GB2085548A

GB2085548A - A Method of Reducing the Total Resistance of Aircraft

Info

Publication number: GB2085548A
Application number: GB8124351A
Authority: GB
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1980-10-17
Filing date: 1981-08-10
Publication date: 1982-04-28
Also published as: FR2492338A1; GB2085548B; DE3039193C2; DE3039193A1

Abstract

In a method of reducing the total resistance of aircraft 10 in accordance with the principle of the active boundary layer thickening, wind turbines 14 are arranged in front of leading edge 11a of the aircraft's wings and on fuselage nose 10a, in the wake whereof the air washed surfaces of the aircraft 10 lie. The shaft power of each wind turbine 14 is transmitted by connection shafts (14a), arranged in each wing or in the fuselage, for conversion into a propulsive force for propellers 15 which are arranged in the region of trailing edge 11b of each wing 11 or fuselage tail 10b respectively. <IMAGE>

Description

SPECIFICATION A Method of Reducing the Total Resistance of Aircraft This invention relates to a method of reducing the total resistance of aircraft in accordance with the principle of active boundary layer thickening.

It is known that a turbulent boundary layer has the property that with a hydraulically smooth wall frictional resistance becomes less with an increasing boundary layer thickness. The frictional resistance of an aircraft could accordingly be reduced in that the air washed surface is embedded into a boundary layer which is thickened in the region of the aircraft's leading edge. If, now, the boundary layer thickening is effected with passive means, then the pressure drag or resistance of this mean exceeds by a multiple the saving in frictional resistance. The flow or aerodynamic resistance of an aircraft, which is composed of the pressure resistance and the frictional resistance, can therefore not be reduced by passive boundary layer thickening.

In German Offenlegungsschrift No. 24 28 683 reference is made to obtuse and sharp-edged vehicles, where the resistance reduction is supposed to be achieved by flow-mechanical cladding of the obtuse bodywork. The frictional resistance or its reduction is not discussed therein.

From German Patent No. 603 034 there has become known a wind turbine which is intended to serve to increase lift in that compressed air is directed by nozzles over the upper side of the wing. This depicted turbofan cannot, however, lead to a reduction in the total resistance (indeed it is not intended for this purpose) since the thrust or cruising fan is acted upon with the full dynamic pressure of the free incident flow. A gain in thrust is thus not achievable, since the propulsion-producing blade wheel (or rotor disc or impeller is not arranged in the wake of the wind turbine. Only in this case, as proposed by the present invention, is the afflux impulse and thus the intake resistance or drag of the thrust generator considerably less than the resisting powers VOO originating in the case of undisturbed afflux from the intake impulse.

The previously-known prior art, with regard to which here additionally German Patent No.

542,471, German Auslegeschrift No. 25 06 974 and United States Patent No. 4 149 688 should be mentioned, has not recognised the possibility of a considerable reduction in resistance.

The problem underlying the present invention is, by active boundary layer thickening, to reduce considerably the resistance of an aircraft and to convert the shaft power produced into a propulsive force and to utilize same.

This problem is solved in that the present invention provides a method of reducing the total resistance of aircraft in accordance with the principle of active boundary layer thickening, characterised in that arranged in front of a leading edge of the aircraft's wing and on the aircraft's fuselage nose are wind turbines in the wake whereof the washed surfaces of the aircraft lie, and the shaft power thereof being transmitted by connection shafts arranged in the wing or in the fuselage for conversion into a propulsive force for propellers which are arranged in the region of a trailing edge of the wing or fuselage tail respectively.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a preferred embodiment of the aircraft in accordance with the present invention; Fig. 2 illustrates the principle of active boundary layer thickening by means of a blade wheel of the aircraft of Fig. 1, said blade wheel being designed as a wind turbine; Fig. 3 is a diagram of the comparison of the velocity profiles downstream of the wind turbine with the boundary layer profile of the same impulse loss thickness; Fig. 4 is a diagram comparing the influence of the wind turbine surface ratio on the drive power ratio for the case of thrust=resistance; Fig. 5 is a diagram comparing the dependency of the drive power ratio on the boundary layer thickening parameter;; Fig. 6 is a diagram comparing the influence of the three efficiencies T, p, and VTW on the power ratio in the case of thrust=resistance; and Fig. 7 is a diagram comparing the resistance ratio in the case of turbine power=propeller power and the power ratio in the case of thrust=resistance.

The present invention provides for the fact that with active means, in the form of a wind turbine, the boundary layer is thickened and the surplus power of the wind turbines, arranged in the region of a wing leading edge of an aircraft, is transmitted by way of connection shafts to propellers arranged in a trailing edge region and is utilised to generate propulsion.

Referring firstly to the preferred embodiment of aircraft shown in Fig. 1, said aircraft 10 is driven by two propeller turbojet engines 13, has on its nose 1 Oa a wind turbine 14, a blade wheel or impeller of which wind turbine 14 is connected by way of a connection shaft 1 4a to a driving propeller 1 5 which is arranged on the aircraft's tail lOb. The shaft power of the two propeller turbojet engines 1 3 is supplied by way of cross shafts 14b, a gearing 1 6 and the connection shaft 1 4a to the driving propeller 15.

Similar blade wheel arrangements are arranged on both sides of the aircraft's fuselage on wings 11, namely at wing leading edge 11 a, the wind turbines 14, at wing trailing edge 11 b and at the propellers 15, in which respect each turbine 14 and propeller 1 5 is connected to one another by way of a connection shaft 14a. In this latter arrangement, however, no additional drive power is supplied to the blade wheel pairs.

Making a start from the fact that the turbulent boundary layer has the property that, with a hydraulically smooth wall, the frictional resistance becomes less with increasing boundary layer thickness, the following conditions apply to the plane or flat plate:

in these equations, Tw designates the wall shear stress, v designates the impulse loss thickness of the boundary layer and P=V/P designates the kinematic viscosity of the air.

Equation (3) shows that the wall resistance at the plate rear edge amounts to only 87 percent of the corresponding value with x equal to 0.5 L and 63 percent of the wall shear stress with x equal to 0.1 L. From this it emerges that the frictional resistance of an aircraft is thereby considerably reduced when in accordance with the aircraft of the present invention the surface area is embedded into a boundary layer which is thickened in the region of the leading edge. If, however, this boundary layer thickening is effected with passive means, then the pressure resistance of these means exceeds the saving in frictional resistance by a multiple. As a result of the measures proposed here, with a relatively slight expenditure in weight the frictional resistance in the case of aircraft is reduced by from 35 percent up to 50 percent.

The present invention will be further explained with reference to an analysis which is shown diagrammatically in Fig. 2. The incompressible plane or flat flow about a thin non-pitched plate of length L, at the leading edge of which there is disposed a non-shrouded wind turbine 14 of height 2H, is to be investigated, in which respect in the wind turbine 14 respectively the total pressure is reduced by 1/2[V002-(V00-Au)2] In the plane of the wind turbine 14 the flow velocity amounts to V=V00-Au/2. Per unit of time, the air mass rnT experiences a specific decrease in energy, so that for the shaft power PT there applies the equation:: P=pHAu (V00-Au/2)2 TiT (4) in which respect TiT designates the turbine efficiency. From the theory of momentum one then obtains for the impeller resistance WT""T Au =pH Au (V00-Au/2) (5) Downstream of the wind turbine 14, the flow velocity drops to the value VOObu. At this location the wake of the wind turbine 14 has a height or level of: H (V00-Au/2)/(V00-Au).

The wake or reciprocal depression has an impulse loss thickness of

If one disregards the transition of the boundary layer profile from the rectangular shape directly downstream of the wind turbine (Fig. 2) to the known shape of the fully fashioned turbulent boundary layer u(y) (=Y) 1/7 (6) Voo # then for the further boundary layer calculation it can be assumed that the boundary layer between leading and trailing edge behaves as if its origin were to lie in the point

At the plate trailing edge then the impulse loss thickness amounts to H Kv=0.03 6 (L0-L)0,8 (#/Voo)0,.2 (7) From this there is calculated the frictional resistance WR:: WR=p V200(VHK - -VVK); (8) for the boundary-layer velocity profile in accordance with equation 6 there applies: 72 AHK= VHK (9) 7 in which respect from the equations 6 and 9 the velocity distribution directly in front of the tail propeller 15 can be calculated.

The individual propeller 1 5 is so designed that the air flowing in the so-called stream tube assumes, far downstream of the aircraft 1 0, a velocity of Voo. The increase in velocity is achieved by raising the total pressure in the impeller by the amount: 1 --P [Vȏo-uHK (y)] 2 From this the energy increase of the air per unit of time is calculated and the impulse rise thereof, from which then thrust Sp and the power consumption of the propeller Pp can be calculated.From the turbine resistance WT, the impulse loss thickness VVK, the frictional resistance WR, the propeller thrust Sp and the propeller power consumption Pp, the resulting propulsive force SRES in the case of PT=PS or respectively the driving power PpPT in the case of Sp=WT+WR can be ascertained.

For the drive power of the conventional comparison aircraft there applies the equation: PR=WR VOJ?1TW; (10) In this respect, designated by WA is the resistance of the flat plate without boundary layer thickening, by PR: the pertinent drive power and by rl, the drive efficiency of the engine.The ratio of the drive power with or without boundary layer thickening in unaccelerated horizontal flight thus amounts to: Pwith 0.9#HK/#P-#T#VK(1-#u/2Voo) (11) Pwithout 0.036 L08(v/VOO)0,VTiTW From this it can be seen that the power ratio depends upon the dimensionless parameters #VK/L, VT, P, TiTw H/UVK and Re,.

The fundamental difference between the efficiencies #T and Tip on the one hand and #TW on the other hand must be stressed. Whilst the drive efficiency 71TW not only takes into account the power losses through friction, shock waves and wake twist, but also the jet loss, this loss is not contained in the impeller efficiencies #T and gp.

The engine efficiency TiTw will therefore generally be considerably less than #T and Tip.

Upon the derivation of the resulting power of resistance WRES for the case that the propeller power consumption is equal to the turbine power delivery, the defining equation for the propeller surface Hpx can be established.

In Fig. 3, the velocity profiles downstream of the wind turbine 14 for several values of the surface ratio H/VvK are compared with the boundary layer profile of the same impulse thickness into which the rectangular profile merges after a certain length of run. In view of a high outer turbine efficiency Tijet=1 -A/2V00, H/VVK should be selected as large as possible (see also Fig. 4). On the other hand, the transition to the resistance-favourable boundary layer profile requires all the more length of run the further the outer wake edge is remote from the wall. A rapid transition to the boundary layer profile is ensured, for example, by a surface ratio of H/VVK=7.

Fig. 5 illustrates the dependency of the power ratio upon the boundary layer thickening parameter sVK/L for two combinations of the efficiencies 71TT )7pr and ijTW It is evident that even without active boundary layer thickening a gain in power is thereby realised, since the propeller 1 5 which is arranged in the "wake depression" or "reciprocal depression" of the aircraft 10 has, by virtue of the slight afflux velocity, a very high propulsive efficiency.

With increasing boundary layer thickening, a further gain in power can be registered, which at a certain thickness ratio reaches a maximum value. Upon a further thickening, however, then the drive power again rises, since now the power losses in both impellers gain importance to an ever stronger degree. Generally it can be said that the most power-favourable thickness ratio (rvK/L)opt is all the greater the less the impeller losses are.

Fig. 6 illustrates the crucial influence of the efficiencies of turbine QT, propeller razz and engine QTW on the power gain which is realisable with the measures proposed in the present invention and in accordance with the applied principle. In Fig. 6, whilst the solid curved lines indicate the respectively greatest power gain, the dotted curved lines apply to the limiting case vvK=O, that is to say that no wind turbine is present. As can be seen, for all conceivable efficiencies a gain in power can be achieved.The present principle becomes particularly interesting in the region VT, Qp O9 and gTW < p In the case of an impeller efficiency of 0.92, which corresponds to the state of the propeller technology about 40 years ago, and an engine efficiency of 0.75 (modern transport aircraft with a two-circuit engine in cruising flight) the resistance saving amounts to 36 percent. Of this, 11 percent can be attributed to the active boundary layer thickening, 10 percent to the favourable propulsive efficiency of the propeller 1 5 working in the wake and 1 5 percent to the low propulsive efficiency of the comparable jet drive.

The impeller efficiency would seem, in the case of modern aircraft, to be able to be raised in 0.96.

In this case, with the aid of the method of the present invention, it would be possible to reduce by 47 percent the power consumption necessary for overcoming the boundary layer friction. As illustrated in Fig. 5, the greatest power saving would be achieved with a boundary layer thickening ratio of vvK/L=0.0035. With H=7VVK, this corresponds to a wind turbine height of H/L=0.0245.

In Fig. 7, the resistance ratios of PfflPp and the power ratio for 5RES=0 with in each case optimum thickening ratio VvK/L are compared with one another. It is shown that the relative reduction in resistance is generally somewhat less than the relative gain in power. This is because the propulsive efficiency, which is considerably more favourable in comparison with the conventional jet drive, of the propeller in the case of free-wheeling impellers is not fully utilized.

Through the proposal, in accordance with the present invention, of thickening artificially the flow boundary layer with the aid of wind turbines 1 5 arranged in front of wing leading edge 11 a and the fuselage nose 1 Oa, it has become possible to reduce the frictional resistance of aircraft quite considerably. The power output of each impeller of the wind turbines 14 is transmitted through fuselage and wing 11 to propellers 1 5 which are arranged in the region of the trailing edge 11 b of each wing 11 or the fuselage tail 1 Oh, where it is converted into a propulsive force. It is self-evident that the achieved reduction in the frictional resistance by from 35 to 50 percent can be converted either into an increase in the aircraft's range or into a corresponding enlargement of useful load which the aircraft can carry.

Claims

1. A method of reducing the total resistance of aircraft in accordance with the principle of active boundary layer thickening, characterised in that arranged in front of a leading edge of the aircraft's wing and on the aircraft's fuselage nose are wind turbines in the wake whereof the washed surfaces of the aircraft lie, and the shaft power thereof being transmitted by connection shafts arranged in the wing or in the fuselage for conversion into a propulsive force for propellers which are arranged in the region of a trailing edge of the wing or fuselage tail respectively.

2. A method as claimed in claim 1, characterised in that, to achieve a highest possible propulsive efficiency, the propellers are operated completely in the total-pressure wake depression of the aircraft, in which respect the propeller area in its blade tip region is reduced relative to that of the root region and thus a velocity distribution which is as uniform as possible is achieved downstream of the aircraft.

3. A method as claimed in claim 1 and 2, characterised in that the aircraft is optionally driven with the aid of PTL engines by way of driving shafts, gearing the connection shaft and tail propeller.

4. A method as claimed in claims 1 and 2, characterised in that a wind-turbine surface ratio of H/bvK=""' is selected.

5. A method of reducing the total resistance of aircraft substantially as hereinbefore described with reference to the accompanying drawings.