May 19, 2020 / XB-1

What are wing bend tests and how are they performed?

Boom’s mechanical engineer, Nate Hepler, explains what it takes to test the limits of a wing structure.

Before any aircraft can be approved for flight, it must first go through rigorous testing on the ground. For commercial airliners, this process includes everything from wing bend and structure temperature tests, to engine ingestion, windshield impact, and even lightning tests.

While Boom’s supersonic demonstrator, XB-1, will not fly passengers, it’s still critical the aircraft pass many of the same tests with flying colors. In early March, the team successfully completed one of the largest test milestones: wing bend, or static load, testing.

The project, led by mechanical engineer Nate Hepler, involved three days of simulation, testing, monitoring, and evaluation.

By simulating the most extreme flight conditions imaginable, the team validated the wing design and officially approved the structure as safe for flight.

Here’s how it went:

The wing is restrained in a large steel fixture for phase one of testing — the “up” bend test. As part of the process, an array of 26 hydraulic actuators will simulate maximum aerodynamic forces the structure could experience in flight.
An early morning debrief, led by Hepler, sets the stage for what’s to come.
Engineers conduct final inspection of the wing, which has been instrumented with acoustic sensors. The sensors will pinpoint the location of any sounds that may be heard within the structure during testing.
As the initial load tests begin, tracking up to 5% of maximum force, engineers measure the wing’s deflection and strain at various locations. They compare said measurements against predictions.
Load tests are conducted incrementally, tracking up to 5% force and down to 0%, before increasing to 10% and greater. As the structure is loaded into new territory, the team keeps a constant pulse on how the structure responds, tracking differences in strains, displacements, and noise.
Pausing at 20% of upright limit load, the engineers continue to monitor deflection and noise.
Engineers pay attention to both the structure and computer data in real time, as the structure reaches its first target of 40% upright limit load.
A timelapse of the deflection for the 40% of limit load case reveals the durability of the structure under intense stress.
Tracking up to 60% of limit load case, acoustic engineers continue to monitor noises from the structure. Noises are expected when a composite structure is loaded for the first time. The difference between a “good” and “bad” noise has much to do with its acoustic signature. To a trained acoustic engineer, a microcrack developing in the matrix reads differently than a disbond or defect.
The process of incremental load testing continues through the first phase of bend tests. After successfully completing and repeating tests for 100% of limit load case, the structure is validated and approved by engineers as safe through 3.8g.
Engineers review “up” bend test results, as the team prepares to rotate the structure for phase two.
The team unloads the wing, detaches actuators, and removes sensors, in preparation for the “down” bend test. In the final test, actuators will be reattached to the upper wing structure to test the wing up to its -1.5g load case.
Carefully, the wing structure is lifted out of the steel fixture.
XB-1’s 21 foot wingspan comes into full view as the structure is slowly rotated.
The wing structure is lowered into an inverted position back into the steel fixture, with the support of a forklift.
Boom’s Chief Test Pilot, Bill Shoemaker, marks the location of where actuators will be positioned against the upper wing skin.
The team works to reapply the actuators and begin final testing.
With the “down” bend test underway, the incremental load testing process is repeated. Engineers track changes as the load force is increased from 10% to finally 95% and 100%.
Team lead Hepler reviews computer data, as the team confirms all deflection and strain measurements aligned against prediction.
Final notes and reporting is pulled, as down bend tests complete. With static wing test objectives complete, the structure is approved to meet its -1.5g extreme load case.
A final celebration by the team brings this significant milestone to a close.

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