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The following presentation offers the operational considerations of the 138S airship as a research platform. The main focal point is a discussion of methods for integrating sensors and equipment with the airship. Other items include payload capabilities, airship performance, platform advantages for aerial research and a brief review of past experiments performed.

The 138S airship is a non-rigid airship160 feet long and 41 feet in diameter. It is propelled by a reliable 300 horsepower Lycoming aircraft engine and has a hydraulic flight control system. The 138S car has 241 cu. ft. of volume in the cargo compartment; maximum payload of 3000 pounds, which allows approximately 1500 pounds for equipment, depending on mission requirements; a maximum range of 400 miles; and an average endurance of 17.5 hours for past experiments. Figure1.1 provides detail on the configuration and terminology of the airship.

A significant capability of the 138S airship is ease in integrating equipment. Due to the airships slow speed, antennas, samplers, and meteorological instruments can be mounted on the car or the envelope with minor concern for aerodynamics and structural strength. The strength requirements for installations on an airship are 3.0g down, 2.5g forward, and 1.0 g sideward. The 138S is a very stable platform with low vibration in the car and the envelope. Both low loads and low vibration allow the use of laboratory apparatus in equipment installations with minimal modification.
 

The 138S airship is a good airborne platform for low speed, high resolution data acquisition. The vast array of installation possibilities give the researcher great latitude in the design of sensor installations. The low structural requirements encourage the use of laboratory instruments and lab prototypes with minimal reconfiguration. US LTA also offers design and manufacturing services to researchers for fast, cost effective sensor installations on airborne platforms.

Many different types of scientific research platforms are currently in use, such as: ships, towers, buoys, airplanes, satellites, etc. Each platform has advantages and disadvantages for different scientific research. An airship's ability to move slowly in an air mass or slowly over an area without disturbing the air is unique to airships and in demand in the research community. An airship also has advantages of high stability at low speeds which allow the deployment or installation of equipment in the airstream with little aerodynamic concern, long endurance, high payload, the ability to remain in a given air mass, and the ability to travel at low altitudes.

Surveys in the research community show potential operations in the following fields:

Airborne gravity measurements
Mesoscale oceanographic phenomena
Langrangian trajectory
Propagation studies in acoustics
Calibration of ground based remote sensors
Calibration of orbital remote sensors
Atmospheric wind shear
Boundary layer inversion studies of turbulence, profiles and cloud/radiation properties
Atmospheric internal boundary layer at coastal zones
Whitecap coverage vs. wind shear stress
Turbulent humidity exchange
Vertical gas chemistry measurements
Marine mammal tracking
Iceberg tracking

In general these experiments fit into the categories of remote sensing, high resolution/calibration, and observation. Although there are a great number of programs that only an airship can perform, it is important to include that an airship can also fly missions that can be flown by other existing platforms. This can help take some of the workload off of existing dedicated platforms, allowing them to be more effectively utilized.

The 138S airship has been used by researchers from the Applied Physics Laboratory at the University of Washington and from the Naval Research Laboratory to investigate various atmospheric and oceanographic phenomena. Both of these groups installed equipment inside the car, on the outside of the car, and on the envelope. An external generator and a hoist system were developed for use by researchers.

Those successful experiments with the 138S airship demonstrated its viability as an airborne scientific platform. In a recent paper by Glen Frick and Bill Hoppel, they concluded:

The airship's ability to perform high-spatial-resolution profiling both vertically and horizontally makes it the platform of choice to study aerosol, gas-phase chemistry, and dispersion occurring in PBL plumes downwind of ships, power plants, and cities.

"...we would anticipate that the airship would serve as an ideal platform to study fair weather cumulus, and the chemistry and aerosol interactions of cumulus"
 
 

The integration of equipment into the138S airship can be classified into three categories, car interior installations, car exterior installations, and envelope installations. Although every installation is unique, attachment methods for equipment generally follow standard methods. Before discussing various installations and guidelines for installing equipment an overview of the structure is in order.

The 138S consists of three major components, a helium filled envelope, rigid tail surfaces, and a control car.  The envelope includes the hull fabric, two air filled ballonets, and 16battens that are attached to the nose dish.  The tail surfaces are fabric covered aluminum trusses independently attached in an inverted Y configuration.  The car consists of a welded steel framework, a foam core composite shell, and aluminum honeycomb flooring and all of the controls for the aircraft. The passenger/cargo area has seating for 6 including pilot or 147 cu. ft. of cargo room with an additional 94 cu. ft. reserved for access.
 

2.1 CAR INTERIOR INSTALLATIONS

Installing equipment in the car is usually accomplished by attaching standard 19 inch equipment racks or otherwise housed equipment to the existing seat track. The cargo area has two seat tracks that are parallel and 1.27m (50in.) in length. Each set of seat tracks are on 35.56cm centers (14.0 in) and accept standard Accra seat track stanchions. The maximum height for racks is 1.82 m (72 in); after the racks are installed, there is an additional 38 cm (15 in) of height within the ceiling truss. A provision near the top of the racks should be made to allow them to be more secured with cable to an aft truss point.  The car has room for up to 4 racks, but if more than two are installed, the rear rack(s) should be less than 51 cm (20 in.) deep.   Careful attention should be given to what side(s) of the racks will require access for installation and which side will require monitoring in flight.   Equipment must be able to pass through doors on the car which are 66 cmx 1.73cm with a 15cm radius at the corners.   The Model 138S Car Shell can be removed to permit special installations.

When equipment cannot be attached to the seat track, it can be bolted to the floor, clamped to the truss, or attached to the car shell.  When equipment is bolted to the floor, the seat tracks under the equipment are removed and the floor panels are replaced if necessary. Items under 14 kg can be clamped to the truss or attached to the shell.

Equipment that requires frequent monitoring or adjustment yet needs to be exterior for data readings can be located on the handrail by the door.  This allows both access and the ability to store the equipment inside the car for take-off and landing.  Access to the equipment exterior to the car can be provided by removing one or both of the doors or a window.  If doors or windows are removed, equipment facing them will need to be environmentally protected.  It is recommended that rack equipment not contain knobs or wiring exterior to the rack to avoid entanglement in the aisle ways.

For power, the airships removable generator has two circuits: one 20 amp and one 30 amp 120VAC 60Hz.  The connections for these are located at 25cm above the floor and accept MS 3100F16-10Pcannon plugs.
 
 

2.2 CAR EXTERIOR IN STALLIONS

Their are many different methods for attaching equipment to the exterior of the car. To date, equipment has been attached to the handrail, the shell, the generator, and to a boom attached to the truss. Figure 2.2 shows some of the equipment that has been attached exterior to the car. The boom can be used to rigidly attach equipment or with the hoist, it can allow the deployment and retrieval of 317 kg (700 lbs) up to  61 m (200 ft) beneath the airship.

Equipment under 68 kg can be attached to the handrail and also supported with cables to the truss at the top of the carshell.  This type of installation is similar to the attachment of the generator.  Hinging to the handrail of a rigid platform permits raising the platform for take-off and landing.

The aerodynamics of external equipment is generally not of great concern, however at 80 kph, the equipment will see moderate aerodynamic forces and proper fairing should be considered to reduce drag.  In all cases engineering supervision from US LTA is required.
 

2.3     ENVELOPE INSTALLATIONS

Attachment to the envelope can be done by either lacing or suspension patches. Small light items, under 22 kg(50 lbs), are usually laced on as in Figure 2.3. This installation is typical for equipment located anywhere on the envelope.  Items that are slung away from the envelope are mounted with a series of fan patches as shown in fig 4.1.  Items over 33 kg will require a specific platform.

The model 138S was designed to be a working airship.  For its medium airship size, it provides the payload of larger airships and combines excellent endurance and range.  This section details the operational aspects and variable constraints researchers may encounter.

The 138S with support equipment including mobile mast can fly to remote locations and operate for extended periods.  The 138S can operate at any airfield with a minimal 1000' airstrip (grass or paved) and the operations equipment is fully self contained.

3.1     PAYLOAD/EQUIPMENT SPACE

The 138S airship has a maximum of 1360kg(3000 pounds) of disposable payload. Because airships have different lifting capabilities under different atmospheric conditions, total disposable payload changes based on mission requirements. For example, a mission that requires4000 feet of altitude, full fuel, 3 crew members, and the APU has approximately1200 lbs for equipment; where as a mission that required 1000 feet of altitude,1/2 fuel, 3 crewmembers, and the APU has approximately 1900 lbs for equipment. In general the more altitude required or the higher the temperature, the lower the payload. Figure 3.1 demonstrates payload vs. altitude.
 

3.2     PERFORMANCE

Scientific missions usually have a flight profile comprising; take-off, cruise to station, data runs on station, cruise to landing site and landing.  For the maximum on station time it is recommended that the take-off site be as close to the operating station as possible. Moving the crew to a new landing site during a flights easy to do. A suitable take-off and landing facility consists of a flat area, preferably asphalt or concrete or short grass, 300 m or1000' in diameter.  A larger area is always preferred and an approved airfield even better.

The endurance of the 138S airship is between 4.5 and 20 hours depending on airspeed requirements, as illustrated in figure 3.2A.  During operational flights endurance averaged 17.5 hours.  Greater endurance can be obtained with auxiliary fuel tanks which mount in the car.  The airship can fly with full control at 25 kph (15 mph) airspeed and has a top airspeed of 87 kph (54 mph).  During operational flights, the 138S was maintaining an altitude of 70m above ocean within 5m for 1 hour data runs in large part because of the ease provided by the hydraulic flight controls (end excellent pilot effort).  During missions that are flown near equilibrium, the 138S has the ability to hover over a target area and remain stationary in an air mass.  The maximum range of the 138S is 650 km (400 mi) and varies with airspeed/fuel consumption (see figure 3.2B).
 
 

3.3     RELIABILITY

Experiments performed using the 138S as an aerial platform have demonstrated its dependability under demanding conditions.  During one research project a total of 45 working days had 5 installation days, 5 weather days and 2 maintenance days with the balance available for data collection.  Compared to other fixed wing aircraft this represents a significant improvement in time utilization.

3.4 AUXILIARY EQUIPMENT

Hoist System

The 138S airship has a removable boom and hoist system that allows the deployment and retrieval of multiple payloads from the gondola to 61m (200 feet) beneath the airship. This system is capable of handling 300kg (650 pounds) of equipment in multiple configurations. The hoist operates at 4.6m/min (15 feet/min) and is controlled from the gondola. An equipment platform has been developed for NRL for use with this hoist system. If that platform does not meet mission requirements suitable platform can be developed.   This system is not FAA approved and is operated under experimental limitations or under military flight rules.

Generator

An auxiliary generator can be installed on the starboard side of the gondola. This generator provides 5500 VA of120AC power supplied into the car in two circuits, one 20 amp and one30 amp. The control of the generator is from a remote that fits in a standard 19 in radio rack requiring 4.5 cm of space.
 
 

The use of the 138S as an aerial research platform has been established during the last three years for both the Naval Research Laboratory, and the Applied Physics Laboratory at the University of Washington.

4.1     NAVAL RESEARCH LABORATORY

Bill Hoppel, of the Naval Research Laboratory, utilized the blimps ability to travel at slow speeds within an air mass during the summers of 1992/1993/1994 during MAST (Monterey Area Ship Tracks) in Monterey, CA. Detailed vertical and horizontal profiling of the marine boundary layer was performed, including aerosol size distribution, gas chemistry, and aerosol particle nucleation. Extended periods in clouds allowed for detailed analysis of cloud droplet spectra and cloud droplet collection for analysis. The installation of up and down looking radiometers gave albedo, ocean surface temperature and UV irradiance.

These experiments required the installation of three, 1.8m racks of equipment in the car, a radon sampler outside the car, radiometers and a GPS antenna on top of the airship - mounted similar to Figure 2.3, a down looking radiometer and IR radiometer, and a particle sampler suspended beneath the nose of the airship as in Figure 4.1.  The particle sampler had a 5 cm tube and cables running sample air, data and power from the apparatus to the control car.

4.2 APPLIED PHYSICS LABORATORY

Bill Plant, of the Applied Physics Laboratory at the University of Washington, successfully utilized the 138S airship for three different experiments sharing data gathered at the same time aboard the 138S. All three experiments used the Airborne Sensor Platform ASP) developed for NRL by Aeroenvionment. The ASP was designed to be tethered 65 meters beneath the airship placing it outside of the flow distortion pattern of the blimp. Various meteorological instruments were fitted to the ASP with a data cable sending information up to the blimp. The first experiment measured microwave backscatter of the ocean to determine surface parameters including wind, wave spectra, and surface currents. The second experiment measured direct eddy-correlation air-sea interaction surface flux measurements. The other experiment measured surface IR signatures including, fronts, upwellings, and wave breaks.

Peter Pupator, M. Berry, J. Larsen, "AnOverview of FAA Type Certification of the US/LTA 138S Airship." In AIAALighterThan Air Systems Technology Conference, April 1991

FAA report P-8110-2, "Airship Design Criteria", Rev A.

US LTA report, 138S-001 "Flight Test Plan for FAA Certification of the US LTA Non-Rigid Airship Model 138S.", revA, 1989.

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