good morning in the last lecture we are talking
about how to get an idea of the weight of
the airplane you are going to design typically
if we start thinking in terms of lets say
passenger aircraft and we decide maybe hundred
passengers two hundred passengers we have
an idea what sort of a range i want what sort
of a speed i want if you have got few parameters
as customers requirement then todays scenario
we immediately see the database we try to
select a baseline aircraft whose performance
parameters are almost like whatever you are
looking for
generally what happens for any new design
you try to add some new aspects to that new
design but overall requirements based on mission
requirements you will find some aircraft already
in a is existing which is closer to what requirement
we have because lots of designs are available
lots of airplanes are available they are flying
there is no point in reinventing will as we
understand for aircraft if i talk about material
there is so much development in material and
that impacts the strength to weight ratio
there are sensors there is huge development
in sensors so you will find over a period
of time dynamically there is a change in the
overall configuration of an airplane we say
this is material property we say which is
sensor capability
so since all these data are available which
are result of lot of work done by generation
after generation it is always wise to look
for a baseline airplane to get enough idea
what will be the weight just to give an example
if we are planning for a four seater or a
five seater low speed airplane which is like
a business propeller driven airplane then
immediately we think of maybe cessna two zero
six piper saratoga all this class you get
an idea the weight will be around fifteen
hundred to sixteen hundred kgs right if we
are talking about bigger aircraft you have
to see whether it is close to eight three
one nine three twenty or boeing immediately
you have got the weight and basic parameters
available right
so the whole point what we are contesting
is once we have got a machine requirement
you can always find out some baseline airplane
and in baseline airplane one of the [pri/primary]
primary parameter which you look for is what
is the weight of that airplane and why weight
is important we know when i say weight then
i know trust equal to drag lift equal to weight
after all from ground to air i am flying so
there is a requirement of lift and lift will
lift requirement will be more as the weight
increases so weight decides what sort of lift
i need to generate a lift means it is function
of density of the air the speed the wing area
so all these things get connected to weight
right similarly if we see drag drag has component
with c l square right
so in in a very simplistic term the lift is
more drag also will have its more contribution
and since weight increment results is more
lift so i can always assume that will also
make ourselves to look for more trust or more
power to work on the drag or drag related
power so finally if you find weight gets linked
to everything which is ideally true also without
going into all these equation after all i
am going to able to lift something so more
weight means more lift more effort right
so that is where weight is extremely important
and we will be now asking ourselves how do
i get initial estimate of the weight knowing
very well through a baseline approach if i
select a baseline aircraft i have rough idea
about the weight class i am talking about
what is the method routinely we follow to
get an hang hang of what is the weight we
are looking for and when i talk about the
weight is weight is what what i am talking
about weight is takeoff weight that the gross
weight what it is composed of it will have
weight crew member weight payload then weight
fuel and weight empty
so this has been broken up very smartly into
these four categories what is the first one
the crew member crew member weight we always
will know its a human weight and there are
a vision standards and they have to follow
that that when you design an aircraft of particular
type you have to take weight of the human
being to be seventy five kg or hundred kg
depending upon the regulations then payload
also we know because whether the passenger
whether the cargo we know a priori if it a
two hundred seater so i know average weight
of one person i will add those i know how
much baggage will i will allowed to them to
carry
so this w crew and w payload they are in our
hand we know a priori right known lets say
so life is not that difficult now come back
here the third one the fuel weight how much
fuel i should carry how much fuel is required
who will decide that natural position ground
how far you want to go at what speed you want
to go how much time you want to loiter all
these thing will decide how much fuel is required
because for every operation fuel will be consumed
during taxing acceleration takeoff climb cruise
loiters landing sometimes combat all these
things are necessitated by power delivered
by the engine and which is at the cost of
fuel being burnt
so if i want to have an idea about fuel i
need to know what is the mission profile this
is extremely important its as simple as this
if you are driving a car and if from kanpur
if you want to go to delhi how much fuel you
should carry in your fuel tank who decides
that first thing i need to know what is the
distance from kanpur to delhi the road distance
and what is the fuel consumption rate of the
car also you know that we need to also know
how is the road condition why that is important
because we know that there is a particular
speed at which fuel consumption is minimum
and the question comes whether i will be able
to drive my car meeting those requirements
are not because road condition traffic conditions
may not be what i am thinking to have an optimal
speed so all these things will be considered
then you say ok i if have thirty liter forty
liters fifty liters and if it is not sufficient
how do you plan you plan for a refueling you
plan that ok after this many distance nearest
fuel pump or petrol pump i will fill my car
these all mission similarly for a aircraft
also we have to have some mission profile
what this airplane is going to do and that
will primarily decide weight of fuel but it
is just not mission profile you understand
the mission profile you have to visualize
through the weight of the airplane because
we have seen as weight increases drag increases
power increases and though naturally your
fuel requirement will also increase
so there will be if a separate approach for
estimating w fuel and this one w e which is
w empty 
that has been also added separately for simple
reason that w empty means it is more driven
by your structural design what material you
have use what sort of stress relieving structure
you have and those will those things will
decide what is the empty weight ok again you
will find this empty weight will also be somehow
i can link to w naught may be linearly proportional
based on the statistical data these are all
initial statistical data driven inferences
which will be used to get the first parameter
for a conceptual design
now i come back here again so i write w naught
equal to w crew plus w payload plus w fuel
plus w empty and this i can write w f with
assumption that i will be working more with
the fraction of gross weight and w e also
i will model at w e by w naught into w naught
please understand this approach here we are
trying to find out how much percentage of
the gross weight is the fuel weight how much
percent gross weight is the empty weight and
then i multiply by w naught to get fuel consumption
weight this is purely based on the statistical
data that makes life simpler ok if i substitute
this here then i can write you can do yourself
w naught equal to w crew then w payload by
one minus w f by w naught minus w e by w naught
keep this back of your mind will see how to
use this
so let me write this expression here because
you will be using this w naught equal to w
crew plus w payload divided by one minus w
f by w naught minus w e by w naught we will
first discuss about w e by w naught that is
empty weight fraction and these are estimated
using i told you using historical data please
understand all this efforts are to get the
initial numbers for the design to start the
design conceptual design
generally you find from historical data this
w e by w naught vary from point three to zero
point seven 
and another important observation is that
it diminishes with increase in w naught what
is w naught so for i am taking w naught w
naught w naught is the gross weight everything
included right just to give an example historical
data representations typically if i take this
is point seven summer it is point five and
i am just trying to give a qualitative representation
of a jet transport jet transport and here
this will be around should have so if you
are designing a jet transport airplane then
depending upon what weight class you want
to design you can get roughly what is w e
by w naught this is
this we can focus on figure the empty weight
fraction trends you could see that for sailplane
powered for thousand pound this w e by w naught
is around point six five again if you go further
you find for a jet trainer for a ten thousand
pound for a jet trainer it is around point
six five to point six two focus on military
cargo bombur weighing around one lakh pounds
the w e by w naught can vary between point
four five to point one so this figure empty
weight fraction trend which is based on historical
data is a guideline for it ok
based on whatever figures we have seen the
author has done a good work in [git/giving]
giving us a correlation where it will been
a into w naught c in to kvs where you could
easily see w naught is the gross weight and
c at a are constants they vary for different
type of configurations for example for a sailplane
for a sailplane unpowered a value is point
eight six and c is minus point zero five and
for a jet transport a is one point zero two
and this minus point zero six
what is interesting to note here that the
c is negative thats exactly i was telling
as w naught increases w e by w naught decreases
that we should be very very clear given it
was shown here as well but you can use this
correlation but only my request is use fps
unit fps unit to all this calibration chart
which i am using from ah book aircraft design
by raymer all the details i will be giving
you they are available with me in fps unit
so i strongly advise you you will use fps
unit unless otherwise i give you those collaborations
or those correlations in m k s the new book
of raymer does give equivalent m k s constants
but i will be it using fps units right you
can refer this empty weight fraction versus
w naught and you could see that for sailplane
unpowered value of a is point eight six c
is minus point zero five for a gel aviation
single engine it is two point three six two
the value of a and minus point one eight is
value of c for the jet trainer it is one point
five nine and minus point one zero like this
this values have been generated using the
historical data using the figure which i was
shown earlier right and thats the huge work
just to repeat these correlation ah this empirical
relation is derived using all though historical
data and it for a designer it is very handy
ok you can even even write your own code ah
using a lookup table and get these numbers
easily but it is designer you must understand
that w e by w naught if it is between point
three to point four around this thats a good
one first with class of around one lakh pound
or less than that then what is kvs this kvs
factor you see because there is a change in
the material from metal to composites and
then there has to be some reduction in the
weight and this value would be somewhere around
point eight five that is fifteen percent reduction
i generally take ten percent reduction at
the initial stage or sometime i may take it
as one ok because i dont know any initially
how many components i will be making using
composites
so do not give too much advantage on that
to have an initial design we take around one
right ok and as you grow you will develop
feel and then you can play around with that
so once w e by w naught is over our next attempt
is w f by w naught that is the fuel fraction
now if i write fuel fraction estimation i
need to be clear about number one mission
fuel i need to be clear about reserve fuel
and also third one trapped fuel it is very
clear when i talk about mission fuel the fuel
required to complete the whole mission from
takeoff to landing back right
what is use of fuel that when you are going
for a mission you need to give something reserved
because of so many emergencies contingencies
so you can think like this ok i should have
thirty minutes extra fuel or i should plan
reserve fuel based on the nearest airport
where i can be diverted based on the contingencies
so that sort of planning you have to do and
the trapped fuel is some fuel which you cannot
take it out from the fuel tank so all this
three you have to scatter for because when
you are flying a machine please remember safety
is the at most criteria you should not start
because of fuel and made the ultimate conclusion
so we have to be careful about this things
and no a priori we will be focusing more on
machine fuel how to systematically calculate
that that is a fuel and trapped fuel they
get to come back by experience what sort of
planning you have got it may be around six
to seven percent depending upon the situation
ok those also we will highlight but more importantly
now we will talk about a mission fuel then
what is the mission if you are talking about
mission fuel what is the mission
for example i give you an example if i am
going from kanpur to delhi how much fuel i
should carry i should know from what which
point to which point i am going and what i
am doing in between right what should have
speed i will be flying how many times i will
switch off the car and start all these issues
will come right and how is the road conditions
so mission fuel when i am trying to look for
i need to know what are the possible missions
(Refer Time:24:00) and i will now talk that
is generic mission one what i will be talking
about a simple cruise what is that now you
start from here climb cruise do a loiter and
land this climb cruise loiter and with land
that is i am going from here lets say from
kanpur i am gaining a height around eight
kilo meters cruising and then i come near
the delhi airport loiter get there air traffic
lens and land
so from one point to another point i am going
but please understand if i am designing a
military aircraft the situation may not be
always this because i take off from here beat
the job and i come back to the base this is
one possibility there are other possibilities
i do the mission then go to the another base
so the mission profile will change even if
it is a simple cruise right it is like for
transport aircraft general aviation design
what is the main aim for such aircraft is
basically required range that is a fore most
strain at what speed etcetera etcetera will
follow but main aim is have to move from one
point to another point no other primary issues
right
so these are covered under the mission was
simple cruise mission like our elevation cessna
saratoga this belong to this we take off from
here go to and land right the mission two
that could be here i am now going towards
air superiority right now you can see that
from sybil we are not talking about little
military ok and such mission requirements
demands the airplane takes off climb to altitude
cruise turns back do a combat drop bomb weapon
anything goes up loiter and land do you see
this mission here it takes off from here goes
to your cruise altitude dives right with our
cruise and then do some another combat drop
repent whatever it is and again goes altitude
goes and loiter and land
please understand assume that the aircraft
is having a jet engine right so jet engine
efficiency will depend upon the altitudes
flying the speed the temperature right now
for a military air type aircraft you dont
have option if you want to do a low altitude
combat or some undigit combat you have to
be there that did not be here right type of
condition for the fuel efficiency so fourth
assumption will change here some weapon drop
will be there or when you do a combat a lot
of high acceleration may happen so fuel consumption
here will be all together different so its
in this area may not be a very highly ah fuel
efficient domain so fuel consumption will
be more then you have to crew out and then
loiter and land right
so that way when i will do calculate the fuel
fraction w f by w naught i need to know what
are the mission requirement what the operation
is they are do right here it is not that fuel
efficient operation correct sometime after
charger may be used extra fuel may pumped
in so i need to know all these things before
i know how much fuel is required so i need
to calculate w f by w naught taking this sort
of a mission if i am planning for it air superiority
aircraft which is different from simple cruise
type aircraft ok
the third category so before i ah come to
third category for air superiority mission
we have one is cruise out these are the primary
manual work cruise out one second is combat
which we have seen third word is weapon drop
and fourth second cruise because the aircraft
must return to the base that is when i am
going like this this this this i do something
here load and i need to come back to the base
it is not for landing at different other bases
where far we have taken off to come back there
so whatever rage you have gone if you want
to estimate how much fuel is required from
a point a to point b whatever range you have
covered you have ensure that you have to come
back same another distance almost same distance
maybe flat conditions are different because
while return you are not doing combat right
so we some time what do you do whatever cruise
distance is there we duplicate that to ensure
that this part is used to compute the fuel
assumption for return flight if a air superiority
then we have this is another important i am
giving military example but please understand
we dont advocate who are right ok but same
time we should be enough prepared if somebody
is below our belt we should be able to retaliate
correct but all these lectures are primarily
for to be understanding what a technologies
involved where the method involved we do not
forget most of the civilian ah things which
we were using they are actually product of
military technologies ok here because then
military technology the quality assurance
is pretty high and at an adverse condition
to design things ok
so third one i will talk about is low level
strike mission 
the low level strike the word which is being
traced is low level but it was the word radar
signature you want to be very clear see what
you want to see so you have to flag every
low altitude the moment you want to flag low
altitude you if that may not be at all aerodynamically
very efficient ah profile these low level
means fifty feet hundred feet lets hundred
feet under fifty feet two hundred feet we
are talking about those distances right we
got is this strike mission you see a strike
and come back right but same time avoid capture
by radar
so what are the criteria there it is let me
draw the first profile it is again almost
similar the look loiter then come to fifty
or hundred do whatever you want to do those
an here the condition is a must be flown at
just few hundreds feet 
of the ground this is to improve survivability
that is avoid radar and also it helps in locating
the target what is the problem problem is
l by d goes down engine efficiency goes down
i say high speed flight you can imagine at
low altitude high speed so how much fuel will
be consumed l by d is low engine is performance
is low you want high speed
many times it happens the amount of fuel which
will consume here that may be almost equal
to the amount of fuel is consumed here so
such mission profile when you are calculate
fuel fraction you have to give a dew weight
age here your most of the fuel will be consumed
here right and the last one which we will
be talking about is strategic 
bombing i wanted to ah omit this because i
did not like the word bombing but then you
should know it the deal it is it has a new
technology so very important you should notice
so what is this again same start from air
cruise to altitude not very high altitude
come down and this r means refueling right
so get aircraft refuel here we have another
aircraft again you cruise out then come back
to around thousand feet maybe and then do
whatever drop operations again you cruise
now we land at some other base not you are
coming back to the original base right to
go from delhi are landed somewhere in ahmedabad
or somewhere else it is not you are coming
back to the base right this is important please
understand many times in operation what happens
you are carrying atom bomb from the base you
have taken off but for some reason it could
not release the bomb
so it is not advisable that that airplane
lands back to the same base because if there
is accident it may create a problem so there
are many [sa/such] such thing happens whether
it is this type or not so even if it is supposed
to come back to the base depending upon situation
it may be advisable that you land somewhere
else right but for a strategy bombing mission
this is very the important part is that refueling
so that you can have an extended range so
you can understand refueling i am doing that
means i am going for a longer mission so there
is no point in coming back and land back here
better i land back nearest to that which is
not the criteria other low level strike mission
basically coming back to the base right
so you can very well understand and if you
want to really calculate the fuel consumption
for this and if you follow simple cruise method
you will not get because there are a huge
things are happening ok so all these things
need to be known before we try to follow the
methods which are listed what we will do we
will take simple example of a cruise mission
and a loiter mission if you know cruise or
loiter for that signal that concept you apply
everywhere here basically it has two primary
operation one is cruise one is loiter and
there will be something for acceleration phase
dye phase which will be added but primarily
crews and loiter that is why primarily range
and endurance
once you have identified mission requirements
to ensure that whether we can use these mission
requirement to calculate fuel fraction what
we will do our approach will be very simple
we will take a example and we will actually
find out fuel fraction and empty weight fraction
for a given mission requirement and that will
be a simple cruises mission requirement so
it will be like we will be actually computing
w e by w naught and w f by w naught by for
before that we will postulate what is the
mission requirement for the airplane we are
going to design and that is also a solved
example from this book so that you can refer
that book and learn ah more beyond this lecture
ok that will be my next part of my lecture
tomorrow
thank you very much
