Units in Design

[TFF]

GM started mixing hardware because they saw that metric could fit between two standard sizes so they could have enough strength and loose weight than use the next standard up. Essentially they doubled the off the shelf hardware available to them.

 

[Pilot-34]

I got an inkling of why they add those software "kill switches."

This reminds me of an automotive engineer who worked for a small company that built automated manufacturing equipment for the Canadian automobile industry. His company designed and built some automated production tooling and sold it to a major manufacturer. After they built 100,000 widgets on the machines, the larger company refused to pay because some obscure reset function was slower than a specification. So the smaller company was forced to go broke after the larger company completed all their production run. My buddy got laid-off and left the automotive industry in disgust.

Wouldn't it have been wiser to add an obscure line of software .... After too many customers complain about silent automobiles, the complaint gets back to the company that made the production tooling. "Oh? He does not work here any more. Click!"
Cue evil laughter.

That obscure reset function is only obscure when you’re not using it

 

[bhooper360]

I’ve seen projects dropped because the [justification was “ we did it that way before”].

In literature, the strength of character motivation goes like this:

1. (weakest) man vs world
2. man vs man
3. man vs self (strongest)

Building an airplane is difficult, probably it's one of the most difficult things a person can do. When your confronted by all the things you don't know, it's convenient to blame some guy in a high tower: the guy who designed the part, the engineer, the guy who knows how to use solidworks. You can even blame the world, at least some part of it-- Haas, or some engineering company, the people who set the precedents, or the FAA regs. Better yet, blame the whole **** economy for making some technique or material too expensive!

One one hand,you say you have seen projects fail because those evil engineers don't understand what they're asking the builders to do. That's a man vs. man level of motivation. On the other hand, Jay says that it gives him a sense of personal gratification to pull the bearing out of the freezer and drop it into a well-machined part. Who has a stronger motivation?

Also, if you don't want the mill to get shut off, you can use GNU/Linux to run it. If you don't want your tractor to shut off, you ought to use Linux to run that tractor, too. If you have a relationship with the engineers at the company and you like what they do, then by all means buy a Haas or a John Deere, whatever gets the plane to the fly-in. But you always have an opportunity to take ownership of the software, if you're willing to put in the work. And yes, it's real work, brain uses a lot of calories too.

 

[Jay Kempf]

It's all about context. I have been yelled at by machinists that said that a 4 decimal nominal (basic) dimension was just ridiculous. But a basic dimension has no tolerance. This is why we develop systems for these things and then train people to use them.

Was in a situation years ago where a segment of the manufacturing group had been harboring a misunderstanding of a fundamental tool they were using to measure coating thicknesses. The supervisor was protecting all his people but he was the problem. Basically the meter read in thousandths of an inch which is standard in paint on metal. That mean 1 on the machine meant .001 inch. 1.5 meant .0015 inch and so on. When I got sent in to see why the coatings weren't passing QC first thing I found was no one could answer the simple question "what does the number on the machine mean?" Went around the room. No one knew. Because the machine had "mil" printed on it they had all assumed it was some millimeter thing but they couldn't make any sense of it. So they all covered for each other. After that we trained them all and did some basic arithmetic proficiency checking and put up a series of posters that explained everything. Wasn't their fault. The guy that got the training was long gone. This is just one example.

An engineers job is to give the LOOSEST tolerance possible. No one understands that that complains about how engineers over tolerance things. Tight tolerances are just costly so the idea is to avoid them unless you need them and then to apply them properly and clearly.

One off airplanes have very few tolerances that matter. Most of those are sub assembly hole patterns between major parts or round fits. Where 1 individual rivet is is not important. The size of the rivet hole is.

 

[Pilot-34]

The guy that got the training was long gone. This is just one example.

Lol Those words have ruined many a company!
They are the seed of “we have always done it that way “

 

[bmcj]

I had such an instructor once too. He would make up random units, with conversion factors as needed, and expect the final answer in odd units such as 'napkins per cat drool^2'. It does help with the understanding rather than just memorizing formula.

One of my first aero books emphasized the importance of units, dedicating the entire first chapter to a discussion of units and conversion factors. They even presented (quite well) an example of a system where all measurements of length, time, mass and force could all be expressed in units that were solely as various powers of SECONDS (time).

 

[bmcj]

Utterly Useless Trivia! Roman Legions in N. Africa made ice using radiant cooling. Dig a pit in the sand just outside camp, line with straw, ( carried to feed mules ) set shallow dishes filled with water in pit, expose to space at night, cover with tarps and straw at dawn. Officers could thus have frozen beverages in the Sahara.

The Romans had tarps???

 

[Pilot-34]

How do you explain the Fahrenheit temperature logic to a Tibetan who has never seen the ocean???????

They’ve seen people and blood.

 

[Pilot-34]

The Romans had tarps???

Of course
And traps

 

[AeroER]

Ages ago the founder of Sverdrup Corporation told us a mass flow measure he invented.

The Gulf Stream is something like 12 Sverdrups.

An odd case in a paper about orbital mechanics that included a "hurds" unit. I don't think anyone really nailed that one down, and we had three or four premier experts in the topic on that project. The best guess was "hundreds of radians per second, maybe". That's when the paper is turned into a novelty, then shelved and forgotten, because it's useless at that point

 

[BJC]

An odd case in a paper about orbital mechanics that included a "hurds" unit.

A dollar is a unit of reactivity in a nuclear reactor.


BJC

 

[ragflyer]

I was trained in the 70's and already then we did all calculations in Newtons. We used some old material charts that used kp/mm2 for strength and kp/cm2 for pressure. No big deal as 1 kgf = 1 kp ≈ 10 N. I must say I haven't seen a new text book or a scientific paper for decades that uses kgf or kp.

Thank you and bonus points for being the first one to directly answer the question . Your experience is what I expected but was surprised to find the Pajno books published since 2016 to use the kgf (MKFS system) for force and kgf/mm^2 for stress. Perhaps it has more to do with the vinrage of the author . Good books though and worth reading. Any other with a European aircraft design background?

 

[Wanttaja]

The Romans had tarps???

Well, not blue plastic ones, of course. But Roman ships had canvas sails, made from linen. They would probably do quite nicely.

The Roman army used goatskin for their tents, which probably would have worked as well.


Ron "Veni, Vidi, Vici" Wanttaja

 

[ragflyer]

One of our spacecraft designers at Boeing held solid to English units, and the guys he trained kept that up as well. We made sure our Government contracts left the unit decision to us. Right to retirement, I did all my orbital stuff based on nautical miles.
Ron Wanttaja

Ron, Did he use the BG system with mass being in slugs and force being pounds or the EE system where mass is in pound mass and force pound force with Gc (as Monty points out) as a conversion factor?

Most of my reference books in aero have been with BG, seems more cleaner/elegent. Wonder which system is more common?

 

[Wanttaja]

Ron, Did he use the BG system with mass being in slugs and force being pounds or the EE system where mass is in pound mass and force pound force with Gc (as Monty points out) as a conversion factor?

Most of my reference books in aero have been with BG, seems more cleaner/elegent. Wonder which system is more common?

EE system. Never heard slugs mentioned, though possibly the propulsion guys dealt with them.

Much of the specific design work was done by catalog; I'd compute how much delta-v would be needed over the life of the vehicle, what kind of acceleration would be needed, and pick a likely specific impulse to run the basic equations so we had a ballpark as to the quantity of fuel needed. Would pass that, along with other factors, to the designer and he would pick out the thrusters, propellant tanks, etc. and tell me if the design was still closing. Would then bring in the system experts (including propulsion) and let them hone the design parameters.

Most of the work was pretty cut-and-dried; most of the details of space operation have been known for decades. I put together a set of cheat sheets that followed me whereever I went, gave a leg up on new programs. For instance, here's one page from it to make it simple to determine how much delta-v was necessary to keep a geosynchronous satellite on station....

Also needed ~150 fps/year to handle inclination drift.

Ron Wanttaja

 

[AeroER]

Ron, Did he use the BG system with mass being in slugs and force being pounds or the EE system where mass is in pound mass and force pound force with Gc (as Monty points out) as a conversion factor?

Most of my reference books in aero have been with BG, seems more cleaner/elegent. Wonder which system is more common?

Thermodynamics, fluid transport, heat transfer, aerodynamics, and others apply pounds-mass, pounds-force, and slugs.

The weights guys use slugs. Non weights designers on advanced programs are likely to use pounds-something, and the strength guys in my group had better check the numbers before I do.

When I joined this forum last fall I noticed confusion on the topic, so I wrote a short tutorial to help members understand and sort the problem out. I received a bunch of flack from the dreamers content to wallow in ignorance, so now I barely GAF about the topic.

 

[Jay Kempf]

Heard a new unit for torque a week back..... large values of torque..... n*elephant*mile

Load driving stress at root of a 747 wing in the center at MTOW and max gust load.

 

[wrmiles]

Burned into my memory:

32.2 lbm-ft/lbf-sec2

 

[bmcj]

Why haven’t I heard anyone mention the PDL yet (poundal)? I’ve never had to use it in my calculations, but it is a further complication in the British system.

For those that are scratching their heads, a poundal is a unit of force that can accelerate one pound mass at one foot per second^2 acceleration rate.

 

[raytol]

When I joined this forum last fall I noticed confusion on the topic, so I wrote a short tutorial to help members understand and sort the problem out. I received a bunch of flack from the dreamers content to wallow in ignorance, so now I barely GAF about the topic.

I found your tutorials incredibly informative. Please don't give up on us!