Right Wing Skins Done

Time: 22:30 hrs

Work was done Nov. 17th – Nov. 24th 2018

In a meeting of IECA (Igo Etrich Club Austria, the Austrian counterpart of EAA) I sought approval from my Austro Control technical examiner (the Austrian authority) to put the wing skins on. He ok’ed this as long as it was possible to inspect the interior with a mirror / borescope.

With all the prep work done, putting on the right wing skin was straight forward and easy. The only prep work left to do was to apply Ardrox AV30 to the mating surfaces with a foam paint roller, which I did immediatly before clecoing the skins.

WNG R Skins Ready (3)
Ardrox AV 30 applied to the mating surfaces

One thing I did different compared to the testfit was to start with first clecoing the overlap of the top skins to rib 8 where I had some tension during test fit. It was much easier and no noticeable tension anywhere.

There is a line of three unused rivet holes behind rib 4. Mike at TAF confirmed, that nothing is attached there and that they should just be riveted closed.

WNG R Skin 004 Unused Rivet Line
Unused Rivet Holes

Since I knew that I would not have enough hands to turn the wing upside down in order to rivet the bottom skins, I have raised the wing jig off the table a couple weeks ago by about 25 cm.

Using three boxes strategically placed underneath the wing structure to lift the bottom skin off the table close to the underside of the ribs made it easy to cleco the bottom skins in place myself.

I had to rivet the whole bottom skin in a bent forward and often twisted, overhead position, which – considering the number of rivets – was somewhat inconvenient. Stretching the work over a period of three days and frequently changing position this was ok.

WNR Riveting Nose Skin (2)
Shooting The Last Rivets Of The Nose Skin

It was quite a satisfying feeling to see the skins on. The fit seems to be quite good – no noticeable bulging or waviness.

WNR Skins Done (1)
Right Wing Skins Riveted
WNR Skins Done (4)
Some Excess Ardrox AV30 Still Needs To Be Removed With Acetone

WNR Skins Done (3)

As mentioned above, my technical examiner wants to be able to inspect the interior of  the wing. This would not be possible at the wing step ribs once the tank is on. We therefore agreed on having the next inspection once the tank is finished but not yet riveted to the wing. So let the Proseal fun begin!

As a side note: it’s been almost exactly two years since I started the build. Thank you darling for your support!

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Making Right Wing Ready for Skin

Time: 60:30 hrs

Work was done Oct. 7th – Nov. 7th 2018

After fabricating the conduit support brackets for the wing ribs, I finally was able to rivet the wing ribs to the main and rear spar.

I had a little problem with the flange of wing rib 16 that attaches to the rear spar being bent slightly incorrect. Due to that the rib was 1,5mm too short pulling the main and the rear spar together. This put a lot of tension on the rivets and resulted in some waviness of ribs 15 to 12. This effect would not show with clecos, because the clecos were just pulled out of the rivet holes a little. Putting a 1,5mm shim between rib 16 and the rear spar solved this.

Peter Calley has reminded me not to forget to put the flap torque tube in place (thanks!). It cannot be inserted once rib 6 is in place unless you unrivet the lever arms.

WNG R Structure Ready (19)
Torque Tube in Place

One thing I also have done is, to replace the old style 3mm Nylon aileron pushrod guide (that was just riveted to rib 6) with the new style Vesconite bush, that TAF supplies now –  much more durable IMHO. Since my rib 6 did not have the holes for the new bracket predrilled, an installation drawing Armin from TAF sent me was a big help .New Aileron Pushrod InstallationBefore drilling the rib I checked the bracket position by temporarily putting the long aileron push rod in place connected on one end to the aileron bellcrank and centered in the rib 12 hole on the other. For doing this, I had to make hole in the jig. Some grease helps with threading the long push rod.  All the dimensions given were spot on, so I match drilled the holes and riveted the bracket in place.

WNG R Aileron Pushrod Guide (1)
Old Style 3mm Nylon Pushrod Guide
WNG R Aileron Pushrod Guide (2)
New Style Vesconite Pushrod Guide

Installing a short stretch of conduit for a test, it quickly became obvious that it flopped around. There is no good way to support the electrical wiring (or conduits in my case) between the ribs. Builders have come up with various solutions.

I have decided to make support brackets attached to the short vertical stiffener channels on the main spar  using existing rivet lines similar to the ones Sling 2 builder Pascal L. has documented here (thanks Pascal).

The following photos show some of these support brackets and the routing of the  installed conduits.

WNG R Structure Ready (1)
Overview of Right Wing Conduits
WNG R Structure Ready (2)
Conduits Exiting at the Wing Root
WNG R Structure Ready (5)
Conduit Support Bracket between ribs 5 and 6
WNG R Structure Ready (12)
Conduit Routing For Ldg/Tx Lights
WNG R Structure Ready (11)
Conduit Near Wing Tip for Strobe / Position Light

I have  installed and torqued the eyebolt bearing now for the torque tube and the aileron bellcrank already now . The manual does not suggest that, but I think it is much easier doing it beforehand on the workbench than later through the inspection panels.

WNG R Structure Ready (13)
Torque Tube Eyebolt Bearings
WNG R Structure Ready (15)
Aileron Bellcrank Eyebolt Bearings

Another thing I should have done at that point is to chamfer the upper and lower wing stringers, so they sit nice and flush on the skin at rib 9. Obviously I have not and I later had to later remove the inboard bottom skin once again, after it had already been clecoed in place for riveting 😦

WNG R Skins Ready (4)
Chamfered Edge of Wing Stringer

Scotchbriting the skins with Prekote and priming with Adler 2k epoxy primer also took some time. Space  is getting somewhat tight.

WNG R Skins Ready (2)
Right Wing Skins Primed
WNG R Skins Ready (1)
Right Wing Skins Primed

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Right Fuel Tank Testfit

Time: 7:15 hrs

As mentioned in my last blog entry, I wanted to have a look at how the fuel tank fits, before taking the wing skins off again to assemble the right wing structure.

To assemble the tanks, a jig is required. The manual has the templates for the jig, which I printed out using the poster option with alignment cross hairs of Adobe Reader to get dimensionally correct templates. The dimensions given in the plans allow checking this.

Adobe Print Tip

I am posting this tip here, since some fellow builders mentioned having a problem printing to scale.

For padding the jig to prevent scratching the skin I used a narrow stripe of carpet glued to the jig with contact glue.

WNG R Tank Jig
WNG R Tank Jig

Clecoing the tank together was quick, everything lined up nicely. I just had to ream out the 3,2mm rivet holes – either the holes were just slightly too small or the batch of rivets too large.

WNG R Testfit Tank (1)
WNG R Tank Clecoed

My wife helped me with my first trial to testfit the tank. But whatever wiggling, pushing or pulling we tried, I just could not push the tank fully towards the main spar and had a gap at the tank /  nose skin overlap and the top main spar rivet line did not align either. So off it came again.

After some head scratching and measuring I found out, that the outboard most tank mount bracket that let’s you bolt the tank to the main spar, was sitting high on a rib flange. This made it impossible to push the tank fully onto the main spar. This was not obvious,  as this area is closed off, when the tank is mounted. Even a miniature borescope would not get you in there. Turning the mount bracket 180° fixed that issue.

WNG R Testfit Tank
WNG R Testfit Tank

As expected, I had some misalignment at the bottom tank / wing skin overlap – nothing serious – a little elongating of a couple rivet holes fixed it.

One hole in one of the tank mount flanges near the wing root needed some filing too, but that’s ok for me, since the AN3 bolt used here will need a washer under the bolt head anyway.

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… For Educational and Recreational Purposes … or “Some Like it Hot”

As the FARs and my national regs state, we have the privilege to build experimental or homebuilt aircraft for “… educational and recreational purposes …” and that we can make modifications for experimental purposes.

Here’s one that falls into the first category – education:

As others have done, I am considering to make the whole pitot mast removable for service purposes instead of just the pitot probe itself. Disconnecting the fittings (that connect the pitot and AOA line) and electrical connections in order  to pull  the pitot probe would be a difficult in the restricted space through the inspection holes from underneath the wing.

While pondering about how to make it removable, the question arouse of how hot the aluminum tubes, to which the pitot and AOA line connect, would become during operation. And I also was interested in how hot the pitot tip actually gets.

So I decided to measure a temperature profile. For the measurements I used a thermocouple hooked to a Fluke 289 DMM. I applied small dabs of thermal grease to the pitot tip and six discrete points along the rear (AOA) connecting tube and also to the thermocouple for good thermal conductivity between the tube and the thermocouple. Measurements were taken at these points at an ambient temperature of 23°C.

EL Pitot Heating Measurement (1)
EL Pitot Heating Measurement Positions

The (unregulated) GAP 26 pitot / AOA probe was wired to a regulated lab power supply set to 12V. Turning on the power showed an initial current of 6A, but the current limit indicator lit up for a second or so – which means that the inrush current was higher than the 10A current limit of the powersupply (the GAP 26 installation manual specifies a min. 15A rating for the connection wires).

EL Pitot Heating Measurement (2)
Pitot Heat Measurement Setup

After a 60 s heating period the power supply was switched off and a temperature profile measurement was taken (took about 60 s). This procedure was repeated three times. After that, the heating was on for 10 min until the heating current reach a steady state and another temperature profile measurement was taken. Here are the results for the five temperature profiles measured.

EL Temperature Profile AOA Tube

Here is the temperature measurement for the probe tip during the heating/measurement cycles. Note the 160°C (320°F) steady state tip temperature (“some like it hot”), so take care during your preflight when checking the pitot heating.

EL Temperature Pitot Tip

In the steady state, I additionally used a Testo 889 thermo imaging camera to document the steady state after 10 minutes heating.

IR Bild
Thermal Image of Unregulated GAP26

The next picture shows the temperature profile along the black line shown in the thermal image above.

Thermal Profile along AOA Tube

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Right Wing Test Fit

Time: 22:15 hrs

Work was actually done August 5th 2018 till September 8th 2018

With the wing rib subassemblies ready and the wing ribs and rear spar parts prepped, the fun part of building started again.

First the rear spar was clecoed and riveted. No problems here, just needed to take a little care, which holes not to rivet, because they either attach to the wing ribs or it would make riveting the wing ribs impossible.

For assembling the wings, the jigs to set the wing twist correctly, first had to be built (no scotchbriting, alodining or priming required 🙂 ).

To bolt the spars to the jig, TAF supplies all the hardware, but the holes in the jig were not to size. When drilling out the holes in the jig, my drill started to chatter on the first hole and it became out of round and oversize. It does not matter much, because there are eight holes in the main spar at the wing root, but of course I was more careful with the other ones.

With the spars in the jig, the root jig and tip jig were crosschecked level again on the top edge before clecoing the wing ribs and rib subassemblies for a testfit.

WNG R Pushrod Alignment (1)
Right Wing testfit

If you look carefully at the photo, you may notice a very dumb mistake I made here:

I wanted to check the alignment of the aileron pushrod holes and for some silly reason I took the wrong turnoff and partially threaded the string through the holes for the flap torque tube. The kink in the string visible in the photo above got me nervous, because I first thought that I may have received a Sling 2 rib or such. Everything is fine now, but it took me a day or two to find out.

Another thing that took some time was fabricating brackets for my electrical and pitot/AOA conduits. Because I want better serviceability once the wings are skinned, I am deviating here from the manual by using conduits (like in the rear fuselage and empennage). With some pushing and pulling, my 13mm corrugated conduit perfectly fits through the holes in the ribs. The same installation method is used in the RV community and Vans advises RV builders to use a dab of RTV or Proseal at the holes to avoid the ribs cutting into the conduits due to vibration over the years. I may still do this, but I also will tie wrap the conduit to tie wrap mounts riveted to brackets.

WNG R Conduits (2)
Electrical and Pitot/AOA Conduits
wng r Conduits (1)
Electrical and Pitot/AOA Conduits

The conduits have not been routed yet, since this has to be done in the correct sequence when riveting the wing ribs.

Before testfitting the skin, I made sure that the transition from the ribs to the main and rear spar was smooth. Two or three ribs were high or low  by 1 or 2mm. Slightly rebending the rib flange took care of that

Test fitting the wing skins was fairly straightforward. I just clecoed each skin at the corners, then bisected the main and rear spar rivet lines a couple of times with a cleco; same procedure with the ribs: first the corners, then one cleco in the middle, 1/4 and 3/4 position and so on.

WNG R Testfit Skins (3)
Testfit Right Wing
WNG R Testfit Skins (1)
Testfit Right Wing

The rivet line at the overlap of the upper skins at rib 8 was a little tight, but I managed to cleco it without upsizing the holes. All in all the skin is very smooth with no visible bulging or waviness.

The front rivet line of the bottom skin, which will be overlapped by the fuel tank skin, has some misalignment.

WNG R Misalignment Bottom Skin.jpg

The reason seems to be a slightly too large bend radius of the front spar cap. Mike at TAF advised me to elongate these holes, since there is almost no load on the skin there and  I think I can live with this advise. It makes sense, as there is a parallel rivet line 2″ further back with twice as many rivets that take the load and the front rivet line just acts as a shim for the overlapping tank skin.

I still have to check though, whether this misalignment will be a problem when riveting the tank skins.

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Preparing Wing Parts

Time: 88:30 hrs

Work was actually done July 21st 2018 till August 3rd 2018

With the decission made, that the wings will have to be built in the garage and the garage rearranged for the wing build, preparing the
– parts for the wing rib subassemblies
– wing ribs
– rear spar channels
– stringers
was next (removing plasting, debur, scotchbrite, alodine, prime …) – a lot of time-consuming work with not much to show.

After the prepwork the subassemblies were riveted together.

WNG Ribs (2)
Wing Ribs and Subassemblies Right Wing
WNG Ribs (3)
Wing Ribs and Subassemblies Left Wing

Also, after consulting TAF, a 0.1 – 0.2 mm scratch in the right wing main spar cap was sanded out and alodined.

WNG R Mainspar Scratch (2)
Scratch in Right Wing Mainspar Cap
WNG R Mainspar Scratch (5)
Scratch Sanded Out and Alodined

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Rearranging the workshop

Time: 15:30 hrs

Work was actually done July 18th 2018 till July 20th 2018

With the CF sections joined my plan was to mate the CF with the RF, install the firewall and the stringers for the CF side skins and possibly the side skins themselves. With this in mind, I have clecoed the CF and the RF together for a testfit (sorry, no photos).

Impressed by the size of the now combined fuselage and the little space left in my single car garage, it dawned on me that – had I joined RF and CF – I actually would have to build the wings in the basement .

I had roughly checked the feasibility of building the wings in the basement before and I knew it would it least be very tight, but I just wanted to make sure. It seems that I then have forgotten to consider the rear step ribs when doing this rough check and … you guess it … a precautionary second measurement showed, that it would be next to impossible to build the wing in the basement and later to get it out of the basement.

So plan B was put into action. I unclecoed the CF from the RF again, put the CF in the basement again and the RF in the corner of the garage. A protective housing, that now simulatiously acts as an additional workbench, was built around the RF from leftovers from the kit crates.

WNG Garage Preparation (1)

The two workbenches were rearranged such that one wing (including the jigs) fits on the table and I still can walk around the wing when the garage door is closed. Of course the workbenches were aligned and levelled again in all directions.

WNG Garage Preparation (4)

When it is getting too cold during winter, I will be able to continue with some work on the CF in the basement and also build the wing tanks in the basement.

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