ARMORED T-28 RESEARCH AIRCRAFT FACILITY

INSTRUMENTATION DESCRIPTION

 

The basic requirement of the instrumentation system was that it must reliably measure and record the selected quantities with minimum attention during flight, since the aircraft was flown solo. Limited capability for monitoring the data system in flight was available. There was also a system to telemeter some data to the ground for examination. The data system was designed to be remotely controlled from the front cockpit with a minimum requirement for in-flight checks. Johnson and Smith (1980) provide a basic description of much of the T-28 instrumentation system, although the replacement of the data acquisition system in 1989 and other more recent equipment additions rendered some of this description obsolete.

The data acquisition system is a single-board "personal computer". It was normally activated on the ground prior to each research flight, although it could be started and restarted in flight. The pilot could monitor certain aspects of data system operation in flight through a programmable CRT display. During flight, he had displays of time, and liquid water concentration from the Johnson-Williams device, and on occasion monitored special user-supplied equipment.

State Variables

Because of the importance of the static pressure and temperature measurements, these variables were measured redundantly. The static pressure instrument, the Rosemount 1301-A-4-B, has a basic accuracy of 0.1% and response time of a few tens of milliseconds. Two of these units were carried on the aircraft. The resolution of the 16-bit analog-to-digital converter in the data recording system is 0.0000152, so the realized resolution of incremental pressure measurements with the Rosemount instrument was about 0.0015 kPa.

Reliable temperature measurements outside the clouds were obtained from both a Rosemount and an NCAR reverse-flow sensor (Rodi and Spyers-Duran, 1972). In-cloud temperature could not be measured as reliably because of wetting of the sensing elements (Heymsfield et al. , 1979; Lawson and Cooper, 1990). The NCAR reverse-flow thermometer has a basic accuracy similar to that of the Rosemount device (+- 0.5 d C) but a slower response, with the Rosemount time constant being ~1 s and the reverse-flow time constant ~3 s. Usually the NCAR device was relied upon for in-cloud measurements because of its superior wetting resistance, but it was not always possible to provide reliable in-cloud temperature measurements under all conditions, particularly at above-freezing temperatures (Lawson and Cooper, 1990).

The T-28 system did not include a humidity sensor. The emphasis of research involving T-28 observations was usually on the interior characteristics of storms, and a robust, generally suitable instrument for measuring humidity in clouds was not found.

Kinematic Measurements

Horizontal winds can be obtained by subtracting aircraft movement relative to the air, based on measured heading and true airspeed, from the aircraft ground track obtained from an on-board Global Positioning system (GPS). Winds obtained in this way are not comparable in accuracy with winds obtained by INS-equipped aircraft, but can yield a semi-quantitative picture of storm circulations.

Updraft measurements can be derived from aircraft rate-of-climb values obtained by differentiating the static pressure (altitude) values or from direct rate-of-climb measurements made with a Ball variometer. The altitude differentiation approach is normally used to get the rate-of-climb values. The variometer is not used as the primary sensor because it has a limited range (+- 30 m/s) and becomes nonlinear near the ends of this range. It also has an inaccuracy near the zero point which results in a reading of -3 m/s when the actual value is between +- 3 m/s. If the rate-of-climb is outside the +- 3 m/s range and within the +- 30 m/s limit, the instrument functions properly and the data can be used for backup purposes.

The simplest approach to updraft calculation is an expansion of the concept used by Auer and Sand (1966); some of the aircraft-induced vertical motions are removed during post-flight processing of the data by correcting the rate-of-climb values for the effects of airspeed and engine power variations. An additional term based on energy conservation considerations (Dye and Toutenhoofd, 1973; Cooper, 1978) can be applied to correct further for pilot-induced effects. In this manner, the larger-scale updrafts can be measured with an estimated accuracy of better than +- 3 m/s or 10%, whichever is larger. Small-scale motions (i.e., gusts) with horizontal dimensions smaller than 0.5 km or so are below the sensitivity of this relatively simple system.

An improved method of calculating updraft speeds from the T-28 measurements has been developed by Kopp (1985) based on earlier work by Lenschow (1976). It uses the aircraft equations of motion to obtain a better correction for the aircraft-induced motions. The required measurements are static pressure, dynamic pressure, and pitch angle. Estimated accuracy is +- 3 m/s and spatial resolution is ~0.5 km.

Redundancy is an important feature of the T-28 instrumentation system. Only a few storms a year can be studied in detail and the data are sufficiently important to warrant the maintenance of back-up instrumentation. For determinations of vertical air motion, for example, the variometer (rate-of-climb indicator) output is used in the event of a malfunction of the Rosemount pressure sensors. If both pressure instruments and the variometer should fail, the accelerometer data could be integrated to determine rate of climb.

Hydrometeor Measurements

One unique feature of the T-28 instrumentation system was its ability to measure the numbers and sizes of hydrometeors over almost the entire size spectrum present within a storm. The particles may have ranged from cloud droplets a few micrometers in diameter to hailstones several centimeters in diameter. Various sensors covered different portions of the size range in an overlapping fashion. Somewhat comparable measurements were obtained for each subrange from two different sensors, again affording a useful degree of redundancy.

The sensors applicable to each particle size category are listed below. The values in parentheses indicate the approximate sampling volume per unit distance along the flight path for each instrument; the sampling volume per unit time can be estimated by multiplying by the nominal T-28 true airspeed of 0.1 km/s.

1. Cloud droplets, up to about 30 um in diameter:
   
   J-W cloud liquid water concentration sensor; Particle Measuring Systems Forward Scattering Spectrometer Probe (FSSP) (3.E-4 m**3/km).
2. Intermediate or "embryo" size particles, 30 to more than 1000 um:
   
   Particle Measuring Systems (PMS) two-dimensional optical array spectrometer (0.1 m**3/km); particle camera (up to 2.6 m**3/km).
3. Raindrops, graupel, and snowflakes, from about 1 mm up to 5 mm or larger:
   
   Continuous hydrometeor sampler (foil impactor; 1.4 m**3/km); particle camera.
4. Hailstones, from 4 mm to more than 5 cm:
   
   Hail spectrometer (100 m**3/km); foil impactor.

The sampling volumes tend to increase for instruments designed to sample larger particles to compensate for the smaller concentrations of such particles. The particle camera and hail spectrometer could not be carried simultaneously because both required the same mounting points under the left wing of the aircraft.

The above allocation of instruments to particle size categories is arbitrary to some extent. For example, the two-dimensional probe (2D-C) provides partial images of particles considerably larger than 1000 mm, while the particle camera can photograph centimeter-size hailstones. However, the instrument sampling volumes imposed serious limitations on the representativeness of the data. It was also generally recognized that all of the available instruments were deficient in the 50-150 mm size range.

Our data system accepted data from a PMS 2D-P probe (covering the size range from ~200 mm to ~6.4 mm) and the T-28 did, in fact, carry a 2D-P on many projects. However, it could carry only one PMS imaging probe at a time. Normally no 2D-P was available for use on the T-28, but one could sometimes be borrowed on a project-by-project basis.

A variety of computer techniques were developed to process the two-dimensional image data to determine particle sizes and crystal habits. A preliminary capability to automate the processing of foil impactor data has been developed but additional work is needed to make this routine. Information about particle size distributions can be obtained from the PMS probes, the particle camera, the foil impactor, and the hail spectrometer. Particle phases (ice or water) can be determined unequivocally from the particle camera data and frequently can be identified from the foil and PMS two-dimensional images as well. (Attempts to identify phases from the foil impactor data occasionally can be suspect, as shown by Knight et al., 1977.) Particles larger than 5 mm, which are measured mainly by the foil impactor and hail spectrometer, are normally assumed to be ice because raindrops of these sizes break up very quickly in nature due to dynamic instabilities.

The hailstone spectrometer, developed at the South Dakota School of Mines and Technology, operated on a "shadowgraph" principle similar to that employed in the PMS probes. It used 128 phototransistors spaced at 0.9 mm intervals in a linear array to count, size, and image hailstones as they passed through a planar beam of laser light perpendicular to the flight path. Shadows smaller than about 4.5 mm were not counted, and the data were usually analyzed with the assumption that all particles larger than this were hail.

A device has been developed by NCAR scientists to capture hailstone samples inside the thunderstorm. Frozen particles are decelerated and captured in a chilled receptacle for later analysis in the laboratory. This device was available for use with the T-28.

Electric Field Measurements

The T-28 carried five cylindrical rotating-shutter electric field mills, located at: (1) the upper rear canopy facing upward; (2) the lower fuselage baggage-bay door facing downward; (3) and (4) the wing tips facing outward; and (5) the outboard hail spectrometer pylon, facing downward.

Experience and in-flight intercomparisons with other aircraft using the five mill locations have shown that reliable estimates of the electric field components in the vertical and transverse directions could be obtained in clear air and in the presence of light precipitation. It was also possible to derive an estimate of charge on the aircraft using field mill readings or instrumented discharge wicks on the fuselage. More work is required to provide reliable interpretations of observations obtained during penetrations of severe storm interiors when the aircraft becomes highly charged (see, e.g., Jones, 1990).

Navigation and Performance Variables

The aircraft carried a GPS navigation system as well as a VOR/DME system including two DME's. There was no on-board radar. The aircraft navigation equipment was used by the pilot to arrive at the desired initial point for a cloud penetration, but instructions relating to penetration headings were transmitted from colleagues with access to ground radar. The equipment on board was not considered sufficient for precise navigation in regions of mature storms where heavy precipitation zones and strong up- and downdrafts had sharp boundaries which didn't always correspond to visible features. Real-time tracking on the ground coordinated with a state-of-the-art meteorological radar display, with aircraft position data based on the GPS system, FAA surveillance radar, or other precision ground-based radar or radio-direction finding systems, was therefore required for operations in mature storms. Telemetry of the position data from the T-28 to the ground was available to assist in this process (see Sec. 4). The position data were also recorded on the aircraft data system for use in later analyses.

A gyro-stabilized platform and accelerometer system was available to provide aircraft pitch and roll data as well as vertical accelerations. No angle-of-attack or yaw data are available with the present system configuration.

Dynamic pressure (indicated airspeed) and aircraft heading data were recorded routinely, the former redundantly. A real-time true airspeed computer supplied data to timers in instruments requiring this synchronization, such as the PMS imaging probe or the particle camera. Post-hoc true airspeed calculations were made to determine the exact sampling volumes of other instruments. As indicated in Sec. 3.2, the rate-of-climb data serve mainly a backup function.

User-supplied Instrumentation

The facility was able to accommodate user-supplied instrumentation. Space in the rear cockpit was available, as well as various hard points and pylons on each wing. The aircraft normally flew near its maximum allowed gross weight and could only carry an additional load of 70 kg (about 150 lbs); however, further capacity to carry user-supplied instrumentation could be made available if some of the standard instrumentation was removed. About 500 W of 28V DC and 700 VA of 115 VAC (400 Hz) power were available above the requirements of the standard instrumentation and other aircraft systems. The instrument operating environment was unheated, unpressurized, and subject to significant levels of shock and vibration.

The T-28 carried more than a dozen different precipitation, cloud, and aerosol particle samplers over the years, in addition to the suite of instrumentation described above. In some instances the T-28 carried dedicated data acquisition systems associated with this equipment. It also carried an SF6 analyzer during three field campaigns in which tracer techniques were used to study cloud circulations and precipitation development.

REFERENCES

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