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Aviation History
1948
1948 - 1747.PDF
OCTOBER 2IST, 1948 FLIGHT ON THE LEVEL A New Electrically-actuated Gyro Horizon by Sperry " £T HE artificial .horizon, or gyro horizon, lias been a member of the blind-flying instrument family for so many years now that one has grown to accept it, and its limitations, so naturally that it is almost with surprise that one learns of the advent of a new and im- proved version. In presentation, size and general ap- pearance, the new Sperry instrument is essentially similar to its predecessors and, unless previously told, a pilot would not be aware of any change—until lie started aerobatics; '«, the change in performance would then become marked. Previous designs of artificial horizon have all been vacuum-operated, and have thereby suffered certain opera- tional limitations, particularly at high altitudes and during aerobatics. It was to overcome these limitations that the Sperry Gyroscope Co., Ltd., designed the H.L.3 electric artificial horizon. In the standard vacuum-operated hori- zon, the gyro assembly is enclosed in an airtight case, the interior of which is connected to a venturi. Reduction of air pressure in the interior induces an airflow through jets in the case directed on to notches in the rim of the gyro wheel, so that the induced airstream will spin the rotor at about 12,000 r.p.m. Understandably, with air- craft flying at great heights, the reduction in ambient den- sity will affect the pressure differential through the case, and thus affect the driving air supply to the gyro. Fluc- tuations in ambient temperature and the filtration required to protect the delicate bearings from dust carried in by the airflow are troubles which <k> not attend the electric horizon. It is independent of air supply and operates with maximum reliability over a wide range of tempera- tures ; additionally, the impossibility of dirt entering the instrument gives improved efficiency, and considerably longer operational life. JS^T^ As well as improvements in agr^fcipnal conditions, the H.L.3 also offers improvem^ji^'m operational range. The mechanical design of th&*fitSSmal vacuum-operated horizon was such that indica^y«> were limited to 90 deg. in roll from the hori7.tfft1,8l>mnd 6^deg. in pitch for climb and Shown here mounted on a test stand, the gimbai assembly of the instrument may be discerned. The gyro axis is normally vertical, and at the side and back of the instrument can be seen the sttttnr windings of the erecting motors. Although cleaned up in detail, the dial of the new electric gyro horizon con- forms to the established presentation. A divided hori- zon bar is used to permit clearance of the front gim- bal ring bear- ing. dive. For example, a hali-loop and roll off the top in- volves changes of attitude beyond the limits of freedom of the ordinary gyro horizon, and, to overcome this, the gyro assembly must be caged, i.e., locked mechanically in position. Unfortunately, if uncaged when the aircraft is flying on anything but an even keel, the air-driven instrument will indicate a false horizon and, in addition, perhaps take some few minutes before it finally stabilizes to give correct indications. During this time, the pilot cannot rely on the instrument's truthfulness. If, on th<e other hand, the gyro is not caged before executing aero- batics, the instrument will topple, through what is known as gyroscopic lock, which occurs when the gyro spin and mounting axes become coincident. Operational Improvements Improvements have been effected in operational limita- tions with the H.L.3 electric horizon. It provides com- plete freedom in roll through 360 deg., and freedom through 170 deg. in pitch, i.e., up to 85 deg. climb and dive. This does not circumvent the possibility of toppling, but, in place of the mechanical caging normally employed, an electric erection system is used. Ordinarily, the gyro is erected to an accuracy of 2 to 3 minutes of arc at a rate of 3 to 5 deg./min., but when fitted in fighter aircraft, the H.L.3 horizon can be provided with an erec- tion control switch to give rapid erection at the rate of 180 deg./min. By means of this, if the air- . ... .... craft exceeds the gyro pitch limits of 85 deg. -.•••-...' -.. .-..•: in climb or dive, the quick-erection switch ;;:'.;"-"• can be operated to bring in fast erection for a- "",-• .'••••; •'_... brief period, thus almost immediately re- ",: - erecting the gyro to give correct indications. This quality is of very great value in that it quite eliminates the possibility of the instru- ment presenting the pilot with false data. The gyro rotor consists of a three-phase, squirrel-cage induction motor, contained in a "cup" rotor of G.E.C. heavy alloy. This method of construction ensures that the weight of the rotor is concentrated as near to the periphery as possible, and so provides maxi- mum inertia. This last quality is further en^ hanced by the rotational speed of 23,000 r.p.m.—nearly twice the speed of air-driven types—and, as gyroscopic inertia increases as the square of the speed of rotation, the in- herent stability of this new Sperry horizon can be appreciated. The rotor is carried in compound ball races compensated for temperature, so that the fric- tion in the bearings remains constant through- out a wide range of temperatures. Electrical contact is xnade through the bearings. Any tendency for the gyro to drift, which may be caused by bearing friction or other factors, is overcome, by the erection system, which
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