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.

18th October 2009 - Ribs Completed

Finally the last rib is glued, cleaned up, and final sized. Next job is to clean up the workshop which seems to have largely disappeared under a mountain of sawdust. The sacrificial board on the worktable will then be replaced with a new sheet of 9mm MDF and then the actual wing construction can start - BUT I'm still waiting on some Douglas Fir for the spars. The company cutting it for me had prettty much completed the job when a resin pocket appeared at the end of the cut length so it is back to waiting for some more lumber.

12th October 2009 - More Rib Progress

It's difficult to say much interesting about building ribs or to take interesting pictures. However, the ribs for the right wing are now complete and there are just 6 remaining to make for the left wing - the end is getting closer.

4th October 2009 - Rib Progress

Work continues making the wing ribs but the end seems in sight now. 15 ribs are now fully complete out of a total of 26. There are four variants: the root rib which is smaller and has the larger openings to allow the spar attach brackets to pass through, rib 2 which is a fully sheeted but has the same basic structure as the normal ribs, rib 13 which is also fully sheeted and also has a solid wood nose section 4" long to allow the wing tip to screw into it (if I decide to build the metal wingtip), and ribs 3 to 12 which are the standard open section ribs.

I have also now fully kitted the remaining ribs with the exception of the diagonals which are cut to fit in situ. The bottom picture shows the four sheets for the left wing ribs 2 and 13, bags containing all the rib uprights cut to length, and all the required gussets. At current rate of progress which seems to be two ribs every three days I should finally have all the ribs done in three weeks.

30th September 2009 - Identical ribs

The production of ribs is underway again and I've also started the process of getting them precisely identical and perfectly matching the outline on the plan. First one of the ribs was sanded using a disc sander to final shape. This was then designated as the master. Using stub sections of front and rear spar the other ribs can then be located below the master. A trimming bit is then used in a bench mounted router to match each rib to the master. The picture shows the forward end of the lower rib being trimmed to match the master. The router cutter has a small bearing of exactly the same diameter as the cutter itself. The router is positioned to let this bearing ride on the master and the cutter then matches the profile of the new rib to the master. Using this approach the ribs can be matched perfectly ready for final installation on the spars. The use of the router table ensures that the sides of the ribs are perfectly at right angles to the faces. Moreover the process is very quick - a couple of minutes per rib yields the final result. The router is also very useful for trimming any excess gusset from round the edge of the rib. Run the bearing on the capstick and any overlap of the gusset or any glue is quickly removed.