AIAA-2002-5882AIRSHIPS AS UNMANNED PLATFORMS

CHALLENGE AND CHANCE1

Dr. Ingolf Schäfer, AIAA member, Engineering Services, Lahnau, Germanye-mail Ingolf.Schaefer@isuk.de

© 2002 by the authors. Published by the American Institute of

Aeronautics and Astronautics, Inc. with permission1

AbstractThe paper deals with the design considerations to be taken into account for airships used as unmanned platforms. Starting with a brief description of possible applications, specific aspects of lighter-than-air technology are described. This does include aspects such as aerostatics and buoyancy control, aerodynamics and flight mechanics. Performance, behavior and materials are other aspects such as structural concepts. Propulsion is covered as well. It is shown that the key to a successful design is a multi-disciplinary design optimization approach. Finally, various achievements and lessons learned from already build solar powered RC airships are listed before a brief outlook closes the paper.

IntroductionLighter-than-air vehicles have been in the past on several occasions pioneers for developing and introducing new technologies into the world of aviation. The Zeppelin airships at begin of the 20th

century were the first aircraft which used primary metal structures based on aluminum. Airships were the first vehicles which made scheduled intercontinental flights and developed long-distance navigation. The demand for light-weight and fuel efficient engines was a pacemaker which achieved results which have not beaten till then. An airship was the first civil type-certified fly-by-wire aircraft (The GA-42).Unmanned Aerial Vehicles (UAVs) are an upcoming trend in Aviation. Beside the military applications more and more civil applications are within reach. Introducing UAVs into an existing and well developed airspace with manned aircraft is a challenge in itself. Several questions and problems are existing which need an answer before commercialization is possible. This starts with questions about see-and-avoidance strategies, showing equivalent level of safetey and ends with new strategies for software development and pilot workload controlling more than one vehicle at a time.Airships are still serving a niche market using their specific capabilities. Using airships as unmanned

aerial vehicles is another possibility where airships may find a niche, but as well prepare the ground and serve as a cheap and low-risk flying test vehicle in order to learn from it and develop in a cheap and efficient way techniques and technologies for UAV’s in general.

ApplicationsAirships are by the physical principles they are using different to airplanes. The key aspects of difference are:- large volume in order to generate aerostatic lift- low speed due to the large volume and the

resulting drag- no power required for staying aloft.This results in advantages in some specific applications, from which some of the more obvious ones shall be discussed briefly.

High Altitude Long Endurance PlatformsHere, the intention is to place a vehicle geo-stationary above one point as a relay or communication platform. For indefinite endurance, solar power seems to be the most promising way to generate power for propulsion. Airships do not need speed to keep altitude but only to counteract wind. And they provide the area required for the solar panels as well as the space for storing the reactants of fuel cells.

SurveillanceThis type of application asks primarily for endurance instead of range. This means that the low airspeed of an airship is not a disadvantage. Additional, the envelope allows to install large radar antennas or sensors without causing additional drag. This has been done in the 40’s and 50’s as well in the U.S. with the Navy blimps.

Earth MonitoringUnmanned Airships can operate close to the ground at very low speeds. This results in much better resolution and higher quality of the data generated compared to higher and faster flying planes or satellites.

AIAA's Aircraft Technology, Integration, and Operations (ATIO) 2002 Technical 1-3 October 2002, Los Angeles, California

AIAA 2002-5882

Copyright © 2002 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

Fig. 1: Solar Airship “Lotte” during take-off

Environmental ControlDue to the low pollution level (or no pollution with electric propulsion) and the much smaller disturbance of the surrounding air compared to helicopters, airships have much better capabilities for detailed measurements. Airships have been used as well in rainforests for observations from the tree-tops, which is by other means difficult.

Lighter-Than-Air Design and Technology Aspects

Engineering of airships has a history of 150 years by now. Unfortunately, this history has a lot of ups and downs and is by far not that continuous as it is for airplanes. Therefore the engineering sciences behind airships are not as deeply developed as for other aircraft.One of the key systems engineering aspects is that the typical allocation of specific functions to systems as it has been successful for airplanes is not possible for airships. This is based on the fact that often functions are shared between systems (for example trim capabilities with the control surfaces or c.g. shift using the ballonets) or that different systems are used at different phases of operation (motion control with propellers at low speed and with the fins at higher speeds).This makes as well safety assessment processes more complex than for airplanes and results in larger efforts for investigating these aspects. The general advantage is that the level of redundancy is much higher due to the fact that there are alternative systems which could act as a (partial) replacement and especially because of the lift generation which

is independent from speed and present just by the physical nature of the lifting gas.

Structural Design ConceptsIn the early days of airship design, three major structural design concepts have been developed. Although modern designs do not fit perfectly into these basic types, it is quite helpful to understand the structural design aspects. Non-rigids (also known as blimps) are based on the usage of internal pressure in the envelope for maintaining the shape. As the volume of the lifting gas is dependent on altitude and gas temperature, there have to be installations for controlling the pressure. This is usually done with air-inflated bladders within the envelope, the so-called ballonets. The volume of these changes accordingly in order to hold the internal pressure within the design limits. The typical internal pressure required is equal to the altitude pressure difference of about 50 m. The advantage of this design is the simple and straightforward layout, the major disadvantage is that it is difficult to introduce local concentrated loads such as those coming from the fins or propulsion into the envelope. This is done in most cases with a system of internal and external suspension, where the latter increases the drag.Rigid airships are the well-known Zeppelin-types of airships. Here the outer shape is defined by a structural framework of rings and longitudinal girders protected by an outer cover. Individual gas cells containing the lifting gas are installed inside this framework. This concept avoids the problems of maintaining pressure, but the weight of the structure is high. Therefore the advantages of the

rigid design can be used much better on large sized ships than on smaller ones. Here, the structural weight fraction reaches values around 50%, a level which has never been reached by the other design concepts.An approach to combine the advantages of both systems is the semi-rigid design. Especially in the early days of aviation, non-rigids had limits in size due to the low stiffness of the envelope fabric and the low stress levels of these materials. In order to stiffen the shape on the lower side, where compression loads do occur, a structural rigid member was attached. This so-called keel of the ship allows to attach most of the propulsion and installations to a rigid component, reducing design complexity and assembly.With the use of modern structural calculation methods and the available materials, it is now possible to optimise the use of flexible and rigid elements to a higher degree. This leads to pressure-supported designs where rigid members are only used where they are required for load distribution, as a parallel load path or for additional stiffening. The Zeppelin NT 07 or the solar airship Lotte are typical representatives of this type of design. For unmanned applications, the non-rigid type is the most promising one due to its simplicity and the size of such craft.

AerostaticsThe basic difference between HTA and LTA is the usage of aerostatic lift. Aerostatic lift can be explained using the law of Archimedes which defines the lift as the weight of the displaced fluid minus the weight of the displacing fluid. Looking at the equations in detail, one can see that the static lift is independent from altitude. Further on, it is simple to calculate that the specific lift is very small. As a rule of thumb, one cubic meter of helium lifts about

1 kg, a cubic meter hydrogen about 10% more.A well known aspect with aerostatic lift is the effect of superheating. Small differences in air and gas temperature will give remarkable changes in the static lift conditions. This difference may be caused by solar radiation on the envelope, quick changes of the air temperature, etc. This change in lift has to be neutralised. Another important point which has to be kept in mind is that the volume of the lifting gas increases with increased altitude. The altitude at which the gas reaches its maximum available volume is called the pressure ceiling, from which on gas needs to be vented. All these effects together show that aerostatics very much dependent on outside conditions which explains partly the weather dependency of airships. This can be overcome by proper flight planning, which is different to those for airplanes.

AerodynamicsThe shape of the envelope as a “cigar-shaped” body of revolution is mainly driven by a compromise between low drag and aerodynamic stability of the airship. Low-drag bodies with small areas of separated flow do have a unstable moment about their lateral axis. This is compensated by stabilizing fins at the stern of the hull. This leads for typical airship designs to drag coefficients of about 0.02 ... 0.03 (The reference area for this is volume2/3). Due to the large size of airships and the absence of requiring speed to generate lift, flight speeds are low compared to airplanes. As the power demand follows a cube law, airships are usually operating between 60 ... 100 km/h (38 ... 63 mph).Aerodynamic forces are used primarily for controlling the movement of the airship and to compensate changes of the static lift. The airship hull is also able to generate dynamic lift of up to 10% of the total lift without a major drag penalty.

Fig. 2: Coupling of the different design aspects for airships

Flight MechanicsLTA systems need to take into account virtual mass effects. Bodies accelerated in a fluid have to accelerate the surrounding flow field, too. For heavier-than-air craft the effect is neglectable due to the small fraction of the overall forces. Airships, as they are based on the principle of being of equal density as the surrounding air, have a different behavior. This virtual mass is a multi-dimensional tensor. As an example, the translational movement in lateral or vertical direction results approximately in a doubled mass of the body for this movement, which dampens the motion.Combined with the aerodynamic characteristics, airships can be dynamically stable without being statically stable. This is the preferred choice in order to achieve reasonable handling characteristics, resulting in a different type of stability modes. With proper sized fins, an fixed-stick piloted airship disturbed by a horizontal gust flies into a stable circle. The smaller the fins are, the smaller the radius of this circle becomes, up to a point where the flow separates and the airship stalls. As a typical requirement derived by practical experience, the flight path circle radius with neutral rudder should be larger than 7.5 ship length and as a measure for controllability, it should be less than 2.5 ship lengths for full rudder deflection.

Structures and Loads CalculationStructural elements are required in order to collect, transfer and distribute the mechanical loads acting on the ship. The main loads in operations are the static loads of the ship itself, which are up to 70% of the maximum load for most of the structures. Inertia loads from maneuvers or gusts are are seldom higher than 0,5 g Areas such as the fins have primarily aerodynamic forces to counteract, but still here the loads are spread out over a large surface with a low load level. This means that usual structural elements based on rigid members cannot be used as the required thickness is far below the level which is producible and additional stability problems such as buckling or crippling would primarily drive the design. Pre-stressed membranes are proven to be the lightest solution to these design problems in most areas.

EnvelopeThe main structural element of an airship is the envelope. Environment, application and size result in special requirements which have to be fulfilled at the same time. The answer developed over time for this is a multi-layer material with a fabric as the main load-carrying element and different coatings for UV-protection and gas retention. Laminated films such as Tedlar® for the outer skin and Mylar® as gas barrier are another approach. The

material for the ballonets has to be flexible in order to change shape depending on inflation ratio of the ballonet.

Propulsion ConceptsA propeller is the preferred choice due to the low speeds of airships, but there are different possibilities to generate the required torque. The most obvious one uses a combustion engines and does not need further explanation.Electric powered airships are slightly different. Due to the fact of not burning fuel, the weight of the airship remains constant during operation, which allows to fly always closer to EQ and therefore more efficient. Due to the low energy density of batteries or other storage systems an optimized energy management is required. Batteries can be supported by solar cells attached on top of the envelope, taking advantage from the large surface of the hull.A much further developed concept is using regenerative fuel cells in combination with solar cells to allow indefinite endurance. Airships have the advantage that the storage of the reactants is much simpler because enough volume for unpressurized storage is available.

Flight behaviour of airships

Take-offMost airships are taking off heavy, e.g. the static lift is lower than the weight of the ship. This simplifies the ground handling. In order to overcome the heaviness, either dynamic lift is required or the usage of tiltable propellers. Both ways have been used successfully. For a dynamic take-off, a short start distance is required (about two or three ship lengths).For small airships such as the solar airships “Lotte” and “Speedy” a different procedure was developed: The nose of the ship was pushed up in the air with propeller at full throttle. This results in a spectacular take-off with a ship at an attitude angle of up to 80 degrees.

TurnsAfter initiating a turn it takes some time before a stable condition is achieved due to its inertia. The turn is usually initiated with full rudder deflection for maximum acceleration and after 30° change the rudder is reduced to the required angle for the stable turn. Due to the fact that the center of gravity lies below the volumetric center of the airship and therefore the aerodynamic center of pressure, the turn is combined with a roll. The roll results then in a vertical force generated from the vertical fins which pushes the nose of the ship down.

Fig. 3: Stability modes depending on tail size

In addition, the turn starts with a small move to the opposite direction due to the force generated by the vertical fins before the ship moves into the right way.For airships, the turn radius is direct dependent on the rudder deflection and nearly independent from speed. This is based on the fact that all forces are scaling with v2. This means that the turn rate is proportional to the speed for a specific rudder deflection. Typical values are in the range of 6°/sec ... 10°/sec.

Ascent and DescentAs the change of altitude requires no additional work for airships at equilibrium, large ascent and descent speeds are possible. Especially the descent is limited for non-rigids by the capabilities of the pressure control system and the performance of blowing air into the ballonets. Lotte has reached ascent speeds of 12 m/s at airspeeds of 14 m/s.

Approach and LandingManeuverability at lower speeds (below 15 km/h for smaller ships) is limited as long as only aerodynamic means are available. Without additional means such as tiltable propellers, the typical procedure is to align the airship with the mean wind direction at higher speed and safe altitude before letting the airship decelerate towards the landing point. If changes are required later this is usually handled by the ground crew or the landing is aborted. For ground handling, several handling lines, especially at the nose of the ship, are installed.

Multi-Disciplinary Approach in the Design

As being common for complex systems, an overall optimization and tuning of the whole concept is important in order to achieve a balanced and efficient design. Work on this aspect has been

carried out over the last couple of years. This is based on the experiences made with the solar airships Lotte and Speedy and being converted into several computer codes.The general approach for this is as follows: 1. A mathematical representation of the different

aspects such as aerostatics, aerodynamics, systems performance and structural capabilities has been worked out.

2. Based on the practical experiences in design and operation, the interdependencies between the systems have been described.

3. Different input parameters defining a specific design point or a mission have been defined.

4. The link between the input parameters and the variation of related data is set up.

5. The program has then used an optimization algorithm for finding the best performing solution.

Initial results of this calculation scheme were used to build the solar airship Speedy and as will be shown later, this resulted in major improvements of performance. The algorithm used is based on the “Threshold Accepting” method, a simple an quick adaptable one [Dueck90], [Dueck93].A typical example for a cross-system trade-off is the voltage level in the electric propulsion system. Higher voltage level means lighter cables with a defined loss, which is an important factor due to the size of the ship. This high voltage level would mean that the solar panels are working on a high voltage level as well, e.g. the size of the individual subgenerator is large. Due to the curved shape, the cell with the worst inclination angle towards the sun defines the current output. Therefore here a lower voltage level is preferable. This problem can be reduced by separating the voltage level of the solar generator from that in the propulsion system, but this needs a DC-DC converter which adds losses in the system.

Fig. 4: HALE cornerstone missions as been proposed by esa [Kueke 99]

HALE AirshipsUsage of high altitude lighter-than-air vehicles for different purposes is not a new idea. The first serious study of the high-altitude station keeping powered balloon concept was performed by General Mills, Inc. during the late 1950. This study described a powered airship that has to carry a payload of 30 – 100 kg to an altitude of 18 – 24 km for a duration of 1- 8 h [Anderson60]. High Platform I was the first attempt to fly in the stratosphere. The ship was launched in 1968. It was a balloon with an electric driven propeller in the gondola. The program was continued in 1970 with High Platform II, a solar powered airship flown for 2 hrs. in 20 km altitude [Beemer73]. Further activities were done by Goodyear in 1972 with the powered balloon (POBAL). Raven Industries continued with two studies: A parametric study of high-altitude station keeping vehicles (HASKV) followed, and an additional study of super-pressurized POBAL (POBAL-S) overlapped. A demonstrator was built and tested within the HASPA program undertaken by Martin Marietta/ Sheldahl Corporation in 1976. The concept was to use an airship powered by fuel cells with 100 kg payload. The most aggressive design was the 1982 Lockheed HI-SPOT. The airship was 142 000 m3, 154 m by 42.2 m at a gross weight of 11,750 kg. The body was a low drag “dolphin” shape with long laminar flow in the boundary layer. HI-SPOT was powered by four new generation liquid-cooled, turbocharged liquid hydrogen fuel IC engines.

Currently, HALE airships are of interest due to the usage for mobile communication. Several studies and activities have been initiated in the U.S., Europe and Japan. These activities are supported by the improvements made in the area of fuel cells, solar power and electronics.HALE airships are large due to the fact that at 20 km altitude the specific lift reduces to about 60 g/m3. The decision for this altitude is based on the fact that here the wind speeds have a minimum. Nevertheless, there are large variations with season and latitude. This drives the design of the propulsion system and the size of the ship. Although these high wind speeds are seldom and the average speed required is much lower (15 – 20 m/s), the layout needs to take top speed demand into account. Finally, the goal is to have a craft with indefinite performance. This can be achieved with solar power and a regenerative fuel cell system but has as well major demands to the reliability of key systems and the envelope. Usually, inspection and regular maintenance are a key element for the high level of reliability in aviation which is not possible if several years of station-keeping are intended.

Solar Powered RC airships

LotteEnd of 1991 at the University of Stuttgart a project was initiated to build and operate a solar powered airship. Due to the limits in budget and time, an unmanned and radio-controlled version was built.

The ship, called “Lotte” made its first flight 18 months after start of the project. Later, in autumn 1993, it was transported to Australia and operated there under better solar but more challenging environmental conditions. Here, different operational procedures were improved and endurance tests had been carried out.Based on this, an improved version was build begin of 1994 and is operated commercially till then. The improvements did primarily affect handling aspects for transportability, ground infrastructure for control and simplified handling of the ship.The ship has a volume of 100 m3 and a length of 16 m. It is fully solar-powered with batteries as a backup. The number of batteries and solar panels can be chosen based on the specific application, which gives as well more flexibility for payload. Top speed is at about 45 km/h with an engine of 1kW. Payload weight is up to 20kg. The ship is controlled via a redundant radio-control link working at 35 MHz and 430MHz. Independent from this, key ship parameters such as internal pressure, charge level of the batteries, power output to the engine, etc. is down-linked via a separate telemetry.Different system architectures for the electrical power supply have been tested and compared. The final version has a fully integrated system were power is transferred between the main circuits of 90V for the propulsion, 12V for gas management and systems and 6V for the RC-system. This required careful design and setup in order to reduce EMC problems.

SpeedyBased on the experiences and the improvements made with “Lotte”, a further step to the technical limits with a solar RC ship was taken. For this, the focus was set on maximum speed capability as a simple performance measure with a limit for volume of 20 m3. Using the multi-disciplinary approach as being described above, the configuration was defined and further optimization of the systems took place. This all together resulted in a ship which was then able to reach a top speed of more than 60 km/h – 50% more than Lotte but wich 20% of the weight. The ship was flown 1996 in Australia for further operational tests and to improve handling characteristics. Currently, the ship is operated as a demonstration vehicle.The system layout was much simpler than for Lotte. The gas management system for example has no trim capabilities and a single center ballonet instead of two. The pressure control is much simpler by allowing a larger variation of the internal pressure. This was possible as the lightest envelope material available was still much stronger than required. The different parts of the electrical system were much more separated which reduced the effort for EMC design. Due to the large size of the solar generator, which was more than one third of the overall weight, the center of gravity need to be adjusted as well in the vertical direction, finally resulting in putting the batteries in an outrigger at the nose below the ship.

Fig. 5: Solar RC airship “Speedy” during landing

Unmanned Aerial Vehicles and the Role of Airships

Unmanned Aerial Vehicles have received an increased level of interest in the last years. Based on the technological achievements in the last decades it seems sensible to investigate the application of such vehicles not only in restricted airspace for military applications but as well for civil applications, using the same airspace as manned aircraft do.This by itself leads to some serious questions. One of the safety fundamentals in airspace is see and avoid, which is much more difficult if the pilot is not on board of the aircraft. In order to reach an equivalent level of safety, the data collection and the datalink to the ground control station have to reach a very high level of reliability. If a safety assessment shows that failure of this system would give a catastrophic condition, a failure probability of 10-9 per flight hour needs to be achieved, based on the well-known FAR/JAR 1309 paragraph and its related standards. This will be difficult with a wireless datalink. Higher level of redundancy will increase operational cost which then beats one of the key advantages of UAVs.If the solution will be to have a more autonomous aircraft, new types of software standards are required. Current software in aviation is non-decisive and predictive in its behavior. TCAS is a good example by giving the pilots only recommendations based on the negotiation process with the other aircraft, the final decision is still made by the pilot. Autonomous flight will lead to control functions which need decision-making software. This is not covered so far in RTCA DO-178B as been used for software certification inaviation today.Development of systems providing such capabilities will at a certain stage request practical tests. Due to the effects of malfunction, serious effort is required for backup solutions and intensive ground testing in advance. This makes the whole development costly and time consuming.Airships, by their physical nature, have less functions which lead to hazardous or catastrophic events. This gives relaxed requirements on their reliability and the related MTBF values. The capability of using different systems for serving the same function, as been explained above, allows to use them in a very efficient way as one step in generating the data required for full certification or to set up a proper certification strategy. Due to the large time constants of airships, performance demand on software is lower than in airplanes, giving more possibilities for check-routines or for manual override before entering a critical flight condition. The complete loss of all flight control systems on board of an airship will not result in immediate uncontrolled ground contact but convert the airship into a balloon, which can be seen on

radar screens well due to its size and is a less dangerous vehicle due to the low speed. A controlled descent could then be initiated either by the operating limits (flying the ship heavy at all times) or with a fully independent system which starts to vent gas.These properties would make airships a useful tool in the development of UAVs, especially at an early stage where basic information needs to be gained.

Conclusion and outlookUnmanned airships have been operated successfully in the past. There are promising applications for which they can be used. Additional, they can act as a pioneer for other UAVs as well because of their specific system properties. The independence of lift generation from propulsion and the low speeds allow to use airships as a sensible development step for fully autonomous aircraft. Specific systems can be tested in airships in order to learn how they work in a real environment.Nevertheless, proper understanding of the specifics of airships is a prerequisite for proper design. The basic principle they are using is simple, but the complexity increases on the next level of detail. Knowledge of the interdependencies and the limitations of lighter-than-air is important to identify those applications where they are beneficial compared to other systems.

References

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[Kroeplin95]B. Kröplin, I. Schäfer:Experiences by Design and Operation of the Solar Powered Airships „Lotte 1 -3“Proc. of the 11th AIAA Lighter-Than-Air Systems Technology Conference, Clearwater, FA, May 15 - 18, 1995

[Kueke99]Reimund Küke, Per Lindstrand, Peter Groepper, Ingolf SchäferHigh Altitude Long Endurance Aerostatic Platforms: Past, Present and FutureDGLR Jahrestagung Berlin 1999October 1999

[Rehmet94]M. Rehmet, C. Bauder, I. Schäfer, B. Kröplin:Solar Powered Airship ProjectInternational Conference of Remotely Piloted Vehicles (Bristol, Sept. 1994)

[Schaefer97]Ingolf SchäferEin Verfahren zur Entwurfsoptimierung von LuftschiffenDissertation, Universität Stuttgart 1997