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Aviation History
1941
1941 - 0315.PDF
FEBRUARY 6TH, 1941. Ill JLOAD FACTORS IN GUSTS Piloting Procedure in Rough Weather : The V-G Recorder : Pull-out Tests THE U.S. Civil Aeronautics Board has issued' areport, principally for the information of pilots, onwhy and how air loads are encountered by aero- planes when they fly into gusts. Written by Francis R. Shanley, of the Aircraft Airworthiness Section, the report is No. 7 of the series, prepared for the Civilian Pilot Train- ing Programme. The paper sets out to present the theory of load factors due to gusts in a simplified manner, and a briei summary of it follows hereunder. '*4ffi an aeroplane is in straight and level flight, it is flying at a load factor of 1. That is to say, it is developing an upward aerodynamic Lift on its surfaces exactly equal to its Weight, which acts downward. Load Factor is, of course, Lift/Weight. But if the aeroplane now enters a sharp-edged current oi air flowing upward (an up-gust), the angle of incidence of the wing is increased and the Lift is thereby increased. Thus the aeroplane starts to accelerate upwards and the Load Factor is increased above 1. For a given forward speed and a given gust speed, the increase of Lift will be directly proportional to the wing area. Therefore, the increase in Load Factor (bearing in mind the above defini- tion) is inversely proportional to the wing loading. So considering two different aeroplanes which are both flying into the same gust, and are both loaded to their normal flying weight, that one which has the lower wing loading will experience the higher Load Factor and the greater acceleration. But one must not fall into the error of thinking that by reducing the load carried by an aero- plane and then flying it into a gust that the stresses to which the wing structure are subjected will thereby be increased. The reverse is the case, and the stresses in the gust are reduced by lightening the load. This is so because the increased Lift on the wings in entering the gust is the same, but the Lift on the wings in the straight flight previous to encountering the gust was lower. Therefore, to reduce stress on an aeroplane it is sound to reduce its load when flying in gusty air. But it is also true that an aeroplane designed with a heavier wing load- ing is more able to resist gust effects than one designed v^jth a light wing loading. W Effect of Air Speed Load factor added by a gust does not increase as the square of the airspeed, as might be expected. This is so because increase of forward speed reduces the effective angle of incidence at which the wing meets the gust. The nelt result is that the added load factor is directly pro- portional to the aeroplane's airspeed. So it becomes neces- ssry to specify a maximum speed in connection with gust load factor requirements. Conversely, if an aeroplane has been designed for a t certain gust velocity at a given forward speed, it is capable - of withstanding higher speed gusts at lower forward speeds AEROPLANE . FOR UP-GUST OF 30 FT/SEC " ""•"• M.P.H SPEED 204O60 80 100 1E0 140 160 180 ZOO Z20 24O 260 280 AIRSPEED M.P.M. Fig. I. An aeroplane designed for a certain gust velocity ata definite forward speed can withstand higher gust velocities at lower forward speeds, and vice versa. Fig. 2. Actual trace on a V-G recorder installed on a D17S Beechcraft for diving tests. Load factors are given vertically, airspeed horizontally. with the same safety, and vice versa. (This effect is seen in Fig. 1.) From this springs the safety precaution—in gusty weather which is so fierce that the structure of the aeroplane seems in danger, reduce speed. Reducing the weight of an aeroplane will not always permit operations at higher gust speeds, since this weight is probably all removed from the fuselage and there may be other parts of the structure, such as the engine mount- ing, whose load has not been reduced, and these may be critical. The elementary conception for gust formulae has been based on the idea that a gust is absolutely sharp-edged, and that the aeroplane is out of it at one instant and com- pletely in it at the next. This is not quite true, and there ih a transition stage or " gradient" between the area of zero vertical velocity and that of the fully developed gust. So the aeroplane feels the effect of the gust not quite instantaneously, though in many cases nearly so. The V-G Recorder Research work has been done on gusts and statistics have been collected from numerous flights to determine the gust velocity which may be regarded as the reason- able maximum which is likely to be encountered 1 nder various conditions of flight. The accelerometer alone is not an adequate instrument for this purpose, as angle of incidence cannot be deduced from its readings. But by coupling the accelerometer to the airspeed indicator so that simultaneous readings are obtained on two ordinates at right angles, all aerodynamic quantities and load factors can be obtained. Such an instrument is known as a V-G recorder, and gusts and all about them can be studied by means of it. By installing such an instrument on an aeroplane, the gusts encountered may be studied and the maximum "effective sharp-eaged gust" velocity determined. Infor- mation on which to base tr^e design specifications of the future is thus stored up. The V-G recorder is also used in test-flying, and Fig. 2 shows an actual trace of such a recorder installed in a D17S Beechcraft for diving tests. The load factor is recorded vertically and the forward speed horizontally by the pointer, this being able to move in both directions. The figure 1 on the vertical scale corresponds to straight and (vertically) unaccelerated flight. The load factor on the aeroplane is 1. The figure 2 indicates that the aeroplane is now being accelerated vertically upward at a value of ig, that is, approximately 32ft. per sec. per sec, and its load factor is 2. (It should be clearly noted that accelera- tion in terms of g is not synonymous with load factor.) The figure o indicates the aeroplane is being accelerated downward at the rate of xg and that the load factor is zero. Under these circumstances the pilot is iust on the point
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