Determination of Aircraft Antenna Loads Produced by Natural Icing Conditions

RM No. E7H26a

NACA

RESEARCH MEMORANDUM

DETERMINATION OF AIRCRAFT ANTENNA LOADS PRODUCED BY NATURAL ICING CONDITIONS By William L. Kepple Flight Propulsion Research Laboratory Cleveland, Ohio

NATIONAL ADVISORY COMMITTEE

FOR AERONAUTICS

WASHINGTON February 18, 1948

NACA RN No. E7g26a NATIONAL ADVISORY CONMITTEE FOR AERONAUTICS REARCE MEORAIWUM PRODUCED BY NATURAL ICING CONDITIONS By William L. Kepple §WjT V1_WMyW4 A flight investigation was made to determine the effect of distance flown in the icing region, antenna length, and antenna angle on the tension occurring in aircraft antennae while in regions of aircraft icing.

The, experimental antennas were of lengths ranging from 15 to 00 to 64 0 with the airplane 43 feet and were placed at angles of Distances up to 256 miles were flown in diverse thrust axis.

icing conditions at true airspeeds from 157 to 214 miles per hour and pressure altitudes at which icing conditions were encountered.

The results indicate that: The effect of ice formation on antenna tension increased with the angle of the antennas with the longitudinal axis of the airplane. The maximum tension for was 68 pounds, whereas the antennae having angles from 00 to 150 and 640 was maximum tension for antennas having angles of 274 and 438 pounds, respectively.

INTRODUCTION The current use of improved airplane ice-prevention equip- ment has extended operations in icing conditions and thus accen- tuated the need for protecting aircraft antennas against structural failures resulting from ice accretions. A flight investigation was therefore conducted at the NACA Cleveland laboratory to deter- mine the tension in aircraft antennas produced by ice formations.

The investigation was conducted in natural Icing conditions over a range of true airspeeds from 157 to 214 miles per hour and altitudes from 3500 to 9500 feet. Seven experimental antennas of 00 to lengths ranging from 15 to 43 feet were placed at angles of of the airplane. The effect of the axis 640 with the thrust following factors on the antenna tension was Investigated: antenna length, included angle between the antenna and the thrust axis of the airplane, and distance flown in the icing region.

NP.CA RM No. E7H26a APPARATUS AND PROCEDURE The location of the seven experimental antennas on the airplane is shown in figure 1; the length and the angle of each experimental antenna are also presented and are listed In the following table: Antenna Antenna length angle (ft) (deg) Inasmuch as It was anticipated that the tension which would be imposed on the antennas would exceed the breaking strength of the ordinary antenna wire, the experimental antennae used were of aircraft-control cable. Turnbuckles 7 by 1/16-inch diameter ..

7, and spring-tension units were mounted on the rear of the antennas (fig. 2) In order to impose a static tension of 30 pounds.

The tension In the antennas was measured by strain gages located at the front of the antennas (fig. 3). The strain gages were temperature-compensating and were sealed In order to keep them moisture-free. Electrically heated anti-icing boots were installed, around the body of the strain gages. A recording galvanometer indicated, the tension in the antennas. The strain- gage system was accurate to ±2 percent.

Multirotating cylinders, as shown In figure 4, were used to determine the average droplet diameter, the drop-size distribution, and the liquid-water content. These cylinders were similar to those used in reference 1 with minor mechanical modifications.

Pressure altitude and indicated airspeed were measured by service instruments and the ambient-air temperature was measured by a shielded resistance-bulb thermometer.

A series of four flights was conducted. Into areas of pre- clicted icing. Table I presents the meteorological conditions for the respective flights. The values shown in this table are the averages of Individual values recorded' at intervals throughout

NACA RM No. E7E26a each flight operation. The notation for the drop-size distribution corresponds to that given in reference 1, table I, page 6. The factors that made possible the existence and the suspension of eupercooled. liquid-water droplets encountered during the flight operations are presented in the appendix.

RESULTS AND DISCUSSION Photographs of ice configurations on the 8 0 , 41-foot antenna and the 150 , 32-foot antenna are presented in figures 5 and 6, respectively, and are representative of Ice formations on antennas 00 to 150 . Figure is a photograph, taken in having angles from flight, of the ice formation on the 15 0 , 32-foot antenna and the 640 , 40-foot antenna. The, ice configuration on the 64 0 , 40-foot antenna is 'illustrative of configurations collected on the 640, 40-foot and the 44 0 , 42-foot antennas during, most of the flight operations.

The variation of antenna tension with distance flown in the Icing region for the antennas is presented In figure 8 for the four flight operations. The shaded areas on the figures for the 440, 42-foot antennas represent the maximum and 640 , 40-foot and minimum tensions produced by whipping. The variation of antenna tension due to whipping for the 00 and 150 antennas was insignifi- cant. Figure 8 shows that, in general, for the 44 1 and 64 0 antennas, antenna tension increases considerably with the distance flown In the icing region; whereas, for the antennas of 15 0 and less, the variation of tension with distance is Insignificant. The change in the slope of the antenna-tension curve from positive to negative indicates that whipping of the antennas broke office formations and thereby temporarily reduced the antenna tension.

The variation of antenna tension with antenna angle for antennas of approximately 42 feet in length Is presented in fig- ure 9. The tension values presented in this figure were obtained from figure 8. Figure 9 indicates that the antenna tension increased with increasing antenna angle for antennas of nearly con- stant length. With increasing angle, the antenna tension increased eight times faster in . the 440 to 640 range than in the 00 to 150 range. The slope of the curve for antenna angles greater than 150 indicates that the required antenna strength increases rapidly as the antenna angle Is increased beyond 15 6 . The Investigation presented in reference 2 also recommends a limiting antenna angle of 150 . The variation of antenna tension with antenna length at a constant antenna angle of 15 0 Is presented in figure 10. The

NACA RM No. E7H26a data were obtained from figure 8 and indicate that the tension increases nearly linearly with antenna length. The maximum-ten- sion measured for each of the seven experimental antennas during the four flight operations is given in table II.

SUM41RY OF RESULTS From an an of the tension occurring In aircraft antennas during flights in regions of aircraft icing made for antennae with 00, 80 , 150 , 440 , and angles to the thrust axis of the airplane of 640 and lengths from 15 to 43 feet, the following results were obtained: and 640 antennas increased consid- 1. The tension in the erably with distance flown In the icing region, whereas the tension for antenna angles of 15 0 and-less did not increase appreciably with distance flown in the Icing condition.

2. The rate of change of antenna tension with antenna angle in the 00 to 150 range was one-eighth the rate of change of antenna tension with antenna angle In the 44 0 to 640 range. The slope of the curves of tension as a function of antenna angle begins to change appreciably above 15 0 , which indicates that the required strength of antennas will increase rapidly if the antenna angle is increased beyond. 150.

3. Antenna tension varied approximately linearly with antenna length.

00 to 150 antennas The maximum recorded tension for the was 68 pounds. The maximum tension for the 44 0 and 64 0 antennas was 274 and 438 pounds, respectively.

Flight Propulsion Research Laboratory, National Advisory Committee for Aeronautics, Cleveland, Ohio.

NACA RM No. E7H26a APPENDIX - MBYTEOROLOGIOAL HISTORY OF FLIGBT OPERATIONS Flight operation 1. - This flight was conducted in a region having meteorological conditions that resulted. from: (i) a conver- gence of several air masses; (2) the lifting of relatively warm, moist air over the upper cold frontal surface; and (3) an unstable condition in the lower 4800 feet. These factors made possible the existence and suspensiOn of supercooled liquid-water droplets.

Flight operation 2. - Flight operation 2 was conducted in an area ahead of cold-front weather. The suspension of supercooled liquid-water droplets was enhanced by the following factors: (1) a strong convergent air flow in the deepening pressure system associ- ated with the front; (2) a temperature inversion through which a strong vertical ascent of saturated air occurred; and (3) the evap- oration of melting snow plus an unstable lapse rate to the cloud base.

Flight operation 3. - The flight was made into a region doin- mated by a deck of altostratus clouds caused by the overrunning of maritime tropical air over a quasi-stationary continental polar front.

The flight was conducted near the northern periphery of the precip- itation area, in which it appeared that nearly colloidal unstable conditions existed.

Flight operation 4. - This flight was conducted in a convective-type cloud of limited geographical extent. Strong ver- tical air motion carried the cloud tops several hundred feet above the temperature inversion, which was conducive to the suspension of a large amount of supercooled water droplets in the cloud.

1. Vonnegut, B., Cunningham, H. M., and Katz, B. E.: Instruments for Measuring Atmospheric Factors Related to Ice Formation on Airplanes. M.I.T., De-Icing Res. Lab., April 1946.

2. ilaller, George.L.: Icing of AircraftAntenna Wires. Jour.

Aero. Sci., vol. 6, no. 1, Nov. 1938, pp. 27-28.

NACA RM No. E7E26a TABLE I - FLIGHT AND MMEOROLOGICAL, VARIABLES aDrop Flight Distance Pressure Ambient- Droplet Liquid- opera- in icing altitude air temp- diameter size water tion region (ft) erature (microns) • dietri content (miles) (°F) bution (g/cu in) E 1 153 3500 26 8 0.25 E 2 115 3700 25 13 .30 E .21 3 256 9500 20 16 4 1 121 5700 1 21 15 A . .26 aSee reference 1, table I, page 6.

TABLE II - MAXIMUM ANTENNA TENSION Flight Antenna Antenna Antenna opera- angle length tension tion (deg) (ft) (ib) 64 40 438 •44 42 274 15 43 68 15 32 . 57 15 15 43 8 41 56 1 0 41 .50 National Advisory Committee for Aeronautics

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0 ^ Flight operation 1; pressure altitude, 5500 feet; true (a) airspeed, 155 miles per hour; ambient-air temperature 26 0 F; liquid-water content, 0.25 grains per cubic 640, meter; average droplet diameter, 8 microns; 40-foot antenna not installed for the flight operation.

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40 80 120 Distance flown icing in region, miles (b) Flight operation 2; pressure altitude, 3700 feet; true airspeed, 177 miles per hour; ambient-air temperature, 25 0 F; liquid-water content, 0.30 grams per cubic meter; average droplet diameter, 13 microns.

withdistance flown in Icing tension Figure 8. - Variation of antenna region for experImental antennas.

NACA RM No. E7H26a NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS Antenna Antenna angle length - (deg) (ft)

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o 43 V ' 8 41 V 0 QeLil .-1 .1 Cd [I] V 1kV 120 160 200 Distance flown in icing region, miles (c) Flight operation 3; pressure altitude, 9500 feet; true airspeed, 214 miles per hour; ambient-air, temperature, 20 0 F; liquid-water content, 0.21 graris per cubic meter; average droplet diameter, 18 microns; data for 44 42-foot antenna not obtained owing to strain-gage failure.

Figure 8. - Continued. Variation of antenna tension with distance flown in icing region for experimental antennas.

NACA RM No. E7H26a NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS - TntennaKti angle length (dog) (ft) o 64 44 42 o 15 43 - - V 15 15 - ' 8 V 0 - 41 - 50C ri a ----- I- 4) co 4) - I _ 0 tSu 120 160 Distance flown in icing region, miles (d) Flight operation 4; pressure altitude, 5700 feet; true air- speed, 182 miles per hour; ambient-air temper&ture, 210 F; liquid-water content, 0.26 gram per cubic meter; average droplet diameter, 15 microns.

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Concluded. Variation of antenna tension with distance flown in icing region for experimental antennas.

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06---- 60 80 20 40 Antenna angle with thrust axis of airplane, deg Figure 9. - Variation of antenna tension with antenna angle for anten- nas of 40. 41, 42, and 43 feet in length. Distance flown in Icing region, 115 miles.

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0 20 40 Antenna length, ft - Figure 10. - Variation of antenna tension with antenna length at an- tenna angle of 15 0 . Distance flown in icing region, 115 miles.