Aircraft control system

(12 ) (1o) Patent

United States Patent No.: US 7,802,756 B2

Kendall et al. (45) Date of Patent:

Sep. 28, 2010

(54) AIRCRAFT CONTROL SYSTEM (56) References Cited U.S. PATENT DOCUMENTS (75) Inventors: Greg T. Kendall, Glendale, CA (US); Walter R. Morgan, Simi Valley, CA 1,288,384 A 12/1918 Coakley (US) (73) Assignee: AeroVironment Inc., Monrovia, CA (US) (Continued) (*) Notice: Subject to any disclaimer, the term of this FOREIGN PATENT DOCUMENTS patent is extended or adjusted under 35 U.S.C. 154(b) by 344 days.

DE 582542 1/1934 (21) Appl. No.: 11/732,109 (22) Filed: Apr. 2, 2007 (Continued) (65) Prior Publication Data OTHER PUBLICATIONS US 2008/0001028 Al Jan. 3, 2008 Flittie, K. and Curtin, B., "Pathfinder Solar-Powered Aircraft Flight Performance," AIAA Paper 98-4446, Aug. 1998, pp. 618-632.

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(74) Attorney, Agent, or Firm Erie J. Aagaard, Esq.; The 16, 2000, now abandoned, application No. 11/732, Law Office of John A. Griecci 109, which is a continuation-in-part of application No.

10/600,258, filed on Jun. 20, 2003, now Pat. No. 7,281, (57) ABSTRACT 681, which is a continuation-in-part of application No.

10/073,828, filed on Feb. 11, 2002, now abandoned, which is a division of application No. 09/826,424, filed A span-loaded, highly flexible flying wing, having horizontal on Apr. 3, 2001, now Pat. No. 6,550,717.

control surfaces mounted aft of the wing on extended beams (60) Provisional application No. 60/182,165, filed on Feb.

to form local pitch-control devices. Each of five spanwise 14, 2000, provisional application No. 60/241,713, wing segments of the wing has one or more motors and filed on Oct. 18, 2000, provisional application No.

photovoltaic arrays, and produces its own lift independent of 60/194,137, filed on Apr. 3, 2000.

the other wing segments, to minimize inter-segment loads.

Wing dihedral is controlled by separately controlling the local (51) Int. Cl.

pitch-control devices consisting of a control surface on a B64C 3138 (2006.01) boom, such that inboard and outboard wing segment pitch (52) U.S. Cl . .................................................... 244/45 R changes relative to each other, and thus relative inboard and (58) Field of Classification Search ..................... 244/5, outboard lift is varied.

244/16, 25, 13, 45 R, 36, 15, 55, 34 R, 47, 244/181 4 Claims, 5 Drawing Sheets See application file for complete search history.

/10 f

US 7,802,756 B2

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4,742,977 A 5/1988 Crowell OTHER PUBLICATIONS 4,768,738 A 9/1988 Weinert 4,781,341 A 11/1988 Kasper Djuknic, G. M., Freidenfelds, J., and Okunev,Y, "Establishing Wire- 4,863,813 A 9/1989 Dyer less Communications Services via High-Altitude Aeronautical Plat- 4,907,764 A 3/1990 Long forms: A Concept Whose Time Has Come?," IEEE Communications 4,928,317 A 5/1990 Franchini Magazine, Sep. 1997, pp. 128-135.

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Brown, Stewart F., "Eternal Airplane A Solar Electric Wing Takes 5,356,094 A 10/1994 Sylvain Off," Popular Science, Apr. 1994, pp. 70-75 and 100.

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Walter, Katie, "The Unitized Regenerative Fuel Cell," Science & 5,379,969 A 1/1995 Marx et al.

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application PCT/USO4/19746 (a PCT application corresponding to a 6,076,766 A 6/2000 Gruensfelder parent, U.S. Appl. No. 10/600,258, of present application), Dec. 23, 6,126,111 A 10/2000 Burcham et al.

2005.

6,178,754 B1 1/2001 Dujarric Graage cited by examiner 6,296,957 B1 10/2001 *

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AIRCRAFT CONTROL SYSTEM (i.e., vertical forces generated on empennage horizontal sur-

faces and elevators, with a moment arm that is the distance The present invention relates to aircraft. More particularly, from the wing center of pressure to the empennage vertical the present invention relates to aircraft having unique control center of pressure).

mechanisms, and related methods of controlling an aircraft. 5 To minimize the torsional loads, the Pathfinder, Centurion The present application is a continuation in part of U.S. patent and Helios aircraft include "wing-mounted elevators" along a application Ser. No. 10/310,415, filed Dec. 5, 2002, now U.S.

substantial portion of their trailing edges (i.e., the trailing Pat. No. 7,198,225 which is a divisional application of U.S.

edges of each flying wing segment). These aircraft do not patent application Ser. No. 09/527,544, filed Mar. 16, 2000, include rudders or ailerons, and the wing-mounted elevators now abandoned, which claims priority from U.S. Provisional 10 are not designed as elevons (i.e., they cannot move in contrary Patent Application Ser. No. 60/182,165, filed Feb. 14, 2000, directions near opposite wingtips). Roll is passively con- each of which is incorporated herein by reference for all trolled by the dihedral of the wing, which is developed in purposes. The present application is also a continuation in flight. Sideslip is also passively controlled by the dihedral of part of U.S. patent application Ser. No. 10/600,258, filed Jun.

the wing. As discussed above, the allowable wing dihedral is 20, 2003, now U.S. Pat. No. 7,281,681 which is a continua- 15 limited by the structural strength of the wing.

tion in part of U.S. patent application Ser. No. 10/073,828, Given the broad range of functions that a long-duration, filed Feb. 11, 2002, now abandoned, which is a divisional of suborbital platform has the potential to perform, it is desirable U.S. patent application Ser. No. 09/826,424, filed Apr. 3, to design such high-altitude platforms to be capable of han- 2001, now U.S. Pat. No. 6,550,717, issued Apr. 22, 2003, dling larger payloads and power demands. The platforms which claims priority from provisional application Ser. No.

20 could be variations of existing platforms, such as larger varia- 60/241,713, filed Oct. 18, 2000, and which also claims prior- tions of the Pathfinder, Centurion and Helios aircraft, but such ity from provisional application Ser. No. 60/194,137, filed platforms will likely have to handle increased bending loads Apr. 3, 2000, each of which is incorporated herein by refer- along the wing as such larger aircraft have to react against ence for all purposes.

dihedral-causing forces over a larger wingspan.

This invention was made with government support under 25 There exists a definite need for a multipurpose aircraft that ERAST JSRA Contract NCC-04004 awarded by NASA. The can remain airborne for long durations. Preferably, such an United States Government has certain rights in the invention.

aircraft should be able to operate up to very high, suborbital altitudes. Importantly, it is desirable for such an aircraft to BACKGROUND have the capability to meet larger payload and/or power sup- 30 ply requirements. Furthermore, there exists a need for such an Aircraft are used in a wide variety of applications, includ- aircraft to be structurally light weight and well controlled.

ing travel, transportation, fire fighting, surveillance and com- Various embodiments of the present invention can meet some bat. Various aircraft have been designed to fill the wide array or all of these needs, and provide further, related advantages.

of functional roles defined by these applications. Included among these aircraft are balloons, dirigibles, traditional fixed SUMMARY OF THE INVENTION wing aircraft, flying wings and helicopters.

One functional role that a few aircraft have been designed The present invention addresses the needs mentioned to fill is that of a high altitude platform. Operating from high, above by providing an aircraft that can operate at high alti- suborbital altitudes, such aircraft can monitor weather pat- 40 tudes, carry substantial payloads, and/or remain aloft for long terns, conduct atmospheric research and surveil a wide vari- periods of time.

ety of subjects.

The aircraft of the invention typically includes a laterally Three high altitude aircraft that have been constructed are extending wing, a plurality of pitch-control devices, and a the well-known Pathfinder, Centurion and Helios aircraft, control system configured to control the plurality of pitch- which have set numerous flight records. The basic design 45 control devices. Each pitch-control device is mounted at a concepts underlying these aircraft are discussed at length in separate lateral location along the wing. Each pitch-control U.S. Pat. No. 5,810,284, which is directed toward an unswept device is configured to apply pitch-control torque at its lateral flying wing aircraft having a very high aspect ratio and a location, and the wing is characterized by a torsional flexibil- relatively constant chord and airfoil. While these aircraft are ity high enough for each pitch-control device to separately quite noteworthy for their long term flight potential, they do and substantially control localized pitch at its lateral wing have limits in their available power and payload.

location, i.e., to a degree substantial enough to be significant Such aircraft may be designed as flying wings that include for flight control.

a number of self-sufficient wing sections, each generating The pitch-control device may feature a body, e.g., a boom, enough lift to supportits ownweight. To minimize weight, the connecting the wing to a control surface aft of the trailing aircraft structure is highly flexible, and is designed to with- edge of the wing. Advantageously, the control surface is posi- stand only relatively small torsional loads and moderate 55 tioned at a distance from the wing adequate to provide the bending loads along its lateral axis (i.e., its wingspan). The aerodynamic forces from the control surface with a pitching aircraft's wing has little or no dihedral while on the ground.

effect on the wing to cause changes in the local lift that However, due to high flexibility, the large aspect ratio and the dominate (i.e., are much larger than) the changes in lift that constant chord, in-flight wing loads tend to cause the wing to 60 occur from the redirection of air by the control surface (i.e., develop a substantial dihedral angle at the wingtips, which the flap effect), over the entire flight envelope. Thus, aileron may not be optimal for a given wing strength. Thus, there is a reversal is not an issue.

tradeoff between the structural weight of the aircraft and the desirability of the wing shape. The invention further features that the control system is There is an inherent relationship between an aircraft's configured to operate the pitch-control devices under proto- overall airframe geometry and the design of its airfoils and 65 cols that will actively control wing dihedral. Advantageously, control surfaces. Typical aircraft offset negative (i.e., nose- under such predetermined protocols, a highly flexible wing down) pitching moments through the use of tail moments can be used while limiting the risk of excessive wing bending.

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Other features and advantages of the invention will become of the wing was not utilized. The torsional flexibility of the apparent from the following detailed description of the pre- Pathfinder, Centurion and Helios aircraft made the use of such ferred embodiments, taken in conjunction with the accompa- control surfaces relatively impractical. Lacking the torsional nying drawings, which illustrate, by way of example, the rigidity of a normal aircraft, the Pathfinder, Centurion and principles of the invention. The detailed description of par- 5 Helios aircraft could suffer from significant control reversal ticular preferred embodiments, as set out below to enable one problems if the control surfaces operated as ailerons. Under to build and use an embodiment of the invention, are not some circumstances, these difficulties also might affect the intended to limit the enumerated claims, but rather, they are operation of the control surfaces as elevators. Thus, the con- intended to serve as particular examples of the claimed inven- trol reversal issue potentially limited the operability of the tion. io aircraft.

For example, a downward control surface deflection on a BRIEF DESCRIPTION OF THE DRAWINGS normal, torsionally stiff wing, would typically be expected to cause additional airfoil section lift (an effect that will be FIG. 1 is an elevational view of an aircraft embodying the hereinafter referred to as a "flap effect"). However, such a invention.

15 deflection will likely cause a significant nose-down pitching FIG. 2 is a plan view of the aircraft depicted in FIG. 1.

(twisting) moment on the wing, which on a torsionally flex- FIG. 3 is a perspective view of the aircraft depicted in FIG.

ible wing can lead to a decreased angle of attack, and thereby 1, in a flexed position that creates moderate dihedral typical of a reduction in overall lift (an effect that will be hereinafter loading under mild flight conditions.

referred to as a "pitch effect"). Under various flight condi- FIG. 4 is a perspective, cutaway view showing the con- tions, a control surface on the trailing edge of a torsionally struction of one portion of one wing segment of the wing of flexible wing can experience one, the other and/or both of the aircraft depicted in of FIG. 1.

these two contrary effects to a significant degree.

FIG. 5 is a block diagram showing a control system and As a result, the response to a movement of the control related components from the aircraft illustrated in FIG. 1.

surface on a highly flexible (in torsion) winged aircraft can be FIG. 6 is a partial plan view of a second aircraft embodying unpredictable. Moreover, over the flight envelope (e.g., the invention.

through variations in flight speed), the response can vary FIG. 7 is a partial plan view of a third aircraft embodying between having one of the effects dominate, having the other the invention.

dominate, having the two cancel each other out, and having the two cyclically operate with one lagging the other to drive DETAILED DESCRIPTION OF THE PREFERRED so the wing in a potentially unstable forced vibration (i.e., flut- EMBODIMENTS ter) having both bending and torsional components.

With reference to FIGS. 1-3, a first preferred embodiment The invention summarized above and defined by the enu- is a flying wing aircraft 10, i.e., it has no fuselage or empen- merated claims may be better understood by referring to the 35 nage usable to control the overall pitch of the aircraft (as a following detailed description, which should be read in con- typical aircraft would have). Instead, it consists of an junction with the accompanying drawings. This detailed unswept, laterally extending wing 12 similar to that of the description of a particular preferred embodiment, set out Centurion aircraft, having a substantially consistent airfoil below to enable one to build and use one particular imple- shape and size along the wingspan. Fourteen motors 14 are mentation of the invention, is not intended to limit the enu- situated at various locations along the wingspan, each motor merated claims, but rather it is intended to serve as a particular driving a single propeller 16 to create thrust. Four vertical fins example thereof.

18a-18d, or pods, extend down from the wing, with landing In accordance with the present invention, a number of gear at their lower ends.

preferred embodiments of an aircraft of the present invention The aircraft 10 is longitudinally divided into preferably are of designs similar to those of the Pathfinder, Centurion 45 five modular wing segments sequentially located along the and/or Helios aircraft, as mentioned above in the Background lateral wingspan. These include a center segment 20, left and of the Invention. While the embodiments' designs, and varia- right intermediate segments 22, 24, and left and right wingtip tions of them, are described below, further details useful for segments 26, 28. These wing segments preferably range from the practicing of this embodiment of the invention are pro- 39 to 43 feet in length, and have a chord length of approxi- vided in U.S. Pat. No. 5,810,284, which is incorporated mately eight feet. Alternative variations of the embodiment herein by reference for all purposes. Nevertheless, it is to be understood that designs for other embodiments of the inven- may be highly flexible flying wing aircraft that are unitary (i.e., not segmented), but are nevertheless highly flexible.

tion can differ substantially from the described aircraft.

Like the Pathfinder, Centurion and Helios aircraft, the pre- With reference to FIGS. 2, 3 and 4, one or more of the wing ferred embodiments may be flying wings. These embodi- segments of the aircraft 10, and preferably at least three wing ments include a plurality of laterally connected, wing seg- 55 segments (as depicted) (and/or up to and including all of the ments that preferably can each support their own weight in wing segments) each include a pitch-control device 42, each flight so as to minimize inter-segment loads, and thereby pitch-control device being mounted at a separate lateral loca- minimize required load-bearing structure. These embodi- tion along the wing. The pitch-control device is preferably a ments have aircraft control systems configured to control the boom 44 extending longitudinally aft and holding a prefer- flexible development of wing dihedral during flight, and 6o ably horizontal control surface 46 in a position preferably aft thereby further control inter-segment loads. of the trailing edge of the wing 12. For the purposes of this The Pathfinder, Centurion and Helios aircraft had trailing- application, it should be understood that a "horizontal" sur- edge control surfaces configured as trailing edge flaps (or face is one extending in a direction having a horizontal com- "wing-mounted elevators" on the trailing edge of the wing). ponent, that is adequately horizontal to impart control forces These control surfaces were not configured to act differen- 65 having a relevant vertical component. In alternative embodi- tially. The coordination of the wing trailing edge control ments, the pitch-control device could include both a fixed surfaces to prevent contrary movement on different portions horizontal surface and an active control surface.

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The three wing segments having pitch-control devices are Thus, the aircraft of this embodiment might have a chord- preferably an inboard wing segment (e.g., the center segment wise length of roughly 20 feet, with a wing segment chord- 20) and two outboard wing segments (e.g., the end segments wise length of eight feet, and a wingspan of approximately 26, 28). Thus, the flying wing preferably includes at least 3 200 feet. The structure is configured to be lightweight, with pitch control devices, which are preferably located symmetri- 5 significant flexibility in vertical bending (allowing for signifi- cally across the wing. cant dihedral bending) and spanwise torsion (allowing for significant relative pitching).

Each such pitch-device control surface 46 is configured for With reference to FIGS. 2, 4 and 5, the embodiment rotationally deflecting relative to the boom 44 such that a includes an electronic aircraft control system 52 configured to controllable, preferably vertical aerodynamic force is applied control the operation of the aircraft. The aircraft control sys- to the boom aft of the trailing edge of the wing. The force 10 tem includes a structural control system 54 configured to applied to the boom is preferably normal to the longitudinal control structural bending of the aircraft, and a flight control dimension of the boom, and at a distance from the wing system 56 configured to control the flight of the aircraft.

segment on which it is mounted, such that a torsional force is Because these two functions may be significantly interre- applied to the wing segment at or about the lateral location to 15 lated, the structural control system and flight control system which the boom structurally connects to the wing segment.

are likely to significantly interact within the overall aircraft Moreover, the wing 12 is characterized by enough torsional control system 52.

flexibility in the lateral locations of each pitch-control device Both the structural control system 54 and the flight control 42 to separately control localized pitch of the wing at and/or system 56 receive data from numerous sources. One such near its lateral wing location. In this application, the termi- source is a communications unit 61 configured to receive nology "separately control' should be understood to mean 20 instructions from a ground controller (e.g., a ground-based that the pitch-control devices are physically independent such pilot). Another source is a plurality of flight parameter sensors that each could in theory be commanded to operate in a 63, preferably including one or more of the following sensors: manner different from the others.

a positional sensor (e.g., a GPS), a heading sensor, a pitch This control over localized pitch is to a degree substantial 25 sensor, a roll sensor, a yaw sensor, an altimeter, a flight speed enough to be significant for flight control (i.e., for control of sensor, a vertical speed sensor, a slip sensor, a pitch rate the response of the aircraft structure to aerodynamic forces, sensor, a roll rate sensor, and a yaw rate sensor. A third source so as to change the aircraft structural configuration (e.g., wing is a plurality of structural sensors 65, preferably including one dihedral and/or bending load) and/or the aircraft flight or or more of the following sensors: vertical wing bending sen- orientation). The position and configuration of each pitch- 30 sors, fore-and-aft wing bending sensors, wing torsion sen- control device preferably limits any flap effect it has on the sors, motor speed and/or thrust sensors, control surface wing segment (in response to deflection of the control sur- deflection and/or force sensors, and solar sensors configured face) such that the pitch effect is dominant over the entire to detect the exposure of the structure to sunlight. Each of flight envelope of the aircraft. In other words, the change in these sensors is of a type either known in the art (e.g., strain vertical force from movements of the pitch-control device 35 gauges and positional sensors), or that can be formed with a control surface, are significantly less than the change in lift combination of known sensors.

experienced by the wing due to the resulting change in local In some cases, one or more sensors of one type may serve wing pitch.

the function of the sensor of another type. For example, a Each pitch-control device boom 44 connects the control plurality of pitch sensors and/or pitch rate sensors laterally surface 46 to the wing 12 at a distance aft of both the spar 40 40 positioned along the wing may provide data to analytically and the trailing edge 48 of the wing adequate to cause the determine wing torsion, which might otherwise be detected control surface pitch effect to dominate the control surface with strain gauges.

flap effect. This is distinctive from a normal aircraft, for The structural control system 54 and the flight control which wing-mounted control surfaces are intended to operate system 56 may each contribute to command instructions sent using a dominant flap effect.

45 to a number of aircraft systems. The systems receiving com- Optionally (as depicted in FIG. 4), additional, flap-effect mand instructions to control their operation include the con- control surfaces 50 could be incorporated into the trailing trol surfaces (e.g., pitch-control device control surfaces 46, edge of the wing, particularly in locations structurally close to and flap-effect control surfaces 50) and the motors. As noted (e.g., within a spanwise area torsionally affected and/or con- above, in some cases the structural sensors will be of a type to trolled by) a pitch-control device 42. These trailing-edge 50 sense the operation of the control devices (e.g., the control control surfaces could be limited in use to flight regimes surfaces and/or the motors).

where in their response would be predictable, or could be Using the aircraft control system 52 and the pitch-control used in concert with a pitch-control device to produce desired devices 42, aircraft dihedral is controlled by having the struc- effects (e.g., the trailing edge control surface could control lift tural control system 54 cause aircraft control system com- while the pitch control device limits the wing pitch resulting mands to be sent to the pitch-control devices to initiate control from movements of the trailing edge control surface). Alter- movements of their control surfaces 46 using a protocol that natively, the pitch-control devices may be the only control controls the pitch of their respective lateral locations on the surfaces (or the only horizontal control surfaces) on the air- wing, and relatedly affect their wing segments and/or nearby craft.

portions thereof (and possibly the pitch of nearby wing seg- The overall length of the pitch-control device as measured 60 ments). In particular, outboard pitch-device control surfaces back from the elastic axis of the wing, and its control surface 72 are directed to actuate downward (i.e., trailing edge down), size, may be experimentally or analytically determined to causing their respective wing segments 26, 28, or portions of meet the criteria of minimizing overall weight and drag, while their respective wing segments to pitch downward (i.e., lead- providing for the pitch effect to be the dominant effect over ing edge down) and thereby decrease the overall lift generated the entire desired flight envelope. Possible pitch-device 65 by the respective outboard wing segments.

lengths that might be considered, as multiples of the wing Simultaneously, inboard pitch-device control surfaces 74 fore-and-aft length (i.e., chord length), include 1.5 and 3. are directed to actuate upward, causing their respective wing

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7 8

segments, or portions of their respective wing segments 20 to on the wing 12 (i.e., the use of paired motors and pitch- pitch upward and thereby increase the overall lift generated devices limits the shear forces and fore-and-aft bending of the by the respective inboard wing segments. As a result, with wing due to moment arms between the thrust of the nearest inboard lift increased and outboard lift decreased, overall motor(s) and the drag of the pitching device). The depicted wing dihedral may be controllably reduced, eliminated, and/ 5 outboard pitch-control devices are paired with motors.

or controlled to achieve desired wing configurations and Optionally, the wing may include additional motors that are desired wing stress levels. not paired with pitch-control devices (as depicted for the The aircraft control system is thereby configured to control inboard pitch-control device). The motors may optionally be the plurality of pitch-control devices under a protocol (i.e., a controlled by a motor control system 58, (which may be part detailed plan or procedure) that controls wing dihedral io of the aircraft control system) that is configured to control the according to a predetermined program. Such a program will operation of the motors such that the unpaired motors (i.e., typically include dihedral limits (e.g., maximums dictated by motors not paired with a pitch-control device) are operated at flight efficiency and structural limits, and optionally mini- a lower thrust level than the paired motors, the difference mums dictated by flight control issues, possibly varying over being at or about the anticipated or actual level of pitch-device the entire flight envelope), and dihedral schedules (such as 15 drag, which may vary by flight condition and control surface ones based on maximizing the exposure of wing solar cells to position. Likewise, two or more motors near an unpaired sunlight, ones based on optimizing the positions of onboard pitch-control device may be controlled by the aircraft system instrumentation, or ones based on stability and control param- controller to provide relatively increased thrust in a propor- eters). The protocol may include control inputs that are sym- tional amount based on their lateral positions relative to the metric, such as ones to increase or decrease dihedral, control 20 pitch-control device.

inputs that are inverted on opposite sides, such as ones to roll As a result, the motor control system is configured to the aircraft, and possibly even control inputs that are asym- separably control the thrust from the plurality of motors to metric. reduce fore-and-aft wing loads between the motors. Option- In order to optimize flight efficiency by reducing drag, the ally, the motor control system may optimize this function aircraft control system dihedral schedule may be configured 25 using flight data and sensory information regarding wing (i.e., the protocol may include command procedures) to cause strain, actual thrust and actual structural configuration (e.g., the dihedral to be less when the sun is high in the sky, or when wing bending, wing torsion and other related parameters).

it is night. This allows the aircraft to optimize the tradeoff The aircraft 10 controls yaw, and thereby turns, using dif- between power generation and flight efficiency. To accom- ferential thrust from varied motor torque on the propellers 16.

plish this end, the control system determines a dihedral con- 30 It uses a combination of sideslip and dihedral to control bank figuration to increase the power generated by solar cells, angle. Optionally, the pitch-control devices could be used to should they be present. This can be done by simply reading a create varied lift over the wingspan, and thereby control bank clock signal from a clock within the aircraft control system angle without large side slip issues. Other known methods or and adjusting the dihedral (and possibly the heading) based mechanisms for creating differential thrust could also be on the anticipated light conditions. More preferably, the con- 35 used.

trol system can detect the light conditions, either through The aircraft relies upon its large wingspan and relatively signals from light sensors, or from indications of the power low velocities to avoid yaw instability. Roll may be controlled levels generated by one or more of the solar cells. passively by the wing being maintained with a positive angle As suggested above, in some situations it might be desir- of dihedral, and/or by using the pitch-control devices to create able to increase wing dihedral. To do so, the reverse of the 4o differential lift across the wingspan.

above-recited operation is conducted. More particularly, out- The aircraft may further include inter-segment hinge board pitch-device control surfaces 72 are directed to actuate mechanisms and hinge locks, as described in U.S. patent upward, causing their respective wing segments, or portions application Ser. No. 10/310,415, filed Dec. 5, 2002, which is of their respective wing segments, to pitch upward and incorporated herein by reference for all purposes. The struc- thereby increase the overall lift generated by the respective 45 tural control system may further control the pitch-control outboard wing segments. Simultaneously, inboard pitch-de- devices to actuate the inter-segment hinge mechanisms (i.e., vice control surfaces 74 are directed to actuate downward, acting as hinge actuators), as described in that application.

causing their respective wing segments, or portions of their The hinge locks (i.e., hinge-rotation locks) can be either respective wing segments, to pitch downward and thereby within the hinge mechanisms, or otherwise controlling them.

decrease the overall lift generated by the respective inboard 50 When a rotational lock is in an unlocked configuration, hinge wing segments. actuators allow the relative rotation of respective wing seg- As a result of the above design, the preferred embodiment ments. When the rotational lock is in a locked configuration, of the aircraft is light, travels at relatively slow air speeds, and the hinge mechanism is restrained, and the respective wing has a configuration controllable to limit stresses on its indi- segments are prevented from rotating with respect to each vidual components. Optionally, the control system may other, thereby maintaining the wing's dihedral configuration.

receive input from sensors configured to detect the configu- The aircraft may optionally feature additional, non-aero- ration (e.g., the relative position, orientation, bending and/or dynamic mechanisms (as described in the above-noted appli- torsion) of the aircraft and/or individual wing segments cation), configured to affect the local wing pitch (i.e., pitch- thereof. Thus, the aircraft control system may actively control control devices) and/or to control the rotation of the hinge the aircraft configuration to be maintained within structural 60 mechanisms, thereby adding further controllability to the safety limits (e.g., for the bending stresses to be maintained wing configuration and/or the operation of the hinge mecha- within safety limits) and within an optimum flight configura- nisms. These mechanisms may include CG-movement tion range, even when the aircraft encounters undesirable devices (i.e., devices configured to change the center of grav- flight conditions such as turbulence. ity in a particular area of the wing so as to affect its pitch Preferably the pitch-control devices 42 are each paired 65 and/or roll). It is preferable that there be asymmetric arrange- with (i.e., located substantially aft of) a motor 14, thus poten- ment of hinge mechanisms on the aircraft, along with a sym- tially limiting the effects of drag from the pitch-control device metric arrangement of pitch-control devices.

US 7,802,756 B2

9 10

Additional configurations, such as aircraft configured to It should be understood that a wing that is uniformly (and deflect into W-shapes or M-shapes are also within the scope highly) flexible can be considered as having a number of of the invention. Such configurations having alternating posi- highly flexible regions. The term highly flexible should be tive and negative dihedral can reduce wing loading for flight understood to represent a level of torsional flexibility wherein conditions in which it is desirable to have significant side 5 but for any pitch-limiting devices (i.e., if they weren't there), exposure of the wing surfaces (such as when the sun is low on one or more ailerons would experience aileron reversal over the horizon). Furthermore, aircraft with only two pitch-con- some portion of the flight envelope.

trol devices or only one pitch-control device are also within While the pitch-limiting devices couldbe active horizontal the possible scope of the invention, particularly when com- control surfaces controlled by a control system to limit wing bined with a structural control system implementing proto- io pitch, or a combination of a control surface and a fixed hori- cols as described above. zontal surface, preferably the pitch-limiting devices include While the described embodiments of active dihedral con- only one or more fixed horizontal surfaces mounted aft of the trol are employed on an aircraft having numerous, flexible, wing. More particularly, each pitch-limiting device prefer- non-swept wing segments of constant airfoil and chord, they ably includes a body (e.g., a boom) connecting the wing to a can likewise be employed on other aircraft designs including 15 fixed surface aft of the trailing edge of the wing at a distance conventional aircraft, and even biplanes. adequate to cause the flap effect of the proximate ailerons to More particularly, with reference to FIG. 6, another dominate the pitch effect over the entire flight envelope. The embodiment may be a conventional aircraft provided with a primary function of the pitch-limiting devices 507 is control- flexible wing 401, which supports a fuselage 403, and ling and/or preventing local wing torsion and bending, and includes a number of highly flexible regions 405 capable of 20 thereby allowing ailerons to function properly without expe- significant independent wing torsion. Eachregion has a pitch- riencing aileron reversal.

control device 407 that controls the pitch of that region, and Advantageously, the features described above with respect reacts any negative pitching moments of that region's cam- to the various embodiments can provide various advantages.

bered airfoil. The aircraft wing 401 will preferably include at By allowing for high torsional flexibility, torsion-carrying least one pitch-control device 407 on each side of the fuselage 25 wing structure can be limited, reducing the weight of the 403 in a symmetric formation. Preferably (though not neces- aircraft and thereby potentially increasing its payload capac- sarily), the fuselage carries an empennage (not shown) that ity. Moreover, by controlling wing bending loads, wing spar includes typical horizontal control surfaces, and/or other weight can be reduced. Furthermore, by providing control fuselage-mounted pitch-control surfaces (e.g., a canard). over the structure, potentially expanded flight envelopes are Preferably, the primary function of the pitch-control 30 available to the aircraft. Improved stability and control may devices 407 is controlling and/or preventing local wing tor- be obtainable using controlled wing shape (e.g., dihedral), as sion and bending, but overall flight control can also be a well as improved flutter characteristics (which again provide primary or secondary function. Overall aircraft pitching for expanded flight envelopes). Moreover, the increased moments can also be reacted by the fuselage-mounted pitch- structural weight of the devices may be partially offset by the control surfaces. An aircraft control system preferably con- 35 elimination of ailerons and/or wing-mounted elevators.

trols both the pitch-control devices and any fuselage-mounted From the foregoing description, it will be appreciated that pitch-control surfaces to those ends, and preferably receives the present invention provides a number of embodiments of a input from various sensors, as described with reference to the lightweight aircraft capable of both stationkeeping and flight first embodiment. over a wide range of speeds, while consuming low levels of While the above-described pitch-control devices actively 40 power, for an extended period of time, while supporting an control local wing pitch, another embodiment of the invention unobstructed communications platform, and while exhibiting uses passive controls (i.e., pitch-limiting devices) so as to simplicity and reliability allow the use of ailerons on a highly flexible wing without Other embodiments within the scope of the invention experiencing aileron reversal. While an aircraft with a fuse- include devices comprising forward extending booms con- lage is described in the embodiment below, other embodi- 45 figured with canards, and CG-movement devices. Likewise, ments may be of other configurations, such as flying wings other embodiments of the invention could have other numbers like those described above. of wing segments, including variations with an even number With reference to FIG. 7, another embodiment may be a of wing segments (e.g., six wing segments), and other num- conventional aircraft provided with a highly flexible laterally bers of motors. For example, an embodiment similar to the extending wing 501, which supports a fuselage 503, and 50 Helios aircraft might be configured with six wing segments, includes a number of highly flexible regions 505 capable of 10 motors, and anywhere from two to six (or possibly more) significant independent wing torsion. A plurality of ailerons independent pitch-control devices. Likewise, a simple 506 are mounted at various lateral aileron-locations in the embodiment might include three wing segments with one to highly flexible regions along the wing. three motors and two or three (or perhaps even one) indepen- A plurality of pitch-limiting devices 507 are mounted at dent pitch-control devices, or might even be a very long separate lateral pitch-limiting-locations along the wing. Each unsegmented wing with one or more motors and a plurality of pitch-limiting device is configured to apply a pitch-limiting independent pitch-control devices.

torque at its pitch-limiting-location. Each pitch-limiting-lo- While a particular form of the invention has been illus- cation is proximate the aileron-locations of one or more aile- trated and described, it will be apparent that various modifi- rons. Thus, each region has a pitch-limiting device 507 that 60 cations can be made without departing from the spirit and limits the pitch of that region in response to aileron deflection. scope of the invention. Thus, although the invention has been The aircraft wing 501 will preferably include at least one described in detail with reference only to the preferred pitch-limiting device 507 on each side of the fuselage 403 in embodiments, those having ordinary skill in the art will a symmetric formation. Preferably (though not necessarily), appreciate that various modifications can be made without the fuselage carries an empennage (not shown) that includes 65 departing from the invention. Accordingly, the invention is typical horizontal control surfaces, and/or other fuselage- not intended to be limited by the above discussion, and is mounted pitch-control surfaces (e.g., a canard). defined with reference to the following claims.

US 7,802,756 B2

11 12

We claim: one or more control surfaces would experience aileron 1. An aircraft characterized by a flight envelope, compris- reversal over some portion of the flight envelope.

ing: 2. The aircraft of claim 1, wherein the pitch-limiting • laterally extending wing; devices include only one or more fixed horizontal surfaces.

• plurality of control surfaces mounted along the wing; and s 3. The aircraft of claim 1, wherein each pitch-limiting • plurality of pitch-limiting devices, each pitch-limiting device includes a boom connecting the wing to a fixed surface device being mounted at a separate lateral pitch-limit- aft of the trailing edge of the wing at a distance adequate to ing-location along the wing, each pitch-limiting-loca- cause the flap effect of the proximate control surfaces to tion being proximate the location of one or more of the dominate the pitch effect over the entire flight envelope.

control surfaces; io 4. The aircraft of claim 3, wherein the control surfaces are wherein each pitch-limiting device is configured to apply a configured to operate as ailerons.

pitch-limiting torque at its pitch-limiting-location; and wherein the wing is characterized by a torsional flexibility high enough such that but for the pitch-limiting devices,