Thanks for sharing, guys. Ninja history is best history 
I love all the little design touches that used to be commonplace in tools. Joinery and precision tuned springs for a ruler? Custom ball bearings for a spinning converter disk? Nice.
It’s a bit sad to see so much craftsmanship replaced by digital, “good enough” slop.
I’m younger than you two, but hey, I still mourn the rotary phone!
I’ve always found it fascinating that the Apollo spacecraft, with its thousands of interacting components, systems and structures, got humans to the moon and landed with ~20 seconds of fuel left - all designed by engineers using slide rules.
During missions, ground control and even the astronauts themselves used slide rules for various purposes, notably as backup to the guidance computers and during emergencies.
Back in my private pilot days, an important item in the flight bag was the E6B flight computer. Used for calculating fuel burn rates, wind correction, ground speed, etc. it was really just a circular slide rule that was guaranteed to work even if you lost all power, batteries and so on for any instrumentation or handheld electronic calculators. No idea if it’s still used or taught in ground school today. (Crazy kids with their newfangled gadgets invading my lawn’s airspace.)
They’re still sold at least. See this page and ignore anything electronic: Sporty’s Pilot Shop - Search Results.
I carried one of these Vintage Jeppesen Model CR-5 Compact Flight Computer Round | eBay from the start of my civil flying career as a high schooler until I got trained on my first computer-equipped airliner in 1999. That’s ~25 years. Nice to see an everyday tool of half my life called “Vintage”. Not. Ref the other current thread, now I feel old.
I was one of the last in my school to get an electronic calculator. That slide rule had some sentimental value. On a student’s budget, I was careful with my money. Last year’s silicon is free so I gave the price a chance to come down and finally saved another $20 with TI instead of HP. That would have been in about '74 or '75.
They were a standard piece of equipment for engineering going way back. 19th century at least.
The scales you used the most were always in the same place, no matter how many additional scales it had. One of the ones for basic multiplication was on the bottom of the sliding part. The other was just below it at the top of the lower fixed part. They were always labeled ‘C’ and ‘D’.
I convinced my parents to buy me an HP 25 in 1976. That was a lot of money. But I loved that thing. I have a facsimile version on my phone. My fingers still know where everything is, so it is faster to use than other calculators. (Now also own an HP 35 and 45 as museum pieces. Sadly neither work.)
The demise of slide rules caused the loss of teaching the allied skills. Slide rules don’t manage the magnitude of the answer. So mantissa, but no exponent. One was taught how to keep track and get the right answer. This translates into any time an order of magnitude estimate is needed. Same deal using books of log tables. Which was the step up from a slide rule. Just more accuracy.
Calculators also don’t provide any physical intuition. I still remember a physics test in high school where the difference between two values was very small, and a few fellow HP owners got the wrong answer because they did the calculation in the default 2 decimal digit fixed point display. They got the same results for the two values and proceeded to get the wrong final answer. Point being that that result was unphysical. So there was a clue that something was wrong.
No tool avoids needing an understanding of the problem.
I do feel that despite the unsophisticated nature of slide rules and log tables, as compared to electronic devices, we have lost something as well.
Really interesting. Could you use one of those in MS Flight Simulator with a vintage plane to navigate around the world?
Also… why is this a separate standalone thing instead of being part of the cockpit as a built-in redundancy?
[At the risk of getting this whole sub-thread moved to “What was once ubiquitous…”] I assume that slide rules and log tables are no longer introduced in high-school classes, but neither are scientific calculators. So what is being used on the paedagogical side? What are you allowed to bring to a high-school physics test?
Oh man. Sorry about the ninja, @Whack-a-Mole ! I had mistakenly thought this was my own thread because of post #18 and its follow-ups… I apologize. Maybe the sub-thread does need to be broken out.
I am not sure I understand your question about being “part of the cockpit”. That flight computer is a mechanical thingy you hold in your hands. You mean having some marks and rules like the old Apollo capsules had on some of the windows and hatches?
I know how to reply in a new thread but to move an entire sub-tree I believe a moderator has to do it.
I live about 1 mile from the high school. On football night I can loudly hear the cheering of maybe 1,000 people. But if the loudest one in the group were to scream all by himself, I could not hear him. It always puzzled me why the sound carried further with 1,000 people screaming at the same volume.
I don’t have a clear mental picture of this, but yeah, I just mean a discs-and-dials setup that’s already built into the cockpit as a permanent fixture, kinda like how some manual navigational tools were built-in to ships before electronic displays.
(And sorry, I can’t be clearer about this because I don’t know much about plane or ship or lander control surfaces!)
Yep, probably. That might be a good idea at this point, but I’ll leave it up to them to decide (and someone to report).
That’s most of why I still teach my high school math students how to use them. I have my own collection of slide rules, plus a class set I made that just have the simple scales for multiplication and division. Granted, I only spend one day on it for each class.
Even more true now than ever, with the rise of AI.
Pretty much every high schooler any more has a TI-84 graphing calculator, and all tests allow use of it. But that’s (finally) going obsolete now, too, because of Desmos, a free online graphing calculator which is much better, which is also allowed on almost all tests.
Great question. I’d like to understand this better too.
(What is actually happening with the vibrations and waveforms? Do 1000 people cause some sort of constructive interference that travels further? What’s louder, 1000 people screaming underwater or a single whale?)
(And how we do calculate it with a slide rule?)
How do you know?
I wonder if you could hear the pre-game national anthem any better/worse than the crowd noises. (I’ve never lived near a stadium)
I think the 1000-people-shouting thing is a matter of signal to noise. There are always a lot of sounds in the environment, so a sound that’s much quieter than the ambient background won’t be noticed. But a sound louder than the background will be.
Each person outputs the same power. With 1000 people, you have 1000X the power. The logarithm of 1000 is 3.0. Multiply it by ten to get 30 dB.
It’s 30 dB louder.
Keeping the non-decibel hijack alive …
It’s not a complete navigation tool. At least not in the sense of finding direction or location.
The front side is predominantly a simple multiply/divide slide rule (C & D scales). But a major use of multiplication & division while flying is solving problems involving time. e.g. At 400 knots, now long to go 37 miles? At 3000 gallons per hour fuel consumption, and 7200 gallons in the tank, when do we run out of fuel? A common factor of these calcs is one dimension is time. Time might be an input or an output, but it’s (almost always) in there somewhere.
And since time isn’t reckoned in decimal subdivisions, the slide rule scales have special markings to implicitly convert e.g. 1.5 hours into 1 hour 30 minutes into 90 minutes. Another special feature is that often we’re converting between statute or nautical miles or kilometers, or perform a computation in one unit set but need the answer in the other. A couple special marks deal with that too. Ditto feet & meters, gallons and litres, pounds and kilograms.
Unrelated to all that …
Before the advent of complicated but still mechanical air data systems on planes, so pre 1965-ish, and even in the early more-electronic airplanes up through 1980-ish, there’s a need to apply corrections to the values shown on the airspeed indicator or altimeter to correct for temperature, altitude, etc., to derive how fast you’re actually going and how high. All that’s just another number on another screen today. But back then you had to run that multi-step math, and specialized scales down on the inner part of the “whiz wheel” did those specialized calcs.
The back side was completely different in look and function. It was all about doing vector math where we have 2 velocity factors and need to derive the third. The airplane is aimed north at 200 knots, the wind is from the west at 20 knots. What path are we really taking across the ground? If I want to correct the path to actually travel north, how many degrees westward do I need to aim the airplane to offset the wind? If the wind includes a headwind or tailwind, what does that do to our speed?
More specifically, back in the day you couldn’t directly know what the wind was right here right now. But you could know which direction you’re pointed at what airspeed. And by careful measurement of your progress versus the ground or navigation beacons, derive what speed and path you were actually covering versus the ground. The vector difference between those two must be the actual wind. And once you know that, you can make informed adjustments to how the next 1-2 hours will play out versus your plans. Lather rinse repeat every half-hour-ish until arrival.
Totally necessary work in the pre-GPS, pre-fully computerized nav systems era. Lest you get lost or run out of fuel.
It’s kinda fiddly work. Especially as you got older, you needed your bifocals and up close to your face to read it accurately. A hand-held tool worked better than a similar disc bolted to the cockpit wall someplace. By the time it would have been made large enough to be easily read, it’d be too huge to fit anywhere.
Also, back in the day all these calcs were done in advance on the ground. Starting with a map, figure out what route, what the courses are between all the points, then do time/distance math to figure out time & fuel consumption step by step. Oh yeah, then take a second pass through the plan to adjust all those leg by leg calcs for the forecast wind. You ended up with a giant paper spreadsheet with one row per leg from liftoff to touchdown and with 10 - 15 columns of numbers for each leg. On a long airline flight that might be 30 or 40 rows and take a couple hours’ diligent effort to work out. Lotta stubby pencil work. Having your trusty tool in your tool bag while you’re in the office doing all this before going to the plane was helpful.
Nowadays software at HQ (or for hobby pilots, an app on their tablet) whips all that crap out in milliseconds. And the computers in the airplanes update all those numbers continuously as the flight progresses.
Lot’s changed in 40-60 years.
I also live near a racetrack. About 3 miles away. I can only hear them on motorcycle night, and they are loud.
A mass of people all yelling at in a stadium won’t create correlated sound. Each person just adds power to the total sound. So a thousand people is 30dB louder than one person.
In a carefully crafted situation audio sources are correlated. Arrays of loudspeakers for instance. Then there are directions where you get constructive interference. But not in all directions. In other directions you get destructive interference and sound nulls. Done right and you get useful control of sound over an area. But sound is difficult. The range of wavelengths involved is huge. If a human hears across a nominal 20Hz to 20kHz, wavelengths range from 17 metres to 1.7 cm. How one wrangles acoustic energy varies dramatically through this range.