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US LTA MODEL 138S SPECIFICATIONS
1. INTRODUCTION
2. GENERAL DESCRIPTION
3. DIMENSIONS AND CHARACTERISTICS
4. AIRSHIP GENERAL ARRANGEMENT
Envelope
Nose Dish and Battens
Envelope Pressure System
Control Car
Suspension System
Propulsion
Flight Controls
Electrical
5. OPERATING LIMITS
US LTA (US Lighter-Than-Air) is a Eugene, Oregon Corporation dedicated to the design, manufacture, and operation of airships for the commercial, civil, and scientific marketplaces worldwide. The company has developed and manufactures the Model 138S Airship, a 138,000 ft.3 single engined airship. This 6 passenger craft is certified to F.A.A. regulations under Type Certificate AS2NM of July 24, 1990. A versatile and reliable airship, it is ideally suited for a wide variety of applications and mission requirements and with the photographic capabilities of the nightsign it is also the most spectacular.
Listed among the design features
are:
The US LTA Model 138S is a helium-inflated airship measuring approximately 160 feet long and containing 138,000 ft.3 of helium. It has a disposable payload of over 3000 pounds, maintains speeds in excess of 50 miles per hour, and can accommodate various combinations of equipment and personnel for different mission configurations. The 138Sconsists of three main components: a helium-filled outer hull ("envelope"),attached tail fins ("empennage"), and a control car. Figure1 is a general view of the US LTA 138S airship.
The 138S is a non-rigid airship, meaning it has no internal frame or supports to maintain the shape of the envelope. The pressure of the helium gas within the envelope keeps it taut and rigid and provides the "structure" which allows the airship envelope to carry wind/air loads and travel through the air at relatively high speeds.
The control car is attached to the envelope via a series of cable attachments dispersing the loads into curtains attached to the envelope, the "internal" and "external catenaries". The pilot controls the airship with a single conventional stick located in front of the pilot's seat on the port side of the control car. Stick movement is transmitted through hydraulically boosted cables to hinged control surfaces mounted on each of the three fins. The fins are mounted to the envelope in an inverted "Y" configuration and together with the attached control surfaces, "ruddervators", are called the "empennage".
The interior of the control car contains the flight controls, instrumentation and passenger and crew accommodations. The 6 seats are easily removable and the control car interior can be configured for various mission payload requirements.
Propulsion is provided by a single fuel-injected 300 hp six cylinder engine mounted to the aft end of the control car. The engine drives a three-blade reversible-pitch propeller which "pushes" the airship through the air.
The various components of the airship are outlined in figure 1 and are discussed in greater detail in the following sections.
Overall Dimensions
Length:
160.0 ft (48.8 m)
Width:
41.6 ft (12.7 m)
Height (wheel to top):
56.7 ft (17.3 m)
Empennage area
Total Fin Area:
649.5ft
(3 fins each at 216.5 ft) (60.3 m)
Total Movable Surface Area:
278.1ft (3 ruddervators each at 92.7
ft) (25.8 m)
Envelope Dimensions
Volume:
138,000 ft3 (3,908 m3)
Surface Area:
16,787 ft (1,560 m)
Length:
158 ft (48.2 m)
Maximum Diameter:
41.6 ft (12.7 m)
Fineness Ratio:
3.8
Maximum Section:
63.3 ft (from theoretical bow) (19.3 m)
Center of Buoyancy:
72.2 ft (from theoretical bow) (22.0 m)
Control Car Dimensions
Overall Length:
21.5 ft (6.6 m)
Interior Width at ceiling:
5.8 ft (1.8 m)
Interior Width at floor:
5.4 ft (1.6 m)
Height from Bottom of car:
9.0 ft (2.7 m)
Overall Height with Landing Gear:
11.7 ft (3.6 m)
Cabin length:
13.4 ft (4.1 m)
Cabin headroom
6.3 ft (1.9 m)
Weights
Maximum design gross weight:
9300 lbs (4213 kg)
Empty weight:
5900 lbs (2673 kg)
Ballonet Volume
Forward:
18,000 ft3 (510 m3)
Aft:
18,000 ft3 (510 m3)
Percent of Envelope Volume:
26%
The envelope is constructed using 16 panels of fabric bonded into a complete strip called a "gore". This gore spans the length of the ship. These 16 identical gores are then bonded together into a complete envelope. The envelope material consists of very thin layers of lightweight woven fabric laminated to composite and non-woven material to provide high strength and low permeability to helium. The gores and panels are bonded together using both cement and heat sealing methods.
Sixteen aluminum battens laced to the envelope fabric stiffen the nose top permit higher airspeeds and distribute mooring loads.
Top
NOSE DISH AND BATTENS
Located at the nose of the 138S are sixteen long, curved aluminum tubes, called "battens", which radiate like spokes from the steel nose dish located at the extreme forward tip of the envelope. The nose dish is laced to the envelope. The battens are bolted to the nose dish and laced to the envelope along their length. The battens and nose dish serve to stiffen the nose area of the airship to permit higher airspeeds without deformation of the hull. The nose dish/batten assembly also distributes mooring loads to a large area of the envelope when the airship is moored to a mast.
The major components of the envelope pressure system include the air duct and electric blowers, the forward and aft ballonets, two inlet air valves, and two outlet air valves. Figure 6 shows the complete envelope pressure system. The envelope pressure system has two important functions; pressure control and trim control.
As the airship flies through different altitudes and temperatures the envelope containing the helium gas is subjected to changing forces which create changes in the internal pressure of the envelope. These variations in internal helium pressure are stabilized and compensated for by the pressurization (inflation or deflation) of two self contained air compartments within the envelope called" ballonets", one located at the fore and one at the aft end of the airship. This action exemplified in figure 4 permits the airship to fly without the need for releasing helium.
As the airship gains altitude and the envelope pressure increases (external pressure is low), air is expelled by valves from the ballonets reducing the internal pressure. The ballonets are deflated using air valves located in the bottom of each ballonet. These valves are set to open and close at a predetermined pressure for automatic hull pressure control or they can be opened and closed manually by the pilot. This constant fluctuation is regulated automatically by the pressurization system. Eventually the airship may reach a height at which the ballonets are completely empty of air; this is called the "pressure height" of the airship and defines the maximum safe operating altitude at which the airship can fly.
The ballonets serve a secondary function in providing for fore/aft trim control by adjusting their fullness relative to each other. In other words, if the pilot wishes to fly the airship slightly nose-heavy, air is pumped into the forward ballonet, thus making the forward portion of the airship "heavier" than the aft. Figure5 shows the function of the ballonets as trim control devices.
The ballonets are typically inflated by ram air from the propeller wash entering the air duct mounted directly aft of the propeller. The air is routed by the duct to two damper valves which can be opened to allow the air to enter either or both of the ballonets, as directed by the pilot. If the air flow from the prop wash is not sufficient to keep the ballonets inflated, or in the event of an engine failure, electric blowers mounted in the air duct can be used to keep the ballonets inflated. The complete system is outlined in figure6 below.
The control car is located just forward of the midpoint of the envelope on the underside of the airship. The outer shell of the car is made of a light, strong fiberglass/foam laminate material which allows the car to have graceful curves and a smooth, aerodynamic exterior (see figure 7). The shell includes the doors, windows, various access panels, and the engine cowlings. The shell is attached to a welded tubular steel truss, which also provides the anchoring points for the cables which suspend the car from the envelope.
The control car contains the flight controls, instrumentation, and accommodations for pas- sengers or crew totaling up to six people. Alternatively, it can be configured to accommodate scientific instruments, cameras, a night sign operator's console, or any other arrangement of equipment and/or personnel that falls within the payload and specifications of the airship. The flight controls include all necessary instrumentation of day or night VFR operations.
A landing light, antennae, and a swiveling non-retractable landing gear are located along the bottom exterior of the car. A handrail for the ground crew runs around most of the car exterior just below floor level.
The control car contains provisions for ballast (both solid and water), fuel, hydraulic fluids, the battery, and various other items.
The control car is suspended from the envelope using two systems of cables, one located inside the envelope (internal catenary system) and one outside (external catenary system). The internal catenary system consists of steel cables which attach to the upper portion of the control car frame, run upwards through the helium compartment inside the envelope, and attach to nylon fabric catenary curtains attached at the upper interior of the envelope as shown in Figure 10. The internal catenary system supports about 85% of the weight of the control car.
The external catenary system absorbs forward, reverse, and side-to-side forces created by the propeller and ground handling while also supporting about 15% of the weight of the control car. It consists of stainless steel cables attached at one end to the upper exterior perimeter of the control car frame and at the other to a series of stainless steel cables cemented into the envelope just above the control car. Figure 11 shows the external catenary system.
The airship is powered by a single fuel injected six cylinder Lycoming engine, rated at 300 horsepower with a TO (Time Between Overhaul) of 2000 hours. The engine is mounted on a steel dynafocal-style frame which in turn is mounted to the control car frame at the aft end of the control car. The engine drives a 3-blade reversible pitch Hartzel propeller. Figure 12 shows the engine and propeller.
The pilot controls the 138S using a single conventional stick. Stick movement is first boosted by a hydraulic system and then translated to cables. The cables run through the bottom of the control car, exit the car at the upper aft end, and run to the fins through a series of pulleys attached at various points to the envelope and fin framework. The control cables end at the control surfaces, which deflect according to the movement of the stick.
The control stick functions much like a conventional aircraft stick (except no pedals); push the stick forward and right, and the airship will go into a dive while turning right; pull it back into your lap, and the airship will climb. The control system is designed to minimize pilot fatigue while remaining manually operable should the hydraulic system fail. All controls and instrumentation are located forward of the pilot and crew seats. A centrally located pedestal contains the engine, propeller, and fuel controls. An instrument console on top of the pedestal contains the radio and navigation controls and the flight and engine instrumentation. An upper console contains electrical, hydraulic, and envelope pressure systems controls and instruments.
The electrical system is a standard light aircraft 28-volt, 100-amp negative ground type pow- ered by an engine-driven 100-amp alternator regulated to 80 amps of continuous current.
A general aviation 24-volt lead acid battery provides electrical power for engine starts and emergency operation of essential equipment in cases of engine or alternator failure.
Engine, Propeller speed and pitch limits
Engine Type: Textron Lycoming IO-540-K2A5
Propeller Type:
Hartzell HC-E3YR-7LF
(3 blades, 78" diameter, constant speed, reversing pitch)
Max take-off power* 2700 rpm, 29.5 in. Hg
Max continuous power 2700 rpm, 29.5 in. Hg
Max reverse prop speed 2100 rpm, 23.0 in. Hg
Max prop cycling speed
1400 rpm
* There is no time limit
for the use of this power limitation.
Weight and balance limits
Max static heaviness
400 lbs (181 kg)
Max static lightness
200 lbs (91 kg)
Max landing weight
8900 lbs (4,032 kg)
Max take off weight
8900 lbs (4,032 kg)
Max car weight
5356 lbs (2,426 kg)
Max useful load
3017 lbs (1,367 kg)
Range limits
Max range (approximate)
400 nautical miles at 35 mph (740 km at 56 kph)
Max duration (approximate)
16 hours at 29 mph (46 kph)
Airspeed limits
VH (Max operating limit speed)
*
54 mph EAS (90 kph)
Max speed for water ballast drop
35 mph IAS (56 kph)
Max wind speed for ground handling
25 mph
Max wind speed for airship on
the mast 80 mph
* This speed limit may not be deliberately exceeded in any regime of flight (climb, cruise, or descent) unless a higher speed is authorized for flight test or pilot training.
Flight crew and passenger limits
Min flight crew
1 pilot
Max seating configuration
6 (including pilot)
Operating condition limits
Max operating ambient temperature
120° F (38° C)
Min operating ambient temperature
using water ballast
33° F (1° C)
Limitations
VFR, Day, or Night
Flights into known icing conditions prohibited
Climb and descent limits
Max climb rate
1800 ft/min (732 m/min)
Max descent rate
2000 ft/min (366 m/min)
Max pitch attitude
30° nose up
Min pitch attitude
30° nose down
Max operating altitude
9000 ft (2743 m)
Envelope pressure limits
Max hull pressure
2.8 in. water
Min hull pressure
1.1 in. water
Water ballast limits
Nominal discharge rate
231 gal/min (874 l/min)
Max capacity
54.5 gal (436 lbs.) 206 l. (198 kg))
Electrical system limits
Max electrical load 100 amps
Engine limits
Max continuous horsepower
300 at 2700 rpm
Max cylinder temperature
75° F
Min oil pressure (idling)
25 psi
Min oil pressure (normal)
60 psi
Max oil pressure (normal)
95 psi
Max oil pressure (starting, warmup,
taxi, and take-off)
115 psi
Min oil temperature
60° F
Max oil temperature
118° F
Max fuel consumption
150 lbs (25 gal)/hr 68 kg (95 l)/hr
Fuel Pressure
0-35 psi
Fluid types
Fuel
100 LL aviation fuel
Oil:
30° to 90°F (-1°
to 32°C)
MIL-L-6082 Grades: SAE 40
MIL-L-22851 Ashless
Dispersant Grades:
SAE 40
0° to 70°F (-17°
to 21°C)
MIL-L-6082 Grades: SAE 30
MIL-L-22851 Ashless
Dispersant Grades:
SAE 40, 30
Hydraulic fluid:
MIL-H-5606
Capacities
Max fuel capacity
106 gal (409 l) 100 gal useable
Max oil capacity
12 qts (11.4 l)
Min oil
9 qts (8.5 l)
Max hydraulic fluid capacity
0.72 gal (2.73 l)
Fuel Flows indicated are at
best economy power settings corrected
to 2000' MSL.
Airship at Max-takeoff weight of 8900 lbs. (400 lbs. heavy)
Fuel Flow may vary due to
changes in payload, trim and leaning of
engine.
Endurance Shown for full tank (100 gal.) upon take-off
The endurance shown is for fuel flow rates shown in FUEL FLOW graph (p.21)
Endurance is in hours and excludes time and fuel needed for take-off and 45 minute emergency reserve.
Range shown in statute miles for fuel flow rates shown in figure 1.
Range shown for a full tank (100 gal.) of fuel. This range excludes take-off, climb to 2000' MSL, and 45 minute reserve.
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