am taking about if it is one k g per
meter square typical aero model which you
know it just takes off what will be v stall
v stall will be under root two into one by
in to nine point eight by one into one this
is how much
four to five
four to five meter per second as where you
say easy to fly such machine when i am saying
v stall from four to lets say hundred and
for a bigger airplane this w by s will become
three hundred four hundred five hundred so
if you see the v stall will go on increasing
and the increase in v stall means what v stall
means minimum speed required to maintain lift
equal to weight and who will ensure this v
stall who will ensure that yes the airplane
is moving we can't if capable of flying with
this much of speed this this or that what
you require you require this is the airplane
we require the thrust of the power should
be capable enough to overcome the drag experience
by it so as to maintain the speed which we
are demanding
if this number is more it means you need to
high power engine that also means the weight
of the engine will increase the weight increases
me again you see your wing loading will change
everything goes into a ah iterative mode right
but as a good designer you should understand
what are the implication of wing loading is
there another ah ah observation which designer
need to have prairie on wing loading is if
i increase ah wing loading that means relatively
ah ah the area of the wing i am reducing right
if i want lesser wing loading so that v stall
is less for lesser wing loading s is related
relatively it should be higher for a given
weight right
what is the implication of s being higher
s being higher be larger area that means larger
area ok that is good as for as lift is concerned
but what is the problem as there is a larger
area that means
the drag also will increase skin friction
drag primarily ok so that means if drag increases
that tells you you need more thrust or power
right so you could see that there are conflicts
a designer has to satisfy everybody and depending
upon the mission requirement will satisfy
one aspects more than the other for example
if it is a fighter airplane where i need larger
maneuver i will not fly at a smaller wing
loading i will fly so that wing loading is
low so if wing loading is low i can accelerate
fast because the drag part will be less ok
and also if wing loading is low means relatively
the wing is smaller means the aero plane is
compact
so i can roll very fast if the wing is very
large rolling will become difficult ok so
all those conflict will come but we need to
know the ah we need you should be able to
smell a wing loading if a number comes to
a figure ah ah on your table this wing loading
is this immediately should know that oh this
has the this has that this will have this
problem this will have this advantageous that
is why i am revising few concepts like wing
loading etcetera after a wing loading is a
another concept we will be talking about we
will be going in detail about wing loading
and all in each lecture and before i go for
those lecture i just thought
i revise few things there is something t by
w if w by s is wing loading what is t by w
t by w is also some loading but this is thrust
loading 
what is the implication of thrust loading
for example wing loading we have seen wing
loading has direct relationship with the v
stall minimum speed to fly level ah lift equal
to weight thrust loading if i want to see
again you have to go to your performance lecture
where for a steady steady climb with all the
diagram which all this diagram that we are
flying we are flying such that it is climbing
at a climbed angle gamma gamma is the climb
angle and we have explained about steady climb
for a steady climb i can easily write t minus
d minus w sin gamma equal to m and d v by
d t since this is a steady climb so i equated
this equal to zero and is climbing at a constant
speed length
now if i see this equation i can write t by
w equal to sin gamma plus d by w which i can
approximately write because the sin gamma
plus one by c l by c d this part is approximate
i am assuming lift equal to weight but here
you could see lift is not equal to weight
the component if you see like this lift will
be w cos gamma but i am assuming gamma small
so i am taking their liberty just for a designer
he does lot many approximation to get field
for some number ah this equation is very very
handy for a designer once he wants to know
what should be the thrust loading typically
c l by c d maybe around ten around fifteen
depending about what type of aircraft you
are designing
so even if it is ten so this contribution
will be at the at most point one if c l by
c d will be more than ten actually we will
find fifteen twenty so this component will
be at the most point one and ah this is straight
forward so how much t by w you require sin
of if i want to climb by thirty degree so
this is point five plus point one point five
six right no point six this is point six because
sin that is point five if i am climbing at
smaller angle ah accordingly i know what is
the t by w required because thrust is primarily
required for climbing for cruise the thrust
regard will be much less
because in the cruise thrust will try to only
balance the ah the drag of the machine but
while climb it will not only try to overcome
the drag but also take the weight upward please
understand that when i have written here d
by w and d by w i have approximated as one
by c l by c d but they have assumed w equal
to lift which is an approximation where clearly
you know that l equal to w cos of gamma right
ok we have just the designer we know that
ok gamma ah around fifteen degree then cos
gamma is almost one so some from the approximation
we are done for analytical solution of course
you should ah find out exactly by putting
these values so that is that should be ah
kept in mind but from designers perspective
please understand that this value c l by c
d ah we will flying at a c l by c d much higher
than ten fifteen twenty so the designer ok
maximum point one this one what is our aim
ah we are trying to find out what is the t
by w rough value i want who dominates it we
could see that it is dominated more by the
climb angle
so directly from here you find the climb fifteen
degree sin fifteen plus this value add roughly
you will get the value of t by w for the mission
because you know that t by w is important
for climb phase because thrust required during
climb is much more than thrust required during
cruise but this is another -[cl]ash point
you understand when i talking about t by w
and w by s that is also a design i need to
keep back of his mind so you are loosely talking
about very very loosely you are talking about
wing loading and thrust loading just to give
you ah ah understanding introductions because
you have to go in detail about those things
in the design but one thing you understand
that as i am going higher and higher the w
will go on reducing do not forget that ok
similarly as i am going higher and higher
even if i maintain same thrust if it is ok
but w will go on increase so t by w requirement
will go higher right then there is another
thing as i am going higher and higher the
thrust will also drop dynamic thrust will
drop with the propeller driven airplane you
could see that ah ah density of air outside
will be reduced right so the thrust available
from thrust or power when you talk about propeller
engine you know that we talk in terms of power
and thrust will get engine so t by w as i
am going higher and higher the thrust part
also will vary with the altitude
so as a designer i need to know how much it
will change and keep that has a margin right
so it is always better to have excess margin
on this so all those points also we will be
discussing so for today just for introduction
we have just gla[ss]- cruise w by s and t
by w and i will request you to go through
my earlier lectures and performance and come
prepared for next lecture
