Push It to the Limit: Push It to the Limit: Airframe Structural Test By Charlotte Adams
AEROSPACE TESTING

Push It to the Limit: Push It to the Limit: Airframe Structural Test By Charlotte Adams

Before a new military or civil aircraft can take to the skies it must run a gamut of ground and flight tests to verify that it functions as intended and is safe to fly. Ground-based structural tests are an important element as material “coupons,” components, and entire airframes are subjected to forces, stresses, and conditions that simulate conditions they will encounter in flight. For this article we consulted experts in military and business aviation.

Strength and fatigue tests are key parts of the regimen. Static strength testing involves “applying intense load conditions to the structure to ensure it can withstand load[s],” explains Albert Dirkzwager, director structural integrity, test, and simulation for Textron Aviation. Fatigue testing involves “applying multiple cycles of normal flight conditions to the airframe to simulate thousands of hours of flight time to prove the durability of the airframe,” he says.

Similarly U.S. Air Force ground testing validates that the strength, durability and damage tolerance, stiffness, and mass balance requirements have been met, says Gregory Schoeppner, chief of the Structures Branch, in the Air Force Life Cycle Management Center.

He describes a “building-block approach,” moving from testing material coupons, subelements, elements, subcomponents, components, and finally to complete airframes. This approach is “typically used to mitigate manufacturing scale-up risks while maturing design and analysis tools to accurately predict the behavior of increasing[ly] complex structures.”

The most granular level of structural testing is at the coupon level, agrees Victor Alfano, senior director of strategic programs for NTS, a testing company with 28 labs in North America. These small material samples provide customers an understanding of fundamental material behavior under different conditions that can be applied to the design of aircraft components and structures. Coupons can be tested for characteristics such as tensile strength, fatigue, and crack propagation, and loads can be applied under extreme temperature conditions. NTS recently tested an engine cowling that required temperatures as high as 1,400-1,500 degrees F. while applying loads, Alfano says.

Test Articles

USAF full-scale strength test (FSST) and full-scale durability (fatigue) test (FSDT) require a dedicated airframe, each, Schoeppner says. These tests also require specially designed load frames that can apply distributed loads to the fuselage, wings, and empennage; discrete loads to the landing gear attach structure; and pressure loads to pressurized compartments in the airframe. “The load frame and facilities for each of these tests can take many months to a couple of years to design, manufacture, and set up.”

Scott Maher, a staff scientist at Gulfstream, says they typically dedicate one airframe, including all the associated components, to static strength testing and another airframe to fatigue and damage tolerance testing.  Gulfstream images.
Scott Maher, a staff scientist at Gulfstream, says they typically dedicate one airframe, including all the associated components, to static strength testing and another airframe to fatigue and damage tolerance testing.
Gulfstream images.

The same is true for all-new bizjet designs. Typically one airframe, including all the associated components, is dedicated to static strength testing and one airframe, to fatigue and damage tolerance testing, says Scott Maher, staff scientist, Gulfstream. (Some of the miscellaneous components may be tested separately from the overall airframe for convenience, he says.) The static strength testing begins and concludes first, as a portion of it typically supports first-flight safety, and all of it must be completed before certification. Fatigue testing usually takes longer, but only a portion of the fatigue cycling needs to be complete for certification. Many of the separate test schedules overlap.

Dirkzwager says that while the number of articles depends on the program, the Citation Longitude used “multiple different test articles and full-scale airframe articles… .”

Test Gamut

In addition to basic static strength and fatigue testing, Textron lists additional tests, such as:

  • Residual Strength Testing – static strength testing while simulating failed structural components to prove redundancy of the structure.
  • Bird Strike Testing – FAA requirement to demonstrate the safety of the aircraft if impacted by a bird.
  • Operational Tests – to demonstrate that flight control and other systems operate properly under various loading scenarios.
  • Tire Burst Testing – to demonstrate reliability of the airframe in the event of a blown tire.
  • Landing Gear Drop Testing – to verify that the energy absorption of the landing gear system behaves properly under landing impact.
  • Impact Testing – to impact various components with possible real-world scenarios to ensure the robustness of the design.
  • Extreme High/Low Temperature Testing – involving some of the preceding tests.

Scott Maher, a staff scientist at Gulfstream, says they typically dedicate one airframe, including all the associated components, to static strength testing and another airframe to fatigue and damage tolerance testing. Gulfstream images.

Although there are no requirements to test to failure for any component, according to Albert Dirkzwager, director structural integrity, test, and simulation for Textron Aviation, occasionally the company will intentionally test a component to failure to help refine their analytical techniques and ensure they are designing the most weight-efficient structure. Textron images.
Although there are no requirements to test to failure for any component, according to Albert Dirkzwager, director structural integrity, test, and simulation for Textron Aviation, occasionally the company will intentionally test a component to failure to help refine their analytical techniques and ensure they are designing the most weight-efficient structure. Textron images.

Analysis

“Since it is impractical to test the response of the airframe to the thousands of different load cases within the design envelope, analysis and design models are relied upon to predict the response of the airframe to the full-range of loading conditions,” Schoeppner says. The main objective of the structures testing is to validate analysis and design models for the airframe loads, strength, durability and damage tolerance, and dynamics.

“Advancements in structural analysis and the ability to model individual parts with more accuracy and in greater detail have allowed us to minimize design margins and perceived design conservatism,” he says. This is “particularly beneficial for reducing structural weight to meet design goals.” But it makes strength and fatigue testing all the more critical in discovering design shortfalls and oversights, he adds.

Textron Aviation’s external loads team analytically “flies” the aircraft through possible gusts, maneuvers, and landings which it could experience to compute the maximum forces on the airframe, Dirkzwager says. The structural analysis team narrows these cases down to a few dozen for each component and specifies the forces to be applied during the test.

“For cyclic tests, we develop typical usage profiles based on measured flight data of similar aircraft” to help “develop how often and at what load levels to flex the wings and the number of pressurization/depressurization cycles.”

Extra Margins

Fatigue testing builds in wide margins. “We are required by the FAA to test three-times the number of cycles as the life of the aircraft to capture variability in the materials and flight profiles,” Dirkzwager says.

For static strength, limit loads are the maximum loads to be expected in service, Gulfsteam’s Maher says. The regulations require a 1.5 factor of safety beyond limit loads (ultimate loads). “The structure must be shown by analysis, supported by testing, to have no detrimental permanent deformation under limit loads and to not fail under ultimate loads,” he says.

Generally testing is completed to the prescribed design static load or cyclic load duration and it is not required to test until failure, Maher says. “That said, it is often advantageous to test to failure to confirm failure modes and validate analysis methodologies. Also, any test data beyond the original prescribed design load is like money in the bank if future variants have higher design loads.”

There are no requirements to test to failure for any component, Textron’s Dirkzwager says. “Frequently bird strike tests do significant damage to the airframe that requires repair prior to running subsequent tests.” And occasionally, “we will intentionally test a component to failure to help refine our analytical techniques to ensure that we are designing the most weight-efficient structure.”

The design service life of an aircraft can be defined in flight hours, flight or pressurization cycles, or years, Schoeppner says. The FSDT simulates the cyclic loading that the aircraft will experience during service and demonstrates that the aircraft design is sufficient to meet the design service life. At a minimum, the durability test demonstrates that the airframe can survive two lifetimes of usage.

The requirement for the FSST test is to demonstrate that the airframe will not fail when loaded to design ultimate load (DUL). “There is no requirement that any components of the airframe be tested to failure.” However, after demonstrating DUL capability, the FSST is often loaded to failure to determine the ultimate strength capability of the airframe and to provide data to further validate strength models.

Shown here is NTS’s state-of-the-art Lightning Center of Excellence laboratory. The electrical characteristics of the different types of lightning flashes, and the resulting surges and fields from a strike, are complex. The company’s engineers have studied the effects of lightning on a structure or system by isolating the components of the lightning waveforms and electrical/magnetic fields, and NTS evaluates their effects through individual simulations. The company’s labs include specialized equipment to simulate the electrical characteristics of natural lightning as well as the transients it induces in electrical and electronic systems. NTS says it offers lightning testing at multiple facilities across the United States. NTS image.
Shown here is NTS’s state-of-the-art Lightning Center of Excellence laboratory. The electrical characteristics of the different types of lightning flashes, and the resulting surges and fields from a strike, are complex. The company’s engineers have studied the effects of lightning on a structure or system by isolating the components of the lightning waveforms and electrical/magnetic fields, and NTS evaluates their effects through individual simulations. The company’s labs include specialized equipment to simulate the electrical characteristics of natural lightning as well as the transients it induces in electrical and electronic systems. NTS says it offers lightning testing at multiple facilities across the United States. NTS image.

Facilities

Gulfstream’s building covers more than two acres – 92,252 sq. ft., with a volume under the roof of 3,690,080 cu. ft., says John Kenan, director, flight test operations. The facility has a structural floor with concrete and steel reinforcement 5 ft. thick to support the loads that are required for structural testing. Landing gear loads are on the order of 100,000 pounds. Gulfstream has tested components and coupons to loads in excess of 500,000 pounds, he says.

Gulfstream also has a negative pressure chamber for flammability tests that is vented and filtered to exhaust outside the building, he says. Burners are positioned by a robot so that personnel are not required to enter the chamber to move from calibration thermocouple/calorimeter to the test article and back. “We use an infrared camera to record and monitor temperatures across the entire test article in real time.”

Many coupons are either conditioned and/or tested at temperatures and/or humidity other than room temperature, Kenan says. Environmental chambers vary in size from a few cu. ft. to 16 ft.x40 ft. and 8 ft. high. “We test at [temperatures] … from -70 to +700 degrees F.”

Textron Aviation’s structural test facility includes about 51,000 sq. ft. of test floor to a ceiling height of 50 ft., Dirkzwager says. The building features an overhead crane hook, an integrated hydraulic supply system, and an air system. It also includes a physical test lab, instrumentation lab, and engineering offices, plus an additional 20,000-sq.-ft facility for systems testing, including environmental/bleed air testing and fuel system testing.

Equipment

Gulfstream uses Moog digital load control equipment with more than 584 channels, Kenan says. “Our transducers are monitored using 15,500 channels of HBM computer-controlled data-acquisition systems.” A typical full-scale test will use over 100 actuators and 5,000 channels of data acquisition. Tests are monitored and displayed using more than a dozen HD video cameras, several at frame rates up to several thousand frames per second. “As we set up a test, we can choose from an inventory of 940 hydraulic actuators, 800 load transducers, and 550 deflection transducers.”

Textron cites MTS and MOOG/FCS control equipment and the ability to run 20 large-scale independent tests as well as to control position, pressure, and load. The company also has “drop towers” for drop-testing landing gears. One of these handles heavier-gross-weight landing gear testing and another, lighter-weight testing, Dirkzwager says. Textron also uses multiple load frames for material and small component testing, temperature/humidity/altitude environmental chambers, birdstrike canon, hail gun with the ability to shoot 2-in. hail, and a burn test chamber.

Increasing computing power and the development of dedicated tools for life analysis and derivation of non-standard stress concentration factors make estimates more accurate, Gulfstream’s Maher says. Control parameters can be changed on the fly, whereas “in the analog days, this would have been accomplished with a jeweler’s screwdriver one channel at a time.”

Better computing power and advanced finite element analysis allow us to model the structure with more precision and detail to eliminate high-stress areas that could result in lower-than-desired life span, Textron’s Dirkzwager says.

Direct digital controllers allow the addition of features to stabilize the test article, automatically speed the cycling rate, and tune the system, which have made a dramatic impact on the calendar time required to run tests – while at the same time enhancing accuracy and redundancies, Dirkzwager says. Test control systems can control over 100 load points with additional accuracy, and durability tests have become increasingly efficient.

“Right-sizing load cylinders, servo valves, and transducers to maximize performance and to take advantage of the improved accuracy of the test control system and data acquisition has also played into improved test performance.”

The number of available channels and the reliability and performance have all increased massively, Gulfstream’s Kenan says. In the mid-80s we had 32 channels of load control and 1,000 channels of data acquisition. “Due to the limited number of channels, we had to rewire the load and data systems between each test condition.” During more recent test campaigns, however, “we used 115 channels of load control (with 160 available) and recorded 6,200 channels of data.” Modern data acquisition systems “have allowed us to significantly increase the number of transducers, the frequency of their sampling, and the accuracy of those readings. Incidentally, all transducers can now be simultaneously sampled virtually, eliminating data slew.”

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