Abstract:
A rotor blade for a reaction drive type helicopter is provided. The rotor blade includes a main duct extending from a proximal end, couplable to and for fluid communication with a rotor hub, to a distal end for ducting a first air/gas stream from the rotor hub to the distal end. A nozzle is attached to an outlet of the main duct at the distal end for receiving the first air/gas stream from the main duct and releasing the first air/gas stream to propel the rotor blade. A circulation control is carried at a trailing edge of the blade. A trailing edge duct is carried intermediate the trailing edge and the main duct and is in fluid communication with the main duct by a partition with a plurality of orifices formed therein to bleed air from the main duct and generate a second air/gas stream therein with a pressure less than the pressure of the first air/gas stream. The trailing edge duct supplies the second air/gas stream to the circulation control.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 62/015,254, filed 20 Jun. 2014. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of aviation. 
     More particularly, the present invention relates to propulsion systems for helicopters. 
     BACKGROUND OF THE INVENTION 
     A reaction drive, also known as a pressure-jet and a tip-jet system, have been used successfully in the past to provide rotor power for helicopters. Reaction drive helicopters differ from conventional helicopters in that the rotor power is provided by the thrust of jets mounted at the blade-tips. This eliminates the mechanical transmission systems of conventional helicopters leading to a much lighter aircraft, requiring less energy to move. Reaction drive helicopters have a number of variants which, for the purposes of this invention, are considered to be divided into a first type in which air or gasses are directed through the blades and out a nozzle at the blade tip, and a second type in which a motor is positioned at the blade tip. The first type is typically differentiated on the basis of the air or gas temperature exiting through the jet nozzle at the tips of the helicopter blades. Usually these are labeled hot, warm or cold cycle tip-jet systems and are generated remotely from the blade tip. It is recognized that reaction drive helicopters are part of a larger group of related propulsion units that are generally termed reactive jet drive rotor systems. This larger group encompasses other helicopter rotor tip driven systems including the second type, in which motors such as turbojets, rockets, ramjets, pulse jets and other combustion engines attached to the blade tips have been used to provide rotor power for lifting and forward flight purposes. This invention is concerned with the first type of reaction drive helicopter. 
     In the field of aeronautics, circulation control is an approach used to modify an airfoil&#39;s aerodynamic forces using a specially shaped trailing edge instead of moving surfaces such as flaps. The main purpose of circulation control is to increase the lifting force of the airfoil at times when large lifting forces at low speeds are required, such as takeoff and landing. Circulation control airfoils take advantage of the Coanda effect which increases lift through the interaction of an air jet flowing through a slot in the trailing edge of the airfoil and a free air stream moving over the airfoil&#39;s upper surface as the airfoil moves through the air. A jet of air flows out of the slot and follows the curvature of a highly curved lower surface of the airfoil. The jet of air from the slot entrains the free air stream moving over the airfoil to create a laminar flow around the curvature, creating lift. 
     While circulation control systems work well on conventional airfoils, the use on a reaction drive helicopter is problematic. The air flow supplied to the rotor blades is employed for powering the rotors by being released through nozzles at the blade tips. Another problem using circulation control on rotor blades is flow separation. The flow over the Coanda surface typically separates at an angle between 120-125° from the slot. This separation can become periodic in nature with the separation point alternating between the two angles. This separation “flipping” can cause vibration and periodic lift forces. 
     It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art. 
     An object of this invention is to use circulation control on a reaction drive helicopter. 
     Another object is to minimize the unsteady lift phenomenon that occurs due to separation over the Coanda surface. 
     SUMMARY OF THE INVENTION 
     Briefly, to achieve the desired objects and advantages of the instant invention, provided is a rotor blade for a reaction drive type helicopter. The rotor blade includes a proximal end couplable to a rotor hub, a distal end terminating in a blade tip, and a trailing edge extending from the proximal end to the distal end. A main duct extends from the proximal end, for fluid communication with the rotor hub, to the distal end. The main duct is for ducting a first air/gas stream from the rotor hub to the blade tip. A nozzle is attached to an outlet of the main duct at the blade tip for receiving the first air/gas stream from the main duct and releasing the first air/gas stream to propel the rotor blade. A circulation control is carried at the trailing edge. A trailing edge duct is carried intermediate the trailing edge and the main duct and separated therefrom by a partition. A plurality of orifices are formed in the partition to bleed air from the main duct and generate a second air/gas stream therein with a pressure less than the pressure of the first air/gas stream. The trailing edge duct supplies the second air/gas stream to the circulation control. 
     In a specific aspect, the circulation control includes an upper surface of the blade terminating in a lip edge at the trailing edge. A lower surface terminates in a Coanda surface at the trailing edge. A slot is defined between the Coanda surface and the lip edge at the trailing edge in fluid communication with the trailing edge duct for receiving the second air/gas stream from the trailing edge duct and releasing the second air/gas stream to produce a Coanda effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the drawings, in which: 
         FIG. 1  is a representation of a reaction drive helicopter incorporating circulation control according to the present invention; 
         FIG. 2  is a cross-sectional partial view of a rotor blade according to an embodiment of the present invention; 
         FIG. 3  is a perspective partial view of the rotor blade of  FIG. 2 ; and 
         FIG. 4  is a graph that can be used to find the required area for a desired pressure drop and mass flow. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings in which like reference characters indicate corresponding elements throughout the several views, attention is first directed to  FIG. 1  which illustrates a reaction drive helicopter, generally designated  10 . Helicopter  10  includes a fuselage or body  12  carrying an engine  14  producing a stream of compressed air and/or gas  15 . The air or gas flow path for reaction drive helicopters originates at either a driven load compressor  16  or a bleed from a gas turbine engine (not specifically shown). The air is ducted from engine  14  and/or compressor  16  to a hollow rotor mast  18  where it flows vertically upward to a hub  19  of a rotor  20 . Hub  19  has air channels that divide and transmit the air/gas to rotor blades  22  coupled to hub  19 . Each blade  22  includes a proximal end  24  coupled to hub  19  and a distal end  25  terminating in a blade tip  26 . Blades  22  are hollow and define a main duct  23  extending from proximal end  24  to distal end  25  and are in communication with hollow rotor mast  18  through hub  19 . The air/gas flow from mast  18  is turned through 90-degrees and split by hub  19 . The air/gas is redirected and split between blades  22  where it is ducted through main duct  23  to blade tips  26  and discharged, through nozzles  28 . The discharged air/gas induces rotational movement of blades  22 . Specific details of the reaction drive helicopter  10  and details of the production of the air/gases ducted to the blade tips have not been provided, since the blade tips, according to the present invention, will function with substantially any reaction drive helicopter discharging air/gas through the blades. How the air/gas is generated can be accomplished in a variety of methods. 
     Turning now to  FIGS. 2 and 3 , also provided is a circulation control  30 , carried at a trailing edge  32  of each rotor blade  22 . Circulation control  30  produces a jet of air  33  along trailing edge  32 . A portion of the air/gas traveling within main duct  23  is diverted to circulation control  30  to produce jet of air  33  along trailing edge  32 . Circulation control  30  includes a trailing edge duct  34  in fluid communication with main duct  23  of blade  22 . In the preferred embodiment main duct  23  is separated from trailing edge duct  34  by a partition  36  that has orifices  38  of known coefficient of discharge. Trailing edge duct  34  is formed between an upper surface  40  and a lower surface  42  terminating at trailing edge  32  in a highly curved Coanda surface  43 , having a radius of curvature R. Upper surface  40  terminates in a lip edge  44  of thickness T. Lip edge  44  and Coanda surface  43  are spaced apart to form a slot  45  having a height H running the length of blade  22 . In the preferred embodiment the lip edge is relatively sharp, and the thickness T is less than 0.4 times the slot height H to avoid a wake region downstream of the lip that disturbs the flow of air over Coanda surface  43 . 
     In operation, as rotor blade  22  passes through ambient air, a free air stream  58  travels along upper surface  40 . As free air stream  58  reaches slot  45 , it is entrained by air jet  33  and follows the curvature of Coanda surface  43  until air jet  33  and entrained free air stream  58  separate at separation point S. The separation point is typically at an angle between 120-125° from slot  45 . This range of separation locations can be problematic, and will be addressed presently. 
     Jet  33  is blown from slot  45  over a highly curved aerodynamic surface (Coanda surface  43 ) to increase or modify the aerodynamic forces and moment with few or no moving surfaces. In general, the driving parameter of Circulation Control is the jet momentum coefficient, Cmu: which is defined as the mass flow of the jet (m jet ) times the velocity of the jet (V jet ) divided by the dynamic pressure (q) and the area of the airfoil (S). The value of jet momentum coefficient (Cmu), the direction of jet  33 , and the total temperature of jet  33  are usually specified based on past experimental work. In most cases the direction of jet  33  is set normal to slot  45 , and thus tangential to Coanda surface  43 . For a typical reaction drive rotor blade chord length of 8 inches the standard slot height is about 0.2% of the chord or 0.015-inch. The slot is typically located at x/c=88.75% on the upper side of the airfoil. Most of the studies involve sea level standard day conditions with free stream velocities between 65 and 105 miles per hour. It can be shown that the coefficient of lift (Cl) is nearly constant over this wide range of speeds and it is generally concluded that the lift is independent of free air stream  58  velocities so long as Cmu is held constant. At very low momentum coefficients, the tangential blowing of jet  33  will add energy to the slow moving flow near the surface. This will delay or eliminate the separation, and is called Boundary Layer Control. When the momentum coefficient is high, the lift of the blade will be significantly increased. This is called Circulation Control (CC). The lift augmentation, which is defined as ΔCL/ΔCmu, can exceed 80. In the preferred embodiment of the present invention, the thickness T of lip edge  44  at the flow exit point is relatively critical and thickness T of lip edge  44  at trailing edge  32  will not exceed 1.4 times the slot height. If the ratio is larger, then a wake region may form downstream of the lip. 
     In many instances the power of rotor  20  is desired to be greater than the power available when circulation control system  30  and nozzles  28  employ the same pressure expansion ratio. To increase rotor power, rotor blade tip nozzles  28  need to have a higher pressure ratio than the circulation control jet  33 . The pressure of air/gas supplied to circulation control  30  is generally limited to pressure ratios less than 2.5 atm. To properly increase power to blades  22 , nozzles  28  are designed as supersonic nozzles that can stably produce very high velocities typically using pressures greater than 3 atm. This type of operation provides high take-off and hover power levels. The pressure to Coanda jet  33  would ideally be reduced to a level around 2 atm. This latter pressure avoids bifurcated shock waves forming in Coanda jet  33  leaving slot  45 , which is not designed as a supersonic nozzle. At the high isoentropic velocities produce by expanding from high pressures the jet velocities can undergo local supersonic to subsonic velocity changes through local shock structures that negatively impact the attachment of jet  33  to Coanda surface  43 . Besides loss of performance this can also create significant noise levels which need to be addressed. 
     Thus, the high pressure air/gas stream entering main duct  23  from hub  19  must be separated into two streams, one a high pressure stream (&gt;3 atm) of high mass flow and the other a low pressure stream (&lt;2.5 atm) of low mass flow, before they are allowed to expand to atmosphere through two separate nozzle systems. The high pressure stream provides rotor power to reaction drive helicopter  10  through its expansion to supersonic velocities through nozzles  28  at tips  26  of blades  22 . The low pressure stream (should be less than 2.2-atm) is to create lift through the use of circulation control provided by Coanda surface  43 . Coanda surfaces  43  do not work well with the supersonic velocities that would be created if the high pressure main stream was used. Thus, a lower pressure air stream for circulation control must be created from the high pressure stream exiting the hub. 
     The approach to separating the flows in the present embodiment is to use a series of orifices  38  located in partition  36  separating main duct  23  carrying the high pressure flow, with edge duct  34 . Partition  36  can be a plate or sheet in the form of perforated sheet stock. This plate or sheet runs from proximal end  24  to distal end  25 . A non-dimensional pressure loss parameter (ΔPc/P) can be defined in terms of the total effective open hole area (A eff ) and the desired Coanda mass flow (Wc) for a given main stream pressure (P) and temperature (T) shown as Equation 1 (Imperial units). The actual pressure loss over the plate (ΔPc) is typically non-dimensionalized for ease of use.
 
Δ Pc/P =(0.829)*( Wc*T   0.5   /A   eff   *P ) 2   Equation 1
 
     The effective open hole area is equal to the geometric open hole area multiplied by a “discharge coefficient” (Cd). The value of Cd is a function of the hole diameter (d) based Reynolds Number (Red), the pitch to diameter ratio (P/d) of the holes and the ratio of plate thickness to the hole diameter (T/d). Correlations for the discharge coefficient (Cd) are available from a number of investigators for triangular hole pitches. These are typically provide as a function of K which in turn is a function of the ratio of the hole diameter to the hole pitch (d/P). The empirical equation that best describes the relationship between Cd and K is provided here as Equation 2.
 
 Cd=K ( d/P )0.10  Equation 2
 
     For design purposes (T/d) is taken to be greater than 1.6 which is usually required to provide the necessary strength to withstand the pressure drop over the holes. If it is also assumed that the flow involved is fully turbulent (Red greater than 20,000) then K is approximately constant at 0.965 for T/d&gt;1.6. With these design assumptions the discharge coefficient can be determined with high accuracy well below the effects of hole eccentricity or other manufacturing flaws. 
     Turning briefly to  FIG. 4 , a graph provides the relationships of Equation 1 in graphical form and is loosely based on past inlet pressures and temperatures together with estimates of the Coanda mass flow for a small reaction drive (2-seat) helicopter. The graph can be used to find the required area for a desired pressure drop and mass flow. For a hole diameter of around 0.1-inch the required number of hole is on the order of thousands. More typically because the blade spans can be around 9-ft even for small reaction drive helicopters, the hole diameters will probably be closer to 0.05-inch. For higher mass flows (larger helicopters) the holes will be larger in diameter for a given pressure drop. The characteristics shown in  FIG. 3  relate to perforated sheet/plate stock where the holes are arranged on a triangular pitch basis. 
     In addition to the above circulation control system  30 , a trip-strip  50  has been added to Coanda surface  43 . The flow over the surface typically separates at an angle between 120 to 125-degrees from slot  45 . This separation can become periodic in nature with the separation point moving between the two angles. This separation “flipping” may cause vibration and periodic lift forces. To reduce or avoid this unsteady flow phenomenon, a trip-strip  50  consisting of a length of wire is attached to the surface at or about the 120-degree point to force the separation to consistently take place at this specific point. The shape of the wire depicted is generally circular in section but can be almost any shape. The height of the trip strip will be derived for each specific application as height depends on and varies with the Coanda surface shape and surface finish. 
     Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof, which is assessed only by a fair interpretation of the following claims. 
     Having fully described the invention in such clear and concise terms as to enable those skilled in the art to understand and practice the same, the invention claimed is: