Stabilator airbag dynamic load behaviour in the ground vibration test of F18A-HORNET FIGHTING AIRCRAFT

Stabilator airbag dynamic load behaviour in the ground vibration test of F18A-HORNET FIGHTING AIRCRAFT. In the F18A Hornet fighting aircraft full scale Ground Vibration Test (GVT), several airbags were installed to simulate the loading data obtained from 256 flights. In the port and starboard stabilators, six airbags were attached, three airbags in each stabilator. To be able to assess the dynamic fluctuation loads in the airbag, a set of load cells in the airbag, on the stabilator and some optional accelerometers were used. Calculated channels were defined to calculate the unmeasured loads of the airbag. These channels were calculated during the test running.The work was required to understand the dynamic fluctuation loads of the airbags and to detect the maximum dynamic loads occur.

This paper presents the anlyses result of the airbag dynamic loads and the airbag feedback (net) loads which are calculated by an upper-lower equation in the time domain.

The maximum dynamic load to the stabilator in band 1(10 – 20 Hz) occurs on airbag no.61 with the value of 1982 Lbs. The maximum dynamic load to the stabilator in band 2 (32 -52 Hz) is 2254 Lbs, located on airbag 75..

Keywords : Ground Vibration Test, Fighting Aircraft, Dynamic Loads, Stabilator

INTRODUCTION

In the full scale ground vibration test (GVT) of the FA18 Aircraft (see Figure 1), dynamic loads data from real flights in various manoeuvres identified by the AOA-Q (angle of attack – pressure) regions are simulated to the tested aircraft. The dynamic loads data are obtained from 256 flights as analysed by the author, Siswanto and Conser (1997). These data are the continuation of the previous data gained by Mouser and Conser (1996). Other reports related to the dynamic loads are documented by the author, Siswanto (1997) as well as Siswanto and Mouser (1997).

As the airbags are expected to transfer the loads to the stabilators accurately, it is therefore required to review the airbags dynamic behaviours during the test running. The maximum loads occurred to the stabilators are also critical as they are directly applied to the structure of the stabilators.

Figure 1. FA-18 Aircraft

AIRBAG CHANNELS

In order to assess the dynamic fluctuation loads in the airbag locations, a set of load cells in the airbag and on the stabilators. Some optional accelerators were used to monitor the mode of the stabilators dynamic behaviours. The numbering system of the airbags uses number 71 to 75 for starboard and 61 to 65 for the port stabilator. The position of the airbags is illustrated in Figure 2.

Figure 2. Airbag positions and numbering

In the first 10 flights, airbag 61, 71, 65 and 75 were examined as well as the calculated force channels in airbag 61 based on accelerometers. The purpose of using the calculated force channels was to ensure that the load read by the load cell in airbag 61 was comparable with the force which was calculated by multiplying the displacement with the stiffness of the airbag 61.

DYNAMIC LOAD CALCULATION PROCEDURES AND RESULTS

Airbag stiffness and stabilator displacement were used to calculate the dynamic loads. By multiplying the displacement of the stabilator and the stiffness of the airbag, the predicted net load was obtained. As the airbag stiffness with specification 40 psig that was used in test was not available, a linear extrapolation based on available data was conducted. The calculation results were then compared to the measured load in channel 61.

The calculation procedure was conducted in time domain using data from the first 10 flights. Once the measured were assured to be comparable with those from calculation, the test went to 256 flights. The displacement was calculated at frequency 13 Hz which is the dominant frequency of the stabilator.

The calculated results were comparable with those measured using accelerometer. The average calculation from all AOA-Q regions was used to give general information instead of comparison in each region. The calculation was conducted by summing all non zero values divided by the number of AOA-Q regions which have no zero values. The differences were only 4.1% and 0.1% for RMS and maximum values, respectively. The calculated and measured loads are shown in Table 1.

Table 1. Load Comparison: Calculated - Measured

Type	Calculated (Lbs)	Actual (Lbs)	Error (%)
RMS	213.55	222.68	4.1
Maximum	651.36	651.95	0.1

CONCLUSION

Even though the net loads to the stabilator in airbag 61, 63, 65, 71, 73 and 75 fluctuate, the average value in each AOA-Q region is comparable with what should be measured. In terms of average (mean) values, the net loads to the stabilator are accuarate.

Airbag 61 and 71 fluctuate dominantly in band 1. They behave in a simple way. The upper load and the lower load to the stabilator oscillate at the same frequency in band 1. Other airbags behave more complicated, and dominantly fluctuate in band 2.

Generally the RMS dynamic load fluctuations of the lower loads are greater than the upper loads except in airbag 65 and 75.

The maximum dynamic load to the stabilator in band 1 occurs on airbag 61 with the value of 1982 Lbs. The maximum dynamic load to the stabilator in band 2 is 2254 Lbs located in airbag 75.

The motion of vertical tails vibrates the upper reservoir in band 1, the motion contributions in band 2 is not significant. The motion of the reservoir in band 2 is dominantly due to the stabilator motions. The stabilator motion contribution in frequency range 38 to 46 Hz (within band 2) to the net load on the stabilator is 58%.