US1853694A - Counter running double propeller - Google Patents

Description

A ril 12, 1932. F. ME LCHER 1,353,694

I COUNTER RUNNING DOUBLE PROPELLER Filed Aug. 14. "1930 I l l l Patented Apr. 12, 1932 UNITED STATES PATENT oFncE I rnlmz mcaaaor ausram I I COUNTER RUNNING DOUBLE PROPELLEB 1 Application fled August 14, 18 80, Serial Io. 475,838, and in Austria March 17, 1828.

This invention relates to adouble propeller with counter-rotation for operation in media pf all kinds. Double counter-running pro- .pellers have hitherto been used only in cer- 5 tain special applications and in water, as for example in connection with torpedoes for the purpose of eliminating the reaction efiect of the ropeller which is assumed to be a left hand one and consequent turning of the torpedo bod ;but no attention has hitherto been paid to t e exploitation of this counter-rotation from a purely hydraulic point; of view.

There are many known forms of construetion for single ropellers in which the pitch of the blades bot in a radial and in a circumferential sense is adapted to be adjustable a,

suit altering conditions of flow. v

The present invention'has for its object to achieve the highest possible propeller efliciency by the suitable choice of the respective pitch of the juxtaposed counter-rlmning propellers and by a particular construction of the d ad acent entry anddischarge edges of the blades of these propellers. A further particular object of the invention is to employ the front propeller as a guide apparatus for effecting as favorable an uniform an admission as possible to the rear propeller.

In its application to aircraft this inventlon aims at the provision of propellers with approm'mately twice as great a thrust but w1th approximately the same disc-area and w1th at least the same degree of efliciency as single propellers of the usual type. Y

An example of a form of construction em- .bodying the subject matter of the inventlon Y is illustrated in Figs. 1 to 4 of the accompanying drawings, in which a Fig. 1 is a longitudinal section of acounterrunning double propeller; A

Fig. 2 is a side elevation peller. V

Fig. 3 is an'elevation of the rear propeller which is assumed to be a right hand one.

Fig. 4 represents develo d rojections of cylindrical sections of the lad s of the'fore and rear propeller respectivel The s ds of the medium relatively to t e prope as a 50 and b are shown diagrammatically by means of radius-vectors.

of the fore pro- The propeller a is keyed to the hollow shaft and the propeller b to the inner shaftql. he two shafts are driven iniopposite directions. The locking of the propeller bosses on the tapered ends of the shafts can be effected in any desired manner, for instance by means of driving nuts 0 and p. v

Given the resistance of the vessel or torpedo, and given the speed of rotation and the proportional working capacity of each propeller, all the relative entry and discharge speeds of the traversing medium for the entire range of the blades of both fore and rear propellers can first be calculated point for point inaccordance with the impulse rule,

provided the initial angle of the relative speed of entry to the pressure side is also given. After the plotting of the radius-victors of the relative entry and discharge speeds in diagrammatic form (Fig. 4 the varying degrees of pitch of the prope er can be doneed In the diagram of Fig. 4 the axially directed absolute rate of flow-speed of the medium on entering the propeller is represented by the radius-vector o,=o. (Zw), 42 being the speed of the ship and w the wake co- In the diagram u is the circumferential speed of the medium at the point concerned (A-A in Figs. 1 to 3), after allowance for reduction on account of slip which increases from the centre towards the circumference. The relative flow-speed w. in the propeller a at its entering edge It is the resultant of the absolute speed at entry '11,, and the negative circumferential speed -u.

The local pitch at the entering edge It is as follows H.=9nrr. tang a rbeing the radius of the propeller at the point concerned1(A-A) and a, the angle of pitch at the entering edge and on the pressure side D of the propeller; c' is tang (ae= (tang or-tang e) (Z+tang e1 tan a) 1 and 1rD.n be substituted for tang or. and u respectively it follows that tang e= r u From the absolute discharge speed v of the front propeller only the discharge pitch H, can be calculated.

v,=k. n. H,'; whereby n represents the number of revolutions and k a constant de-.

pending on the angle of convergence between the surfaces of the blade in cross-section at the discharge edge a; the angle of incidence q90 between the direction of the absolute discharge speed '0. and the direction at the circumference is known to be somewhat variable in a radial sense.

If the above described conditions be fulfilled the apressure side D of the developed cylindric section presents a flattened curve corresponding to the varying pitch in a circumferential direction.

' Theoretically the merely approximately axially directed absolute discharge speed '0,"

of the fore propeller a would have to e equal to the absolute admission velocity '0 at the entering edge (1 of the rear propeller b. '22., will however be less than '0. to the extent of a slight loss of impact in the gap 9 (Fig. 1)

In the dia rammatic determination of-the speed and pitch triangles particular attention must be paid to the twist component in the discharge speed of the fore propeller, which component increases towards the boss of the propeller.

The pitch of the pressure side of the rear propeller b is again variable, so that the deducedaxial discharge speed 4)," =70. n. H. is the resultant of the circumferential speed of the propeller.

' increases,

- aeesaeea u and of the relative rate of outflow speed w at the discharge-edge i.

If the above elucidated fundamental conditions be fulfilled the fore propeller a acts as a guide apparatus for the rear propeller b, with the ob ect of providing a favorable flow to the latter.

The double propeller a, b, which is to be regarded as constituting a single self-contained unit, thus describes in running an enclosed surface of rotation h-e-f-i, as if the latter were the enveloping surface of only one single propeller. 1

In the constructional example shown the projections shown in Fig. 1 of the juxta osed inner discharge edge 0 and entering e e d are straight and uniformly distanced om each other; they can however be inclined to each other or curved in any desired manner, and the intermediate gap g can also be varied within slight limits, so that this gap is somewhat larger in the vicinity of the boss, where irregularities of flow can occur, while in the most effective portion, for about the outer two-thirds of the length of the blades, the width of, the gap remalns uniform. It is advantageous to reduce the width of the gap 9 or the clearance between the two propel ers to less than 5 per cent of the greatest diameter The external shape of the entering edge h of the fore propeller and of the discharge edge i of the rear propeller is immaterial. The diameter of the rear propeller b can also be somewhat less than that of the fore propeller a in accordance with the contraction of the screw-water as the speed v so that the tangent t to the enveloping surface in the gap is ipclined towards the rear (Fig. 1).

I claim 1. A counter-running double propeller for operation in media of every kind characterized by the essential feature that the angle v of pitch on the pressure sides at the entering edges is. greater to the extent of a uniformly equal initial angle than the appertaining angle of incidence between the direction of the relative inflow speed to the working propeller and the negative direc-- tion at the circumference, whereby the local variable pitch of the discharge edge of the propellers corresponds to a given speed of rotation and a certain definite distribution of working capacity of each propeller.

2. A counter-running double propeller according to claim 1, characterized by the essential feature that the variation of the pitch of entry from the blade tip to the boss is of such a decreasing tendency that throughout this range'the initial angle is approximately 4.

3. A counter-running double propeller characterized by the essential feature that 'the clearance between the two propellers is less thanper cent of their greatest diameter.

4. A counter-running double propeller characterized by the essential feature that 5 the diameter of the rear propeller is less than that of the fore propeller to such an extent that the tangent to the blade tips of both propellers at a given speed is parallel to the local direction of flow, which direction is conditioned by the contraction of the screw-jet.

5. A counter-running double propeller for operation in media of every kind, consisting of a lefthand and a righthand propeller characterized by the essential feature that the angle of pitch on the pressure sides is greater in the amount of approximately 4 degrees than the appertaining angle of incidence between the direction of the relative inflow speed to the working propeller and the negative direction at the circumference by a certain definite distribution of working capacity of each propeller, whereby the clearance between the two propellers in the extent of about two thirds of the length of the blades of the propellers is less than 5 per cent of their greatest diameter and the diameter of the rear propeller is less than that of the fore propeller to such an extent that the tangent to the blade tips of the propellers at a given speed is inclined toward the rear according to the contraction of the screw-jet.

In testimony whereof I have aflixed my signature. 7

, FRANZ MELGHER.

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