1.0 FIELD OF THE INVENTION
This disclosure relates to a novel aviation propeller of unconventional design using thick airfoil blades configured at constant pitch angles and arranged at varying circumferential angles. More specifically, to a device which produces high thrust and reduced acoustic signatures, equivalent to toroidal propellers, and scalable for use from small drones to large hovercraft.
2.0 BACKGROUND OF THE INVENTION
Multi-rotor drones are used for a large range of applications: search and rescue, cinematography, industrial inspection, airborne surveillance, aerial delivery, first person view (FPV) drone racing, and even air taxis. Annoying sound levels are an urban use issue, evoking the primal cringe against giant mosquitoes. Increasing flight times and decreasing refueling or recharging times, as well as reducing acoustic engine noise, are desired improvements. Lightweight mufflers and anechoic coating materials can mute engine noise. Drop-in propeller replacements with higher thrust, quieter acoustics, and more energy efficiency, are keys to widespread multi-role acceptance. This invention describes a novel and unconventional propeller design to address these challenges.
3.0 RELATED ART
With reference to FIGS. 1a-1d, four main categories of aviation propellers are shown. Included are standard geometry propellers, shrouded propellers, ducted fans, and the newer non-Euclidian geometry toroidal propellers. FIG. 1a is a conventional 3-bladed propeller as disclosed in U.S. Pat. No. 4,445,817 “Propeller Construction” (R. J. Wethern, 5/1984). FIG. 1b is a shrouded propeller design disclosed in U.S. Pat. No. 5,096,382 “Ring-shrouded Propeller” (L. B. Gratzer, 3/1992). The propeller shroud is to control tip vortex drag and noise while providing structural integrity.
FIG. 1c is a general ducted fan propeller design consisting of a multi-bladed hub assembly enclosed in a duct as used on small drones. The duct functions to control tip vortex effects on thrust and also provides reduces acoustic signature. Intl. Pat. No. WO2017049135 “Ducted Fan Propulsion System” (K. Manning, 9/2015) discloses one such system wherein multiple ducts, motors and rotatable fan blades are housed within a single outer cowling.
FIG. 1d is an example of non-Euclidian geometry toroidal propeller design. Two somewhat similar toroidal designs used for analysis and performance comparisons herein are disclosed in U.S. Pat. No. 6,736,600 “Rotor with a split rotor blade” (R. Bannasch, 5/2004) and U.S. Pat. No. 10,836,466 (T. Sebastian, C. Strem, 11/2020), assigned to MIT and hereinafter known as the MIT patent. U.S. Pat. No. 9,926,058 (G. C. Sharrow, 03/2018), hereinafter known as the Sharrow device, discloses some features of a marine toroidal propeller similar to the present invention. The device has a large diameter hub of approximately 25% of the overall device diameter, although the hub is elongated volumetric tube geometry rather than a thin porous disc as in the present device. The blades all exhibit large pitch angles with a longitudinal displacement between the leading and trailing spans, but have the standard spanwise twist thereby reducing the pitch angle toward the blade tips. Further, the blades exhibit the standard spanwise taper in both chord length and thickness.
Thick airfoils, having thickness to chord ratios up to 20%, exhibit desirable high lift, low drag, and desirable aeroelastic stiffness characteristics. They are generally used as aircraft wings or for compressor and turbine blades (“Mechanics and Thermodynamics of Propulsion”, P. Hill and C. Peterson, pp. 294-307), but not for small drones or general aviation propellers. The example NACA 633-318 used herein is an 18% thick high lift/low drag airfoil (“Theory of Wing Sections”, I. Abbott and A. Von Doenhoff, pp 538-539) which exhibits a large, nearly constant lift coefficient (flat stall) over a large angle-of-attack range above 10 degrees. Another example is U.S. Pat. No. 6,905,092 “Laminar-flow Airfoil” (D. Somers, 6/2005), is the basis for the new 15% thick NASA Natural-Laminar-Flow NLF(1)-0115 airfoil.
4.0 SUMMARY OF THE INVENTION
The present invention discloses a novel aviation Mantavian Propeller of unconventional design with thick airfoils and constant pitch angles on rotor blades which are arranged at differing circumferential angles. This novel propeller design operates at high thrust and reduced acoustic signature comparable to current toroidal propellers. The design comprises a 3-part hub and two or more separate attachable rotor wings each comprising at least two rotor blades and a hub connector. The design is scalable from 5-inch-class drones to large piloted hovercraft due to the use of basic aerodynamic geometries incorporating deviations from standard design practices.
By way of example, Da Vinci's “air screw helicopter blade” concept still pervades modern propeller design in the form of a “constant pitch” (“Propeller Geometry”, navalex.com), which results in a spanwise blade twist. This is fine for wood screws and marine propellers (hence the term “screws”). By contrary, the present invention uses a large constant spanwise pitch angle concept. By way of further example, standard design practice states that usually 1 to 4 degrees of propeller blade pitch angle provides the most efficient lift/drag ratio (“Propeller Blade Thickness Chart”, thenavalarch.com). By contrary, the best mode of the present invention successfully uses non-twisted rotor blades at a constant spanwise 27 degree pitch angle. The standard ratio of hub diameter to propeller diameter is on the order of 10% to 18%. The best mode of the present invention uses a significantly larger ratio of 50%.
The goals and objectives for the present invention include quiet 2000-6000 rpm operation, structural strength, scalability up to large piloted hovercraft and helicopters, competitive thrust, power and acoustics with toroidal propellers, low cost manufacturing, and ease of repair. Achieving the stated goals and objectives required both ingenuity and violation of many standard propeller design practices; such as, designing the rotor blades as aircraft wings rather than propeller blades. The present invention incorporates 1) simple geometric elements, 2) large diameter hubs, 3) thick, high lift/low drag airfoils with large flat stall regions, 4) small non-tapered constant thickness chord lengths, and 5) large constant pitch angles.
Subsequent sections demonstrate to those skilled in the art that the resulting design meets the aero-acoustic performance expectations. Further, it is assembled from separate replaceable parts, thereby facilitating ease of manufacturing and lower manufacturing expense compared to conventional propeller designs or concepts with non-Euclidian geometries.
5.0 BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form part of the specification, illustrate various principles of operation and examples of the present invention, including a preferred embodiment of the invention, as well as alternate embodiments, and, together with the detailed description, serve to explain the principles of the invention.
FIGS. 1a-1d illustrate prior art propeller propulsion systems.
FIG. 2 presents the principal components of the present invention.
FIGS. 3a-b illustrate conventional versus Mantavian Propeller geometries.
FIG. 4 illustrates rotor wing attachment to a hub.
FIG. 5 compares lift coefficients for thin and thick airfoils.
FIG. 6 compares experimental measured thrust for 7 inch diameter propellers.
FIG. 7 compares thrust characteristics for 12 inch diameter propellers.
FIG. 8 presents experimental thrust coefficients for several propellers.
FIG. 9 presents experimental decibel levels for several propellers.
6.0 ENGINEERING EVALUATION EQUATIONS
The prior art devices cited above for modern propellers have demonstrated high thrust coefficients. Reported levels vary from 0.10 to 0.16 at power coefficients Cpow between 0.5 and 3.0. U.S. Pat. No. 10,836,466 to MIT previously cited disclosed a toroidal configuration with a value of 0.16 while operating at a power coefficient of 0.75 and having a low acoustic signature.
A general form for the nondimensional thrust coefficient Kt as given in “Thrust Coefficient” (web.MIT.edu), and as used for the toroidal propellers in U.S. Pat. No. 10,836,466, is given as:
Here T is the thrust in pounds, ρ is the air density, D is the diameter in feet, and the revolution rate ω in revolutions per minute is reduced to revolutions per second. Similarly, a nondimensional power coefficient for the ratio of thrust produced to power required is defined as
Equations (1) and (2) are used hereinafter to analyze experimental data and compare performance of various configurations.
7.0 DETAILED DESCRIPTION OF THE INVENTION
The present invention and principles of operation will now be described more fully hereinafter with reference to the accompanying drawings, in which various exemplary embodiments of the invention are shown. It is recognized that modifications and variations of the present invention will be apparent to those skilled in the art as obviousness considerations, and that all such modifications and variations are included within the scope of the appended claims. Like numbers refer to like elements throughout. Hereinafter, the term “axial” refers to a direction parallel to the general axis of symmetry of the hub 10, while the terms “radial” and “circumferential” refer to directions perpendicular to said general axis of symmetry of said hub 10, with the direction “circumferential” also being perpendicular to any radial line.
FIG. 2 presents scale top view drawing of the general embodiment of the present invention, illustrating both the geometry and the major elements of the Mantavian Propeller 120. The Mantavian Propeller 120 comprises one hub 10 comprising outer rim 12, central core 13, and radial vanes 11 as shown, and three each rotor wings 20, each said rotor wing 20 having two each 1 inch chord by 3 inch span rotor blades 21,22 of NACA airfoil 633-618 18% thick cross-section, with each pair of said rotor blades 21,22 sharing a hub connector 24 and attached at spanwise constant pitch angles of 27 degrees.
FIGS. 3a-b illustrate the major geometric difference between the Mantavian Propeller and a standard geometry propeller. With reference to FIG. 3a, a standard geometry 4-bladed propeller with equally spaced rotor blades 21 and 22 is shown. A radial reference line passes horizontally through the center of rotor blade 21 which is positioned at the 3-o'clock position of the propeller. Rotor blade 22 is at the 12-o'clock position. The differential sweep angle 99, denoted ψ1, is obviously 90 degrees. Moving counter-clockwise, the differential sweep angle between each successive pair of blades is also 90 degrees. This is the exact result obtained from Standard Propeller Theory where all differential sweep angles are equal to 360 degrees divided by the number of blades.
With reference to FIG. 3b, the Mantavian Propeller of FIG. 2 having two rotor wings 20 with two each rotor blades 21 and 22 attached to hub 10 by hub connector 24 is shown. This Mantavian is also a 4-bladed propeller. A radial reference line again passes horizontally through the center of rotor blade 21 which is positioned at the 3-o'clock position of the propeller. By way of example, rotor blade 22 is shown at the 1-o'clock position. The differential sweep angle 97, denoted ψ2, is obviously 60 degrees. Moving counter-clockwise, the differential sweep angle 98, denoted ψ3, between the next successive pair of rotor blades, which is rotor blade 22 and the following blade at the 9-o'clock position, is now 120 degrees. Obviously, when ψ2 is 90 degrees, the standard 4-bladed propeller thrust results. Further, when ψ2 is minimized such the rotor blades 21 and 22 just touch and are as one blade, the propeller thrust is that of a standard 2-bladed propeller with a doubled 2-blade planform area. However, unexpectedly discovered during an experimental study, for one specific combination of values of ψ2 and ψ3 this geometry has a positive viscous wake interference which produces a maximum thrust from the Mantavian Propeller that is greater than either a 4-bladed standard geometry propeller or the 2-bladed standard geometry propeller with a doubled blade planform area.
FIG. 4 illustrates one method for attaching separable rotor wings to hubs. Shown is hub 10 having outer rim 12, and rotor wing 20 having two rotor blades 21, with rotor wing 20 attached to outer rim 12 by way of example using two screws 44 inserted through the hub connector 24 and into the outer rim 12.
FIG. 5 presents a comparison of lift coefficients versus angle of attack for a thick airfoil and two thin airfoils. Data set 51 is for the NACA 633-318 18% thickness airfoil shape in the upper left. Overlapping data sets 50 are presented for two thin airfoils, NACA 0006 and 0008 shown in the lower right, having thicknesses of 6% and 9% respectively. There is relatively little difference between the two curves or the airfoil cross-sections. It is obvious from the figures that the thick airfoil has superior lift characteristics and has an extremely long flat stall region when compared to the thin airfoils. The advantage of the flat stall extending the operability envelope is obvious to those skilled in the art.
8.0 BEST MODE
Initial testing showed that a baseline test unit without any special blade tip design equaled or exceeded the performance measures of the best comparative designs. Therefore this baseline unit is presented as a best mode. Experimental performance data for four test devices manufactured by 3D printing was collected using a test assembly. The test assembly consisted of a PVC pipe frame having a thin “cross-blade” motor mount secured to a postal scale having 0.2 ounce resolution. A non-contact downwash deflector plate was mounted ½ inch above the scale surface. An EFlite Rimfire 60 outrunner electric motor with a 60 amp speed control connected to a 22.2 volt LiPo battery rotated the test devices. Battery current and voltage was measured for power calculations using a multi-meter. Propeller speeds in revolutions per minute (RPMs) were measured with a laser RPM meter, and sound levels in decibels were measured with a digital decibel recorder. All tests were conducted at an altitude of 5 Kft, ambient air temperature of ˜105 F, and relative humidity of 15%, with the exception of the experimental data of FIG. 7 at ˜84 F and relative humidity ˜90%. A single standard air density value of 0.00189 lbf-sec2/ft4 at 5K feet (“Mechanics and Thermodynamics of Propulsion”, op. cit., pp. 542) was used for all coefficient calculations.
The four test units were 3-D printed using ABS filament: a 7 inch diameter B090 toroid, a 5.1 inch diameter best mode Mantavian Propeller 120, hereinafter noted as Manta 5, a 6.75 inch diameter best mode Mantavian Propeller 120, hereinafter noted as Manta 7, and a 12 inch diameter best mode Mantavian Propeller 120, hereinafter refered to as Manta 12. Both the Manta 5 and the Manta 7 were fabricated as single units as illustrated in FIG. 2. The B090 toroid, believed to be the B100 design disclosed in U.S. Pat. No. 6,736,600 (op.cit.) was fabricated from an STL file obtained on the web. The Manta 12 was fabricated as four separate components and assembled into a complete Propeller 120 as per FIG. 4. Manta 12 included a hub 10 and three attachable rotor wings 20 as shown in FIG. 4. Experimental data was produced, using the test assembly previously described, for the four test devices fabricated as noted above. The MIT patent mentions fabricating toroids and standard geometry propellers, but does not provide any information on airfoils, blade twist or dimensions.
FIG. 6 presents experimental data for two 7 inch diameter propellers. The ordinate axis is rotation speed in RPM, and the abscissa axis is thrust in pounds. Experimental data for the Manta 7 is presented as data set 81. Curve 82 is a thrust curve produced from Eq. 1 using a thrust coefficient of Kt=0.197 and the Manta 7 geometry. Data set 83 is experimental thrust and RPM points for the 7 inch diameter B090. Curve 84 is produced from Eq. 1 using thrust coefficient Kt=0.09 and the B090 geometry.
FIG. 7 presents experimental data for one 12 inch diameter Manta and theoretical curves for both the B160 toroid and the SG30 standard geometry propeller from the MIT patent (op.cit.). The ordinate axis is rotation speed in RPM, and the abscissa axis is thrust in pounds. Data set 91 is experimental data for the Manta 12. Curve 92 is a thrust curve produced from Eq. 1 using thrust coefficient Kt=0.29 and the geometry of the Manta 12. Curves 93 and 94 are theoretical thrust curves produced from Eq. 1 using a published thrust coefficient of Kt=0.16 for the B160, a published thrust coefficient of Kt=0.135 for the SG30 and an equivalent diameter of 12 inches for both B160 and SG30.
FIG. 8 presents experimental thrust coefficients Kt (Eq. 1) plotted versus experimental power coefficients Cpow (Eq. 2) for several propellers, including Manta 5, Manta 7, Manta 12, the B090 toroid, plus the B160 toroid, the B100 toroid, and the SG30 standard geometry 3-bladed propeller from the MIT patent. Experimental data for the Mantas and the B090 was produced using the method and test assembly previously described. Data set 105 is for the Manta 5, data set 101 is for the Manta 7, and data set 102 is for the B090 toroid. Data sets 104, 103, and 106 are published experimental test data for the SG30, the B160, and the B100. Data set 107 is experimental test data for the Manta 12. The Manta 12 is the exact same geometric planform as the Manta 5 and the Manta 7, data sets 101 and 103, but scaled up to a 12 inch diameter and assembled from three rotor wings 20 attached to one hub 10. It is apparent from the data comparison that all three Mantas have higher thrust coefficients than the MIT toroid B160, and that Mantas 7 and 12 are in the same range of power coefficients.
FIG. 9 presents experimental measurements of decibel levels plotted versus thrust coefficients Kt for the Manta 5 (data set 118), the Manta 7 (data set 114), the Manta 12 (data set 111), the B090 toroid (data set 112), as well as the B160 toroid (data set 113), the B100 toroid (data set 115), and the SG30 three-bladed propeller (data set 116) from the MIT patent. The data for the three Mantas and the B090 were produced using the digital decibel recorder previously described. Sound levels were measured in propeller planes of rotation at a distance of 12 inches from the outer diameters. Other propeller data shown was reproduced from the MIT patent. Line 117 is the pre-test measurement of background sound level in dB during the present tests. The Mantas appear to have lower acoustic signatures at higher thrust coefficients than the MIT test devices.
In summary, the experimental data for all three Mantas and the B090 toroid show excellent agreement with thrust coefficient theory using appropriate Kt values for each device. The behavior of the Mantas is interesting in that Kt appears to be nearly constant with respect to power coefficient Cpow, and that the acoustic signatures are all roughly within the same narrow band. Experimental data comparison implies that both Mantas have superior performance to the B-series toroids and the SG30, even though the present tests were conducted in thin air at 5000 ft altitude with air temperatures greater than 100 F. Due to a lack of detailed knowledge about the MIT test units, conditions and apparatus, some differences in decibels would be expected. Any such differences between the present B090 test data and the MIT toroid test data may be attributable to different altitude effects, open air versus closed room background noise, motor noise, propeller design, or any combination thereof.
This best mode Mantavian Propeller 120 offers several advantages compared to other propeller designs. The simple flat rectangular blade geometries allow ease of manufacturing using conventional manufacturing techniques. The Mantavian Propeller 120 has demonstrated lower acoustic signatures at higher thrust coefficients, as well as higher thrust coefficients at comparable input power coefficients, than equivalent diameter toroids. The smaller blade planform areas of the Mantavian device also result in lower overall propeller weight.
Patent Application
20250052157
