Saturday, May 1, 2010
Testing for Rotor Reliability
With so much riding on them, helicopter rotor blade testing is critical.
A helicopter rotor is the rotating component that generates an aerodynamic force. Also called the rotor system, it usually refers to the helicopter’s main rotor mounted on a vertical mast over the top of the helicopter, but it also can refer to the tail rotor. A rotor is generally made up of two or more rotor blades, although several earlier helicopters featured designs with a single main rotor blade. The main rotor provides both lift and thrust, while the tail rotor provides thrust to compensate for the main rotor’s torque.
Tail rotors are generally simpler than main rotors since they only require thrust control via change in pitch. The rotor head is a robust hub with attachment points for the blades and mechanical linkages designed to control the pitch of the blades.
Helicopter rotor blades are highly sophisticated products, consisting of a matrix of various materials and composites. Each rotor blade is manufactured as a composite, with foam or honeycomb materials forming the core of the blade. The blades are covered with one or more layers of fiber-reinforced plastics on the outside. For further reinforcement, carbon, Kevlar or glass fibers are used. In highly stressed areas, such as the leading edge of the blade, titanium erosion shields provide additional reinforcement.
Because so much is riding on them, helicopter rotors are safety-requisite components. They are exposed to high stresses caused by centrifugal, inertia and aerodynamic forces. They must combine low weight with a high degree of fatigue resistance. There are many ways to test rotor blades, which can verify that the blade laminations layers do not separate. Testing also can verify that the fibers do not break under repeated stress.
Rotor blades can be visually inspected. Visual checks aim at detecting surface cracks and corrosion. Manual acoustic inspection is done by tapping the rotor blades with a hammer and then noting the impact sound to determine whether the substructure’s bond is intact or has separated. This tests for delamination between the alloy skin and the leading edge spar or blade root reinforcing strip.
But these tests are time-consuming, cannot be automated and can have a high incidence of error. Also, these tests make it very difficult to discover any damage not visible on the outside. For these reasons, it is necessary to have nondestructive and automated testing systems that can test with greater reliability and efficiency.
Infrared thermography can test rotor blades. A rotor blade’s surface temperature is increased using a heat pulse to reveal local build-up of heat. If a defect with reduced heat conductivity prevents heat dissipation into the object’s interior, then the surface above the defect remains warm for a longer time, and the defect’s location can be seen in a thermographic image.
An infrared image of a defective rotor blade could show air inclusions and structural dampening. It can indicate an area of delamination or an area that is moving toward the breaking point for the fibers. With infrared thermography, rotor blades can be tested in a semi-automatic process and surface areas of several square meters can be completely registered in one minute. Even defects that are invisible on the outside can be detected with improved clarity. Image processing software assists in automating the evaluation.
Wind Tunnel Tests
Wind tunnel test measurements, flight test measurements and analytical prediction play a key role in the developing and testing rotor systems. Tests are typically performed using a range of sizes and wind tunnel test facilities to provide precise, repeatable control of rotor operating conditions. Airspeed, rotor speed and rotor thrust are easily controlled in a wind tunnel, and can be set to values not possible in flight.
Wind tunnels can test aerodynamics, dynamics, model noise, and full-scale aircraft and its components. The aerodynamic characteristics and aeromechanical stability boundaries of new and advanced rotors can be explored, as well as rotor-fuselage interactions. Stability and control derivatives can be determined, including the static and dynamic characteristics of new aircraft configurations.
Models for wind tunnel testing fall into three categories: rigid, Froude-scaled and mach-scaled. “Rigid models only simulate aerodynamic profile and are used to study aero performance under ideal conditions,” says Edward C. Smith, professor of aerospace engineering at Penn State University. “Froude models study rotor dynamics, but are complex and expensive to build. They can simulate steady elastic deformations under airloads. Mach-scaled models study performance and aero characteristics; they simulate compressibility. Their higher rotations per minute can match full-scale tip speeds in hover and forward flight.”
Despite its many advantages, accurate wind tunnel noise measurement testing can be difficult. Wall effects can prevent the rotor wake from developing exactly as it does in free flight. This is crucial because an important contributor to rotor noise is the interaction between the rotor and its own wake (such as blade-vortex interaction).
In many wind tunnel tests, the rotor test stand is not the same shape as the fuselage, hence aerodynamic interference between the test stand and rotor is different than in flight. The wind tunnel walls cause reflections that may corrupt the acoustic signals. The wind tunnel has its own background noise, caused by the wind tunnel drive and by the rotor test stand. The turbulence level is rarely the same as in flight. The rotor is frequently trimmed differently in a wind tunnel test than in flight.
SMART Wind Testing
Testing on a full-scale, state-of-the-art helicopter rotor system for a Department of Defense customer is currently underway at the U.S. Air Force Arnold Engineering Development Center (AEDC)’s National Full-scale Aerodynamics Complex (NFAC) at Moffett Field, Calif. The testing is a two-part collaborative effort between the Defense Advanced Research Projects Agency (DARPA), NASA and the U.S. Army.
The test article, a Boeing Smart Material Actuated Rotor Technology (SMART) helicopter rotor, is being tested in the NFAC’s 40-by-80-foot wind tunnel to study the system’s forward flight characteristics and to collect data to validate cutting-edge aero-acoustic analysis codes.
“DARPA has a program called the helicopter quieting program,” says Jeffrey Johnson, Arnold’s test engineering group lead at NFAC. “Their overall goal is to try and develop codes for predicting the acoustics of helicopters. One component of the program is to take this active flap rotor system, and run it at several specific test conditions to get acoustic measurements so they can have that as a database to compare their acoustic prediction codes against.”
Dr. William Warmbrodt, NASA Ames Research Center’s Aeromechanics Branch chief, says the NFAC is uniquely qualified to conduct testing of the SMART rotor. He is the acting program manager for DARPA on the test and helps to execute the technical aspects of NASA’s and the Army’s part of the project.
“This is the only facility in the world that can test full-scale helicopter rotor systems,” he says. “We are testing rotor systems that have never been operated within simulated forward flight conditions and to do this safely and still get the kind of quality data out of them is a unique challenge.”
Warmbrodt describes helicopters as a unique flying platform requiring all the tools available to properly test to improve system designs and capabilities. “Helicopters are the most complex mechanical systems flying today,” he says. “In the wind tunnel you can only simulate steady operating conditions and as such the rotors get tested in a certain way and then you rely on your analysis to take it beyond what you have demonstrated in the tunnels, understanding dynamic behavior in a real fight environment.”
Helicopters are the most complex mechanical systems flying today.
Electronic speckle pattern interferometry (ESPI), also called electro-optic holography or TV holography, is an effective way to test rotors. It can measure a rotor’s surface roughness, shape and slope contours. ESPI produces results in the form of images called fringe patterns. It displays fringe patterns in real time on a TV monitor without the need for photographic processing or optical filtering. It is an especially powerful test technique to characterize rotors at elevated temperatures providing full field displacement and strain information.
Rotors are illuminated using two laser beams, which make equal angles with the surface normal to the object. The image of the object is formed on the detector of a video camera using a system of lenses. The image captured by the video camera is processed using a digital image processor.
To obtain the fringes corresponding to the displacement field, two images of the object are needed: one before deformation, and the other after deformation. The image of the object illuminated by laser light has a random distribution of speckles, which are extremely sensitive to the motion of the object’s surface. Speckles are the bright and dark spots that appear on a diffuse scattering surface when illuminated by laser light. This phenomenon occurs because waves of laser light scattered by various surface elements are superimposed with a random phase relationship in space.
Although very similar to ESPI, Shearography is typically used for nondestructive testing instead of material analysis and strain measurement. The shearography method is less susceptible to environmental noise and operating the equipment typically requires less technical understanding. It is generally used qualitatively because additional information and processing is required to determine the absolute value of the deformation. In comparison with conventional testing techniques, shearography offers the advantages of a noncontact, full-field test and an overall significantly increased testing speed.
Laser shearography is an inspection technique where rotor blades are positioned in a huge vacuum chamber and loaded with a relative pressure difference. The pressure change in the vacuum chamber will produce slight deformations on the rotor blade’s surface. A laser shearography system can observe these deformations and automatically indicate defects, such as delaminations and debondings, which show up by typical deformation patterns.
A ventilation system facilitates fast pressure sequences during operation. The typical pressure difference is a few mbar. Despite the large dimensions of the vacuum chamber, the test pressure difference is realized within five seconds.
The rotor blade is positioned on a sledge and fixed by automatic clamps in the center and the sledge is driven into the chamber. The internal chamber dimensions are designed for inspection of large main rotor blades. Two shearing cameras are positioned on a separate guiding system on each side of the rotor blade. They allow simultaneous inspection of both sides of the rotor blade during one pressure cycle.
Automatic defect analysis can be facilitated with software that controls the exact positioning of the shearing cameras, the automatic pressurization of the vacuum chamber and automatic evaluation of the measuring results. Monitors present the inspection results of both cameras on the control panel. Three different operation levels allow automatic inspection (operator level), definition of new rotor blades and teaching of “good” rotor blades (specialist level) and complete access to all data structures and free configuration of the complete system (expert level).
For each rotor blade, inspection sequences are defined and stored in a database. This data contains all parameters for definition of the optical and mechanical properties during the inspection cycle. Using the rotor blade code, all parameters are automatically loaded and the inspection is started. If similar rotor blades have been inspected earlier, the measuring results are automatically compared with the stored master data of earlier inspections. Deviations are automatically indicated on the monitor.
The test results are automatically analyzed by comparing the measured data with a set of earlier taught master data. This allows the operator to distinguish between structural information and anomalies. The automatic anomaly localization is carried out during the test cycle and indicates the anomaly position on the screen. Sizes and positions of the anomalies are printed in a test report, which is automatically prepared after every test cycle.
These are only some of the most popular helicopter rotor blade testing methods. There are others, and because rotor testing is such an important part of the helicopter development process, new testing techniques are always being developed.