[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.