Engineering topics

Ormiston, R. A.

Publications and source records attributed to Ormiston, R. A..

11 records

NASA NTRS1980

On the nonlinear deformation geometry of Euler-Bernoulli beams

Nonlinear expressions are developed to relate the orientation of the deformed beam cross section, torsion, local components of bending curvature, angular velocity, and virtual rotation to deformation variables. The deformed beam kinematic quantities are proven to be equivalent to those derived from various rotation sequences by identifying appropriate changes of variable based on fundamental uniqueness properties of the deformed beam geometry. The torsion variable used is shown to be mathematically analogous to an axial deflection variable commonly used in the literature. Rigorous applicability of Hamilton's principle to systems described by a class of quasi-coordinates that includes these variables is formally established.

Hodges, D. H.↗

NASA NTRS1979

Wind tunnel tests of four flexible wing ultralight gliders

The aerodynamic lift, drag, and pitching moment characteristics of four full scale, flexible wing, ultralight gliders were measured in the settling chamber of a low speed wind tunnel. The gliders were tested over a wide range of angle of attack and at two different velocities. Particular attention was devoted to the lift and pitching moment behavior at low and negative angles of attack because of the potential loss of longitudinal stability of flexible wing gliders in this regime. The test results were used to estimate the performance and longitudinal control characteristics of the gliders.

Ormiston, R. A.↗

NASA NTRS1976

Hingeless helicopter rotor with improved stability

Improved stability was provided in a hingeless helicopter rotor by inclining the principal elastic flexural axes and coupling pitching of the rotor blade with the lead-lag bending of the blade. The primary elastic flex axes were inclined by constructing the blade of materials that display non-uniform stiffness, and the specification described various cross section distributions and the resulting inclined flex axes. Arrangements for varying the pitch of the rotor blade in a predetermined relationship with lead-lag bending of the blade, i.e., bending of the blade in a plane parallel to its plane of rotation were constructed.

Ormiston, R. A.↗

NASA NTRS1975

New design of hingeless helicopter rotor improves stability

Cantilever blades are attached directly to rotor hub, thereby substantially reducing cost and complexity and increasing reliability of helicopter rotor. Combination of structural flap-lag coupling and pitch-lag coupling provides damping of 6 to 10%, depending on magnitude of coupling parameters.

Ormiston, R. A.↗

NASA NTRS1975

Flapping response characteristics of hingeless rotor blades by a gereralized harmonic balance method

Linearized equations of motion for the flapping response of flexible rotor blades in forward flight are derived in terms of generalized coordinates. The equations are solved using a matrix form of the method of linear harmonic balance, yielding response derivatives for each harmonic of the blade deformations and of the hub forces and moments. Numerical results and approximate closed-form expressions for rotor derivatives are used to illustrate the relationships between rotor parameters, modeling assumptions, and rotor response characteristics. Finally, basic hingeless rotor response derivatives are presented in tabular and graphical form for a wide range of configuration parameters and operating conditions.

Peters, D. A.↗

NASA NTRS1974

Techniques for Improving the Stability of Soft Inplane Hingeless Rotors

The influence of basic parameters that govern flap lag stability of hingeless rotor blades in hover is reviewed, and potential methods are studied for improving the lead lag damping of soft inplane configurations for low thrust conditions. These conditions are relevant for ground and air resonance stability of coupled rotor body dynamic systems. Results indicate that the isolated rotor blade lead lag damping can be usefully increased by a combination of flap lag elastic coupling and pitch lag coupling. For a typical soft inplane configuration, 6% of critical damping can be obtained for moderate pitch lag coupling. For large values of the coupling parameters, the lead lag frequency is substantially reduced at high pitch angles and airfoil stall effects also reduce the lead lag damping.

Ormiston, R. A.↗

NASA NTRS1973

Rotor dynamic considerations for large wind power generator systems

Successful large, reliable, low maintenance wind turbines must be designed with full consideration for minimizing dynamic response to aerodynamic, inertial, and gravitational forces. Much of existing helicopter rotor technology is applicable to this problem. Compared with helicopter rotors, large wind turbines are likely to be relatively less flexible with higher dimensionless natural frequencies. For very large wind turbines, low power output per unit weight and stresses due to gravitational forces are limiting factors. The need to reduce rotor complexity to a minimum favors the use of cantilevered (hingeless) rotor configurations where stresses are relieved by elastic deformations.

Ormiston, R. A.↗

NASA NTRS1973

An unsteady wake model for a hingeless rotor

A simple nonsteady wake model derived from the unsteady moment of the momentum equation for zero advance ratio is correlated with cyclic pitch frequency response tests conducted with a small hingeless rotor model. Two and three or more bladed rotor analyses are presented.

Crews, S. T.↗

NASA NTRS1973

Nonlinear Equations for Bending of Rotating Beams with Application to Linear Flap-Lag Stability of Hingeless Rotors

The nonlinear partial differential equations for the flapping and lead-lag degrees of freedom of a torisonally rigid, rotating cantilevered beam are derived. These equations are linearized about an equilibrium condition to study the flap-lag stability characteristics of hingeless helicopter rotor blades with zero twist and uniform mass and stiffness in the hovering flight condition. The results indicate that these configurations are stable because the effect of elastic coupling more than compensates for the destabilizing flap-lag Coriolis and aerodynamic coupling. The effect of higher bending modes on the lead-lag damping was found to be small and the common, centrally hinged, spring restrained, rigid blade approximation for elastic rotor blades was shown to be resonably satisfactory for determining flap-lag stability. The effect of pre-cone was generally stabilizing and the effects of rotary inertia were negligible.

Hodges, D. H.↗

NASA NTRS1972

A Theoretical and Experimental Investigation of Flap-Lag Stability of Hingeless Helicopter Rotor Blades

The stability of hingeless rotor blade oscillations in hover is examined theoretically using a simplified centrally-hinged, spring-restrained, rigid blade to approximate the deflections of actual elastic blades. The aerodynamic and inertial coupling between the flap and lead-lag degrees of freedom is primarily responsible for instability, however elastic coupling and kinematic pitch-lag coupling both exert a powerful influence on hingeless rotor blade stability. Experimental results obtained from a two-bladed 1.81m diameter model rotor designed for minimum elastic coupling have confirmed the results of linear theory. For this model configuration rotor blade stall at high pitch angles was found to counteract the destabilizing flap-lag coupling and increase the damping of lead-lag oscillations. It was possible to account for this effect with the theory by using drag data for stalled airfoils.

Ormiston, R. A.↗

NASA NTRS1972

Experimental investigation of stability and stall flutter of a free-floating wing V/STOL model

An experimental investigation was made of the static and dynamic stability characteristics of a one-fourth scale model of a tilt-propeller free-wing V/STOL aircraft. The effects of wing pivot location, wing chord, trailing-edge angle, propeller tilt angle, and thrust were studied, and a limited evaluation was made of high-lift devices. A dynamically similar wing was used to measure frequency and damping ratio from transient response data in the linear aerodynamic regime and the results were compared to quasi-steady and unsteady aerodynamic theory. It was found that at high trim angles of attack, in the nonlinear aerodynamic regime, stall flutter oscillations occurred with typical amplitudes ranging from 15 to 20 deg. Wing control tab deflection was effective in initiating and terminating stall flutter but variations in configuration or operating conditions did not greatly influence the occurrence of characteristics of the oscillations.

Ormiston, R. A.↗