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# Why C not F?

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# Why C not F?

8th Nov 2013, 07:54

Join Date: Jul 2013
Location: RU
Age: 36
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Why C not F?

Hey guys!

Just have being watching old documentary about how to operate F6F Hellcat, and noticed that all gauges concerning temperatures measures in Celsius no F.
So the question is, why would pioneers of aviations US would switch their F measures onto C?

Thanks.
8th Nov 2013, 08:51

Join Date: May 2001
Posts: 10,823
Because maybe that period there was a lot of European designers on the team. The aircraft was also in service with the French and The British.

Its not really a headache what ever the gauges are calibrated in. You just keep it in the green band and do something when its in the red.

The only time F is better than C for a pilot is with dew points. C is to big a unit apart from that you could have all the gauges in bunches of bananas per donut squared or even a none dimensional % of the max and it would do for operating. But the engineers like numbers which can relate to thermodynamic output so C or F it is.

To be honest the yank engineering has always been a bit of a mixture of imperial and decimal more decimal these days. Some industry's are more likely to use it than others depending what their export market is. The only one in my experience which is 100% imperial in the main is the oil industry.

But all US international company's are struggling remaining on imperial. Nobody else has machine shops etc setup or calibrate to manufacture in imperial. Most engineers from the 1970's onwards in the rest of the world don't have a clue about imperial measurements or working with them. The 787 had issues with it which caused delays with it coming in service due to this. So if they intend to make use of foreign production facility's they virtually have to work in metric.

One of the lads off my degree course went on to work in NASA. And when being asked about getting his brain screwed with converting everything said that they worked in metric all the time. And the only time stuff got converted was for outside NASA reports for US consumption. I presume he was correct as they had quite a famous screw up and managed to crash a satellite into a planet due to one team working in imperial and another in metric.

Now I am a metric trained and educated person so will admit I am a bit bias. I can't see the rest of the world changing back to imperial. Personally I agree the C unit is to large for aviation and F works better for one situation. But the rest of the time it doesn't matter. I have flown both F and C calibrated aircraft.
8th Nov 2013, 09:30

Join Date: Apr 2010
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'Green bands' and the like seem to have been a luxury in WW2 aircraft instrumentation - amongst the major combatants only the USAAF used them with any real regularity. The Luftwaffe, RAF, VVS, etc evidently expected pilots to memorise the numbers. Of course, if you were a VVS (Soviet) pilot, you might have to fly a lend-lease aircraft with Imperial gauges - in a whole new alphabet.
8th Nov 2013, 09:48

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More than likely they were standard instruments over multiple types so they could be mass produced.
8th Nov 2013, 09:57

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Large unit

P-40 gauges are in Celsius too
8th Nov 2013, 10:58

Join Date: May 2001
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Lumps I don't think the METAR and TAF system can deal with decimals.

And the P40 was an allied aircraft as well.

And strangely enough the name rang a "bell"

The company's roots come from a Scottish Engineer called Alexander Bell of telephone fame.

And these days they have ditched aircraft and are in the business of actuators and nuclear components and car parts.

Car parts have been metric for 20 odd years now. And I have only ever seen metric product from them.

It was more than likely a legacy dating back to bell.

Alexander Graham Bell address to Congress

It appears that bell was lobbying for metric in 1906 I wouldn't be surprised if the whole company was being pushed metric anyway.

Last edited by mad_jock; 8th Nov 2013 at 11:14.
8th Nov 2013, 16:51

Join Date: Jul 2013
Location: Everett, WA
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Aviation has always been a mixed bag of units.

At least for the last 40 years that I've been involved, temperatures were nearly always C (EGT and TAT/SAT), distances and speeds in nautical miles (or Mach, which is dimensionless), and altitudes in feet. Three different systems

Knots or nautical miles is really weird since, outside of aviation (and a handful of sailor types), hardly anyone knows what it means even if they've never used metric.

However the inertia of the system is such that I doubt it'll ever change, at least not during my lifetime.
8th Nov 2013, 17:04

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Three different systems
Why the ""?

Knots and nautical miles makes sense for speed and distance because 1 nm = 1 minute of arc (1/60th of a degs). Hence, those are reasonable units for navigation.

Feet for altitude makes sense, because meters is not a fine enough scale, and doesn't make out levels in round numbers.

Metric for everything else makes perfect sense too...

I doubt it'll ever change, at least not during my lifetime
Hopefully not..
8th Nov 2013, 17:14

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Celsius is still rubbish though for dew points due to being overly large unit.

But hey we can live with it, just listen for those magic words RVR.
8th Nov 2013, 18:33

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Why C not F?

8th Nov 2013, 18:59

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For a lot of the time its quite common to be given say 5/5 on the METAR and it to be sever cavok or thick fog.

If you are working in F you have 1.8 covering 1 Deg Celsius.

so 5/5 Cavok would be 42/41 and thick fog would be 41/41 but it would still be record as 5/5 in Celsius.
8th Nov 2013, 19:04

Join Date: Jan 2008
Posts: 10,684
Feet for altitude makes sense, because meters is not a fine enough scale, and doesn't make out levels in round numbers.
Don't speak too soon - 250m is around 820ft, just think how many extra RVSM flight levels could be squeezed in.
8th Nov 2013, 19:30

Join Date: Mar 2006
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Originally Posted by levashov

Just have being watching old documentary about how to operate F6F Hellcat, and noticed that all gauges concerning temperatures measures in Celsius no F.
So the question is, why would pioneers of aviations US would switch their F measures onto C?
Небось что под рукой лежало то и ставили, а головные боли у тогдашних летчиков похлеще этой были.

Translation: they likely just installed whatever instruments they had on hand, and as for headaches, the pilots had bigger things to worry about.
8th Nov 2013, 20:46

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Location: The Netherlands
Posts: 246
Why C not F?

8th Nov 2013, 21:31

Join Date: Nov 2007
Location: Texas
Posts: 1,798
Don't speak too soon - 250m is around 820ft, just think how many extra RVSM flight levels could be squeezed in
Even the Chinese admit meters don't work for RVSM.

CAAC RVSM Flight Level Allocation Scheme
9th Nov 2013, 23:33

Join Date: Jan 2010
Location: USA
Posts: 48
But all US international company's are struggling remaining on imperial. Nobody else has machine shops etc setup or calibrate to manufacture in imperial. Most engineers from the 1970's onwards in the rest of the world don't have a clue about imperial measurements or working with them. The 787 had issues with it which caused delays with it coming in service due to this. So if they intend to make use of foreign production facility's they virtually have to work in metric.
That was the case 20 years ago and maybe for engineers trained 20 years ago but it's much less of an issue today. The reason is computers. Engineers don't deal with numbers in their head, they look at the 3D model on the computer and click around. Any serious manufacturing is all CNC which could care less. Need to measure something? Use a CMM and check if the measurements are red or green.

There's definite benefits to this. In the old days, you have to stick to whole numbers and fractions. In aerospace especially, this automation lets you go ahead and throw in curves, thin and shell parts to the minimum and so on. "Nice" numbers in any system are now gone.

Besides, there's a number of other issues beyond measurements that globalization has caused. One is datums and another is documentation. The Mars Orbiter crash you allude to was not caused by units. It was caused by documentation. The error was in the update of an ICD. Even if everybody was metric, you still need to document it. Force can be mN, N, daN, kN, kgf. Metric doesn't solve the problem.
10th Nov 2013, 07:41

Join Date: May 2001
Posts: 10,823
Nah its not really.

With CNC you can't get a true curve you can only get a surface with steps in it, very small ones I might add but if you need a true curve for say a lens you have to use something else.

My field was none linear finite element analysis and pretty quickly you learnt to only work in metric SI whole units. It didn't really matter to much when doing linear models you can just post process but when you then went on to failure mode analysis or critical crack intensity you were wide open for human errors to creep in even keeping within the metric system of units.

If it was a pure none linear analysis such as contact, mooney rivilin materials or large deformation you needed to run it again to get units that would make any sense if you went away from metric SI.

If you did that you were truly international.

So if it was an oil job I would run everything in metric SI then get it all working then change the input deck base units by conversions or remove the SI conversions from the customer data and then run it again to get output for the report.

But oil was the only work which was imperial. The US engineering market must be loosing billions of dollars per year do to trying to stick to imperial. And American export products would be more viable as well in an international market.

Like it or not virtually all CNC machines are designed in metric. All the shafts are in metric, all the actuator screws are in metric. Its a metric binary device.

yes I agree that a stiffness matrix doesn't know or care what the units are. But I did notice the last time I saw a young engineer doing an analysis that he was forcing a solution through with an ill conditioned matrix and the mesh was dirty with no attempt to quad/cube areas of interest. So I agree that datums and standard boundary conditions is still a problem as its always been. But there also seems to be an attitude that the computer can solve anything. The lessons of engineering discovered 100's of years ago by the likes of Alexander Bell have been forgotten.

The marine engineering side of things are the only ones who seem to have a handle on standard datums and BC's

I think Boeing is the last none metric manufacturer out there of large airliners. And its pretty well documented that they are having problems with international suppliers and also part tolerances that are supplied due to this.

So yes metric isn't the solution, but metric SI as was adopted by US congress in 1866 which then they went further in 1875 to define the imperial system using the metric system is. You might be surprised how much in the US is metric already but just labelled in Imperial. All computers are metric, all cars since the 80's, bottle sizes are actually liters but packaged in the nearest Fl/oz.

Even ASME base units are metric SI now and the imperial versions are converted for the home market.

The US was actually metric before the UK by nearly 100 years.

I think the only reason why you don't move is public outcry. But you must be nearly the only country left on imperial. It must be costing your nation a fortune.

Personally I don't like some of the metric measurements for a consumer usage point of view. A pint of beer is a pint of beer and 500ml is not a pint.

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