Question

Course: Mechanical Vibration

Report Paper ( 1000 words)

Topic is Shakers for Aircraft Suspension Durability Testing

All material must be referenced correctly. Reference could be from Journals, Articles, Books.

Please include at least 2 or 3 reference.

Answer 1

Topic : Shakers for Aircraft Suspension Durability Testing

It is a device used in vibration testing to excite the structure either for endurance testing or modal testing.

Vibration testing is done to introduce a forcing function into a structure, usually with the use of a vibration test shaker or vibration testing machine. These induced vibrations, vibration tests, or shaker tests are used in the laboratory or production floor for a variety of things, including qualifying products during design, meeting standards, regulatory qualifications (e.g. MIL-STD 810, etc.), fatigue testing, screening products, and evaluating performance.

Most often electrodynamic and servo-hydraulic shakers are used, depending on the frequency range and displacement required.Electrodynamic (ED) shakers are well suited for most vibration tests and offer several advantages over alternative approaches. Compared to hydraulic shakers, ED shakers are capable of much higher frequencies. These high frequencies can be very important in testing electronics and electronic assemblies. ED shakers are also capable of reproducing a wide range of shock and SRS pulses in addition to general vibration testing. Lastly, ED shakers have very linear behavior, so controlling the vibration test may be easier or possible when not otherwise possible.

Captive flight – external carriage include stores carried externally on a jet aircraft. The overall vibration profile arises from four sources; engine noise, in-flight aerodynamic turbulence, vibration transmitted through attaching structures and internal store vibration. Engine noise is highest at the boundary layer of the exhaust jet plume. At this point there is turbulence between the exhaust air and ambient air and is at its maximum during takeoff. In-flight aerodynamic turbulence do not greatly affect overall store vibration, however, they may cause other structures such as tailfins to produce vibrations that are then transmitted to the store. These vibrations are a “low frequency system” and are often characterized as buffet vibration, Figure 2. “Buffet vibration is typically concentrated between 10 and 50 Hz” and “is dominated by store structural resonances.” Finally stores are also susceptible to internal vibration from elements such as rotating machinery.

Captive flight – internal carriages generally don’t experience harsh vibration levels unless the bay is opened during flight. “This event is referred to as a cavity resonance and results in high levels of turbulence inside the bay.” This vibration is characterized as a wide band spectrum with high spikes across it. Low frequency spikes (<100 Hz) are not expected to be of issue for most stores, however, high frequency spikes can be very dangerous.

Figure 1. Figure 514.7D-5 from MIL-STD-810G w/ Change 1 – Jet Aircraft Store Vibration Respon

Vibration testing simulates vibration conditions of cargo in jet aircraft. These vibrations are broadband random. Typically the maximum vibration levels produced are caused by the exhaust noise generated during takeoff. Vibration during normal flight is substantially less. Figure 1 detail the Acceleration Spectral Density (ASD) and frequency levels of various aircrafts and the vibration exposure of cargo carried on those respective aircrafts during takeoff. The General Exposure curve can be used when the specific aircraft is no known.

The test duration is 1 minute per takeoff times the number of takeoffs the product may see in its lifetime.

vibration environment for cargo carried in propeller aircraft. The vibration environment generated from propeller aircraft are dominated by high amplitude approximately sinusoidal spikes at propeller passage frequency and harmonics. Lower level wide band random vibration is also present due to air flow over the aircraft. Collectively, these phenomena are superimposed to create the vibration profile for cargo carried on propeller aircraft resulting in a Mixed Mode Vibration Test.

Figure 1 and Table 1 show the vibration profile for this environmental condition. The test durations are obtained from the products life cycle environment or one hour per axis if the duration is unknown.

Figure 1. Figure 514.7C-8 from MIL-STD-810G w/ Change 1 – Propeller Aircraft Vibration Exposure

Table 2. Table 514.7C-VIII from MIL-STD-810G w/ Change 1 – Vibration Exposure based on Location.

References :

1. https://www.bksv.com/en/products/shakers-and-exciters/LDS-shaker-systems/high-force-shakers/V964
2. https://www.nts.com/services/testing/dynamics/vibration/