Robert A. Ormiston

Hover Testing of a Small-Scale Rotor with On-Blade Elevons

Abstract : A two-bladed, 7.5-ft diameter dynamic rotor model with 10% chord on-blade elevons driven by piezoceramic bimorph actuators was designed and tested in hover at tip speeds up to 298 ft/s. The elevon actuator succeeded in achieving deflections of plus or minus 5 deg at the nominal rotor speed of 760 RPM. Aeroelastic and structural dynamic response characteristics were evaluated over a wide rotor speed range using frequency sweep excitation of the elevon up to 100 Ha. The CIFER(registered) post processing method was very useful for determining frequency response magnitude, phase, and coherence of measured blade root flap bending and torsion moments to elevon input, as well as elevon response to actuator input voltage. Experimental results include actuator effectiveness, effects of low Reynolds number on elevon pitching moments, elevon reversal, and variation of flap bending mode responses with rotor speed and elevon excitation. The active rotor performed satisfactorily and the results provide an encouraging basis for future wind tunnel testing that will evaluate on-blade elevon effectiveness for reducing rotor blade vibratory loads.

Theoretical and experimental aerodynamics of the sailwing

The sailwing is a unique type of semiflexible foldable wing. A brief description of its construction, basic properties, and some past research, is used to introduce an analytical and experimental study of its aerodynamic characteristics. Emphasis is placed on an approximate structural analysis which treats the nonlinear behavior of the sail deflection. Twodimensional flexible airfoil theory and Prandtl lifting-line theory are used to establish the aerodynamic loading. The results allow prediction of the induced tensions, the nonlinear life curve, induced drag, and aeroelastic divergence of the sail chordwise deflection mode. Selected experimental results are presented for comparison with the theory, along with a brief discussion of the implications of the study regarding flight vehicle applications of the sailwing.

Investigation of Rotor Blade Structural Dynamics and Modeling Based on Measured Airloads

The work presented herein treats measured airloads from the UH-60A Airloads Program as prescribed external loads to calculate the resulting structural loads and motions of a rotor blade. Without the need to perform any aerodynamic computations, the coupled aeroelastic response problem is reduced to one involving only structural dynamics. The results, computed by RCAS and CAMRAD II, are compared against measured results and against each other for three representative test points. The results from the two codes mostly validate each other. Seven more test points, with responses computed by RCAS, to form thrust and airspeed sweeps are evaluated to better understand key issues. One such issue is an inability to consistently predict pushrod loads and torsion moments well, and this is found to be amplified at the two test points with the highest thrust coefficient. For these two test points, harmonic analysis reveals that the issue is due to excessive amounts of 5/rev response that stem from high levels of 5/rev pitching moment excitation. Another issue that concerns all test points is that the phase of the 1/rev blade flapping motion is not predicted well, which reflects the high sensitivity of this quantity that is developed due to having a first flap frequency of approximately 1/rev. Results also show that current force-velocity relationships, used in describing the lead-lag damper, are not satisfactory to consistently yield accurate inboard chordwise bending moment predictions. Overall, the investigation here, conducted with numerous test points, further confirms the methodology of prescribing measured airloads for assessing the structural dynamics capability of a computational tool.

Stability of Hingeless Rotor Blades in Hover with Pitch-Link Flexibility

A stability analysis of a single cantilevered helicopter rotor blade in hover is presented. The blade is represented by an elastic uniform beam, cantilevered in bending and having a torsional root spring to simulate pitch-link flexibility. Nonlinear equations are adapted for a linearized stability analysis about the blade equilibrium operating condition. Numerical results are obtained for hingeless rotor configurations having pitchlink flexibility, precone, droop, twist, and flap-lag structural coupling. The results indicate that hingeless rotor stability characteristics are sensitive to changes in most configuration parameters. For a given torsion frequency, the effect of pitch-link flexibility is generally found to be similar to the effect of blade torisional flexibility. Droop and precone, although physically similar, exhibit different effects on stability when pitch-link flexibility is present. Twist is shown to influence the stability by altering the flap-lag structural coupling.

Assessment of 1-D Versus 3-D Methods for Modeling Rotor Blade Structural Dynamics

Comparisons between 1-D and 3-D analyses are conducted systematically for several problems ranging from simple isotropic beams to realistic composite blades in order to better understand the differences between the two approaches and physics behind them. 1-D beam analysis is conducted using the RCAS rotorcraft comprehensive analysis with VABS calculated 2-D cross-sectional properties. 3-D finite element analysis is conducted using a commercial code MSC/Marc. Natural frequencies are calculated at various rotor rotational speeds for various beam lengths and the differences are quantified. There is very good agreement between the 1-D and 3-D analyses for isotropic beams when the beam length is greater than ten times chord (difference is less than 3%). Even as the beam length decreases, there is still good agreement for the bending modes. However, the differences between the two analyses become larger for the torsion modes. For composite beams and blades, there is excellent agreement between the 1-D and 3-D analyses except for the torsion modes. A large difference is observed for beams with [15]24 layup and the largest differences occur for the flap bending modes. Ply orientation (and thus coupling) appears to play an important role. Torsion frequencies predicted by the 1-D analysis are always lower than the 3-D analysis for all the cases examined.

Assessment of Beam and Shell Elements for Modeling Rotorcraft Blades

A geometrically exact shell element is developed within the finite-element, multibody dynamics-based Rotorcraft Comprehensive Analysis System. The shell element accommodates transverse shear deformation as well as arbitrarily large displacements and rotations. The shell element is developed using an approach that allows for compatibility with other structural elements in the Rotorcraft Comprehensive Analysis System. It is validated by comparing its predictions with benchmark problems. The two-dimensional shell and one-dimensional beam finite-element analyses are compared for three typical blade configurations of varying slenderness ratio: a swept-tip blade, a blade with discontinuous chordwise elastic axis and center-of-gravity locations, and a blade with a flex beam. The purpose is to quantify the differences between two-dimensional-shell one-dimensional-beam finite elements for modeling rotor blades. There is good agreement between the one- and two-dimensional analyses in predicting the natural frequenc...

Optimum Rotor Performance in Skewed Flow Based on Actuator-Disk Theory

Recent studies have pointed out that conventional lifting rotors in forward flight have efficiencies far lower than the optimum efficiencies predicted by theory. This paper presents a closed-form optimization of induced power with finite-state models is expanded to obtain the results for pressure and induced velocity distributions for a rotor in forward flight. Results for induced power efficiency (IPE) variations in forward flight and for different conditions are presented.

Comparison of One-Dimensional and Three-Dimensional Structural Dynamics Modeling of Advanced Geometry Blades

Comparisons between one-dimensional and three-dimensional analyses are conducted systematically for advanced geometry blades, which have tip sweep, tip taper, and planform variations near the root with various materials and effects of boundary conditions in order to better understand the differences between the two approaches and the physics behind them. One-dimensional beam analysis is conducted using the rotorcraft comprehensive analysis system with variational asymptotical beam sectional analysis calculated two-dimensional cross-sectional properties. Three-dimensional finite element analysis is conducted using a commercial code MSC/Marc. Natural frequencies are calculated at various rotor rotational speeds, and the differences are quantified. There is very good agreement between the one-dimensional and three-dimensional analyses for free–free aluminum beams, even for a very short beam with beam length five times chord (L=5×c). The one-dimensional analysis accurately captures the planform variation near...

Advancing State-of-the-Art Unsteady, Multidisciplinary Rotorcraft Simulations

To address the complex multidisciplinary nature of rotorcraft analysis, high-fidelity computational fluid and structural dynamics models have been developed to investigate a range of challenging rotorcraft issues. First, an advanced technology, active flap rotor (Boeing SMART) is investigated, and performance, aerodynamic and structural loads, vibration, noise prediction and flow physics mechanisms are shown. The rotor model includes complex and detailed flap and flap-gap modeling. Second, analyses on an advanced dynamics model (ADM) research configuration rotor investigate regressing lag mode (RLM) aero elastic instabilities. Tightly-coupled computational fluid dynamics (CFD)/computational structural dynamics (CSD) stability calculations show noticeable improvement over lower fidelity methods. Third, the state-of-the-art capability of CFD methods to directly predict low frequency in-plane noise on realistic lifting rotors is benchmarked for the first time. In all cases, comparisons are made between CFD/CSD, comprehensive analyses, and experimental data. Taken together, these works offer an important advancement in rotorcraft analysis capability for advanced technology rotor configurations under study for future Army rotorcraft, and highlight future needs in next-generation rotorcraft analysis software.