Abstract
Out-of-autoclave(OoA) processing has the advantages of low cost, light weight and environmental protection, and has become a hot spot in the field of composite materials worldwide. This paper investigates the application of OoA processing in the gondola of the AS700 civil manned airship. The production cost of gondolas is reduced by selecting low cost materials such as glass fiber, PVC foam and OoA processing. The porosity of parts is reduced and controlled at about 2% by optimizing the edge breathing of prepreg during curing. The maximum tensile strain of the glass fiber is 4 593; its maximum compressive strain is 3 680; and its maximum shear strain is 4 884. The maximum Von Mises stress of the foam is 0.70 MPa. These settings all meet the margin requirement of safety. Finally, the ultimate load test of the gondola is carried out to verify the safety of the gondola structure. Our study presents critical parameters for the gondola design, including load, structure, strength, and manufacturing process test, and provides certain references for the design of similar products.
With the development of composite technology, composite materials have been increasingly applied to aviation structure
The ratio of the void volume to the material volume, commonly known as the void volume ratio or the void volume fraction, is a key parameter to characterize the quality of part
Main drivers of the porosity formation are the length of the part or its distance from the vacuum sourc
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The new generation of OoA prepreg can produce part
Gondolas are the important part of airships, but little relevant literature has focused on them. Based on the AS700 civil manned airship, the engineering application of low-cost composites and OoA processing in the airship gondola is carried out in this paper. First, the gondola design and the finite element model analysis results are introduced. Next, the fabrication of the gondola is described. And then the test results are analyzed. Finally, the material selection and process selection for low-cost design are recommended.
AS700 civil manned airships can be used in tourism, aerogeophysical prospecting, aerial survey, aerial photography, emergency rescue, etc. For tourism, the gondola is located in the lower abdomen of the airship and carries two pilots and eight passengers, or one pilot and nine passengers.
According to its function, the gondola can be divided into the cockpit, the passenger compartment and the fuel compartment, as shown in

Fig.1 Layout of the gondola
According to the general layout and load distribution of the gondola, the gondola framework structure is arranged. The gondola framework is composed of the frame (common frame and reinforcing frame), the top ring beam, the window frame, the door frame and the floor beam. The reinforcing frame adopts metal structure to bear concentrated load, and the other structures adopt foam sandwich composite structure to reduce structural weight and obtain structural stiffness. In order to facilitate molding and reduce the deformation of parts, the foam sandwich structure uses symmetrical pl
According to the airworthiness provisions, there are nearly 30 load conditions for the gondola. They are divided into three groups:Structural mass inertia force (including overload coefficient), landing gear load and propeller load. The overload coefficient includes flight maneuver overload, gust load overload and emergency landing overload. This paper does not discuss the emergency landing conditions, but introduces three severe conditions:The combination of the maximum continuous power of the engine and the designed maneuvering conditions(load condition No.1), the landing gear horizontal landing(load condition No.2), and the landing gear sideslip landing(load condition No.3). The loads of each condition are shown in Tables 1—3. The coordinate system is shown in
(1) |
where is the maximum calculated stress.
The calculation results of load condition No.1 are shown in

Fig.2 Cloud diagram of the glass fiber tensile strain under load condition No.1
The calculation results of load condition No.2 are shown in

Fig.3 Cloud diagram of the glass fiber tensile strain under load condition No.2

Fig.4 Cloud diagram of the glass fiber tensile strain under load condition No.3
In order to reduce the void content, the impregnation rate of prepreg resin is improved by optimizing the curing temperature curve to reduce the flow-induced void content. By optimizing the VBO layup of parts, the air entrapment in the curing process, and the gas-induced void content are reduced. Using small samples to analyze the void content of different processes, the curing temperature curve of parts is obtained, as shown in

Fig.5 Curing curve of prepreg

Fig.6 VBO layup schematic
In addition to optimizing the edge breathing of prepreg during curing, the following aspects should be paid attention to:
(1) When the prepreg is taken out from the cold storage, the packaging bag cannot be opened. It needs to be placed at room temperature for several hours until the prepreg is heated to room temperature.
(2) It is necessary to vacuum in the process of laying when there are many layers.
(3) As shown in

Fig.7 Stepped edge with dry fiberglass strands
(4) Multiple vacuum valves need to be arranged for parts with large area.
(5) The thickness of the tool plate need to be uniform to ensure that the temperatures rise of different parts of prepreg plies rise uniformly.
(6) The air tightness of the mold is tested. The vacuum should be more than 9.2×1
Through the above measures, the void content of the parts can be effectively reduced to about 2% by micro-photograph image analysis. The void content of aerospace structures is acceptable at levels below 2

Fig.8 Tool with lay-up and vacuum bag
The gondola is assembled by assembly frame, and the assembly sequence is from bottom to top, from inside to outside. The order is the bottom skin, the frame, the floor beam, the floor, the side skin and the top plate. Most parts are located by face and pin holes on the assembly frame. After positioning, the parts are assembled by cementation or screw connection. When the cementation gap is greater than the design value, the composite gasket is added to reduce the gap. Each skin and frame is arranged with four pin holes for positioning, and three pin holes can be inserted smoothly during assembly, which shows that the manufacturing accuracy of parts is high.
In order to verify the safety of gondola structure and the rationality of strength calculation method, the above three severe conditions were tested, and the test loads were set as the loads in Tables 1—3 multiplied by the safety factor of 1.5.

Fig.9 Actual gondola within the test fixture
The gondola was fixed and restrained with the longitudinal steel beam through the top joint. The landing gear load and the propeller load were loaded by the hydraulic actuator, and other loads were loaded by the counterweight. Several strain gauges were arranged on the outside of the gondola skin, the frame and the beam to monitor the stress of the structure. The test data were collected and stored in real time through the data acquisition box, and the actuator was loaded by the coordinated loading system. Data acquisition and actuator loading were carried out by the operator in the control room.
For each load condition, the pre-test of 40% limit load was carried out to eliminate the test gap. debug The test equipment was debugged, and unloaded to 0 after loading. Then, the structure was loaded step by step to 67% ultimate load for 30 s to observe whether there was harmful deformation. Then, the structure was loaded step by step to 100% ultimate load for 3 s to observe whether the structure was damaged and then unloaded to 0.
Three test conditions were loaded successfully, and no harmful deformation and damage were found, which indicated the safety of the gondola structure. The tensile, compressive and shear strain values of the composite measured by the strain gauge under load condition No.2 were compared with the predicted values of the finite element model, as shown in Figs.

Fig.10 Comparison of tensile strain under load condition No.2

Fig.11 Comparison of compression strain under load condition No.2

Fig.12 Comparison of shear strain under load condition No.2
The gondola design always implements the requirements of low-cost design, and comprehensively considers the aspects of material selection, process, weight, mechanical properties and porosity.
(1) In the manufacturing process of composite parts, raw materials and labor account for a large majority of costs, with contributions from equipment, tooling, energy and consumables being comparatively smal
(2) The selection of VBO prepreg curing rather than autoclave curing is based on the following considerations. First, the cost of autoclave curing in equipment, mold and energy is higher than that of VBO prepreg curin
(3) VBO prepreg cure improves the autoclave cuing in several environmental performance metrics (greenhouse gas emissions, resource use, ecosystem quality and human health) by between 10% and 20% (as measured using each category’s appropriate unit), primarily through reductions in energy consumptio
The AS700 civil manned airship gondola is designed and fabricated using low-cost composites and OoA processing. The cost of product development and batch production can be effectively reduced by selecting glass fiber prepreg, PVC foam and OoA processing. The composite parts are manufactured by the layer method, and they are only compacted and cured in the oven under vacuum pressure. The porosity of the parts can be reduced to about 2% by optimizing the edge breathing of the prepreg during the curing process, which meets the requirement of engineering application. During the assembly process, the positioning hole of the part fits well with the positioning pin of the mold frame, indicating that the part has high precision.
Three severe load conditions are selected to carry out the limit load test. The test shows that the gondola structure has no harmful deformation under 67% limit load and no damage under 100% limit load, which verifies the safety of the structure.
Considering the impact load of the landing gear, the reinforcing frame connected with the landing gear adopts metal materials. The metal frame and the composite skin adopt cementation and bolt connection, which brings about structural weight gain and cost increase. Further research is needed to ensure that the reinforcing frame adopt composite materials can meet the requirements of the whole life cycle.
This paper introduces the load, structure, strength, manufacturing process and test of the manned airship gondola, and demonstrates that it is feasible to choose low cost materials such as glass fiber and PVC foam, and use OoA processing to produce manned airship gondolas. The research results of this paper are consistent with the development prospect of low cost and green production of aircraft, and can provide certain references for the design of similar products.
Contributions Statement
Mr. ZHU Qiang designed the study,complied the gondola structure design,OoA processing research and experimental organization, and wrote the manuscript. Prof. TONG Mingbo contributed to the discussion and revision of the study. Mr. XIAO Peng contributed to the discussion and background of the study. Mr. LIU Chong contributed to the discussion and gondola fabrication. Mr. QIU Chen contributed to the gondola test. Ms. MA Jingjing contributed to the finite element analysis. Mr. JIN Tao contributed to the discussion. All authors commented on the manuscript draft and approved the submission.
Acknowledgements
This work was supported in part by China Special Vehicle Research Institute and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Conflict of Interest
The authors declare no competing interests.
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