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HOP Capabilities

HOP Specifications

The Jet Ranger adopted for the HOP is a light, single engine (turbine) helicopter that was originally designed as a light observation helicopter for the US Army. Its first commercial version was certified in 1966, and while many of its components have been improved over the years, its conceptual design dates back to the early sixties. It is simple, robust, and based on the US National Transportation Safety Board (NTSB) statistics, it is the safest, single-engine aircraft (including airplanes!) flying today. It has been used extensively for military, police, news gathering and many other applications all over the world and, as a result, it benefits from a very broad international network of technical support.

Jet Ranger CapabilitiesA full description of the Jet Ranger characteristics and performance is available on the manufacturer’s website at www.bellhelicopters.com, and only the most relevant characteristics for its use as a HOP are summarized in Table 1. While its available payload capability (APC) for scientific instrumentation is limited, when compared to its hourly fuel consumption, it is one of the most efficient turbine helicopters. Thus, it is comparatively cheap to operate, which is another reason (in addition to safety record and technical support), to adopt it as a HOP. Seats, all unnecessary plastic covers and sound-proofing material were removed from the 40-ft3 aft cabin to reduce its weight and to make room for instrument and computer racks. This resulted in an increase of the APC by nearly 100 lbs. The 16-ft3 baggage compartment can also be used for instrument and computer racks. Two power inverters (from 28V DC to 110V AC) provide a total of ~2 kW for instruments, sensors, and computers.

Sensor Suite

Permanent sensors The figure at right shows pictures of the HOP as it is currently equipped with its permanent scientific instrumentation, i.e., the sensors that are expected to be used for any scientific mission. This set of sensors consists of the following (all sensors have a data output rate of 40 Hz or better):

AIMMS-20

The Aventech Research Inc. (www.aventech.com) AIMMS-20 measures the three components of the wind, temperature and relative humidity. It consists of four modules:

  1. An air-data probe (located on the nose of the HOP) that senses temperature, humidity, barometric pressure, the three-dimensional aircraft-relative airflow vector and the three-axis acceleration and magnetic field measurement;
  2. An inertial measurement unit that provides three-axis acceleration and three-axis angular rates;
  3. A dual processor global positioning system that includes dual antenna inputs for differential carrier-phase measurement (one antenna is located on the nose and the other one is on the tail of the HOP); and
  4. a central processing module that, among other functions, converts the inertial and GPS phase/position/velocity data into precise attitude data (roll, pitch, true heading).

This processed information is shared with all other sensors and, therefore, the AIMMS-20 is operated during all research missions. It is also used to coordinate the clock between the different sensors and to trigger data storage (see below).

LI-7500 Open Path CO2/H2O gas analyzer

The Licor (www.licor.com) LI-7500 measures water vapor and CO2 concentrations. It consists of two components:

  1. The analyzer sensor head that is mounted on the nose of the HOP, and
  2. the control box, which houses the electronics and is located in the aft cabin (see figure above).

The sensor head has a 12.5 cm open path, with single-pass optics and a large 1 cm diameter optical beam. Reference filters centered at 3,950 nm and 2,400 nm provide for attenuation corrections at non-absorbing wavelengths. Absorption at wavelengths centered at 4,260 nm and 2,590 nm provide for measurement of CO2 and water vapor, respectively. These features minimize sensitivity to drift and dust, which can accumulate during normal operation.

Ultrasonic Velocimeter (USV)

An Ultrasonic Velocimeter (USV) prototype developed by the Kaijo Sonic Corporation in collaboration with Japan Aerospace Exploration Agency (JAXA) (Matayoshi et al 2005) is also mounted on the nose of the HOP to measure the three components of the wind and temperature. As this information is crucial for the calculation of all turbulent fluxes, it is beneficial to have this duplication, especially because the AIMMS-20 and the USV are based on different technologies. The USV is based on a conventional ultrasonic anemometer that consists of two main components:

  1. A probe (also located on the nose of the HOP), which senses the three-dimensional aircraft-relative airflow vector and ambient temperature by measuring ultrasonic pulse transit time between three mounts; and
  2. a control box and a junction box (located in the aft cabin), which control ultrasonic pulse emissions, and output the measured data via RS-232C.

The main advantage of the USV as compared to a pitot-static system is that it can provide accurate measurements at low speed. This is obviously important for helicopters. Unlike conventional ultrasonic anemometers, the USV uses high-frequency (200kHz) ultrasonic pulses to reduce acoustic noise, and its probe shape minimizes airflow disturbance at high airspeeds. These modifications allow a broad range of airflow measurements, from 0 to 70 m/s, which covers the entire flight envelope of the HOP.

Onboard Computing and Real-time Data Visualization

A computer is used to run a National Instruments LabVIEW (www.ni.com/labview) program that reads the data input from each instrument, parses and displays data, and determines when to log the data to file. The AIMMS and USV communicate via individual RS232 serial lines to the PC. The Licor outputs two 0-10V analog signals (proportional to water vapor and CO2) that are connected to the PC through a National Instruments USB-6008 Data Acquisition (DAQ) Card. An independent pressure sensor that is used to calculate potential temperature in real time provides a 0-10V analog signal that is also wired to the DAQ Card. Finally, a 0-5V signal is fed through a switch in the cockpit and back to the DAQ card so that the pilot can easily “mark” the beginning and the end of a measurement flight leg by creating a signal in the log file. This is a useful marker when processing the data after the flight.

The real-time visualization that is displayed on the ultra-bright monitor located in the cockpit includes the potential temperature profile, which is calculated from the temperature measured with the USV and pressure from the sensor that has a static port below the helicopter. This real-time profiling capability is useful for the assessment of the height of the ABL and, accordingly, for the selection (in real time) of relevant flight altitudes. It also displays the DAQ information in graphic form.

Modular Design

It is also important to note that the HOP has a modular design and, therefore, sensors and instruments can be mounted on it for specific experiments and dismounted afterward. It also has an attachment device under its belly, which includes power and data connectors. Thus, instruments can be mounted inside pods that can be rapidly attached to that universal device and that can communicate with the on-board data acquisition system, if desired. For instance, an aerosol lidar has been constructed in such a pod (Eichinger et al 2008).

Navigation System

Finally, it should be mentioned that the HOP is equipped with the Chelton Flight Systems (www.cheltonflightsystems.com), which is a state-of-the-art navigation system that provides three-dimensional synthetic vision of the terrain with all its obstructions (including antennas, buildings, etc), a complete flight/navigation instrumentation system, and the “Highway-In-The-Sky”(HITS), which depicts a perspective-like tunnel. This system helps perform very precise flights according to preselected altitudes and coordinates of the path to be flown. It also includes traffic awareness and real-time satellite weather for enhanced safety. It is backed-up by a battery-operated portable GPS Garmin 496 (www.garmin.com) in case of electrical power loss.

References

  • Eichinger, W., R. Avissar, and H.E. Holder, 2008. Development of a Helicopter Borne Lidar for Boundary Layer Studies, Proceedings, 24th International Laser Radar Conference, 23-27 June 2008, Boulder, Colorado, USA.