Ground Vibration Testing Aircraft - The XB-1 recently achieved another milestone on its way to its maiden flight. This time it was stopped by waves as part of a ground vibration test (GVT).
As ground testing of the XB-1 nears completion, several physical tests on the ground are required to confirm the aircraft is safe before flying to Mojave.
Ground Vibration Testing Aircraft
One of the most relevant tests is the ground vibration test or GVT. This is a unique test that gently supports the plane. The plane is then "excited" by attaching an electrodynamic shaker. A shaker is similar to a music speaker, but instead of a cone, the shaker has a thin rod. In a speaker, a cone pushes air back and forth to create sound waves, while in a shaker, this cone is replaced by a simple rod that repeatedly hits the plane back and forth.
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This force profile can take many forms, but all of them induce different vibrational modes of the aircraft, allowing the analyst to confirm that the predicted modes agree with the tested modes. Modes of vibration are the unique forms of excitation that a given structure acquires. Having a fine-tuned model is important because these predictions inform the flight shake tests that will be performed later in the flight test program.
Simply put, the GVT is completed to avoid flapping after passing the aircraft. Flight is an "aeroelastic" phenomenon in which the structural state of the aircraft interacts with the aerodynamics to generate large forces that can cause structural damage.
We spoke to Eric Gustafson, the load and dynamics engineer who spent the last two years planning these five-day tests. His team is located at the intersection of aerodynamics and structural disciplines, and specializes in flight science, which aims to create and improve external loads (flight maneuvers, turbulence, landings, etc.) and evaluate aeroelastic properties (deformation of the vehicle under load and how it behaves). is a branch. load affects aerodynamics, handling and structural stability).
Ground vibration testing validates our understanding of the aircraft's fundamental behavior. Each of these vibrational modes has a resonant frequency, damping, and characteristic.
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Imagine a violin string. Vibration characteristics depend on mass and stiffness. In a sense, ground vibration testing is "playing" with an airplane. We stimulate it and it vibrates in different modes or resonant frequencies, which are measured by sensors. Conceptually, it's similar to a violinist stringing a bow across a string to create pitch; the wire breaks and vibrates.
In a dynamic sense, the behavior of the aircraft is determined by the vibration mode. Everything that happens in the plane over time can be described by the mode. For example, the response over time to a gust of wind hitting an airplane can be described by the following mode. In the case of acute flapping, the oscillations of the wing begin to interact with the unsteady aerodynamic changes above the wing. A common example you may have heard of aerodynamics interacting with structures is the Tacoma Narrows Bridge, where the bridge's rotational mode was the cause of the collapse.
We use real-world experiments to calibrate and validate our theoretical aeroelastic models, managed by the Load and Dynamics team. If there is a discrepancy between how we think the aircraft will react at takeoff and what our models show, we can adjust the model before taking the XB-1 to 35,000 feet for flight testing.
GVT is a way to find out how the XB-1 will perform in flight from the safety of our hangar. What we're really trying to avoid here is shaking. The goal of aircraft design is to create a structure that is inherently safe from any tendency to exceed reasonable limits.
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Any good plan begins with a statement of purpose. It separates a necessity from a nice-to-have. Every program wants to do a quick GVT to minimize the time it takes to declare the aircraft ready to fly. Therefore, for effective testing, we do a number of things, including creating a clear, organized, and efficient test plan. As each aircraft application uses GVT, it may have a specific formula, but each aircraft has unique needs such as shaker distribution and sensor placement. It is important to determine the optimal place to trigger the plane (ie, like jumping off the end of a diving board instead of the base) and where to measure the response. We can use predictive analytics to plan for this.
Writing such a test plan is complex and involves multiple stakeholders in the test, from avionics, flight control, structural, test engineering, and even facilities. Members of this team include test fabricators, production/test engineers, flight control engineers, avionics engineers, and load and dynamics engineers. These partners enable us to test through the necessary test equipment, flight hardware, flight software, structural configuration, and even building power, space, and equipment.
In general, we planned to run the test for 5 days (including setup), and the test was successfully completed in this window.
It is obvious that the behavior is in the air as evaluated using a flexible model of the aircraft. So we aim to test how to fly. This includes the status of the included chassis. However, because the gear usually supports the weight of the aircraft on the ground, this presents a problem. So we decided to support the plane from above to overcome this. Two large portal frames higher than the aircraft supported the vertical lift at the front and rear of the aircraft. We isolate the plane with rubber to reduce the unwanted stiffness of the bridge system, which can prevent a successful conclusion from the test. The components of this "suspension system" are tested separately before the GVT because of the dangers associated with lifting such a mass off the ground.
Ground Vibration Test
The Overture will be a much larger aircraft than the XB-1. Once an aircraft of this size is ready for GVT use, engineers will have twice as many sensors and shakers as the XB-1, using support systems like the airbags below to replicate the flight. As with the XB-1, the test flight will be very close to the first flight, so it will be an interesting time during the Overture program. As a certified program, all Overture tests will be increasingly comprehensive.
Strictly speaking, any physical object can have an infinite number of vibrational modes, but the first modes with the lowest frequencies are usually the most important. Each mod contributes to the overall response of the aircraft in some way, but it is difficult to describe them all at once. Instead, we trigger the modes at regular intervals to show the behavior of each mode individually. For the XB-1, we only care about 30-40 modes. These include: body flex, wing flex, wing roll (rotation), control surface rotation mode, etc. The relationship between these modes is important. For example, if certain frequencies of the modes are close, they can cause unwanted interactions.
We enter the model correlation phase, where we update our theoretical aeroelastic model to match the behavior of the tested aircraft. After this correlation step, it is understood that the model may represent aircraft configurations not tested on the ground, with minor variations. This allows for a final flight prediction that will give us the evidence we need to keep the XB-1 in the air and achieve our supersonic mission!
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