Skybolts [7.5 hours]

I installed the top cowling Skybolts.

Since I didn’t use the Skybolt flanges, I needed to drill out both the cowl and the custom built flange holes that I’ve been cleco’ing so far for holding the cowl in position. I used a step drill to take a section of holes out to 15/32″, removed the cowl, and drilled the flange holes further to provide enough clearance for the retainers. A tapered hand reamer is handy for touching up the 15/32″ cowl holes slightly to fit the rings.

I used floating retainers for the lowest position on each side. These allow a bit of up/down movement, not sure they were necessary but I put them in anyway as they were part of the kit.

  • f53a
    f53a
    Installing Skybolt retainers
  • f53b
    f53b
    Installing Skybolt fasteners
  • f53c
    f53c
    Every second fastener done
  • f53d
    f53d
    Most done
  • f53e
    f53e
    Clamping retainer prior to riveting
  • f53g
    f53g
    All Skybolt retainers installed
  • f53h
    f53h
    All Skybolt retainers installed
  • f53f
    f53f
    Skybolt fasteners complete

 

Cowling bump, oil door etc. [16.5 hours]

While waiting for the 3D printed inlet plenum, I went back to complete a few cowling jobs.

The hole that I cut out of the lower cowling to clear the A/C compressor needed to be filled in. To do this, I taped a 1/4″ thick cardboard spacer in front of the A/C compressor, then applied play-doh to build up a shape that looked aerodynamically reasonable. I applied more masking tape over this, and then packing tape to act as a release agent. I then took the bottom cowling off, and applied two layers of fiberglass cloth over the taped off area. Once this cured, I pried it off, trimmed the excess, and scuffed the resulting shape all over.

After removing the tape and packing, I epoxy’d and cleco’d the shape piece in place, turned the cowl over, applied thickened epoxy to fill the gaps around the cutout, and then applied two layers of glass cloth over the entire inner area. Once cured, I applied a layer of micro to the outside, and sanded it back to form the final shape. There were a lot of crater sized pin holes and these will have to be filled, apart from this the A/C compressor bump on the lower cowling is done.

I also did the oil door. I stiffened the door with a piece of 0.063″ Alclad,  bent into shape by hitting it with a rubber mallet. In retrospect, I should have either drilled some lightening holes in it or perhaps 0.032″ would have been OK, the door is quite heavy. I used a pair of Cessna KM610-64 Camloc’s for the catch, building up the button area with thickened epoxy to match the oil door thickness.

I also fitted the Aerosport RV-10 emblem covers to the cowling halves. Just follow the instructions, being careful not to epoxy the cowl halves together anywhere except where you intend to.

  • f52a
    f52a
    Taped 1/4" cardboard spacer in front of A/C compressor
  • f52b
    f52b
    Essential materials
  • f52c
    f52c
    Building up the shape
  • f52d
    f52d
    Taped up with packing tape
  • f52e
    f52e
    Two layers of glass cloth
  • f52f
    f52f
    Trimmed shape in place
  • f52g
    f52g
    Applied two layers of glass cloth to inside cutout
  • f52h
    f52h
    After one round of micro, needs more work
  • f52j
    f52j
    Oil door and stiffener plate
  • f52i
    f52i
    Alclad plate to stiffen oil door
  • f52k
    f52k
    Setting stiffener plate in place
  • f52l
    f52l
    After epoxy set
  • f52m
    f52m
    Drilling oil door hinge
  • f52n
    f52n
    Drilling holes for release catches
  • f52o
    f52o
    Release catch buttons built up with thickened epoxy
  • f52p
    f52p
    Release catches in place, roughly sanded
  • f52q
    f52q
    Release catches in place, roughly sanded
  • f52s
    f52s
    Stainless steel catch plates in place
  • f52r
    f52r
    Almost completed oil door
  • f52t
    f52t
    Glassing in Aerosport templates
  • f52u
    f52u
    After curing, with holes drilled and template punched out
  • f52v
    f52v
    After initial cut
  • f52w
    f52w
    Aerosport emblems in place

 

Compressor stall [105.0 hours]

Over the past month I’ve had to deal with something that was always going to stop the project dead in its tracks – sorting out how to mount an Airflow Systems A/C compressor inside a Showplanes cowl. The main problems are:

  • The A/C compressor doesn’t fit – it smashes into the cowl
  • The A/C compressor occupies almost all of the space normally used for the left hand air inlet plenum

I’ve seen other build efforts that range from leaving out the left hand induction air plenum entirely, to an A/C compressor mount position that occupies a good part of the area of the left hand inlet duct – in turn compromising the cooling air for cylinders #2, 4 and 6.

Recent efforts in Australia, such as this one, use a longer arm on the A/C mount to drop the compressor down low, and make a bump on the bottom cowling to suit. Armed with the latest mount kit from AFS, I proceeded to test mount the A/C compressor, and immediately ran into quite a bit more trouble than I had expected.

Tensioner wheel, bracket

The supplied drive system simply didn’t work. The tensioner (idler) wheel, using the supplied bracket, hits the starter motor. I can see that with the previous “short” arm, the system would  swing up higher and that would free up enough room for the idler wheel to swing past the starter motor. I sent feedback to AFS about this but received no reply. After consulting with the Australian builder referenced above, it turns out the solution was to:

  • Add spacers, for a total of 7/16″ of spacing, to the engine mount. This moved the entire compressor assembly “away” from the starter motor by 7/16″.
  • Machine a replacement idler pulley, which was the same size as the AFS supplied pulley, but without the ridges on each edge – saving around 1.5mm of edge distance.
  • Make a longer arm, even longer than the “long” one AFS supply

I started making up additional spacers (AFS supplies three 0,063″ spacers), using 0.063″ Alclad, but I was only able to add two of these before I ran out of threads on the engine studs used for the A/C mount. I didn’t want to replace these studs, and I didn’t want to switch over to low profile lock nuts – the A/C compressor is quite heavy. So, my spacing limit without resorting to these measures was 0.3″.

The guy who did the VH-BKK installation referenced above graciously had another idler pulley made, and sent it down to me. Armed with this, I still needed to change the position of the idler mount, which means I have to build a new idler mount bracket. I’ve ordered some 6061-T6 plate to do this, in the meantime I drilled a new hole in the existing bracket (rendering it structurally unsound) just to prove I have the hole position correct. This is shown in the following picture, along with the new idler pulley. The clearances everywhere around this are very small, but this is typical and they will be adequate. The idler pulley can now swing up past the starter motor with about 1.5mm of clearance.

I also had to change the Serpentine belt, for this arrangement I fitted a 4PK1130 belt, rather than the 4PK1113 belt provided with the kit – just 17mm longer.

It is necessary to cut out a section on the front of the Showplanes cowl and fiberglass a bump in place to provide adequate clearance for the front of the A/C compressor. There’s nothing hard about this, and one good thing about the Showplanes cowl installation is that the Serpentine belt does not overlap the inlet at all (unlike with the Van’s cowl) – so there is no air leakage associated with treatment to clear the Serpentine belt, and no compromise to cooling air inlet area.

  • new_idler_bracket
    new_idler_bracket
    New location of idler hole (dimensions in mm)
  • f50i
    f50i
    Modified tension pulley position

LHS Inlet Plenum

I had expected to solve this problem with various cuts and balloons etc. to the existing inlet plenum. After looking at the problem, I didn’t even attempt it because (a) this was going to beyond my fiberglass skills, and (b) this is probably beyond anyone’s fiberglass skills. There simply isn’t a clear enough path through the maze to make an adequate shape, and unless I could turn the plane upside down, gravity was always going to work against me.

After some soul searching, which included consideration of leaving the A/C system out, I decided to go ahead and create a 3D model of the entire system as a means to 3D print a replacement inlet plenum. The top end is identical to the Showplanes system, using the lower 1/3 of the circular inlet hole for induction air. The plenum has to step down and change shape, wrap around the bottom of the A/C compressor while clearing the bottom cowling, and then join in to a replacement shroud around the existing Showplanes air filter system. Here are some screenshots which include various early prototypes and the final model, which includes an air takeoff to account for the standard Van’s 2″ scat tube outlet for air to a heat muff.

  • ip1
    ip1
    Early model - rear view, showing heat muff scat tube attach point
  • ip2
    ip2
    Early model - Front view, without compressor
  • ip3
    ip3
    Early model - top view showing heat muff air inlet - slot is for Van's supplied mesh screen
  • ip4
    ip4
    Early model - left side view showing heat muff air inlet - slot is for Van's supplied mesh screen
  • ip5
    ip5
    Early model - Induction air inlet, detail showing clearance for Serpentine belt and A/C compressor lug
  • ip6
    ip6
    Early model - Rear view with A/C compressor model in place
  • ip7
    ip7
    Early model - Top view with A/C compressor model in place
  • ip8
    ip8
    Early model - Right side view with A/C compressor model in place
  • ip9
    ip9
    Early model - Front view with A/C compressor model in place
  • f50g
    f50g
    Inadequate clearance for botton A/C fluid connection (early model)
  • f50h
    f50h
    Prototyping with small sections taped together
  • f50f
    f50f
    Prototyping with small sections taped together
  • ac_hole
    ac_hole
    Location of A/C cutout in Showplanes cowl
  • lhs_inlet_view1
    lhs_inlet_view1
    Final model
  • lhs_inlet_view2
    lhs_inlet_view2
    Final model - Serpentine belt and A/C compressor lug clearance
  • lhs_inlet_view3
    lhs_inlet_view3
    Final model - Rear air takeoff for heater SCAT hose
  • lhs_inlet_view4
    lhs_inlet_view4
    Final model - Air filter shroud, note drain hole provision at bottom
  • lhs_inlet_view5
    lhs_inlet_view5
    Final model - Viewed from bottom front
  • lhs_inlet_view10
    lhs_inlet_view10
    A/C clearance (note: not fully rendered)
  • lhs_inlet_view11
    lhs_inlet_view11
    A/C clearance (note: not fully rendered)
  • lhs_inlet_view12
    lhs_inlet_view12
    Bottom showing fluid connections (note: not fully rendered)
  • lhs_inlet_view13
    lhs_inlet_view13
    View showing clearance for fluid connections (note: not fully rendered)
  • lhs_inlet_view14
    lhs_inlet_view14
    Separate shape under design for attachment around Governor

The inlet plenum as shown can be 3D printed in one piece. It is secured at the bottom by three screws in the LH air filter shroud, using nutplates, same as the standard Showplanes arrangement. Around the compressor, two places mount to the A/C compressor lugs using Buna-N rubber spacers, AN3 bolts and washers. The vibration-damping spacers are rated to 200 degrees F which should be adequate. At the top, a flat Alclad plate which extends down from the existing #2 cylinder mounting flange is secured across the heat muff air inlet area, and at the very front of the inlet, using #6 countersink screws with nutplates on the underside of the inlet plenum. One of the front positions uses a low profile nut, embedded in a hex shaped hole, instead of a nutplate.

For the plenum material, the choices are currently between UV resistant ASA, or the more expensive UV resistant tough resin. Both have high glass transition temperatures and adequate chemical resistance. The plenum does not extend back too close to cylinder #2.

To prototype the system, I printed the plenum in sections on my cheap consumer grade 3D printer, using PLA, and taped the parts together. It takes about 100 hours of printing time to do the complete model, and I’ve made several revisions to refine the design and fix up the various maths mistakes I made along the way. It’s been a laborious process but I’m happy enough with the end result. It takes a high end desktop platform 15 hours to render the model in suitably fine detail, and a bit of repair processing to derive the final STL file. I uploaded the (64MB) STL file to http://www.craftcloud3d.com and was quoted just over U$200 to print the model in Black UV resistant ASA, with 40% infill.

Just today I pulled the trigger and ordered the final article, it’s being printed by a vendor in Sweden, and will be delivered here within two weeks. There is still a chance that the final piece will require some small adjustments. There’s a limit to how far I can go with taped together sections, and how many times I can re-measure everything. The final model is way more complex than I had expected, and has consumed far too much time, I’m tired of fretting about it so it’s a relief to get it off my desk.

In the meantime, there are a few other, much simpler pieces I can design to help with the baffling around the governor etc. I’ll complete these, but mostly I’ll be getting back to actual, real, physical work on the project – there is a lot still to do.

 

 

 

Showplanes cowl – initial fit [48.0 hours]

Very slow progress recently due to various issues, travel and otherwise. I’m also way behind with build log posts, so here’s the first catch-up one.

I started looking at the engine baffles, and fitted some of the rear ones. I can’t trim the baffles until I fit the cowling. I can’t fit the cowling without knowing where the spinner is located, and I can’t figure that out without mounting the propeller. So, out of the farm shed came the propeller crate, and I unpacked and fitted the Hartzell 3-blade composite propeller. Then I was able to bolt the spinner in place, and measure the distance from the flywheel to the spinner.

Just about every builder seems to make up a simulated spinner from a circular piece of wood, as described by VAN’s. I didn’t have any bolts on hand that would suit such a thing, and it seemed to me the real spinner hub was already a pretty good reference for setting up the cowl. So that’s what I did, and it worked out great. There is of course the issue of also having the real composite propeller blades in harm’s way during the cowl fitting process. This is just a workshop discipline issue as far as I’m concerned, and simply means being clean and methodical – so maybe it actually helps.

The Showplanes cowl itself is a work of art – a very high quality moulding and the fit to the fuselage is excellent. The instructions that come from Showplanes, though, are quite poorly written. For the initial cowl fitting the message is to simply follow the VAN’s instructions from the RV-10 build manual. That’s what I did, and if I was going to do it over, I would do the top cowl aft edge final fit differently, as I’ll note below.

Here then is the complete procedure I used to fit the cowl. With this procedure, I found that I only had to get the bottom cowl in position once, i.e. trimming of both the aft and side edges were done in-place, so by the time I removed the bottom cowl for the first time, it was fitted with all four side and rear hinges riveted in place.

  1. Trim the front flange of the upper cowl, so that the two cowl halves fit together with a circular front surface that is a good fit with the spinner. I drilled six #30 holes at the positions recommended in the Showplanes instructions, so that I can easily fit the two halves together in the same position with cleco’s. Don’t drill out for nutplates yet.
  2. Run a piece of 2″ masking tape across the top of the fuselage, with trim lines at 3″ and 4″ back from the edge of the fuselage where the top cowl aft edge will need to be trimmed. After marking these trim lines, I also ran a piece of 3/4″ masking tape around the top of the fuselage, about 1mm back from front edge. This was simply to protect the top skin when I did the final sanding of the top cowl aft edge “in place”.
  3. Put the top cowl in position. I wadded up a towl around the top of the flywheel to get the top cowl in place. For the cowl to spinner gap, I used 3/16″. There’s a debate about spinner gaps on this VAF thread and by the time I was all done with both cowls the gap was more like 7/32″. Tape the cowl in place, and measure back from the 4″ trim line to establish the trim line on the  aft edge of the top cowl.
  4. Take off the top cowl, and cut off the excess, leaving around 2mm aft of the trim line to allow sanding back to the final fit.

Here’s why I would do it differently a second time around. I sanded the top cowl back to the trim line at this point, per the instructions. Then I drilled (from underneath) and cleco’d the aft points of the top cowl into place. The problem with this is, when I put the bottom cowl into place, and cleco’d the two cowl halves together at the front, I found that the position I had the top cowl wasn’t quite right. It only takes a tiny deviation at the front, to change the aft corners a lot. In the end, I decided the cowl had to be right, so I set it up in the correct position around the spinner (allowing for a bit of engine droop over time), adjusted the holes on the top aft edge by drilling (most) of them out to 3/16 (gold cleco’s, and I’m left with a gap of about 1.5 mm across the right rear corner of the top cowl where I’m going to have to do a layup to extend the cowl back to where it needs to be, because as it turns out I trimmed too much off it in the first place. So step 5 below is what I SHOULD have done:

  1. Back when I made the brackets for the Skybolts, I would have drilled the center locating hole #40, instead of #30, just to give more options if I had to finesse locating holes. Now, before putting the top cowl back in place (still with 2mm of excess material on it), I would have made up a few “flaps” of cardboard that are taped to the top fuselage and can be flipped over to show the location of several of these #40 holes across the top cowl aft edge – so that enough holes can be drilled through from the top (I’d do #40, then #30) to secure the top cowl in position as it gets sanded back to fit against the fuselage. Now, with this all prepared, set the top cowl aside, still with ~2mm to sand/trim back on the aft edge. This sanding to size will only be done with the bottom cowl in place – because that’s the way we can guarantee that the top cowl is in the correct position.
  2. Prepare the bottom cowl. I used a laser to establish a center line, by trial and error of the cowl position on a workbench while checking the horizontal measurements at (a) the front, and (b) the rear of the cowl. This center line turned out to be almost exactly in the center of the slot in the lower part of the cowl, but I didn’t want to rely on that – too used to VAN’s mouldings I guess. Removing the lower cowl with a 3 blade propeller can be a problem – I chose one of the common solutions which is to cut the slot for the nose gear leg fairly deep, and I’ll need to install a cover over it as part of the installation procedure. Before cutting the slot, I match drilled four #40 holes into a piece of scrap 0.032″ Alclad, that can be used to retain the aft edge in place after the slot has been cut. Without this retainer plate, after the slot is cut, any attempt to pull up the aft edges of the lower cowl will distort the cowl by pulling the two halves apart around the slot. Now cut the slot. I used a 2″ hole saw, and located the center of the hole 2″ down from the forward most part of the solid surface on this part of the cowl – see the pictures. This places the top part of the hole 1″ from the edge where the honeycomb filler starts. I then used a jigsaw to cut out both sides of the resulting 2″ slot. This created a long slot, wide enough to get around the gear leg without the gear leg fairing – I’ll widen the slot later to fit the fairing. That gives me a bit of wiggle room if the slot is not quite in the center, since this can be checked against the un-faired gear leg once the lower cowl is in position. Tape up the front gear leg with some masking tape, to protect against scratches, and sand any rough edges off each side of the slot.
  3. The untrimmed lower cowl, even with this long slot, is still difficult to get into position with the 3 blade propeller. I used an engine lift to lift the engine up until the nosewheel was off the floor by about an inch, this gives just enough room to get the untrimmed lower cowl into place. By the time I took the lower cowl off again, it is trimmed and all hinges are fitted, and I was able to get it out without the engine lift. This is how I want it – no special tools required to get the cowl off for maintenance. To get the cowl on this first time though, it’s a bit of a trick and definitely takes two people. I positioned the propeller blades as shown in the pictures – the blades are not quite at the 4 and 8 o’clock position – measure from the blades (at around the places where the cowl sides will have to pass) to the floor and get them about even. This was perhaps 5 degrees after the 4  /  8 o’clock position, in terms of normal engine rotation, because of the blade twist. Protect the propeller blades with some tape.
  4. Now, with one person each side, slide the lower cowl up between the blades. It’s a tight fit, but by going a bit sideways and bending the aft corners of the cowl as they pass the blades, you can do it. Another option would be to temporarily remove the bolt holding the gear leg up, but I didn’t need to do this. Once the cowl had cleared the propeller blades, I slid a short table with some padding under the cowl just so it can’t slide back down into the propeller blades.
  5. Crawl underneath, and cleco the retaining plate into place on the bottom aft center of the cowl, behind the gear leg, to hold the two halves together at the constant slot gap of 2″.
  6. See the pictures. I drilled two small holes at the top rear of the lower cowl excess, put a loop of 0.04″ safety wire through each as shown, and then used a light 1″ tie down strap run across the top of the fuselage to hold the lower cowl in place. Don’t tighten it down yet, and when you do, don’t tighten it down much at all – it’s just to hold the aft of the lower cowl in place. You could also use rope, if you’re good at knots.
  7. Lower the engine lift, so the nose wheel is back on the ground, and remote it, we don’t need it any more.
  8. Place the top cowl back on, and insert the six #30 cleco’s that hold the front of the two cowl halves together near the spinner. I had placed a folded up towel around the top of the flywheel so the front cowl assembly sits in around about the correct place.
  9. Make up a pair of metal brackets as shown in the pictures, I made mine in an “L” shape out of scrap Alclad, drilled a #40 hole in the end that will retain the bottom cowl, and carefully drilled two holes (By memory they were 75mm apart) to match the existing holes in the spinner backplate. See the pictures. Screw these brackets, one on the left and one on the right, as shown in the pictures. The bracket part that touches the lower cowl will bend against the lower cowl, but do not drill the #40 holes into the lower cowl yet.
  10. Now, get the front of the cowl in the correct position with respect to the spinner – with the spinner gap you want, even all the way around, centered left/right, and with top/bottom alignment according to what your selected guess is for the amount the engine will drop over time. Most people seem to use about 1/8″ for this guess. Spend plenty of time with this. Also check the alignment of the entire cowl. I didn’t use plumb bobs like the VAN’s instructions, I previously used a self leveling laser to check the left/right alignment of the door sills at the same point on each (rolling one main wheel up onto a piece of 1/8″ thick wood got this alignment right), and then set up the same laser to check the left/right alignment of the cowl intake holes. After making a dozen or so adjustments, most of which simply undid prior adjustments, I tightened up the bottom cowl rear strap (just enough to retain it in place), and carefully drilled #40 through the two spinner brackets and installed cleco’s. This secured the lower cowl into place, and the front of the upper cowl as well.
  11. Never bump the prop from this point on – you want to keep the alignment set up by the two cleco’s into the lower cowl.
  12. Now, using the flip-over cards previously taped to the top of the fuselage, drill #40 through the top center hole of the top cowl, and a hole about 12″ each side of the top. These three cleco’s set the position of the top cowl, although at this point the rear edge of the top cowl is not yet trimmed. Drill these holes out to #30 since the alignment may have been out slightly from the flip cards.
  13. Remove the three cleco’s. Check the top cowl aft edge and convince yourself that the trim line is still correct. Fix it up if it is not. Now, remove the top cowl by removing the six front #30 cleco’s, put it on a bench, and begin sanding down to the trim line across the center section of the cowl.
  14. Place the top cowl back in position, cleco the front, check the aft edge, remove the cowl and continue sanding down to the trim line. I probably did this about 8 times to get the top cowl trimmed. The best tool to use for this is a long perma-grit sanding block. I always hand sand edges that I want to be accurate. Leave the machine sanders in the cupboard. Work from the center of the top cowl, out to each side. As you do so, you might want to drill more cleco holes, if you had done more flip cards. You can’t really trim the side corners of the top cowl unless it is properly secured in place across the top. I don’t know how you do this if you use the Skybolt mounting plates that already have the large hole drilled in it. Tape the cowl in place I guess. I’m glad I made my own brackets.
  15. Once you’re happy with the aft edge of the top cowl, you could release the lower cowl, including the two #40 cleco’s at the front, let it slip down out of the way, and then using a long flexible 90 degree drill extension and the top cowl held in place with the holes already drilled, get your arm up under the aft end of the top cowl and drill/cleco the remaining top cowl aft edge holes from the inside out. This only works if you make your own brackets with #40 (or #30) holes. The top cowl aft edge is now done, and since it was done with the lower cowl in place, it will be right – unlike mine that I need to fill a bit.
  16. Remove the top cowl and set it aside. Move the bottom cowl back up into position, cleco’d to the spinner brackets at the front and held with the strap over the top at the rear. I used a couple of high lift jacks on each lower aft edge to take up any gap at these points.

Enough talk for now, here are a first set of pictures:

  • f48a
    f48a
    Time to get the propeller out!
  • f48zh
    f48zh
    Propeller on, ready for cowl fitting
  • f48b
    f48b
    After trimming cowl to suit 15" spinner
  • f48c
    f48c
    Top cowl after initial trim of aft edge
  • f48d
    f48d
    Material removed from aft edge of top cowl
  • f48e
    f48e
    Top cowl after sanding aft edge to match fuse
  • f48f
    f48f
    Match drilling (from underneath) eventual SkyBolt positions
  • f48g
    f48g
    Establishing centre line of bottom cowl
  • f48zi
    f48zi
    Bottom cowl initial cutout, will need a cover
  • f48zj
    f48zj
    Holding plate used to retain shape during fitting. Marks are from gear leg position.
  • f48h
    f48h
    Bottom cowl in place. Note propeller position.
  • f48i
    f48i
    Bracket to retain front of bottom cowl - right side
  • f48j
    f48j
    Bracket to retain front of bottom cowl - left side
  • f48k
    f48k
    Setting up to mark trim line on bottom cowl AND top cowl over-trim error

Continuing now with the bottom cowl:

  1. Mark the aft edge trim lines, per the plans, using lines previously measured back 4″ on the fuselage.
  2. Remove the bottom cowl, and trim to within under 1/8″ of these lines. Using a long straight (course) permagrit sanding block, sand down close to the trim lines on each side.
  3. Fit the bottom cowl, check for alignment, and continue sanding until the cowl fits properly. I did the final sanding to fit “in place”, by just allowing the cowl to drop down a bit, and spread out on the side I was working on. I stopped at the point where the fit was good, I’ll worry about paint gaps closer to painting time.
  4. With the bottom cowl supported in place, and each pre-drilled aft edge hinge pin fitted, match drill the aft edge hinge pins from the inside out using a right angle drill extension. There may be a few that are hard to get to, skip these and do them once the cowl is removed.
  5. Drop the bottom cowl out, match drill any holes that had to be missed, clean out all debris, and refit the cowl putting a #40 cleco in every hole. Using a countersink cage and a 3 flute countersink with #40 pilot, countersink each hole in turn for an ad3 rivet head.
  6. Clean out any final debris, and rivet the hinge on each side. I used a pneumatic squeezer, and actually did this in place with the cowl dropped down and stretched out a bit.
  7. Refit the cowl with the hinge pins on each side, and the two cleco’s on the front (to the temporary spinner brackets).

Now it’s time to do the side hinges:

  1. Refit the top cowl, allowing the sides to overlap on the outside of the bottom cowl. Now mark a trim line on each side, using a long straight edge. I used a 40″ steel ruler, and a laser which I set up so that the trim line was in the same pitch as the door sill on each side – this is the horizontal pitch when the aircraft is in cruise. This is not horizontal on the ground.
  2. Remove the top cowl, cut and sand down to this trim line on each side, using a long straight sanding block. Refit the top cowl.
  3. Mark a trim line on each side of the bottom cowl, using the top cowl as a guide. Cut and sand down close to this line, but not to it. At this time I also marked in the position of the Aerosport cowl hinge pin covers, and left material forward of this hole on the lower cowl as shown in the pictures.
  4. Fit the top cowl, and keep sanding the bottom cowl sides until they fit properly, with a small gap. At some point during this process I also cut out the holes for the Aerosport cowl hinge pin covers, and trimmed the front section each side of the lower cowl so that it was a good fit with the upper cowl, which wraps around the bottom cowl at each front corner.
  5. Prepare the hinges for each side, and drill #42 all rivet holes in the lower hinge halves. I used a drill press set at 4,000 rpm for this step, to allow drilling accurately and without distorting the hinge half. I elected to use AN257-P4 hinge halves on the upper cowl, so that the hinge eyelets on the lower cowl were below the edge of the cowl. This gives a little bit of extra room when sliding the lower cowl out under the three blade propeller. Clamp the AN257-P3 hinge halves onto the lower cowl (with the upper cowl removed), using a spacer block to set the desired depth all the way along the hinge (I used a scrap of 0.063″ Alclad). Using a right angle drill extension, drill #40 from the inside of the hinge through the lower cowl on each side. Clean out debris, reattach the hinges, and countersink the outside of the cowl for -ad3 rivets. Rivet each bottom cowl side hinge in place. Again I used a pneumatic squeezer.
  6. Set the top hinge half in place on one side, and determine the exact height above the split line where the rivet holes will go. Select a position around the midpoint, and using a foam block wedged/taped between the hinge and a cylinder head (without pushing the side of the cowl “out”), sit the upper cowl in place. Mark where this first hole will be in the upper cowl half, using the previous measurements.
  7. Using an air drill (=> high speed), with a #42 bit, drill this hole through the cowl and hinge. Apply very little pressure, take your time and let the drill bit do the work. I used a 0.025″ metal spacer between the cowl halves for this first hole, which allows for a slightly wider gap after a bit of “spring back” occurs. Pop the cowl up, clean away any debris and de-burr this hole in the hinge, reassemble and put a cleco through the cowl and hinge for this first hole.
  8. Mark hole positions along the length of the top cowl, using a piece of masking tape (see pictures). Working forward and aft of this hole, drill through the cowl and hinge #42 and cleco each hole. I stopped using the 0.025″ metal spacer at this point, and simply eye-balled the gap because I didn’t have three hands. Once all holes are drilled, remove the top cowl, clean up the debris and cleco the top hinge half in place. Ream #40 all holes, clean up, and refit the top cowl, hinge and all clecos. Now, remove one cleco at a time, countersink, and reinstall the cleco. Once all holes are countersink, remove the top cowl, clean up, deburr the hinge holes, and rivet the hinge in place.
  9. Refit the top cowl, slide the hinge pin in for the side just completed, and repeat steps 33-35 for the other side.

That’s it for the initial cowl fit. Next step is to fit the two support fairings supplied by Showplanes that fit around the front gear leg, that support the center section of the lower cowl.

  • f48l
    f48l
    Bottom cowl trim line
  • f48m
    f48m
    Trimming bottom cowl
  • f48n
    f48n
    Sanding bottom cowl aft sides to match fuse
  • f48o
    f48o
    Establishing side trim line
  • f48p
    f48p
    Trimming top cowl sides
  • f48q
    f48q
    Trimming top cowl sides
  • f48r
    f48r
    Riveting hinge to aft edge of bottom cowl
  • f48s
    f48s
    Trimming and sanding sides of bottom cowl
  • f48t
    f48t
    The best tool for sanding long straight edges
  • f48u
    f48u
    Side edges trim/sand complete
  • f48v
    f48v
    Front corners of bottom cowl
  • f48w
    f48w
    Fitting hinge to side edge of bottom cowl
  • f48x
    f48x
    Fitting hinge to side edge of bottom cowl
  • f48y
    f48y
    Match drilling first hole, bottom cowl side hinge
  • f48z
    f48z
    Drilling bottom cowl side hinge
  • f48za
    f48za
    Finished drilling bottom cowl side hinge
  • f48zb
    f48zb
    After drilling first hole #42 through top cowl side hinge
  • f48zc
    f48zc
    Riveting right side hinge to top cowl
  • f48zd
    f48zd
    Right hand side cowl hinge complete
  • f48ze
    f48ze
  • f48zf
    f48zf
    Drilling top cowl left side hinge
  • f48zg
    f48zg
    Cowl sides complete
  • f48zh
    f48zh
    Propeller on, ready for cowl fitting