Brake Lines

The brake lines were mostly done in parallel with the fuel lines, so the time mentioned in the Fuel Lines post include the time for the brake lines.

The Parflex semi rigid Nylon tubing and the Parker Poly-Tite fittings that TAF uses make a fairly high quality impression to me, but the PVC thickwall tube, that TAF uses to sleeve the Nylon tubing would not be my first choice material in the cabin (PVC). The thickwalled PVC weighs in at 0,087 kg/m. Add the Parflex Nylon tubing and you are on par with D-4 braided steel Teflon lines. For reasons pointed out below, I propably would go with Teflon brake lines had I to do it again.

The tubing comes in a spiraled bundled approx. 30cm in diameter and it maintains the spiral shape, not matter what you do with it. It’s just a stubborn bunch (not sure if “stubborn” is the correct word here). One cannot pull it straight or unbend it and it is not possible to twist it. In hindsight I propably should have just asked TAF, how they handle it.

With that said, working with Parflex Nylon tubing is not my favorit task. I finally managed to work it by carefully warming it up piece by piece with a hair drier, lightly bend it and cool it down with cold air again. I used the lowest temperatur setting – just warm enough to not burn my fingers. The Parflex is rated -51°C to 93°C (-60°F to 200F°) operating temperature. I assume, that heating it to higher temperatures will compromise its burst pressure.

Rod L. has pointed out on his blog that he had a problem with an air bubble at the highest spot in the system (the brake cylinder in his case), when filling the brake lines. I therefore took care to position all the mounting brackets and zip tie down mounts such, that I can lay the whole loom down sideways on the CF floor if I run into this problem.

Slide show of the brake line routing:

Some comments on the center console:

As explained in previous posts, I must use a cerfied engine – propeller combo (Rotax 914F with Muehlbauer MTV34). This necessitates a full throw propeller lever on the center console. The propeller lever operates the hydraulic governor via an additional bowden cable. As I do not have the second lever yet, it is not shown in above photos.

I therefore have to fabricate a new cover plate CF-CHL-016-C-A- anyway, to which the throttle and prop levers are mounted. I will mount the parking brake sideways. This gives the parking brake lever a forward<->backward movement. Additionally it opens up space to place the flap switch to the left of the parking brake lever.

Originally I wanted to use the left and right buttons on my Tosten CS8 stick grips for the flaps. This seems to be a common practice for US builder, but my build supervisor from AustroControl did not approve of that. With the panel already being fairly crowded, we finally agreed on above position.

It is almost impossible to inadvertently operate the flap switch, but I most likely will add a switchguard similar to the ones used on the panel.

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Fuel Lines

Time:   40:00 hrs

The time shown above does not reflect the actual hours. I have been working on and off half an hour here and an hour there for months but I have neglected logging the work, so above numbers are just an estimate.

The main reason for ordering the aviation and electrical system from Midwest Panel builders early in the build (as described in the last post) was, that I wanted to install the wiring for the electrical system before closing the CF by installing the side skins and canopy.

Same is true for the brake lines and the fuel lines. Normally this is part of the firewall forward and finishing work late in the build. Other builders have commented on how tight it gets in the cabin, when tackling this work. Or to state it more clearly:

It can be a pain (on your spine) if you are
– my age
– as tall or taller than I am (1,86cm /6 ft 1)
– not as flexible any more

Therefore installing the fuel and the brake lines was next on my list before continuing on the CF with the side skins and the firewall. Here is the lengthy story on that topic.

Very early in the build I have already decided that I did not want to use rubber fuel hoses in the cabin. They normally have I five or six year replacement mandate on certified aircraft. With the tight space in the cabin and especially in the center channel replacing those would be (you guess it) – a pain.

Moreover I did not like the fuel filters down low in the center channel. Replacing these and spilling fuel inside the cabin did not appeal to me. For a long time I have been weighing aluminum against teflon fuel lines and had decided to go with teflon and putting the fuel filters into the wing root near the inspection panel.

After reading on Pat S.’s blog about using 3/8″ OD 3003-0 aluminum tubing (which seems to be standard on RV aircraft) I was unsure again and contacted TAF boss Mike Blyth. Here’s his answer:
” ….Using the aluminium 3003 fuel lines is actually a better solution than using the rubber lines. During this year we will be changing over and using them on all our aircraft in future. …”
I was sold.

I did however rethink the idea of putting the fuel filters into the wing root. I did not like the idea of fuel spilling into my face when lying under the wing to exchange a filter with full or half empty tanks. Add a shutoff valve at the tank? No. Besides the added complexity, the fuel from the lines would still drain into my face. I therefore decided to put them in the cabin, but directly connect the filters to the fuel selector valve as high as possible. This will put the filters above the tank fuel level. With the fuel selector valve closed, the fuel will drain back into the tank when the filters are disconnected at the fuel selector valve.

This all was just decided and then Corona set in – a year ago from now. It took all of five months till I had the Andair fuel selector valve, the Andair fuel filters, the 3003-0 aluminum tubing and all AN6 fittings in hand to start with the fuel lines. Make sure you get a 37° flaring tool when you buy one (ask me, how I know). Builder friend Pascal L. was kind enough to lend me his Rolo-Flair flaring tool after I got myself the wrong (45°) one (Thanks Pascal!). It works really nice. After a few practice pieces, I was able to produce consistent flares.

Putting the fuel filters up high at the selector valve resulted in complex 3D curves of the fuel lines.

I was not able to produce these 3D curves with sufficient precision to make them fit nicely into the center console (i.e. routing them in a way to have other areas like rudder cables still serviceable). Plus, it would be almost impossible to replace them without tearing half the aircraft apart, should they ever leak.

After several attempts I gave up …

… and returned to my original decision to use Teflon hoses.

I went with Goodridge G-line XF 811 series hose, which is a highly flexible, antistatic, stainless steel braided smoothbore PTFE hose. I was able to source it within reasonabe time from an Austrian motorsport supplier – a factor in Corona times.

I am pointing the antistatic out here, because I’ve read that cheap Teflon hoses can lack this property. Fuel flow in these hoses can cause static discharge, which over time will micro perforate the hose and make it leak, which will be difficult to find.

The AN6 fittings are reusable and screw in type. I am copying the (original Goodrich) assembly instructions here for information.

It took a couple trials to find out how to best make a clean square cut without fraying the braided steel and how to best feed the braid into the socket.

For me, using a single layer of paper masking tape and an angle grinder with a 1 mm cutting disk for stainless steel worked best. Going slowly down and around and cutting the last strands upwards made a nice cut without fraying the braided steel.

To guide the hose into the socket, for the first millimeter, I used a strip of aluminum. Then I removed the masking tape and continued with a turning motion. Once I got that sorted out making one connection went faster than making a good flare on a tube.

My tank lines (feed and return lines) are AN6, so I have ordered the Andair fuel selector valve with AN6 90° angle fittings for the tank lines and straight AN6 fittings for the engine lines. The Andair part no. is FS2020-D2-TMT. Originally the return lines were smaller diameter using AN4 fittings at the tanks, but I have changed those to AN6, which is standard now.

The fuel filter is a 62 micron stainless steel screen inline filter (Andair FX375-M) with AN6 male fittings. It is directly connected to the fuel selector valve using a AN6 female to AN6 female 45° adapter.

Here’s a slide show of the routing:

A note on the diagonal brace Sling 4 builders may have noticed in one of above slides:

In 2019, when TAF did their Sling 4 build at the LAA Sywell 70th anniversary rally in GB (10 people building a Sling 4 in 7 days), the LAA issueing the permit to fly made a remark about the movement in the center console in this area, when the flaps where extended. As a result of that remark I added the brace then and it does reduce movement significantly.

It does not really make servicing this area easier, but it came in handy now for a bracket to secure the fuel return lines (and other stuff later). I have made the brackets removeable by using M4 screws with nyloc nuts shoult it ever be necessary.

Rotax states in the 914 installation manual
“…In the feed line from tank to the fuel pumps an additional fine filter with mesh size 0.1 mm (70–100μ) has to be provided. The filter has to be accessible for service. A combination of filter/watertrap (gascolator) is recommended…”
The Andair GAS375 with its 70µ filter would fullfill this requirement by itself. Other builders have omitted the additional inline filters upstream of the fuel selector valve.

I have installed them by now, but (theoretically) I could remove them and reroute the feed lines to directly connect to the selector valve without the inline filters. Because they eliminate the gascalator being a single point of failure, I see some advantage of the inline filters.

I was interested in what the pressure loss in my configuration would be. Since son Klaus currently is doing a course in fluid dynamics at University, I asked him to calculate that for me.

Here is what he came up with for the feed (=suction) line:

And here it’s for the return line:

The numbers shown are for :

  • zero pressure head
  • turbulent flow assumed (Re=2940 for the suction line)
  • “…negligable pressure loss…” for the inline filters (as stated by Andair)

For the return line Rotax states in the installation manual:
“…The fuel return must be a line of low flow resistance. Max. tolerated pressure loss is 0.1 bar (1.5 p.s.i.) between fuel pressure control and tank inlet with both electric fuel pumps in action. Otherwise the carburetors could flood. …”, so we are definitely on the safe side here.

For anyone interested to check the calculations for correctness, please feel free to download the Fuel System Pressure Loss Excel spreadsheet.

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Avionics and Electrical System

(… countless…,  but no work actually done on airplane)

To let the cat out of the bag right at the start: although I am an electrical engineer myself, I have decided not to build the avionic/electrical system ourselves, but have Midwest Panel Builders do that for us.

This blog will not document any actual work on the avionic/electrical system – just describe the decission process for our avionc system up to taking delivery of our avionic/electrical system from Midwest Panel Builders.

For the better understanding of non EU (or likely also non Austrian) readers, it is propably necessary to point out differences to other (especially non EU) countries.

Experimental aviation in EU is still subject to national legislation. Each EU country (even such a small one like Austria) has their own laws that apply to experimental and homebuilt aircraft and some can vary significantly (e.g. PPL training on an experimental aircraft is possible in France, in Austria and most other EU countries it is not).

Fortunately national airworthy certificates for experimental and homebuilt aircraft are now officially recognized EU wide, so we now can fly experimental and homebuilt aircraft in most EU countries without the need of a special permit for each country.

But national laws apply and can have a big impact on the build . In Austria one needs to apply for a build permit, which will be issued by the authorities (Austro Control). As part of the build permit Austro Control can mandate things according to national law. Here is an excerpt of my build permit:


A summary of the major items mandated by my build permit is :

  • Airspeed indicator, altimeter and compass must be certified analog instruments
  • Radio, transponder and ELT must comply with national/international airworthiness standards, which I learned translates into an EASA form 1 requirement (more interesting details on that later)
  • Use of a certified engine and certified propeller, which in combination must be torsionally vibration tested. This translates into the certified Rotax 914F / MTV34 or Rotax 915iS / MTV34 propeller combination (I must not use the 914ULS or the Airmaster propeller)
  • CS23 (which is the certification standard for certified aircraft) is mentioned as a framework for proofing airworthiness

The reference to CS23 as framework for airworthiness certification is not a big deal as far as it applies to the airframe. At the beginning of my build TAF has handed over a comprehensive package to Austro Control containing information such as
– design airspeeds and permissible loads (v-n-diagram)
design loads and CG position
– structural proof of main structure
– ………….
and much more.

Therefore I will not have to do any structural proof (like wing load test, drop tests or such) or e.g. spin testing. This package, by the way, was highly lauded by Austro Control as the best that they have ever received for an experimental aircraft).

One consquence of the build permit by the way is the need for a traditional prop lever on the throttle quadrant in place of the brake lever (because of the hydraulic governor the MTV 34 propeller uses). This entailes the necessity to have toe brakes instead of the hand brake lever.

Direct consequences on items mandated by my build permit for the avionic system and the instrument panel are:

  • The use of certified analog instruments as primary flight instruments, which also must be placarded as such and the Garmin G3X system only being secondary instruments.
  • No electronic prop controller on the panel, since the MTV 34 has a hydraulic governor
  • The reference to CS23 as framework for airworthiness certification in my build permit basically means that Austro Control has approve of the avionic and electrical system and panel layout (at the time when applying for the airworthiness certificate)

With that said:

I believe (and – in retrospect – so far can confirm this impression) that Midwest Panel Builders build and deliver an excellent product.


With what I have explained above, I really wanted to get it right at first attempt down to the proverbial “dot on the i”. I really did not want to find ourselves in a situation, where we have to change this or that on a finished panel/electrical system, when it comes to airworthiness certification.

Just imagine the consequences of a missing (mandated) analog gauge or a radio/transponder not in compliance with the “national/international airworthiness standards” requirement.

Even when only looking at placarding: the powder coated, laser engraved panel Midwest supplies does not really lend itselves to add placards or just change the wording of a placard.

So here are a few pics of my initial panel design based on what Midwest Panel Builders had on their homepage, one Garmin-only version:

Instrument Panel Garmin
and one using a Trig TT21 transponder and Trig TY91 radio as a plan B alternative (since they both have an EASA form 1):
Instrument Panel Trig

Designing the panel took a lot of communication between MWPB/me and Austro Contol/me respectively (“Time: ….countless ….”).

To avoid any missunderstanding here: communication with Austro Control / Midwest Panel Builders has been very constructive, but often things required going into the details and finding a non standard solution.

Understandably Steve and Adam could not know about details in Austrian/EU law but they always were very helpful to find a solution that met Austro Control’s requirements. Propably I jangled their nerves at times, so: thanks for your patience and great costumer service, Adam and Steve!

Thanks also go here to my build supervisors at Austro Control, who at times went the extra mile to help.

To give an example:

At one point one of my build supervisors saved me from having to overthrow the whole final design which had already been set.

He found out that my transponder (Garmin GTX35R) “only” has an FAA 8130-3 form, but no EASA form 1. This by itself would not be a problem as far as airworthiness goes, but I would not be allowed to fly in controlled airspace (practically impossible here in the EU with its high intensity of controlled airspace).

The regulative authority to get a radio station license is not Austro Control, but the Austrian counterpart of the American FCC that would not issue a radio station license without an EASA form 1 – at least that’s what we thought.

My build supervisor then (May 2020) checked the current EASA regs and found out, that in Sept. 2019 FAA and EASA had signed a mutual acceptance of TSOs/ETSOs (without the need of any additional paperwork). So: any avionic device that has an FAA 8130-3 form must now automatically be accepted without explicitly having an EASA form 1 and vice versa. My build supervisor also crosschecked and confirmed this with the responsible person at FCC (“..going the extra mile…”).

Long story short: we can use the GTX35R in controlled airspace and no change in panel design (see pics below) was needed. Without my build supervisors input a major redesign would have been necessary.

Another example of how deep into the details communication went between MWPB, Austro Control and me (or why I said, that I”propably jangled their nerves at times”):

With reference to “CS 23.991 Fuel pumps” an EASA Special Conditions Order exists for aircraft with electrical fuel pumps only, that the main pump must have the characteristic of a directly driven pump:
– runs when the engine runs
– cannot be switched off inadvertently and permanently

This would have necessitated major changes in the whole electrical system using additional relais (propably more failure prone in my opinion).  The problem with that mandate is that it would not be possible to test the main pump during startup.

Discussing this with Austro Control, my build supervisors accepted a solution using a switch guard for the main pump, that meets the following requirements:
– ascertains that the main pump cannot be switched off inadvertently
– switches the pump on in the default position
– the guard must be actively operated in order to turn the main pump off

Steve and Adam at MWPB then helped to find such a switch guard, that fulfills these requirements and also works with the switches they use – It is an APEM Series 40 switch guard:

AV Main Fuel Pump Switch guard

Here’s the final design of the panel (without said switch guard):

Midwest Instrument Panel 3 (2)

Side note:
For lack of space on the panel, the flaps switch will be placed on the center console behind the throttle lever.

The certified analog instruments will be:

  • Winter 4550 Altimeter
  • Winter 5713 Air Speed Indicator
  • Winter C2300 Compass

Major Garmin equipment consists of:

  • Dual GDU 460 G3X 10″ Touch Displays
  • GTR 225A 8.33KHz Aviation COM Radio
  • GMA 245R Remote Audio Panel
  • GTX 35R ADS-B Out Capable Remote Mode S Transponder
  • GMC 507 Auto Pilot Control Panel
  • GEA 24 Engine/Airframe Indication System
  • GAD 27 Trim, Flaps and Light Controller
  • GSU 25C AHRS
  • GMU 11 Magnetometer

Major non-Garmin equipment items are:

  • IBBS Backup Battery System
  • Artex 345 406/121.5 ELT
  • Aithre Shield EX 2.0 carbon monoxide detector
  • Tosten CS8 stick grips

The Aithre shield carbon monoxide detector is mounted behind the panel and connected to the GEA24 via an analog input. The CO level will be shown on the display and will trigger an aural alarm when exceeding critical levels

AV Athre Shield EX2.0 Carbon Monoxide Detector

Observant readers will propably note our call sign above the radio OE-KVP, which (for nonpilots) is spelled
O scar E cho K ilo V ictor P apa
on the radio, or according to standard radio procedures in the EU abbreviated to
O scar V ictor P apa
after initial radio contact.

During phase 1 a temporary call sign is needed here in Austria. For aircraft with more than three seats, that temporary call sign has to start with OE-V, for which I have been able to reserve OE-VVP. So the abbreviated call sign will stay the same.

In the sense of the project name “Sling 4 Family Project” the call sign stands for the initials of the familiy members

  • OE sterreich (=Austria)
  • E lisabeth, my wife
  • K laus, our younger son
  • V era, our doughter
  • P aul, our elder son (and co-builder)
    P eter (me)

Getting the call sign from Austro Control took less then one workday:
first email at 9:30pm to having the reservation for OE-VVP and OE-KVP in the email at 2:25pm next day.

Similarily for getting the Mode S transponder Code: four hours from first contact till having it in the email. Amazing, great service!

Here is a photo of the panel from MWPB shortly before it was disassembled again for shipment:

AV Midwest Panel

The analog instruments still need to be ordered. They are produced in Germany, so it was easier (handling reimport) and cheaper to buy them over here.

Unfortunately one of the two big boxes was badly damaged, when I took delivery of the shipment Aug. 25th & 26th 2020:AV Shipment (4)
AV Shipment (10)

It must have already been damage by the UPS truck driver, when it was picked up from MWPB in Michigan USA, because a small “stowaway” parcel had slipped inside the box with a Michigan senders address.

I’m so glad that Steve and Adam of MWPB have stuffed so many “airbags” and damping material into the box, that nothing was damaged. The “valuables” fortunately were in the undamaged box.

Steve and Adam did a great job supplying all required paperwork to prove that this all is actually airplane stuff, so I “only” had to pay 20% import sales tax.

Steve and Adam are already familiar with all this now. I am documenting the following here however for other EU Slingers following in my footsteps (took me “some” research time to find out):

It is important that the costum import declaration form shows the TARIC (= international customs tariff code) number 8803300010  (= “Other parts of aeroplanes for use in civil aircraft”) and that all 8130-3 forms are attached. Failing to do so would cost additional 7% costum duties!

Other than that, the costum handling agent also needed my EORI number. He was surprised that I had one as a private person. Normally you have that only as a business enterprise. This number was also needed, when I imported the Sling 4 kit from South Africa as a private person, so I did not have to apply for it again.

This is, what came in the two big boxes:AV Shipment (1)

AV Shipment (2)

AV Shipment (3)

Here is what the panel currently looks like:
AV Shipment (7)

AV Shipment (9)
AV Shipment (8)

After having installed the brake lines (which I am working on right now), I will install the wiring harness and antenna cables in the CF next. This bunch of wire weighs in at 6 kg (13 lbs):AV Wiring Harness

I am really looking forward to flying behind this panel.

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Fuselage Join, GPS antenna bracket, Torque Tube Stop Collars, SB #0018 …

Time:   19:00 hrs

Work was actually done July. 24 th 2020 till July 29th 2020 and Aug. 12th 2020

Before actually doing the fuselage join, I installed a mounting bracket for the diversity GPS antenna. If son / cobuilder Paul ever decided to go IFR with our Sling 4 (I won’t), he will need a certified ADSB transponder with diversity. There are certain requirements for placement of the diversity GPS antenna. The top of the arch, where the RF joins the canopy would be the best place and will meet this requirements.

We can place the GPS antenna here, because we do not have any cables for the parachute passing here (no BRS). Originally I intended to simply use M4 rivnuts with M4 taper head screws, but Midwest Panel Builders have advised not to do so. It is critical that the taper angle of the screws matches the countersink of the antenna, else it can crack the antenna if incorrect.

According to the Garmin manual the GPS antenna requires #8-32 UNC-2A x 1.0 screws, which have a different countersink angle than the M4 taper head screws.

I therefore had to fabricate a mounting/stiffener bracket using appropriate nutplates. Thanks go to RV12 builder friend Wolfgang for the nutplates, the solid rivets and lending me his rivet squeezer/dimpler – couldn’t have done without!

In the last and next picture you can see the conduit I ran up behind rib 2 for the antenna cable. The conduit will be covered with leather or carpet when doing the upholstery work in the cabin on rib 2.

Though the manual mostly finishes the CF before joining the fuselage sections, this would be very difficult for me. The garage I’m building in is very small and I need more space for the upcoming work on the brake lines and on the firewall in the near future.

Also I have heard that TAF USA now does the join before doing the sidewalls and the firewall. With the carpets and the sound insulation mostly installed in the luggage compartment , I decided it was time to join the rear and center fuselage now.

At least with the riveting tools I have, some rivets are impossible to set with the short elevator torque tube (CT-AEL-002-C-C) underneath the back seat in place and I had to remove that once again. I therefore suggest to not rivet it in place until having joined RF and CF.

Another reason to not rivet it in place early in the build are the stop collars. They also are fairly difficult to install with the torque tube in place.

Even with the short elevator torque tube removed, some of the rivets were really difficult to set (at least with my riveting tools). The backseat strengthening channel (CF-CHL-039-C-C-) forward of the torque tube is in the way. Do yourself a favor and also cleco that in place until after the fuselage join.

The stop collars which center the long elevator torque tube (CT-AEL-001-C-C-) forward of the landing gear channel are doable with the torque tube in place, but I did it with the flap torque tube removed (see reason below).

Being at it, I also wanted to install the two stop collars that I still had in my bags. Could only be the flap torque tube, right? Not! I later noticed in the manual that the flap torque tube do not have stop collars, but it was to late. I had already unriveted the torque arms, removed the torque tube and drilled the holes for the collars, so I installed them. With approval of TAF I used the Avinox BE61-0512 (that were used for the Service Bulletin #0014) as a suitable replacement to rivet the torque arms back on.

On July 28th 2020 TAF issued Service Bulletin #0018 concerning insufficient swaging of oval sleeves on the rudder cables.

Checking the crimps on my rudder cables, all measurements were out of range. After contacting TAF I was given the options:
1.) recrimp the sleeves myself using a Nicopress swaging tool
2.) have TAF send me a new set of rudder cables

Option 2.) would have meant to uninstall the cables. This would require to turn the fuselage sideways and undrill the rivets for the cable guides through the small inspection panels on the floor (without actually being able to see the rivets other than with my borescope camera) – not really a thing I was keen to do.

Besides other information about cable crimps, TAF has sent me a copy from a Dec. 2013 Kitplanes article by Dan Horton which advices against using cheap copycat crimping tools, because they do not produce consistent crimps. I was lucky being able to borrow an original Nicopress swagging tool (thanks Josef!) as the originals run at ~350€.

So with the original tool in hand I preferred option 1.) and with the help of my Head Supervisor and World’s Best Copilot this job was done within one hour. When visiting Sling 2 builder friend Alois B. in Munich first week of September, I took the tool with me and we were able to also perform the SB #0018 on his Sling 2.

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Luggage Compartment Sound Insulation and Carpets

Time:   26:00 hrs

Work was actually done July 21st till July 29th 2020

Remembering fellow builders my age “lamenting” what a pain it was to do the carpets in the baggage compartment with the side skins on (or even worse with the cannopy installed), I’ve decided to do the soundproofing and the carpets in the baggage compartment now and install the baggage floor, while I still had good access (RF and CF still apart).

It is definitely much easier to do that job standing comfortably on the ground in front of the luggage compartment and have everything at a fairly comfortable workheight compared to kneeing on the back seat, working in a bent forward, twisted overhead position. I can only advise doing it before joining RF and CF.

I started with first installing the parts that cover the stringers. For this work I used a contact spray adhesive, that has a fairly high temperature range. For documentation purposes I am posting the photos here.

I liked the instant strong adhesion here, as no clamping was needed. Any slight imperfection in positioning the part did not really matter, because it would be either invisible (on the inside of the stringer) or be covered by the next to install soundproofing. An experience I’ve made here: it makes things easier, if you rehearse putting the part in place before actually applying the spray adhesive.
… Sorry, no photos here for that part of work …

After I had the stringer parts in place next was to line the upper area of the luggage compartment with soundproofing. Im using selfadhesive 1/4″ SUPER SOUNDPROOFING from AC which has an FAA certificate that it
“…meets or exceeds. the flammability test criteria that is contained in CFR 25.853(a) APPENDIX F, PT l(a)(l)…”

To make templates for the soundproofing (and carpets – see comment below) I just cut some thin flexible cardboard slightly oversize, placed it on the stringer and squeezed the excess into the corner. To put the soundproofing in place I first did an Aceton wipe, pulled the rear bottom corner of the adhesive backing back, positioned it on the stringer and pressed the rear bottom corner in place. Working upwards and forward I pulled more and more of the backing off.

For glueing the carpet onto the sound proofing I used a PU construction adhesive I could source locally. It did not have the highest temperature range (-30 to +80°C) but it was the only one which explicitly stated suitability for Aluminum. With a fairly thick consistency applying it with a cartrige makes this a “no mess” job even when working overhead ( … as long as you do not to touch anything else with it 😉 …).

I applied the adhesive to both, the soundproofing and the carpet. For the soundproofing I sparingly spread it full surface with a spatula, richer at the edges. On the back of the carpet I laid a roughly 4 cm crosshetch pattern with the cartrige leaving out ~2 cm at the edges. Doing so allowed me to put the carpets in place without making a mess. Except for that narrow overlap at the parachute box I did not have to fixate the carpets.

A word of warning here:
Whatever adhesive you use – do a test first. Do not use contact adhesives on SUPER SOUNDPROOFING.

The producer of SUPER SOUNDPROOFING describes the product as a “… closed cell vinyl/nitrile insulating material, which will not absorb water or oil…” .
I noticed that my spray adhesive will dissolve the top layer – a µm thin foil. It will not attack the foam, just the foil, whose main purpose (I think) is to inhibit moisture intrusion.

When doing my tests it was not really obvious that the foil had dissolved. The contact adhesive spray I am using disperses in fine droplets, which dissolved the foil into small lumps hardly different than the droplets themselves. Just when accidentily touching the glue at some stage I pulled off a patch of foil. I assume the thin foil to be PVC since the contact spray adhesive is not suitable for that (see photo above)

To be able to continue with carpeting Paul next riveted the luggage floor in place. I am glad he did that work, which I have dreaded a little for a while. One can reach back there with only one arm in the narrow gap underneath the floor and there is hardly enough space for the pneumatic rivet gun. It all worked better than exspected.

Since I expect someone climbing back there someday I have added small brackets (red circles in photo above) to stiffen up the area, where the luggage floor stringers rest on the bulkhead. These brackets also made it easier to set the rearmost rivets, which connect stringers and bulkhead.

Once the luggage floor was in place I could finish with what I had left.

I could not find the lock for the luggage compartment door and I don’t have weather proofing material to seal the door yet.

I would like to have a lock that uses the same key as the cabin door. Most likely I will have to fabricate some sort of striker plate for that and therefor did not yet install a strip of carpet above the door.

The grey gaps will not be visible any more once I have the door seal and that last stripe of carpet in place.

Unfortunately, at the time when I ordered my kit, TAF did not consider the fact that I do not have a parachute box. Many of the carpets had the cutouts for the parachute box and I was able to make a couple pieces frome these unusable parts. TAF will send me carpets for the bulkhead and the floor, when I have my cowling shipped in the near future.

Overall I am quite happy with the outcome so far. It’s amazing, what a difference the soundproofing makes.

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RF Top Skins

Time:   24:30 hrs

Once upon a time there was a Sling 4 builder, who wanted to finish the rear fuselage ….

OK, ….. let me start another way:
More than two years ago I wanted to finish the RF by riveting on the top skins, but some rivetlines were off by >3mm and at the rearmost rib (601) not a single rivet hole even overlapped. The details can be found here: RF Testfit of Top Skins

Fast move forward two years:
After discussing the problem with technical, TAF has sent me

  • new RF top skins that were 3mm longer at the overlap with undrilled rivet lines, where there was a mismatch and
  • undrilled longerons 301 and 302 (at the overlap between side and top skins)

I first matchdrilled the undrilled longerons 301 and 302 to my side skins. The starboard side longeron had the taper at the front pointing the wrong direction. After consulting with TAF, I cut the taper on the correct side and fabricated a doubler inside the longeron.

RF Longeron 301

During the very early stages of my build, when browsing through the RF Factory build photos on Craig’s build blog (Thanks Craig – they still are a big help at times!) I noted this small detail.

RF Shoulder Harness Mod (4)

At first I thought I was missing some parts and contacted TAF. They told me, that this was a special order for a costumer, who wanted a shoulder harness but they could send them to me. This mod of course has to be installed before closing the RF.

RF Shoulder Harness Mod (8)

RF Shoulder Harness Mod (9)

Above photo I think gives a pretty good idea, how the mod works. The harness bolts to that eyeplate at the baggage bulkhead.

The standard sling 4 never had a shoulder harness for the back seat passenger safety belts. I have seen the slot for the eyeplate on current TSi build photos, but the eyeplate was not in place, so I not sure, wether this mod is standard on the TSi now.

Once the shoulder harness mod was in place, I started matchdrilling the top skins. Actually I did not matchdrill, but used a modified matchdrilling procedure to match the rivet holes of the top skins, the side skins and the longerons at the overlap area.

With all other rivet lines already drilled in my new skins (and fitting well to my RF ribs), I was able to cleco the new top skins together and to the ribs. But instead  of overlapping the side skins, I slid them inside behind the side skins, marked and drilled 5 resp. 6 holes at the rib positions. I then slid the longerons in place and clecoed them at these holes thus sandwiching the top skin between the longeron and the side skins.

Marking and drilling holes between the ribs then allowed me to cleco the top skin in place firmly sandwiched between the side skins and the longerons. This now allowed me to exaktly mark all the other holes. I drilled those holes slightly high to achieve a tight fit of the skins.

For the missing rivet holes at the rearmost rib (601) I fabricated a little drill jig to dublicate the rivet holes to the top skins.

Match drilling with the top skins clecoed in place resulted in an almost perfect fit when it was time for a testfit. There was only one spot where minute bulging was visible, which may need some attention again.

RF Top Skin Testfit (2)

RF Top Skin Testfit (3)

When testfitting the top skins I noticed that my cabletray was obviously a narrower version, than is currently used with the newer skins.

RF Cable Tray (1)

Though I will not install a parachute (and therefor will cut the cables off for weight), I still will install the cable tray because it does structurally stiffen the top skins significantly.

At first I thought of fabricating a wider version myself (using my old top skins),
but then I decided to try and bend the tapered sidewalls apart.

Bending the sidewalls apart while keeping the rivet lines parallel creates twisted complex 3D compound curves, both on the side walls and the flange. Making twisted flanges, that fit to the top skin while simultaniously achieving the target distance between the parallel rivet lines along their entire lenght on both sides required some patience and time.

RF Cable Tray (3)

But I’m quite happy with the outcome and it still took a little less time, than fabricating a new tray. And I think it even looks better than the original 😉 .

Next I had to take care of  the antenna brackets. Our cocktail bar …ah … parachute box is in the original location behind the luggage bulk head and the antenna behind the box. When I started building the RF, I already have installed the conduit for the RG58 antenna cable delivered with the kit for that position.

TAF has later moved the box and antenna forward and the antenna bracket was made significantly stronger. TAF has later also sent me the stronger version, so I have both antenna brackets now.

The RG58 would do the job for COM, but higher quality RG400 seems to be the standard now. I decided to put the antenna in the more forward position because of the shorter cable run and  better accessability (via the cocktail bar) using an RG400 cable.

Having both antenna brackets, I decided to just leave the conduit and RG58 in place as a reserve. The next photos show the routing of the conduits and  final location of both antenna brackets.

RF COM Antenna Cable Routing (2)

Note: the white cable above is just to document the cable run and will be replaced with an RG 400 later

RF COM Antenna Cable Routing (4)

The top skins act  as a ground plane for the COM antenna. It is important to have a good electrical connection between the antenna mount and the top skin. I therefore did not prime the brackets and skins in that place and alodined them on the inside.

RF Top Skin Primed

Today I did a (DC-) resistance measurement and I am not so sure any more if alodining was a very good idea. The Ω reading was 2 to 5 times higher compared to raw 6061-T6. Nothing like anodized aluminum, which is a good insulator but still. I’m aware this is all very unscientific and the electrical connection may behave much different at VHF.

I’ll leave it at that for now, but it is definitely something to keep in mind, should I ever have problems with transmission or reception quality of the COM. Since I still have access to the antenna bracket via the cocktail bar, I can drill out the rivnuts, sand the alodine off  at the rivnut holes and set new rivnuts.

Before riveting the top skins I also remembered that all fellow builders whose blogs I’m following have mentioned that they have doubled the number of rivets at the joggle where the composite canopy joins the RF top skins. Most of them also have upsized to 4mm rivets at least in the upper part of the arch. I remember that this has been ok’d by TAF. Since it is much easier doing it on the workbench, I drilled all these extra holes now – just 3,2 mm  for now, which I will upsize, when matchdrilling the canopy to the top skins.

RF Top Skin Testfit (4)

After all those preparations the fun part, namely riveting on the top skins was quick and easy.

RF Top Skins Riveted (1)

RF Top Skins Riveted (2)

I could not find the small antenna mounting plate in my parts bags so I could not set the rivnuts yet and had to leave that area unriveted.

RF Antenna Mounting Plate

Observant  readers propably will notice the bulge at the top of the arch immediately. The bulging occured already before  I attached the cable tray, so it definitely is not a side effect of me bending the tray incorrectly. I have tried a little, but any effort to get rid of it would cause wrinkles somewhere else (and would propably drive me crazy).

Again, reading fellow builder’s blogs paid off here. I think it was on Peter C.’s blog, where I read about a trick first when fitting the canopy. Thanks Peter! He mentioned that he firmly had to push the canopy upwards to get a decent fit at the top of the arch – even using some “brute force” methods like using a car jack 😉 .

Remembering this I tried to push the rib upwards at the top by hand and voila –  the bulge was almost gone.

When the skins were riveted, Paul tried to install the luggage door frame the next day.
No matter how he tried to install it, he always had some bulging or wrinkles somewhere.

When I looked into it a day later, it still took me another half hour to find out just by pure luck. The two halves of the door frame spacer were slightly to long at the top (~1mm ; exaggerated for photo sake below).

RF Luggage Door Frame

The oversize was small enough to still allow fitting the frame with clecoes except for one or two rivet holes. But as already said, it would cause bulging or creases – depending on where one started clecoing. Removing just a mm immediatly solved the problem.

I have used Sikaflex to seal the door frame spacer against the skins and the door frame.
Being on my own, working with Sikaflex was a mess here and cleanup took a while.

The luggage door itself is delivered flat and has to be bent to match the curvature of the fuselage. The plans recommandation to ” …hold the door over your thigh or a suitable high density foam roller and press firmly to create a gradual bend. … “ did not work well for me. The door is fairly stiff and it was difficult to get the bend in the right place using this method.

Here’s what worked for me using things I had lying around:

RF Luggage Door Bending

I used different thickness shims to clamp a wood block to the workbench (with  an offset from the workbench edge) thus creating a wedged gap. After shooting the photo I increased the wedge (appr. 10mm gap at front, 2mm at the back end). The edge of the workbench sets the position of a bend, the wedge defines the curvature.

Sliding the door into the gap and evenly  pressing down with flat hands gave very good control about the position and amount of bend. Once I had this fixture it took about 15 minutes and four or five iterations, to get below fit (which I am happy with).

RF Luggage Door Testfit (2)

It’s hard to tell from the photo below, but:
the top corners are slightly overbent and the vertical sides are slightly underbent. After adding a 1-2 mm foam insulation layer for weather proofing to the inside of the door, the door latch will pull the door flush with the skin when closed.

RF Luggage Door Riveted

Overall I am quite happy how the door came out. I still have to install a striker plate for the door latch.

One more thing I did is getting back to the RF luggage floor. That’s been already finished, but I did not like the tie down rings I’ve used very much. When I saw the tie down system, that Jim P. was using (thanks Jim!), I was sold. The system is called L Track or Airline Track Anchor Rail System

On Ebay I was able to source a kit for 55€ consisting of 4x1m rails, 10 tie down fittings, 8 end caps and a glue cartridge. I bought the slim version of the rail (25 mm wide x 10mm high)

RF Baggage Tie Down System

What I like about it is that one can position the tie down rings in the rail anywhere you need them and they are removable. I am installing the rail with 4mm countersunk rivets plus glue. The rail at the back can only be installed after the luggage floor is installed, which I will do after the fuselage join.

RF Baggage Tie Down System (2)

Please note that the line of 6.35mm (sic!) holes in the luggage floor channel is not required, when the backseat mod is applied (those Sling 4 that have the parachute box forward of the baggage bulk head)

RF Luggage Shelf Support Bracket (1)


I have also stiffened up the luggage shelf since I expect someone climbing back there someday to access the “cocktail bar”.

RF Luggage Shelf Support Bracket

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Motivation Photos

Shortly after New Year fellow member of my aero club and RV12 builder friend Wolfgang invited me for a flight in his RV12 (Wolfgang’s RV12), for which he has received the airworthiness certificate last June.

Flying on a cristal clear winter day here in the Austrian Alps is one of the most beautiful flying experience one can imagine, so no way I could refuse this generous offer.

So here are a few motivation pictures:

2020 Flug mit Wolfgang (1)

2020 Flug mit Wolfgang (2)

2020 Flug mit Wolfgang (3)

Thanks Wolfgang for offering this flight!

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Three years after …

Time:   8:00 hrs

Work was actually done Dec. 27th 2019 till Dec. 31th 2019

With aileron and flap aligned, the left wing is mostly finished too. This is almost exactly three years after taking shipment of my kit Nov. 16th 2016

With all the AirVors crew being home for Christmas, I had helping hands to lift the wing off the workbench and put it into storage.

First we had to take the rear fuselage out of storage again to gain storage space for the wing on the rigtht side of the garage. Then we lifted the wing off the workbench, put it leading edge down on soft styro foam and bolted the jig to the far end wall with three wall plugs.

wnl wing Storage (1).jpg

Son Paul had this great idea, which made building a wing cradle or wing dolly unnecessary and also saves some space.

To protect the wing from accidental damage, we again built a blind wall and a small workbench from plywood out of the crates the kit originally was shipped in. Again I painted the blind wall white (with the cheapest wall paint I could find). Though I have bright led ceiling lights the white side walls really improve illumination of the workspace significantly.

We also shortened the legs to lower my two workbenches and make work on the fuselage easier. Lastly son Klaus and his friend helped me to carry the center fuselage upstairs from the basement and carry it to the garage.

Below photos show my rearanged “hangar”. The left wing, vertical stabilizer and horizontal stabilizer are stored behind the blind wall on the left side of the photo, the right wing behind the right.

I could have stored the elevator on top of the right wing, but (with consent of the World’s Best Copilot) putting it on top of the wardrobe in the master bedroom was much easier 😉 (it’s hardly visible).

WRKSHP Wing Storage (3)

WNL Wing Storage (3)

It takes some organizing, but yes, a Sling 4 can be built in single garage – provided it is long enough. This unfortunately (being “only” 5,9m long) is not true in my case.

For the time being I will be able to finish work on the fuselage, but when time comes to hang the engine and fit the cowling, I will have to exchange the window on the back wall for a door. That’s ok for me, as my garage currently does not have a second entrance. I will then let the tailcone protrude out of the door by ~0,5 m.

With the wings now in storage,  open items before paint will be:

    • fill rivet heads
    • install rivnuts for service panels
    • pull pitot and AOA lines

Open items after paint will be:

    • install taxi and landing lights and lens and connect
    • install nav/pos/strobe lightand connect
    • install push rods

Putting the top skins on the rear fuselage and joining the rear and center fuselage will be our next jobs.

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Right Wing Aileron and Flap

Time:  7:00 hrs

Work was actually done Nov. 28th 2019 till Nov. 30th 2019

The method I used on the left wing is somewhat tedious, so I tried another method on the right wing, which was much faster, even though the aileron did not align immediately and also required adjustment (the left aileron did not need any adjustment).

I am therefore describing here the procedure I used this time:

First thing to do is to rivet the flat parts of the aileron. Do not rivet the holes in the rounded areas yet. Then you need to check, how well the aileron aligns at the trailing edge using the prepunched rivet holes.

To lock the twist for the alignment check, I’ve removed the stems from five rivets, pushed the stemless rivets into place and held them in place with Ducktape (or painters tape will do).

As it takes only 1/10th of mm to cause a misalignmnet of 3 or  4 mm at the trailing edge, to my experience just using Ducktape alone does not lock the twist enough, as it leaves some wiggle room for lateral movement.

wnl Align Aileron (1)

Then I mounted the aileron (notice the painter tape, where the five rivets are) …

WNL Align Aileron (1)

… and measured the misalignment using an appropriate drill bit (from my drill bit set).

WNL Align Aileron (3)

I overcompensated here (0,5 mm thicker drill bit) to allow for some springback.

Then I put the aileron on my workbench, removed the five stemless rivets, placed that very drill bit at the oposite end to shim the trailing edge (and thus correct for the misalignment, as measured with this drill bit before).

Weighing down the inbord and outboard end of the trailing edge, I took a look of how much the leading edge rivet line was out of alignment. This was laterally off by only ~0.1 mm, so within tolerances to still use the 3,2 mm rivets.

I therefore reamed out two holes and set these two rivets.  I then mounted the aileron again to check alignment, which was near perfect. So I took it off again, put it on the workbench like before and continued reaming out and setting the other rivets.

WNL Align Aileron (5)

With the leading edge riveted, I installed the aileron again and clamped it at the wingtip end.

Next I prepared the flap in the same fashion: insert five stemless rivets to temporarily lock the twist for measuring the misalignment and flip the flap over to find a shim with correct thickness.

WNL Align Flap (8)

WNL Align Flap (7)

Flip flap back again, clamp at the inbord end and measure how much the alignment is off  (used an appropriate drill bit again).

WNL Align Flap (4)

Flip flap over again, remove the stemless rivets, increase thickness of shim by adding the drillbit, weigh down both ends to set the twist and set one rivet. Then flip back, clamp the flap and check alignment. Continue riveting if alignment is ok, which it was at that point (or drill out rivet and adjust accordingly).

WNL Align Flap (9)

Seems, like I’ve become a little slapdash here and riveted away without checking once more – which I probably should have. The alignment is not as perfect as on the left wing, but it’s not off enough to warant undrilling all rivets again.

In hindsight looking at one of above photos I should have overcompensated also by 1 mm instead of the 0,5 mm, since the flap is much longer than the aileron.

WNL Align Flap (10)

When aligning the aileron and the flap I observed that both, the aileron and the flap have a hump on the bottom side at the trailing edge even though the skins are in line.

wnl aileron Flap Trailing Edge Hump

I was worried about that and contacted technical at TAF. Technical informed me that this will be found on all Sling 2 and 4 and that I need not be concerned about directional stability, drag or stall behaviour as all flight testing had been done with flaps and ailerons like this.

Before putting the wing into storage, I twisted the ldg/taxi light wires and pulled them into the conduit. Remember to add some lenght, as the wire gets shorter by twisting (if memory serves me right, I think I added ~10%).

WNL Twisting Wire


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Left Wing End Rib and Wingtip

Time:  13:30 hrs

Work was actually done Nov. 9th 2019 till Nov. 24th 2019

With the help of my younger son Klaus and some of his friends (thanks guys!) we repositioned the wing front to back and upside down to make work on the wing tip easier.

After removing the wing tip jig I first installed the end rib (WG-RIB-011-R-A-). This required some adjustment of three flanges that were slightly incorrectly bent, but after that the fit was perfect. Like on the right wing I fabricated a bracket for the conduit and pulled the nav/strobe/pos light cable.

WNL Wing Tip (1)

Upon a first testfit of the wing tip the fit and alignment was quite good, but the wingtip was short at the aft end by 8-10 mm. So in the sense of  FAR §21.191 (g) (“….solely for their own education or recreation…”) it was time for my first composite layup.

After sanding off the gelcoat / primer and cleaning with an acetone wipe I built a dam out of stiff plastic.

WNL Wing Tip Epoxy Work (1)

First  I cut thin fiberglass mat which I wetted with epoxy resin using a paint brush. I then layed theese out on a sheet of plastic in a staggered manner to achieve a smooth transition. After adding another ply of plastic I squeeged out the air bubbles and excess resin, removed the plastic again, transfered the layup to the wing tip and weighed it down to get a good bond.

WNL Wing Tip Epoxy Work (3)

WNL Wing Tip Epoxy Work (4)

After two days of curing I turned the wing tip over and made a layup on the other side and let it harden again. Before sanding the layup into a smooth transition I brushed on a layer of resin to get rid of some pinholes.

Making the layups was actually was quite fun and quick. Not so the sanding to achieve a smooth transition, which took two or three hours and another layer of epoxy resin.

The translucent area in above photo shows, who much the wingtip was to short.

Again like on the right wing I added some aluminum reinforcement strips to the inside of the wing tip with Sikaflex 252i (underneath the recessed area, where the tip is rivetted to the wing skin).

WNL Wing Tip (8)

After installing the mounting brackets for nav/position/strobe lights, some sanding of the recess, edge forming and light bending of the wing skin I was quite happy with the fit and rivetted the wing tip in place (again sealing it with a thin layer of Sikaflex).

WNL Wing Tip (5)

Again like on the right wing I could not get perfect alignment along a small area near the leading edge, but a little body work will take care of that, when it comes to painting.

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