Patent Publication Number: US-2012025016-A1

Title: Aircraft propeller

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of invention relates to propellers for an aircraft 
     2. Description of Related Art 
     In order for an aircraft propeller to be more easily balanced for smooth and effective use, it generally has an even number of blades. Each pair of blades is arranged in longitudinal alignment with each other on opposite sides of and radially of the axis of rotation of the propeller. When the blades of the propeller are equally spaced, the discrete frequency noise of the acoustic spectrum is characterised by a single fundamental blade passing frequency and its harmonics. The blade passing frequency is the product of the number of blades and the rotational tip speed of the propeller. The harmonics are integer multiples of the blade passing frequency. Hence, the acoustic spectrum of a propeller with equally spaced blades is typified by regularly spaced pronounced “peaks” of sound energy, coinciding with the fundamental blade passing frequency and its harmonics. However, with the blades equally spaced, the fundamental frequency and its harmonics essentially reinforce each other, adversely affecting the perceived noise. 
     By spacing the propeller blades unequally, a fundamental blade passing frequency is generated for each unique angle between the blades. In turn, each fundamental frequency generates a set of harmonics. Whilst spacing the blades equally distributes the sound energy over a single fundamental blade passing frequency and one set of harmonics, unequally spaced blades distribute the same acoustic energy over a broader range of frequencies. Unequally spaced blades may also modify the decay rate with frequency of the sound pressure levels of the harmonics. Thus, whilst the sound pressure level of the fundamental frequency may be higher and more harmonics may occur when the blades are unequally spaced, the overall perceived noise may still be reduced, due to the favourable interaction of the fundamental frequencies and/or the harmonics. it has been found that unequally spaced blades do not adversely affect the propeller performance. U.S. Pat. No. 5,096,383 discloses a propeller with six blades which are unsymmetrically arranged around the axis of rotation of the propeller. 
     Whilst six bladed propellers are known it would to be desirable to have a propeller with more blades to increase efficiency. Minimising the number of blades increases reliability and reduces cost. Therefore, having the opportunity to use an odd or even number of blades allows full optimization of the propeller design. 
     It would be desirable to provide a propeller in which at least some of the problems discussed above were at least reduced. 
     BRIEF SUMMARY OF THE INVENTION 
     According to embodiments of the present invention there is provided a propeller for an aircraft, the propeller comprising an odd number of blades and wherein the odd number of blades are unequally spaced. 
     It has been surprisingly found that a propeller can be provided with an odd number of unequally spaced blades which may be angularly spaced relative to each other such that the propeller is balanced. Providing an odd number of blades makes the propeller more efficient than the corresponding conventional propeller with one less blade whilst reducing the disadvantages associated with designing a propeller with an increased number of even blades. 
     It is even more surprising that a balanced propeller may be provided with an odd number of blades that are unequally spaced. The unequal spacing enables the perceived noise to be reduced as discussed above. Each blade is preferably mounted radially about an axis of rotation such as by being mounted on a rotatable hub. At least one blade is preferably angularly separated from its two neighbouring blades by two different circumferential angles in the plane of rotation. 
     The propeller may comprise five unequally spaced blades which is more efficient than a four bladed propeller and less expensive and complicated than a six bladed propeller. The propeller may comprise seven unequally spaced blades which is more efficient than a six bladed propeller but is less expensive and complicated than an eight bladed propeller. 
     The propeller may be arranged with another propeller to provide a contra-rotating propeller arrangement which comprises a second propeller positioned axially aft of the first propeller, rotating in an opposite direction. 
     The propeller or contra-rotating propeller arrangement may be provided to operate aft of a pylon attached to an aircraft such that the frequency of interaction of the pylon wake with the rotor blades would be variable reducing both the sound levels and perceived noise. 
     The propeller may be provided with a controller for controlling the speed of the propeller in use to actively control the noise produced by the propeller. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a propeller with five unequally spaced blades; 
         FIG. 2  shows the propeller of  FIG. 1  with the perpendicularly resolved forces of each blade illustrated; 
         FIG. 3  shows a propeller with seven unequally spaced blades. 
         FIG. 4  shows a contra-rotating rotating propeller arrangement; 
         FIG. 5  shows a propeller provided aft of a pylon attached to an aircraft 
         FIG. 6  shows a contra-rotating propene arrangement provided aft of a pylon. attached to an aircraft and 
         FIG. 7  schematically shows an arrangement for actively controlling the speed of the propeller. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the example of  FIG. 1 , the embodiments of the present invention provide a propeller  10  for an aircraft with an odd number, in this case five, unequally angularly circumferentially spaced blades  20 ,  21 ,  22 ,  23 ,  24 . The circumferential angle between each pair of blades in this example is 71.42°, 73°, 70.96°, 72.69° and 71.93°. It has surprisingly been found that the unequally circumferentially spaced blades produce a balanced propeller for smooth and efficient use, being more efficient than an equivalent four bladed propeller whilst reducing noise levels due to the unequal circumferential spacing and being less expensive and complicated than a six bladed propeller. 
     In this example each of the blades  20 ,  21 ,  22 ,  23 ,  24  is mounted to a rotatable hub  30 . The hub may be rotated by any suitable means as is well known to a person skilled in the art, such as an engine. Examples of engines may include a piston or turbo prop engine. Although all of the blades are shown mounted to the hub  30  in  FIG. 1 , any suitable arrangement for mounting the blades to a suitable axis of rotation may be used as will be known to a person skilled in the art. 
       FIG. 2  illustrates how the spacing of the blades around the hub may be determined to ensure that the propeller is balanced when in use. The positioning of the blades is such that the resolved perpendicular forces in the plane of the blades is balanced. For example, in the embodiment of  FIG. 2 , the resolved vertical and horizontal forces for each blade are illustrated schematically. The total of the forces  20 V,  21 V,  22 V,  23 V and  24 V is zero and the total of the corresponding resolved horizontal forces  21 H,  22 H,  23 H and  24 H is also zero ensuring that the overall propeller  10  is balanced in use. Clearly the precise positioning of the blades may be varied, for example to optimise noise reduction, provided that the resolved perpendicular forces in the plane of the blades is balanced. 
       FIG. 3  shows a further example of a propeller  10  with an odd number, in this case seven, unequally circumferentially spaced blades  31 ,  32 ,  33 ,  34 ,  35 ,  36  and  37 . As in the example of  FIGS. 1 and 2 , the blades are unequally circumferentially spaced but have balanced resolved perpendicular forces in the plane of the blades such that the propeller  10  is balanced in use. It has surprisingly been found that the unequally circumferentially spaced blades produce a balanced propeller  10  for smooth and efficient use, being more efficient than an equivalent six bladed. propeller whilst reducing noise levels due to the unequal circumferential spacing and being less expensive and complicated than an eight bladed propeller. 
       FIG. 4  schematically shows a side view of a contra-rotating propeller arrangement with two propellers  10  arranged to be driven in opposite rotation, Contra-rotating propellers have been found to be more efficient than a single propeller since a single engine can be used to drive the two propellers  10 . However, contra-rotating propellers can be noisy. 
     It has been found that a contra-rotating propeller in which at least one of the propellers, and preferably both, comprise a propeller with an odd number of unequally spaced blades as in embodiments of the present invention provide a contra-rotating propeller with enhanced efficiency but with reduced noise levels, The example shown in  FIG. 4  has two propellers  10  arranged one behind the other on a coaxial shaft  40  driven by the engine  41  via a gear transmission (not shown) such as a planetary gear or spur gear transmission for example. The use of propellers  10  of embodiments of the present invention in a contra-rotating propeller arrangement as shown in  FIG. 4  reduce the noise problems associated with efficient contra-rotating propeller arrangements. 
       FIG. 5  shows a propeller of an embodiment of the present invention arranged to operate aft of a pylon  50  attached to an aircraft  51 . By spacing the blades non-symmetrically, the frequency of interaction of the pylon wake with the rotor blades would be variable, Hence, unequally spaced blades have the effect of reducing both the sound levels and perceived noise of a propeller installed aft of a pylon  50 . 
       FIG. 6  is similar to  FIG. 5  except that it shows a contra-rotating propeller as in  FIG. 4  installed aft of a pylon  50 . The arrangement of  FIG. 6  comprising a contra-rotating propeller installed aft of a pylon  50  would normally suffer from noise problems caused by the interaction of airflow over the pylon  50  and between the two propellers  10 . However, by using at least one, and preferably both propellers  10  of embodiments of the present invention with an odd number of unequally spaced blades it has been found that the noise levels are significantly reduced. 
       FIG. 7  schematically illustrates a controller  61  for controlling the speed of a propeller or a contra-rotating propeller arrangement in use to actively control the noise produced by the propeller. One or more sensors  60  such as microphones which may be provided within the aircraft, for example in a passenger area, may be arranged to provide output signals, either directly or for example over a wireless link, to the controller  61 . The controller  61  may then be arranged to control the speed of the propeller to change the phase of the propeller blades relative to the other propellers on the aircraft, to actively control the amount of noise produced. In this way, the one optimum phase position can then be found between 0 degrees and 360 degrees. The active control arrangement of  FIG. 7  may be used to further enhance the noise reduction provided by the propeller  10  of embodiments of the present invention. 
     Many variations may be made to the examples described above without departing from the scope of the present invention. For example the blades of the propellers illustrated in  FIGS. 1 to 3  may have any desired circumferential separation angles provided that the propeller is arranged, in use, to be balanced. The blades and hub  30  may be made from any suitable materials as will be well known to a person skilled in the art.