[CLICKING] PHILIP GREENSPUN:
All right, folks. Tina is going to tell you more. We had a little bit
of weight and balance before that flight
planning example, but Tina is now going to
give you the real story. TINA SRIVASTAVA: So are guys
all ready to go out and get your drone license? Good. I'm really glad. PHILIP GREENSPUN: [INAUDIBLE] TINA SRIVASTAVA: All right. Weight and balance So
we talked a little bit. We've actually had quite a
few discussions throughout about stability, about
where the center of G is, how that affects
your controllability of the aircraft. So weight and balance is
not only an important thing to know, but it's
a required thing that you have to do before
you go flying each time. And one thing
that's also relevant is to understand the
center of gravity, and you want to make sure
that center of gravity is within the limits
for your given aircraft. So we talked about-- yesterday, when we were talking
about aerodynamic flight, that certain aircraft
have to be reinforced to do certain operations. So again, it really
has to do with that. And by the way,
from yesterday, mark left behind some of his
aerodynamic procedure diagrams. So if you want to take a look
at that, those are up front. Well, one thing
that really you have to keep in mind in terms of
what the aircraft can handle is just what is the
max gross weight. So what can this airframe
sustain, in terms of stresses on its body. We also talked this
morning about performance and discussed certain
things like the distance-- the length of the runway and the
distance you need to take off are affected by how the
airplane is weighted. So we're going to talk about
those things specifically, and then one thing
I have on here is if you go over
your gross weight, then you get into
test pilot territory, and right after this-- we'll grab lunch and
come back-- we'll hear from [INAUDIBLE],,
a test pilot who's going to tell you some
really cool things as well. All right, so it's just some
basic terms to get started off. So when you're thinking
about an aircraft, one piece of information is
its standard empty weight. So that's the weight
of the airplane, and there's also
some fuel in the tank that sometimes it's hard to
drain, and when you refill, that fuel sits at the bottom and
can't be used by the aircraft, so that's the unusable
fuel, and then there's some different
fluids and fuel oil inside the aircraft. So all those count
in the calculation of the standard empty weight. There's also the
basic empty weight, where you have some
optional equipment inside, and we'll go to a aircraft's POH
to look into what it actually reports about what it contains. There's also a
good rule of thumb to know about what weight
you're going to associate with the per gallon of fuel. So it's generally a good
number is six pounds. For the fuel that
you'll be using, it's slightly more for
different types of fuel. So right at the
beginning of the class, we talked about weight,
which is the force pulling the aircraft down, which has to
oppose the lift pulling it up. So the weight actually
becomes less as you burn fuel. So when you take off, you have
all the fuel on the aircraft. As you've been flying
for a while, that fuel is used in the
aircraft has less weight. So there's a fully
loaded aircraft. Has something called
the maximum ramp weight. So that has fuel that
will be expended even when it's just taxiing
over to the runway, and so you make certain
assumptions about how much fuel you'll
actually burn off just taxiing up to the runway
and then before you take off. And so when you're calculating
the weight of the airplane when it takes off,
it'll be slightly different than how you fueled
it when it was sitting parking on the ramp. OK, some of the
biggest things that you would keep in mind that
you change every time you get into the airplane. So there's the weight
of the passengers, the baggage you take
with you, and then how much fuel you fill the
plane with before you take off. So the payload in this case is
the passengers and the cargo. So whatever baggage and
the weight of those bags that you take with you. So one thing to keep in mind
in addition to just the weight, we have to figure out where
that vector, that force vector, acts on the aircraft. If all of the weight
is really at the front, you might have a
center of gravity at the front, which will create
a moment on the airplane. So it'll cause it to bend
in a particular direction. And if all that
weight was acting at the back of
the plane, you had a lot of really heavy bricks
at your tail, which is not a good idea, it would
cause that force vector to act over here
and create a moment. So that's all related to torque,
which we talked about before. So it's also important,
not just to calculate what the weight
is of the airplane once everybody and
everything is inside, but also how that weight is
distributed in the airplane and how it affects the
airplane's center of gravity. All right, so I think that
many of you have already-- It looks like most
folks in the rooms were either MIT undergrad
or grad student, which means you already understand
some of these basic physics terms about torque
and moment, so we'll discover them very briefly. The moment is the
weight times the arm. So basically where the wait
is times how far away it is from a particular location
that you're calculating it from. So we're going to do a
couple basic examples. So we have a seesaw here. So in this case, whenever
you're calculating your moment, you have to pick
a reference point and make sure you're
consistent as you use it. So in this case, they have a-- their data point
is at that triangle at that fulcrum in the
middle, and so they have a moment on the right
and a moment on the left. So you see various
rectangles that represent different objects
or different weight, and then they calculate it. So that blue box that's five
pounds is 10 inches away. So to figure out what the
moment is that it applies, you just calculate the
weight times how far away is. OK, so in order to figure out
what the center of gravity is, you might want to
consider where you're doing that reference point. So rather than doing
the reference point from right in the middle of
the aircraft, in this case, they're starting at the end. So where that yellow
line is where it says datum is where they're
starting, and then they're using the length from
that same reference point. So now you have a purple
box of five pounds that's 10 inches from that
datum, and then three pounds then a green weight
that's father, and the red box that's
the farthest away. So here they've listed
all those different boxes and the weight of each of those
boxes times how far it is. You multiply that to get
the moment that it applies. And use that to
calculate the location of the center of gravity. And just in terms of if you've
been on a seesaw before, who has been on a seesaw before? All right, good. I'm glad. So as you know that
that fulcrum or where that triangle is, if
you put that triangle at the center of gravity. So all the kids sitting
on one side of the seesaw weigh the same as
the kids sitting on the other side of the seesaw
at the appropriate lengths away, so they're
applying the same moment. Then the fulcrum balances. So if we determined what
the center of gravity of this airplane was and
I put my finger there, then it would balance
and it wouldn't tilt one way or the other. So that's the whole idea. So for a given aircraft,
there is not only the center of gravity, but
in the testing process, they understand what is
that approved range of where that center of gravity can be
such that the aircraft is still stable. So if you're thinking about
putting a lot of weight at the front of the
aircraft and less weight at the back of the aircraft,
then the center of gravity moves forward, and
there's a limit to how far forward
that center of gravity can be that allows the
plane still to be stable. So that's the forward
CG limit, and then you see the same thing at the back. Aft just means back, so how
far back the center of gravity can be. All right, so we've
already discussed some of the performance things, but
just to just to remind you, what are some of the things that
can happen if your air plane is overloaded? You want to just a
shout out some things that you would
experience if you have a very, very heavy airplane. AUDIENCE: Longer take off. TINA SRIVASTAVA: Yes, very good. Just to repeat it,
longer to take off. So we have takeoff and
landing distances increase. So if you have a
very heavy airplane, you may not be able to take
off from a short field runway. And then of course
it was talking about if your aircraft gets
outside the approved envelope, it will talk about what
we mean by envelope, then it hasn't been tested
to be stable at those areas, and so now you've become a
test pilot, which you may not want to do. Again, we're going to hear
from a test pilot very soon. OK, so what happens
if-- not just that it's overloaded,
maybe it's still within the envelope for
the total amount of weight, but what if the
center of gravity has moved too far forward and
pass that forward CG limit. So your plane naturally
wants to be nose down. So what is the impact of that? Well, it'd be harder
to climb, right? It's harder to climb
because your nose keeps pulling it down, for example. What does it mean to have
a stall speed increased? Is that a good thing? AUDIENCE: No. TINA SRIVASTAVA: And why not? Why is it not a good thing? AUDIENCE: [INAUDIBLE] TINA SRIVASTAVA: Yeah, so
basically, if your stall speed has increased, that means you're
likely to stall at a higher speed. So if traditionally
can fly at 60 knots without stalling but that
stall speed has increased from say 40 knots up
to 60 knots, that means that when
you traditionally have been able to fly
safely at 60 knots, you could actually
stall at 60 knots. So an increased
stall speed is bad. And what about an aft CG? Having the weight
too far to the back. What is the impact there? AUDIENCE: They pitch up. TINA SRIVASTAVA: They
tend to pitch up. Right. And so one thing that they are
talking about here is flare. So this is when you're coming
into land, you go down, but then you flare up, and
it could actually over flare, and that can cause some really
strange things when you're trying to land,
such as bouncing, and you definitely
don't want to do that. But all of these things
you might actually just notice when you're
feeling the airplane. It acts differently
than you want it to act. So here's just basically
a cross section. If you took off the wings
and the top of the fuselage and you were looking
at the plane, it shows you the pilot
sitting in the left seat, a passenger in the right seat,
two passengers in the back. This one actually has a third
row of passengers, and then two baggage compartments. So this is just showing-- traditionally, when you're
loading up an airplane, they payload again is your
passengers and your baggage, where do they sit? So now I'm going to go over
to this document viewer and talk about
specific airplane. The plane I like to
fly, a Cessna 172. OK, so this book is talking
about a particular aircraft, a Skyhawk, so it's a Cessna 172. This is a plane that
many of you guys might use as a flying aircraft. We've also talked
about for example a Piper, which is a low wing
airplane that you might use. So this is what a
Cessna looks like, and it has a tricycle
gear at the bottom. Whatever plane you are
using for training, I would get very
familiar with this book, and in particular, remember
what the table of contents is. Because when you get your oral
exam on your FAA examination, it's OK if you don't
remember everything. What's really great is if you
know how to find information, and you can say, hey, if
I didn't remember this, I would go look at my
POH, and I would go look at the weight and balance
section, section 6 of my book, and I'd be able to flip and find
this information very quickly. So oftentimes, you'll get
questions in that oral exam where they would
ask how you would go about finding information. We discussed that a lot
with regard to weather data. What are the sources
of weather data, so with regard to
weight and balance, the information
about your airplane is found with a book made
about your particular airplane. And if you're at a
particular flight school, and we'll show you
that in an example, they might have a-- they most
likely have all of this weight and balance information
listed in a spreadsheet, sometimes it's
accessible online so you know for your exact airplane
what its gross weight is. So if we flip to
that section 6, it has a fair amount of
information, including the picture we just showed you. So it talks about-- in this case, there are
only two rows of seats. So that the pilot and
passenger, one row of seats, so it seats four people, and
then it has a baggage area. But what's really
important to understand is that even though the
Cessna can fit four people, it will show you that if you put
four 200 pound people in there with a bunch of bags, you'll
actually exceed the allotment. So just because
there are four seats, doesn't mean you can take
four large people on a flight. Some of it is something
we'll keep in mind, and maybe you can reduce bags
and reduce fuel such that you can make that weight workout. So we talked about envelopes. So the envelope is
basically a line on this graph that shows where
the center of gravity can be, and the envelope is
where is it safe to be. So if you're inside
the envelope, so inside this line, if
your calculations result in a center of gravity
inside, then you're within the envelope. If you're outside of
it, then you're not. You can see a little
pencil mark towards the top where I took a flight
recently, and I was trying to calculate
whether or not I was inside and in fact I was. And it's OK that I'm right
at the edge of the envelope. As long as you're
inside, you're still with inside the tested limits. Now, there are two
different categories or two different lines here. There's a bigger line
that says normal category, and then there's
a smaller line-- smaller envelope-- that's
talking about utility category. Does anyone know
what the difference is between normal and utility? AUDIENCE: G limits. TINA SRIVASTAVA: G limits. Is that what you're
going to say? So there's different
restrictions. One question you might
get asked is, how do you find out where the answer is? And it's right in here. Right where you think it is. And I'm actually going to
show you real examples, so we're going to go through
an example of loading up an airplane. Don't want to embarrass
anyone, but we might ask some of you
what your weight is. So beware. So every time you have
to calculate your weight and balance, you
can do it by hand, or you can use a
computer to do it, and I would just caution
that you have to double check all the information. So here's a great
link that tells you for a particular Cessna 172, a
weight and balance calculator. So here, you can actually
type in the empty weight. Now, there's a big Warning
here, use your own AC data. What is that saying? What is AC? AUDIENCE: Aircraft. TINA SRIVASTAVA: Aircraft. Use your own aircraft's data. So look at your
actual information from your school to
figure out for the plane that you're renting what those
pieces of information are. So here is a link from
east coast aero club just as an example,
and they have provided for every
aircraft that they have a bunch of information. So for each aircraft, they
have provided the empty weight and the CG, the moment,
gross weight, useful load, and then they also talk
about two different settings for fuel. So you can have full
fuel or fuel to tabs. So that is something you'll
hear pretty frequently. Fill the fuel to tabs. So if you want to have
extra mass, extra weight in your plane, you have some
heavier folks flying with you, you might only want
the fuel to tabs. But if it's just you or
you and your instructor and you want to fly a
cross-country flight, you might want to fuel full. So these are all
things you can change. This calculator has
some restrictions, which is that the weight is
identified here in terms of-- so they have the oil. So you might want to
check what is the oil level inside your engine. For a Cessna 172, six quarts is
a really good number to have, but you check it every time
and during your pre-flight. It could be five quarts,
it could be seven quarts, so you want to
double check that. And of course, if you
see that it's low, four quarts, you really
want to ask someone to fill that up
before you go flying. But the other thing
here is you actually list the weight of the
people that are sitting in the front and the back. So I talked about
embarrassing folks. Well, let's not let's not
embarrass folks right now. So let's say the
pilot and let's say you're doing some great
resource management. You have another pilot
sitting next to you, so your co-pilot is
there, and you also have some big folks sitting
in the back, both the left and the right side. And you want to take a
bunch of pictures and stuff, but you want to get
as low as you want, so you put in some bags. So this-- it's actually
hard to see because there's too much weight. This red little
plus sign is showing where your center
of gravity fell and whether it's within that
utility or normal category. So in this case, we
have three people. Front two are 200
pounds each, one person in the rear with 200, and bags
there weighing about 10 pounds. Is this safe to fly? No, so you're a little red
dot went outside the limits. If you have a much
smaller person sitting in the back seat, you
cross over that line, and now you're back into
the normal categories so you can fly. Again, another
thing you can alter is, of course, you
fill the fuel to tabs. So this is also talking about-- so most of the time in the
POH book, so the section 6 is about weight and balance. So although the
one example I just showed you was for a Cessna
172, the Piper Warrior also has that
consistent, so really recommend just memorizing the
table of contents for your POH. So the way that you go
about calculating it is the same way we entered it
into their little calculator. So first you have to determine
the basic empty weight of the aircraft and
then the moment. Section 6 almost always
also has a place for you to do that calculation,
and so they have a place right
there where you can enter in your
airplane, the moment, and it does the same steps
that we just saw on the slide. So you start with that
basic empty weight, then you focus on the
fuel, and it gives you the 6 pounds per
gallon is what you're going to most likely be using. What is the max
number of gallons? What is the amount for tabs? You want to actually look
this up for your airplane. The weight of the pilot
in the front passenger, rear passengers, and the
baggage area, and then you get this ramp weight. And as I talked about,
you can actually have your plane be a little
heavier than the envelope right when it's fueled on the
ramp because there's some allowance for the fuel that
you'll burn off during a taxi, and so that's right here. So fuel allowance for engine
start, taxi, and run up. So they took off about
eight pounds over there, and then you can calculate it. And once you locate
the point, then you go to that diagram right here. So you go to the
weight and the moment, and you figure out whether
or not you're in the safe-- whether or not you're
in the envelope for that normal
category of aircraft. So what we just went through
is the same procedure that's listed right here. And so here is it just
doing that same example for a particular Piper Warrior. So they have the basic empty
weight of the aircraft. Just as a hint, if it's
something round like 1,500, then you probably didn't get
the exact accurate amount. So again, if you look
at the real thing, you're getting it down to some
very specific accurate numbers. So 1,196.63. That means you
got a real number. Here's our Piper Warrior. And so they have the basic
empty weight, the passengers in the front and the rear. They don't have any
bags here right now, and they basically multiplied
the weight times the arm. The arm is really
indicated in the book and in these
calculators as to where the location is of the chair. Now, of course, the seats
slide forward and back, but they usually don't--
they don't get into that many specifics, and then you can
calculate here the moment. So next, we do the
ramp condition. So ramp condition
is basically, what's the weight and loading
of your airplane once it's fueled up and
sitting on the ramp? Then you, in this
case, they took off seven pounds for
that taxi, and run up and then you're takeoff
condition, in this case, is 2,440 pounds, and it has
a moment here as calculated. So you basically do the
same thing we just showed. So you you look at the
weight on the left, and the CG location
as the vertical line, and you see where those
two lines intersect and you see whether or not
it's inside the envelope. You can also check
the landing condition. In general, you will have
burned fuel during your flight, and so your landing
condition will be lower, but you might have
heard big jets, and you've heard that sometimes
they have to do a fuel dump. Why did they have
to do a fuel dump? Well, maybe they
calculated their landing based on having flown
across the country, but their flight
across the country was canceled because
of weather or whatever, and so they had to dump
their fuel in order to land, which you don't like to hear is
once they've dumped their fuel then you have another delay in
the air, and you're like, hey, what happened? I thought we dumped
all of our fuel. But of course, as a
small little airplane, dumping fuel is not
really a procedure you're going to be doing,
so it might be a good idea to just calculate the landing
and the runway lengths with the full fuel in case you
have to make it landing right then and there before you've
flown all the distance you thought you were
going to get to fly. So in general, as you can see
that where we were beforehand was higher and the
weight goes down, so your landing condition
is almost always going to stay
within the envelope. There's some places,
where if you're CG is too far aft or
too far forward, where actually that
reduction in weight could cause you to come
outside the envelope. All right, so there are also a
lot of spreadsheets available. You can make your own
spreadsheet pretty quickly in Excel using the data
from the particular aircraft that you're renting or buying. All right, and these
are just some snapshots from the book that
we just looked at and the show pictures
of where the seats are in the airplane in order
for you to calculate that. You could also calculate
the moment left to right. It's pretty easy to
think about, though, that in that tiny
little airplane, if you're sitting on the
front in the left and no one's sitting on your right and
you have someone sitting in the back on the left, then
you're going to be tilted over. It's very easy to
fix that problem, just make sure you have someone
sitting on the other side. I have noticed some of
these effects before. I'm a little bit of
a smaller person, and there was a very, very large
instructor sitting next to me, and I actually could tell
as I was flying that it was a little bit tilted. And so with the future
flights, we put some more bags on my side, and it was a
little bit more evened out. Usually, you won't run into
that too much as an issue. In some of the small puddle
jumper aircraft and so a commercial aircraft, they
have sometimes a situation where there's only one
row of seats on one side and then there's
the aisle and then two seats on the other side. Have you guys seen
that configuration? I'm getting a lot of head nods. So in that situation,
they might enforce that you have to sit in the
seat that was assigned to you. And when people try to
move from the side that only had one seat in
the row to the side that has two seats because they just
want more space to spread out, they might say, hey, for
weight and balance reasons, you have to sit in the seat
that was assigned for takeoff and so that's what
they're talking about. There any questions
about weight and balance? Do you guys think you
could do weight and balance calculations? Yes? OK, I'm getting a lot of head
nods, so everyone took physics. That's good. Good job. Yes? AUDIENCE: So I assume that
[INAUDIBLE] the helicopter has lateral elements in the chart. [INAUDIBLE] TINA SRIVASTAVA:
Helicopter question. I will actually defer to
Philip, who flies helicopters quite frequently. So once he's back, we'll ask
him about helicopter CG limits. Good question. Anything else? Yes? AUDIENCE: [INAUDIBLE] TINA SRIVASTAVA:
So the question is, when you're actually
flying, do you do this calculation
every single time, or do you mostly only
do it when you're really loading the aircraft? So it's actually
required that you have the weight and balance done. The reason you don't do it
every single time, though, is you frequently fly in
the same configuration. So if you rent from
a flight school, you might frequently rent
you know a couple aircraft, and you might frequently
fly just yourself, doing a solo flight, or
fly with your instructor. So you might just keep
it stored that you have a couple of pieces of
paper in your flight bag that are the weight and balance
calculations for what you almost always fly
and is almost always within the normal
category envelope. But you definitely want
to do those calculations if you're getting
passengers in the back seat. Whenever we go on
these MIT fly outs, we actually ask people
for their weights, and it's actually a
big restriction too. So when we're trying to
get passengers to sit down, just because I fly
assessment with four seats, as we just saw from the example,
I don't have four pass-- I don't have three
passengers to offer. I often say, I have
220 pounds available, and so they then have to sort
out, Sebastian and his team, as to who to assign to my
plane versus another plane. And also, if people
aren't being-- if you notice people aren't
being overly forthcoming about their weight,
you might want to increase the amount
that they gave you. PHILIP GREENSPUN:
Yeah, let me add that-- obviously, have a
lot more experience being over overweight than
Tina might have, but also-- actually, these pants
are a good example. 20 years ago, some
friends and I, we made a web front end to
a Levi Strauss factory that could make custom cut khakis. So we asked people
in the web form when they ordered what's
your waist size, what's your inseam, what's
your height and weight. And from that, we
had software to try to infer their actual weight,
their actual waist size, and then we make them--
you can't really see this, but these pants just
say pair number 7. They don't say anything about
what size it actually is. They would get their pants-- this was in 1997. They would get their
pants, if they didn't fit, they'd say, look, it needs to
be a little looser in the waist. It needs to be a little
shorter, and then a revised pair would be
made and sent to them, but we never relied
on their stated weight or their stated
waist size, exactly. TINA SRIVASTAVA: Philip,
there was a question for you that I wasn't able to
answer about helicopter CGs, and really thinking
it was the lateral CG. What was your
question specifically? AUDIENCE: [INAUDIBLE] PHILIP GREENSPUN: It's very
hard to get a-- the question is it's very hard to get a
helicopter out of lateral CG, as with an aircraft. If you have to have you people
in the back and a light person in the front, it's easy to
get out of front back CG, longitudinal CG, but-- TINA SRIVASTAVA: He's talking
about, what we just discussed, it's frequently the
case that you might have a forward CG or an aft
CG, but you don't usually have an issue with the CG on
the left or the right, unless-- it's very visible in
a four seat aircraft how to fix those
types of problems. PHILIP GREENSPUN: One
issue with the Robinson is they put the main
fuel tank on the left, so they're budgeting for-- in a solo situation, a pilot
of at least a certain minimum weight in the front right. So very lightweight,
sometimes when teenagers are learning
to fly helicopters, they have to add a
little bit of ballast, let's say like a big
heavy gym weight. A 50 pound gym
weight in the front left footwell or something. AUDIENCE: The other thing
that could go wrong is-- so when you do your pre-flight,
one of the things that you do is that if you're sitting
in the pilot's seat, so on the left side,
so to your right in the middle at
the bottom, there's a little knob where you
can say whether you want the fuel to be coming from
the left tank, the right tank, or both. And pretty much, all
the time, the checklist says that after you've
parked the plane, you want to twist it to
the left or the right, and the reason is
you don't want-- while it's sitting
there, fuel to be sloshing across the plane,
and so you set it to left, for example. Now, if you skipped that
step, even though it comes up several times throughout
your pre-flight and then your run up check,
if you didn't put the fuel to draw from both tanks and
it's only drawing from one tank, then you might experience
that instability. PHILIP GREENSPUN: All
right, why don't we-- should we take a 15-minute
break so people can run out, grab food, bring it back,
and then we'll hear from [INAUDIBLE] about-- TINA SRIVASTAVA: The test pilot
and life potentially outside of the limits.
