Showing papers on "Airfoil published in 1970"

Two-Dimensional Transonic Wind Tunnel Tests of Three 15-Percent Thick Circulation Control Airfoils

Abstract: : Two relatively thin Circulation Control (CC) elliptic airfoils were tested subsonically to determine their characteristics as proposed helicopter rotor tip sections. These airfoils, employing tangential trailing edge (Coanda) blowing, had shown very promising transonic characteristics in previous tests. It was the purpose of the subonic retests to determine if these thin sections could generate low speed characteristics which would be equally impressive. Due to its more forward slot location, the 15-percent thick pure elliptic section displayed effective subsonic operation at positive angle of attack, reducing drag while producing lift coefficients up to 3.5. The rounded trailing edge configuration, with further aft slot and better Coanda deflection of the jet, generated lift coefficients up to 4.25 (with a preference for negative incidence), but experienced higher drag levels. As a result of the small nose radii and low test Reynolds number, both sections were limited in performance by leading edge separation. At a fixed momentum coefficient, variation in slot height indicated that better performance was obtained for reduced heights. This was due primarily to higher energy levels in the jet sheet, but the lower bound on slot height was limited by boundary layer building in very small nozzles. Comparison of both CC sections to the more conventional NACA 0012 blade section indicated far greater lift capabilities with circulation control. However, due to blowing power requirements, equivalent efficiency was less at positive incidence, than for the conventional section.

Trailing-edge stall

Abstract: A study is made of the laminar flow in the neighbourhood of the trailing edge of an aerofoil at incidence. The aerofoil is replaced by a flat plate on the assumption that leading-edge stall has not taken place. It is shown that the critical order of magnitude of the angle of incidence α* for the occurrence of separation on one side of the plate is , where R is a representative Reynolds number, for incompressible flow, and α* = O(R−¼) for supersonic flow. The structure of the flow is determined by the incompressible boundary-layer equations but with unconventional boundary conditions. The complete solution of these fundamental equations requires a numerical investigation of considerable complexity which has not been undertaken. The only solutions available are asymptotic solutions valid at distances from the trailing edge that are large in terms of the scaled variable of order R−⅜, and a linearized solution for the boundary layer over the plate which gives the antisymmetric properties of the aerofoil at incidence. The value of α* for which separation occurs is the trailing-edge stall angle and an estimate is obtained from the asymptotic solutions. The linearized solution yields an estimate for the viscous correction to the circulation determined by the Kutta condition.

Optimization of airfoils for maximum lift

Abstract: The pressure distribution which provides the maximum lift without separation for a monoelement airfoil in an incompressible flow is determined using existing boundary-layer theory and the calculus of variations. The airfoil profiles corresponding to these pressure distributions are determined using second-order airfoil theory. The results indicate maximum lift coefficients as high as 2.8 for Reynolds numbers between five and ten million, and the corresponding drag coefficients are on the order of 0.01. Compressibility has not been considered directly, however the form of the optimum pressure distributions suggests that the critical Mach numbers should be on the order of 0.35.

Unsteady airfoil stall review and extension

Abstract: Unsteady airfoil stall in incompressible flow, including pitch rate induced accelerated flow effect on leading edge and trailing edge stall

Aircraft fan with outflow deflectors

Abstract: The cruise and lift fans of an aircraft are provided with an array of deflectors of the jalousie type forming a directed outflow path. The deflectors are generally of airfoil or aerodynamic profile and have flexible skins with an internal mechanism enabling the curvature to be altered to adjust the outflow direction through at least 105 DEG .

The Two-Dimensional Oscillating Airfoil in a Wind Tunnel in Subsonic Flow

Abstract: A method is developed for computing the forces acting on an oscillating two-dimensional airfoil in a wind tunnel in subsonic compressible flow. The cases of closed, open, and ventilated tunnel walls are considered. The method is exact in that it provides a complete solution to the linearized mathematical model of the problem. The partial differential equation for the pressure potential is solved by an integral transform technique. This leads to an integral equation relating the known oscillatory motion of the wing to the unknown pressure acting on it. A numerical method is presented for the solution of this integral equation.

Control flap arrangement

Abstract: A control flap arrangement for an airfoil of an aircraft has a control flap with a nose surface defined by a center of curvature. The control flap is mounted to the airfoil for pivoting movement with respect thereto, and is actuated by a fluid piston-cylinder device which is connected to the airfoil and to the control flap at a point offset from the pivot point thereof. Cover members are mounted in the slot formed between the control flap and the airfoil in such a manner that when the control flap is pivoted with respect to the airfoil the respective one of the cover members arranged in the slot on the side to which the control flap is pivoting will move away from the airfoil to divert air through the slot and the control flap will move away from the other device to form the slot.

Two Dimensional Subsonic Wind Tunnel Tests on a 20 Percent Thick, 5 Percent Cambered Circulation Control Airfoil

Abstract: : An experimental program was undertaken to develop circulation control, high lift airfoils for rotary wing vehicle application. The basic method used to eject a thin jet sheet of air tangentially over the rounded trailing edge of a thick airfoil, usually of modified elliptic cross section. The jet sheet remains attached to the rounded trailing edge, separating, eventually, on the underside. The report presents results for cambered ellipse. Lift, drag and section equivalent lift-drag ratio data are presented which indicate that this model is one of the most efficient high lift airfoils yet tested.

Reverse velocity rotor and rotorcraft

Abstract: A rotary wing aircraft and rotor control system for such aircraft, which uses the reverse velocity airflow produced by high airspeeds of the aircraft across the retreating blade to generate positive lift on the retreating blade. Blade pitch control means and method for such aircraft providing a two/revolution, cam or higher order cyclic pitch input to generate more uniform lift in the advancing and retreating blade sectors of rotor motion. A main rotor for such aircraft having a new airfoil shape adapted to more efficiently generate positive lift from the reverse velocity airflow, and also having slotted flaps or other auxiliary lift devices for increasing lift at low aircraft airspeeds. A twistably-mounted tail rotor for such aircraft adapted for counter-torque operation at low aircraft airspeeds and for forward propulsion at high airspeeds.

Aircraft wing airflow control system

Abstract: A system for controlling flow through a passageway located in the aft 30 percent chord region of an aircraft wing airfoil to change the aerodynamic flow pattern during ground roll conditions, and thereby spoil lift, generate a downwardly acting force to increase the effective aircraft weight on the wheels, increase drag, and create thrust reversal by redirection of engine exhaust gases. The preferred embodiments involve unique flap means which may be actuated into position to intercept flow beneath the wing and turn the flow upwardly through the passageway. Disclosed flap members include a modified Fowler flap with provision for increased rotation forwardly to an acute angle with respect to the wing, and several optional cascade vane flap devices. An improved spoiler member incorporating an aft facing convex surface is also presented.

Sears function and lifting surface theory for harmonic gust fields

Abstract: Sears' function for the lift on a two-dimensional airfoil induced by a harmonic gust field in an incompressible flow is reviewed. It is observed that a much simpler function can be defined by a transformation of the gust reference point from the airfoil midchord to its leading edge. The simplicity of the modified Sears function permits accurate interpolation for the large number of reduced frequencies required in a gust frequency response analysis. Approximations to the modified Sears function are discussed for use in preliminary analysis. The form of the transformation appropriate for interpolation of gust loads obtained from lifting surface theory for finite aspect ratio wings is then discussed. Both oneand twodimensional gust fields are considered. It is shown by example calculations for compressible subsonic flow that the transformation required by lifting surface theory is analogous to that required by the two-dimensional incompressible case. It is concluded that the computational savings permitted by interpolation makes lifting surface methods economical for gust frequency response analysis. It is also concluded that the Doublet Lattice Method for subsonic flows has the versatility required for load calculations for both oneand two-dimensional gust fields.

The response and airloading of helicopter rotor blades due to dynamic stall

Abstract: : An aerodynamic model is constructed for the application of the properties of dynamic stall of airfoils to the calculation of the airloads and blade motion of helicopter rotor blades. Dynamic stall occurs on an airfoil undergoing pitching motion at high angle of attack, and is characterized by peak section lift and moment much larger than the corresponding static stall loads. A method is developed for the solution of the equations of motion of a rotor blade by means of harmonic analysis. The effect of dynamic stall on the blade torsional motion at high advance ratio is examined, and comparison is made with the limited experimental data available. An increase in the dynamic stall angle is shown to significantly decrease the amplitude of the pitch motions.

A review of para-foil applications

Abstract: and agree well with the semiempirical expression previously determined. Most encouraging results were obtained from an experiment performed on the elliptical airfoil of 18% thickness ratio with dual symmetrical jets. These results indicate that the leading-edge jet does not disturb the flow and actually furnishes some additional reaction force to the lift. Hence, the important application of the elliptical airfoil (or oval airfoil) with dual jets to the retreating blade of a helicopter rotor is evident. Furthermore, the results of aerodynamic response measurements of the model to cyclic changes in the blowing jet are surprisingly encouraging. The cyclic valve was tested at frequencies equivalent to twice that of the rotational speed of a conventional helicopter blade, and the response of the lift was found to be excellent with negligible delay. The response of the drag as well as the chordwise pressure distribution to the cyclic changes in the blowing jet were also found to be very good. These results clearly indicate that the periodic variation of lift on the airfoil can be fulfilled by cyclic variation of the jet momentum; hence, the circulation control problem is reduced to simply the problem of pressure control inside the model.

Circulation control rotor system

Abstract: The rotating blades on a helicopter are in the shape of a cambered elliptical airfoil with blunt edges. A plurality of slots are employed on the upper surface of the air foil to blow a thin sheet of air tangentially across the surface of the foil and around the trailing edge. Circulation control is achieved by the tangential blowing as the sheet of air adheres to the surface and travels around the trailing edge, detaching beneath the trailing edge at a location determined by the intensity of blowing. The effect of the tangential blowing is relocation of the stagnation stream lines producing increased lift on the foil. The air supplied to the slots is modulated with respect to the azimuth position of the rotor and the speed of the rotating blade so that cyclic control over the lift of the rotor can be accomplished without altering the blade attitude.

Compressibility effects on oscillating rotor blades in hovering flight

Abstract: : A theory is developed and used for determining the influence of compressibility on the aerodynamic forces acting on helicopter rotor blades when oscillating in hovering flight. The usual two-dimensional mathematical model of the flow is adopted in which a section of the reference rotor blade and its helical wake is replaced by an airfoil with a straight wake and a system of regularly spaced infinite wakes below, the amount of spacing depending on the downward flow induced by the rotor and the number of blades. (Author)

Slotted diffuser system for reducing aircraft induced drag

Abstract: A wing assembly for increasing lift and reducing drag is disclosed comprising (1) an inboard conventional primary wing panel, and (2) an outboard secondary wing panel which is aftswept and comprised of a cascade of airfoil elements. The inboard panel is provided with a constant lift distribution which is dropped sharply at the knee or juncture with the outer panel, shedding a substantially concentrated vortex at the knee rather than at the wing tip. The sweep of the outer panel deflects the flow carrying this vorticity outboard, and its cascade airfoil elements then operate in the upflow outboard thereof. The cascade elements of the outer panel are stacked vertically above to the rear, so that the vorticity shed from each element generates a spanwash providing an incremental lift and thrust on the next element aft and above, which, in turn, because of its sweep deflects the vorticity underneath outboard, providing a greater effective span. The cascade splits the vortex into a vertical stack of vortex sheets, which laminate into an expanded size, slowly turning vortex core. The energy and corresponding induced drag of the vortex pair shed from this improved wing assembly is less because the vortex cores are (a) expanded, and (b) displaced outboard.

Numerical investigations of an airfoil in a nonuniform stream

Abstract: The nonlinear differential equation for the flowfield around an airfoil in a two-dimensional nonuniform parallel stream is solved by the finite difference method. The numerical results show that even for a thin airfoil at small angle of attack, the stagnation pressure and the vorticity carried by the streamline passing around the airfoil are significantly different from the corresponding values carried by the undisturbed streamline through the body under the linearized approximation. When the deviation upstream from the uniform flow is represented by a Gaussian profile to simulate the velocity increment behind a propeller, the numerical results show that there is an optimum vertical location of the airfoil relative to the upstream profile for maximum lift. A correlation between the maximum lift and a nonuniformity parameter of the upstream profile is obtained. This parameter is a combination of two parameters, the maximum velocity deviation and the spread of the nonuniformity. When the upstream velocity profile changes from one uniform stream to another through a layer with a steep velocity gradient, there is a significant gain (loss) in lift, if the vorticity is in the same (opposite) sense as the circulation around the wing. A more drastic change in lift occurs, however, when the circulation is in the opposite sense as the vorticity. This occurs when the layer with a steep velocity gradient passes over the upper surface of the airfoil. This phenomenon of a sudden change in lift is of importance when an airplane encounters an atmospheric disturbance.

Angle of attack indicating system

Abstract: An electronic angle of attack indicating system is provided which may be used, for example, to indicate the sail trim of a sailboat, or the angle of attack of an aircraft, or the like, so that the sail or the aircraft may be maintained at any preset attack angle. The system of the invention uses a pair of temperature sensitive devices, such as thermistors, which are mounted on either side of the leading edge of an airfoil, such as a sail or wing, and which sense the air flow differential on the opposite sides of the airfoil, by virtue of the resulting temperature differences. An electric indicating system coupled to the temperature sensitive devices indicates the existence of the air flow differential, so that the appropriate adjustments may be made to maintain the preset attack angle.

A free-streamline model of the two-dimensional sail

Abstract: The two-dimensional sail is considered in a free-streamline model to complement the oft-considered airfoil model which is limited to small angles of attack. The shape of the sail, the lift and drag coefficients, and the moment are obtained for various angles of attack and states of tension.

Visualization of heaving aerofoil wakes including the effect of a jet flap

Abstract: Theoretical solutions exist for the flow about aerofoils in various modes of unsteady motion. This is also true for aerofoils whose lift is augmented by a jet flap. A common feature of these theories is the assumption of small amplitude displacements from the mean path and the generation of a continuous vortex sheet wake. However, in practice when the motion is oscillatory and of finite amplitude, an alternating trail of individual vortices is often found. The present visual investigation includes the measurement of some vortex street parameters associated with heaving aerofoils. The results are compared with those of conventional thin aerofoil theory in order to assess the significance of this departure from the conventional small displacement flow model.

Pivotal Rigging for Sailing Boats and Sail Propelled Vehicles

Abstract: 1,184,914. Wind driven marine and ice boats. A.M.G. DE GALBERT. 26 April, 1967 [28 April, 1966; 16 Feb., 1967], No. 19295/67. Headings B7B and B7V. A sailing boat or sail propelled vehicle comprises an airfoil carried by mast a and rotatable with the mast about a substantially vertical axis. The airfoil is tiltable by means of sheet g about substantially horizontal carrier arm e which is centrally disposed with respect to, and transverse to, the span of the airfoil and which passes through the top of the mast. In the embodiment shown the airfoil consists of detachable sail i fixed to the two rigid end ribs r which fit over the extremities of the spar f.

Airfoil including fluidically controlled jet flap

Abstract: Controlled deflection of a jet flap emanating from an airfoil is accomplished by control fluid streams aligned at a substantial angle to the fluid jet which produces the jet flap. Disclosed embodiments show the use of the fluidically controlled jet flap to adjust the attitude and/or airfoil loading of an aircraft, to generate oscillatory lift forces for flight flutter testing of an aircraft, and to generate and control gusts in wind tunnels.

Investigation of Helicopter Control Loads Induced by Stall Flutter

Abstract: : An analytical study was conducted to determine if available unsteady normal force and moment aerodynamic test data could be used in conjunction with existing helicopter rotor aeroelastic and variable inflow analyses to provide a method for predicting the stall flutter response of a helicopter rotor blade. For this purpose, incompressible unsteady aerodynamic data for an NACA 0012 airfoil executing pure sinusoidal pitching motions were employed. To apply such data under rotor blade operating conditions where multiharmonic motions and velocity variations exist, the data were expressed as functions of instantaneous section angle of attack, angular velocity, and angular acceleration. In addition, scaling procedures were developed in an attempt to account for the effects of compressibility. Limited application of the resulting analysis to define the aeroelastic characteristics of several blade designs showed that significant self-excited torsional oscillations of the stall flutter type could, in fact, be predicted for certain combinations of flight conditions and blade designs. Correlation studies, to evaluate the ability of the analysis to predict control loads, were performed with CH-53A maneuvering flight test data and with level flight test data from the NH-3A (S-61F).

Hypersonic similarity solutions for airfoils supporting exponential shock waves

Abstract: : A study was made of airfoil optimization, using the equations of hypersonic gas dynamics to explore the 'Newtonian chine strip' theory that airfoil concavity enhances the lift-to-drag ratio for a fixed drag penalty. The flow behind concave and convex exponential shock waves is investigated, and the corresponding airfoil surfaces are determined. The calculations show that the optimum lifting surface for fixed drag is only slightly more concave than a flat plate and that the improvement in performance is small. A limit line is shown to exist in the flow field behind convex exponential shock waves, so that is is not possible to construct a convex airfoil that supports an exponential shock wave over its entire length if the nose curvature is too large. (Author)

Measurement of Aerodynamic Forces on an Oscillating Airfoil

Abstract: : A literature survey was conducted to determine the state of the art of measuring and predicting aerodynamic characteristics of oscillating airfoils. Aerodynamic forces on a two-dimensional NACA 0012 airfoil oscillating sinusoidally in pitch were measured by two techniques. The forces were obtained from pressure measurements and by means of strain gage balances. Pressure measurements were made on the airfoil oscillating in pitch about the quarter-chord point at various mean angles of attack. Strain gage balance readings were obtained for models with pitch axis located at 25, 37, and 50 percent chord points oscillating about various mean angles. Direct force measurements were employed in an effort to obtain drag data. Instantaneous pressure distributions are presented for representative oscillating conditions.

Airfoil web dryer

Abstract: An airfoil nozzle is disposed adjacent the moving web to be dried and is constructed with a substantially flat planular guide surface trailing the nozzle, facing the web and substantially parallel thereto.

Faired Towline Hydrodynamics

Abstract: A theoretical analysis for determining the hydrodynamic forces on a faired towline is developed. The analysis requires that the hydrodynamic forces acting on an element of the towline be defined as a function of the towlines local inclination angle to the flow, and of the Reynold's number. These hydrodynamic forces are known as loading functions. The analysis is based on consideration of the boundary layer formed on the airfoil shaped cross section used for the towlines. The Reynold's number effect on the loading functions is derived using momentum theory, and requires that the pressure distribution around the airfoil section be defined. Potential flow theory is used, and gives excellent results when compared with pressure distribution measurements made on models. The theoretical loading functions are compared with model data obtained from water channel, tow tank and wind-tunnel tests. The analytical results show excellent correlation with the data. The analysis has been termed Boundary-Layer Loading Functions (BLLF).