US LTA 138S AIRSHIP AS AN AIRBORNE
RESEARCH PLATFORM1
Micah H. Hamley
US Lighter Than Air
Eugene, Oregon, USA
ABSTRACT
This paper presents 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. Detail is presented on attachment systems for interior and
exterior installations of equipment. Other items include payload capabilities, airship
performance, a review of past experiments performed, and approximate pricing
information.
1.0 INTRODUCTION
The 138S airship is a non-rigid airship 160 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 ft3 of volume in the cargo compartment; a
maximum payload of 3000 pounds, which allows approximately 1300 pounds for
equipment, depending on mission requirements; a maximum range of 400 miles; and an
average endurance of 17.5 hours for past experiments. Figure 1.1 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 g down, 2.5 g
forward, and 1 g sidewards. 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 has been used by researchers from the Applied Physics
Laboratory at the University of Washington and from the Naval Research Laboratory in
Washington, D.C. 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.
1Presented
at the First International Airborne Remote Sensing
Conference and Exhibition, Strassbourg, France, 11-15 September,
1994.
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Figure 1.1 US LTA 138S Airship General Arrangement
2.0 EQUIPMENT INTEGRATION
The integration of equipment into the 138S 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 airship consists of three major components, a helium filled envelope,
rigid tail surfaces, and a control car. The envelope includes the coated fabric hull, two airfilled ballonets, and 16 battens 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, aluminum
honeycomb flooring, and all of the controls for the aircraft. The car is partitioned between
the cockpit and the passenger/cargo area. The passenger/cargo area has seating for 4,
(another passenger seat is in the cockpit), or 147 ft 3 of cargo room with an additional 94
ft3 reserved for access.
2.1 CAR INTERIOR INSTALLATIONS
Installing equipment in the car is usually accomplished by either attaching
standard 19 inch equipment racks or otherwise housed equipment to the existing seat
track. The cargo area has two sets of seat track that are 1.27 m (50 in) long. Each set of
seat tracks are on 35.56 cm (14.00 in) centers and accept standard Ancra seat track
stanchions. The maximum height for racks is 1.82 m (72 in); after the racks are installed,
their is an additional 38 cm (15 in) of height within the ceiling truss. A provision near the
top of racks should be made to allow them to be 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
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Figure 2.1 Car Floor Layout and Isometric View of Car
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side(s) of the racks will require access for installation and which side will require
monitoring. Equipment must be able to pass through the doors on the car, which are 66
cm X 1.73 cm (26 in X 68 in) with a 15 cm (6 in.) radius at the corners.
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. If equipment is bolted to the floor, the
car floor panels will have to be replaced after the equipment is removed. Items under 14
kg (30 Ibs) or items that require additional support can be clamped to the truss or
attached to the shell.
Equipment that requires frequent monitoring or adjustment yet needs to be exterior
for 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 takeoff and landing. Access to equipment
exterior to the car can be provided by removing one or both of the doors, which is easy or
a window, which is quite complicated. If doors or windows are removed, equipment facing
them will need to be environmentally protected. The simplest way to environmentally
protect a rack is to make a clear acrylic panel the same size as the rack with Velcro
around the acrylic and the edges of the panel. It is recommended that the isle side of
equipment not have knobs or wires sticking out without some form of cover since people
walking down the isle will tend to brush against them and can unknowingly turn knobs
and pull wires.
The wiring of scientific equipment by scientists is often more functional than
efficient. It is recommended that all wiring be of adequate length for the job and not
longer. Another important consideration when wiring is that it weighs a lot. Most
laboratory equipment comes with a standard 2 m (6 ft) power cord, this is both bulky and
unnecessary. Instead of running all of the equipment off of bunches of power strips,
wiring each piece of equipment with correctly sized single wires to enclosed buss bars
can save significant weight and be considerably safer.
The airship’s removable generator has two circuits: one 20 Amp and one 30 Amp
120V AC at 60 hz. The connections for these are located 25 cm (10 in) above the floor
just aft of the starboard door. Both circuits accept MS 3100F16-10P cannon plugs.
2.2 CAR EXTERIOR INSTALLATIONS
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 300 kg (650 Ibs) 61 m (200 ft) beneath
the airship. Equipment under 68 kg (150 lbs) can be attached to the handrail and also
supported with cables to the truss at the top of the car shell. This type of installation is
similar to the attachment of the generator. Another similar method of supporting
equipment is to hinge it at the handrail, on a rigid mount midway up the car shell, or on
the generator and run rope to pulleys and cleats inside the car. This allows
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Figure 2.2 Car Exterior with Various Equipment.
the equipment to be raised -for takeoff and landing and is illustrated in Figure 2.2.
The aerodynamics of external equipment is generally not of great concern, however
at 22 m/s (50 mph) the equipment will see moderate aerodynamic forces. It is advisable to
fair the equipment so that the entire item is smooth and rounded. An easy method to
accomplish this is to glue or Velcro styrofoam to a metal enclosure, form it by sanding or
cutting, and glue plastic sheeting to the foam. A well faired item can have 5 times less drag
than an unfaired item and will experience far less aerodynamic vibration. Most exterior
installations require engineering assistance from US LTA.
2.3 ENVELOPE INSTALLATIONS
Attachment to the envelope can be done by either lacing or suspension patches. Small
light items, under 33 kg (75 Ibs), are usually laced on as in Figure 2.3. Equipment
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should be mounted on a base plate with nothing protruding on the envelope side of the
plate. The plate can be constructed of 1.3 mm (.050 in.) aluminum or 3.2 mm (.125 in.)
PVC plastic with fasteners countersunk on the envelope side. The envelope side of the
plate should be completed covered with felt or similar padding. The perimeter of the plate
should have 6.3 mm (.25 in.) holes 25 mm (1 in.) in from the edge evenly spaced
approximately 7.6 cm (3 in.) apart. The plate should be sized so that the plate area results
in loads less than 73 kg/rn2 (15 Ibs/ft2). If the equipment protrudes away from the envelope,
a series of patches can be attached and cabled to the equipment to provide stability.
Items that are to be slung away
from the envelope are mounted with a
series of fan patches or can be slung
from the battens. This method is shown
in Figure 4.1
Items over 33 kg (75 Ibs) or bulky
equipment require specific engineering
based on their location and configuration.
3.0 OPERATING ASPECTS
The 138S airship was designed to
be a workhorse. For its size, it has a
large payload, excellent endurance, and
reasonable range. This section details
the operating constraints of the airship.
Figure 2.3 Typical Lacing
The 138S airship, with its support
Installation.
equipment, can fly to remote locations
and operate for extended periods of
time.
The 138S airship can operate at any airfield with a minimal 1000 ft airstrip and the
operations support equipment is fully self-contained.
3.1 PAYLOAD/EQUIPMENT SPACE
The 138S airship has a maximum of 1360 kg (3000 Ibs) 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 requires 4000 feet of altitude, full fuel, 3 crew members, and the APU has
approximately 1200 lbs for equipment; where as a mission that required 1000 feet of
altitude, 1/2 fuel, 3 crew members, and the APU has approximately 1900 lbs for
equipment. In general the more altitude required or the higher the temperature, the lower
the payload. The payload versus altitude is illustrated in Figure 3.1.
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3.2 PERFORMANCE
Scientific missions
usually have a mission
profile
that
contains
these elements: takeoff,
cruise to station, data
runs on station, cruise to
landing site, and landing.
For the maximum on
station
time
it
is
recommended that the
takeoff site be as close
to the operating station
as possible. Moving the
crew to a new landing
Figure 3.1 Payload vs. Maximum Altitude.
site during a flight is
easy to do. A suitable
takeoff and landing facility consists of a flat area, preferably asphalt, concrete or short
grass, 300 m (1000 ft) in diameter, a larger area is preferred.
The endurance of the 1 38S airship is between 4.5 and 20 hours depending on
airspeed, as illustrated in figure 3.2A. During recent scientific research our 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 24 km/hr (15 mph) airspeed and
has a top airspeed of 87 km/hr (54 mph). During flights flown for APL, the 138S was
Figure 3.2A Range Vs Airspeed
Figure 3.2B Endurance Vs
Airspeed
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maintaining an altitude of 70 m (230 ft) within 5 m (15 ft) for 1 hour data runs. During
missions that are flown near equilibrium, the 138S airship has the ability to hover over a
target area or remain stationary in an air mass. The maximum range of the 1 38S is 640
km (400 mi) and varies according to airspeed, illustrated in Figure 3.2B.
3.3 RELIABILITY
Experiments performed using the 1 38S as an aerial platform have demonstrated
its dependability under demanding conditions. During research projects, a total of 45
work days had 5 installation days, 5 weather days, 2 maintenance days, and 2 days
working on scientific equipment. The 138S was designed to meet the stringent
requirements of the Federal Aviation Administration and holds a Type Certificate, number
AS2NM, for its design.
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 61 m (200 ft) beneath
the airship. This system is capable of handling 300 kg (650 Ibs) of equipment in multiple
configurations. The hoist operates at 4.6 m/mm (15 ft/mm) 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, a suitable platform can be
developed.
Generator
An auxiliary generator can be installed on the starboard side of the gondola. This
generator provides 5500 VA of 120 AC power supplied into the car in two circuits, one 20
ampere and one 30 ampere. The controls for the generator are from a remote that fits in
a standard 19 inch radio rack and requires 4.5 cm (1.75 in) of space.
4.0 EXPERIMENTS
The use of the 138S as an aerial research platform has been established during
the last three years for both the Navel Research Laboratory, and the Applied Physics
Laboratory at the University of Washington.
4.1 NAVAL RESEARCH LABORATORY
Bill Hoppel, of the Navel Research Laboratory, utilized the blimps ability to travel at
slow speeds within an air mass during the summers of 1992 and 1993. Detailed vertical
profiling of the marine boundary layer was performed, including aerosol size distribution,
gas chemistry, and aerosol particle nucleation. Extended periods in clouds is illustrated
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The installation of up and down looking
radiometers gave albedo, ocean surface
temperature and UV irradiance. A similar
experiment to be performed in 1994 will
measure similar properties of ships
exhaust over time.
These experiments required the
installation of 3, 1.8 m 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, a downward looking IR
radiometer, and a particle sampler
suspended beneath the nose of the
airship. The particle sampler wind, wave
Figure 4.2 ASP Suspended
Beneath 138S
in Figure 4.1. The particle sampler had a
5 cm (2 in tube and 5 cables running
sample air, data, and power to the car.
4.2 APPLIED PHYSICS LABORATORY
Bill Plant, of the Applied Physics
Laboratory
at
the
University
of
Washington, successfully utilized the 1
38S
airship
for
three
different
experiments sharing data gathered at the
same time aboard the 1 38S. All three
experiments used the Airborne Sensor
Platform (ASP) developed for NRL by
Aeroenvionments.
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 allowed the
detailed analysis of cloud droplet spectra
and cloud droplet collection for analysis.
Figure 4.1 Particle Sampler
Suspended From Nose Battens
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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.
5.0 REFERENCES
Peter Pupator, M. Berry, J. Larsen, “An Overview of FAA Type Certification of the
US/LTA 138S Airship.” In A/AA Lighter- Than-Air Systems Technology Conference, San
Diego, California, USA, p. 94-100, April 9-11, 1991.
FAA Report P-81 10-2, “Airship Design Criteria”, Revision A.
US/LTA report 138S-001, “Flight Test Plan for FAA Certification of the US/LTA Non-rigid
Airship Model 138S”, Revision A, December, 1989.
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