...

1. Take mold from CAD

2. Put into machine

...

Seats Fastening Research and Proposal

OVERVIEW

The purpose of this document is to serve as documentation for the decision making process behind the choice of fasteners and how the seats would be installed in MS XIV

CONTEXT

It is important to note that this was not originally considered in the design / manufacturing of the seats for MS XIV. This was due to an oversight in looking through the regulations, not having been fully aware of specific best practices and changes to the bottom panel (which was integral to the original plan)

Original Plan

The original plan was to have the seats (and ballast) attach solely via adhesives. 

For the seatback, the rear of the seat would be attached to the bulkheads they would be laying on (either the B Panel for the Front Seat, or C Panel for the Rear Seat)

For the seat pan, they would be attached along the bottom edge that interfaces with the bottom panel as well as the edge of the seat pan supports.

The seat pan supports would be attached along the bottom edge that interfaces with the bottom panel

Regulations

While regulations within the Seats and Ballast specific sections were met, there were regulations within the fastener section that affected the seats and ballast. They pertained to how certain components (designated “critical areas”) could not be solely attached via adhesives.

...

Thus, our previous fastening plan was in direct violation of this regulation and had to be modified.

Best Practices

When coming up with designs it is important to keep in mind best practices or guidelines regarding components, fasteners or designs in general. In this case, with adhesives, it would be best to adhere to design guidelines regarding these in order for them to perform their function (securing the seat) to the best of its ability.

When designing a joint using adhesives, it is best to minimize any tensile loading as it will cause cleavage stress to develop. This is “highly unfavourable for adhesive bonding” [Remove Stress from Adhesive Joints for a Stronger Bond]. 

With the previous fastening plan, in the event of any impact from the rear (e.g. a rear collision), some of the adhesive joints (such as the seatback to the bulkhead) would be under a tensile loading as the seatback is trying to pull away from the bulkhead.

Thus, it was evident that either a new geometry for these adhesive joints would need to be made, or another fastening method (besides adhesives) should also be utilized in order to prevent high levels of cleavage stress developing within the adhesive joint.

Note: While implementing other design guidelines for adhesives (such as increasing the bond area between for the adhesive) could minimize the effect of cleavage stress, this would still be putting the adhesive in its least favourable loading condition and may cause a larger bonding area to be required (increasing material usage)

Manufacturing Changes

Due to manufacturing constraints / issues, the bottom panel would no longer have a layer of core material. This greatly reduces the load bearing properties of the bottom panel. Read this confluence page on Composites for more background info about this: Basic Composites Knowledge  

Originally, the weight of the occupant would be transferred via the seat pan, to the seat pan supports and to the structural bottom panel. However, with the bottom panel no longer being structural, transferring the loading of the occupant’s weight onto this panel is risky and may result in failure of the panel.

Instead, transferring the loading of the occupant’s weight onto the chassis or another structural member is a much safer avenue to pursue. In addition, to provide a further safety factor a new structural bottom panel will be laid over the chassis tubes. This will help take some of the loading from the occupant’s weight and transfer loading to the chassis.

Revised Plan

Based on the regulations, design for adhesive best practices and manufacturing changes, it was ultimately decided that an additional means of attaching the seats (likely fasteners) would be used. In addition, the seats would also have to be attached to structural members such as chassis tubes along with the previously chosen mounting points (e.g. bulkheads)

Front Seatback

In addition to being adhered to the B Panel Bulkhead, it was determined that the front seat back should be attached to the chassis tube that intersects near the shoulder level. 

This was chosen as it is easily accessible to the existing front seat design and it also provides an additional mounting point/area higher up on the seat. This helps prevent lateral movement of the seat and further fixes it in place

Seat Pan Supports (Front and Rear)

The seat pan supports will still remain adhered to the seat pan panel itself but instead of resting (and adhering) to the bottom panel, it would rest on top of the new structural bottom panel.

In addition, the seat pan supports will be attached to the anti-sub chassis tube via a bracket and fasteners and flanges will be added preferably to both the top and bottom to either increase bond area (attaching to new structural bottom panel) or to provide a surface for fasteners and the aforementioned reason (attaching to seat pan panel)

Seat Pan (Front and Rear)

The seat pan will largely remain unchanged, save for any mounting holes that may be needed to attach to the seat pan supports.

Rear Seatback

In addition to being adhered to the C Panel Bulkhead, it was determined that the front seat back should be attached to the chassis tube that intersects near the shoulder level. 

This was chosen as it is the most accessible chassis tube available for us to connect the rear seat to. That being said, the rear seat is curved at this height and this may pose some challenges to fastening. If needed design changes to the rear seatback may be done to overcome / mitigate this challenge.

FASTENING SOLUTIONS

This section will cover how various fastener types may be applied to help fasten the seatback, seat pan and seat pan supports.

The list of fastening solutions comes from this Confluence page: Fastening Methods For Carbon Fiber  

In addition to this video: Assembly Solutions for Honeycomb Composite Panels: Surface Mounted Fasteners and Deform-nut Inserts

And finally this product: Composi-Lok® | Monogram Aerospace Fasteners | Composi Lok 

List of Fastening Solutions:

• ASP (Adjustable Sustained Preload) Fastening System

  • Composi-Lok (seems to be a derivative of the ASP system)

• Lockbolt Fasteners

  • May be known as the trade name “Hi-Lok”

• Eddie-Bolt Fasteners

• Embedded Fasteners

  • bigHead is one such example

• Surface Mounted Fasteners

  • Note: Loading would still have to go through an adhesive, not sure if this is still considered an adhesive mounting method as per ASC regulations

• Deform-Nut Inserts

  • The threaded insert is attached to the rivet nut via an adhesive, as the threaded insert is what the screws threads are grabbing onto, we are still going through an adhesive

    • More info: Deform-Nut threaded insert system for sandwich panels 

• Rivets

  • Based on the Confluence page however, it seems this would require special riveting equipment (not your standard rivet gun)

MILESTONES