Mitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (March 2019)
1Development of Advanced Aircraft Structures with HighVibration Damping Composite of VECTRANTMKIYOKA TAKAGI*1SOTA KAMO*2

KIYOTAKA FUJITA*3

NAOTO AZUSAWA*4TOSHIAKI KOBAYASHI*5 KAZUMASA KUSUDO*6

The buffeting phenomenon, the vibration of the tail unit occurring when a small aircraft isflying at a high angle of attack, accelerates fatigue damage to the structure. Therefore, if it is foundduring the development stage, it would cause an increase in costs and delays in the project due tothe necessary design changes. Accordingly, Mitsubishi Heavy Industries, Ltd. (MHI) has beenworking on the development of a damping structure with high vibration damping efficiency which
could be added to the existing structure with minimum weight gain. We have successfully
developed a new damping structure that can be attached to the tail structure based on the basic
characteristic testing and finite element method (FEM) modelling, utilizing VECTRANTM composite
materials with excellent vibration damping characteristics. In this new structure, we haveconfirmed a 10% reduction in distortion (doubling the projected fatigue life) at the tail fitting
portion in our vibration tests using full-scale models.

|1. Introduction
When a small aircraft is flying at a high angle of attack, the whirling airflow breaks awayfrom the surface of the primary wings and continuously hits the tail. This is known to cause thephenomenon called buffeting, which vibrates the tail. Even when the angle of attack is low, if theaircraft’s velocity is high, a similar phenomenon could occur due to strong shock waves on thewing surface which would cause the airflow to break away. Vibration causing a repetitive load on
the tail would accelerate the fatigue damage to the tail structure. Unexpected fatigue damage
reduces the service life of aircraft and increases the maintenance costs(1). When fatigue damage due
to buffeting is found during the development of an airframe, an increase in the development costs
and falling behind schedule due to the necessary design changes need to be dealt with. Therefore, itis necessary to have a vibration damping structure which would efficiently reduce vibration byadding it to the main structure at a later stage while minimizing the weight gain. This reportintroduces the development of a new vibration damping structure made of VECTRANTM(2)composite materials (hereinafter referred to as VFRP, VECTRANTM Fiber-Reinforced Plastics)
which have excellent vibration damping characteristics for the purpose of improving the fatigue
strength against vibration loads in the tail fitting structure of small aircraft.

|2. VFRP overview
VECTRANTM is a high-strength polyarylate fiber made from liquid crystal polymers inwhich molecules are bound with each other in an extremely strong manner. It has been
manufactured by Kuraray Co., Ltd. since 1990 for various purposes ranging from fisheries to space*1

Chief Staff Manager, Fixed Wing Aircraft Engineering Department, Aircraft & Missile Systems Division, Integrated Defense &Space Systems*2

Fixed Wing Aircraft Engineering Department, Aircraft & Missile Systems Division, Integrated Defense & Space Systems*3

Vibration Research Department, Research & Innovation Center*4

Strength Research Department, Research & Innovation Center*5

Vectran Production & Technology Department, KURARAY SAIJO CO., LTD.

*6Industrial Materials Research and Development Department, KURARAY CO., LTD.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (March 2019)2(2)development . VFRP have been attracting a significant amount of attention as vibration damping
composite materials since they are high-tensile and highly elastic and have excellent vibration
damping characteristics. Figure 1 shows a comparison between VECTRANTM and other types offibers in terms of vibration damping.

Figure 1 Features of the machine body

|3. Development process of vibration damping tail structure
Figure 2 illustrates the development process of a vibration damping structure made of VFRP
to be applied to the tail structure of small aircraft. In this process, the vibration characteristics
obtained in material characteristic tests of VFRP and component tests simulating portions of the tailstructure (fastener joints, inter-beam structure and lugs) were incorporated as basic data into Finite
Element Method (FEM) models as necessary. Next, the damping effect was examined in vibration
tests with full-scale models where an effective vibration damping structure that can be added to thetail was designed based on FEM. Overviews and outcomes of the individual tests are provided fromChapter 4.

Figure 2

Development process|4. Various vibration characteristic assessment tests
4.1

VFRP material characteristic test

We first studied the VFRP material characteristics. As noted in Figure 3, a test coupon piece
was created to confirm the material strength (tensile and compression performances as well asinter-layer toughness) and stiffness in strength tests. Next, as shown in Figure 4, a model in acantilevered state where one end was fixed with jigs was created in which the natural frequency

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (March 2019)3and vibration damping rate were measured through excitation accelerated by hammering (blowswith a hammer). The test model is made of VFRP and multiple carbon fiber-reinforced compositematerials (hereinafter referred to as CFRP, an abbreviation of Carbon Fiber-Reinforced Plastic)
with different fiber types, combinations and orientations. The test results confirmed that VFRP
indicated a high vibration damping rate equivalent to that of general high-damping alloys (i.e.,manganese alloy M2052, etc.) Furthermore, we learned that in VFRP the fiber orientation had asignificant influence on the vibration damping rate.

Figure 3

Results of tensile strength test

Figure 4

Vibration test

4.2

Vibration characteristic test of fastener joints

To examine the changes due to VFRP application in vibration characteristics at a part joint
with a multi-row fastener, a model in a cantilevered state where one end was fixed with a multi-row
fastener as illustrated in Figure 5 was created using the processing method and a fastener for anMitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (March 2019)4

actual aircraft structure, and the vibration damping rate and distortion were measured byhammering. In this test, a total of 4 models made of 3 types of composite materials (CFRP, VFRPand CFRP + VFRP) and aluminum materials, respectively, were created for comparison purposes.
The test results confirmed that the vibration damping rate of VFRP was approximately 4 times
higher than that of aluminum materials.

Figure 54.3

Vibration characteristic test of fastener joints

Vibration characteristic test of lugs

To examine changes due to VFRP application in the vibration characteristics of the lug
structure at the tail fitting portion, the model presented in Figure 6 was created using theprocessing method and pins for an actual aircraft structure. In this test, VFRP was utilized as shim
where vibration tests were carried out in 3 different structural patterns to examine the impact on thevibration characteristics due to the shim position and lug eccentricity. The test results confirmedthat the shim position and lug eccentricity had no significant impact on the vibration characteristics
at the lug portion. Instead, it is assumed that the tiny backlashes between lug holes and pins, theso-called gaps, have a significant impact on the vibration characteristics at the lug portion.

Figure 6

Vibration characteristic test of lugs

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (March 2019)54.4

Vibration characteristic test of inter-beam structure

To examine changes in the vibration characteristics due to VFRP application in theinter-beam box structure of the tail, inter-beam box structure models simulating an actual aircraft
structure were created as can be seen in Figure 7. The models consist of an aluminum internalframework structure and outer plates. In this test, a total of 3 models made of 3 types of different
outer plate materials (aluminum, CFRP and CFRP + VFRP) were created for comparison purposes.
The CFRP + VFRP outer plates have a structure where VFRP is sandwiched between two CFRP
layers. The test results confirmed that the model with VFRP outer plates demonstrated an increased
vibration damping rate as well as a decrease in distortion by about 15%, compared with the modelwith aluminum outer plates.

Figure 7

Vibration characteristic test of inter-beam structure|5. Vibration damping assessment test of full-scale tail unit
Utilizing the FEM model in which the vibration characteristics obtained from the various
vibration damping characteristic tests described in Chapter 4 above are incorporated, 3 patterns ofVFRP vibration damping panels with different application ranges were designed as depicted inFigure 8, which would effectively dampen the vibration of the tail. Based on these patterns,
full-scale models simulating an actual tail structure were created. The models consist of analuminum internal framework structure and CFRP outer plates. VFRP vibration damping panels areattached to the outer surface of the CFRP outer plates. A model to which no VFRP panels wereapplied was also created for comparison. The bottom portion of the models is attached to a jig withthe lug structure located in 2 positions where vibration phenomena were simulated by applying
excitation to the jig. According to the test results, Pattern 2 has the highest vibration damping effectwhere distortion decreases by approximately 10% compared with Pattern 1, to which no VFRPvibration damping panels are applied. This means that, referring to the S-N Curve(3) in Figure 8, thefatigue life is almost doubled. Consequently, we can conclude that VFRP vibration damping panels
have a significant effect in terms of improving the fatigue life.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 1 (March 2019)
6Figure 8

Vibration damping assessment test of full-scale tail unit

|6. Conclusion
This project verified the vibration damping effect achieved by attaching VFRP vibration
damping panels to the tail structure, which reduced distortion by about 10%. This technology isexpected to contribute to improved airframe fatigue life. This technology could also be applied toother structures exposed to vibration, and is expected to be applicable to a wide range of technicalfields including general machinery.
* VECTRAN is a registered trademark of Kuraray Co., Ltd.

References(1) S.A.Barter and L.Molent(2012) “Investigation of and in-service crack subject to aerodynamic buffet andmaneuver loads and exposed to a corrosive environment”, ICAS2012(2) Kuraray Co., Ltd. Web Site http://www.kuraray.co.jp/vectran/en/index.html(3) U.S. Department of Transportation Federal Aviation Administration (2003) “Metallic Materials Properties
Development and Standardization (MMPDS)”