Mk 4 Dead Reckoning Flight Computer

Tonight we are looking at an analogue computer used by navigators in the RAF to make calculations on speed, fuel consumption and the effects of wind upon a flight:imageThe reverse of the computer allows different calculations to be made on the same instrument:imageThis computer was first developed by a US airman called Phillip Dalton in 1937 and came into widespread use in the USAAF in 1940. Over 400,000 of the computers were made for the US during World War II and the RAF soon started developing its own versions of the little analogue computer. Tonight we have an example form the 1950s or 1960s marked up as a ‘Computer Dead Reckoning Mk 4’ and with a /|\ ownership mark:imageThe computer is made up of two parts, an aluminium set of revolving dials:imageAnd a plastic insert panel with a graduated scale on it:imageThe computer can be used on the front side for altitude calculations:imageAnd for Air Speed calculations:imageNote the words ‘Mach Speed’ etched into the aluminium indicating that this computer was indeed used and probably by the pilot of a supersonic fighter like the English Electric Lightning. The rear of the panel is for the calculation of drift caused by wind. I must confess I have absolutely no idea as to how the slide works, but this description I found online explains how it is used:

The front side of the flight computer is a logarithmic slide rule that performs multiplication and division. Throughout the wheel, unit names are marked (such as gallons, miles, kilometers, pounds, minutes, seconds, etc.) at locations that correspond to the constants that are used when going from one unit to another in various calculations. Once the wheel is positioned to represent a certain fixed ratio (for example, pounds of fuel per hour), the rest of the wheel can be consulted to utilize that same ratio in a problem (for example, how many pounds of fuel for a 2.5-hour cruise?) This is one area where the E6B and CRP-1 are different. Since the CRP-1s are made for the UK market, they can be used to perform the added conversions of Imperial to Metric units.

The wheel on the back of the calculator is used for calculating the effects of wind on cruise flight. A typical calculation done by this wheel answers the question: “If I want to fly on course A at a speed of B, but I encounter wind coming from direction C at a speed of D, then how many degrees must I adjust my heading, and what will my ground speed be?” This part of the calculator consists of a rotatable semi-transparent wheel with a hole in the middle, and a slide on which a grid is printed, that moves up and down underneath the wheel. The grid is visible through the transparent part of the wheel.

To solve this problem with a flight computer, first the wheel is turned so the wind direction (C) is at the top of the wheel. Then a pencil mark is made just above the hole, at a distance representing the wind speed (D) away from the hole. After the mark is made, the wheel is turned so that the course (A) is now selected at the top of the wheel. The ruler then is slid so that the pencil mark is aligned with the true airspeed (B) seen through the transparent part of the wheel. The wind correction angle is determined by matching how far right or left the pencil mark is from the hole, to the wind correction angle portion of the slide’s grid. The true ground speed is determined by matching the center hole to the speed portion of the grid.

These computers are no longer used by the RAF but many other air forces and civilian airlines still train their navigators and pilots in their use and it remains perhaps the last slide rule based computer still in use in the air.

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