FLIGHT. JUNE 9, 1938.
celestial sphere is similar to the triangle PXZ on the surface
of the globe.
pxq = Celestial Meridian of Star = QO°.
•pzq = Celestial Meridian of Observer = oo°.
= Declination of Star, as given in the Air Almanac.
= Latitude of Observer.
= 90
0
— qx.
= 90
0
— Declination
= 90° — q z.
= 90
0
— Latitude.
= Angle between the Meridian of the Observer and
the Star.
= Local Hour Angle of Star (L.H.A.).
= Zenith Distance.
As the diagram perhaps looks rather complicated, let us go
to the surface of the globe, and stay there (Fig. 7). Putt ing
in the Greenwich Meridian PG, what do we find ?
qx
q x
Then xp
And zp
A nd xfrz
A n d
ig 7
a) We
k n o w
the
a n g l e
GPQ ;
t h i s
is the
G r e e n w i c h H o u r
A n g l e t a b u l a t e d in the Air Almanac, for X.
b ) We k n o w th e a n g l e GPQ' ; t h i s is the l o n g i t u d e (W, in
t h i s c a s e f r om G r e e n w i c h of the O b s e rv e r at his e s t i m a t e d
p o s i t i o n
Z.
c) Hence, by subtracting one from the other (they are
each in degrees and minutes) we know, Q/PQ or the
Local Hour Angle of X, or ZPX.
We have now a spherical triangle PXZ of which we know two
sides, PX (90-Dec.) and PZ (90-Lat.) and the included angle ZPX.
We must find the third side ZX (the Zenith Distance) and
also the angle PZX, from which we shall obtain the bearing of
Z to X.
It may sound complicated to solve a spherical triangle in
order to find ZX and PZX. People start to be frightened at the
mere sound of the words spherical trigonometry. However,
even if ZX is found the long way of logarithms, it is only a
question of knowing how to use a short formula, and of being
able to look up a few statements in books of tables, put them
down on paper, and add up three short lines of figures—at the
most. With a little practice, the procedure becomes automatic.
A deep knowledge of spherical trigonometry is not required to
be able to look up tables and add together a few figures.
Actually, there are in existence over a dozen short methods,
all evolved with the object of cutting out even this small amount
of work.
To find ZX, then, becomes merely a matter of inspec tion
of the special tables developed for these short methods
with the known arguments of Declination, Latitude, and
Local Hour Angle. It can be done in a couple of lines, and in
less than a couple of minutes.
The bearing PZX (or Azimuth) is similarly found by the use
of Azimuth Tables, in one opening of the book.
To summarise : v - '•
(1) To find the Calculated Distance, Usually known in text-
books as the Calculated Zenith Distance, or Calc. Z.D., you must
make use of the following arguments :
(a) The Local Hour Angle of the body (found by applying
the longitude of your D.R. position to the Greenwich
Hour Angle given in the Air Almanac for that time).
(b) The Declination of the body (given in the Air Almanac).
(c) The D.R. lati tude (which you give yourself).
How you make use f these three arguments is a matter
of choice. The Calc. Z.D. can be found either by long
(i.e., logarithmetic) methods or by short methods (i.e., looking
up special tables designed to cut out logarithms).
In any case, finding the answer should not take more than
two or three minutes.
(2) To find the Azimuth, or bearing : Look up Azimuth
Tables, with the same three arguments. Time required—
less than one minute.
T o
be concluded next week. .->•-•
H EN S HEL P R O D U T IO N
German Use of Pressings and Castings
•• THE HenscheJ HS126 parasol general purpose military mono-
-*- plane is being produced in very large quant ities in a
very short time, due to the pressing of most of the sheet metal
parts and the extensive use of castings. There are about
3,000 sheet metal parts, of which 1,000 are pressed in dies.
The fuselage comprises transverse frames of sheet metal
and heat-treated Elektron castings. The frames are formed on
a machine designed by the manufacturers ; this is quite auto-
matic and is fitted with accurate steel templates. It is not
influenced by the operator's skill and its capacity is 30 frames
an hour. The covering is pressed sheet metal. The main
frames for the attachments of the wings, the controls and the
engine are pressed solid members of sheet metal, with cas1
heat-treated bridges secured by screws with elastic stop-nuts
The two halves of the undercarriage are cast integral with
each other and a cast-steel journal is screwed to the ends of
each half. There are more than 400 light metal castings in
the machine. All wing ribs are die-pressed.
(a) Forming the transverse frames on an automatic machine.
(b) Pressing wing ribs in a i,0DO-ton hydraulic press.
(c) Showing the use of pressings and forgings. The door is
spot-welded.