Sunday, February 21, 2016

Autopilot servos, torque tubes, and undercarriage

Autopilot servos and control surface torque tubes were installed as described below. The area around the servos and torque tubes looks to be pretty tight, so I decided that I would install the servos now and hopefully prevent some scraped knuckles down the road.

I am planning for Garmin avionics, so the servos I installed are Garmin GSA-28's.  TAF has optional brackets specific for these servos.  The first snag I ran into was that the rear spar did not have pre-drilled holes for the elevator (pitch) servo bracket.  I mounted the servo on the brackets to get the proper alignment of those components, clecoed one bracket in its correct location to the seat rib, and then aligned the servo so that it was parallel to the seat ribs.  I then used a number of cleco clamps to lock the front bracket in position and removed the servo without disrupting the bracket location. With some careful drilling and clecoing I was able to match-drill all the bracket holes through the rear spar. Then it was easy to rejoin all the components and rivet in place.  The aileron (roll) servo attachment was much simpler - just a matter of getting the bracket holes to align with the servo mount holes.   One point I did notice while mounting the aileron servo was that the position of the aileron control stop is reversed in the Garmin install instructions compared to the visual depiction of the control stop in the fuselage instructions - not sure if other brand servos need the stop to be mounted differently, but do keep this in mind if you are using Garmin servos.

Garmin GSA-28 servos - just over 1% of the total plane cost in this photo!!!

Drilling holes in the rear spar for mounting the bracket of the autopilot pitch servo

Autopilot pitch servo riveted in place

Hidden on the top left is the mounted autopilot roll servo
This is the correct orientation of the aileron control stop bracket for the Garmin servo.
With the servos mounted I transitioned to the torque tube brackets.  The torque tubes are held in position by vesconite bushings and aluminum brackets.  For the front and rear elevator torque tubes and the flap torque tube, I found that there was quite a bit of friction on the tubes that prevented them from being relatively easily rotated.  Since we want the control surface connections to be free of any binding, I carefully trimmed the bracket edges to reduce the pinching of the torque tubes by the bushings.  By selectively excluding bushings one at a time, it was relatively easy to identify which bushing/brackets were binding.  I was careful to remove as little of the bracket as possible since if the bushing was too loose it would just rotate in the bracket, instead of the torque tube rotating in the bushing.  The front elevator torque tube in particular was a real pain to get in and out - it has a narrow gap to get past before being seated properly.  After a lot of  fine-tuning, I was finally satisfied that the torque tubes were rotating freely enough and riveted the brackets in position - a lot of hand-riveting. Note that I found that the bracket/bushings tightened up a bit with rivets compared to their movement with clecos - the rivets pulled the brackets together a little more tightly than did the clecos.

Riveting the elevator control stop tot he torque tube - this step was not described in the
instructions, but it is barely visible in one of the instruction images. Saw this correct positioning
 initially on Peter's site
Test fit of the torque tubes (and components of the center console). Flap tube is on top,
elevator tube on the bottom.

Another view of the clecoed flap and elevator torque tubes

Rear elevator torque tube just prior to riveting
Riveted rear elevator torque tube
About the only other thing I could do on the fuselage while I wait for the replacement parts was to cut out the soundproofing for the forward fuselage floor.  This won't get installed until final assembly, but they are ready to go when that happens.  Note that the soundproofing was not described in the Sling 4 instructions, but it is in the Sling 2 instructions, so I followed the Sling 2 instructions for this aspect.

Soundproofing cut out and ready to install.  It will get affixed to the bottom of the floor skin
that is riveted on top of these channels.
And then it was time for something completely different - the undercarriage.  I have never changed an airplane wheel before, so this was all new territory.  As airplane folks know, in contrast to automotive rims, the wheel rims for airplanes come in two halves that are bolted together on the tire. Dismantling the rims was straight-forward, though if I had taken a photo of the front rim before dis-assembly, the re-assembly would have gone a bit quicker.  I found the hardest part was to get the valve correctly positioned.  I ended up putting the tube around one rim half, with the valve in position, and then setting them both together into the tire (being very careful not to pinch the tube). I made sure to coat the inside of the tire and the outside of the tube with Johnson and Johnson baby powder aviation talc before assembly to help the tube slide and position itself properly in the tire during inflation.  I used hand clamps to squeeze the tire together so that the both halves of the rim could be placed in position and bolted.  The final hurdle for the front tire came with the nut and washer used to lock the valve stem in position.  I simply could not get any threads on the nut to engage the valve stem threads.  After doing a little on-line research, I found someone else that had this problem and they solved it by grinding down the washer.  So - off to the Scotchbrite wheel with the washer - I ended up carefully thinning it down to about half it's original thickness.  That freed up just enough thread on the valve stem to allow the nut to engage the threads - once started it was then easy to tighten it in place.

Front wheel rim is pretty simple.
Completed front wheel - note that the valve stem feeds through the rim to the other side.

The main undercarriage wheels were done in a similar manner, with the added complexity of the brakes. However, having figured out the steps on the front tire it was very straight-forward putting the main wheels together. 

Both main wheel rims before assembly

Using hand clamps to narrow the tire so that  the two rim halves can be bolted together

Wheels need to be mounted on something, so out came the components for the front tire strut and the main undercarriage.  The front tire strut goes together nicely just as shown in the instructions.  The threads for the bolts holding the top bushings on were pretty cruddy, so I ran some spare bolts through the threads a couple times to clean them up before final install.  As the photo below shows, I also have some rust on the strut that I will clean off and repaint before final install - it had been sealed in plastic shipping wrap and this seems to have trapped the moisture in and facilitated the rust formation. Should be easy to fix.  I struggled initially with trying to figure out how to mount the wheel on the main undercarriage. I just could not figure out how to mount the rim and brakes in the same orientation as shown in the paper Sling 4 undercarriage manual I received. After a while I remembered to check the dropbox to see if there were any updates to the instructions.  Sure enough, the orientation of the brakes in the paper manual is opposite that in the updated electronic Sling 4 manual.  Once correctly oriented, the wheel assembly slid on just like that (though admittedly, the cut-out in the undercarriage for one of the brake stems should have clued me into the proper orientation sooner).    

One main wheel mounted to undercarriage. Front is to the left.

Both main wheels on undercarriage and completed front wheel.
Nose wheel strut.  This will be dis-assembled, rust removed, and re-painted.

Although I will not mount the undercarriage for a while, I checked the fit of the 10 mm bolts that are used to bolt the undercarriage to the fuselage.  I have previously drilled these out, but did not have the bolts to check the fit.  Now, with bolts in hand, I found that my holes were just slightly too small for the bolts.  I  turned a 10 mm reamer  (from Amazon) using a hand wrench to take the holes to final size - all bolts fit now.

Using a hand wrench to turn a 10 mm reamer for the undercarriage bolt holes.
Fit of one bolt shown after hole was reamed

Sunday, February 7, 2016

A long overdue update!

At long last I sat down and prepared an update of all the progress over the last year.  Look below for updates categorized as following:
  • Fuel cells
  • Seats
  • Finishing the wings
  • Tailcone
  • Center Fuselage
Note that all the updates do not fit on one page, so be sure the click on the older posts link to see all the updates.

Next on tap is the undercarriage and working with the fiberglass components while I await replacement components to enable completion of the fuselage.



Starting the fuselage - center assembly

Center assembly construction started with verification of the proper torquing of the main spar assembly.  As with the other components, a dry run fitting of the center fuselage was very helpful to help understand the interrelationships of all the components and to assess how much  space was available for riveter access.

The actual center fuselage construction started with riveting and bolting the rear spar assembly together.  Seatbelt brackets were then installed on the the seat ribs and the rear spar and ribs were riveted to the rear center skin.  The main spar was then clecoed to the forward center fuselage skin and the forward and rear assemblies were joined via the the undercarriage channel. The center console, rudder pedals, firewall and side skins were then clecoed together to verify the fit.

Fuselage construction is currently stalled while I await shipment of replacement parts.

Verifying proper torquing of the main spar assembly.
Riveting and bolting the rear spar components.
Riveting the seatbelt brackets on the seat support ribs
Riveting the rear spar assembly and ribs to the rear floor skin.

Clecoing the front and rear spar assemblies via the undercarriage channel.
Center fuselage components clecoed in position.

Riveting rear spar assembly

Clecoing the center fuselage control stops is position. 
Clecoing the front fuselage bottom skin to the main spar.

Riveting the center console seat brackets. Although I an not sure if it was required, after taking
this photo, I drilled out the rivets with tails in the undercarriage channel and installed
them the opposite direction, with the smaller, smoother heads in the undercarriage channel.

Undercarriage assembly was riveted using countersunk rivets.
Clecoing the center console
Clecoing the rudder pedal assembly
Clecoed front console assembly, rudder pedals, and firewall.
Side view of the front fuselage assembly.
Testing the forward fuselage side skin fit.
Front fuselage side skins and firewall test fit.
Lots of rivnuts required for the center console assembly.
Front and rear fuselage assemblies waiting for replacement parts to enable completion.

Starting the fuselage - tailcone

As with the other components, the fuselage build begins with a lot of cleaning, deburring, test-fitting and priming of parts.  Something that became obvious right away is that there are a lot more pieces in the fuselage, and many of them look very similar.  I got in the habit of taking a photo of the the individual components in each labeled bag provided by the factory, so that I would be able to identify the pieces after the labels were removed - this ended up being very helpful when the actual riveting began.
Test fit of the tailcone assembly prior to priming

Test fit of the step brackets.  The test fit identified a few edges that needed to be trimmed
back to allow for proper fit, as well as some holes that needed match-drilling.
All the various channels for the front seat backrest (Bag 38).
Photo helped identify the channels during actual assembly.

Primed fuselage components - a lot of different channels

The fuselage build begins with the rear cone.The Sling 4 fuselage instructions are not quite as mature as those for the wing and empennage, so ensure you look at the more mature Sling 2 fuselage instructions as well - not everything is the same between the Sling 2 and 4, but reviewing the Sling 2 instructions will provide a little extra guidance.  First component riveted is the tailcone assembly.  I riveted the fuselage ribs at the same time using the provided jig.  The longerons were then clecoed to the tailcone bottom skin. Riveting the longerons was one of the areas that the current instructions are wrong.  The instructions for the Sling 4 indicate that all the longerons are to be riveted to the bottom skin, whereas the Sling 2 instructions correctly indicate that the outer longeron should not be riveted at this time since the lower side skin overlaps it for final riveting.  I had to drill out a number of rivets due to this mistake in the instructions.  The instructions also call out for the tie-down ring to be installed on longeron 8 before attaching the rear fuselage assembly - however, I was unable to do it in this order, so I drilled out the longeron 8 rivets and attached the tie-down ring after attaching the rear assembly to longeron 8.

Installing nutplates on tailcone rib 9.
Tailcone ribs 7, 8 and 9 riveted.
Riveting fuselage ribs using the supplied jig.

Riveted fuselage ribs all lined up.

Longerons clecoed to the tailcone bottom skin

Longerons riveted to the tailcone bottom skin.  Note that the outer logeron rivets needed
 to be drilled out as those holes are also used for attaching the side skins

Instructions call for the tie-down to be riveted to longeron 8 before attaching the tailcone
assembly.   However, the assembly does not fit over the riveted longeron 8 - easy
 to fix by drilling out the longeron 8 rivets, attaching longeron 8 to the tailcone
assembly, and then re-riveting the tie-down.
Having sorted out the tailcone floor instructions, the tertiary structure of the tailcone was then built.  Ribs were clecoed to the tailcone, using zip-tied upper longerons to keep the structure stable.   The front and rear lower side skins were then clecoed in position after attaching the rear assembly doubler (not described in instructions, but obvious where it goes - see also the Sling 2 instructions).  The step brackets were then installed.  Holes for the step brackets were not pre-drilled in my longerons, so it took some careful drilling to line up longeron holes to the step brackets.  The side skins were then riveted on.  The support channels for the luggage floor were also riveted at this stage.  The tailcone was then set aside while I await replacement components for the updated parachute box location.

Clecoing on the tailcone ribs

Clecoing on the lower side skins of the tailcone.

Installing the right step bracket.

Front of the tailcone looking towards the rear.

Riveting the tailcone skins.

The luggage floor assembly support channels (below the skin in the photo) were clecoed
and riveted while the assembly was in position in the tailcone to help ensure proper
alignment during final installation.  Note that the luggage floor was not riveted
to the tailcone during this step (that will happen later).