WW2 Speed Measurement

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May 26, 2015
Hi guys,

How did Second World War aircraft measure speed?

I always assumed the Americans and British would use miles per hour and the Germans use kilometres per hour but I recently read something that suggested airspeed was measured in knots?

I'm interested to know - specifically what German bomber crews used (which I assume would be the same for all branches of the Luftwaffe anyway).

Any clarification?

The RAF used m.p.h., while Seafires were calibrated in knots from the start, so it would appear that the whole F.A.A. were the same, and the Air Ministry said the same of Coastal Command. Bomber Command converted to knots from April 1st., 1945, with the rest of the RAF following on April 12th.
The airspeed indicators in US planes were in mph. We have several original P-51 instument panels as wella s panels from many WWII aircraft, and all the ASIs were in mph. Our museum has two flyable AT-6s and there must be another 4 - 6 around the airport at Chino, and all have or had ASIs in mph. Some of the P-51s have been retrofitted with ASIs in knots because ATC assigns you speeds today in knots, but the originals were in mph. I am not sure about the museum P-51s because they are flyable and so we don't do much "restoration" work on them.

To tell the truth, I have not looked at a Corsair, Bearcat, Avenger, or other Navy plane with enough attention to notice what units their ASIs are calibrated in, but I will this weekend since I am now curious.

We have an Avenger, an SBD, a Corsair, a Skyraider, a Ryan FR-1, as well as some Navy jets. I assume the later planes were in knots, but probably won't look at any planes that aren't flying since their cockpits may or may not even be complete enough to HAVE an instrument panel. Static planes sometimes are VERY incomplete inside. We have a Gloster Meteor, for instance, but it is a shell. It is complete from the outside but there are no engines, no hydraulic lines, and nothing inside the cockpit. You'd never KNOW that unless you got a ladder and looked inside.
Pilot manuals for F4U-1, Avengers and SBD-3 all give ranges in Nautical miles and give aircraft restrictions (dive speeds, lowering landing gear, flaps etc) and landing speeds in knots. One would assume that the airspeed indicator was knots (photos are too small to tell) to prevent mental gymnastics in the cockpit. When or even if there was a change over I don't know. US Navy may have used Knots in the 30s if not before.

Manual for the TBD-1 (Douglas Devastator) uses knots.
I'm glad someone looked in a flight manual ;)

Generally naval aircraft were in knots. I remember reading a NAVAIR design standard document that specified this. When flying the mental conversion is quite simple (about .15 or .85) 100 knots is about 115 MPH
A nautical mile is the distance subtended by one 60th of one degree. IE one minute of arc. Thus the nautical mile related directly to the angular measurements made by sextants. In fact if you have an old style moving hand watch set to Greenwich Mean Time or some other reference you can use your watch as a sort of astrolabe to work out your longitude.

It thus makes sense to use nautical miles and knots.

The knot was literally a knot in a rope to which a bucket was attached. When speed needed to be known it was thrown into the water and a sandglass or water clock used to count of the number of knots in a minute Of time. Thus we would have you speed which the navigator integrated to dead reckon (estimate) position using an hour glass or a clock between navigation fixes. Before chronometers the only navigation fixes were sightings of land.

As you can see time, angular measurement, distance and speed (in knots) all line up on earths surface. Aircraft and ships until recently still navigated with celestial or solar measurements and keep track between fixes by dead reckoning speed in knots. A machine that does this integration automatically is an odograph it uses a compass as well as TAS true air speed.

This dead reckoning was not very accurate and ships often ran ahead and therefore ran aground.

Having an accurate time piece would solve this and an Englishmen named Harrison eventually produced an accurate chronometer resistant to ships motion, temperature and pressure changes. They eventually ended up looking like a giant fob watch.

Dava Sobel in her book "Longitude" relates the story of an British fleet of 4 warships that ran aground in Sicily this way. An able seamen had actually kept track of the ships himself by integration and had become increasingly concerned that a collision with land was imminent. He tentatively approached the captain and was hung by the admiral aboard ship that day (there was fog). Navigating yourself was considered as undermining the officers so severely it was a hanging offence. The ships collided with Sicily shortly thereafter and all ships lost. Chronometers allowed longitude to be measured accurately. Captain James Cook navigated to Australia with Chronometers as well as a new purely celestial system to calculate longitude.

The meter was designed to be 10 million from equator to nth pole so 10,000km.

A unit of measure was called the gradian in which a right angle consists of 100 gradian instead of 90 degrees exists and so one kilometre would be 0.01 gradians of the earths surface. The gradian is used by many military to reduce confusion in transmitting firing orders. Of course to work nicely a decimal system of time would be needed, probably a 40 hour day.

The English in fact tried a decimal system of measures well before the French but then they ended up with a king again and their well thought out system died.
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Measuring knots wasnt done with a bucket you would never be able to pull a full bucket of water inboard at even a few knots. It was done with a quadrant board called a Chip the rope was knotted every 47 to 48 feet and how many knots in 30 seconds gives you your speed in knots. Nowadays its 47 feet 3 inches between knots and 28 second timing.

It should never be called knots per hour its Knots on its own or Nautical miles per hour but never Knots per hour.

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Speed though water and speed "over ground/bottom" :)

Anybody who has spent time in a 5 knot boat in 4-5 knot currants knows the difference :)

That was the problem Adolfs planners faced in the English Channel. Crossing a body of water at 2 knots when tidal currents run between 0 and 6 knots to port and starboard would be like bombing south England with hot air balloons.
We went throgh Deception Pass, Washington, U.S.A. in a 6 knot sloop running the old iron spinnaker as hard as it would go. We made about 1 - 2 knots progress and made it out, but I'd hate to try that and lose power!

Looked at our planes and they have all been replaced with instruments in knots, so I think it would be a guess on my art to extrapolate from that. None of main guys were around since they are mostly over at Duxford for the airshow.
It doesn't matter what unit you use as long as you know how fast you are going.

In the U.S.A. we've had car speedometers with both mph and km/hr for more than 35 years. We know how fast we're going in either unit and see no reason to change the units in our own country. If foreign people come here and rent a car, the speedometer is clearly marked in both units. There is NO excuse for speeding and saying you didn't know. The cops don't buy it.

As FlyboyJ showed above, we have airspeed indicators that show both knots and mph, too, and even a third unit if you fly jets ... Mach number. If we come across a need for kph, It isn't too difficult to add that unit, especially in a digital cockpit ... press a button. So everyone can be comfortable.

If you fly here in the U.S.A., altitudes will be assigned in feet and airspeed in knots IF it is assigned at all. I have flown metric-instrumented aircraft and it was never a problem excpet for altitude. So ... this one had an altimeter in feet while everything else was metric. It was a Yak. All we did was to fly the speeds in the book using the book units except for altitude. It was simple. We KNEW we weren't gloing to bust the speed limit in a Yak with an M-14P engine in it.

We accept metric if we buy off the shelf from Europe and specify SAE if we are doing the specification instead of buying off the shelf. It isn't especially difficult to convert a ¼-28 bolt into exact metric, which is 6.35 x .907143 or back. You figure it out in Excel once and it works forever.

I could do all my posts in metric, but the units used during WWII, which is the main subject of this forum, were not metric on the Allied side. They were on the Axis side for the most part, so we don't usually ask for a conversion from anyone else since we do it all the time.
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Sure, but in ye good old days of WW2 a man called a navigator would still use a chronometer and sextant to workout longitude and latitude. If he knew his speed in knots as well as heading he could conveniently estimate his change in position, between fixes, in terms of degrees, minutes and seconds. The knot and nautical mile is superior for this. In many cases fixes were via radio navigation beacons.

You see plexiglass bulbs in the dorsal position of many bombers to take a celestial reading, the end of the glass house in the Lancaster I assume but often evident in flying boats.
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What a confusing mess. Everything would be so much simpler if everyone FINALLY just accepted SI units

To all intents and purposes the Knot is an SI unit. Its never going to go away sea and air navigation is awkward enough when one knot at the equator is one minute of latitude and on a Mercator projection you simply use dividers and the latitiude scale. Using Kilometers and conversion scales just adds unnecessary and potentially dangerous extra steps in chartwork.

I am not against Metric measures but I use whichever measure seems best I found myself measuring a worktop the other day in milimeters and the timber to make the base in feet and inches.

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