[A]Improper W&B procedures - rant[/A]

[Sac Arrow]

Long story short. Okay it is a short story. I have been invited to participate in an informal ground school, to present a specific topic. That topic is weight & balance. Note the graph below, for a 172R. Many people are familiar with that graph. In fact most people are, or another Cessna flavor. I'm too lazy to look at other AFMs to see if this error is propagated by other manufacturers.


Okay here is my issue: The sample procedures are given in English units. That's all good. Great. But as you can see, the graph is presented both in English units, lbs and lb-inches, and in SI units.

Sort of. Airplane weight is given in kilograms. We generally use the term "weight" when referring to our body mass in kilograms, but a kilogram is not a weight, it is a mass.There is nothing wrong with referring to a material quantity in terms of mass, as long as we understand it is a mass, or, it truly doesn't make a difference. But, the moment is given in 'kilogram-millimeters.'

THERE IS NO SUCH THING AS A F(*&R&*(#$ING KILOGRAM-MILLIMETER!! No. The SI unit of moment is the newton-meter. One kilogram weighs 9.8 newtons. If you were to suspend a one kilogram mass at the end of a one meter pole, it would exert a force of 9.8 newtons, and result in a moment of 9.8 newton meters at the other end of the pole. If your body mass is one hundred kilograms on earth, it is still one hundred kilograms on the moon, on Jupiter, or in outer space. Your weight in pounds, or newtons on the moon will be one sixth of what it is on earth.

Now I know why they did this. They feel that converting the plane's mass to an actual weight (and force) is super fluous, so treating the kilogram as some sort of pseudo-force is harmless and it is going to yield the same result anyway, so why bother.

If one of my engineers gave me a calculation that involved kilogram-meters, he or she would be fired on the spot. That's the kind of **** that causes structures to collapse and people to get killed, so why not get the physics right?

 

[MauleSkinner]

so you’re thinking that pilots, who generally struggle with the simple math required to do a weight and balance, should do more mathematical conversions?

 

[Tarrow]

kg works as long as you are on earth with 9.8 m/s^2 acceleration due to gravity. That even varies some depending where you are on earth and what altitude you are at. The weight and c.g. locations are probably off by more than that.

 

[Sac Arrow]

so you’re thinking that pilots, who generally struggle with the simple math required to do a weight and balance, should do more mathematical conversions?

No, I think either do it right, or don't do it.

Let's talk cranes. Go to Europe. Crane capacities are given in kilonewtons. You have a 9.8 KN crane. You think, why not just give the crane capacity in kilograms? 1,000 kilograms is the same as 9.8 kilonewtons, right?

The problem is, what if this crane is mounted on a ship? Every ship's engine room has a gantry crane, and you may have cargo cranes on the deck. That 9.8 kn limit is absolute. If you're adrift at sea, pulling a 1,000 kg piston and rod assembly from your Sulzer engine, wave action is going to cause the force on that crane to fluctuate between something below 9.8 kn, to something above 9.8 kn. You just broke your crane. Now you're stranded in the Gulf of Aidan, and you are boarded by Somali pirates, who keep you hostage for 112 days until Seal Team Six moves in, taking out all of the pirates and half of the hostages (technically a success) which could have been avoided if you didn't try to lift the entire two piece con rod at once.

 

[murphey]

I love my cherokee with common sense charts.

 

[Bill]

Dude, you need a Snickers.

 

[MauleSkinner]

No, I think either do it right, or don't do it.

if we applied that to all parts of flying an airplane, most pilots would have to quit.

 

[Salty]

If you’re going to take off in your Cessna on the high seas balancing off a crane, then yeah, better eat your fig newtons. Otherwise……

 

[Let'sgoflying!]

Is this one of the Reims 172 built in France?

 

[Capt. Geoffrey Thorpe]

Kilograms "force" is pretty commonly used.
Stupidity is also common.
Words like "Weight" existed long before inertial mass and force due to gravity were known to be different things resulting in more confusion and misunderstanding than you can shake a stick at.

Given that NIST defines the pound in terms of the Kilogram, instead of calculating weights and balances, think of it as calculating mass and balance. Pounds mass * inches or Kilograms * mm gives you a mass moment.

Oh, and for pilots who struggle with stupid simple math such as basic algebra...

Tells you how to "solve" for either moment, arm or "weight" without having to remember the algebra. Just remember "MAW".
You have probably figured out how it works already, so I won't insert a link to this video that explains it.

Or, will I?

(can't find the evil laugh emoji - use your imagination)

 

[Sac Arrow]

Kilograms "force" is pretty commonly used.
Stupidity is also common.
Words like "Weight" existed long before inertial mass and force due to gravity were known to be different things resulting in more confusion and misunderstanding than you can shake a stick at.

Given that NIST defines the pound in terms of the Kilogram, instead of calculating weights and balances, think of it as calculating mass and balance. Pounds mass * inches or Kilograms * mm gives you a mass moment.

I get the concept of mass centroids. I get the concept of shape centroids. I just think applying a "mass moment" is a fundamental misapplication in weight and balance. I also agree that 'kilogram force' is commonly, and incorrectly used.

Back in the English world, our base unit of mass is the slug. We also have a secondary unit called a 'pound mass', which is an lbm. It is one slug numerically divided by 32.2, or 1 lbf/1 ft2. It is sometimes used in engineering calculations which more often than not end up being off, by a factor of 32.2.

If Cessna wants to define a new unit, the kilogram force, kgf, and derive it from the newton or kilogram to make clear the relationship, I'm fine with that.

 

[Capt. Geoffrey Thorpe]

Back in the English world, our base unit of mass is the slug. We also have a secondary unit called a 'pound mass', which is an lbm. It is one slug numerically divided by 32.2, or 1 lbf/1 ft2. It is sometimes used in engineering calculations which more often than not end up being off, by a factor of 32.2.

Back in the olden days when I was working on my engineering degree (mechanical was the only option since electricity had not yet been invented), everything was in lbm (and lbf) and slugs were just mentioned in passing once or twice and 32.2 pounds mass per pounds force feet per second squared is permanently burned into my brain. Permanently. I will never forget that.

And, I see no issue with calculating a mass moment.

 

[kgruber]

The one that gets me is "heat."

One day one of Thermodynamics we learned there is no such thing as "heat."

 

[MauleSkinner]

Problem solved (by another aircraft manufacturer):

 

[Sac Arrow]

Problem solved (by another aircraft manufacturer):View attachment 124078

That didn't solve a damn thing. We know one kilogram weighs 2.2 lbs. We don't need a chart to go further.

 

[MauleSkinner]

That didn't solve a damn thing. We know one kilogram weighs 2.2 lbs. We don't need a chart to go further.

I think the problem is you’re trying to buck decades of engineering tradition.

 

[Sac Arrow]

I think the problem is you’re trying to buck decades of engineering tradition.

I am the product of decades of engineering tradition!

Okay maybe two.

 

[L J Donelson]

No, I think either do it right, or don't do it.
.
.
.
... which could have been avoided if you didn't try to lift the entire two piece con rod at once.

See, this is where you are going off the rails by overthinking. It's not avoided if you didn't try, it's - if the crane was in imperial units and you had an appropriately designed factor of safety applied that limited the entire system, then "do it right, or don't do it" would have applied.

As for the extremely detailed scenario ending with Seal Team Six, if the end of their mission is me walking then I do count it as a success.

 

[Jim_R]

Long story short.

If you are so worked up about this that you can't just ignore it, then during your presentation say something like:

"This chart shows 'kilograms' as a counterpart to 'pounds', but it neglects that the 'pounds' on this charts are 'pounds force', not 'pounds mass'. That will drive some engineers and scientists crazy, but really it makes no practical difference when you're trying to figure out if your W&B is in the safe zone or not, especially since no one in this room is likely to pay any attention to the 'kilograms' part of the chart anyway.

If you have no idea what I just said, just pretend I didn't say it. Moving on..."

In the meantime, I'm wondering how much sleep you've lost over the years by people referring to the "center of gravity", since that implies that somehow gravity has a shape, and clearly that's as insane as saying that yellow has an aroma.

 

[Sac Arrow]

It's fairly apparent that Cessna doesn't actually intend on having anyone actually use SI units for W&B calculations. If they did, they would also include loading station arms in mm rather than having you have to manually convert them, and they would also include another sample calculation in SI units. If they aren't going to do that, why even bother annotating the loading envelope graph?

I think I'm just going to chuck the 172 AFM and go with a Piper Arrow example.

 

[MauleSkinner]

Good thing we didn’t go metric 50 years ago.

 

[Sac Arrow]

Good thing we didn’t go metric 50 years ago.

We do a lot of work in SE Asia and Australia. I'm comfortable with either. I know more cardinal values in English units, but the math is simpler in metric.

 

[Albany Tom]

I'm agreeing with Sac here. Most people think that the metric system is magically easier. It isn't. So to those that want to use metric, suck it up and use Newtons. It shouldn't be a surprise to anyone that's either had 30 seconds of physics or worked in any trade for more than a week. Probably every torque wrench sold today is marked in in-lb or ft-lb and Nm, or exclusively Nm.

Besides, w&b is all about cg and ratios. You could do it with bricks per broomstick, if you standardized the values of bricks and broomsticks.

The climbing people already get this. Rope and hardware is measured in kN, not kg.

 

[Half Fast]

Rope and hardware is measured in kN, not kg.

Makes sense. I assume climbers must also weigh themselves in Newtons?

 

[Half Fast]

@Sac Arrow , you’re absolutely correct.

How goes your battle with the pharmaceutical industry, which weighs dosages in milligrams?

 

[Albany Tom]

Makes sense. I assume climbers must also weigh themselves in Newtons?

It's probably safer that they have to do the math, or be aware of the Newton rating they need. Otherwise you'd have 150 lb people thinking that a 200 lb rope was just fine.

 

[Dana]

Besides, w&b is all about cg and ratios. You could do it with bricks per broomstick, if you standardized the values of bricks and broomsticks.

The climbing people already get this. Rope and hardware is measured in kN, not kg.

For w&b it makes sense to use kgf, because [I presume] metric scales give weight in kg. Adding an additional conversion to newtons just adds another opportunity for error.

For climbing (and cranes) using N or Kn makes sense as there are dynamic effects beyond just the mass being held.

 

[Lindberg]

Long story short. Okay it is a short story. I have been invited to participate in an informal ground school, to present a specific topic. That topic is weight & balance. Note the graph below, for a 172R. Many people are familiar with that graph. In fact most people are, or another Cessna flavor. I'm too lazy to look at other AFMs to see if this error is propagated by other manufacturers.
View attachment 124055

Okay here is my issue: The sample procedures are given in English units. That's all good. Great. But as you can see, the graph is presented both in English units, lbs and lb-inches, and in SI units.

Sort of. Airplane weight is given in kilograms. We generally use the term "weight" when referring to our body mass in kilograms, but a kilogram is not a weight, it is a mass.There is nothing wrong with referring to a material quantity in terms of mass, as long as we understand it is a mass, or, it truly doesn't make a difference. But, the moment is given in 'kilogram-millimeters.'

THERE IS NO SUCH THING AS A F(*&R&*(#$ING KILOGRAM-MILLIMETER!! No. The SI unit of moment is the newton-meter. One kilogram weighs 9.8 newtons. If you were to suspend a one kilogram mass at the end of a one meter pole, it would exert a force of 9.8 newtons, and result in a moment of 9.8 newton meters at the other end of the pole. If your body mass is one hundred kilograms on earth, it is still one hundred kilograms on the moon, on Jupiter, or in outer space. Your weight in pounds, or newtons on the moon will be one sixth of what it is on earth.

Now I know why they did this. They feel that converting the plane's mass to an actual weight (and force) is super fluous, so treating the kilogram as some sort of pseudo-force is harmless and it is going to yield the same result anyway, so why bother.

If one of my engineers gave me a calculation that involved kilogram-meters, he or she would be fired on the spot. That's the kind of **** that causes structures to collapse and people to get killed, so why not get the physics right?

No, I think either do it right, or don't do it.

Let's talk cranes. Go to Europe. Crane capacities are given in kilonewtons. You have a 9.8 KN crane. You think, why not just give the crane capacity in kilograms? 1,000 kilograms is the same as 9.8 kilonewtons, right?

The problem is, what if this crane is mounted on a ship? Every ship's engine room has a gantry crane, and you may have cargo cranes on the deck. That 9.8 kn limit is absolute. If you're adrift at sea, pulling a 1,000 kg piston and rod assembly from your Sulzer engine, wave action is going to cause the force on that crane to fluctuate between something below 9.8 kn, to something above 9.8 kn. You just broke your crane. Now you're stranded in the Gulf of Aidan, and you are boarded by Somali pirates, who keep you hostage for 112 days until Seal Team Six moves in, taking out all of the pirates and half of the hostages (technically a success) which could have been avoided if you didn't try to lift the entire two piece con rod at once.

Is this a physics or engineering lecture? If not then the above seems to be irrelevant. If the Cessna POH creates a hazardous condition for its specific application, you should certainly discuss that and why it is so.

 

[Capt. Geoffrey Thorpe]

For w&b it makes sense to use kgf, because [I presume] metric scales give weight in kg.

Yes, when you weigh out potatoes you get a number in kilograms. And...
Modern scales with load sensors do indeed "react" to a force. But...
Big but...
But they are all calibrated by placing a calibrated MASS on them. Not a force.
So, if you put a bag of potatoes on a calibrated scale on earth, you find that it "weighs" 5 kg. Take the same scale, calibration masses, and potatoes to the moon, and once you have calibrated your scale (which you should always do after moving it) and you will find that your bag still "weighs" 5kg.
Even on earth, if you calibrate a scale in Newtons, you would get slightly different "weights" for the same object depending on where you are due to local variations in the acceleration of gravity.
Or if you have an older balance style device, you are balancing your potatoes directly against your reference mass and changes in gravity (which would change the force) will not change the measured weight.

So, does a scale measure force or mass? Depends on the technology used. But in either case, they are calibrated with a reference mass and are used to give you a "weight" in kg (mass). Also, NIST (the legal national authority on weights and measures in the United States) defines the "pound" in terms of the kilogram - not the Newton - so for legal purposes, pounds are a measure of mass, not force.

The problem, as previously mentioned, is the word "weight" pre-dates the distinction between inertial mass and force due to gravity and can be ambiguous so one needs to be clear which is intended - force or mass.

Kilogram force is an abomination unto God and causes way more problems than it solves.

 

[Half Fast]

So, does a scale measure force or mass? Depends on the technology used.

To state it simply:

A spring scale measures force and will give different readings in different gravities.

A balance scale measures mass (using a calibrated counter balance mass) and will give the same reading in any gravity.

When I took physics (shortly after the apple whacked Sir Issac), an answer would be marked incorrect if we put “lb” for the unit of measure. It had to be “lbm” or “lbf” to show we really knew which property, force or mass, was being considered.

 

[Domenick]

... In the meantime, I'm wondering how much sleep you've lost over the years by people referring to the "center of gravity", since that implies that somehow gravity has a shape, and clearly that's as insane as saying that yellow has an aroma.

Au contraire. Tasting/smelling/hearing colors and other stimuli is called "synesthesia." It's rare, but real.

Colour-smell synesthesia

A website about the different types of synesthesia, with descriptions and real examples of each one. Discover your type of synaesthesia!

www.thesynesthesiatree.com

https://www.npr.org/sections/thesalt/2013/03/12/174132392/synesthetes-really-can-taste-the-rainbow

Synesthesia - Wikipedia

en.wikipedia.org

And more or less on topic, here's an Excel workbook to calculate your aircraft W&B:

https://www.dropbox.com/scl/fi/p9b4...ees.xlsx?rlkey=dxdz8du0jk7to9s64wlqtn6ef&dl=1

It currently has sheets for PA-28-161, -140 (2 seats), -140 (4 seats), -180, -236, but given the W&B envelope data given in your plane's TCDS, it can be setup for many GA aircraft.

I hope the units are unoffensive.

 

[Jim_R]

Au contraire. Tasting/smelling/hearing colors and other stimuli is called "synesthesia." It's rare, but real.

I intentionally used that comparison and I was curious how long it would be before someone mentioned that. It still supports my point: Only a small fraction of the population experiences that phenomenon...just like only a small fraction of the population shares the OP's complaint.

 

[Capt. Geoffrey Thorpe]

only a small fraction of the population shares the OP's complaint.

I attribute that shortcoming to the so called "new math".

 

[Jim_R]

I attribute that shortcoming to the so called "new math".

I attribute at least some of it to people having good judgment about what's important to get upset about and what isn't.

 

[Sac Arrow]

@Sac Arrow , you’re absolutely correct.

How goes your battle with the pharmaceutical industry, which weighs dosages in milligrams?

We normally dose chemicals in milligrams per liter in water treatment.

 

[Capt. Geoffrey Thorpe]

I am the product of decades of engineering tradition!

 

[Sac Arrow]

View attachment 124186

Well not that many decades...

 

[Half Fast]

View attachment 124186

An elegant weapon for a more civilized age.

Well not that many decades...

Barbarian.

 

[Sac Arrow]

An elegant weapon for a more civilized age.




Barbarian.

In fairness, we we learned to use one of those in an archeology class. Along with a trebuchet and a longbow.

 

[Half Fast]

In fairness, we we learned to use one of those in an archeology class. Along with a trebuchet and a longbow.

I am a staunch advocate of the right to keep and bear a trebuchet.