Why don’t airports have scales?

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Dan Thomas

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Got ya.
Not calculating the actual CG IS the Reason that dead level is important.
The actual acceptable center of gravity range is probably a football shaped three dimensional space with the long axis parallel to the wings axis.

I wasn’t thinking about large transports
More small aircraft not needing a type rating.
For them a relatively small scale would be enough to give you the gross weight

And of course once you put them on that platform it’s such a minor thing to get a vertical line of center gravity it just seemed like the thing to do.

Well of course not knowing the height of the center on that line is what causes the problem
Finding the vertical location gives us nothing worth knowing. The airplane responds safely with the CG within its longitudinal limits, and the vertical location cannot change very much in any foreseeable loading situation. The manufacturer has already been through all that. Even in a high-wing airplane, with the extremes of full fuel and a light pilot, and low fuel and full people and cargo, the vertical CG movement won't be much because the loads aren't that far apart vertically to start with. The airplane's weight is the larger fraction and damps the displacement. Furthermore, the wing and tail both act in directions roughly perpendicular to the aircraft's longitudinal axis, and pitch changes don't change that much at all. The wing's center of pressure moves a bit with changes in angle of attack, but the response would be essentially the same wherever the vertical location of the CG might be.

So, see, the vertical CG, for our little airplanes, is irrelevant. Learning to fly involves enough difficulty without adding irrelevant stuff to the learning process. In fact, angle of attack (AoA) is a critical part of understanding flight, and yet students in recent decades haven't been getting it in flight training. Maybe ten minutes is spent on it in groundschool, and a few more in flight, and then it's ignored. That brevity in coverage gives the student the impression that it isn't important, and it kills some of them. They do buzz jobs and pull up hard to impress the folks, inadvertently getting the accelerated stall that they asked the airplane for, and it spins into the ground. They had been fed stall speed numbers without the crucial detail that stall speeds go up with wing loadings, and directional changes involve loading changes. Carb ice kills a few too, and wrecks a lot of airplanes, because they've come away with the impression that carb ice is a winter thing. More dysfunctional education.

There are a lot of subjects we need to enhance in training, but vertical CG isn't one of them.
 

Pilot-34

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The point of my post was it’s easy for a single platform scale to calculate and project a vertical line containing the center of gravity.
But without knowing the height above the scale that point is at you can’t know for certain it is within the acceptable center of gravity limits.
And that what limits the utility of the scale for weight and balance information.

now if there was just a simple way to determine the center of gravity height. ......
 

Angusnofangus

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The point of my post was it’s easy for a single platform scale to calculate and project a vertical line containing the center of gravity.
But without knowing the height above the scale that point is at you can’t know for certain it is within the acceptable center of gravity limits.
And that what limits the utility of the scale for weight and balance information.

now if there was just a simple way to determine the center of gravity height. ......
I seems to me that Dan Thomas in post #101 pretty well explained why the vertical c of g doesn't really mean much. Even I understand what he is saying, and I'm just an old tin-basher.
 

12notes

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Vertical cg is not taken into consideration when calculating a safe weight and balance the standard way. Dan Thomas's post explains why the vertical cg isn't something that needs to be calculated except in extreme circumstances (e.g. probably not a good idea to strap a deer on top of your Quicksilver at the longitudinal cg).

Vertical cg location does not matter much if you weigh the airplane in level flying attitude and use the standard cg calculations. When the plane is level, the weight vector is perpendicular to the bottom of the airplane, and will go to the same point on the ground no matter where it is vertically in relation to the airplane floor.

Vertical cg location, however, does become an issue if you weigh the airplane in it's ground attitude and try to correct back to level attitude. It affects the math, not the plane. When the plane is not leveled, the weight vector is no longer perpendicular to the bottom of the airplane, and moving the cg up and down in relation to the airplane floor will move the weight vectors direction back and forward along the ground. Without knowing the vertical cg, calculating the longitudinal cg cannot be accurately calculated by weighing an aircraft in ground attitude.

Calculating the weight distribution involves the following term (simplified for clarity):
((cg longitudinal)*(cosine(angle))) + ((cg verticals)*(sine(angle)))

If you measure it at 0 degrees (level) , since the cosine of 0 degrees is 1, and the sine of 0 degrees is 0, this term simplifies to:
((cg longitudinal) * 1) + ((cg vertical) * 0) = cg longitudinal

If you measure it level, the vertical cg isn't needed for the calculation. However, if you measure it at any other other angle (except level upside down) you would need to know the vertical cg to get an accurate weight distribution correction, since the sine of the attitude would not be zero. At 10 degrees ground attitude, for example, the term becomes:

(.98 * (cg longitudinal) + (.17 * (cg vertical))

The vertical cg would also change with different loadings (e.g. 100 lbs of baggage would change the cg height differently than 100 lbs in a high wing fuel tank), so you'd need to take into account the individual weights at different locations to get the correction factor for weighing loaded at an angle. The plane can tolerate the difference in cg heights between the fuel and the baggage, but the math requires the exact position of the vertical cg location to be accurate in calculating longitudinal cg when weighing in ground attitude.

Extra, extra simplified:
Plane don't care, math do.
 
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Pilot-34

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I seems to me that Dan Thomas in post #101 pretty well explained why the vertical c of g doesn't really mean much. Even I understand what he is saying, and I'm just an old tin-basher.
I understand what he is saying too. And he is correct.
But in the context of why there are not small plane scales at airports and why those scales can’t give us balance information he is misleading.
Not because he is wrong in his comments in someway but because he is on a different conversation.
The CG height IS THE critical factor. And why one set of flat scales can’t give us the critical balance information we need for a safe flight.
But he set his comment up in terms of teaching students weight and balance.
And in that Context he’s absolutely correct.
 

Angusnofangus

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I understand what he is saying too. And he is correct.
But in the context of why there are not small plane scales at airports and why those scales can’t give us balance information he is misleading.
Not because he is wrong in his comments in someway but because he is on a different conversation.
The CG height IS THE critical factor. And why one set of flat scales can’t give us the critical balance information we need for a safe flight.
I am by no means an expert, but do know that longitudinal C of G is critical to safe flight. I believe that point has been made here numerous times. Also the point has been made that vertical C of G does not much matter. This has been done by people who are a lot smarter on the subject that I am. While drive-on scales are important and useful for trucks, trucks aren't concerned with their C of G. Drive-on/drive-off scales for aircraft will tell you zero about your C of G.
 

Pilot-34

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Right. Most private owners don't check where the CG is before flight.
And that brings us back to the point of this thread .
I guess I’ve always thought it was odd that we don’t even check our gross weight figure.

Think about lots of planes get an annual inspection every year that is a real life physical check the airplane is in good operating condition and for the rest of the year we have that paperwork . It is a check our plane is in good operating condition.
Yet before every flight even though we have that paperwork we can go out and physically examine the plane.
Every flight.
But when it comes to wheight and balance settle for a little bit of paperwork.
That’s just always seemed odd to me.
Now I get it that on a home building forum , builders and pilots Maybe a little more familiar with the aircraft in the average pilot.
No disrespect was meant by the question. It just seemed like one of those oddities of aviation.
 

Pilot-34

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I am by no means an expert, but do know that longitudinal C of G is critical to safe flight. I believe that point has been made here numerous times. Also the point has been made that vertical C of G does not much matter. This has been done by people who are a lot smarter on the subject that I am. While drive-on scales are important and useful for trucks, trucks aren't concerned with their C of G. Drive-on/drive-off scales for aircraft will tell you zero about your C of G.
Drive on scales can tell you exactly where your center of gravity is.
They can’t do it in one weigh but they can do it in two.
A drive on scale can Calculate and display by projection the vertical centerline of weight.
Lift the tail up or hold it down and it will now display a new center of gravity at a slight angle to the first one.
Where those two lines cross is the exact center of gravity
Most projection skills of that nature could dust display your exact center of gravity in less than a minute.
The downside of course is it’s a two step process either you or a line boy will need to move the tail.
That’s how simple it is

The wheels don’t need to be in a precise place just on the scale. you don’t have to do any calculations at all just look for the spot that the two lines crossed.
 

Dana

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Yet before every flight even though we have that paperwork we can go out and physically examine the plane.
Every flight.
But when it comes to wheight and balance settle for a little bit of paperwork.
That’s just always seemed odd to me.
Now I get it that on a home building forum , builders and pilots Maybe a little more familiar with the aircraft in the average pilot.
Not really. A wire or fairing might come loose at any time, or a tear in the fabric, or whatever, so you check for it every time you fly. The aircraft's empty CG isn't going to change between flights unless you've installed or uninstalled equipment (at which point you recalculate it and come up with a new empty CG). Sometimes you might reweigh it when you're doing the annual if you suspect it's off. But otherwise, if the plane is well designed with fuel and passengers/baggage in reasonable places, on most flights the CG won't change much, and the empty CG won't change at all.

So you run the numbers a few times when you first get the plane, as I did with mine, and you have a pretty good handle on how you can load the plane while keeping it within limits without having to calculate it every time. In my case I physically can't load it out of CG aft unless I stuff the baggage compartment with gold bullion or lead bricks, and the front cockpit won't hold a passenger heavy enough to make it too far forward. If you're hauling heavy baggage or people, for example four big guys in a C-172 with full fuel and baggage, then you might want to run the numbers for that flight. But you still don't need a scale, that's a solution in search of a problem.
 

Dana

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Some pilots don't check the weather either.

Normally, I don't know the destination.
Exactly, for a short flight I check the weather only if it looks questionable, just like W&B.

I always know my destination... it's the sky.
 

Pilot-34

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Not really. A wire or fairing might come loose at any time, or a tear in the fabric, or whatever, so you check for it every time you fly. The aircraft's empty CG isn't going to change between flights unless you've installed or uninstalled equipment (at which point you recalculate it and come up with a new empty CG). Sometimes you might reweigh it when you're doing the annual if you suspect it's off. But otherwise, if the plane is well designed with fuel and passengers/baggage in reasonable places, on most flights the CG won't change much, and the empty CG won't change at all.

So you run the numbers a few times when you first get the plane, as I did with mine, and you have a pretty good handle on how you can load the plane while keeping it within limits without having to calculate it every time. In my case I physically can't load it out of CG aft unless I stuff the baggage compartment with gold bullion or lead bricks, and the front cockpit won't hold a passenger heavy enough to make it too far forward. If you're hauling heavy baggage or people, for example four big guys in a C-172 with full fuel and baggage, then you might want to run the numbers for that flight. But you still don't need a scale, that's a solution in search of a problem.
Do you see that you are trusting paperwork for your weight and balance but you won’t trust it for The rest of your planes operating condition ?

I know something unexpected affecting either your way or your balance is unusual
But so is finding something that will keep the airplane from flying on your preflight.

How many preflight have you done in a row without finding something to keep you from flying? And yet you still keep doing those pre-flights don’t you?
 

Dan Thomas

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Do you see that you are trusting paperwork for your weight and balance but you won’t trust it for The rest of your planes operating condition ?

I know something unexpected affecting either your way or your balance is unusual
But so is finding something that will keep the airplane from flying on your preflight.

How many preflight have you done in a row without finding something to keep you from flying? And yet you still keep doing those pre-flights don’t you?
You really, really, REALLY need to go get some groundschool and flight education. You have so little idea of what you're talking about that it's embarrassing. You keep digging, trying to save face or something, but it just gets worse. The idea that we do preflight inspections repeatedly without ever finding anything, for example, is ludicrous. We DO find stuff sometimes, and correct it, or have it corrected, before we fly. An airplane tied down outside, especially, can have all manner of damage done to it. Water in the fuel from rain getting past a leaky cap or from some malicious person. Birds' nests in the engine cowling or tail or wings. Mice in the airplane, The wind can bend things like control surface control rods and travel stops. Tires get soft. Some things can work loose. Engine oil needs checking. Inspection panels fall off and get lost. People steal stuff. Hoses start seeping fuel or oil or hydraulic fluid. Brake calipers start leaking. There are lots of things you'd rather not find in flight. I once found the head broken off a bolt holding the landing gear on. The bolt was loaded in shear, but vibration and gear movement was working it out and sooner or later it would have departed and the airplane would likely have been wrecked. If you're flying at night you check all the lights. The propeller has to be checked for nicks caused by stones or other debris, since a nick can become a crack, a crack becomes a break, and a few inches of propeller blade missing results in such severe vibration that the engine can be torn out of the airplane. There's a real CG problem for you. What happens to the CG when 300 pounds of prop and engine fall off the nose? It won't even glide. You're dead.
A pilot who walks up to his airplane, unties it, climbs in and takes off is not really a pilot. He's a driver. I know a guy who did that and he took off with a big bird's nest in the cowling, right on top of the cylinders, stopping all cooling flow over a couple of cylinders. Didn't do that engine any good at all.

Read this: See How It Flies

And then realize that you still have to study weather and aviation law, too.
 
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Dana

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Do you see that you are trusting paperwork for your weight and balance but you won’t trust it for The rest of your planes operating condition ?

I know something unexpected affecting either your way or your balance is unusual
But so is finding something that will keep the airplane from flying on your preflight.

How many preflight have you done in a row without finding something to keep you from flying? And yet you still keep doing those pre-flights don’t you?
It's not the same thing. Finding something seriously wrong on a preflight is unusual, but I have caught things that have prevented me from flying numerous times. Normal wear and tear makes things break. But finding something suddenly wrong with the weight and balance is inconceivable. Unless somebody has snuck into my hangar and secretly bolted a lead brick to the tail of my plane, the weight and balance is not going to change.
 

mcrae0104

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Drive on scales can tell you exactly where your center of gravity is.
Nope.

They can’t do it in one weigh but they can do it in two.
Nope.

A drive on scale can Calculate and display by projection the vertical centerline of weight.
Nope.

That’s how simple it is
Nope.

The wheels don’t need to be in a precise place just on the scale.
Nope.

you don’t have to do any calculations at all...
Nope.
 
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