US3771922A

US3771922A - Stabilized rotary blades

Info

Publication number: US3771922A
Application number: US00301827A
Authority: US
Inventors: A Tracy
Original assignee: McDonnell Douglas Corp
Current assignee: McDonnell Douglas Corp
Priority date: 1972-10-30
Filing date: 1972-10-30
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

Projections near the tips of high speed rotary blades are actuated by centrifugal force to ensure uniform spacing between the blade tips as a means of reducing buzz-saw noise.

Description

United States Patent [191 Tracy STABILIZED ROTARY BLADES [75] Inventor: Alfred C. Tracy, Los Angeles, Calif.

[73] Assignee: McDonnell Douglas Corporation,

Santa Monica, Calif.

[22] Filed: Oct. 30, 1972 [21] Appl. No.: 301,827

I521 U.S.Cl ..4l6/196,4l6/190,416/191 [51 j Int. Cl. F0ld 5/22 [58] Field of Search 416/190, 191, 196,

[56] References Cited UNITED STATES PATENTS Anxionnaz 416/196 X [451 Nov. 13,1973

2,914,299 11/1959 Mitchell 416/196 UX 3,104,093 9/1963 Craig et a1. 416/210 FOREIGN PATENTS OR APPLICATIONS 1,426,798 3/1969 Germany 416/196 Primary Examiner-Everette A. Powell, Jr. Att0rneyWa1ter .1. Jason et a1.

[57] ABSTRACT Projections near the tips of high speed rotary blades are actuated by centrifugal force to ensure uniform spacing between the blade tips as a means of reducing buzz-saw noise.

3 Claims, 5 Drawing Figures STABILIZED ROTARY BLADES BACKGROUND OF THE INVENTION In modern high speed turbo engines for aircraft, the tendency is to use turbofan engines with as high an engine bypass ratio as possible. This calls for increasingly larger fan blades. Through the necessity of damping and because of other stress problems, the root support structure of the blades must be loose. Dimensional variations in spacing at the tips of the blades thus may become quite large, producing uneven acoustical pulses with a fundamental or lowest frequency starting with the first rotational frequency and all harmonics thereof.

The highest frequency would be the frequency of a.

blade passage over a point on its periphery and in the case of an aircraft engine would be on the order of 2,000 cps. In a perfect situation, in which there were no buzz-saw noise, there would be no variation in the blades characteristics, such as angle of attack, tip spacing, camber, contour, etc. By stabilizing the tips of the blades in accordance with the present invention, there is great improvement in the reduction of buzz-saw noise generated by the rotor tips. Theoretically, blades spaced perfectly and of identical manufacture will produce no harmonics below the 38th in the case of the DC-l jet engines and it is the fifth through the 20th harmonic that causes the buzz-saw noise. Attempts have been made to reduce the intensity of this noise through treatment in the cabin side walls and attempts have been made to reduce the compressor whine and the turbine whine by perforated linings in the in-take and the exhaust. While these treatments affect the harmonics of the buzz-saw noise, they do notreduce the lower frequency buzz-saw noise source through the stabilization of rolling deflections of each blade and through the control of spacing between the blades.

SUMMARY OF TI-IE PRESENT INVENTION In accordance with the present invention, turbine blades are designed with side arms near the threequarter span position. These arms are deflected slightly inwardly toward the rotor hub and utilizes the centrifugal force that occurs during the normal running of the engine to bend the arms in an elastic deflection such that adjacent blade arms come in contact with each other and thus limit the spacing between the blades. These elastically deflected arms stabilize the dimensions between the blades and will also add to the damping of each blade, reducing the blade vibration an appreciable amount. A bend relief-at the root of the arm may be desired in some instances forgreater flexibility and greater deflection under the centrifugal loading.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view with parts broken away to more clearly show the rotor blades in an aircraft turbofan engine;

FIG. 2 is an enlarged view of a portion of the blades showing the extended arms near the blade tips;

FIG. 3 is a front view enlargement showing the arms in normal position and their deflection position due to centrifugal force;

FIG. 4 is a view similar to that of FIG. 2 wherein the arms have been extended by centrifugal force upon rotation of the blades; and

FIG. 5 is a plan view taken along the line 5-5 in FIG. 4.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT Reference is now made to FIG. 1 wherein there is shown the housing 10 about an aircraft engine and within which the blades 12 of the turbine are rotatably mounted upon a rotor or hub 14. The housing 10 typically may have a diameter 8 or 9 feet and the blades may have a length on the order of 3 or 4 feet. These blades should be as light as possible, yet must be free from destructive vibration or flutter. These blades are subjected to large twisting and bending moments. Because of stress reasons the blades are loosely attached to the rotor within hub 14 such that considerable mechanical clatter may be heard when the engines are being shut down such as when the aircraft has taxied to the unloading station at the end of the flight. However, it is not this clatter that the present invention overcomes but it is the uninherent, uneven spacing of the blades as they pass a point on the periphery of the housing that generates the low frequency sawtooth noise when the engines are operating. For example, if there were 39 blades and the rotational speed were cycles per second, then the blade passage frequency would be 2,340 cps. Any variation in angle of attack or tip spacing, camber, contour, or other characteristic will produce a buzz-saw noise within a range of from 200 cps to 1,000 cps. Just forwardly of the blades are

engine housing extensions

16 and 18 having an outer surface compatible with the outer surface of the housing 10 and inner surfaces compatible with the inner surface of housing 10. These sections preferably are made of high strength-to-weight panels to form rings having a honeycomb or cellular material sandwiched between the inner and

outer faces

22 and 24. By providing a plurality of openings 26 within the inner surface 24 some of the sound waves may be damped within the inner cavities. This particular approach is more clearly set forth in co-pending patent application Ser. No. 286,457 filed Sept. 5, 1972 by Arnold W. Guess and Guenter M. Schindler and forms no part of the present invention except in combination therewith.

Referring now to FIG. 2 there is shown a plurality of blades 12 having laterally extending arms 28 extending from both sides of the .blades within the outer onefourth of the length of the blade 12. These arms are of a generally flat plate-like structure having an outer edge surface 30 extending relatively parallel to the surface of the blade 12 from which the arm '28 extends. The outer edge 30 of arm 28 is bent downwardly somewhat in the direction of the hub and in this position the adjacent anns will have a small gap therebetween.

A front enlarged view may be seen in FIG. 3 wherein

adjacent blades

12 and 12A are shown spaced apart but there is a clearance or a spacing on the order of 0.001 to 0.005 inch between the

free edges

30 and 30A of the

adjacent arms

28 and 28A when in this inwardly directed non-deflecting position. The elasticity of the

arms

28, 28A at their juncture with the

blades

12 and 12A is such that there may be an outwardly directed deflection on the order of 0.015 to 0.020 inch due to centrifugal force when the blades are rotating at conventional speed. This outward deflection moves the

arms

28, 28A to their dotted line positions 28D and 28AD with the letter D designating its deflected position. These

arms

28 and 28A may have a bend relief 32 on the compression side and a bend relief 34 on its tension side of the juncture between the arms and the blades. The extent of this bend relief, of course, depends upon the elasticity of the material used and the amount of deflection desired for the amount of centrifugal force determined by the rotation of the blades. It is conceivable that in some cases such bend reliefs may not be necessary and in fact fillets may be used.

In FIG. 4 there is shown a fan of blades 12 having a plurality of arms 28 extending therefrom about a circumference near the outer ends 36 of the blades. These arms 28 are shown extended outwardly and in nearabutting relationship due to centrifugal force as the blower blades rotate. These arms 28 have filleted connections to the blades since a bend relief is not necessary in this particular application.

Reference is made to the plan view in FIG. 5 taken along the line 5-5 in FIG. 4. Inasmuch as the turbine blades 12 are angularly oriented relative to the axis of the hub 14, the arms 28 also are angularly oriented such that the outer edges 30 are generally parallel with the surface of the blades as previously mentioned until centrifugal force deflects the

adjacent arms

28, 28A into abutting contact there is a gap between the adjacent

outer edges

30 and 30A. This gap, of course, closes when the rotors reach conventional speed and centrifugal force causes the outward deflection of the arms.

Having thus described an illustrative embodiment of the present invention, it is to be understood that other modifications and variations will occur to those skilled in the art and it is to be understood that these deviations from the illustrative embodiment are to be considered part of the present invention as claimed.

I claim:

1. In combination with a turbofan engine having a plurality of blades attached to a rotor hub and a housing having a circumferential inner surface in close radial proximity to the tips of said blades,

means for reducing noise caused by irregular spacing of said blade tips and thus varying the frequency of blade tip passage over a point on the inner surface of the housing about the periphery within which said blade tips rotate,

said means comprising side arms extending laterally outwardly from the surfaces of said blades and radially inwardly toward said hub,

said arms having outer edge surfaces positioned radially inwardly from the connection of said arms to said blades,

said arms deflecting radially outwardly due to centrifugal force when said blades are rotated at a preselected frequency, the outer edge surfaces of adjacent arms on adjacent blades abutting each other when said blades rotate at said preselected frequency.

2. The combination as in claim 1 wherein said arms are of equal length thereby to cause equal spacing of said blade tips when said blades are rotated at said preselected frequency.

3. The combination as in claim 1 wherein said outer edge surfaces are substantially parallel to the surfaces of said blades in the arm thickness direction and in the arm length direction to define abutting planes parallel to said blade surfaces.

Claims

1. In combination with a turbofan engine having a plurality of blades attached to a rotor hub and a housing having a circumferential inner surface in close radial proximity to the tips of said blades, means for reducing noise caused by irregular spacing of said blade tips and thus varying the frequency of blade tip passage over a point on the inner surface of the housing about the periphery within which said blade tips rotate, said means comprising side arms extending laterally outwardly from the surfaces of said blades and radially inwardly toward said hub, said arms having outer edge surfaces positioned radially inwardly from the connection of said arms to said blades, said arms deflecting radially outwardly due to centrifugal force when said blades are rotated at a preselected frequency, the outer edge surfaces of adjacent arms on adjacent blades abutting each other when said blades rotate at said preselected frequency.