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JayinKitsap
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Metric vs Imperial in aircraft

Mon Mar 07, 2022 3:56 am

I saw in a post that the F-15 and F-16 are all in imperial sizes and dimensions. I am sure the US made aircraft from old (like the B-52 & KC-135) are Imperial sizes but the US Federal Government starting in the 80's tried to change all of its construction projects to metric, after two decades of trying they gave up and went back to Imperial there, but all of the flow meters, pressure transmitters, and industrial equipment is totally metric. Car production around then went metric, I believe Cat has been metric for 40-50 years.
So which aircraft are still Imperial dimensions and which are metric. I would venture the B-737 could be Imperial, but I expected Metric. I would be blow away if the 787 wasn't metric. Positive the F-35 is metric. Is the F-15 Imperial.

Does the instrumentation change from Imperial for WN to Metric for EK? Settable in the cockpit? Do flight computers use kg or lb for weights of fuel, etc?
 
bluecrew
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Re: Metric vs Imperial in aircraft

Mon Mar 07, 2022 7:56 am

JayinKitsap wrote:
I saw in a post that the F-15 and F-16 are all in imperial sizes and dimensions. I am sure the US made aircraft from old (like the B-52 & KC-135) are Imperial sizes but the US Federal Government starting in the 80's tried to change all of its construction projects to metric, after two decades of trying they gave up and went back to Imperial there, but all of the flow meters, pressure transmitters, and industrial equipment is totally metric. Car production around then went metric, I believe Cat has been metric for 40-50 years.
So which aircraft are still Imperial dimensions and which are metric. I would venture the B-737 could be Imperial, but I expected Metric. I would be blow away if the 787 wasn't metric. Positive the F-35 is metric. Is the F-15 Imperial.

Does the instrumentation change from Imperial for WN to Metric for EK? Settable in the cockpit? Do flight computers use kg or lb for weights of fuel, etc?

Can't speak to the design blueprint stuff. I would assume anything recent would have a multitude of measurements available, as they've all been designed with computers. That data is available in every imaginable unit.

Obviously, for InHg/HPa, it's a toggle on any newer avionics. To change between kg/lb or ft/m, it's probably just a software update, just a little update to the box. Meters are a switch in the FD, depends on the airplane though. You'd never convert the avionics to meters only though - only two countries (afaik) use them. More efficient to use a conversion table at that point, or fly the FL+1 model from China.

Also, flying in China is terrifying, if the opportunity comes, don't do it.
 
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fr8mech
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Re: Metric vs Imperial in aircraft

Mon Mar 07, 2022 2:25 pm

Speaking from a maintenance perspective, the only time I’ve come across a piece of metric hardware is when we take something apart at a sub-assembly level, on an Airbus. Example, originally, Airbus considered the main cargo door actuator, along with its block valves, as a single assembly, to be replaced as such. All that hardware is in imperial.

Soon after we had the aircraft in service, we found that one of the block valves had a tendency to leak. Instead of changing the entire actuator, we just change the valve. That valve is installed on the actuator with metric hardware. At least it was a decade or so ago. We may have asked for a change to the component spec to change the hardware to imperial.

All our Airbus drawings are in metric with imperial in parentheses.
 
Thrusty69
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 3:54 am

All the big commercial western jet stuff is not metric, but sometimes you run into the exception, and I question whether it’s actually a /32nd measurement….

I frequently see metric dimension tires on airliners from Europe etc…

I’ve worked on some smaller German built piston aircraft and metric fasteners were norm.
 
M564038
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 7:18 am

Everything in aircraft are in metric. Everything.
 
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Starlionblue
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 8:02 am

I'll nitpick.

Those units are not "Imperial". They are "US Customary".




bluecrew wrote:
JayinKitsap wrote:
I saw in a post that the F-15 and F-16 are all in imperial sizes and dimensions. I am sure the US made aircraft from old (like the B-52 & KC-135) are Imperial sizes but the US Federal Government starting in the 80's tried to change all of its construction projects to metric, after two decades of trying they gave up and went back to Imperial there, but all of the flow meters, pressure transmitters, and industrial equipment is totally metric. Car production around then went metric, I believe Cat has been metric for 40-50 years.
So which aircraft are still Imperial dimensions and which are metric. I would venture the B-737 could be Imperial, but I expected Metric. I would be blow away if the 787 wasn't metric. Positive the F-35 is metric. Is the F-15 Imperial.

Does the instrumentation change from Imperial for WN to Metric for EK? Settable in the cockpit? Do flight computers use kg or lb for weights of fuel, etc?

Can't speak to the design blueprint stuff. I would assume anything recent would have a multitude of measurements available, as they've all been designed with computers. That data is available in every imaginable unit.

Obviously, for InHg/HPa, it's a toggle on any newer avionics. To change between kg/lb or ft/m, it's probably just a software update, just a little update to the box. Meters are a switch in the FD, depends on the airplane though. You'd never convert the avionics to meters only though - only two countries (afaik) use them. More efficient to use a conversion table at that point, or fly the FL+1 model from China.

Also, flying in China is terrifying, if the opportunity comes, don't do it.


Flying in China can be terrifying. But metric altitudes aren't really the reason... :D
 
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WesternDC6B
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 1:35 pm

Something that has passed through my mind a time or two, so, here goes.

BACKGROUND: I seem to recall some Imperial/Customary-based hardware that was called "Whitworth". Sears's Craftsman Tools offered Whitworth wrench (spanner) and other such tools sets.

THE QUESTION: Did Armstrong-Whitworth, or anyone else, use Whitworth hardware?
 
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fr8mech
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 4:19 pm

M564038 wrote:
Everything in aircraft are in metric. Everything.


Sure, but all the 12.7mm nuts and bolts are easily removed or installed with a 1/2inch wrench/socket.
 
e38
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 5:31 pm

JayinKitsap wrote:
Does the instrumentation change from Imperial for WN to Metric for EK? Settable in the cockpit? Do flight computers use kg or lb for weights of fuel, etc?


The software is installed per the operating specifications of the individual airline. In most cases, the parameters are set and cannot be changed without a modification to the software. At my company, weights are in pounds (lbs), altitude in feet, speed in knots, distances in nautical miles (n.m.), etc.

The only parameter we can adjust in the flight deck (Airbus A320 series) is altimeter setting. There is a knob that can switch between in Hg and hPa.

We use charts/calculators for items that may require a conversion; for example, gallons to pounds (fuel).

e38
 
Lpbri
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 6:12 pm

As was stated before, on Airbus, all maintenance manual data such as torques and dimensions are given in metric, with imperial values in parenthesis. Honestly, when working with small dimensions, millimeters are much easier to work with. Brake wear limits are given in millimeters, and I have a millimeter scale that I use. Oddly, torque wrenches give Newton-meter values in addition to imperial, but Airbus gives torque values in dyn- something.
 
M564038
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 6:56 pm

Well, that sounds about right.
1/2 inch is just funny phrase for 12.7mm anyway.

fr8mech wrote:
M564038 wrote:
Everything in aircraft are in metric. Everything.


Sure, but all the 12.7mm nuts and bolts are easily removed or installed with a 1/2inch wrench/socket.
 
Thrusty69
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 8:56 pm

My Snap-On 1/2” socket is clearly engraved with “1/2-12.7” right on it. Swear to god. … …
 
kalvado
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 10:32 pm

Thrusty69 wrote:
My Snap-On 1/2” socket is clearly engraved with “1/2-12.7” right on it. Swear to god. … …

Was it supplied with a bag of M7.8 nuts?
 
JayinKitsap
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 10:44 pm

Starlionblue wrote:
I'll nitpick.

Those units are not "Imperial". They are "US Customary".

:D


In the US building construction industry it really is just the normal units, sometimes English or Foot Pounds, I only see Imperial when in Canada, although they are very metric on all of their meat prices, but the sale signs are Sirloin $14.99 /lb which is quite shocking as in Seattle its $ 5.99 /lb (until our recent crazy runup).

Thanks to everyone with their replies, it does seem that aviation from the mechanics point of view is metric, with some soft metric thrown in (the 12.7mm socket).

As an engineer one thing that throws me with metric is the distinct difference between force and mass. In "US Customary" when acceleration is 1 g a pound mass = a pound force, so everything is in pounds. It takes a double check converting to metric to be sure which pound is being referenced. It is so important to have intuition going, I instinctively know whether a steel beam W14x22 or W21x44 should be used. (Wide Flange shape, sorta a 14" deep beam, but the really heavy ones can be 28" deep and weigh 750 PLF) x the weight per foot in pounds. I'm usually quite close with my initial sizes that get optimized in my FEM program called Risa 3D.

In US construction, our plywood, drywall, wood studs, windows, ceiling tiles, and the like are set up on a 2 foot module, well a 2x2 ceiling tile is easy to note, but its 609.6 mm across is a pain, so they did soft metric (epic #fail) 600x600 tiles, not 610x610. Well with 40 tiles the actual length is 18.29 m, but the soft dimensions is 18.0 meters and there is a gap of over 1 foot due to the screwy soft dimensions. I blame the soft minds of Architects for this.
 
Thrusty69
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 11:01 pm

Jay, there’s no metric on airliners. You may have missed the jokes. No mechanic anywhere working on airliners has metric tools anywhere.
 
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DL_Mech
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Re: Metric vs Imperial in aircraft

Tue Mar 08, 2022 11:32 pm

Aircraft with non-switchable altimeters will have separate metric altimeters if needed ( seen here to the left of the Captain’s control wheel).

 
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Starlionblue
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Re: Metric vs Imperial in aircraft

Wed Mar 09, 2022 12:01 am

Dimensions of bits and bobs aside, some things on Airbus are in customary units. Just a few examples
- Altitude and VS in feet, obviously
- Brake pressure in PSI
- Hydraulic pressure in PSI
- Bleed pressure in PSI

Things we can't see in the cockpit:
- Thrust in ft-lb.
- Tyre inflation pressure in PSI (I think)

e38 wrote:
JayinKitsap wrote:
Does the instrumentation change from Imperial for WN to Metric for EK? Settable in the cockpit? Do flight computers use kg or lb for weights of fuel, etc?


The software is installed per the operating specifications of the individual airline. In most cases, the parameters are set and cannot be changed without a modification to the software. At my company, weights are in pounds (lbs), altitude in feet, speed in knots, distances in nautical miles (n.m.), etc.

The only parameter we can adjust in the flight deck (Airbus A320 series) is altimeter setting. There is a knob that can switch between in Hg and hPa.

We use charts/calculators for items that may require a conversion; for example, gallons to pounds (fuel).

e38


There's also a button to turn on metric altitude display below the altitude tape on the PFD. :)
 
M564038
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Re: Metric vs Imperial in aircraft

Wed Mar 09, 2022 6:33 am

And you missed the definitions of the units.

Thrusty69 wrote:
Jay, there’s no metric on airliners. You may have missed the jokes. No mechanic anywhere working on airliners has metric tools anywhere.
 
Tristarsteve
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Re: Metric vs Imperial in aircraft

Wed Mar 09, 2022 12:11 pm

Thrusty69 wrote:
Jay, there’s no metric on airliners. You may have missed the jokes. No mechanic anywhere working on airliners has metric tools anywhere.


When we changed our first Electric Hydraulic pump on an A320, we had trouble with the bolts mounting the bracket to the pump. We had to go and get a spanner out of our car tool kit to get them off. It was the first time ever I used a metric spanner on an aircraft, and I have never used one anywhere else.
After that we bought a metric tool kit for our van, but never used it, except when changing over these brackets, which I did about four times.

WesternDC6B wrote:
Something that has passed through my mind a time or two, so, here goes.

BACKGROUND: I seem to recall some Imperial/Customary-based hardware that was called "Whitworth". Sears's Craftsman Tools offered Whitworth wrench (spanner) and other such tools sets.

THE QUESTION: Did Armstrong-Whitworth, or anyone else, use Whitworth hardware?


We used whitworth spanners on the Vickers Viscount and Vanguard when I started at BEA at LHR in 1969. I had loads of Whitworth spanners, but never used them since.
 
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WesternDC6B
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Re: Metric vs Imperial in aircraft

Wed Mar 09, 2022 12:58 pm

M564038 wrote:
Well, that sounds about right.
1/2 inch is just funny phrase for 12.7mm anyway.


I prefer metric for mass. In customary I am 269, but in SI I'm a slim, trim 122.
Last edited by WesternDC6B on Wed Mar 09, 2022 1:06 pm, edited 1 time in total.
 
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WesternDC6B
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Re: Metric vs Imperial in aircraft

Wed Mar 09, 2022 1:04 pm

Tristarsteve wrote:
We used whitworth spanners on the Vickers Viscount and Vanguard when I started at BEA at LHR in 1969. I had loads of Whitworth spanners, but never used them since.


Of course! Why have two standards when three are so much better!
 
M564038
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Re: Metric vs Imperial in aircraft

Wed Mar 09, 2022 3:32 pm

It’s a valid point, absolutely.

WesternDC6B wrote:
M564038 wrote:
Well, that sounds about right.
1/2 inch is just funny phrase for 12.7mm anyway.


I prefer metric for mass. In customary I am 269, but in SI I'm a slim, trim 122.
 
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Starlionblue
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Re: Metric vs Imperial in aircraft

Thu Mar 10, 2022 2:35 am

WesternDC6B wrote:
M564038 wrote:
Well, that sounds about right.
1/2 inch is just funny phrase for 12.7mm anyway.


I prefer metric for mass. In customary I am 269, but in SI I'm a slim, trim 122.


Or just over 19 Stone.
 
celestar345
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Re: Metric vs Imperial in aircraft

Fri Mar 11, 2022 4:41 am

Thrusty69 wrote:
My Snap-On 1/2” socket is clearly engraved with “1/2-12.7” right on it. Swear to god. … …


Never seen any metric equivelent on my snap-on set... are yours custom made?

Thrusty69 wrote:
Jay, there’s no metric on airliners. You may have missed the jokes. No mechanic anywhere working on airliners has metric tools anywhere.


The trim panel above entry doors on a 787 is fastened with 3mm hex drive - it's even mentioned in the AMM.
 
M564038
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Re: Metric vs Imperial in aircraft

Fri Mar 11, 2022 6:45 am

12.7mm isn’t 1/2” , but 1/2” IS 12.7mm.

An inch is metric.

celestar345 wrote:
Thrusty69 wrote:
My Snap-On 1/2” socket is clearly engraved with “1/2-12.7” right on it. Swear to god. … …


Never seen any metric equivelent on my snap-on set... are yours custom made?

Thrusty69 wrote:
Jay, there’s no metric on airliners. You may have missed the jokes. No mechanic anywhere working on airliners has metric tools anywhere.


The trim panel above entry doors on a 787 is fastened with 3mm hex drive - it's even mentioned in the AMM.
 
phugoid1982
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Re: Metric vs Imperial in aircraft

Fri Mar 11, 2022 10:14 pm

Oh my goodness. I just had nightmares about a Professor who studied at Caltech giving us mixed units problems in my Aero Engineering ugrad. Seriously, slugs as a unit of mass? Pound mass vs pound force. SI was just simply more elegant. Unfortunately, having grown up in the US I do think in the so called imperial units. Hard for me to think in meters and km/hr. That can be overcome with time I guess
 
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ehbowen
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Re: Metric vs Imperial in aircraft

Sat Mar 12, 2022 4:15 am

phugoid1982 wrote:
Oh my goodness. I just had nightmares about a Professor who studied at Caltech giving us mixed units problems in my Aero Engineering ugrad. Seriously, slugs as a unit of mass? Pound mass vs pound force. SI was just simply more elegant. Unfortunately, having grown up in the US I do think in the so called imperial units. Hard for me to think in meters and km/hr. That can be overcome with time I guess


No Big Deal. US Navy Nuclear Power School (circa 1984) was all US Customary...PSIG, degrees Fahrenheit and Rankine, BTU/hr, pound-force and pound-mass. Never had a problem with it after it got through to me that a pound-mass does not EQUAL a pound-force, it WEIGHS a pound-force (in a 1G field).

On The Other Hand, we never used 'slugs'....
 
Lpbri
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 4:03 am

phugoid1982 wrote:
Oh my goodness. I just had nightmares about a Professor who studied at Caltech giving us mixed units problems in my Aero Engineering ugrad. Seriously, slugs as a unit of mass? Pound mass vs pound force. SI was just simply more elegant. Unfortunately, having grown up in the US I do think in the so called imperial units. Hard for me to think in meters and km/hr. That can be overcome with time I guess


When I studied Aerospace Engineering, all my AE classes and texts used SI units. When I took Fluid Mechanics, which was offered by the ME department, imperial ( or British Engineering ) units were used. All of us AE students in the class were confused when the professor tried to explain what a pound-mass was.
 
flipdewaf
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 7:07 am

phugoid1982 wrote:
Oh my goodness. I just had nightmares about a Professor who studied at Caltech giving us mixed units problems in my Aero Engineering ugrad. Seriously, slugs as a unit of mass? Pound mass vs pound force. SI was just simply more elegant. Unfortunately, having grown up in the US I do think in the so called imperial units. Hard for me to think in meters and km/hr. That can be overcome with time I guess

We had the same thing during my degree too, we all chuckled at the slug until we were told about the unit of torque that is the ‘slug-inch’ more commonly referred to as the ‘slinch’.

I’m in a different field from aero at the moment but needed some information around a process recipe from the US. The volume information was given in ‘pales’….

I once heard it said that if you invent/commercialise a technology then you get to choose the technical language around it, including the units…

Fred


Sent from my iPhone using Tapatalk
 
gtae07
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 10:49 am

I was an AE major, too. Our professors used both--one problem would be in metric, the other in US units. I like that approach; keeps you fluent in both. Except orbital mechanics; that's the only place in the AE school that was all-metric.

I now work at a well-known aircraft OEM. Everything is in US/imperial units. The drawings/models, the hardware, the stock thicknesses of material, and more. It works just fine. And contrary to popular belief, we still use decimals (many people seem to think that inches can only be expressed in fractions...). The kit airplane I'm building is all imperial, too. I think the only metric fasteners are the nut holding the alternator pulley on the shaft, and the battery terminal screws.

I can work in metric--math is math--but I find it a lot harder to *think* in metric. But that's probably because I've spent most of my life designing, fixing, building, and operating things in imperial units.
 
M564038
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 11:03 am

Imperial is also metric.
It does however not use the metric units.

One thing is how weird it is for the americans to insist on using their own units with all the extra effort that ensues for the majority.
The thing that gets me, is how they don’t even see that imperial at this point is a sub-set of metric. Everything is defined in metric, but doesn’t use the normal metric units.

It’s like, everything else equal, counting 0.9, 2.5, PI, 5.5, 7.7, 9.3, 11.5 instead of 1-10 and insisting it is a completely equal, practical alternative to regular counting of how many eggs you have left in the fridge.

gtae07 wrote:
I was an AE major, too. Our professors used both--one problem would be in metric, the other in US units. I like that approach; keeps you fluent in both. Except orbital mechanics; that's the only place in the AE school that was all-metric.

I now work at a well-known aircraft OEM. Everything is in US/imperial units. The drawings/models, the hardware, the stock thicknesses of material, and more. It works just fine. And contrary to popular belief, we still use decimals (many people seem to think that inches can only be expressed in fractions...). The kit airplane I'm building is all imperial, too. I think the only metric fasteners are the nut holding the alternator pulley on the shaft, and the battery terminal screws.

I can work in metric--math is math--but I find it a lot harder to *think* in metric. But that's probably because I've spent most of my life designing, fixing, building, and operating things in imperial units.
 
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WesternDC6B
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 12:21 pm

flipdewaf wrote:
I once heard it said that if you invent/commercialise a technology then you get to choose the technical language around it, including the units…

Fred


That may be so, but, whatever happened to speed in furlongs per fortnight?
 
gtae07
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 12:49 pm

M564038 wrote:
It’s like, everything else equal, counting 0.9, 2.5, PI, 5.5, 7.7, 9.3, 11.5 instead of 1-10 and insisting it is a completely equal, practical alternative to regular counting of how many eggs you have left in the fridge.

That's a poor analogy, not least of which because eggs are countable items, not dimensions of arbitrary length. If I'm working in inches and such, it's all still practical. Especially on the aero engineering side, we do all the designing in decimal inches and don't insist on trying to divide feet by 10. We also don't dig up absurd units (the tired old "furlongs per fortnight as one example :roll: ), nor do we insist on using fractions everywhere. Or do you think we design to standard metric sizes and use inches out of spite?

Yes, metric is computationally convenient if you wish to convert between different units of the same dimension (length, mass, etc). Beyond that there is no inherent advantage or superiority--a centimeter is not inherently "better" than an inch, a gram is not inherently "better" than a slug, Kelvins are not inherently "better" than Rankine, any more than two is inherently "better" than five. Both are arbitarily-defined units of measure, and one is not inherently superior (morally or otherwise) because one happens to be originally based off some poriton of the earth's circumference, or off the boiling and freezing points of water at standard earth atmosphere, rather than some other equally-arbitrarily-chosen thing. Go anywhere else in the universe and those reference points are meaningless. Even the sacrosanct base 10 numbering system we use day-to-day is just an artifact of the standard number of digits on our hands--an alien culture with 12 digits on their primary manipulating limbs would probably find inches and feet an entirely sensible system and would think we're crazy for trying to use base 10 everywhere.

If you stick to one unit system it really is transparent to the user. You can work just as effectively in one as you can in the other.

As for why the US hasn't switched... two factors, I think:
1. Primacy of educaton - people are typically going to be resistant to changing away from that which they learned and used first. And even when you learn a different system, unless you are constantly using the new one in a variety of situations, you wind up continually doing mental conversions to get that intuitive grasp of "what does that physically look like?".
2. Existing industrial standards - trying to switch to new standards is a massive undertaking. The capital investment in machinery and equipment (sawmills for wood, all manner of fastener production, standard sheetmetal sizes, tubing forms, etc) is enormous and most of that can't simply be reprogrammed at will. Construction in the US is heavily built around "dimensional lumber" ("2-by") and 4ft by X products (drywall, plywood/OSB, etc). Cinderblocks are a standard 8x16 size. Plumbing is in inch sizing. Building codes are standardized around these materials, and the prescriptive building codes are all based on those materials.

In the aerospace industry the fasteners, sheet sizes, material allowables, and more are all based on US/customary/imperial units. The entire supply chain for decades' worth of aircraft production is based on that, to avoid having to requalify all of your materials and go through ridiculous import troubles and supplier difficulties, everyone keeps using this hardware.

As for why these things stuck around in the US in particular... I suspect a lot of it has to do with the post-WWII era. The US industrial machine (and the US mainland itself) did come under direct attack during the war. Unlike almost all the other combatants, the US came out of the war with a massive and undamaged industrical base, a strong economy, and little to no basic rebuilding of social infrastructure required. We got a huge head start in the aerospace field because of that, and by the time other nations were able to catch up the units we were using at the time were entrenched.
 
M564038
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 1:33 pm

It is not an analogy. It’s an illustration.
Since 1963, the standard is metric, imperial is just a name for a few random points with weird traditional names somewhere on scales of the proper metric standard units.

I’ll give you an actual analogy.
The C# lydian dominant scale is not a competitor to the western 12 note chromatic tonal system. It is a small subset of it, requiring unusual notation.

So if it is decided the meter needs to change, that also changes not only the millimetre, centimeter, kilometer and so forth, it also changes the inch, the foot, the yard, the mile etc., but it is not the other way around.

gtae07 wrote:
M564038 wrote:
It’s like, everything else equal, counting 0.9, 2.5, PI, 5.5, 7.7, 9.3, 11.5 instead of 1-10 and insisting it is a completely equal, practical alternative to regular counting of how many eggs you have left in the fridge.

That's a poor analogy, not least of which because eggs are countable items, not dimensions of arbitrary length. If I'm working in inches and such, it's all still practical. Especially on the aero engineering side, we do all the designing in decimal inches and don't insist on trying to divide feet by 10. We also don't dig up absurd units (the tired old "furlongs per fortnight as one example :roll: ), nor do we insist on using fractions everywhere. Or do you think we design to standard metric sizes and use inches out of spite?

Yes, metric is computationally convenient if you wish to convert between different units of the same dimension (length, mass, etc). Beyond that there is no inherent advantage or superiority--a centimeter is not inherently "better" than an inch, a gram is not inherently "better" than a slug, Kelvins are not inherently "better" than Rankine, any more than two is inherently "better" than five. Both are arbitarily-defined units of measure, and one is not inherently superior (morally or otherwise) because one happens to be originally based off some poriton of the earth's circumference, or off the boiling and freezing points of water at standard earth atmosphere, rather than some other equally-arbitrarily-chosen thing. Go anywhere else in the universe and those reference points are meaningless. Even the sacrosanct base 10 numbering system we use day-to-day is just an artifact of the standard number of digits on our hands--an alien culture with 12 digits on their primary manipulating limbs would probably find inches and feet an entirely sensible system and would think we're crazy for trying to use base 10 everywhere.

If you stick to one unit system it really is transparent to the user. You can work just as effectively in one as you can in the other.

As for why the US hasn't switched... two factors, I think:
1. Primacy of educaton - people are typically going to be resistant to changing away from that which they learned and used first. And even when you learn a different system, unless you are constantly using the new one in a variety of situations, you wind up continually doing mental conversions to get that intuitive grasp of "what does that physically look like?".
2. Existing industrial standards - trying to switch to new standards is a massive undertaking. The capital investment in machinery and equipment (sawmills for wood, all manner of fastener production, standard sheetmetal sizes, tubing forms, etc) is enormous and most of that can't simply be reprogrammed at will. Construction in the US is heavily built around "dimensional lumber" ("2-by") and 4ft by X products (drywall, plywood/OSB, etc). Cinderblocks are a standard 8x16 size. Plumbing is in inch sizing. Building codes are standardized around these materials, and the prescriptive building codes are all based on those materials.

In the aerospace industry the fasteners, sheet sizes, material allowables, and more are all based on US/customary/imperial units. The entire supply chain for decades' worth of aircraft production is based on that, to avoid having to requalify all of your materials and go through ridiculous import troubles and supplier difficulties, everyone keeps using this hardware.

As for why these things stuck around in the US in particular... I suspect a lot of it has to do with the post-WWII era. The US industrial machine (and the US mainland itself) did come under direct attack during the war. Unlike almost all the other combatants, the US came out of the war with a massive and undamaged industrical base, a strong economy, and little to no basic rebuilding of social infrastructure required. We got a huge head start in the aerospace field because of that, and by the time other nations were able to catch up the units we were using at the time were entrenched.
 
phugoid1982
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 3:48 pm

gtae07 wrote:
I was an AE major, too. Our professors used both--one problem would be in metric, the other in US units. I like that approach; keeps you fluent in both. Except orbital mechanics; that's the only place in the AE school that was all-metric.

I now work at a well-known aircraft OEM. Everything is in US/imperial units. The drawings/models, the hardware, the stock thicknesses of material, and more. It works just fine. And contrary to popular belief, we still use decimals (many people seem to think that inches can only be expressed in fractions...). The kit airplane I'm building is all imperial, too. I think the only metric fasteners are the nut holding the alternator pulley on the shaft, and the battery terminal screws. I can work in metric--math is math--but I find it a lot harder to *think* in metric. But that's probably because I've spent most of my life designing, fixing, building, and operating things in imperial units.


Oh!, it was worse than than mixed unit separate problems. The professor would mix units in the same problem. It was Flight Mechanics. It would involve some long problem calculating some dynamic response starting from scratch basic Aerodynamics. Weight would be in lbm and he would do cruelly terrible stuff like use density in (slugs/m^3). Not only that, eventually when you needed the solution we couldn't use Laplace transform tables. Convolution integral by hand. It was a horrible class and as you can probably guess most people including me lost points from mess of so much unit conversions and not having enough time. One problem with multiple parts, worth 100 points mistakes and carried over. I ended up with an A but the class was massively curved. I don't remember my final raw score but I couldn't have been more than 50%. I still remember this after so long. I wish I had a copy of those exams to post. Might still give me PTSD though.
 
kalvado
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Re: Metric vs Imperial in aircraft

Sun Mar 13, 2022 4:27 pm

gtae07 wrote:
2. Existing industrial standards - trying to switch to new standards is a massive undertaking. The capital investment in machinery and equipment (sawmills for wood, all manner of fastener production, standard sheetmetal sizes, tubing forms, etc) is enormous and most of that can't simply be reprogrammed at will. Construction in the US is heavily built around "dimensional lumber" ("2-by") and 4ft by X products (drywall, plywood/OSB, etc). Cinderblocks are a standard 8x16 size. Plumbing is in inch sizing. Building codes are standardized around these materials, and the prescriptive building codes are all based on those materials.

It's worth remembering, that the size of 2x4 actually changed mid-20th century. By now 2x4 is 38 x 89 mm, not even 1:2 ratio any more. Call it 40x90 mm and it is pretty much the same.
To make things worse, 2x8 is 38 x 184 mm, notably different than double 2x4.
Don't get me started on actual sizes of plumbing threads, those may still have some resemblance to inch dimensions - but I have hard time finding that in 1/8 and 1/4 fittings.
 
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Aquila3
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Re: Metric vs Imperial in aircraft

Sat Apr 02, 2022 4:35 pm

Well i have this problem.
I work now for an EU company that produces machines with some parts made in US.
So i have to carry always both toolsets when i have to fix something.
Inches and feet are not a problem, but one thing throws me out, and that is gauge of the screws.
A 6-32 x 3/8 would be gauge 6 screw with 32 threads per inch, 3/8 long.
But why gauge 6?
How I find out a gauge 6 from a gauge 4? I mean metric screws i can measure them (if i really need to) with a caliper and be sure this is M3 or a M4 . But even with caliper in inches (they have almost always both scales) I cannot get gauges.
Is there a practical way? Do you look at the socket size or hex head size?
Is a gauge 12 always double diameter of a gauge 6 or is it completely arbitrary?
Why don't you use simply 1/8 and 5/32 screws like we do for M3 and M4 ?
 
kalvado
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Re: Metric vs Imperial in aircraft

Sun Apr 03, 2022 2:05 am

Aquila3 wrote:
Well i have this problem.
I work now for an EU company that produces machines with some parts made in US.
So i have to carry always both toolsets when i have to fix something.
Inches and feet are not a problem, but one thing throws me out, and that is gauge of the screws.
A 6-32 x 3/8 would be gauge 6 screw with 32 threads per inch, 3/8 long.
But why gauge 6?
How I find out a gauge 6 from a gauge 4? I mean metric screws i can measure them (if i really need to) with a caliper and be sure this is M3 or a M4 . But even with caliper in inches (they have almost always both scales) I cannot get gauges.
Is there a practical way? Do you look at the socket size or hex head size?
Is a gauge 12 always double diameter of a gauge 6 or is it completely arbitrary?
Why don't you use simply 1/8 and 5/32 screws like we do for M3 and M4 ?

Very roughly speaking, common gauge sizes of 2,4,6,8,10 and 12 correspond to metric M2,3, 3.5, 4, 5 and 6. Those gauge dimensions scale as a+b*x. Metric threads have a=0 in that equation. You can certainly use calliper and use a table to look it up. Actually I have an app with drill sizes and fit dimensions for both thread types on my phone.
Small inch threads like 1/8 are taken by pipe threads, and you don't want to add them to the mess. Having NPT 1/4 and 1/4-20 as totally different things is good enough to make things messy.
 
gtae07
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Re: Metric vs Imperial in aircraft

Sun Apr 03, 2022 11:05 am

kalvado wrote:
Small inch threads like 1/8 are taken by pipe threads, and you don't want to add them to the mess. Having NPT 1/4 and 1/4-20 as totally different things is good enough to make things messy.


Pipe thread (NPT) has nothing to do with standard UN (Unified National) threads like 1/4-20. UN thread is based off of the nominal OD of the fastener, so the major diameter of the external thread is going to be a tiny bit less.

Pipe threads (NPT) are tapered and made for sealing fluid connections. One should remember that pipe is specified by nominal inner diameter, not outer (like tubing). That's why a 1/8 NPT male fitting has a much larger thread than 1/4".

Also remember that the #6, #10, etc. thread as well as NPT were carried over from outside the aerospace industry. You also don't see a lot of NPT on newer transport-category equipment, though it's still common on light aircraft.
 
kalvado
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Re: Metric vs Imperial in aircraft

Sun Apr 03, 2022 1:34 pm

gtae07 wrote:
kalvado wrote:
Small inch threads like 1/8 are taken by pipe threads, and you don't want to add them to the mess. Having NPT 1/4 and 1/4-20 as totally different things is good enough to make things messy.


Pipe thread (NPT) has nothing to do with standard UN (Unified National) threads like 1/4-20. UN thread is based off of the nominal OD of the fastener, so the major diameter of the external thread is going to be a tiny bit less.

Pipe threads (NPT) are tapered and made for sealing fluid connections. One should remember that pipe is specified by nominal inner diameter, not outer (like tubing). That's why a 1/8 NPT male fitting has a much larger thread than 1/4".

Also remember that the #6, #10, etc. thread as well as NPT were carried over from outside the aerospace industry. You also don't see a lot of NPT on newer transport-category equipment, though it's still common on light aircraft.

That is true (although I didn't quite get 1/8-1/4 NPT part. Certainly once you know the deal (aka made enough mistakes) things are pretty straightforward. I am talking about the learning curve and the cost - not so much monetary, but time and personal embarrassment - to reach that point and absorb consequences.
 
ZaphodHarkonnen
Posts: 1238
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Re: Metric vs Imperial in aircraft

Tue Apr 05, 2022 10:20 am

M564038 touches on it but doesn’t make part of their point clear. For decades the US Customary Measures as defined by NIST are defined in reference to Metric/SI. So 1 inch is so many millimetres. A pound is so many grams. Basically making the conversions multiplication by various constants.

This is tangential to the rest of the discussion. But still an interesting point. Also UK (Imperial) measures are defined differently for some measures. So the US and UK measure could appear to be the same but be slightly different in reality.
 
kalvado
Posts: 3728
Joined: Wed Mar 01, 2006 4:29 am

Re: Metric vs Imperial in aircraft

Tue Apr 05, 2022 3:14 pm

ZaphodHarkonnen wrote:
M564038 touches on it but doesn’t make part of their point clear. For decades the US Customary Measures as defined by NIST are defined in reference to Metric/SI. So 1 inch is so many millimetres. A pound is so many grams. Basically making the conversions multiplication by various constants.

This is tangential to the rest of the discussion. But still an interesting point. Also UK (Imperial) measures are defined differently for some measures. So the US and UK measure could appear to be the same but be slightly different in reality.

In theory, that is true, It is only a matter of coefficients - which are ugly anyway. 1 meter is, by the current definition, a distance that light travels over 1/299 792 458 second in vacuum. That makes a foot - distance light travels over ~ 1/983 571 056.43s ~1.0167 nanosecond in vacuum. In fact, I do use "1 foot = 1 light nanosecond" pretty often.
However, those conversions have limit practical consequencies in the field. For aircraft maintenance - and to a lesser extent assembly - standard is the name of the game. You cannot shrink 1/4"=6.35 mm wrench by 5% to fit a 6 mm nut. Great to know the conversion, but doesn't hemp with the nut.
This issue is not limited to different standards. I ran into a fine thread, namely M6x0.5mm the other day. It is uncommon, but perfectly standard flavor of metric threads.
You may be surprised, but McMaster Carr - a major US hardware supplier - has no such hardware in the catalog, nor I could find anything anywhere else. It could actually be alien-made left handed thread measured in alien units, I would be unable to get any hardware for that as well. Good thing that upon some search, adapters were included into the set by the manufacturer - and they were not lost over time.
 
StTim
Posts: 3990
Joined: Thu Aug 08, 2013 7:39 am

Re: Metric vs Imperial in aircraft

Fri Apr 08, 2022 3:18 pm

The customary units vs imperial units that is commonly misunderstood is the use volume one.

A US Gallon = 3.78541 Litres
An Imperial Gallon = 4.54609 Litres

Also I heard recently that there are three separate miles in the US.

1 Nautical miles = 1,852 Metres
1 Mile = 1,609.34 Metres
1 U.S. survey Mile = 1,609.347 Metres. This is what is commonly called the statute mile. Not much difference but it is there.
 
M564038
Posts: 1122
Joined: Sun Jan 03, 2016 11:16 am

Re: Metric vs Imperial in aircraft

Fri Apr 08, 2022 7:01 pm

Just replace it with a scandinavian mile. Nice and round number that fits in with the metric system.

StTim wrote:
The customary units vs imperial units that is commonly misunderstood is the use volume one.

A US Gallon = 3.78541 Litres
An Imperial Gallon = 4.54609 Litres

Also I heard recently that there are three separate miles in the US.

1 Nautical miles = 1,852 Metres
1 Mile = 1,609.34 Metres
1 U.S. survey Mile = 1,609.347 Metres. This is what is commonly called the statute mile. Not much difference but it is there.

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