GB2328657A - Aircraft with orientable solar-power panels - Google Patents

Abstract

A solar-powered aircraft comprises wings 10, 11 secured to twin booms 12, 13 which are connected by a stabilator 15. Propellers 31 to 34 powered by electric motors are provided at the wings 10, 11 and at the forward ends of the booms 12, 13. For energy supply of the electric motors, panels 21, 22, 25, 26 equipped with solar cells are arranged on the booms 12, 13. The panels 21, 22, 25, 26 are pivotable about their longitudinal axes 23, 24, 27, 28 and thus orientable relative to the aircraft to improve the energy yield from the sun. A neutral point of the panels 21, 22, 25, 26 is arranged to coincide with the centre of gravity for stability. Alternatively a thin solar film (60 fig 2e) is secured to the aircraft by a guy wire and orientated by control tabs. Various other arrangements of solar panels are described (figs. 2a-2d) as well as details of the ways in which the panels may be folded or telescoped to reduce drag (figs. 3a-4b).

Description

1 2328657 Solar-Powered Lightweight Aircraft The invention relates to a

solar-powered lightweight aircraft, more particularly a solar-powered drone, as set 5 forth in the preamble of claim 1.

Such aircraft are known, for example, from DE-OS 26 16 000, US 4,697,761 or DE 296 16 989 Ul.

Solar-powered drones are sola r-powered lightweight aircraft which are employed at altitudes exceeding 10,000 m for surveillance purposes. Solar-powered drones find application e.g. in monitoring the weather, the earth%s surface, air space surveillance, for telecommunication purposes or as defence system components. Furthermore, solar drives are also used for powered gliders, for example.

Solar-powered aircraft comprise wing-mounted photovoltaic solar cells for generating electric current made use of to power pro pellers by means of electric motors. Since solar- powered aircraft need not carry fuel they can be configured as lightweight structures; long flight durations thus being possible with. such aircraft. Where aircraft of this type are equipped with an additional stored energy facility, excess energy obtained during the day can be stored for use at night, thus making night flights possible. Accordingly, flight durations of several months are achievable.

The photovoltaic solar cells are horizontally arranged on the upper side of the wings. Thus, the energy obtained when the sun is low is exceptionally small so that flight durations of several months in the northern hemisphere are possible only to a limited extent.

To make longer flights possible also in winter or to enable the aircraft to carry high payloads the surface area of the wings needs to be made larger so that a sufficient number of solar cells can be provided. Increasing the wing 2 surface area has, however, the disadvantage that the speed in flight is reduced. To be able to "park,' a drone in a certain area its speed in flight needs to be higher than prevailing wind velocities.

Since the upper side of the wings is curved, and the wings need to be elastic, arranging the solar cells on the upper side of the wings is very difficult since solar cells are highly sensitive to pressure. So as to influence the aerodynamics of the wings by providing expansion gaps between the solar cells as well as by the flatness of the solar cells as little as possible, the solar cells need to be arranged with the aid of extremely complicated embedding systems on the wings.

Embedding the solar cells calls for a rugged wing design, this, however, increasing the weight of the wings thus making optimization of the wings as regards weight and surface area impossible. It is particularly in the case of solar cells mounted skew on the wings that additional pertubations materialize, prompting undesirable swirl. In addition, this complicated system of embedding the solar cells is a nuisance in replacing damaged solar cells.

Known from DE 29 51 699 A1 is to apply relatively thin path-shaped strips of metal to each of the left and righthand w:ings of a powered glider on which semiconductor solar cells are arranged. Solar strips of this kind tend to flatter severely in flight, causing a very high drag. on top of this, such thin-film cells have only a very low efficiency so that exceptionally large surface area solar strips would be necessary. as a result of which the drag would be increased even more. In addition to this the solar strips are attached only to the leading edge of the wings, i. e. they needing to develop the aerodynamic lift for their dead weight themselves.

3 It is thus the object of the invention to define a solar-powered lightweight aircraft on which the solar cells are arranged so that a high energy yield for a simultaneously low drag is achievable.

This object is achieved in accordance with the invention by the features as set forth in claim 1. Advantagous further embodiments are the subject matter of sub-claims relating back thereto directly or indirectly.

In accordance with the invention the solar cells arranged on appendages provided outside of the wings, total neutral point of these appendages being located at center of gravity of the aircraft, as a result of which wing structure can be optimized as regards airfoil weight. More particularly the curved wing surface is detrimented by solar cells.

are the the the and not Furthermore, it is now no longer necessary to enlargen the wing surface for transporting high payloads or for flying when the sun is low, so that higher air speeds can be achieved, as a result of which positioning the drone is facilitated. In addition to this there is now no need for the weight of the solar cells to be borne by the wings, thus enabling them to be configured in optimized lightweight design.

Locating the total neutral point of the appendages preferably at the center of gravity of the aircraft has the major advantage that any changes in the angle of attack, due to gusting, for instance, do not result in forces being produced by the appendages generating free moments about the center of gravity and thus irrespective of the position of the appendages their influence on stable flight and steering of the aircraft is very slight.

The appendages are arranged directly on the fuselage, 4 for example, however they may also be provided on the aircraft so that they form the fuselage or parts thereof of the aircraft. Furthermore, the appendages in accordance with the invention can be oriented to face the sun, thus enabling the inclination of the appendages to be oriented as regards the sun so that the surface of the solar cells is always arranged perpendicular to the solar radiation. Being able to orient the solar cells in this way permits an optimum energy yield.

This is why the lightweight solar-powered aircraft in accordance with the invention can also be put to use for long flight durations even when the sun is low, thus making lengthy flights possible in winter, too. Energy yield early in the morning and in the evening is optimized also in summer so that the aircraft can be parked at high altitudes, or the energy required to power the aircraft during the night can be made available by a lesser number of solar cells, this in turn having the advantage that the all-up weight is reduced.

Preferably the appendages are orientable by being pivotable about at least one axis, they being provided, for example, as rectangular surface areas the longitudinal axis of which runs parallel to the longitudinal axis of the aircraft. To minimize appendage drag the long itudinal axis of the aircraft needs to run parallel to the trajectory in the design point of lightweight aircraft. Such appendages may be secured by mounting fixtures running parallel to the fuselage. Furthermore, the fuselage itself may be configured as a tubular structure serving to secure the appendages.

For their orientation the appendages are pivotable about their longitudinal axis. One such mounting fixture may also be provided to mount several appendages, the longitudinal axis of which is oriented parallel to the longitudinal axis of the aircraft. Such appendages are preferably pivotable both about a longitudinal axis and about an axis running parallel to the aircraft longitudinal axis. Due to this second axis of rotation the appendages can be set independent of the flight regime of the aircraft so that no forces materialize thereon, the reponse of the lightweight aircraft thus being independent of the size, location and position of the appendages. Mounting fixtures for the appendages may also be provided on the wings of the aircraft.

Preferably, adjusting the appendages is done via self- retarding gearing powered by electric motors, thus resulting in only minor forces in setting the appendages for a low power consumption. It is furthermore possible to arrange control tabs on the appendages for orientation. Orienting the appendages may be done by computing the level of the sun relative to the aircraft, this requiring the precise position and attitude of the aircraft, the time of day and date to be known.

In one preferred aspect of the appendages, the appendages are configured so that their surface area can be made smaller, as a result of which the drag caused by the appendages, for instance in night flight, can be considerably reduced. For diminishing the surface area of the appendages they consist for example of several interarticulated subappendages rendering the appendages collapsible.

In accordance with a further embodiment the appendages consist of several telescopic sub-appendages, it being furthermore possible to provide flexible solar foils as the appendages which can be rolled up to reduce the. surface area.

To boost the energy yield solar cells having two-sided effectiveness may be applied to a transparent panel structure, as a result of which use can be made of the radiation both directly and reflected from the earth with practically no change in weight.

6 When the aircraft is employed for surveillance the location and attitude of the aircraft is continually sensed for instance by a GPS system - so that this data can be made use of in computer tracking the appendages relative to the level of the sun. As an alternative, brightness sensors may also be provided on the appendages for tracking.

For boosting the energy yield solar cells may also be provided on the wings and/or empennage and/or fuselage in addition to the appendages. Since these solar cells merely serve to generate extra energy they may be arranged on the wings for example so that the airfoil geometry is not effected, or only to a negligable degree.

is The invention will now be detailled on the basis of preferred embodiments with reference to the attached drawings in which:

Fig. 1 is a schematic perspective view of a preferred embodiment of a solar-powered lightweight aircraft in accordance with the invention; Figs. 2a thru 2e are schematic plan views of various possible arrangements of appendages; Fig. 3 appendage; is a schematic cross-sectional view of an Figs. 3b thru 3e are cross-sectional views of various embodiments of collapsible appendages; Fig. 4a is appendage, and a cross-sectional view of a telescopic Fig. 4b illustrates the appendage as shown in Fig. 4a but in the telescoped condition.

7 Referring now to Fig. 1 there is illustratedthe embodiment of a solar-powered lightweight aircraft comprising wings 10, 11. Connected to the wings 10, 11 is a twin boom fuselage, a first tubular boom 12 being connected to the wing 10 and a second boom 12 to the wing 11. The two tubular booms 12, 13 are oriented parallel to the longitudinal axis of the aircraft 35 and are connected to each other by means of a stabilator 15 to stabilize the aircraft. A vertical tail 16 and 17 respectively is provided at each aft end of the booms 12, 13.

In the region of the booms 12, 13 appendages 21, 23 are joined to the booms 12 and 13 respectively between the wings 10, 11 and the stabilator 15. These appendages 21, 22 are equipped with solar cells (not shown). The rectangular configured appendages 21, 22 are pivotable about their longitudinal axes parallel to the booms 12, 13.

Rectangular appendages 25, 26 are likewise arranged on the portions of the booms 12, 13 protruding forward beyond the wings 10, 11. Corresponding to the appendages 21, 22 the appendages 25, 26 too, are pivotable about their longitudinal axes 27, 28 oriented parallel to the booms 12, 13. Accordingly, the appendages 21, 22, 25, 26 can be oriented about their longitudinal axes 23, 24, 27, 28 to face the sun.

The energy having been generated by the solar cells arranged on the appendages 21, 22, 25, 26 mainly serves to power the propellers 31, 32, 33, 34 as well as to pivot the appendages by electric motors (not shown) Each of the propellers 31, 32, 33, 34 is powered by an electric motor (not shown) The propellers 31, 34 are provided at the forward end of the booms 12, 13 and thus arranged directly ahead of the appendages 25, 26. The two other propellers 32, 33 are arranged on the wings 10 and 11 respectively.

The aircraft as shown in Fig. 1 comprises furthermore an 8 energy storage element (not shown) which is charged by means of the excess energy generated by the solar cells during the day. The solar cells provided on the appendages are designed for this purpose so that at least enough excess energy is generated during the day needed to power the aircraft during the following night flight.

An electrolyser is employed, for example, as the energy storage element in which water is dissociated into H2 and 02, During the night flight the stored H2 and 02 gases are converted in a fuel cell into electric current for driving the propellers 31, 32, 33, 34. These gases may be stored, for example, in the tubular wing struts or the booms 12, 13.

is Since the booms 12, 13 are arranged symmetrical and parallel to the longitudinal axis 35 of the aircraft and the appendages 21, 22, 25, 26 are arranged on the tubular booms 12, 13 the total neutral point is located at the center of gravity of the aircraft when the appendages are correct in attitude and size.

Calculating the total neutral point can be done by means of the following equations relating to calculating a total neutral point of the appendages 21, 22, 25, 26 as shown in Fig. 1, the parameters indexed 1 each relating to the forward appendages 25, 26 and the parameters index 2 to the aft appendages 21, 22.

The meanings of the individual parameters along with their units are listed in the following Table:

9 Parameter Meaning Un i t CA S lift coefficient (aircraft as a whole) reference surface area m2 speed m/s plate width m plate length m pitot pressure neutral point location aspect ratio Apt-bpt/lpt angle of attack downdraught angle total value plate v bpt lpt q XN 10 A kg/ (m. S2) m (X rad rad (XW ges Pt The total neutral point xN,Ptges can be calculated from equation (1) as follows:

CAa.ht - XN.M + CAa.Pa - qpa Spw aaw). XN.Pa qpt, SPO qpa Spa CAa.Ptl CAa.M - qpt, SPU X14 = (1) In a first approximation the pitot pressure qptl of the 25 forward appendages 25, 26 can be made equal to the pitot pressure qpt2 of the aft appendages 21, 22. Furthermore, in a first approximation the change in the downdraught angle relative to the angle of attack can be made zero, equation. (1) for the total neutral point then being simplified to equation. (2) CAa.PO XKNI + CAa.PL2 tu. X N.M Shl SPt2 CAC.Ptl + C-Aa^2 S XN.Ntes = (2) The lift coefficient CA(x,pt of each appendage 21, 22, 25, 26 contained in equation (2) as a function of the angle of attack can be calculated from equation (3):

2. z - Ap, 2 + 4 + A2pc (3) The distance XN,pt of the neutral point of a flat plate away from the leading edge of the plate can be approximated by equation (4):

N.Pt 2 (4) 1.5+4225+16 Apt The arrangement of the appendages 21, 22, 25, 26 as shown in Fig. 1 has the advantage that the weight of the appendages can be maintained relatively small since the longitudinal axes and thus the resulting forces are small.

Furthermore, the drag of the appendages is small and the effect on the flow at the wings due to the appendages 25, 26 is likewise small.

Furthermore, hardly any additional weight results from the mounting fixtures of the appendages since the mounting fixtures serve as the booms 12, 13. In addition to this, the pivotability of the appendages 21, 22, 25, 26 is easy to achieve since the corresponding devices for pivoting the appendages can be arranged in the booms 12, 13.

Referring now to Figs. 2a to 2d various embodiments for attaching appendages to receive solar cells on an aircraft are illustrated. The aircraft as evident from Fig. 2a configuration of the corresponds substantially to the 11 aircraft as shown in Fig. 1. Illustrated on the booms 12, 13 are the two appendages 25 and 26 forward of the wings 10, 11. Aft of the wings 10 and 11 the two appendages 21 and 22 are evident on the booms 12 and 13. Furthermore, in the plan view the vertical tails 16 and 17 as well as the stabilator 15 are illustrated. Unlike the Fig. 1 only two propellers 31 and 34 are represented in the schematic illustration of the aircraft as shown in Fig. 2a.

The configuration of the aircraf t as shown in Fig. 2b corresponds to that of the aircraft as evident from Fig. 1 except that two appendages 51 each instead of the integral appendages 25, 26 are attached to the booms 12, 13 forward of the wings 10, 11, i.e. a total of four appendages is provided. Aft of the wings 10, 11 three appendages 52 are attached to the booms 12, 13 instead of each of the integral appendages 21, 22, i.e. a total of six appendages is provided, the longitudinal axis of each being oriented perpendicular to the booms 12, 13. Due to the low profile of the appendages they can be designed to minimize the drag of the appendages.

Each of the appendages 51, 52 is arranged symmetrical to the booms 12, 13 so that each of their centerlines coincides with the longitudinal axis of the booms 12 'and 13 oriented parallel to the direction of flight. All appendages 51, 52 in this embodiment are configured pivotable about both their centerlines and their longitudinal axes, thus making orientation of the appendages along the momentary trajectory irrespective of the direction of flight, as a result of which the appendages 51, 52 produce practically no induced drag in all attitudes.

orienting the appendages 51, 52 about their longitudinal axis is possible simply by the air flow, for the purpose of which the axis of rotation of these appendages needs to be freely movable and located forward of the corresponding 12 neutral point of the appendage. Furthermore, the center of gravity of these appendages should be on their axis of rotation in each case.

The aircraft as shown in Fig. 2c comprises wings 10, 11 which are secured to a fuselage 40 as a single boom 40, the longitudinal axis of which coincides with the longitudinal axis of the aircraft 35. Arranged at the aft end of the fuselage 40 is the tail 41 whilst at the forward end of the fuselage a propeller 42 is provided. Arranged on the wing 10 via mounting fixtures 43 (indicated only schematically) forward of the wing 10 are flat rectangular appendages 44 which are pivotable about their longitudinal axis running parallel to the direction of flight.

is Aft of the wing 10 further appendages 46 are arranged on mounting fixtures 45 level with the appendages 44 so that the longitudinal axis (not shown) of these likewise flat and rectangular appendages 46 coincides with the longitudinal axes of the appendages 44.

Arranged on the wing 11 via mounting fixtures 47 corresponding to the appendages 44 are appendages 48 likewise pivotable about a longitudinal axis (not shown). Additionally arranged on the wing 11 via mounting fixtures 49 corresponding to the appendages 46 are appendages 50 likewise pivotable about a longitudinal axis (not shown).

The embodiment as evident from Fig. 2c has the advantage that the mass of the appendages 44, 46, 48, 50 is evenly distributed over the wing span of the aircraft, thus relieving the load on the wing structure. Furthermore, the embodiment as shown in Fig. 2c can also be put to use in flying wing configurations.

Referring now to Fig. 2d there is illustrated a further embodiment for applying appendages to an aircraft. Between 13 the two booms 12, 13 fore and aft of the wings 10, 11 several appendages 55 and 56 respectively are arranged. The appendages 55,56 are likewise configured flat and rectangular. The longitudinal axes of the appendages 55,56 run parallel to the longitudinal axis 25 of the aircraft and are pivotable about their longitudinal axes. The arrangement of the appendages 55, 56 as evident f rom Fig. 2d creates a large surface area which can be equipped with solar cells. To minimize drag the appendages 55, 56 can easily be shifted together transversely to the longitudinal axis of the aircraft.

Referring now to Fig. 2e there is illustrated an embodiment relative to, for example, a conventional aircraft to which a thin-film solar foil 60 is secured by means of a guy wire 61. For orienting the solar foil 60 control tabs (not shown) may be provided on a mounting f ixture 62 of the solar foil.

The embodiment as shown in Fig. 2e has the advantage that the aerodynamics of the aircraft are not effected by the appendage in the form of the solar foil 60 which has an exceptionally low weight. Furthermore, rolling up and deploying the solar foil 60 can be achieved by simple ways and means. Current technology solar foils have, however, a low efficiency.

Referring now to Fig. 3a there is illustrated a cross section of a rigid appendage 29 equipped with solar cells, the embodiment of this appendage corresponding for example to the embodiments as shown in Figs. 1 and 2a of the appendages 21, 22, 25 or 26. The appendage 29 is secured to a tubular boom 12% by means of a U-shaped mounting fixture 65. The mounting f ixture 65 is thus secured to the boom 12% so that the appendage 20 can be pivoted about its longitudinal axis 23% by means of pivoting devices (not shown).

14 Possible embodiments for collapsing an appendage 291 are evident from Figs. 3b to 3e. For this purpose the appendage 291, as evident from Fig. 3b, consists of several subappendages 66, 67, 68 connected to each other via articulated joints 69, 70. it is thus possible to reduce the drag of the appendage 291, for example during night flight, by angling the appendage 29 down from the position as shown in Fig. 3a by the articulated joints 69, 70 so that the three subappendages 66, 67, 68 are arranged about the boom 12 to form in cross-section an isosceles triangle.

In Fig. 3c equally sized sub-appendages 66%, 67% and 68% of an appendage 292 are jointed to each other articulatedly so that the sub-appendage 66% can be collapsed onto the lower sub-appendage 67% connected by the Ushaped mounting fixture 65, and the sub-appendage 68% comes to rest on the subappendage 66%.

Referring now to Fig. 3d there is illustrated an embodiment in which the appendage 293 is divided into seven sub- appendages, one sub-appendage 71 having the width of the U-shaped mounting fixture 65 and the portions of the appendage 293 protruding beyond in both directions each being divided in three, i.e. into three sub-appendages each the same in size. Each of the three sub-appenclages 72 of the appendage 293 divided into three is jointed to the other articulatedly so that they are collapsible to come to rest on each other in a stack of three. In the collapsed condition each of the three sub- appendages 72 laterally adjoins the mounting fixture 65.

In yet a further embodiment as evident from Fig. 3e two sub-appendages 73 can be collapsed onto a sub-appendage 74 connected to the mounting fixture 65. In this arrangement unlike that of the embodiment as shown in Fig. 3c - the subappendages 73 do not coincide, thus simplifying the configuration of the sub-appendages 73 jointed to each other articulatedly, but that the remaining surface area of the appendage 294 is larger.

Referring now to Figs. 4a and 4b another possibility of reduzing the size of an appendage 295 is illustrated. The appendage 29_5 consists of three sub-appendages 75, 76, 77, the middle sub-appendage 76 being connected to the mounting fixture 65. The other two sub-appendages 75, 77 can be telescoped into the sub-appendage 76 as well as in each other so that the surface area of the appendage 295 can be reduced in size, as shown in Fig. 4b, by telescoping the subappendages 65, 67 and shifting into the middle sub-appendage 66.

16

Claims (1)

1. A solar-powered lightweight aircraft including solar cells for generating energy and including at least one propeller driven by an electric motor, wherein the solar cells are arranged on additional surfaces provided outside the aircraft lifting surfaces, the total neutral point of the additional surfaces being located at the centre of gravity of the aircraft and the additional surfaces being orientable relative to solar irradiation.

2_ An aircraft as claimed in claim 1, wherein each of the additional surfaces is pivotable for orienting about at least one axis.

3. An aircraft as claimed in claim 2, wherein each of the additional surfaces is pivotable about an axis extending parallelly and/or perpendicularly to the longitudinal axis of the aircraft.

4.

An aircraft as claimed in claim 3, wherein at least one of the additional surfaces is oriented about a second axis of rotation by airflow during flight.

5. An aircraft as claimed in any one of the preceding claims, wherein sensors responding to brightness are provided for orienting the additional surfaces.

6. An aircraft as claimed in any one of the preceding claims, wherein adjustment of the additional surfaces is effected by way of self- locking gear means.

7. An aircraft as claimed in any one of the preceding claims--, wherein the additional surfaces are flat.

17 8. An aircraft as claimed in any one of the preceding claims, wherein the additional surfaces are orientable by means of control tabs.

9. An aircraft as claimed in any one of the preceding claims, wherein the surface area of the additional surfaces can be reduced in size.

10. An aircraft as claimed in claim 9, wherein the additional surfaces are formed by surface portions, which are pivotably connected together and collapsible to effect said reduction in size.

11. An aircraft as claimed in claim 9, wherein the additional surfaces comprise a plurality of surface portions which can be telescopically moved together to effect said reduction in size.

12. An aircraft as claimed in any one of the preceding claims, wherein the solar cells comprise double-sidedly effective cells mounted on light-permeable panel structure.

13. An aircraft as claimed in any one of the preceding claims, wherein the additional surfaces are mounted on a fuselage or booms of the aircraft by mounting means.

14. An aircraft as claimed in any one of the preceding claims, wherein mounting means for the additional surfaces are provided on the support surfaces.

15. An aircraft as claimed in any one of the preceding claims, wherein propellers are provided at the additional surfaces.

16. An aircraft as claimed in any one of the preceding claims, wherein additional solar cells are provided at the lifting 35 surfaces and/or at a tail and/or at a fuselage of the aircraft.

18 17. An aircraft as claimed in any one of the preceding claims, comprising means for utilising data from a global positioning system sensing location and attitude of the aircraft to compute tracking of the additional surfaces relative to the position of 5 the sun.

18. An aircraft substantially as hereinbefore described with reference to the accompanying drawings.