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.