Claims
- 1. A counterrotating aircraft propeller set comprising:
- (a) a first propeller having N1 blades and
- (b) a second propeller having N2 blades
- wherein N1 and N2 are selected from the following group of numbers: 3, 5, 7, 9, 11, 13, 15, and 17 and wherein the difference between N1 and N2 is 2.
- 2. A counterrotating aircraft propeller set comprising:
- (a) a first propeller having 8 blades and
- (b) a second propeller having 11 blades.
- 3. A counterrotating aircraft propeller set in which the blade crossings which are sequential in time are non-adjacent in space.
- 4. A counterrotating aircraft propeller pair in which the distance between sequential blade crossings is greater than the blade spacing on either propeller.
- 5. A counterrotating aircraft propeller set comprising:
- a first propeller having a first number of blades; and
- a second propeller having a second number of blades,
- in which the blade crossings which are sequential in time are non-adjacent in space.
- 6. A counterrotating aircraft propeller set comprising:
- a first propeller having a first number of blades; and
- a second propeller having a second number of blades,
- in which the distance between the locations of sequential blade crossings is greater than the blade spacing on either propeller.
- 7. A method of reducing the audible noise produced by a counterrotating aircraft propeller set having aft and fore propellers, comprising the step of:
- causing the locations of the blade crossings to rotate about an axis at a speed greater than the quantity: ##EQU5## in which N.sub.a and N.sub.f are the numbers of blades on the aft and fore propellers, respectively, and S.sub.a and S.sub.f are the rotational speeds of the aft and fore propellers, respectively.
- 8. A method of reducing audible noise produced by a counterrotating aircraft propeller set which produces a modulating phenomenon when a fore propeller blade crosses an aft propeller blade, such modulating phenomenon rotating about an axis, comprising the step of:
- causing the modulating phenomenon to rotate about the axis at a speed equal to or greater than the quantity: ##EQU6## in which N.sub.a and N.sub.f are the numbers of blades on the aft and fore propellers, respectively, and S.sub.a and S.sub.f are the rotational speeds of the aft and fore propellers, respectively.
- 9. A counterrotating aircraft propeller set comprising:
- a first propeller having a first number of blades; and
- a second propeller having a second number of blades,
- in which the aircraft propeller set generates a far-field noise having a resultant component described by the following equation:
- P=sin (W.sub.c +M sin (W.sub.m t)),
- in which P represents pressure, W.sub.c represents a carrier frequency, M represents a modulation index, W.sub.m represents a modulation frequency, and t represents time.
- 10. A counterrotating aircraft propeller set comprising:
- a first propeller having N1 blades; and
- a second propeller having N2 blades,
- in which N1 and N2 differ by 2 and have no common factors.
- 11. A counterrotating aircraft propeller set comprising:
- a first propeller having 7 blades; and
- a second propeller having 12 blades.
- 12. A counterrotating aircraft propeller set comprising:
- a first propeller having 8 blades; and
- a second propeller having 11 or 13 blades.
- 13. A counterrotating aircraft propeller set comprising:
- a first propeller having 5 blades; and
- a second propeller having 7, 8, 11, 12, 13, or 14 blades.
- 14. A counterrotating aircraft propeller set comprising:
- a first propeller having 6 blades; and
- a second propeller having 11 or 13 blades.
- 15. A counterrotating aircraft propeller set comprising:
- a first propeller having 7 blades; and
- a second propeller having 9, 10, 11, 12, 13, or 15 blades.
- 16. A counterrotating aircraft propeller set comprising:
- a first propeller having 8 blades; and
- a second propeller having 10 blades.
- 17. A counterrotating aircraft propeller set comprising:
- a first propeller having 8 blades; and
- a second propeller having 11, 13, or 14 blades.
- 18. A counterrotating aircraft propeller set comprising:
- a first propeller having 8 blades; and
- a second propeller having 12 blades.
- 19. A counterrotating aircraft propeller set comprising:
- a first propeller having 9 blades; and
- a second propeller having 11, 13, or 14 blades.
- 20. A counterrotating aircraft propeller set comprising:
- a first propeller having 10 blades; and
- a second propeller having 13 or 14 blades.
- 21. A counterrotating aircraft propeller set comprising:
- a first propeller having 11 blades; and
- a second propeller having 13, 14, or 15 blades.
- 22. A counterrotating aircraft propeller set comprising:
- a first propeller having 13 blades; and
- a second propeller having 14 or 15 blades.
- 23. A counterrotating aircraft propeller set comprising:
- a first propeller having 14 blades; and
- a second propeller having 15 blades.
BACKGROUND OF THE INVENTION
This application is a continuation of copending Application Ser. No. 764,144 filed August 9, 1985 now abandoned.
The present invention relates to the reduction of audible noise produced by aircraft propellers.
Each propeller blade of a rotating aircraft propeller produces a pressure wave which is perceived as sound or noise. The frequency of the noise is equal to the number of blades passing an observer every second, or N.times.S where N is the number of blades and S is the rotational speed.
In the case of counterrotating propellers which rotate about a common axis, such as propellers 1A and 1F in FIG. 1A, an additional source of noise exists. The additional noise is caused by blade-to-blade interactions between the fore propeller 1F and the aft propeller 1A. When an aft propeller blade passes through the wake of a fore propeller blade, a noise pulse is generated. It is desirable to reduce this additional noise.
It is an object of the present invention to provide new and improved noise reduction in counterrotating aircraft propeller sets.
In one form of the invention, a counterrotating pair of aircraft propellers produces noise in the form of a frequency-modulated carrier wave. The frequency modulation allows a designer to manipulate the energy spectrum of the noise in order to, for example, place much of the acoustic energy into inaudible frequency ranges.
US Referenced Citations (11)
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 708096 |
Jul 1941 |
DE2 |
| 521868 |
Jun 1940 |
GBX |
Non-Patent Literature Citations (2)
| Entry |
| NASA Publication N84-13186--Technology and Benefits of Aircraft Counter Rotation Propellers--(NASA CR-168258)-Dec. 1982, p. 10-"Introduction--Acoustics"; p. 30--Acoustic Study Results-Near Field Noise; bottom of p. 31; p. 36--. . . Far-Field Noise; pp. 53--Unequal Blade Numbers; p. 130 (Tables 1 and 2) row--Propeller-Number of Blades; p. 133--Summary, Column Number of Blades FXR. |
| E. J. Richards, P. J. Mead;Noise and Acoustics Fatigue in Aeronautics, 1986, pp. 198, 199. |
Continuations (1)
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Number |
Date |
Country |
| Parent |
764144 |
Aug 1985 |
|
Patent Grant
4883240
