US20130315733A1

US20130315733A1 - Passive thrust enhancement using circulation control

Info

Publication number: US20130315733A1
Application number: US13/678,927
Authority: US
Inventors: Jonathan Kweder; James E. Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-11-16
Filing date: 2012-11-16
Publication date: 2013-11-28

Abstract

A passive thrust enhancement system having a propeller, which includes a propeller hub, a first propeller blade, a second propeller blade, wherein each of the first and second propeller blades include a fluid flow channel within the first and second propeller blades, which is fluidly connected to a flow exit slot, and a flow capture device, which includes a plenum chamber, mounted on the propeller hub, wherein the plenum chamber is fluidly connected to the fluid flow channels.

Description

CROSS REFERENCE

This application claims the benefit of provisional application Ser. No. 61/560,530, filed on Nov. 16, 2011.

TECHNICAL FIELD

The present invention relates to passive thrust enhancement systems using circulation control applied to rotating lift/thrust devices and, in particular, to propellers.

BACKGROUND

The principle motivation in the use of circulation control has been to increase the lifting force when large lifting forces and/or slow speeds are beneficial, such as at take-off and landing. On current fixed wing aircraft, wing flaps and leading edge slats are used during landing and on take-off. The benefit of the circulation control wing is that no additional skin friction drag is created by the movement of conventional surfaces into the airflow around the wing and the lift coefficient is greatly increased. However, as with any lifting surface, the use of circulation control increases the induced drag of the airfoil in proportion to the square of the increased lift coefficient.
Circulation control has also been adapted for use on rotary wing aircraft. In the early years of circulation control there were two main critical design issues with the addition of circulation control to a rotating body such as a helicopter rotor. The first because rotors on helicopters see moderate ranges of angles of attack, 0-50 degrees caused by the inflow of air through the rotor plane. Through the study of high angles of attack in wind tunnel testing, it is possible to predict the behavior of the rotor blade at these higher angles of attack (around 20-35 degrees). The second obstacle in the prior applications of circulation control to a helicopter main rotor was the inability to achieve the response times necessary to effectively use circulation control on a rotary wing aircraft to accommodate the asymmetry of lift experienced during maneuvering flight. With the recent development, in the last ten years, of smart materials, a specifically designed near surface piezoelectric valve (U.S. Pat. No. 6,425,553 B1) has been developed, enabling the concept of using circulation control on the blades of a rotary wing aircraft. Through the use of these and similar types of smart materials, the response time of actuation of circulation control can be reduced thus lending the science to a wider array of applications including rotorcraft and propellers.
With the recent development of unmanned aerial vehicles (UAVs), propeller performance enhancement has become an issue. The use of active circulation control on the propeller, though potentially beneficial is currently envisioned as creating technical difficulties through the supply of air to the circulation control blowing slot. Thus, a passive thrust enhancement system in which air can be supplied to a strategically placed circulation control blowing slot can enhance the performance of a propeller. This passive thrust enhancement system of circulation control for a propeller is described as the preferred embodiment of the device. However, this device functions with any fluid or gas surrounding, and passing through a similar device such as a boat propeller or horizontal axis wind turbine.

SUMMARY

The use of the term passive system indicates that mechanical and/or electrical power is/are not used to supply the airflow to the circulation control sub-system. The use of this system can be applied to any rotating object which generates a fluid dynamic force in any fluid medium, such as an aircraft propeller, a boat propeller, or a horizontal axis wind turbine.
According to a first aspect, the passive thrust enhancement system includes a propeller, which includes a propeller hub, a first propeller blade, a second propeller blade, wherein the first and second propeller blades include a fluid flow channel within the first and second propeller blades, which is fluidly connected to a flow exit slot, and a flow capture device, which includes a plenum chamber, mounted on the propeller hub, wherein the plenum chamber is fluidly connected to the fluid flow channels.
According to a second aspect, the passive thrust enhancement system includes a propeller, which includes a propeller hub, a first propeller blade, a second propeller blade, wherein the first and second propeller blades include a fluid flow channel within the first and second propeller blades, which is fluidly connected to a flow exit slot, a flow capture device, which includes a plenum chamber, mounted on the propeller hub, wherein the plenum chamber is fluidly connected to the fluid flow channels, and a flow control valve.

BRIEF SUMMARY OF THE FIGURES

FIG. 1 depicts a side view of an embodiment of a propeller with a passive thrust enhancement system.

FIG. 2 depicts a longitudinal cross section of an embodiment of a propeller blade.

FIG. 3 depicts a lateral cross section of an embodiment of a propeller blade.

FIG. 4 depicts a cross section of an embodiment of a flow capture device.

FIG. 5 depicts a graph of an embodiment showing the relationship between inlet pressure and forward velocity.

FIG. 6 depicts a graph of an embodiment showing the relationship between plenum pressure and blade diameter.

DETAILED DESCRIPTION

Referring to FIG. 1, an embodiment of a passive thrust enhancement system 100 is shown. The passive thrust enhancement system 100 includes a propeller hub 102, a first propeller blade 104, a second propeller blade 106, and a flow capture device 108. The combination of the propeller hub 102, the first propeller blade 104, and the second propeller blade 106 may also be referred to as a propeller 110. All components of the passive thrust enhancement system 100 may be made out of steel, or any other material known in the art used in the making of propellers or other rotating lift/thrust devices.
Referring to FIGS. 2-4, in FIGS. 2 and 3, cross sections of an embodiment of the first propeller blade 104 are shown. Contained within the first propeller blade 104 is a fluid flow channel 200 and a flow exit slot 202. The design of the second propeller blade 106 is identical to the first propeller blade 104. FIG. 4 shows a cross section of an embodiment of a flow capture device 108, which is mounted on the propeller hub 102. A plenum chamber 400 is located within the flow capture device 108. The plenum chamber 400 is fluidly connected to the fluid flow channels 200 that are located within the propeller blades. For simplicity, FIG. 4 shows one embodiment of the shape of the flow capture device 108. The flow capture device 108 may also be a conical structure with straight sides, or could have sides with a continuously variable curvature, or any other shape that is known in the art.
In an embodiment, the flow capture device 108 has a diameter no larger than the diameter of the propeller hub 102, and the distance the flow capture device 108 extends from the propeller hub 102 is between ¼ and 1 diameters of the propeller hub 102. In another embodiment, the flow capture device 108 has a diameter greater than the diameter of the propeller hub 102 with an upper limit of 1.5 times the diameter of the propeller hub 102. Depending on the design speed of a craft utilizing the passive thrust enhancement system 100, other sizes and shapes of flow capture devices 108 may be used along with other distances from the propeller hub 102, which are known in the art to not negatively impact drag.
In an embodiment, the free stream fluid or gas “pulled” into the propeller 110 imparts a pressure on the propeller hub 102. The pressure on the front side of the propeller 110 is greater than the local atmospheric pressure. In fluid dynamics, pressure flows from high pressure to low pressure, thus for air, or similar fluid, a local velocity will be generated. The flow capture device 108 is attached to the propeller hub 102, and is fluidly connected to the fluid flow channels 200. The higher pressure at the center of the propeller hub 102 is utilized to drive a flow into the flow capture device 108 through the plenum chamber 400 and through the fluid flow channels 200 to the flow exit slots 202. In addition, the fluid will be accelerated due to the centripetal forces applied by the rotational nature of the propeller 110, overcoming the friction in the fluid flow channels 200 and adding to the pressure at the flow exit slots 202.
The higher pressure at the propeller hub 102 is loosely dependent on the propeller performance, only from the standpoint that a higher static thrust propeller will impart more velocity to the surrounding fluid. Thus, any airfoil shape and any propeller diameter can be utilized with the passive thrust enhancement system 100, however, the mass flow rate supplied to the passive thrust enhancement system 100 will vary. The variation in mass flow may require slightly different sized flow exit slots 202 and different configurations of the fluid flow channels 200, however, the passive circulation control air, or fluid, supply concept is unchanged.
In an embodiment, though the circulation control sections of the propeller blades 104 and 106 have a rounded trailing edge. Additionally, the flow exit slots 202 can be placed anywhere along the span of the propeller blades 104 and 106. In another embodiment, the flow exit slots 202 are located between the propeller hub 102 and 33% of the radius of propeller blades 104 and 106. In another embodiment, the flow exit slots 202 are located between 33% and 66% of the radius of propeller blades 104 and 106 measured out from the propeller hub 102. In another embodiment, the flow exit slots 202 are located between 66% and 100% of the radius of propeller blades 104 and 106 measured out from the propeller hub 102.
The sections of the propeller blades 104 and 106 not containing flow exit slots 202 will contain a conventional pointed trailing edge airfoil. In an embodiment, the flow exit slots 202 can be placed at the tips of the propeller blades 104 and 106, which will vent the pressurized air out of the tips of the propeller blades 104 and 106, Venting the pressurized air in this manner may not add to the lift, but will decrease the stagnation pressure from in front of the propeller 110.
The passive nature of the passive thrust enhancement system 100 also has an advantage of being constantly enabled, and with a simple flow control valve (not shown) can be restricted to times where high lift is needed (i.e. takeoff and landing or for extended flight conditions). It is also conceived that the flow control valve incorporated into the passive thrust enhancement system 100 can operate as a throttle to adjust the mass flow supplied to the flow exit slots 202.
The inward velocity through the rotor plane can be converted to a pressure force on the propeller hub 102, and through an analysis of the Bernoulli Equation the relationship between pressure and velocity can be found, see Equation 1 below. With an inlet on the flow capture device 108 feeding to a fluid flow channel 200 in the propeller blade, pressurized air induced from forward thrust (T) can move through the inlet at the higher pressure stagnation location, on the front of the propeller 110, to a low pressure region (atmospheric, or less) at the flow exit slot 202. The Bernoulli Equation can be used to estimate the inlet pressure based on the assumption that the initial conditions are seen as atmospheric conditions and the forward speed of the craft is zero. FIG. 5 shows a graph of the impact of forward velocity on the inlet pressure.
P+½ρV ² +yz=constant t Equation 1
In an embodiment, also taken into account is the internal plenum pressurization due to radial acceleration of the propeller 110 which increases the jet exit mass flow rate and in turn the expansion rate of the exiting fluid flow. The Bernoulli Equation is used to determine the exit velocity of the passive circulation control augmented propeller 110. Equation 2, which is a derived expression from the Bernoulli, Ideal Gas, and Isentropic Equations, is used to determine the magnitude of the jet exit velocity. Equation 2 is based on the specific heat ratio (y), and the gas constant (R) of the fluid as well as the temperature (T) and internal pressure (P) of the plenum chamber 400. FIG. 6 shows the trends of plenum pressurization capabilities due to centripetal acceleration as well as forward velocity, and was performed on varying size propeller blades ranging from 0.1 meters to 1.0 meter. The range of propeller blades shown in FIG. 6 are examples, and not meant as a limitation. The propeller blades could be any size. The trend shows a logarithmic increase in the amount of pressure in the plenum as the rotation increases and the propeller diameter increases. It should be noted that the indicated increase does not hold when the tip speed is transonic or supersonic, that is the tip speed surpasses the local speed of sound. Also note that for simplicity, the fluid flow channel 200 is assumed to be 0.05 by 0.05 meters, but could be any size and shape which fits within the propeller blade.
$\begin{matrix} V = \sqrt{\frac{2 γ RT}{1 - γ} [1 - {(\frac{P_{\infty}}{P})}^{\frac{γ - 1}{γ}}]} & Equation 2 \end{matrix}$
While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by the way of example only, and not limitation. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments.

Claims

What is claimed is:

1. A passive thrust enhancement system comprising:

a propeller, which includes:

a propeller hub;

a first propeller blade;

a second propeller blade;

wherein each of said first and said second propeller blades include a fluid flow channel within said first and said second propeller blades, which is fluidly connected to a flow exit slot; and

a flow capture device, which includes a plenum chamber, mounted on said propeller hub, wherein said plenum chamber is fluidly connected to said fluid flow channels.

2. The passive thrust enhancement system of claim 1, wherein said flow capture device is a conical shape with straight sides.

3. The passive thrust enhancement system of claim 1, wherein said flow capture device is a conical shape with continuously curved sides.

4. The passive thrust enhancement system of claim 1, wherein the diameter of said flow capture device is less than or equal to 150% of the diameter of said propeller hub.

5. The passive thrust enhancement system of claim 1, wherein the distance that said flow capture device extends from said propeller is between ¼ and 1 diameters of said propeller hub.

6. The passive thrust enhancement system of claim 1, further comprising a flow control valve.

7. The passive thrust enhancement system of claim 1, wherein said propeller is an aircraft propeller.

8. The passive thrust enhancement system of claim 1, wherein said propeller is a boat propeller.

9. The passive thrust enhancement system of claim 1, wherein said propeller is a horizontal axis wind turbine.

10. A passive thrust enhancement system comprising:

a propeller, which includes:

a propeller hub;

a first propeller blade;

a second propeller blade;

wherein each of said first and said second propeller blades include a fluid flow channel within said first and said second propeller blades, which is fluidly connected to a flow exit slot;

a flow capture device, which includes a plenum chamber, mounted on said propeller hub, wherein said plenum chamber is fluidly connected to said fluid flow channels; and

a flow control valve.

11. The passive thrust enhancement system of claim 10, wherein said flow capture device is a conical shape with straight sides.

12. The passive thrust enhancement system of claim 10, wherein said flow capture device is a conical shape with continuously curved sides.

13. The passive thrust enhancement system of claim 10, wherein the diameter of said flow capture device is less than or equal to 150% of the diameter of said propeller hub.

14. The passive thrust enhancement system of claim 10, wherein the distance that said flow capture device extends from said propeller is between ¼ and 1 diameters of said propeller hub.

15. The passive thrust enhancement system of claim 10, wherein said propeller is an aircraft propeller.

16. The passive thrust enhancement system of claim 10, wherein said propeller is a boat propeller.

17. The passive thrust enhancement system of claim 10, wherein said propeller is a horizontal axis wind turbine.