AERONAUTICS - NAVIGATION AND INSTRUMENTS The pilot has to find his way in the air as well as control his machine, and for this, like the sailor, he needs special instruments. These are fixed to the dash board of his cockpit.
Here it must suffice to refer to the more important of these, but before doing this, a difficulty peculiar to flight when the hori zon is invisible and the ground cannot be seen, must be noted.
Thepilot, then, has no means of determining the vertical or of knowing for certain whether he is flying straight or in a curve. When the machine is at rest on the ground the bubble of the level fixed in the machine at right angles to its length occupies its central position or a pendulum, a heavy bob suspended from some point in the machine hangs vertically. The same is true if the machine is moving horizontally at uniform speed. Again, if the machine still moving in the same way rolls over to one side the pendulum swings to that side and the pilot can correct the list by the proper use of his controls.
But suppose the machine begins to turn in a horizontal plane (say) to the left. The pendulum bob swings outwards to the right until the direction of the string lies along the resultant of the weight and the centrifugal force now acting on the bob. The pilot, ignorant of the change to curved motion, alters his controls to bring the bob back to its central position, thinking thereby to put the machine on even keel. Instead of this, however, he has nearly given the machine the 'bank" appropriate to the curve in which he has -commenced to fly and proceeds to describe a circle.
Toavoid this difficulty an instrument known as a turn indicator has been devised, and this warns the pilot when his machine begins to deviate from its straight course. By the aid of a compass and a turn indicator he can maintain his direction.
Thecom pass has its own difficulties and for a discussion of these reference must be made to the article on AERIAL NAVIGATION.
To know his height or to fly at constant height the pilot uses an altimeter, a form of aneroid barometer specially designed and corrected for his use (see ALTIMETER).
The speed is given by the speedometer, which acts on the princi ple that the pressure on a surface opposed to wind of velocity V exceeds by
that on a surface tangential to the direction of motion. A rotation counter registers the engine revolutions, while there are gauges to indicate the quantity of petrol in the tanks and the temperature of the water in the radiator system. Sextant: Wireless.—To determine his latitude and longitude when over an unknown country or at sea he must use a sextant and a chronometer. The horizon is often invisible; this and the unsteady movements of the machine make the use of an ordinary sextant difficult, and the R.A.F. bubble sextant has been devised to overcome these.
Wireless telegraphy enables the pilot to fix his position, to fly blind between stations, and to land, using wireless direction finding and marker beacons.
For research work, the measure ment of the forces on the various parts of an aircraft in flight due to the pressure of the air and to its rapid changes of motion, many most valuable recording instruments have been devised, some of which will be found described in the appropriate sections.
For seaplanes and flying boats see SEAPLANE.
As the speed increases from rest when starting, the resistance on the hull increases rapidly until a certain speed known as the hump speed is attained. When this is passed there is a marked drop in the resistance and the machine rises and planes or skims over the surface, the speed increases until the lift on the wings is sufficient to carry the weight, and the aircraft rises from the water and continues its flight through the air. The de termination of this hump speed and the proper adjustment of airscrew and engine to attain a full efficiency at this speed consti tute a serious problem for the designer. AIuch work has been done in the William Froude tank at Teddington on seaplane problems, the conditions of stability, the form of float or boats, the hump speed for various designs, the strength obtained by the different methods of construction and other matters. In a new tank at Farnborough a number of important problems were in 1935 under investigation. Observations in the wind tunnels have been of great value in reducing the air resistance. The high speed attained in the Schneider trophy race of 1929 was due to changes in shape made in consequence of such experiments. (See TRANSPORT BY AIR.) (R. T. GL.)
