A couple of weeks has passed since my last blog post. Christmas is a very busy time for everyone and I'm no different. Two back to back work road trips right before the holidays, but any chance to be in the shop that came up I went.
With the left (pilot) cabin side fit up, time to replicate the right (passenger) side to match.
Used the same method for the matching the cabin longerons, placing them back to back, prior to back drilling them to the skins.
Before heading home one evening, I decided to see what the cabin width would look like. Very rough estimate, but liking what I see - lots of room for two people sitting side by side!
Next time in the shop, I decided to compare the cabin sides and they are perfectly matched - everything will be straight and equal once I create the cabin floor and attach it to the rear fuselage.
Both cabin sides, initial fit-up complete:
A somewhat more accurate placement on the bench using the landing gear channel as a guide. This picture shows the rear cabin longerons as they bend inwards to match the tapering fuselage in the baggage area:
Upsizing the cabin side longerons, doublers and uprights to A4 in preparation for debur, prime and assembly.
Further investigation was required regarding the forward edge of the lower cabin doubler. Shown at bottom right in this photo, it needed to be trimmed back.... but....
The plans show two different things. Circled in red below the plans show the doubler extending forward to the front edge of the cabin skin. However the view from the bottom on the same plan drawing (circled in green) shows that the corner has a relief cut.
The cabin skin plan drawing shows the relief notch at the corner. That seems to match the green circle above. Time to email Zenith.
While waiting for Zenith to respond to my query, I continued to drill out the cabin parts to the correct size (A4)
I heard back fairly quickly from Zenith and it turns out the drawing in the plans is wrong. The doubler only extends forward as far as the front most upright angles. That leaves the notch free to allow the boot cowl skin to form around that corner later. So with that questioned answered, time to pull everything apart for deburring and prime:
With all holes deburred I put the door sill longerons/angles back in place briefly in order to final sand the cabin skins to match them. I also stripped the protective plastic of the the outside of the skin along the rivet lines so I can prime the rivet lines ahead of placing rivets:
Added the last two cabin side rivet holes, but only up to A3. These will be the pilot holes for joining the cabin sides to the rear fuselage. Still need to bend the rear edge inwards to match too:
Debur, scuff with Scotchbright and clean prior to priming:
Decided to change track a bit while I had room to work on the bench and got started on laying out the cabin floor. The camera distortion doesn't show it well, but the cabin floor is a tapering rectangle, narrow end (furthest from camera) being the front end towards the firewall:
Laid out the location of the rudder pedal bearing channel and started to secure it with A3 clecoes:
Next headscratcher.... I see in my plans that there is a gap above the floor skin between the heel support channel (on the left in pic) and the pedal bearing channel (on the right):
Is there any reason this gap needs to be there? I don't see anything in the plans that says that the gap needs to be there for other stuff and to my way of thinking it will just be a place to gather dirt, dropped pens, travel snacks debris on long trips etc.
What I was thinking of doing is widening the heel support channel to overlap the rear flange of the pedal support bearing with his eliminate the gap, then riveting both flanges together (green stars):
I asked in the forums and in an email to Zenith and there is no reason not to widen the heel support. Turns out the gap is a carry over from the 701 design where every chance to save weight was made. The 750 Super Duty model has already covered this gap in the same way I am suggesting, so no issue to proceed, so I will.
Next up is the lower cabin floor corner longerons. Cut them to match (one for each side) and mounted them. This support the cabin sides, so I'll wait to drill them together when fitting the floor into the cabin:
Also on the bottom of the cabin floor is the centre stiffener whih runs from the firewall back to the gear channel. Additional lateral stiffeners will be added later:
Seat support channel is added on the top side of the cabin skin:
Lots left to do on the cabin floor, but off to a good start, next will be the heel support once I fabricate the new wider one:
Back to the cabin sides. Primed and ready for assembly.
Priming of outside of cabin skins:
Final assembly in progress after priming. I'm waiting to add the long forward and long rear longerons until I confirm where they meet the firewall and cabin:
All of the cabin side A4 rivets are driven with the air hammer, similar to the wing spar caps. Fortunately, some of these A4 hard rivets are also close enough to the edges of the cabin skin that I can use the rivet squeezer to complete the assembly:
Forward upright now fully installed:
The others I could reach witht e squeezer are the tops of the rear uprights, the doubler L's and the door sill angle.
I set aside the cabin sides for now to work on the fuselage bottom assembly. First thing was to cleco forward corner doublers to the bottom skin and the fuselage longerons. Next I clecoed all the diagonals and lateral stiffeners to the fuselage skin:
Installed the riv-nuts that will be the mounting points for the stainless screws to hold the hell-hole access cover in place:
All riv-nuts secure and tested with the screws that will eventually be in place - very happy with how these turned out.
Worked the air rivet gun back and forth equally across the diagonals and lateral stiffeners, leaving the ends free to tuck in the side fuselage skins:
Incredible how rigid the lower fuselage skin is now with the rivets lines across the middle of each bay:
I started to work on the end plate gussets and cross angle, but soon realized the plans don't define at what measured location this gets installed. Spoke with another builder and he noticed the same thing with his kit and we agreed this needs to wait until the controls they support in the hell hole are installed ensuring correct alignment.
Next up, the front and rear horizontal tail frames. I'd previously fitted these to the bottom fuselage, now it is time to start building up the box that becomes the tail support area.
Kit builders have this step much easier. The rear side skins and longerons are drilled already, whereas a scratchbuilder like me needs to think a bit how to place them in the right location relative to the rest of the fuselage.
To that end, it is time to start fitting up the side fuselage skin to the bottom skin.
I decided on starting with the right side, mostly because was closest to the edge of the bench!
I added a couple of upright boards which are screwed to the bench, then lightly clamped the skin one of them through the cabin window. The it was just a matter of sliding the skin fore and aft a bit until the front lower corner matched the front right corner of the bottom skin. With the rear side skin clecoed together, I know the distances are correct (and I confirmed this as well).
I drilled the side skin to the lower longeron along several places. As mentioned before, the fuselage has a gentle curve towards to the tail on all four corners. By matching the side skin taper to the longeron and securing it as I went, the fit up was relatively easy to accomplish. I still need to remove each of the diagonals and uprights on the side skin for priming, I just clecoed them in place for now give the side skin some rigidity for this task:
Here is the left side with the rear side skin temporarily clamped in place. I also clecoed on the the top coner longeron to further stiffen up the side skin for back drilling the rear tail frames:
Front and rear tail frames fit up and drilled to A3, eventually these wil be brought up to A5:
A bit hard to see among all the clecoes, but I added the rear frame diagonal as well, these too will be brought up to A5. I've got a clamp holding the ear corner of the skin to the lower longeron and will drill them later once the left side fuselage skin is in place and I confirm everything is square.
Very pleased so far. All the work to get the fuselage skins laid out, cut and replicated side to side is working very well. I'm also pleased that all the fore-thought on order of operations (what to drill now, what to wait for later) is paying off now in the assembly stage.
Thanks for following along, more to come soon!
I've been meaning to publish this edition of my blog since Monday, which was Halloween, but work commitments have kept me busy and I've also been in the shop getting stuff done!
With the lower fuselage skin almost ready for deburring and priming of the joining surfaces, I needed to get the lower fuselage access cover (the Hell Hole cover) final fit up done. I didn't time this to happen on Halloween, but it seems appropriate :)
Before taking the lower skin/longeron/diagonals apart for debur, I fitted the flap/controls mixer cross support by back drilling from the inside of the skin just forward of the hell hole. This cross member provides the rear support structure for the flight controls as they pass through the centre of the cabin space back to the flapperon mixer tube (more on this once completed) .
Also drilled out and deburred the control tube bearing hole in the centre support channel. This mounts centred on cross member to support the control tubes, but I'll wait until I have more of the rear baggage area complete to determine the correct vertical mounting position for it. Nothing in the plans shows a vertical measurement and I want to have it exactly correct so the controls don't bind. There are also some clearance holes to be made once mounted.
With all the diagonals and longerons removed, I laid the lower skin down flat and upside down on the table. The hell hole cover was placed into final position and squared to the hole, diamond bends facing out from the hole.
The goal here is to make the cover lay as flat as possible to the lower skin and use the tension created by the diamond bends to tighten the cover skin as it rebounds once attached. It isn't much, maybe 5mm of depth in the cover from corners to the peak where the diamond bends cross, but it certainly is an effective design by Zenith.
With the cover in position, I placed a bucket of clecos on the centre, which weighted down the panel flat. Then I drilled the cover down where rivnuts and screws will hold it in place, starting at the corners:
With all the holes drilled and fastened (not all shown in this picture) to the lower fuselage skin the cover maintains tension and stiffness once the weight in the middle was removed - excellent!
With everything match drilled to correct size where I can at this point, it was time to strip off the protective plastic and start deburring all the holes. I generally try to leave the plastic on as long as I can to help protect the finish of the aluminum from getting scratched - it's never perfect, but reduces the amount of scratches I'll need to remove before eventual prime and paint.
Once every hole is deburred (both sides of the skin), every inside surface of the skin that contacts another part gets a scuff with ScotchBrite, then a light spray of primer. Like all the other components I've assembled so far, grey self-etching primer in visible/outside areas, zinc-chromate green on inside surfaces.
The inside of the access hole gets the grey:
The rear inside of the fuselage and lower rear skin gets the green:
Had a couple of minutes down time while the primer cured, so I fabricated some nylon bushing plates that will be required for the flapperon mixer assembly. 40mm square, 1/8" thick.
Used a punch to centre a hole in each which eventually will be drilled out to 3/8". More on these later.
For now the lower fuselage skin gets rolled up and put back into storage so I can complete the diagonals and upper longerons for the side skins.
Laying out the right side fuselage skin on the table I needed to complete the cabin window cut-outs. I've been anxious about doing these as there is always a risk of tearing the thin fuselage skin which could render the entire skin un-useable and needing to be replaced. But like anything in this build so far, no risk = no reward, right?
Clamping the skin down to the table with a sacrificial backing board behind it and I used a bit of cutting oil to assist with the round corners:
The pilot holes I drilled previously are exactly the same on both fuselage side skins, but they are being cut individually. I was going to use the fly-cutter tool to form the corners of the cabin window cutouts, but it is too big to cut 25mm radius holes, forcing me to use a bi-metal hole saw. Much more aggressive cutting, but taking it slow and using the cutting oil worked very well.
Corner holes for the cabin window cutouts complete:
Inside hole cuts can be difficult, particularly in small areas as there is no way for the cut-away aluminum to curl out of the way of the shears as it is cut.
Getting metal shears into the 25mm radius holes effectively was too difficult, so I used a Dremel cut off wheel to first cut a relief line wide inside the window cut out. This freed up the tension in the aluminum and I could trim the outer edges of the windows smooth between the corners:
Next all cuts are deburred and sanded smooth. Eventually I'll flange the edges inwards just a little bit so the edges are tight against the Lexan windows. I'm looking into adding a silicone or rubber gasket as well which will keep out the rain and prevent any rubbing on the Lexan.
It took a couple of hours to complete the cabin window cutouts, but with a bit of patience and careful trimming, they both turned out great!
Window cutouts complete, time to start adding the vertical and diagonal stiffeners:
I've done the same as the bottom fuselage skin, adding additional opposite diagonals:
Once the diagonals are in place, flip the right side fuselage skin over and start fitting the longerons. A yet to be drilled longeron for the top edge and the lower right longeron from the bottom skin in place on the side skin for fit up. The lower one is only clamped in place at this time, it will be match drilled during assembly of each of the finished side skins to the bottom skins.
I still need to trim the forward edges of the fuselage side skins at the front cabin join location. I'll wait until later once I have the uprights and spar carry thorough channels fitted in place (just in front of the windows, better view in another picture further down):
Drilled out the upper right side longeron to A3, then alternatively to A4. A forest of clecos!
With the upper longeron final match drilled, the entire right side fuselage assembly is flipped back over and I can start to fit up some of the upright channels.
The baggage area back panel support channel shown here in rough location. I originally thought I should pilot hole fit this in place for future assembly, but I've decided to wait until the fuselage sides are mated to the fuselage bottom to see how everything fits best. Kit builders don't have these issues, they just cleco things together and right size the holes for rivets.
Test fit of other upright channels seems to confirm things will go together nicely when the time comes. The two channels at the left show where the fuselage side skin will be trimmed to match. The channel in the middle behind the first cabin window is one half of the flapperon control tube cover (more on these later).
Next up, everything comes apart for debur and priming. The repeat everything for the left side fuselage skin. With the left side done, I can start assembly of the fuselage and installation of some of the controls like the flap mixer and bearings. I'll likely be fitting the windows up as well, but the final install of the windows happens much later on in the build.
Very pleased with my progress so far on the fuselage. I got discouraged for a bit figuring out order of operations, but managed to get a process together that seems to work well so the left side should go much faster. Another bite of elephant consumed :)
Thanks for following along, more to come soon!
With the wings in storage and the fuselage skins measured and laid out, I can start working on assembling the fuselage! It's been a long time coming, but so looking forward to this step. Big updates as follows.
First up I finished fabricating the last component parts of the fuselage that I can ahead of time. These baggage back support channels will be needed once the skins are together to start building out the baggage area in the rear area of the cabin.
I've said before how handy the CriCut Maker is for taking CAD drawings and cutting them out for use in the project. Here is the template I used for the top ends of the baggage back channels.
I orginally planned to use the crosshairs in the middle of the circle to mark where to bore the hole in the blanks, but I realized I cut the blanks to length first. No way to expand the hole to the correct size by drilling (step drill only works on full area, not edges).
Cut the circle portion away, then traced the curve on the blank.
Carefully cutting them out with the bandsaw and gently finishing with a round file worked fine. The baggage back channels bent up nice.
To measure out the side skins, I used the same method as the lower and upper fuselage skins. They too have a gentle taper curve from front to tail. I adapted the plans into CAD and added 250mm sections, to be measured out on the skins for better accuracy of the taper.
Another template I cut out after CAD entry is are the cut outs for the rear cabin windows.
Unlike the bottom and top fuselage skins, both side skins need to be mirror images of each other. To accomplish this, I stacked two full sheets of 020 and clamped them to the bench.
Then I proceeded to lay out the balance of the measurements on the side skins. To ensure both side skin sheets stayed aligned, I drilled and clecoed indexing holes at the corners together. These index holes are on part of the aluminum that will later be trimmed off. From this point on any holes drilled for windows, supports or other items will be exactly the same on each sheet.
The plans are somewhat difficult to interpret here with regards to where the windows actually are on the side skins. Great measurements if you are using a CNC machine to cut out the holes - not so much for a scratch builder! It took some time and several cross-checks to be sure but they are correct.
Drilled A3 pilot holes along the measured edges of the skin, through both sheets and 10mm in from the skin edges where the fuselage longerons attach:
Balance of pilot holes drilled and clecoed. My camera really distorts proportion on long lengths like this, the taper from front to back of the side skins is much greater than what is seen here (see the CAD diagram above):
Another challenge when scratch building is being sure of the best order of drilling holes - i.e. what will I need to pilot drill, from which side and what attaches here. You can see some notes on the skin in red reminding me to wait on these holes until later assembly as there are doublers here that need to be back drilled on later assembly steps:
Extended the lines on the window templates to confirm they match and line up with what's drawn on the skins:
Taped the templates down in the right locations, then drilled pilot/index holes through both sheets, at the corners of the windows. These pilot/index holes will eventually be widened out to 25mm radius in the skin - but both skins will be exactly the same, just like everything else.
A trace out of the template edges onto the aluminum sheet confirms window orientation to other cabin area components - perfect!
Like the lower skin, the overall dimensions of the side skins are too big for a 4x12 sheet of 020 aluminum, so it requires a extension on the tail end. This doubler skin extension also bolsters the "box structure" of the fuselage tail that supports the horizontal tail and rudder. These side skin extensions are some of the first pieces I made in this project, several years ago - glad to see them being put to use after taking space on the shelf!
Took a few minutes to finished sizing and sanding the rear wing pickups - eventually these plates will attach at the top front corners of the rear fuselage to be mount points for the wings - will be needed very soon, so wanted them done and available.
Unstacked the two side skin aluminum sheets and put the drilled but unmarked lower sheet aside. With the marked sheet back on the bench, I trimmed the edges to the correct shape using shears:
Hard to see in this picture due to camera proportion distortion, but the top edge of the side skin definately has a gentle curve from front to back:
This angle shows the side skin taper well. This is after I trimmed the bottom edge:
Re-stacked the skins again, re-indexing them using the same pilot holes I drilled as before. Then I traced the outer edges of the first skin (now cut to correct size) onto the second skin, making a perfect copy:
Removing the first side skin and returning it (rolled up) to storage, I replicated the layout lines on the second skin - this was easy as the pilot holes already exist where the support angles will be, then this exact copy was cut out using the traced lines from the first skin, then rolled up and put away for now into storage.
Next I got the top skin back on the bench an cut it out from the sheet. Once done, it too was rolled and put into storage.
Bottom skin, back on the bench for trimming to size. Here the pilot side has been trimmed away:
Again, camera distortion at work. Here is the trimmed to size lower skin looking from the tail to where it will join the cabin. It does show however show the curved taper of the fuselage sides. Very happy how this turned out:
It looks really lopsided in this picture - but dimensions between the edges and the access hole are completely equal and square - weird.
Next up, fitting the internal bracing around the access panel (affectionately called "hell hole"). It is supported on 3 sides by Z shaped channels:
The first Z fits laterally across the fuselage skin just aft of the hole. Then one on each side:
Z channel is called that, but it's a bit lopsided to be a true "Z"
Two more overlapping Z's fit laterally on each side, really stiffening up the lower skin:
Next up, the lateral L stiffeners and diagonal L's in each lower bay:
There are literally tonnes of discussions on various forums and websites about "oil canning" of Zenith fuselages. Oil canning is where the skin surfaces between the lateral stiffeners tends to drum a bit as rough air passes over them during certain aerodynamic situations. Some say it's not a Zenith unless it does this but I don't think it needs to be that way. I remember going for a demo flight in a very early model and couldn't believe the noise in the cabin on slow approaches or steep turns (where the airflow over the fuselage is turbulent or "dirty" as they say). Almost too much to endure.
When the original Zenith 701 came out to build it was a plans only design, built in a garage and to be absolutely the the lightest structure possible. Zenith intended it to be be flown as an ultralight on 65HP Rotax two-strokes - so I understand that less weight was important and made it easiest and cheapest for the average person. I guess the drumming of the skins was considered an acceptable trade off.
As the design evolved into what is now the 750 STOL (like mine), the 750 Cruzer, the 750 Super Duty and the 4 seat 801 which all use larger and heavier engines, the drumming remains. In my opinion, Zenith needs to update their designs in this regard. Current larger engine horsepower choices allow for more overall aircraft weight and by extension the reinforcement of these areas - the weight penalty is extremely small for what it resolves. Less drumming is better on pilot fatigue and more importantly airframe metal fatigue.
So to improve my airplane I'm adding additional diagonals to all fuselage skin bays. None of this additional weight is significant nor does it impede any further components form being installed or functioning - all it does is stiffen up the skins to reduce (or hopefully eliminate) skin drumming. Here are the first two bays in the lower fuselage skin with the additional bracing installed:
Next up, I'll finish adding the extra diagonals where needed, then start to prep the lower skin for the addition of the lower longerons. In the meantime, I'm headed to the Zenair/Midland Huronia Airport open house soon and will pick up a couple of more parts from them for the fuselage I can't make in house and some more stuff from Aircraft Spruce - exciting progress ahead.
Thanks for reading along!
Fuel lines and fittings have arrived. Stainless braided Auto-Flex line and Swivel Seal fittings. Expensive but the peace of mind this provides is worth it. This is one area of the plane I refuse to compromise on. Very impressed with the quality of Earl's Performance products. With these in hand I can start laying out the fuel tank plumbing.
I've been wanting to figure out a way to measure the fuel level in the tank that requires no internal sensor - less holes in the tank means less chance for leaks. There are several stories on the builders forums about tank sensors mounted through the sides of the tank leaking or being terribly inaccurate, hard to service and generally failing.
I wondered if there was a better way, and I originally came up with the following. A very sensitive presssure transducer (the chrome unit with yellow label below) that will measure the "head" pressure of fuel in the tank via a "T" in the fuel line next to the tank. In theory this should work using an Arduino microcontroller to read the sensor and output a value to a gauge. Here is the assembly mocked up on the bench. From right to left - finger strainer (inside the fuel tank), tank fitting (through wall of tank), threaded adapter, threaded "T", coupler and pressure sensor), fuel line fitting elbow.
Testing of the system has proven difficult. I've been unable to get consistent readings from the sensor in a static set-up. The programming of the Arduino works fine, I'm just not happy with the accuracy of the output due to the very low spread between empty tank head pressure and full tank head pressure, which given the depth of the tank is about 1.5 PSI. I'd hoped a programming algorithm within the Arduino to amplify the input values and stabilize the readings would work, but it's become too much of a tail chase to get it right. From there I'd still need to figure out how to smooth the values to account for changes in atmospheric pressure (fuel changes "weight" with changes in altitude) and aircraft attitude - banking towards the sensor increases head pressure, banking away reduces head pressure.
In an effort to focus on the build getting moved forward, I've decided to abandon this concept in favour of a top mounted float sensor. Yes, this means another hole in the tank, but if mounted from the top, the risks of leaks is minimized and the ability to service the sensor if needed is much easier. More on this later.
So removing the pressure transducer from the fuel line leaves just the tank strainer, the tank fitting and the elbow. I'll need to order the fitting that goes between them, but they are inexpensive and easy to obtain.
The tank fitting is a one direction NPT thread. This picture shows the fitting as it looks from the outside of the tank.
These fittings were gifted to me by another builder. They are actually the larger style, so I need to modify them slightly to fit the tank side rib so the strainer sits nearest to the bottom of the tank as possible. Trimmed them using the bandsaw, then cleaned them on up the disc sander.
The smaller diameter of the fitting needs to fit through the tank rib from the inside. I don't have a drill bit this size to make the hole, so I had to improvise.
I confirmed the fitting was trimmed enough by laying in on the outer face of the tank rib:
I drilled a pilot hole equivalent to where the centre of the fuel strainer will be on the tank:
Used a large step bit to create the general shape of the hole almost up to full size in the tank rib:
Laid the fitting centred over the hole on the tank side of the rib and traced the final size/shape:
Clamped the rib to the bench elevated on some blocks, then used the Dremel tool with spiral cutting bit to carefully bring the hole close to size using the trace lines as my guide:
Filed the hole to final size to ensure a tight fit:
Here is the fitting in it's final position looking from the inside of the tank rib.
On the outside of the rib, the fit is really good:
With the finger strainer in place
Bending the tank skins can be tricky. It is paramount to ensure the bends are as tight as possible to the corners of the tank ribs to reduce any gaps that will need to be filled when the tanks are welded.
I wrapped and finger clamped one of the ribs with the seamstress measuring tape (very handy long ruler for any project) to get a general measurement of where the corners of the tank skin will be. This picture is of the second tank taken later in the day (left tank had already been through this process and I forgot to get a picture for the blog):
The throat depth of the wide bender isn't deep enough to get to the middle tank skin bends, so I needed to use the heavy bender. To allow full access we moved it out into the open floor area, rather than roll up the paint booth curtain (Ron is painting currently).
First I laid out the 025 skin and marked the approximate location of the first bend. Like the slat skins, order of bending is important here, working from middle out to ends. I've left the skin a bit long at each end to make sure it doesn't come up short once it is bent to shape:
The heavy bender allows for material to pass through, giving full access to the middle of the sheet:
First bend complete, next was measuring the second bend closely to make the corners tight
Finger clamped the ribs in position to get a good measurement where the next bend would be:
Second bend line laid out (note the little circled "2" I used to remind me of the bend order):
Back on the bench, I discovered the second bend was slightly overbent and the process on the bender had opened up the first bend as well.... I corrected both before moving forward, but swapped the bend order on the second tank which prevented this from happening again.
Left tank skin rear corners corrected and test fit shows good:
Marked out the location of the fuel filler neck:
Front side of the tank skin bent up to meet the top skin and close off the tank:
The front side of the tank skin needs to have a 5mm flange bent forwards to create the seam for welding. I puposely waited to bend this flange as I didn't know where the bend would be until I mocked up the tank:
I tipped the tank forward on its nose and clamped the forward tank skin onto a piece of aluminum angle at the bench edge:
Using a deadblow hammer I caefully bent the 5mm flange forwards to match the top corners of the tank ribs:
Reassemble the tank and use Cleco vise clamps to hold it together. Some final tweaking of the rib flanges squares it all up - very happy with the final shape! The extra material overhang on the top side of the skin will be trimmed back to match the flange.
Marked out the fuel tank drain location which is directly below the fuel line fitting. This is the lowest spot in the tank once it is mounted in the wing. It is from here that fuel is drawn during pre-flight to check for contaminants such as debris or water:
The right wing tank went faster now that I have the benefit of a process from the first. Fuel outlets are on the opposite side of the left tank (the face inwards towards the fuselage:
I drilled and sized the fuel line fitting for the right tank after creating the tank so we could move the bender back out of the way. The process for the fitting was the exact same as the left wing, just the opposite side of the tank:
Picture of the top front of the left wing tank. I've laid out the location of the top mount fuel tank sensor and the fuel filler neck.
Both tanks clamped together, resting on top of the wing and awaiting parts for the fuel filler neck and fuel level sensor. These will be fitted before final weld-up.
Very pleased at how the tank assemblies turned out, hopefully welding and subsequent leak testing goes as well. Getting ready to order the fuel filler necks, fuel caps and the parts needed for the fuel level float sensors.
This doesn't fit under either the engine or avionics category, I guess it is part of the wings? Maybe under other......
Thanks for following along, stay tuned for more.
Continued working on the right wing. Got the upper outboard skin mostly matched drilled to the ribs. I've only pilot drilled the skins at the spar and rear channel as I will need to fit the nose and trailing edge skins first, the right size them as a group.
first thing was laying out the rivet spacing on the upper surface as per the plans:
On of the keys to good rivet spacing is knowing where the rib fluting is. Marking it out ensures two rivets between each flute. I marked the distance of each rivet back from the spar centre-line so I can easily transfer the same measurements to the other ribs which are the exact same:
A3 holes clecoed before right sizing to A4 across the top of the wing chord. This really makes the curve of the wing apparent:
With the ribs confirmed as right size, the rear channel is drilled out to A3, waiting for trailing edge skin:
Next up was the upper inboard skin. This makes up the panel that covers the fuel tank. It's installed much the same way as the outboard skin. I'll wait to drill the rear channel here too:
finger clamps help to keep everything straight for pilot hole A3 clecos. Again, I'll wait to drill these up to A4 when I'm ready to add the upper root skin as there is a root angle to attach at the junction of the two skins that help form the taper to the root:
The main upper skins are now complete:
After cutting the 2nd fuel tank skin, I roughly laid out the tank ends and some other parts I needed on 025 for the fuselage. Minimal waste is the goal:
The next steps are joining up the two halves of the fuel tank form template and the two halves of the tank end aluminum templates:
Confirmed the templates match the measurements of the plans. This is very similar to the templates and forms of the wing ribs:
The form template fits well on the outside of the inner wing rib and this confirms the extended tank will fit in the wing bay as I expect. Kinda cool to see it work :)
With that confirmation, I laid out the aluminum templates on the pre sized 025 section. Then I used the centre punch to mark the relief corners:
I also punched the inboard tank ends where the plans show the out-port of the fuel tank. I've yet to completely decide on how this will look on my system with regards to the fittings, lines etc. But the out-port will be here:
To remind me where I made punch marks, I circled them as I went. Always drill and debure the holes before making relief cuts - so much easier
All four ends laid out for the fuel tanks - one left, one right:
Templates cut out, awaiting final relief holes and corner cuts:
Seeing as I only have four ends to make, I decided some pine boards would be just as easy to use and much cheaper than expensive 3/4 inch plywood pieces.
Stacked two boards and traced the form template on the top one. Screwed the boards together.
Cut the template line out on the bandsaw. Fresh pine getting cut smells real nice :)
Once I sanded the edges to the correct size, I marked the edges for rounding off on the router:
Both sides of the form, edge rounded and beveled on the sander for springback allowance on the aluminum blanks:
The rest went the same as the wing ribs, except the forms needed to be clamped around the periphery as there are no tooling holes to use like the wing ribs. Holes in the fuel tanks are not welcome here for obvious reasons! Next step will be start laying out the bends for the wing tank skins.
Before I move forward on the tank construction, I need to finalize what needs to be built into the tanks, including fittings for the filler neck, the out-port where the fuel will travel to the engine via the fuel line and where the quick drains will be mounted (more on this later).
The other thing I need to decide is how to monitor fuel tank levels - this is the kind of stuff I love to figure out, but also keeps me awake at night. The plans call for a float type fuel level sender similar to what you have in a car fuel tank. Essentially it's a float on an arm connected to a sweep contact that changes electrical resistance or voltage in the fuel sensing circuit, which is fed to a gauge on the dash similar (simply) to this:
The drawing above is simple enough, however there are two flaws for this to work in my airplane. There isn't room between the top of the tank and the upper wing skin for the the sliding contact/arm pivot. Second flaw, related to the first is the plans call for the same float arm system, but mounted in the side of the tank. All I can think of is why would anyone want to cut an unnecessary and large hole in the side of the tank? That's just asking for trouble with leaks and the builders forums are chock full of stories regarding just that.
So, like the trim control and lighting, I'm going to create my own Arduino solution. I've been doing some research on other methods to measure liquid quantities (the level) in a container (the tank) without being invasive (cutting holes).
My challenge is to find a method that can provide accuracy over 190mm of fuel tank thickness (top of tank to bottom) at it's thickest point and not require holes in the sides or bottom of the tank where it can leak fuel.
There is some limited information on the interwebs about using ultrasonic sound waves to measure the distance from the sensor to the fuel, but that requires a large range between full and empty to be accurate and again would require a sensor at the top of the tank, something I'm trying to avoid. The math to make this work and the shape of the tank doesn't make this easy.
I briefly thought I could make something like this I found on Amazon. It uses a float that slides up a column open/closing magnetic reed switches as it rises/falls, but it would still require a hole and mount on the top side of the tank and a bunch of circuitry to complicate things:
I've decided to try something like these. A pressure transducer that measures the weight of the fuel in the tank inline with the fuel out port via pressure. These transducers are fuel proof and output proportional voltage in a linear ratio to the pressure sensed - solid state, no moving parts and maintenance free They come in various pressure ratings and configurations, but most importantly are threaded the same size as my planned fuel fittings.
The outputs from the transducers can be read and interpreted by the Arduino microcontroller and with some simple programming the Arduino can output a signal for a readable gauge in the cockpit (one for each left/right tank).
What I want for gauges is really up to me as they can be displayed on a LED panel by simply programming whatever images I want to use as the display.
I could go with something simple such as the traditional automotive gauge on the right, but I kind of like the sweep/ribbon style on the left. The numbers in this example represent percentage, but could be made to show litres/gallons as well - it's all customizable in the programming.
I started to play with LibreCad to make my custom display. I created the sweep and used Microsoft paint to colour each section of the arc. Each arc represents a reading correlating to what the Arduino is reading, giving me a moving gauge as fuel is consumed.
The LED display uses low resolution bitmaps for display, but they can be in colour. I plan on green for anything more than 1/3rd full, yellow between 1/4 and 1/3rd orange then flashing red for anything less than 1/4 to draw attention to it. I might even have the programming sound an aural alarm as well.
A simple animated GIF shows what a declining tank would show (with an added funny at the end):
Progress is leading to more thinking and I love it. It's the true core of what this adventure is about.
Next up, flipping the wing over and fitting the bottom skins and mocking up the fuel tanks for welding and fittings.
Thanks for following along.
Busy couple of weeks since the last blog update, but lots to share.
I continue to assemble the wing spars and gather the remaining materials and make parts for the wings.
With the spar webs cut, it's time to layout the lightening hole locations along the web, and cut the spar cap angles. These form the top and bottom of the spar. It starts with a centre line along the length:
Measuring outboard from the root edge, I made a hatch mark for each of the lightening hole locations:
With the locations laid out, I stacked one spar web on top of the other, secured them with clamps and drilled pilot holes through both - this means all lightening holes in each spar are in exactly the same location.
Next up was cutting the bottom and top spar cap angles using the chop saw. I left them a couple of mm long to allow for filing and sanding the ends smooth as the chop saw cuts fairly rough..
Here are the first pair, roughly laid out on the right spar web. You can see in this picture I've marked up each of the webs with a Sharpie so that I keep everything straight as to which way is up/down/fore/aft and a rough idea of the lightening holes. This is important as I want to use the factory edge on each of the spar webs on the bottom edge of the spar and as my reference for measuring the height of each assembly.
With the lower spar cap lined up with the factory edge and clamped in place, I laid out the rivet lines on the spar cap angles. These holes will eventually be filled by A5 solid rivets. I measured and double/triple checked the layout to ensure everything matches the plans. It's easy to be off a couple of millimetres at the beginning that translates to being off several millimetres at the other. The rivet pitches also vary a bit near the middle of the spar too where the spar web doubler and strut pick-ups are located so those have to be carefully considered too.
Drilling all the A3 pilot holes in the spar caps left a LOT of swarf!
At the bottom of the spar at the root I only drilled one pilot hole to begin the process of lining up the spar cap angle. There are several holes and bolts needed here in the spar cap angle, but I have more components to add including the spar root doubler and the spar root pickup. It will be easier to back drill from the opposite side - pilot holes for spar root pickup will be laid out and drilled on the drill press for accuracy and ease.
To start the process of matching up the lower spar cap to the web, I used a straight steel block. The web sits on a board to back up the drill bit, tight against the block and under the spar cap angle. The spar cap angle is exactly even with the end of the web, forming a perfect corner. Drill through the pilot hole to A3 size - this hole will eventually drilled out and filled with an AN bolt.
I secured the inboard end of the lower spar cap with a cleco, then used the same steel block to line up the web and lower spar cap again moving outboard. A clamp kept everything straight as I drilled the next holes:
Every tenth pilot hole was drilled though the cap and web. A long piece of HSS square tube confirms everything is remaining straight as I go:
With the spar cab and web confirmed as straight and true, I finished drilling the rest of the holes between, checking for straightness each time:
I left the section un-drilled between each end of the spar web doubler location (shown as red angled lines). I'll wait to confirm fit of the doubler and the front strut pick up angle once they are made and fitted. I may back drill these like the spar root depending on how the fit up goes.
With the lower cap in place, i started to layout where the top spar cap will be on the web and the associated rivet lines. yes those are my red Crocs.... don't judge.
The upper spar cap is initially cut long enough to overhang the web where it tapers. This will be trimmed off later to match the doubler which gets added here at the root (more on that later). The rivet layout at this corner is non standard, so for my first hole, I chose the first standard rivet spaced on along the cap. I used a ruler underneath everything to make the spar height exactly 209mm as per the plans and secured it:
Pro Tip: Be careful your pilot hole isn't over top the ruler when you drill through the web! Better that than a finger I suppose!
With the spacing between spar caps confirmed and triple checked, I used a carefully cut wooden spacer to make each of the subsequent holes along the upper spar cap exactly parallel to the bottom one. I started with a wooden block close to the length needed to fit between the caps, squared the ends on the band saw, then slowly sanded each end until it fit snug but perfectly between the two.
I copied the process all the way along, doing every tenth rivet and double checking the spar height each time. The caps are perfectly parallel and the spar height is bang on 209 mm. I finished of the rest of the holes to A3, skipping over the section where the spar web doubler will be. All the holes, top and bottom are A3, eventually will be up sized to A5 for solid rivets. The whole spar assembly as it sits now is already very strong.
Flipping the whole assembly over, I checked the rivet lines and confirmed the spar height as correct. I also started to formulate a plan for the spar root assemblies, spar web doublers and how to trim the upper cap angle taper effectively.
Next up is the spar tips. Made from 025, I bent these a while back when I was working on some 025 sheet work. They too have lightening holes, which I laid out and completed with the fly-cutter on the drill press.
Both tips with lightening holes cut and ready to be flanged. These holes are exactly the same diameter as the ones that will be in the spar web, so I marked the cutter with a flag note stating it was already set. Once I get the spar lightening holes cut, I'll flange them at the same time as these.
To ensure the spar tips are perfectly square and parallel to the spar, I flipped the spar back over and clamped a spare piece of angle to the bottom spar cap angle, measured exactly where the tip should overlap the spar end and marked it for pilot holes.
The red line on the left is the rivet line for the spar tip where it attaches to the spar web. The red line on the right is the rivet line station for the outer wing and nose ribs. It has a different rivet spacing, so I'm leaving that alone until the ribs are ready for installation. This will allow a small adjustment to compensate for any variance on the pickups in the slats, which will be installed on the wings later.
Four A3 holes evenly spaced between the spar caps. These will eventually be A5 pulled rivet holes.
Flip the spar back over. Layout the rivet holes in the ends of the spar caps as per the plans. Clamp it all together. I found it helpful to extend the whole thing over the end of the bench for this.
Back drill through the spar caps through the spar tip and secure with clecos:
Extremely happy with everything so far. The spar is dead straight, dead on 209mm tall throughout it's length and distance from root to tip is exactly as in the plans. Straight and rigid enough to stand on it's own! I'm waiting to pick up some aluminum sheet and flat stock later this coming week to make the spar pick-ups, the spar web doublers and front upper strut fittings..
I had a couple of hours for the shop one morning, so I decided to start modifying my wing rib templates. I've had these made for many months and now that I'm ready to start forming wing ribs I wanted to re-visit their layout. I'd experienced some issues forming the slat ribs and thought I could address this on the Wing ribs. I marked the location of flute relief on both the left and right side templates. This will eventually make forming the curves on the bottom and top of the ribs easier.
I started cutting the flutes using a small drum sander on the Dremel tool. It worked really well (more on these later).
Back in the shop the next evening, I started to form up the 032 spar root doubler. It was relatively easy to make as I had experience from installing a missing one on the 701 wing repair (click here for that part of my story).
It starts with bending a flange on the outboard end, then trimming the doubler to match the taper of the spar web, leaving enough width to bend a second flange to match the taper.
With the doubler bent correctly, I laid out the rivet lines for the upper perimeter and back drilled through the web out to A3, using the bottom spar cap angle as a guide to keep everything straight. (it's hard to see it here as it is underneath the inboard spar web):
Flip the spar over and lay out two rows of rivet lines, 5 rivets between spaced between the spar caps:
With the spar doubler drilled, clecoed and and confirmed as correctly positioned as in the plans, I removed it again in order to better see where I need to trim the upper spar cap angle. I marked a line on the angle using the web as my guide.
The next part was quite challenging - using the chop saw to make the accurate angle cut on such a long and un-wieldly piece of angle. I managed to get it close enough, but boy the chop saw makes ugly work of the cut:
The black line represents everything actually left to trim back for a perfect match to the spar taper. I used an angle grinder to gently remove more material using the spar doubler as a guide until it was perfect:
As I got close, I switched to a hand file, taking it down until it was perfectly level. Some final sanding to round off the sharp edges and it is complete:
Putting it all back together, I began laying out the rest of the root doubler rivets and drilling them out to A3. The plans here are kind of lacking about the spacing, but I believe I got it close to what is intended. These will be A5 rivets and the spacing I've left between them it well withing tolerances. I've written what I've used for measurements on my plans so it will be the same on the left spar.
I upsized these to A4 with the exception of the 3 at the tip. I'll leave these as A3 until I can align the inboard root rib and nose rib. This assembly will only get stronger with the addition of the root attachment plate.
As per the plans, I added two standard L angles on the back of the spar at the required location. These add more torsional rigidity to the spar assembly as a whole.
First I marked the centre line of where the angle attaches at each location on the spar:
I cut and deburred two pieces of L to 209 mm long, then used the rivet holes in the spar tip as a guide as they are the same layout (4 rivets between the spar caps):
It doesn't show here, but I drilled pilot holes in each of the L pieces, then used the layout line on the web to align the L in each of the spots and drilled it out to A3. They too will become A5 eventually.
Ron had a look at the flutes I cut in my rib forms and suggested I widen and soften the edges a bit. To do this I used a hand file.
The file was very effective but left the flutes a bit rough.
A little hand sanding of each and they cleaned up nicely.
Both the wing and the root rib bottoms taper slightly up from the front bottom corner. In order to lay out the lightening hole and tooling hole locations correctly, I set up one of the forms on the bench and used a scrap of angle and a carpenters square as a straight edge for measuring against. As this is my first go at using these forms, I decided to do the two root ribs first in case I discover procedural issues. Better to change plans now if needed, but I think this will work.
The four vertical lines measured laterally from the square end. The tooling hole locations measured vertically up from the straight edge provided by the angle. I drilled the four holes out to 15/64ths diameter, same as the bolts I will use to clamp the forms together when bending the blanks into ribs. Left to right, the first 3 holes are also the centre of the lightening holes, the fourth is a tooling (bolt only) hole:
Flip the stack over, clamp the forms together straight and use the new holes to back drill though the other half of the forms, ensuring both left and right rib consistency.
With the forms and templates ready, I start to stack them and a blank together. From top to bottom in the picture below - right side form, left side form and wing root rib blanks. The blanks don't have holes yet and the stack is now pointing in the opposite direction (left to right - tooling hole, and 3 lightening hole centres).
Line up the root rib blank on one side of the form......
.... followed by the other form, lined up directly over top the other. Normally this alignment is accomplished via the bolts and holes. My blanks don't have tooling holes as I wanted the holes to first match on both forms otherwise what's the point?
With everything lined up exactly where it should be, I clamped the sandwich to the table and using the form holes drilled pilot holes through the blank:
This results in perfectly located holes - all four will initially be bolt holes for forming the rib.
With both root rib blanks having their tooling holes complete, I can bolt it all together and put it in the vise for forming:
Gentle and firm blows with the dead blow hammer, bends the flange over the sides of the form. A piece of hardwood dowel rod helps direct the forming blows, massaging the aluminum into the flutes, taking up the extra aluminum from the curve of the form and creating the desired shape across the top and bottom of the rib:
The flutes really help make the rib nice and straight, but it also make is tougher to remove the form. Not bad enough to avoid the flute work! Once out of the form, fluting pliers can be sued for final adjustment. Once flat and out of the form, the 3 forward bolt holes become pilot holes for the fly cutter.
Knowing the procedure works as I intended with the root rib, I repeated the hole alignment procedure for the wing ribs and it turned out perfectly. I'll get to pilot holing the rib banks soon in preparation to form the ribs..
The nose ribs of the 701 and 750 are close enough that I can use Ron's forms. I remembered this while looking for my nose rib forms - that's why I didn't make them for myself! Ron and two other builders were making their nose ribs at the same time, so they bolstered their form with a metal plate close to the nose. This absorbs and backs the small tinsmith hammer blows required to get the thin nose flange rolled over much better than the wood alone.
Ron's forms are already drilled for tooling and lightening hole centre, so the process changes only slightly. This time, I laid a nose rib blank on the drill backing board and centred the form on the blank.
With it clamped in place, I drilled out the holes, using the form as a guide. Then I repeated this step 11 more times for a total of 6 left and 6 right rib blanks.
Ron's forms do not have flutes cut in them, but Ron says they had no issues forming their ribs without flutes. I will need to know where the flutes need to be crimped using fluting pliers, so I marked out 6 left and 6 right for future forming:
A couple of parts I've yet to make are the front upper strut fitting and the spar root fitting (2 of each, one set for each wing). These are substantially thick pieces of aluminum, each a 1/4 inch thick.
One challenge scratch builders have is a good reference of materials needed for a build. Kits come with everything already cut and mostly bent. To make scratch building affordable, one needs to purchase materials in complete sheets then cut them down to size. Buying in bulk saves major bucks.
Thankfully, I received a really good spreadsheet from another scratch builder early on in my build process, which has been invaluable in giving me some idea of the materials needed.
I've been following along pretty closely to the spread sheet of material, but it sometimes has a bit discrepancy compared to the plans. But as we all know, the plans are king.
My spreadsheet states the spar root fitting is 38mm wide by 240mm long - this coincides nicely with the spreadsheet and can be made from 1-1/2 inch x 12 thick aluminum bar stock perfectly (38mm is 1.49606 inches, close enough for me!)
My spreadsheet also states the front upper strut fitting is 40mm wide by 203mm long. This means I'd need bar stock just over 1-1/2 inches wide (1.5748 inches). This sucks because the next width in bar stock is 2 inches, meaning a bunch of wasted material if I have to cut it to width.
I spent too many hours thinking about this and trying to figure out if maybe I'd be better to order some 1/4 inch plate and cut them all out from that, which means more work and chance for error. It was then I looked again at the plans and realized the spreadsheet is wrong. Both are 38 mm wide, meaning I can make all four from a single strip of 1-1/12 wide bar stock. Cool! (I've adjusted my spreadsheet!)
So the 1-1/2 inch bar stock has been ordered along with some 063 to make the spar web doubler and some 0.188 plate for the wing attach brackets on the cabin/fuselage. It pays to shop around, these materials are about a quarter of the total cost ordering it from Aircraft Spruce and 8 hours closer too! I'll pick it up this week from the supplier. I'll probably get them to quote some 020 that I still need for the wings and fuselage skins.
One material that is cheaper to get at ACS are aircraft grade hard rivets. What you see below is way more than I need, but it's good to have extras. $68 something including tax and the time to go get them. I was going that way anyhow to pick up something for Ron, so it saved us both a little on shipping too. The picture below is what I got for the money. The writing on the label is the weight in pounds, not the cost per rivet.
Another consideration I've been pondering is fuel capacity and what that means for my build. Will the standard size fuel tanks be adequate for my expected fuel burn and range? I need to think about this as it affects how and where the fuel tanks get installed in the wing.
I reached out to William Wynne, the Corvair guy and he advises I can expect to flight plan for an average of 6 gallons per hour fuel burn at normal cruise speeds. Looking at the specs from the Zenith website, standard dual wing tanks are 24 US Gallons (2 x 12 gal.) - meaning not including unusable fuel in the lines and any reserve I can expect about a 4 hour range on average.
The extended tank option from Zenith (plans sold separately?!) increases this to a total of 30 US Gallons (2 x 15 gal.) - an increase of about an hour of endurance. The tanks are essentially a little bigger but still fir in the same wing bay.
Some have added a second standard tank in each wing, meaning a total fuel capacity of 48 Gallons!
That sounds great, but there are some serious pros/cons to consider. Extra range and fuel is always a good thing. But how long do I want to a leg to be - i.e. will I need to stretch/pee/eat before 4 hours? It also costs more to make larger or dual tanks, and it complicates the plumbing of the fuel quite substantially. There is also the consideration it may decrease the usefull load (how much can I take in baggage and gear - fuel weighs a lot) and that it costs fuel to haul fuel.
I'm all for the extra range - it never hurts to have more fuel than I need. I'm just not sure it meets my mission and if I eventually plan to put the plane on floats, then what? That has impacts on gross/empty weight on it's own, without considering the extra weight of fuel.
I don't have to decide yet, but will have to soon. Maybe I'll reach out to Jeff Moores in NewFoundland - he has a 705 Cruzer on floats and see what his experiences are. I'm leaning towards the middle option for a slight increase in range without complicating the plumbing.
So.... long blog today. I hope you are enjoying following along. More to come soon including some decisions on fuel tank size.
I worked this week on getting the elevator trim channel installed on the elevator skeleton. I went over the plans several times to visually ensure I was adding the trim to the correct side of the elevator (remember, I'm building it upside down to take advantage of the flat upper surface of the airfoil).
Again, the plans have to be interpreted correctly - in this case the position of the channel is determined relative to the trailing edge of the elevator. But, that can be difficult without the elevator skin installed as the fold of the skin at the trailing edge extends past the tail end of the elevator rear ribs.
To solve this, I made a small narrow strip of 020 aluminum and bent it exactly as the elevator skin would be - it looks rough but it is exactly the right length to simulate the trailing edge:
I placed the strip in position and clamped it with clecos to the spar as if it was a complete skin. Measuring back from this temporary trailing edge, gives me the position of where the elevator trim channel should be but gives me room to to see my work.
Even with the measurement confirmed, I was having a hard time getting the trim channel to fit correctly, until I got a look at the build pictures that come with the plans. Turns out the kit supplied channel has been joggled at the end, allowing it to sit inside the tip and inner elevator rib.
Once I joggled my channel (that sounds bad as I type it), it fit in the ribs where I needed it to. This automotive body panel air tool is very handy for this:
The middle elevator rib gets trimmed down to fit between the spar and the trim channel. It's attached to the trim channel by an appropriately sized L bracket.
With everything squared up thus far, a quick check of the elevator alignment to the horizontal stabilizer shows extremely close to the plans, so my measurements, cuts and bends are very good and accurate. Very, very happy.
Next step is to start cutting the skins. These are fairly large in size and the bench is pretty crowded at the moment, so I rolled out the 020 sheet and traced out the skin on the floor, leaving it a couple of millimetres wide and long - it can always be trimmed back once I have it fitted to the tail skeletons.
First up is the horizontal stab skin. Making it fit correctly is challenging as you have to make holes AND account for the curvature of the skin across the top (bottom) of the airfoil as well. A kit skin would already be trimmed and holes cut for the front and rear stab brackets. As a scratch builder, this isn't a luxury we benefit from, so we have to come up with a workaround. Time for a template!
First step I did was to mark the location of the front brackets on the spar:
In order to transfer these measurements to the skin, I made a template from scrap 020. I cut out the space needed for the rear bracket, keeping in mind the overlap that is required by the real skin past the spar (15mm):
I removed the front brackets and with the template now in place (clamped) where the skin will be including the curve, I drew a line with a straight edge to represent where the back of the spar is - the goal here is to simulate where the final skin will sit in relation to the brackets. It's better to make mistakes on the scrap than on the full skin!
Knowing where the brackets come through, I was able to measure-mark-create the matching holes in the template and gently open the holes a little at a time with a Dremel tool until the brackets can be reattached where they will protrude through the skin:
I'm very happy how this template fits and I'm very confident it will transfer the positions of the holes to the real skin. I'll use the template to cut the slot for the rear stab bracket before final fitting the skin, but for now I placed the skin across the stab skeleton to check the fit - perfect, nice and square with the outer tip ribs and has the correct overhang of the spar.
Now I've reached a decision point. Do I fit the skin on top first and tighten it down with straps across the flat bottom or vice versa? Both have advantages. I can work form the rear bracket at the spar, fit the skin over the front brackets and pull the skin tight across the nose. Or I can start at the spar on the flat side, secure it and draw the skin tight around the nose, over the curve of the top - essentially working in the opposite direction. I've read that drawing skins tight over a curve is easier, but that means fighting with the brackets.
Either way, the skin will need to be pre-bent at the line that defines the tightest curve first - at the nose with a 27mm radius. The plans show a 90 degree bend in the skin prior to wrapping it around, so I need to get that done first.
I flipped the skin over on the bench to mark the centre of the bend line as per the plans (checking very carefully to mark it in the right spot - right and square:
A long piece of factory edge aluminum clamped down with wood blocks makes a great straight edge:
With the scribed line, I slipped the sheet under the stab skeleton to where it overlaps behind the spar 15mm and the bend line coincides where it should.
The next step will be pre-bending the skin, but I'll need to obtain something close to 27mm radius and more than 8.5 feet long so I can clamp it to the bench. I was thinking a piece of 2 inch ABS plumbing pipe might work, but it may not be stiff enough laterally, so maybe a piece of steel pipe. Unfortunately, Ron doesn't have anything that long in house, so I guess I'm going shopping :)
Thanks for reading, more to come!
Been away from the shop a bit. Christmas with the family, shopping, work etc. There are important things in life besides airplanes I suppose :) That doesn't stop me from doing reasearch. Okay, you can call it browsing if you like.
I wanted to share a website I found called experimentalavionics.com
One of the biggest decisions to be made with my build is what avionics I want in my panel. This of course is guided by the three points of mission, cost and simplicity in that order, although they aren't mutually exclusive either. Simplicity generally leads to lower cost. Mission needs vs wants can also directly influence cost up or down. With a bit of work, the following items can be built very inexpensively, with off the shelf parts and instructions found online.
My aircraft mission is simple enough. I don't need to go fast or high (the Zenair 750 isn't pressurized nor is it a speed demon) and I won't be flying IFR (instrument flight rules). I do want good communications (it's actually what I do for a living!) and the ability to navigate outside the normal ATC coverage areas to some of those good fishing/camping spots.
I'm using a converted Corvair automobile engine. Instrumentation for this is simple too.
The idea of building my own EMS (Engine Monitoring System) from open source electronics/software fits both my budget and interests in learning. I have learned enough electronics skills over the years to build it (thanks to Mom and Dad for starting my learning in basic electronics by buying me this when I was a kid). Whether this becomes my primary engine instrumentation or a back up to the traditional analog engine guages will be decided later after I do some more research. It might look something like this:
A nice, easy to read display suitable for the 6 cylinder Corvair engine. The bonus is how much panel space I'd save and the ability to datalog the information for testing mods or diagnosing trends. Alarm annunciators (flashing warning lights or audio) can easily be added for any parameter that goes out of range. Cool!
The other panel items such as primary flight instruments (altimeter, VSI, etc) require more thought. I like traditional instruments for their familiar simplicity. For the same reasons as the EMS, a EFIS (Electronic Flight Information System) has an intriguing draw, but I'll likely have something like this as my backup instruments:
Again, easy to read, simple and space saving. 6 instruments and a clock all in one place.
A couple of cons that I'll need to consider are temperature operating range and failure modes. It gets real cold where I'll be keeping the plane when it's built (unless I win the lottery, then it's heated floor hanger all the way!)
As for failure modes, how comfortable am I putting all indicators in one place, where a single failure may result in losing everything at once.
The website that I linked above also includes preliminary discussions on intercoms for pilot/passenger communication and a WiFi based AHRS (Attitude Heading Reference System) that could link wirelessly to a tablet for navigation. Perhaps someone will adapt the AHRS to be an inexpensive ADS-B out module!
Lots to think about...
Happy New Year everyone :)
The Google search bots are really going to love my posts now!
Remember my fellow Corvair engine builder Jeff Moores of Newfoundland (see previous post "time-to-get-back-at-it")? While at the Zenair Open House we talked over lunch about the struggles I had been having with head studs and Jeff reassured me that my issues were common issues in both his previous builds. He offered to send me some extra head studs that he had lying around his shop to replace the bad ones from my core. They arrived via mail on Tuesday and they are brand new! All for the price of shipping via snail-mail.
The more I continue pursuing a Corvair as my choice of motor, the more I'm starting to realize the value of getting to know other Corvair builders, both for their experience and generosity. This is the kind of group I want to associate with, not some faceless foreign owned engine maker that just wants my money and couldn't care less about my mission to learn. Thanks Jeff!
Next steps, dealing with the 3 stud holes that need to be fixed (see "progress-sort-of") . I've decided on using TimeSerts which are a threaded barrel insert repair that is accepted in the conversion manual. Definitely more expensive than Helicoils (another possible repair method) but I believe worth the piece of mind. Corvair automotve parts warehouse Clark's Corvairs rents the TimeSert installation tool kit and also sells inserts that are the proper length and a blind nut tool for proper torquing of the head studs. I think I'll order those now and get the repairs done soon in preparation for some case machining work I'm planning.
It's been a while since I had anything to post. Between my paid job, a weekend camping with my daughter's Scout Troop, horse shows and Thanksgiving hikes with the family time has flown by the last couple of weeks.
Found a day free in the schedule on Monday so popped into the shop for a bit. Ron and Donna had been away the previous week so Brenda and I were watching over the shop and property. I was pleasantly surprised when I arrived Monday to see that Donna was kinda enough to put my plans set into a binder for me:
Ron and I had a good chat about my build plans. Comparing the 750 plans to his 701 plans we realized that they shared even more DNA than either of us thought. Obviously we knew that the 750 is the evolution of the 701 design but we are both struck just how common the airfoil (wing shape) and internal structures are. It's easy to see where improvements were made over time and how Zenair has evolved in their kit manufacturing processes (introduction of CNC production of kit parts and CAD drawings). Ron's group of builders are scratch building their 701 planes and have made all the forming blocks for the 701 parts. In other words, rather than buy the kit pieces, they are making (read bending) everything themselves from bulk aluminum sheet and other stock. Time consuming? Yes. Cost savings? Huge (the cost of individual kit pieces from Zenair is in the manufacturing, not the actual materials).
Ron seems to think that his 701 forms are almost exactly the same as those needed for the 750, perhaps with a bit of tweaking. Comparing the plans seems to back up this theory too. So the question becomes one of time vs. money. Scratch building takes time but saves money. Enough money of course always saves time. I'm caught somewhere in the middle, but if I can save some money without too much investment of time there is opportunity there. What makes it better is all the work of the making the forms is already done.
I'm going to defer this decision for now and perhaps order a couple of wing ribs from Zenair. I'll compare the 750 kit pieces with Ron's 701 forms and see just how close they are or what modifications need to be made. From there the decision should be easier. If it turns out the forms aren't appropriate, at least I'm a couple of pieces closer to the end....Ha!
So, onto Monday's task - start stripping the paint off the wing skins anywhere new skins will be overlapping. This ensures good strong joints and provides a clean aluminum surface for anti-corrosion primer.
First, apply the chemical stripper. I'm thankful to have a workshop space that isn't in the basement of my house - this stuff is strong!
After letting it sit for a while, a plastic scraper works fantastic to remove the layers of paint:
I didn't take any final pictures yet as I still have some clean-up to do with acetone and Scotchbrite pad. It's not pleasant work, but I learned the work involved if I change my mind about what colour paint I want for my airplane! While I waited on the stripper to work, I also reworked that rear channel I made that had cracks developing at the corners..... always keep busy.
I'm away next week on a work assignment (near the Zenair Canada factory!) so shop time will be limited again. I wonder if I can order those ribs and have them in time to pick up while I'm in the area? Hmm....
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Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.