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.

29th September 2009 - Remote Compass

Like all tubular metal aircraft, it is difficult in the Tailwind to find a site for a normal compass where it isn't influenced by the magnetic field from the airframe. This will probably be particularly the case for my build as I TIG welded the airframe and TIG is known for leaving steel magnetised. The solution is to have a remote compass situated in one of the wooden wings well away from the fuselage. Most modern electronic flight instruments (e.g. Dynon) have an option for a remote compass but they are expensive and I want to stick with the conventional panel layout. So, I decided to build my own remote compass. The picture shows the actual remote unit I've constructed; for reference the holes in the PCB are 0.1" apart so the entire unit is about 1" x 2.5" and weighs less than 1/2oz. Eventually it will be mounted in a small plastic box and screwed to one of the wing spars. The heart of the unit is an amazing chip from Honeywell - the HMC6343. This has in it 3 magnetometers to measure the earth's magnetic field and 3 accelerometers to measure gravity. These together allow the unit to not only report magnetic north irrespective of the orientation of the unit, but also to output the pitch and roll angles of the unit (and therefore the aircraft). The chip communicates using the I2C standard defined by Philips which uses a two wire serial bus. The four pin connector includes the serial bus wires together with a ground and 5V supply. It will be controlled by a microprocessor circuit installed behind the panel and the compass direction displayed on the display screen in the panel above the radios http://tailwindbuild.blogspot.com/2009/09/14th-september-2009-panel-painted-and.html and also on a separate dedicated 3 digit LED display - location to be determined. The actual orientation of the sensor is not important as calibration of the unit is extremely simple, first tell the chip whether it is on edge or flat, then point the aircraft in one known direction, compare the direction with the compass heading to calculate the deviation and program this into the chip, everything else it does itself and Honeywell guarantee less than 2 degrees of error between the compass output and the true heading with a repeatability error of less than 1/2 degree.

28th September 2009 - The Cowl arrives

I've been away for a couple of weeks so no real progress on the build but today the cowling arrived from the US. I decided to go with the glass fibre cowling produced by Brian Alley at http://www.carbonfibercomposites.net/ and it's a beautiful piece of work. Very light but strong and lovely lines that blend the square front of the fuselage into the round disc behind the propeller spinner. This will now stay in it's shipping crate for a few months until I'm ready to assemble to the fuselage - hope it fits!

15th September 2009 - Rib Rework

As previously recorded I'd made a decision early in the build to final weld the fuselage, including the wing mounts, before starting on the wings so would have to make sure that the wings would fit the fuselage rather than the other way round. The key issue seemed to be to ensure the wing incidence (angle to the air) would be precisely the same on both sides. The root ribs have cut outs that precisely locate vertically the spar brackets that in turn bolt to the fuselage. and therefore set this incidence
http://tailwindbuild.blogspot.com/2008/10/8th-october-2008-right-wing-attach.html
So a few days ago I took the first completed root rib to where the fuselage is stored to measure up. Unfortunately, what I hadn't really considered is that not only does the vertical location of the wings need to be correct but the horizontal spacing between the brackets that bolt to the wing spars has also to match the fuselage very very precisely. In fact this spacing was correct on the left side of the fuselage but there was a difference of about 3/32" on the left side. When I then compared the spacing on a main rib I found that it was about 1/32" longer than the root rib/left side fuselage and so about 1/16" shorter than the right side fuselage. The good news was that the wing incidence was correct on both sides to within 0.1 of a degree - the limits of measuring accuracy.
So, in summary, the spacing of the spars on the plans for the ribs were about 1/32" different between the root rib and the main ribs and I'd built to the plans. Also I'd welded the fuselage brackets about 1/32" shorter than the main rib on the left side and 1/16" longer on the right side. These are small differences in welding terms but would mean that the wings would not bolt up correctly if the wings were constructed to the original spacing.
The solution was as follows:
The first root rib is correct on the left side of the aircraft so leave as is.
Modify the nine finished main ribs by removing 1/32" from the front of the rear spar slot and gluing a section of 1/32" ply to the rear of the slot (4 completed so far). Modify the main rib jig to match and then construct 3 additional ribs which will complete the requirement for the left wing.
Modify the root rib jig to construct the right wing root rib with an inter-spar spacing 3/32" longer than the left and build the right root rib (completed).
Then re-modify the main rib jig to match the right root rib and construct 12 right wing ribs.
Of course the external outline of all the ribs and therefore the wings will be precisely the same only the spacing of the spars internally will vary very slightly.
Perhaps I should after all have built the wings first, but logistically this would have been very difficult.

14th September 2009 - Panel Painted and Layout finalised

I have to confess that I'm finding making ribs pretty boring, so a couple of days ago I painted the panel in it's final matt black and today installed the various instruments in it. Assuming my build sequence for the completed aircraft is correct, I should be able to install the panel complete with instruments and wiring without them having to be removed again so the photo gives a good impression of how the completed panel will look. From top to bottom, left to right the layout is as follows:
Starter engaged warning light
Mag switches (left and right)
Master/Alternator switch
Starter Push Button (NB, the mag switches have to be left impulse mag-on, right-off for this to operate)
Vacuum gauge
Tachometer
Hole for Trig TT21 Mode S transponder (mode S mandatory in Europe)
Pilot headphone sockets
Airspeed Indicator
Trio Autopilot
Artificial Horizon
Directional Gyro
Primer
Altimeter
Vertical Speed Indicator
Warning light Cluster (Low volts, Low Oil pressure, Carb temperature, Autopilot Engaged)
Information Display Unit (GPS Heading, GPS Groundspeed, Air Temperature, Carb Temperature, Bus Voltage, Battery Amps)
Garmin Apollo SL30 Nav Comm
Bendix King KMD150 GPS
Holes for Carb Heat, Throttle, Mixture
Oil Temperature and Pressure gauge
Intercom
Switches for: Strobes, Nav Lights, Auxilliary Fuel Pump, Spare for future requirement
Hole for CHT/EGT gauge
Fuel gauge
Pullable Breakers for alternator field and starter relay
Hole for Cabin Heat Control
Speaker On/Off switch
Chronometer: Time, Stopwatch, Flight-time
Emergency electrical power switch
Passenger headphone sockets
Spare 2-1/4" hole to be covered with panel blank pending any future requirement
12V Power Outlet

The intention of the panel design is to have good functionality but stay in keeping with the ethos of the aircraft and it's design date, hence the predominantly analogue instrumentation. The instrumentation also has to fit in with the physical requirements of the aircraft - particularly the depth limitations created by the fuel tank. I'm pleased with the way it has turned out but obviously the proof will be in flight.
Remember throughout the blog you can click on the pictures to see them enlarged.

10th September 2009 - Right Aileron and Flap completed

A full day's work today although it doesn't really look like it. First job was to weld up three sections of 5/8" * 0.035" tubing to make the aileron mass balance horn. Then this was part filled with molten lead and tack welded to the front of the aileron torque tube to check for balance. A couple of iterations of adding lead and re-tacking saw the balance just slightly nose heavy to allow for the covering and paint. The aileron mass balance horn was then final welded in place. Unfortunately, when the test rib was checked it became clear that the weight would foul on the rear spar so it was cut off, shortened slighty, the top front edge curved, additional lead added to compensate and re-welded into place. It is still very close to the spar but should now clear OK. Next job was to mark on the bench the positions of all the wing ribs and position the flap and aileron over them. The positions of the hinges could then be marked and short sections of 1-3/8" diameter tubing sawn into quarter arcs and welded either side of the four hinges to locate them. Finally eight plates were cut out of 0.032" steel plate and welded round the hinges. The fabric will be glued to these to make a neat junction round the hinges. Some new T-88 epoxy also arrived today so it's back to rib making tomorrow.

9th September 2009 - Right Aileron and Flap continued

Still no glue, so I carried on building the right aileron and flap. First 9 off 6" lengths of 3/8" * 0.035" tubing were welded to the torque tubes, 6 to the flap and 3 to the aileron, to act as ribs. Then the inboard end of the flap and outboard end of the aileron were constructed to length as per the plans and welded to the torque tubes. These are each made from two lengths of 3/8" steel channel. A string was then pulled tight between the ends and used to mark the final lengths of the ribs which were cut off using the angle grinder. The inboard end of the aileron and outboard end of the flap were then also constructed and welded into place sized and positioned to meet the string line. The tubular ribs were then flattened at the rear ends to fit inside the stainless steel "V" section trailing edge. Then the string line was used to ensure all of the ribs were precisely inline and small adjustments made where necessary, Finally the two sections of trailing edge were brazed onto the ribs. Next job is to weld into place locating plates for the hinges and covering support plates around them. Also the aileron has a mass balance welded to the front of the outboard end - details tomorrow.

8th September 2009 - Right Aileron and Flap

It has been a frustrating few days as when I'd finished the first tube of T-88 epoxy I found that the second tube had gone hard so I was unable to make any progress on the ribs. Today, I'm still waiting for fresh glue to arrive so I cleared the worktable and decided to make a start on the ailerons and flaps. I'd constructed the torque tubes back in August last year http://tailwindbuild.blogspot.com/2008/08/8h-august-2008-flap-and-aileron.html so first job was to make the hinges which need to be in place on the torque tubes before the ribs are added. First I added a dummy aileron spar and dummy sections of skin to the rear of one of the ribs. This then gave the spacing and angles for the 0.071" thick 4130 steel plates that will bolt through the wing to secure the ailerons and flaps. The hinge itself is 1-3/8" diameter 0.058" thick 4130 tube which is a good fit around the 1-1/4" diameter torque rods. The 4 hinges for the right wing were constructed in a single piece and then sawn up. The plates were both welded to the tubing and also brazed as it seems pretty important that they don't fail! A 3/16" hole was drilled through the top of the tubing to allow the joint to be oiled. The second picture shows the profile of the weld and the fillet of braze metal. Then, with the hinges installed, the torque tubes were clamped to the bench ensuring that they were exactly straight and level. Finally a jig was made to hold the ribs (3/8" tubing) at right angles to the torque tubes and aligned with the centre line.

1st September 2009 - 2nd root rib

I constructed the 2nd root rib today - seen in the jig before gluing. The problem with this type of jig is that the jig blocks can not be glued to the board as there is a piece of plastic sheet protecting the plan from the glue. This means that when fixing and removing the ribs the nails holding the jig blocks can tend to loosen allowing them to move. OK for two-off root ribs but not for 24 identical main ribs.

31st August 2009 - Ribs and rework

It's been slow progress over the last few days but there are now 7 ribs completely finished and one with glue setting before the gussets are installed on the second side. I've also made a jig for the two root ribs and the first root rib has been constructed, glued, and is setting in it. For the root rib jig I've just pinned blocks of wood to locate the rib structure to a piece of polythene over the full size plan as there only two ribs to make so the jig does not need to be too durable.
I was intending to have a first go at covering starting with the rudder which is the smallest and simplest structure but when working out how to fix the fabric I realised that it was going to be difficult to get a good seal round the rudder horn. In order to give somewhere to glue I welded a small plate either side of the front rudder spar round the horn similar to those installed previously around the hinges. Then this needed the priming touching up where it had been sanded or burnt off so it's another seven days for the paint to completely harden before this is ready to cover.

23rd August 2009 - Priming Complete

Finally the priming is completed, actually the priming itself isn't too bad but cleaning the parts with the Deoxidine is fairly unpleasant. This is basically a phosphoric acid solution which dissolves the aluminium oxide and etches into the aluminium. It does produce a very clean surface for the paint to adhere to but smells terrible and is messy to use particularly on big structures like the fuel tank and combing.

Painting is not my strong point and I usually end up with the surface covered in runs and marks but this time I've invested in a new type of paint gun - LVLP. This stands for low volume, low pressure. Low pressure has been around for some time and has the advantage that the overspray is greatly reduced. Low volume means that the amount of air used is also greatly reduced. I'm spraying using a 1.5HP 50 litre compressor which is not sufficient for conventional guns but works fine with the LVLP gun which uses only 6.5cfm at 22psi. The gun is, predictably, Chinese made but a beautiful piece of engineering and good value - http://www.auarita.com

The rib making has taken something of a back seat recently so still only 4 complete and 1 half done.

19th August 2009 - More ribs and primer

The process of making ribs continues slowly, four complete and one gluing. Similarly the priming seems endless especially as I left out the fuel tank and the panel from the list last time. However, the end is getting nearer. All items except the combing and the fuel tank have been cleaned and at least partially painted so another couple of days should see the priming complete. The primer takes 7 days to completely set so once all the priming is complete I can then start applying top coat to those items where it is needed and that have had the requisite drying time. This is a subset of the items where they are visible or where the better wear resistance of the top coat is useful.

16th August 2009 - Ribs and primer

The first rib was completed today other than tidying up the edges which will wait until all the ribs are complete so that they can be done together to ensure they have exactly the same profile. The second rib was built in the jig and the gussets stapled to the first side. Then it was back to priming. I think my estimate of 2/3rds done yesterday was overly optimistic - perhaps it's 2/3rds now. I still have the stringers, the seat bottoms, the seat backs, the coming, the cowl cheeks and the baggage compartment floor to clean and paint and one side of the doors and outer floor panel to paint.