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Long-Endurance UAV Concepts

Possibilities of the Next Generation of High-Altitude, Long-Endurance Unmanned Aerial Vehicles

Sheila L. Jaszlics, President, Pathfinder Systems, Inc., USA
Dr. Reiner Stemme, President, Stemme Flugzeugbau, Germany

Published in 2004.


High-Altitude, Long-Endurance Unmanned Air Vehicles (HALE-UAV) have the potential to provide surveillance, communications relay, and other military and civilian capabilities at a relatively low cost, without the limitations imposed by satellite orbits. The first generation of these vehicles, integrated with sensors, information distribution systems, and ground stations, is in development under the American “Tier II+” program. The time has come to think of the next step. A second generation of HALE-UAVs, built and operated at a significantly lower cost than Tier II+, is within reach. A propulsion concept, developed in Europe can reduce the gross take-off weight of this next-generation HALE-UAV by a factor of nearly four in comparison with Tier II+, while retaining its payload, range, and endurance.

The Tier II+ HALE-UAV has the following approximate characteristics:

  • Payload mass: 680 kg (1,500 lb)

  • Take-off mass: 12,000 kg (26,500 lb)

  • Nominal altitude for surveillance mission: 19,800 m (65,000 ft)

  • Surveillance area: 5,560 km (3,000 nautical miles) from take-off and recovery airfield

  • Endurance at surveillance area at nominal altitude: 24 hours

  • Propulsion: turbofan


Tier II+ is under construction by the Teledyne Ryan Company of San Diego, California. The objective of the program is to build ten UAVs and two ground systems.


The utility of a second generation of HALE-UAVs, built for a wide-range of services for military and civilian applications in the early 21st century, could equal or exceed the utility of low-altitude satellites. Some of the significant questions relating to this potential next generation are:

  • UAVs of similar performance be developed, built, and operated at a significantly lower cost than Tier II+? Can they be much lighter and smaller?

  • How much of the Tier II+ development can be re-used for the next generation?

  • Will the Tier II+ experience provide adequate methods for integrating HALE UAVs into the Air Traffic Control System?


The weight efficiency of Tier II+ can be improved by factor of almost 400%.

We have concentrated our work on the question of reducing the gross weight needed to carry a given payload, as lower cost is a direct consequence of this. The key to significant weight reduction is in three areas:

  • A propulsion concept that is more appropriate to the HALE-UAV mission at an altitude of 20,000 m than the turbofan propulsion used in the Tier II+ design

  • High-efficiency power plant with low fuel consumption

  • Application of advanced aerodynamic design principles and practice


To explore the possibilities, Pathfinder Systems and Stemme Flugzeugbau have developed a preliminary design, which matches the Tier II+ payload and mission profile, with a take-off weight thatis 27% of the Tier II+ vehicle. The characteristics of this design, the Stemme SX 1500 “Pathfinder Hawk”, are:

  • Payload mass: 680 kg (1500 lb)

  • Take-off mass: 3206 kg (7070 lb)

  • Nominal mission altitude, distance of surveillance area, and endurance at surveillance area at the nominal altitude: identical with Tier II+

  • Maximum altitude: 21,300 m (70,000 ft)

  • Propulsion: Supercharged reciprocating engine, proprietary dual propeller system.


The improvements summarized below made this dramatic weight reduction possible.

A Practical Design of High Propulsive Efficiency

The SX 1500 design employs a patented dual propeller system. At tropospheric altitudes, a low-altitude propeller of small diameter is employed. This propeller is fully retractable. It is based on the propeller system of the existing Stemme S-10 motorglider (Figure 1).

Fig 1. Stemme Retractable Folding Propeller - Front and Side Views

This retractable propeller is in service in many countries, certified in accordance with the Joint Airworthiness Regulations. It is small enough to permit take-off and landing with adequate ground clearance. During low-altitude flight the high altitude propeller, which has a diameter of 5.7 m (18.7 ft), is feathered (Figure 2). It is blocked in the horizontal position, decoupled from the drive shaft by a clutch system.

Fig 2.  SX 1500 Dual Propeller System

At approximately 10,500 m altitude, the large propeller is engaged through a clutch system. At the same time, the low-altitude propeller is disengaged and retracted. When the SX 1500 descends through 10,500 meters, the process is reversed.

The large propeller has a very low propulsive disk loading, less than 48 N/m2 (1 lb/ft2). The Froude Propulsive Efficiency, which is a measure of the energy efficiency of a reactive propulsion process (propeller-driven or turbofan), is around 98% at this disk loading at 20,000 m altitude over a very wide speed range. In contrast, the propulsive disk loading of a turbofan engine at this altitude is 40 to 80 timeslarger. At speeds of 300 to 400 knots, this results in a Froude propulsive efficiency between 55% and 85%. The weight penalty resulting from this reduction of efficiency is extreme, as the increased fuel requirements result in a disproportionately larger aircraft.

The Froude efficiency for a propeller is included in the overall propeller efficiency. In a turbofan, it is reflected in the specific fuel consumption. It is derived from Newton’s law: the propulsive force equals the momentum imparted over unit time. As a consequence, a smaller propulsive disk will impart a larger velocity increment, requiring more power, and a larger fuel consumption, than a larger one. The SX 1500 propulsion system solves this fundamental physical problem by providing the optimal propulsive stream diameter at the cruising altitude of the second-generation HALE-UAV.

A High-Efficiency Power Plant

The choice for the power plant of the SX 1500 is a turbocharged piston engine. It combines excellent specific fuel consumption with the advantage of constant power up to its design altitude.

Two different power plants for the SX-1500 have been considered:

  • The baseline design with the Teledyne Continental TSIO-550 Voyager high-altitude aircraft engine with a dual turbo-charging system

  • An advanced alternative design, using an entirely new Diesel turbo-compound engine


The TSIO-550 Voyager is a conventional, well-proven, liquid cooled, horizontally opposed 6-cylinder aircraft engine with a dual turbo-charging system for a design altitude of 78,700 ft. (24,000 m). This engine is planned for the first version of the SX 1500 and supports the conservative design approach to the aircraft. It allows the development of a HALE-UAV within a short period of time.

A new generation of supercharged, liquid-cooled Diesel engines developed in Europe (by IAV GmbH) is the basis for an advanced version of the SX 1500. The advantage of the Diesel engine over a gasoline engine is improved thermal efficiency. Specific fuel consumption is reduced to about 185-190 g/kWh (0.304-0.312 lbs/BHP/hr) at a specific weight of about 2.0 kg/kW (3.29 lbs/BHP). A special surface cooling system will dissipate the heat generated through the aircraft structure. This arrangement reduces cooling drag and uses engine heat for the temperature control of fuel and aircraft systems.

The large gross weight reduction of the SX 1500 when compared with Tier II+ can be attributed mostly to the propulsion system.

Aerodynamic Considerations

The following design considerations to achieve long endurance are generally well understood by aircraft designers:

  • Maximum endurance is achieved when the quantity CL3/CD2 is at its maximum (CLis the lift coefficient, and CD is the drag coefficient).

  • Aspect Ratio must be as high as possible; span loading must be as low as possible.


The parasite drag coefficient seems to be of secondary importance, as it has only an effect on endurance proportionate to its ¼th power.

These commonly known rules must be modified by many practical considerations. For example, the value of the lift coefficient at which the maximum of CL3/CD2 is achieved goes up in proportion with the square root of the product of the parasite drag coefficient and the aspect ratio. At low aspect ratios (15 or less) this does not present a problem. At aspect ratios exceeding 30, this effect will result in serious performance degradation. Unless a very low parasite drag coefficient (well below 0.01) is achieved, the lift coefficient for best endurance is around 3.0 and higher. Aerodynamic stall over the wing will begin at lift coefficients of less than half of that value. The result is that the “paper” performance cannot be achieved in practice. For this reason, the SX 1500 design has a relatively low aspect ratio (22). At the same time, much effort is applied to reduce the parasite drag coefficient.

Towards the Future

We are confident that the technologies developed in the United States in conjunction with the Tier II+ program, including flight control, communications, sensors, data management, and ground stations can be reused for a second generation HALE-UAV. Through European-American cooperation, we can look forward to a practical unmanned aerial platform operating at altitudes of 20,000 to 23,000 m. Such a platform should provide standard services to its various payloads, including power, communications, flight control, and radar and optical windows. We envision a future in which hundreds of these second-generation platforms will operate in the skies of the next century.

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