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Showing papers on "Blade pitch published in 1991"


Patent
18 Dec 1991
TL;DR: In this article, a machine for converting wind energy into electrical energy comprising in combination a housing, propellers at the forward and rear ends of the housing mounted on shafts for rotation upon a common horizontal axis of rotation, post means supporting a central extent of the house for rotation in a horizontal plane to face into the wind, an alternator within the housing formed of a power take off member coupled to one shaft and a rotor coupled to another shaft for counter rotating when the propellers rotate for generating electricity, and an aerogovernor operatively coupled with the brake mechanisms for control
Abstract: A machine for converting wind energy into electrical energy comprising in combination a housing; propellers at the forward and rearward ends of the housing mounted on shafts for rotation upon a common horizontal axis of rotation; post means supporting a central extent of the housing for rotation in a horizontal plane to face into the wind; an alternator within the housing formed of a power take off member coupled to one shaft and a rotor coupled to another shaft for counter rotating when the propellers rotate for thereby generating electricity; centrifugal pitch control mechanisms for varying the pitch of the propellers as a function of the propeller speed; transmission mechanisms for varying the speed of rotation of the rotor and power take off member as a function of the wind speed; brake mechanisms for proportionately restraining the propellers against rotation; and an aerogovernor operatively coupled with the brake mechanisms for control thereof in response to the speed of the wind as determined by the aerogovernor.

160 citations


PatentDOI
TL;DR: In this article, it has been found beneficial for a forwardly swept leading edge portion to extend at an angle between 30 degrees and 55 degrees from a reference line parallel to a blade pitch change axis and the leading edge of the aerofoil in the notch region incorporates blade droop.
Abstract: Flow separation behind the notch region of a swept tip of a helicopter rotor blade is reduced by features of the geometry and aerodynamic characteristics of the notch region itself. In particular it has been found beneficial for a forwardly swept leading edge portion to extend at an angle between 30 degrees and 55 degrees from a reference line parallel to a blade pitch change axis and the leading edge of the aerofoil in the notch region incorporates blade droop.

66 citations


Patent
13 May 1991
TL;DR: In this paper, an actuator that is responsive to the relative rotation of two input members is used to change the blade pitch in one direction when a braking torque is applied to one member and in another direction when applied to the other member.
Abstract: Blade pitch is changed by an actuator that is responsive to the relative rotation of two input members. Blade pitch is not changed when the members are rotated at the same speed. However, blade pitch is changed in one direction when a braking torque is applied to one member, and blade pitch is changed in an opposite direction when a braking torque is applied to the other member.

53 citations


Patent
19 Apr 1991
TL;DR: In this paper, a variable pitch propeller mechanism includes a plurality of airfoil-shaped blades on three spaced apart blade arms, pivotally connected to the blade arms and interconnected to each other by rigid connecting rods.
Abstract: A variable pitch propeller mechanism includes a plurality of airfoil-shaped blades on three spaced apart blade arms. The airfoil-shaped blades are pivotally connected to the blade arms and are interconnected to each other by rigid connecting rods so these blades move in unison in response to changing wind conditions. Air flow across the airfoil-shaped blades is also controlled to improve the efficiency of the device.

37 citations


Patent
16 May 1991
TL;DR: A helicopter with a retractable rotor for transport wherein the rotor and its blades, the swashplate, the pitch change rods and support mast are lowered or raised in unison by the hydraulic flight control servos as commanded by the helicopter computerized flight control when being commanded by special rotor retraction/extension software.
Abstract: A helicopter with a retractable rotor for transport wherein the rotor and its blades, the swashplate, the pitch change rods and support mast are lowered or raised in unison by the hydraulic flight control servos as commanded by the helicopter computerized flight control when being commanded by special rotor retraction/extension software.

30 citations


Patent
27 Aug 1991
TL;DR: In this paper, the tail rotor of a helicopter is coupled to the main rotor for providing torque compensation so that the helicopter airframe will not counter-rotate under a main rotor.
Abstract: The blade angle controlling pitch beam servo (26) of a helicopter tail rotor (22) is responsive to a main rotor torque signal (92) indicative of torque coupled to a helicopter main rotor (10) for providing torque compensation so that the helicopter airframe will not counter-rotate under the main rotor. The torque coupled to the main rotor is that amount of engine torque (76, 114) in excess of torque coupled to the tail rotor (81, 115) and helicopter auxiliaries (84, 116). The amount of torque compensation provided by the tail rotor is reduced by an amount indicative of aerodynamic forces (130) on the helicopter airframe.

30 citations


Patent
Gregory M. Cole1
27 Mar 1991
TL;DR: An aircraft variable pitch propeller blade was used in subsonic flight at relatively slow turning applications with relatively high power coefficients as mentioned in this paper, where the blade in plan form is tapered at both ends with the maximum chord approximate the mid station of the blade, the leading edge being swept behind the longitudinal axis near its tip and the trailing edge is convex and essentially straight from its maximum chord outward to the tip.
Abstract: An aircraft variable pitch propeller blade utilized in subsonic flight at relatively slow turning applications with relatively high power coefficients. The blade in plan form is tapered at both ends with the maximum chord approximate the mid station of the blade, the leading edge of the blade being swept behind the longitudinal axis of the blade near its tip and the trailing edge is convex and essentially straight from its maximum chord outward to the tip.

19 citations


Patent
13 May 1991
TL;DR: In this paper, a microprocessor is used to determine the relative displacement of the first and second members of a variable pitch actuator, which is indicative of a change in the pitch of the blades.
Abstract: Apparatus indicates changes in blade pitch of variable pitch blades by detecting relative displacements of first and second members of a blade pitch actuator. The members are selectively displaced to change the pitch of the blades. The apparatus includes sensors which provide output signals to a microprocessor, which determines displacement of each member with respect to a frame of reference. The microprocessor takes the difference between the displacements of the two members to determine the relative displacement, which is indicative of a change in blade pitch.

18 citations


Journal ArticleDOI
TL;DR: In this article, a modal control technique based on Floquet theory is used to eliminate the blade's instabilities using collective and cyclic pitch control mechanisms, which is shown to significantly reduce or eliminate regions of flaplag instabilities for a variety of off-design conditions.
Abstract: : The flap-lag equations of motion of an isolated rotor blade and those for a rigid helicopter containing four blades free to flap and lag are derived. Control techniques are developed which stabilize both systems for a variety of flight conditions. Floquet theory is used to investigate the stability of a rotor blade's flap-lag motion. A modal control technique, based on Floquet theory, is used to eliminate the blade's instabilities using collective and cyclic pitch control mechanisms. The technique shifts the unstable roots to desired locations while leaving the other roots unaltered. The control, developed for a single design point, is shown to significantly reduce or eliminate regions of flap-lag instabilities for a variety of off-design conditions. Both scalar and vector control are successfully used to stabilize the blade's motion. Coupling the flap-lag equations of motion of four rotor blades to a rigid airframe alters the flap, lg, and airframes roots. The airframe roots are stabilized using a combination of the body's pitch attitude and pitch rate feedback to the main rotor's longitudinal cyclic pitch. The modal control technique is used to eliminate multiple blade instabilities by first controlling a pair of unstable roots at a specific design point. The resulting closed loop system is a new linear system which periodic coefficients. Another modal controller is designed for this new system to shift a second pair of unstable roots to desired locations. This process is repeated until all instabilities are eliminated. Numerical inaccuracies, however, become noticeable when modal control is used more than once.

16 citations


01 Sep 1991
TL;DR: In this article, a model rotor acoustics test performed to examine the benefit of higher harmonic control (HHC) of blade pitch to reduce blade-vortex interaction (BVI) impulsive noise is presented.
Abstract: Results are presented of a model rotor acoustics test performed to examine the benefit of higher harmonic control (HHC) of blade pitch to reduce blade-vortex interaction (BVI) impulsive noise. A dynamically scaled, four-bladed, rigid rotor model, a 40-percent replica of the B0-105 main rotor, was tested in the German Dutch Wind Tunnel. Noise characteristics and noise directivity patterns as well as vibratory loads were measured and used to demonstrate the changes when different HHC schedules were applied. Dramatic changes of the acoustic signatures and the noise radiation directivity with the HHC phase variations are found. Compared to the baseline conditions (without HHC), significant mid-frequency noise reductions of locally 6 dB are obtained for low-speed descent conditions where GVI is most intense. For other rotor operating conditions with less intense BVI there is less or no benefit from the use of HHC. LF noise and vibratory loads, especially at optimum noise reduction control settings, are found to increase.

15 citations


01 Jan 1991
TL;DR: In this article, a method was developed to analyze the flow around a ducted propeller operating in combination with a set of pre-swirl stator vanes, and the results showed that a very good agreement was achieved in the region of attached flow.
Abstract: A method is developed to analyse the flow around a ducted propeller operating in combination with a set of pre-swirl stator vanes. The method uses a boundary element method to solve for the flow around the duct and the hub and a lifting surface vortex lattice method to solve for the flow around the propeller and stators. The three-dimensional flow around the duct/hub, propeller, and stators is computed separately with the interactions between them being accounted for in an iterative manner. The interactions between the duct and the stators and between the duct and the propeller are treated in a non-axisymmetric manner. However, only the circumferentially averaged interactions between the propeller and the stators are considered. Using this method a duct and set of stators are designed to operate efficiently with an existing propeller. This model was then built and tested in the MIT water tunnel at a variety of stator pitch angles. Comparisons are made between the theoretical and experimental values for the forces on the duct, propeller and stators. A very good agreement is achieved in the region of attached flow.

Journal ArticleDOI
TL;DR: In this article, a computational and experimental method for the evaluation of helicopter rotor tips in high-speed forward flight is presented which uses an unsteady full-potential solver on the advancing side of the rotor disk.
Abstract: A computational and experimental method for the evaluation of helicopter rotor tips in high-speed forward flight is presented which uses an unsteady full-potential solver on the advancing side of the rotor disk. Forces and moments measured during angle-of-attack sweeps reveal the soft-stall phenomenon of the double-swept planforms as well as the delayed stall of the single-swept hyperbolic tip. Double-swept planforms are shown to exhibit the most favorable performance, creating counter-rotating vortices that augment the lifting capabilities of the blade at high angles of attack and delay the onset of stall.

Patent
11 Jun 1991
TL;DR: In this article, the leading edge of a propeller type runner is described in which the leading edges of each blade "lean" forward from the root in the direction of rotation to reduce cavitation and increase efficiency.
Abstract: A propeller type runner is described in which the leading edge of each blade "leans" forward from the root in the direction of rotation to reduce cavitation and increase efficiency. The "lean" provided to each blade of a five blade design is such that the leading edge of each blade obscures a large portion of the trailing edge of the preceding blade.

Patent
15 Jun 1991
TL;DR: In this paper, an aircraft propeller in which the adjustment angle of the propeller blades is variable, with the facility for the propellers blades to be folded inwards during gliding flight is presented.
Abstract: Between the propeller blades is a propeller carrier fixed to the drive shaft. To the outer ends of the carrier the blades are connected, being foldable around the link points to the drive shaft. Between the outer link points and the drive shaft is a link by menas of which the outer link poistion together with the propeller blade is displaceable around an axis (1a) tranversal to the drive shaft. The drive shaft is hollow for fixture and guiding of a flow body fitted in front of the propeller and displaceable in the longitudinal direction of the drive shaft. The link points are formed as shafts which are held in retainers (3) rotatable around the longitudinal axis (1a) of the corresp. propellor blade by means of a positioning device (8). USE/ADVANTAGE - An aircraft propeller in which the adjustment angle of the propeller blades is variable, with the facility for the propeller blades to be folded inwards during gliding flight. It is partic. suited to gliders with auxiliary motor power.

Patent
08 Feb 1991
TL;DR: An asymmetric set of pre-swirl vanes and a specially matched propeller for use on an inclined shaft was designed by considering the mutual interaction of the propeller on the vanes as discussed by the authors.
Abstract: An asymmetric set of pre-swirl vanes (stators) and a specially matched propeller for use on an inclined shaft. The propeller is designed by considering the mutual interaction of the propeller on the vanes and the vanes on the propeller. The propulsor unit provides the following: 1. increased propulsion efficiency due to the reduced rotational (swirl) and axial kinetic energy losses in the propulsor's slipstream; 2. reduction or elimination of propeller cavitation; 3. reduction or elimination of unsteady propulsor forces as well as propulsor-induced hull vibrations. A unique feature of the present invention is that a prior art flat faced commercially available propeller can be modified to match the vane flow field for optimum propulsor performance. The use of commercially available propellers reduces the installation or hardware cost significantly and allows the propeller to be repaired easily if damaged. Another unique feature is that the vanes operate well with an unmodified commercially available prior art flat faced, optimum constant pitch propeller, and that the propeller as modified for use with the vanes also performs exceptionally well without the vanes. The modified propeller without vanes in fact outperformed the prior art flat faced optimum constant pitch propeller used on the 41 foot test craft.

Patent
26 Apr 1991
TL;DR: In this paper, a self-actuating variable pitch marine propeller with two or more blades (20), each independently rotatable, relative to the propeller hub (10), between a first lower and a second higher pitch is provided.
Abstract: There is provided a self-actuating variable pitch marine propeller which incorporates two or more blades (20), each independently rotatable, relative to the propeller hub (10), between a first lower and a second higher pitch. The blades (20) are preferably mechanically linked by coordinating means (25) and are caused to move preferably by a combination of centrifugal force effect resulting from inertial mass means and the hydrodynamic forces acting upon the blade hydrodynamic surface. The rotation of the blades (20) relative to the propeller hub (10) is limited primarily as to speed of rotation by restricted viscous fluid flow damping means (3000) operably connected to the blades (20). In one preferred embodiment, the viscous flow means (3000) further acts as an initial restraint against all motion by closing off the viscous flow orifice (430) until a certain minimum propeller rotational speed is achieved.

Patent
18 Sep 1991
TL;DR: In this paper, a propeller governor control generates a signal which modifies a speed command issued to the propeller, which causes the governor to change blade pitch before reaching its commanded speed.
Abstract: To prevent propeller overshoot, a propeller governor control generates a signal which modifies a speed command issued to a propeller governor. The signal causes the propeller governor to change blade pitch before the propeller reaches its commanded speed. The propeller governor control also compensates for small calibration errors in the propeller governor.

01 Jan 1991
TL;DR: In this article, a wind tunnel investigation has been conducted to parametrically investigate the effect of blade nonstructural mass on helicopter fixed-and rotating-system vibratory loads.
Abstract: A wind tunnel investigation has been conducted to parametrically investigate the effect of blade nonstructural mass on helicopter fixed- and rotating-system vibratory loads. The data were obtained using Mach- and aeroelastically-scaled model rotor blades which allowed for the addition of concentrated nonstructural masses at multiple points along the blade radius. Testing was conducted for advance ratios ranging from 0.10 to 0.35 for ten blade mass configurations. Three thrust levels were obtained at representative full-scale shaft angles for each configuration. Results indicate that proper placement of blade nonstructural mass can provide reductions in fixed-system vibratory loads, but that correct mass placement and the loads reduction realized are dependent upon flight condition. The data base obtained with this investigation provides a comprehensive set of fixed-system shears and moments, blade moment, and blade flap and lag response. The data set is well suited for use in the correlation and development of advanced rotorcraft analyses.

Patent
10 Jun 1991
TL;DR: In this article, a helicopter blade pitch control system is presented, where variable pitch speed paddles attached to rotor blades can change in pitch around the circle of rotation of the blade and also the amount of pitch change can be adjustable by the pilot.
Abstract: A helicopter blade pitch control system allows blade pitch to be controlled with regard to the degree of rotational position of the blade with respect to the direction of travel and also the degree of pitch The invention provides an improved arrangement wherein speed paddles attach to rotor blades can change in pitch around the circle of rotation of the blade and also the amount of pitch change can be adjustably changed by the pilot The apparatus includes a cylindrical member surrounding the rotor of the helicopter and being static with respect thereto so that variable pitch speed paddles connected to and forming a part of the blade assembly and which carry rotating foot members track the upper surface of the cylindrical member changing the pitch of the blade as it moves through a 360 degree rotation The pitch is maximized when the blade moves rearwardly with respect to the direction of travel and the pitch collapses to a horizontal pitch when the blade moves forwardly, in the same direction of travel as the craft

Journal ArticleDOI
TL;DR: In this paper, the blades of two, 8-bladed model rotors with mistuning were compared for measuring vibratory strain amplitudes from off-axis flow, and the results showed that intentional mistuning had a beneficial effect on the aeroelastic response of the propfan motors for a wide range of offaxis flow angles, blade pitch angles, and rotational speeds.
Abstract: Measured vibratory strain amplitudes from off-axis flow are compared for the blades of two, 8-bladed propfan model rotors with mistuning. One rotor had inherent mistuning. The other was intentionally mistuned by replacing every other blade of the first rotor with a blade of same geometry but different frequencies and mode shapes. The data shows that the intentional mistuning had a beneficial effect on the aeroelastic response of the propfan motors for a wide range of off-axis flow angles, blade pitch angles, and rotational speeds. Statistical trends of blade strain amplitudes are compared for both the rotors in terms of the ratio of the maximum to the mean and the coefficient of variation.

01 Jan 1991
TL;DR: In this article, the effect of higher harmonic control (HHC) on a hingeless rotor was investigated in a wind tunnel demonstration with the DLR rotor test rig in the German Dutch Wind Tunnel (DNW).
Abstract: In order to investigate the effect of Higher Harmonic Control (HHC) on a hingeless rotor, a wind tunnel demonstration was perlormed with the DLR rotor test rig in the German Dutch Wind Tunnel (DNW). The test yielded an Impressive result and showed, that a considerable vibration reduction is possible by applying a higher harmonic blade pitch angle to the rotor. Furthermore a simultaneous reduction of all dynamic rotor forces and moments could be achieved with a pure 3/rev or 4/rev blade pitch angle respectively, an unexpected result which can only be explained by aer0elastic phenomena. In order to verify this assumption and to find the reason for the high 3/rev and 4/rev efficiency the wind tunnel data were compared with simulation results attained with different rotor and downwash models. They show the Influence of various aerodynamic and aeroelastlc phenomena on the rotor behaviour and give an explanation for the rotor characteristic ascertained In the wind tunnel. A so called Single Output Controller (SOC) which, although working with a 3/rev blade pitch angle only, succeeds In suppressing the vibrations in all degrees of freedom and, in addition, Is able to keep them small even In case of fast maneuvers finally covers the appflcationar aspect of the new wind tunnel findings.

Patent
12 Jun 1991
TL;DR: In this paper, the side rotor is used as an adjustment propeller to reduce the adjustment angle of the main rotor when flying horizontally at the highest speed through propulsion unloading, by at least one fifth.
Abstract: The propeller used is a normal adjustment type without or with a device for blade inward or outward movement. The helicopter tail outrigger (2) is very asymmetrical in its shape, and the height control tailplane is likewise asymmetrical with a large V-installation angle. The installation angle (3) of the thrust propeller is turned laterally and upwards through 6-10 deg. The side rotor is controllable monocyclically or fully cyclically. The effective rotor adjustment angle of the main rotor when flying horizontally at the highest speed through propulsion unloading is reducible by at least one fifth. USE/ADVANTAGE - For fast flying helicopters in which the side rotor acts as an adjustment propeller.

01 Sep 1991
TL;DR: In this paper, a nonlinear coupled rotor/flexible fuselage analysis has been developed and used to study the effects of higher harmonic blade pitch control on the vibratory hub loads and fuselage acceleration levels.
Abstract: A nonlinear coupled rotor/flexible fuselage analysis has been developed and used to study the effects of higher harmonic blade pitch control on the vibratory hub loads and fuselage acceleration levels. Previous results, obtained with this model have shown that conventional higher harmonic control (HHC) inputs aimed at hub shear reduction cause an increase in the fuselage accelerations and vice-versa. It was also found that for simultaneous reduction of hub shears and fuselage accelerations, a pitch input representing a combination of two higher harmonic components of different frequencies was needed. Subsequently, it was found that this input could not be implemented through a conventional swashplate. This paper corrects a mistake originally made in the representation of the multiple frequency pitch input and shows that such a pitch input can be only implemented in the rotating reference frame. A rigorous mathematical solution is found, for the pitch input in the rotating reference frame, which produces simultaneous reduction of hub shears and fuselage acceleration. New insight on vibration reduction in coupled rotor/fuselage systems is obtained from the sensitivity of hub shears to the frequency and amplitude of the open loop HHC signal in the rotating reference frame. Finally the role of fuselage flexibility in this class of problems is determined.

Patent
12 Sep 1991
TL;DR: In this paper, the mouth of the exhaust tailpipe is located at a small distance from the propeller blade plane and lies on a horizontal plane passing through the axis (6) of a propeller.
Abstract: To reduce the noise of a propeller driven aircraft, the mouth of the exhaust tailpipe is located at a small distance from the propeller blade plane and lies on a horizontal plane passing through the axis (6) of the propeller. The tailpipe is located on the side of the aircraft corresp. to the downwards movement of the blades. The lateral distance of the tailpipe's mouth from the propeller's axis is defined by a mathematical formula dependent upon sound velocity and RPM of the propeller. ADVANTAGE - Noise reduction is optimised.

Patent
07 Jan 1991
TL;DR: In this article, a lead propeller speed feedback loop and a speed derivative prohibition logic based on a detected indicated airspeed VKIAS is proposed to realize highly reliable speed control of those controlling propeller pitch thereby controlling propellers speed.
Abstract: PURPOSE: To realize highly reliable speed control of those controlling propeller pitch thereby controlling propeller speed by providing a lead propeller speed feedback loop and prohibiting looping when airspeed exceeds a specific amount CONSTITUTION: With regard to a propeller speed governor controller of an aircraft flying at high speed, a propeller speed governor for controlling propeller speed by varying blade pitch is provided A lead propeller speed feedback loop is also provided and it is to be prohibited from looping when airspeed exceeds a predetermined amount A lead value of the lead propeller speed feedback loop is set to a higher value than that causing control unstability at airspeed exceeding a predetermined amount A prohibition demanding signal FISD is calculated by a speed derivative prohibition logic based on a detected indicated airspeed VKIAS

Proceedings ArticleDOI
01 Jun 1991
TL;DR: In this paper, a code was developed to predict the periodic aerodynamic loads of an advanced turboprop propeller at a cruise Mach number of 0.813 and 35,000 ft altitude.
Abstract: A code has been developed to predict the periodic aerodynamic loads of an advanced turboprop propeller. The analytical formulation accounts for flow three-dimensionality and flow periodicity due to the propeller inclination. The flow past the blade sections is computed using a thin layer Navier-Stokes solver. An iterative procedure is used to account for the induced axial and rotational velocities. The viscous periodic results are obtained for an eight-bladed Hamilton Standard SR-7L advanced propeller at a cruise Mach number of 0.813 and 35,000 ft. altitude. The results are shown for flow field quantities and performance parameters during the blade passage in the plane of rotation illustrating the periodic nature of blade flow separation and shocks. The time averaged coefficients of thrust and power are computed and compared with available flight test data. The results obtained show excellent agreement at cruise conditions for small nacelle angles of attack.

Patent
07 Feb 1991
TL;DR: The variable pitch propeller for a marine craft has each blade mounted in a bearing in the propeller hub (12) so that the blade can rotate about a radial axis.
Abstract: The variable pitch propeller for a marine craft has each blade (10,11) mounted in a bearing in the propeller hub (12) so that the blade can rotate about a radial axis. Each blade has a spindle which extends radially into the hub and carries a pinion. Each pinion meshes with a toothed rack which moves parallel to the propeller axis in order to change the blade pitch. The mechanism is so designed, that when each blade lies momentarily in a plane tangential to the hull of the craft, each blade, or each group of blades, can be adjusted to a different pitch angle. USE - Variable pitch propeller for marine craft.

01 Jan 1991
TL;DR: In this article, a new method for improving the propeller efficiency due to locating a pre-swirl stator array has been developed by applying linearised lifting surface theory, which has been applied on general stators and in particular stators of low aspect ratio.
Abstract: A new method for improving the propeller efficiency due to locating a pre-swirl stator array has been developed by applying linearised lifting surface theory. This method has been applied on general stators and in particular stators of low aspect ratio. The obtained results of the stator induced velocity are in exact agreement with low aspect ratio wing theory. Incorporating the stator induced velocity together with that of propeller, a new velocity field has been found which led to an improvement of propeller efficiency. Using published theoretical and experimental data of propeller characteristics, comparison has been made between propeller working alone and that working with stator. A gain in propeller efficiency has been achieved at larger advance coefficient and hence a decrease in the required propeller number of revolutions.


Patent
19 Sep 1991
TL;DR: A fixed pitch propeller has blades that are not solely dependent on the blade angle dictated by the helical path of the blade tip as mentioned in this paper, and an outboard portion has a range of blade angles that are relatively flat to operate most efficiently at lower aircraft speeds.
Abstract: A fixed pitch propeller has blades that are not solely dependent on the blade angle dictated by the helical path of the blade tip. An outboard portion of the blade has a range of blade angles that are relatively flat to operate most efficiently at lower aircraft speeds. An inboard portion of the blade has a range of blade angles that are relatively high pitched to allow efficient operation at higher aircraft speeds