The project on a seaplane structure redesign for the purpose of decrease in expenses in production.

The project on a redesign of frames of a seaplane from a riveted design in a design with the greatest possible use of the milled details for cost reduction in the serial production has been carried out by ECAR in 2015.

As the first task the bulkhead frame of the plane has been offered by the Customer. Without waiting for completion of works on the first task in addition to a bulkhead frame redesign 3 type frames at which the lower part was redesigned with riveted on the milled design have been also transferred.

The project implementation.

Work consisted from the following tasks:  

On the bulkhead frame in addition to above mentioned tasks the task of a strength assessment of new developed design, by creation of FE-model of the frame for stress definition of new developed design and its weight optimization was added.  

Existing bulkhead frame (fig. 1) represents the riveted assembly unit consisting of webs, profiles, the reinforced profiles and other parts. Horizontal Beams #1 and #2, vertical Beams #3 are designed as separate assemblies. Unlike a design of bulkhead frame of traditional passenger airplanes – spрerical shape bulkhead frame of this plane is created as flat frame therefore the main function of vertical and horizontal beams is to keep the frame from a bending forces under loading from excessive pressure.

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Figure 1 – Existing frame.
This project was the first in ECAR in which in compliance with the requirement of the customer it was used the software different from used on Airbus – Unigraphics as CAD-system and Team Center as PDM-system.

At the initial stage it has been noted that the task definition offered by the customer – cost reduction in production due to transition to the milled design, is not absolutely correct.

It was obvious that implementation of a task for cost reduction in production can be carried out by performance of the following tasks:

 

Risk of weight increaseing of a design.

At the initial stage of structure redesign serious threat of weight increasing of a design which was defined by technological restrictions, first of all, that minimum possible thickness of webs and pockets at the milled design is 1.5 mm whereas in the existing design it there were 1.2 mm has been identified.

The conclusion has been drawn that it is possible to compensate a tendency of increase in weight of the whole structure of a frame only due to decrease the weight of beams where unlike the frame there is more freedom for changes.

Several concepts for the horizontal beams given on figures below have been considered.

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Figure 2 – Option 1 – The design which is similar to existing as much as possible.
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Figure 3 – Option 2 – With holes for weight reduction.
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Figure 4 – Option 3 – Truss design.
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Figure 5 – Option 4 – Hybrid design.
The carried-out analysis of the offered designs of the Beam #2 has shown that “The hybrid design” (Option 4) doesn’t give any advantages, and the Option #1 is the most conservative, badly corresponding to a way of main part production – milling from a plate.

Thus, Option No. 2 and No. 3 have been offered to the customer as the most suitable for milling and perspective from the weight reduction point of view.

Important point which when determined the design of the Beam #3 is the fact that it participates in a door surrounding. In case if it is produced separately from a design of a frame web there would be additional joint parts usage with the horizontal beams surrounded the cut-out for the door.

The decision has been respectively made that the design of the Beam #3 has to be implemented in such a way that its lower flange has to become a part of the milled design of a frame web to be executed at the same time with horizontal beams surrounding of a the door cut-out but upper flange, beam web and reinficing ribs have to implemented as separate part. It allowed to mill a beam on the one side.  

Two concepts for a vertical beam, similar to Options 1 and 2 for horizontal beams have been considered.

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Figure 6 – Option 1 – Design similar to existing design.
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Figure 7 – Option 2 – With holes for weight reduction.
The design wholes for weight reduction and surrounding reinforcement has been offered as the most preferable from the point of view of the weight reduction and the used manufacturing techniques.  

Nevertheless, the customer has made a choice for the closest to initial design solution on all beams.  

After the choice of design option the work on design study and the strength analysis has begun. First of all the design has included holes and the doublers installed for passing of systems which are installed separately on existing design of the frame.  

ECAR engineers proposed and agreed with Customer’s technology specialists the assembly method with assembly and pilot holes. In this regard in all new designed parts assembly and pilot holes have been implemented.  

The offered assembly order proposed by ECAR engineers looks as follows:  

  1. Assembly holes on a horizontal beam have to be combined with assembly holes on a web of a frame and assembly holes on a flange of an bracket adjacent to a frame web.
  2. The vertical beam is pressed to the web of lower flange of the beam which is part of a design of a frame web, and it’s assembly holes are cooordinated with assembly holes of a bracket.
  3. After when whole structure is clamped by clamps, holes for the fasteners are drilled thrue pilot holes and assembly holes in a frame web, a beams and brackets.
 

The assembly order allows to execute, according to ECAR engineers oppinion, whole frame assembly by one worker.

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Figure 8 – Frame assembly with assembly holes method.

Design of frame web.

At the initial stage several options of placement of stiffening ribs have been considered.

The decision to keep the existing arrangement of ribs has been made to avoid their crossings giving increases in weight.

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Figure 9 – Formation of excess material in a zone of crossing of ribs.
Regular ribs of a frame coincide with edges of an initial design. For prevention of “bubble deformation” when processing on a thin wall it was necessary to execute additional horizontal technological ribs.
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Figure 10 – Main and additional horizontal technological ribs on the frame web.
For new developed design the decision has been made that floor ribs have to become part of the milled web. As a first proposal it was decided to to prolong vertical ribs before crossing them with floor ribs.
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Figure 11 – Design option of floor ribs on the bottom part of a frame.
In a consequence this design had to be modified by results of the stress analysis.

Already by preliminary static stress analysiss have revealed high concentration of stress in zones of the termination of ribs. Were it is decided to use the structure design principles of creation of the high-resource designs applied on Airbus. As a result in a design smooth transitions of thickness and other features have appeared.

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Figures 12-13 – Thickness transitions on the frame web.
One of important specifities of the milled designs is that the design shouldn’t have the protruding elements above the general thickness. It allows to install this milled structure on a table without shims and supports.

The design of a frame web is characterized by the fact that one side is executed with the identical height of protruding elements: flanges, height of horizontal ribs and so on. The opposite side has the elements with exceeding height: increase in flange height, but it is admissible as at observance of the necessary order of milling processing, it was possible to do without shims and supports when milling.

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Figures 14-15 – Final design of Beam #1.
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Figures 16-17 – Final design of Beam #2.
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Figure 18 – Final design of Beam #3.
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Figures 19-20 – Whole design of frame web.

Stress activity.

As initial data for stress analysis the data of analytical calculations results of the existing design have been provided. Besides, to avoid the fatigue analysis of a design the customer as initial data has accepted the allowable stress level in the stretched zone significantly lower than strength of material.  

The strenght of design was estimated for three load cases: excessive pressure, water landing and a raising jack of the plane for the jack point installed on the bulkhead frame.

As the first step the FE-model of the existing riveted frame has been created. Results of loading have compared to data of analytical calculations. Having convinced that levels of stress in FE-model and analytical calculations are the same, the FE-model was a validated then have got to work on performance of FE-model of a new design.

Work on improvement of a design was organized thrue the way of iterations: modification of a design and after this their strength assessment.

As a result of numerous local strengthenings of a wall of a frame, ribs, vertical and horizontal beams it was succeeded to achieve necessary strenth.

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Figure 21 – The picture of frame deformation.
 

The weight of a design was monitored during whole project. Unfortunately, to reduce the weight of a design as it was planned at the beginning of the project, or at least to keep it at the same level it wasn’t achived. Weight increase has made 3,7% that was explained, first of all, by the fact that at a stage of consideration of alternative options of a design the most conservative decisions have been chosen.  

Work delivery to the customer was carried out by ECAR engineers in the customer design bureau office. Process of delivery of works has been difficult, customer representatives have been requested changes in 3D-models as well as changes of 2D-drawings and Bill of Materials. All these changes, in generally, were implemented by ECAR engineers.  

In the official feedback received from the customer ECAR has been specified as the reliable supplier, and ECAR engineers were estimated on highest level with such criteria as quality of the proposed design and concept design solutions, ability to perform work in time, skill level of employees in this technical area and initiative.