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but it's a simple slide rule calculation,
based on your flying time against
fuel consumption.
The Dalton Computer
This is a device with a sliding scale
through it, marked with drift angles
and TAS arcs, with a frosted circular
screen on which you can draw the
business end of the triangle of
velocities:
There is a dot in the centre of the
screen, around which is a compass
rose that can be rotated to bring
your heading or track under the
lubber line. All you need to do is
draw in the wind vector to see how
they all relate to each other.
Note: there will be an instruction
book supplied with your computer,
so the instructions given here will
necessarily be brief.
The first thing to do is move the
sliding scale to make your TAS
appear underneath the dot in the
centre of the frosted screen. Then
156 JAR Private Pilot Studies
rotate the screen so the wind
direction lines up under the lubber
line at the top.
Draw in a line vertically downwards
from the centre dot equal to its
speed in knots. Rotate the screen
again until the track is under the
lubber line. The end of the wind line
will point to a drift arc and a TAS
arc, which you just apply to your
track and airspeed to get the missing
bits, namely the true heading to fly
and the resulting groundspeed which
you use for flight planning. Then
apply the magnetic variation and
compass deviation to get the proper
heading to fly.
CR Series
These were invented by Ray Lahr
and marketed by Jeppesen. They are
circular, with no sliding scale, and
are based on trigonometry (they are
easier to work with one hand, but be
aware that, as the angle of drift
increases, there's a small angular
correction to be applied on top).
Below is a picture of how you would
work out the PLOG above.
The cross between the 10 and 20
under the centre of the instrument is
the wind velocity (180/15). Its
position to the right of the main line
going towards TC (True Course)
means the wind is coming from the
right, and the crosswind component
is 2 kts. Looking across from the 20
on the outside scale (bottom right),
you will see that 1.3° is the
correction to be applied to obtain
the heading (the white arrow above
the letters TAS must be opposite the
aircraft's TAS for this to be correct).
The tailwind component is 15 kts,
which should be added to obtain a
groundspeed of 105 kts.
The very small ones have some
functions left out, which are
unimportant to most aircraft anyway,
to pack everything else in, but don't
get one too small, because your eyes
won't see the print so well at night
(see the Human Factors chapter).
Slide Rule
On the other side of both types,
there is a circular slide rule, with the
60 point on the inner scale
conveniently marked to make speed
and time calculations easier:
Flight Planning 157
It can be positioned against fuel
quantity or distance on the outer
scale to read time on the inner scale.
As with any slide rule, you need the
approximate answer to your problem
first, as a protection against gross
error and to give you an idea where
to put the decimal point (if you were
wondering how it works, you are
adding indices, which is also where
logarithms come from, but that is
outside the scope of this book).
The most common problems
concern time and distance. Just
move the inner scale until the 60
point is opposite the TAS or fuel
consumption, for example. Read the
time on the inner scale against
distance on the outer scale, or fuel if
you are checking how much is being
used. In the above picture, the speed
triangle (60) is opposite 120 (knots or
gallons) on the outer scale, which
means it will take 6 minutes to go 12
nautical miles, or 6.5 to use 13
gallons, and so on. Always reduce
hours (and proportions thereof) to
minutes for simplicity.
To multiply normally (e.g. 2 x 4),
place 10 on the inner scale against
one number on the outer scale, and
read the answer on the outer scale
opposite the other, which will be on
the inner scale.
Square roots can be found easily, too
(useful for finding VHF ranges).
Find the number you want the
square root of on the outer scale,
then rotate the inner one until the
number opposite 10 is the same as
the one against your original
number. For example, 400 will have
2 opposite, as well as against 10
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