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How to get airspeed in knots?


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5 minutes ago, unreasonable said:

You have to throw a log on a knotted string out of the cockpit and count the knots in a specified period of time. ...

It measures the knots over the time frame of an hour. You must at least fly for one hour to get an exact reading like that. Else you have a complicated factoring of numbers. If you are creative and attach an anchor to that knotted string, you get a ground speed reading instead of something like TAS when using a huge drag chute.

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11 hours ago, unreasonable said:

You have to throw a log on a knotted string out of the cockpit and count the knots in a specified period of time. Or just multiply the IAS mph by 1.15 in your head.  

I have this mental image of the navigator winding out a chip log whilst the radio operator holds the hourglass.

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19 hours ago, WWDarkdiz said:

Can't.  I believe planes in that era were statute miles anyway.  At the end of the day, its just a number, you get used to it.  Besides, all of the docs floating around are either metric, or SMPH.

 

US Navy planes had airspeed indicators calibrated in Knots. 

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37 minutes ago, LukeFF said:

 

US Navy planes had airspeed indicators calibrated in Knots. 

Learn sumptin' every day.  Brits were in SM I believe, rest in metric.  Knots are WAY easier anyway, especially at sea, as 1 knot (nautical mile) is 2000 yards at the equator, much easier to figure out range (18nm is 36000 yards.  Try doing that in your head for SM...).  Being Canadian, I speak knots, SM and metric...

 

4 hours ago, Melonfish said:

I have this mental image of the navigator winding out a chip log whilst the radio operator holds the hourglass.

Try that at 360 KNOTS in a Herc doing a 3-star fix with the old bubble sextant... pretty close to that mental image... 🤪

Especially when you really had no idea where you were, just somewhere over Northern Canada...

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13 minutes ago, WWDarkdiz said:

 ...1 knot (nautical mile) is 2000 yards at the equator...

 

To be pedantic, 1 nautical mile is 1852 metres (6076 ft) anywhere. Historically, I nautical mile was defined as one minute (1/60 of a degree) of latitude. A knot is a nautical mile per hour - i.e. it is a measure of speed, not distance. If you are navigating a Herc by sextant though, I don't suppose an error of 76 ft per nautical mile is going to matter much. 😉

 

 

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7 minutes ago, AndyJWest said:

 

To be pedantic, 1 nautical mile is 1852 metres (6076 ft) anywhere. Historically, I nautical mile was defined as one minute (1/60 of a degree) of latitude. A knot is a nautical mile per hour - i.e. it is a measure of speed, not distance. If you are navigating a Herc by sextant though, I don't suppose an error of 76 ft per nautical mile is going to matter much. 😉

 

 

Nope, it doesn't.  We used to use a polar stereographic plotting chart, so 300knots was 30 nm in 6 minutes.  I only ever got one accurate 3-star fix, and that was on my final checkride.  I was happy to be within 5nm of our actual position (VOR/DME fix taken by the instructor, was pre-gps days).

And in terms of 1/60th of a degree of latitude, since the earth is an oblate spheriod (the FIRST term you learn in Nav school, earth is fatter at the equator) a degree of latitude at the equator is different at either of the poles.  If it was a perfect sphere, it would be equal anywhere, but its not.  That's why in cartography, you have to use different projections (transverse mercator etc), because you cannot project a round object's surface onto a flat surface, there is distortion SOMEWHERE, depends on what projection type you use.  But since the actual distance between lines of latitude are NOT equal, 1/60 of a line of latitude is also not equal, it depends on WHICH line of latitude you are at.  2000 yards is a good compromise, and easy to use.  The error introduced is generally negligible, and will cancel out over time.

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Yeah, the nautical mile was eventually defined as 1852 m because nobody could agree as to where you measured the 'degree of latitude'. The French, having attempted to measure the circumference of the Earth (to define the metre), and got close but not close enough (not least because the Earth isn't a sphere, as you say), initially decided the nautical mile would be 1851.85 m. The British Admiralty, not being prepared to take the word of the French for anything, then decided it would be 6080 ft. And just to be awkward, the U.S. decided it was 6080.2 ft. Canadians presumably officially went along with what the Admiralty said, until everyone agreed that we'd let the French be right for once, as long as they came up with a round number. 

 

And yes, cartography gets tricky when you try to map the misshapen-oblate-spheroid-of-sorts onto a flat surface. Or even GPS 3D coordinates. The GPS zero-longitude line doesn't actually concur with the historical Greenwich meridian - they are currently about 334 ft apart at the Greenwich Observatory. GPS is based on the IERS Reference Meridian though, which apparently takes continental drift into account, meaning that as far as it is concerned Greenwich is moving, albeit not very fast.

 

Anyway, what's wrong with navigational errors? Even ones that don't cancel out? If Christopher Columbus had been a better navigator, he'd never have discovered the Americas... 😃

 

 

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In April 1986 we were given a project to complete. Produce a map of Tripoli, Libya using a specific cartographic projection (can't remember which). Being 'enterprising' types, the next day we were going to go off to whoever had a stock of world maps so we could at least get an idea of what we were looking at. Just one problem; the US military bombed Tripoli and suddenly there wasn't a map of Libya to be had - anywhere. So we had to work the problem out for ourselves. We were not amused!

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