Seat Cushion and Citabria Cowl Repair.

I have been waiting for a copper gasket for the Tillotson HR carb needle seat. I have been waiting a month, and believe it or not, it actually arrived while I was writing this blog. In the mean time while I was waiting, I fabricated a new seat head rest and added a leather covered head rest and seat cushion. It really came out nice! Every one who has sat in this seat said it is extremely comfortable.

head rest and seat cushion

I fabricated a light weight head rest support from .020 aluminum  sheet.














 

Shoulder, lap and crotch harness

 Good view of the seat belts and adjustable rudder pedals. The read "Tee" handle is the Ballistic chute deployment handle.

Another view

Citabria Cowl Repair

This is the plane I have been flying at Big Q Aviation in Midlothian Texas http://www.bigqaviation.com/
The Cowl was getting pretty tired and while the wing was being rebuilt, it was decided to repair all of the elongated cowl fastener holes. Once a fastener gets loose, a "sawing" action occurs on the fiberglass holes and the holes start to elongate. Some of the holes were so badly elongated that there was just a thread left on the original cowl. That was the case for 16 of the fastener holes. The rest were just slightly elongated and could be repaired by filling with chopped cotton fiber and epoxy. The original Cowl was hand laid up out of fiberglass cloth and polyester resin. The basic principle of repair says you need to restore the existing structural capability. In the case of a composite repair, that usually means all of the original fibers plus 1 extra. The strength of the original material is the determining factor on what taper ratio will be used to scarf the ply's . High strength uni fiber repairs like on a wing spar will require a taper ratio of at least 100 to 1. The lower modulus materials used on the this cowl could be repaired with as little as 15 to 1. the minimum however per FAA guidelines is 20 to 1.

repair schematic

The basic idea here is to clear out the elongated hole, in this case a slot is opened up along the cowl edge.The edges of the cowl are then taper sanded at a 20 to 1 ratio.  A sheet of waxed .020 Aluminum is then bonded to the outside of the cowl with 5 minute epoxy. 7 plies of equal size semi circle doublers are then laid up into the tapered space. After they cure , the edges of the splice are taper sanded . The process is really simple, only the edges are sanded until a space in the middle equal to the original slot remains. After the splice is sanded, a final overlay ply is added (original ply plus 1 extra)

corner edge repair

 After the inside is repaired, a very slight taper is sanded into the outside surface, and a single ply of fiberglass is laid up. This way the repair is actually a double lap shear/Tapered scarf. This is a very good structural repair because all tendency for the tapered splice to Peel away is reacted by the over lay ply's.

Double hole repair

This is a good example of what the repair looks like after taper sanding. Its possible to actually count the ply's as they drop off. Notice how the center of the repair laminate has not been sanded.  

single hole repair.

 This picture clearly shows the internal taper sanding. One final overlay ply will be added over this repair.

overlay ply with peel ply

a very good technique is to finish the repair lay up with an overlay of Dacron Peel ply. This adds a professional touch and serves a very good structural purpose, the hard edge of the repair laminate is tapered out with a resin rich edge. This serves to smooth the hard edge and eliminates the tendency for the repair to peel away. After cure, the peel ply is pulled off the laminate. This material is the same Dacron I used to cover the Robin.