Vibration Analyses, Vibration and Shock Testing, Mechanical Testing

Semi-Active Side-Lateral Engine Mounts for Control of Vibration and Shock Loading

(Naval Air Warfare Center Aircraft Division Contract)

FFF Engineering successfully completed the option phase of this program with the conclusion of the proof of concept testing. The data collected in this testing revealed that FFF engineering is able to:

1. Produce a random shock signal on a vibration tone.

   

2. Acquire the data
3. Differentiate between a vibration and shock signal and react to the shock signal while providing attenuation for the vibration signal. Thereby, providing evidence of proof of concept.

During vibration isolation is required to reduce the vibration levels from the engine to the airframe. This will result in a mitigation of fatigue damage to the airframe and components installed in the vicinity of the source of vibration.

During shock events, the engine must be held in its position relative to the nacelle to avoid turbine failures experience in the past.

The key result from phase 1 and the option is that we were able to achieve both states using a smart shock absorber. The yellow data is the input signal a 18 Hz signal with a randomly imparted shock event, The magenta data is the signal that the airframe experiences.

Time =0 (ms) to Time =76 (ms)

The yellow signal is the input signal such as the propeller might impart on the side lateral mount. The signal is a 5 g 18 Hz signal up time 76 ms. The magenta signal is the signal after the vibration is isolated through the smart side lateral mount, such as the airframe would experience. The signal is attenuated over 70%. Therefore, the airframe experience 70% less vibration attenuation, which will dramatically reduce the fatigue damage to the aircraft.

Time =76(ms) to Time=201 (ms)

At time = 76 ms a shock transient is being imparted on the engine, at time t = 101 ms the shock transient begins to have substantial displacement. Which is precisely being tracked by the smart damper. The fact that both the input and output are tracking acceleration signifies that the damper is locked (solid mount) and therefore tracking the original shock. There is a lag of approx 5ms but it does not inhibit the smart damper and damper remains locked (tracking) until time t=201.

Time =201 to t=501

At this time the shock transient is over and the damper is unlocked and allowed the vibration isolation system to perform its function of reducing the propeller harmonic by over 70%.

It is important to note that if the 18 Hz signal is attenuated the 74 Hz signal would therefore be further isolation to a value over 95%.

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SHOCK ABSORBING BOLSTER SEAT TECHNOLOGY
Applicable to:
HIGH SPEED PLANING BOATS

This concept discusses a shock mitigating technology and proposed prototype that allows shock loads to be relieved at a predetermined acceleration level and absorbs the excess energy. This technology will mitigate fatigue in occupants due to repeated operational shock loading and reduce impact loads on occupants during high speed planning boat operation.

FFF Engineering Design LLC (FFF) use of a mechanical to thermal energy transfer device to absorb shock loads of 4 g or greater. The mechanical energy to thermal energy transfer device is a mechanism which operates on visco-elastic theory to provide energy absorption due to interface shear damping. This concept will discuss FFF's approach to providing the Navy with a shock mitigating technology that is cost effective, field proven, lightweight, provides low life-cycle cost, and reliable for use on high speed planning boats.

Spring Mass Damper System

Figure 1

The bolster seat system can be described through the components shown in figure 1.

FFF is proposing a shock mitigation device that has been developed for the aerospace industry. The purpose of developing the shock mitigation was to prevent loss of load conditions using wire rope. Wire rope is used extensively to translate loads. Wire rope is an assembly of wire strands that are configured to produce a strong assembly that is capable of withstanding large static loads. Testing has shown that the wire rope assembly is susceptible to short duration shock loads. Once the wire rope assembly was subjected to shock loads the ultimate static strength depreciated significantly. Due to the nature of the loads carried (human cargo) a solution was required. Various different approached were suggested: metallic yielding, spring type shock absorbers, elastomeric shock absorbers, electro mechanical systems, and the METET system. Two approaches were selected based on selection drivers. Two prototypes were developed and tested with the final selection being the METET system. The bolster seat shock mitigation problem exhibits the same characteristics in problem definition that FFF's previous experience with wire rope shock mitigation. The problem can be summarized as high shock loads producing unacceptable high levels of shock loading, which tend to reduce the ultimate strength of the occupant through high fatigue loads.

FFF first concept approached the problem by attempting to provide a semi active band pass filtering system. This band pass system allowed relatively low levels of shock to activate the system. The band pass system was based on position indication rather than the preferred direct acceleration driver. In subsequent discussions with Coastal Systems Station (CSS) FFF learned that the preferred approach was to allow certain levels of acceleration to be absorbed by the seat and occupant system and any acceleration levels over, approx 4 g, would be absorbed by the bolster seat occupant system.

In lieu of previous submittals to CSS by FFF, FFF is presenting this concept for CSS's consideration of the METET system for bolster seat shock absorption. In the following discussion FFF presents a prototype METET system design which is a derivative of the system FFF developed system for aerospace applications that considers: weight, durability, reliability, low life cycle cost and field proven performance as a solution to CSS high speed planning boat bolster seat shock mitigation concerns.

Experimental Data

Figure 2 (below) is the result of a 175 lb weight allowed to free fall 24 inches (485 Joules potential energy) producing an un-attenuated peak load of 2053 lbs.

Figure 2

Prototype System Design

The prototype system design is based on implementing the METET on a seat with unknown translation characteristics. CSS has commented that a translating seat is available for prototype use. FFF believes this design to be the worst case scenario and this design can be implemented on a generic seat. The proposed design is not optimized for weight or for efficiency. If a design for a seat and system would be required then in all probability a different approach would be taken to reduce the amount of gearing and avoid the high torque conditions this design presents. It is FFF's opinion that this design will suffice for evaluation testing.