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

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