17th November 2009 - Covering the rudder

Since I'm now "fully" trained I though I'd better make a start on covering before I forget everything. The smallest piece to be covered is the rudder so that seemed like the place to start. I'm using the Stewart Systems covering process (http://www.stewartsystems.aero/) which has the advantage that the glue and all of the paints are water bourne which results in much reduced smell and hazard. The process is STC'd for use on most if not all certified fabric covered aircraft. Full details of the covering process including the manual are on the web site and there are also links to very helpful videos showing each stage. So it was a case of following through the steps I'd learned but using the Stewart glue. With such a small piece the result seems to be all rib covering tapes and finishing tapes but I'm reasonably pleased with the result and hopefully my inspector will be happy with the standard.

15th November 2009 - Overvoltage crowbar

The alternator I'm using on the Tailwind is internally regulated and provides no protection to the aircraft circuits if it fails. The risk is that the alternator output voltage increases and damages the avionics and even the battery. The way round this is a crowbar overvoltage protection circuit - sounds crude and it is. Basically the circuit monitors the voltage on the supply wires and if it exceeds 16.2V shorts them out through the onboard thyristor. This trips the alternator field/alternator relay circuit breaker which disconnects the alternator from the battery and shuts off power to the alternator field. Many automotive style alternators will continue to generate even after the field supply is removed hence the need for a relay on the output. This is an emergency device which hopefully will not be needed as disconnecting an alternator when it is generating can result in further damage to it. A high power transient voltage suppressor will be wired across the alternator output to protect it if the crowbar operates.

12th November 2009 - Fabric Covering Course

I'm still waiting for the Douglas Fir needed for the wing spars so no progress on the wings but today I participated in a LAA course on fabric covering taught by Dave Almey at Skycraft. There were six of us on the course and each had a PA-25 pawnee horizontal stabiliser to cover. Dave showed us how to do each stage and then we tried it ourselves.
First the frame was glued and the fabric fixed to the first side. Rather than wait for this to set Dave produced another 6 stabilisers that had been glued by a previous course. These were then ironed to tension the fabric and the second side glued into place. This time the glue set during a short lunch break. Next a coat of dope was brushed on and then it was time to learn how to rib stitch. Dave said we had to do both ribs as the second one would be better - it was! Final job was to of glue in place tapes to cover the stitches and to protect the edges of the stabiliser. It was a very good course and having hands on experience under the supervision of someone who really knows how to do it provided the confidence to do it for real on the Tailwind - thanks Dave.