US2020235A - Rotor screw suitable for helicopters - Google Patents

Description

Nov. 5, 1935.

J. H. BRADBURY.

'ROTOR SCREW SUITABLE FOR HELICOPTERS Filed Jan. 20, 1934 Patented Nov. 5, 1935 ROTOR SCREW SUITABLE FOR nnuoorrnas John Hinchcliffe Bradbury, Brisbane,

Queensland, Australia Application January 20, 1934, Serial No. 707,602 In Australia January 26, 1933 1 Claim. (Cl. 170156) This invention relates to rotor screws or rotors, the principal object of which is toprovide a means of vertical lift in air, to any device to which they are attached, and lies in the overcoming of certain difficulties which have been encountered by previous investigators.

The accepted theory of the action of the upper surfaces of aerofoils as used in propellers, tractor screws and like contrivances, as presently understood, rests on three main postulates or assumptions. Firstly, that the kinetic thrust of the propeller has its corresponding reaction in momentum or increased momentum imparted to portion of the fluid medium in which it is working, that is to say, that useful force exertedby a propeller indicates, of necessity, that the slip stream of the propeller contains, in and of itself, the same amount of force, but in the opposite direction. Secondly that an increase of velocity of flow in a fluid is accompanied by a reduction of internal pressure, that is to say, that the well known theorem of Bernoulli is literally true as a law of nature, and moreover that the reasoning by which Bernoulli is supposed to have arrived at the theorem, and to justify it, is true in logic and in fact.

which the aerofoil is moving, is in and of itself an indication of the net force with which the aerofoil is being acted on by the said fluid.

For the purpose of, and strictly in relation of,

and incidental to the invention hereinafter to be described, these propositions or assumptions are denied, as follows:-

Firstly, it is asserted that the momentum of fluids,-as fluids, is of a nature so fundamentally and inherently different from the momentum of a useful empiricism, is utterly fallacious in logic and in fact, and that in general an internal pressure, constant for position, remains constant independent of velocity. In this connection regard must be paid to the difference between internalpressure, which in anincompressible fluid is a mere state of being, void of energy, and state of compression, which implies static energy readily convertible into kinetic energy by release of the compression. Bernoulli considered pressure as "identical with force and altogether neglected the obvious friction between fluid in motion and fluid stagnant in a pressure gauge. Thirdly, that as the readings of pressure gauges on fluids in motion are acted on by two forces, (a) actual compression or rarefaction of the fluid and (b) a suc- 5 tional, frictional, or surface-tensional force, operating to draw the fluid out of the pressure gauge and hence alwaysas if the fluid were rarefied below its actual state; moreover since no force can reside in a vacuum and by the same reasoning a negation of force is implicit in any rarefaction; therefore the force acting on an aerofoil can only be arrived at by resolving the two components of the pressure gauge reading, and rejecting rarefaction, probably compression also, in regions where it is diminishing.

Similarly, and further, and denying accepted theory, it is asserted that the useful force operating on the upper surface of an aerofoil to sustain or lift the aerofoil is substantially due to the following train of circumstances.

(a) The movement of fluids towards the nose of the aerofoil causes a progressively increasing compression of the fluid in the frontal vicinity of the whole vertical height of the aerofoil, and beyond on each side.

(1)) The maximum compression is of the order in conventional dynamic symbols.

(c) The compression is accompanied by a flow of fluid up and along the nose of the aerofoil to the region of the summit or beyond it. The velocity of this flow is quite low at the nose tip, but increases rapidly to a limit which exceeds the general velocity of the fluid, for it is the result of two forces, the general velocity of the fluid, and the compression of the fluid, which, in itself contains force sufficient in a theoretic limit to double the general velocity. Experimental evidence indicates that the maximum velocity of flow is reached at some two thirds of the linear distance to the aerofoil summit, from which point it decreases.

(d) Behind the summit the aerofoil surface is sheltered from the flow, and this sheltered area is, as it were, satisfied, at least in part, by such expansion of compressed fluid into it as the compressibility of the fluid permits, in addition to any actual change of direction of fluid flow. Such expansion is accompanied by a reduction of velocity of contiguous flow. I

(e) The sheltered area is itself completely inert, and the force in the general velocity of fluid is sufiicient to cause a considerable suction on the 5 V lbs. per sq. ft.

fluid in the sheltered area, but is not suflicient to give the general fluid a back flow, unless it has compression, any force available for a back flow residing in the compression only. In gases, the preponderant tendency is for the gas to give up only a small part of its compression force near the summit where the shelter is slight, and substantially maintain a stratum of compression right to the tail. This stratum is some little distance from the surface of the aerofoil, and expends its force in an eddy which curls downward and round, so that incidentally there is contiguous flow from the tail of the aerofoil forward.

(f) On the postulate that a useable suctional, frictional, or surface-tensional force is developed on contiguous matter by a transverse flow of fluid, but no force is inherent in a mere rarefaction of fluid, the aerofoil would have an upward force being exerted on it from the nosetip to the region of the summit, another upward force being exerted on it from the tail tip for a little distance forward, and a substantially inert area between.

(g) Experimental evidence indicates the suctional forces to be of the order 31 lbs. per sq. foot,

where V is the velocity of contiguous flow, and for practical purposes the net vertical components of this force may be taken and vertical components of the compression disregarded.

(h) The extent of the upward forces could not be ascertained from pressure gauge readings unless the actual compression were known for all points on the aerofoil, by experiment orother- Wise.

(2') The reaction of the upward force is on the body of compressed fluid above the aerofoil, and provided the stratum of compressed fluid gives adequate contiguity the static force is no greater for a thicker stratum. The energy dissipated without doing useful work will be represented by the quantity of fluid which escapes under residual compression, and thus the kinetic force available will depend on the quantity of fluid compressed and its compression, and the readiness with which successive strata can absorb compression energy. This absorption of energy would appear to have nothing whatever to do with momentum.

The fundamental difficulty of designing a tractor for vertical lift in air lies in the opposition and mutual antipathy of the two basic premises,

(a) The tractor must exert very great thrust to sustain a dead weight in descent, to lift the same dead weight in ascent, and throughout the whole range between, with steady increase of power consumption from zero at a maximum velocity of descent to a reasonable amount at the maximum velocity of ascent. That is to say the first differential coeflicient of the equation y=f(V) where y is power consumption to develop a constant thrust and V is the vertical velocity, must be positive for all values of V.

(b) The natureof the helical tractor is such that the curve of y=f(V) is paraboloidal, with the first differential coefficient negative for all negative values of V and more than the first half of all positive values.

Briefly and broadly this means that helical tractors required to develop a constant sustaining force have a fatal instability instant upon the slightest variation in vertical or rotational speed.

The invention hereinafter described represents an attempt to overcome the difliculty indicated in the two preceding paragraphs by the application of the principles and views previously expounded in a rotor to develop a force in the same direction as the incoming slip stream, in which 5 the object is to utilize the sustaining and lifting forces known in practice to exist on the upper surfaces of aerofoils in such a way that the expenditure of power to induce the force will steadily increase with the force notwithstanding an 10 accompanying increase of vertical velocity.

In the drawing,

Fig. 1 is a plan view of the device;

Fig. 2 is a perspective view of the upper surface; and 15 Fig. 3 is a perspective view of the lower surface.

The invention is a rotor screw or rotor consisting (1) of a central disc provided with vents, (2) of a number of aerofoils with negative 20 angles of incidence circularly arranged round the central disc and (3) of an annular band between the central disc and the aerofoils. Six is a suitable number of aerofoils and vents, as shown in the drawing herewith. 26

The central disc (A) occupies some 60% of the radial distance outward from the centre of the rotor, and the six vents (B) commence at some 40% and extend to the edge of the disc. The vents are so made that on rotation they pro- 30 vide a cutting edge in continuation of the plane of the undersurface of the disc and are ramped upwards and backwards from the cutting edge to a point immediately above the cutting edge of the next succeeding vent. Adjoining the cen- 35 tral disc is an annular band (0) continuous on both the upper and lower surfaces of the rotor, the lower edge in the same plane as the under surface of the disc. The object of the ventholes aided by the annular band is to discourage cen- 40 trifugal flow, which is undesirable. The cutting edges of the six vents (B) are carried through the annular band (C), thence backward a little, parallelly, to form the leading edges or nosetips (E) of six aerofoils (D) to a radial distance of 45 100%. The aerofoils may most readily be considered in circular arc, but are still of aerofoil shape in any simply curved path. The nose tips (E) are at the same level as the lower edge of the annular band, the tail tips (F) are approx- 50 imately midway of the annular band, and the summits (G) are on the same level as the upper edge of the annular band. The angles of incidence of the aerofoils are therefore negative but the actual angle decreases outwardly. The 55 precise outline of the aerofoils will depend on the practical and dynamic performance required of the rotor but in general the nose-tip angle should be some 45 with the summit well back, as might be obtained by truncating into a negative angle 6 of incidence an aerofoil or good efficiency at a small positive angle, truncation commencing at the nose tip. The space in between the tailtip of one aerofoil and the nose tip of the next should not be great, say sufficient only to form an angle between 45 and 60 in elevation. Solidity ratio is thus very high, close to unity.

The optimum result aimed for when rotation is imparted to the rotor either by engine power or 7 by air pressure against the lower surfaces in descent, is that there should be an incoming stream of air on to the lower surface of the rotor of velocity approximating the mean vertical components of motion of the lower surfaces of the aerofoils, and a stream of compressed air passing up the noses with good contiguous flow to or beyond the summits, thence away from the rotor as it chooses, provided only that it is desired to dissipate as little energy as possible in residual compression of the departing slip stream. In these circumstances a static and kinetic lifting and sustaining force of considerable amount has been shown by experiment to be developed.

Tendencies found experimentally which militate against optimum result appear to arise from the compression of air at the aerofoil noses causing air flow forwards onto both the upper and lower surfaces of the aerofoils in front, in which casethe contiguous flow is of low velocity, and

of very low net power, if any, in the required direction. It is clear however, that for a given peripheral velocity of rotation this tendency will be much less in large rotors than in small ones, the actual magnitude of the zone of compression forwards being probably much the same in each case, and hence less in proportion to size. The tendency will be encouraged if a greater number than six of smaller aerofoils be used in an attempt to secure what advantages there may be in higher aspect ratio 1. e. ratio of radial width of aerofoil to length.

The rotor is here described as in a manner suit able for providing the sustaining or lifting force in air for the purpose of a helicopter, operating horizontally, but it will provide a force in the same direction as the incoming fluid flow in other positions than horizontal, and in other fluids than air, where high power at low translational speed is required, as for example on the bow of a tugboat to dislodge stranded vessels.

The description of the invention herein given large e-ifect on the efficiency of, the rotor, as for.

example the precise relationship necessary between angle of incidence, space between the blades, size of vent holes and so on. It is known that experimental rotors of flimsy construction vary their efiiciency when distorted by centrifugal force on the metal. Moreover some rotors of the general construction herein described have been found to give efficiency as high as in normal rotation when rotated upside down and backwards. It appears also that a sharp tall, as shown, may not be essential and good results may be obtained by aerofoils which have a tail tip in the shape of an inverted nose. Air flow not visibly diiferent in smoke tests from correct flow has been obtained by joining two rotors together, bottom to bottom.

In any case with this rotor as with helical tractors, the particular design for given static and kinetic forces to be exerted with given power consumption will depend on tables obtained by experiment.

The largest rotor so far constructed in the course of experimental work is of 18 inches diameter, whereas a practical size for aircraft, would be about 60 inches as a minimum. Up to 18 inches diameter it appears to be established that assistance to correct flow may be obtained by canting slightly, downwards and outwardly, two diametrically opposite nose-tips, leaving the other nose tips horizontal. The downward cant reduces the tendency for compressed air to flow forward onto the tail of the aerofoil in front, and the air then flowing back causes a pressure over the tail which prevents flow forward from the aerofoil behind, and so on successively.

What I claim as my invention, and desire to secure by Letters Patent is:

A rotor screw or rotor consisting of a central disc ramped upwardly and backwardly near its periphery, with vent holes between successive ramps, an annular band bounding the central disc, and a circularly arranged system of aerofoiis adjoining the annular band and of similar overall height, said aerofoils having negative angles of incidence, noses in direction of rotation, and tail tips, each tail tip being in parallel with and only slightly clear of the nose tip of

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