Uninhabited Air Vehicles Enabling Science for Military Systems (2000) / Chapter Skim
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3 Aerodynamics
3 Aerodynamics
Pages 39-49
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From page 39... ...
The low Reynolds numbers of many UAVs makes the use of wind tunnel models very attractive, and most UAV development involves the creation of substantial experimental databases for performance and control studies. However, very few facilities are suitable for dynamic testing of very maneuverable UAVs (such as UCAVs and HSM vehicles)
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From page 40... ...
This section deals with some of the common aerodynamic challenges. Induced Drag Although HALE UAVs may be required to operate at speeds higher than those for maximum aerodynamic efficiency for reasons of cost or mission effectiveness, the requirement for long endurance leads to lower speed operation, with a subsequent increase in vortex drag.
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From page 41... ...
Reynolds number showing influence of aspect ratio (AR) and laminar flow.
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From page 42... ...
Higher speed UAVs, including supersonic designs, that can gather data efficiently is an intriguing area for future research (Tracy et al., 1999~. Aeroelasticity and Controls Wing flexibility resulting from the requirement for high aspect ratio and low structural weight fraction could cause aeroelastic instability for long endurance UAVs.
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From page 43... ...
Propulsion-Airframe Integration HSM UAVs will probably require highly integrated designs, which will require better modeling of inlet and exhaust flows over a wide operating range. The dimensionless pitch rate is defined as k =——, where a is the angle of attack, t is time, c dt 2U is the mean geometric reference chord, and U is freestream velocity.
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From page 44... ...
PROMISING TECHNOLOGIES This section suggests specific aerodynamics-related technologies and the associated research areas that appear to be promising for the development of UAVs. High-Altitude, Long-Endurance UAVs Section Design Concepts Many current HALE UAVs employ airfoils based on sailplane sections that have been modified for higher Mach number requirements.
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From page 45... ...
Innovative section concepts include very high lift sections; divergent trailing edge concepts (Henne and Gregg, 1989~; continuous-mold-line, variable-camber sections; and slotted sections. Multidisciplinary Design Although some UAVs, such as Global Hawk and Condor, use rather conventional configuration concepts, future HALE UAVs may have very unconventional configurations, including tailless designs, varying degrees of sweep, joined wings, multiple body concepts, oblique wings, formations of cooperating aircraft, and others.
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From page 46... ...
means for much more efficient flow control by applying actuation at the place and time that is most effective. Separation control to enhance maximum lift could improve the loiter performance of HALE vehicles, and modifying the pressure distribution with miniature actuators could extend the region of extensive laminar flow.
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From page 47... ...
The manipulation of separated flow fields may be accomplished efficiently by various flow-control technologies, and active manipulation could permit operation of HSM vehicles in nonlinear flow regimes that would otherwise be avoided, with the goal of increasing mission performance. Because the vortex-dominated flow field arises from separation near the leading edge, subtle changes in boundarylayer properties (due to blowing, suction, or small shape changes)
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From page 48... ...
pressure gradients, Reynolds numbers, and Mach numbers typical of UAV flight conditions and (2) improved flow modeling with part-chord natural laminar flow · techniques for real-time flow sensing and actuation · design architectures for complex multidisciplinary problems, including highly integrated systems
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From page 49... ...
AERODYNAMICS . 49 aeroelastic analysis and design approaches, especially for very flexible, unrestrained, actively-controlled aircraft · novel vehicle control concepts, including flow control · exploitation and modeling of unsteady, nonlinear, three-dimensional aerodynamics · design concepts for very low Reynolds numbers, including steady and unsteady systems · aerodynamic modeling concepts for designing vehicle control systems
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