Navigation Explained - the basics
Dead Reckoning
Dead Reckoning is the basis of navigation.
It simply means that if you know your starting point, air speed, compass heading and time spent flying at that speed then you’ll know where you are (or should be) by plotting this on a map and then confirming this by looking out of the aircraft to obtain a visual confirmation (a fix).
Adding Wind Speed & Direction
Jim's Dalton Flight Computer - Mk.III
The key additional factors are wind speed and wind direction at the height you’re flying at.
A pilot or navigator would therefore apply the forecast wind speed and direction to their compass heading to amend their Dead Reckoning position to an Estimated Position (So taking into account the wind effect or Drift).
So the wind may be blowing behind you and so this would mean you travel further than calculated by your airspeed, or conversely blowing from the direction you’re travelling in and so retard your progress.
The Wind Triangle
By using the forecast wind speed and direction a pilot can calculate either his estimated position or, more usefully, calculate an initial Course to Steer in order to arrive at a desired location.
This calculation is known as the Wind Triangle and pilots during WWII (and still today) use basic manual “calculators” known as a flight computers (which are nothing much more than a circular slide rule) to quickly calculate any aspect of the Wind Triangle, including the estimated position and the course to steer.
These calculators also help solve other frequent calculations such as fuel consumption and airspeed adjustments based on height.
Air Speed
Airspeed is shown by the Air Speed Indicator (ASI) and this measures the difference in the outside air pressure (from the Static Vent) to the pressure at the front of the aircraft (Dynamic Pressure measured from the Pitot tube), with the pressure measured at the front of the aircraft being higher due to the speed of the aircraft through the air. From these differences, the air speed is determined and displayed to the pilot by the Air Speed Indicator.
A further complication for the aviator with Air Speed is that since the Indicated Air Speed is measured using pressure differences, as the aircraft climbs higher the pressure of the atmosphere reduces.
What this means in practice is that the True Air Speed (TAS) increases as altitude increases. Clearly this is very vital to understand as you’re actually flying faster over the ground than your instrument is showing.
In other words the pilot must adjust his Indicated Air Speed as shown on the Air Speed Indicator to take into account the altitude the aircraft is flying at, so he will have to calculate his True Air Speed (TAS – and this another calculation where the flight computer is useful).
Nautical Miles / Statute Miles
Today, speed in an aircraft is usually measured in Knots (Nautical Miles per Hour), a nautical mile being slightly longer than a statute mile. A nautical mile is used in navigation since it has a direct relationship with latitude, whereby a nautical mile equates to one minute of latitude when using navigational charts using a Mercator projection (additionally any line drawn on these charts has a constant bearing).
However, just to confuse things, during much of the war miles per hour and the statute mile were used in all RAF ‘land based’ aircraft with knots and the nautical mile being used in RAF Coastal Command. Bomber Command adopted knots and the nautical mile from 1st April 1945 and this was then rolled out across the RAF as its standard measurement of speed and distance (Air Ministry Order A.365/45 issued on 12 April 1945 formalising this). From a practical point of view the navigator would be governed by what the aircraft's Air Speed Indicator was calibrated to read, either m.p.h. or knots as this is what the pilot would be using. For the navigator converting between the two would have just been part of the job, given that, for example, Astronavigation is done using nautical miles whilst predicted wind speeds were provided in m.p.h.
As you can see, there are already quite a few factors that need to be calculated by the aviator and there are many more, beyond the scope of this brief explainer. Other key things to cover would be the difference between the Magnetic Course shown on your compass to the direction of True North (Variation) and adjustments needed for compass errors associated with where the compass is sited in the aeroplane (Deviation). Other factors to consider would include how much cloud cover there is and its levels as well as dew points for where icing of the aircraft may occur, since icing is a danger to most aircraft.
Obviously as well as doing all these calculations and plotting of positions, there also needs to be the ability to ignore the fact that the Germans are trying to shoot you down whilst you’re busy navigating!
