High-altitude Long-endurance Vehicle Research Articles

Experiments of Propeller-Induced Flow Effects on a Low-Reynolds-Number Wing

Novel findings are discussed in this paper that will be especially beneficial to designers and modelers of small-scale unmanned air vehicles and high-altitude long-endurance vehicles that both operate at low Reynolds numbers (). Propeller-induced flow effects in both tractor and pusher configurations on a rectangular wing using the Wortmann FX 63-137 airfoil (a common low-Reynolds-number high-lift airfoil) are presented in this paper. Significant performance benefits can be found for a wing in the tractor configuration. Experiments, including trip tests and upper-surface oil flow visualization, show and verify that the propeller slipstream induces early transition to turbulent flow in the regions within the slipstream and the premature formation of a separation bubble in the regions outside the slipstream. The result is a reduction of pressure drag and an increase in lift of the wing where lift-to-drag ratios are as high as 10–12 (a maximum of 70% increase in lift-to-drag ratio from a clean wing configuration) and are measured at both low and high angles of attack up to stall (0–16 deg). Similar performance benefits are not observed in pusher configuration results where only increased local flow velocity and varying inflow angle effects are apparent. Thus, contrary to the design rules for optimal performance of wings at high Reynolds numbers, at low Reynolds numbers, a propeller in the tractor configuration exhibits significant performance improvements, especially in cruise configurations (low angles of attack), as compared with a propeller in the pusher configuration or even a clean wing.

Design and flight test of active flutter suppression on the X-56A multi-utility technology test-bed aircraft

ABSTRACTEfforts to develop the next generation of aircraft with ever-increasing levels of performance – higher, farther, faster, cheaper – face great technical challenges. One of these technical challenges is to reduce structural weight of the aircraft. Another is to look to aircraft configurations that have been unrealizable to date. Both of these paths can lead to a rigid flex coupling phenomenon that can result in anything from poor flying qualities to the loss of an aircraft due to flutter. This has led to a need to develop an integrated flight and aeroelastic control capability where structural dynamics are included in the synthesis of flight control laws. Studies have indicated that the application of an integrated flight and aeroelastic control approach to a SensorCraft high-altitude long-endurance vehicle would provide substantial performance improvement(1,2). Better flying qualities and an expanded flight envelope through multi-flutter mode control are two areas of improvement afforded by integrated flight and aeroelastic control. By itself, multi-flutter mode control transforms the flutter barrier from a point of catastrophic structural failure to a benign region of flight. This paper discusses the history and issues associated with the development of such an integrated flight and aeroelastic control system for the X-56A aircraft.

Reexamined Structural Design Procedures for Very Flexible Aircraft

This paper reviews the applicability of some conventional structural design practices to the analysis and design of very flexible aircraft. The effect of large structural deformations and the coupling between aeroelasticity and flight dynamics is investigated in different aspects of the aircraft structural design process, including aeroelastic stability, loads, and flight dynamics and control. This is illustrated with a numerical example of the static and dynamic responses of a representative high-altitude long-endurance vehicle. Suggestions are presented for the development of appropriate frameworks to design and analyze very flexible aircraft.

Impact of solar storms on high altitude long endurance unmanned aircraft and airship design and operations

AbstractThis paper applies existing information on solar storms to unmanned aviation; no new research data is presented. The purpose of this paper is to alert the unmanned aviation community to the potential hazards posed by solar storms, to familiarise it with the effects of solar storms and how to mitigate them, and to encourage research on solar storm effects on high altitude long endurance (HALE) aircraft and airship design and operations. As unmanned aircraft and airships move increasingly into high altitude (50,000+ft), endurance (24+ hr) roles, they will become vulnerable to the effects of space weather, specifically that of solar storms. Although solar storms are commonly associated with their impact on satellites, they affect the routing and timing of airline flights flying for six to eight hours at 30,000 to 40,000ft. Operating twice as high and with flight times twice as long (or longer) than those of airliners, HALE aircraft and airships occupy a middle zone of vulnerability, being more so than airliners but less so than satellites. A key difference however is that satellites are designed for space weather, whereas some current HALE vehicles are not. The paper concludes that unmanned HALE aircraft and airships can be one to three orders of magnitude more vulnerable to solar storms than a trans-Pacific airliner.

Large-Area Aerodynamic Control for High-Altitude Long- Endurance Sensor Platforms

The use of large-area aerodynamic control schemes to enable high-altitude long-endurance sensor platforms is investigated. The focus is on a vehicle with a joined-wing design. The vehicle has two performance shortcomings that are considered typical of the broader class of high-altitude long-endurance vehicles. The first is minimum roll rate at landing due to the large amount of roll damping associated with these configurations. It is shown that multiple distributed control surfaces can help meet the roll rate requirements. The second is sensitivity of takeoff gross weight to maximum lift-to-drag ratio. Notional mission requirements drive the fuel fraction to high levels and small changes in lift-to-drag ratio can enable large changes in the vehicle weight through reduced fuel requirements. It is shown that the same technology used to satisfy the roll requirement can also be used to actively control the twist and camber during cruise and can have a moderate impact on the vehicle weight or endurance.