Hey guys! Jon from Fly8ma.com and today we're going to talk about
the basics of weight and balance onboard our aircraft. Basically where are we
going to put the pilot and the passengers and all that baggage and fuel
to make sure that the aircraft is in equilibrium and within the weight and
balance limitations. Let's get to it.
So let's approach weight and balance in terms of basically a seesaw from when
you were a kid. Obviously when you had a fat friend you
had a hard time weighing down the other side of the seesaw given that it was the
same length. If you wanted to weigh down your side of the seesaw more you would
have to go further away from the pivot point or he would have to come closer to
it to have less of an arm or less an effect over that pivot point. So we're
going to put some things up here. First thing is going to be our wings and
that's our pivot point when we're airborne. That's our center of pressure
and we'll talk more about that later. Then on top of that center of pressure
we're going to go ahead and throw our fuselage there. So you can see this
rudimentary fuselage a little bit of a tail there on the right. Then the next
thing we're going to do is draw an arbitrary line that the manufacturer
chooses and this line is typically just a reference point. Typically the
firewall or basically where the engine compartment separates from the cabin
compartment, so we're going to draw this red line we'll call that our data point.
That's just an arbitrary point chosen by the manufacturer. Everything to the left
of that point forward towards the nosecone is going to be a negative
moment or negative arm. Everything behind it will be a positive arm and that's
what we're going to get into next. So let's say we have a heavy engine sitting
out front there and that engine weighs about 300 pounds and it's 12 inches in
front of the firewall in front of our data point, so that's going to be 300
pounds times minus 12 will give us the moment the weight times the arm equals
the moment. Then we can go ahead and say we have a pilot and he weighs about a
hundred pounds and he's sitting 36 inches behind the data point on the
positive side so it's 100 pounds times 36 inches equals the moment and you can
see that these two moments both cancel out 3600 from the engine and 3600 from
the pilot. One is a negative one is positive, it equals zero. The aircraft's
within balance. And the center of gravity would be 0 inches from the data point.
Now let's take another exam here and say we have that same 300lb
engine, but we have two pilots this time and they're each 100 pounds so now we
have two hundred pounds for our pilots so two hundred pounds times 36 would give us
7,200 inch pounds and you can see that leaves us when we add all this up with
positive 3,600 inch pounds we then divide our moment by our total weight
and we get seven point two and that seven point two number means that the
center of gravity is seven point two inches aft of the data point on the
positive side of the data point, so bringing the center of gravity closer
towards the wings. Now why is this important? Well the center gravity is the
point on the aircraft that if you were to pick up the airplane with your
fingertip it would balance about that point, so it makes sense that we're
trying to make the aircraft balance somewhere in the neighborhood of the
wings since the air is basically acting as your finger tip holding up the
aircraft about the wings as its flying through the air and we'll want to have
that CG somewhere in front of the center of pressure slightly in front of the
center of the wings. So that's enough theory for one video let's go ahead and
jump into doing an actual loading problem for regular old Cessna 172 here.
So we're going to use this loading chart we can see a sample problem
already filled out for us and we're going to do this two different ways. The
first way is going to be using a loading graph that gives us our moments that
it's already computed the arm values for us and then the second way is a little
bit more math but I tend to like it better because you get to see exactly
how you're finding those moment numbers and it kind of spells it all out so
we'll look at it two different ways first way using our loading graph with
the moments, second way doing a little bit more multiplication and doing it
longhand so we can see it ourselves. So for this method using the loading graph
we're simply going to find our weight, say for example the weight of our pilot
in front passenger 340 pounds find that line that says pilot in front passenger.
Follow that line up to 340 and then draw a straight line down from that 340 mark
and we get our load moment over a thousand pounds
inches or thousand inch pounds however you want to say it. Basically that's
giving you an abbreviated moment and so that's where they get their 12.6 from.
Now for a few old same thing 210 pounds of fuel on up the line draw a line down
and you get ten point one so you can see how this method works and at the end of
it all you have to do is then come over here to your moment envelope and you can
simply plot the total moment in your total weight and find out whether or not
you're within the envelope now if you're wondering about this whole normal
category versus utility category in the graph what that's ultimately telling you
is that if you want to operate your aircraft within the utility category
where it's approved for possibly some more maneuvers and a higher load factor
higher G loading, then you're going to have to be below a certain weight in
within a certain CG limit more so than if you were in the normal category so
say if you're going out to maybe do spins and the aircraft was approved for
that in the utility category but not approved to do spins in the normal
category then you would have to be at a lower weight and also within this CG
limitation. So now we can go ahead and use our loading Arrangements page to do
it the harder way, basically where we're going to multiply the weight times the
arm to find our moment and then go ahead and divide all that. So slightly more
math I actually prefer this way. I don't think it's harder. I think I get to see
all of the math myself and I make sure I'm getting the exact numbers. So we can
see here they give us a range for the pilot and front-passenger 34 to 46. We're
going to use the average which is 37 rear passengers they give us the number
73 so these are all pre-measured distances, we don't actually have to go
out to the aircraft with a tape measure and measure from the firewall or from
the data point back to the station where we're putting the weight. Now note
obviously passengers can lean forward and backwards pilot and passenger seats
can move forward and backwards these are rough estimates these are not perfect
numbers and that's why it's so important to be within the envelope when you're
operating the aircraft, not already pushing the envelope when you have not bad
numbers but it's not perfect numbers either. So now we're going to go ahead
and make our little handwritten chart here with our weight, our arm,
and then that'll equal our moment and so we can start with Pilate and
front-passenger we're going to use 150 pounds for our
pilot 150 pounds for our passenger going to multiply that by 37 to get our moment.
Then we can do the same for our rear passengers we'll say we just have one
person flying with us keep it simple there 180 pounds and so we're going to
multiply that by 73. We could go ahead and plug in our known empty weight as
well and the know arm for the empty weight and we'll find that in the POH or
AFM gets changed from time to time and updated in the aircraft maintenance logs
and we'll calculate that out to be sixty-two thousand six hundred inch
pounds for our moment. We also need some fuel to make the engine run and we'll
just say we have say 20 gallons of fuel on board that gives 120 pounds times our
arm gives us our moment. We'll add up our moments will add up our total weights
we'll divide the total moment by the total weight and that gives us our CG
number. So we'll go ahead and draw a horizontal line over from our total
weight we'll draw a vertical line up from our CG location and where those two
lines intersect is where we are located on our CG loading envelope and we can
see that we're slightly outside of the utility category but we're well within
the normal category, so we're safe to go fly we're under our maximum takeoff
weight we're in between our CG maximum forward or aft location. We can't do
those maneuvers are restricted only to utility category operations because we
don't qualify for that today with our current loading, but we can do everything
that we want to do in the normal category. You know that applies to our
aircraft, so we'll have good pitch authority up and down we'll have good
aircraft performance because we're under our maximum takeoff weight and we're
good to go fly. We've seen it two different ways here, either way works.
You're going to get the same answer the same yay or nay answers to whether or
not you can go fly the airplane at that weight, with that CG loading. If any of
this doesn't quite make sense, just leave your questions in the comments below and
we'll get back to you as soon as possible. Make sure you give us a thumbs
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videos. Thanks so much for watching and we hope
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