Aeronautic Instruments Section 4: Direction Instruments Page: 30 of 69

                                
REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS.

DESCRIPTIONS OF BRITISH COMPASSES.
CREAGH-OSBORNE AIR COMPASS, TYPE 5j17.
The Creagh-Osborne air compass, type 5/17 (fig. 6) has been one of the most widely used
aircraft compasses. It is best adapted to service on scout planes where the advantages of a
quick period instrument are desired. The compass is of the liquid damped type (alcohol of
0.84 specific gravity damping fluid) and is equipped with a card totally different from that em-
ployed in most compasses.
The card consists of a pan-shaped thin section of white metal (48 millimeters greatest
diameter), which with the magnetic elements and pivot is so light as to require no float. The
horizontal base surface of the card is cut away so as to leave four spoke members extending
outward from the center to support the rim; the lower edge of the latter'is inclined inward
toward the pivot at an angle of 300 from the vertical. The two bar magnets (40 millimeters
length) are suspended below the card (25 millimeters between centers) upon wire suspensions.
The agate pivot is mounted upon a brass stem attached at the center of the card and rests
in a sapphire cup held on a central post. A vertical adjustable wire extends from the inside
FIGr. 6.-Crengh-Osborne air compass, Type 5/17.  Fro. 7.-Creagh-Osborne aero eampass, Type 259.
upper surface of the bowl to within a short distance from the top center of the card and prevents
the latter from leaving the cup bearing. The lubber-line fixture is mounted inside the bowl,
as shown by the illustration.
The bowl is approximately spherical (80 millimeters inside diameter) except at the front
where a short cylindiical projection extends inclined at an angle of 260 above the horizontal.
This extension is capped by the cover-glass inclined back from the lower edge at an angle of
260 from the vertical. A nonleak joint is made between cover-glass and bezel ring by the use
of a rubber gasket. An air trap and also a chamber for holding the compensating magnets in
proper position are mounted at the top of the bowl. The air trap is arranged so as to collect
any air bubbles which may form in the liquid and to allow for liquid expansion. Two filler holes
are provided, one upon the air trap and the other upon the bowl itself.
The bowl is fitted with three lugs which hold it upon the supporting members of the mount-
ing bracket. Felt washers at the points of attachment beliveen the bracket lugs care for vertical
vibration, while flat spiral springs are provided at the points of support to relieve the hori-
zontal vibrations. A small electric bulb mounted with suitable shield upon the verge ring
provides illumination for the card.
The Creagh-Osborfie air compass has a period of from 8 to 10 seconds and weighs 2.8
pounds.

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