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!
Some quality time in the shop this weekend.
The spar carry-through channel and doublers are critical components that define symmetrical mounting points for the wings and cabin frame. Kit builders truly have an advantage here. Their components come pre-drilled match up perfectly. I originally decided to buy the main channel from Zenith then fabricate the doublers myself once I figured out the order of operations to match drill the other two parts (the factory part is already drilled).
Ron and I had the plasma cutter out for some other fabrication work (engine stand for his O-200 engine) so we decided to give it a try cutting my doubler blanks from 063 aluminum sheet.
We'll need some more practice using the plasma cutter on aluminum (material feed rate, amps, air-pressure) to get nicer results, but with a bit of clean-up, the first doubler angle turned out acceptable. This doubler runs along the upper rear of the spar channel. Here is a picture of it lying flat on the bench in front of the channel and my smaller test piece to confirm bend angle with:
With the test piece confirmed correct, I used a piece of channel as a straight edge to mark the bending line:
Used the heavy bender to form the doubler angle to the correct 65 degrees closed angle. My shorter test piece was perfect, but it took some extra effort and convincing to bend the doubler as 063 is almost too stiff a material to bend at that length with the bender we have.
Once confirmed as correct when matched in position on the spar channel, I clamped the assembly together and elevated it on the bench to allow the clamps to be where I wanted them. Both arms of the doubler angle must lay flat against the spar channel in order to ensure the match drilling works properly.
For balanced drilling, I started in the middle and worked alternatively left and right to drill the rear facing rivet holes to A3 (eventually will be upsized to A5 here):
With the rear side of the doubler angle matched to the spar channel, I clecoed it in position on the upper skin (remember, this is upside down on the bench right now) to check for fit. The sub-assembly is held in position with the outermost A5 clecos that can be match drilled from above. All good so far:
Rear arm of doubler angle is perfectly flat against the channel and also sits flat against the skin under the doubler:
To finish the doubler match drilling on the underside, I flipped the top skin over, clecoed the top fuselage doublers to the skin. Then I elevated the skin onto square tubing (not shown in picture) and added the spar channel sub-assembly under the skin:
The outermost A5 clecos through the top skin doublers, the skin and into the sub-assembly holds things in position. The I used a deep c-clamp and some scrap wood blocks to hold the "sandwich" together for match drilling:
I also added a long thin strip of scrap 063 on the front facing edge as well - this temporary spacer represents where the front doubler channel would be once I have it in place, keeping the sub assembly square to the skin and the holes in lined up perfectly vertical for match drilling. I worked from centre out to the sides, drilling though the existing A5 skin holes in the skin, then the doubler and into the existing spar channel holes:
Rear facing spar channel doubler angle match drilled perfectly. This orientation of the fuselage skin on the table confirms the top doublers are positioned correctly to the drawn skin edges as well.
Doubler is in perfect position flat against the skin and the spar channel. Eventually this will be drilled out to A5 across the spar channel and A6 rivets at the shoulders of the top fuselage doublers at the wing mounting points.
Very happy with how this first doubler turned out. The front facing doubler which is actually another channel, will need to be purchased from Zenith however. We just don't have the ability to fabricate this in the shop accurately enough to ensure good match drilled holes. I believe we could try to do it, but the effort to do so could lead to unacceptable bend and match hole accuracy - not something I willing to save a few dollars on.
More to come, thanks for following along :)
With both the wings into going into storage, I added the fuel tank drain/test ports. I need to make sure that nothing can get into the tanks while they sit in storage, and the any openings on the wings, like the lightening holes at the root have been covered up with tape and plastic to prevent bird or mouse nests!
First time in a very long time the bench has been completely clear of wing stuff! Time to start laying out the fuselage skins.
Had a couple of minutes one afternoon after work to finish one of the horizontal tail fuselage bulkhead frames need for the fuselage:
The plans are a bit deceiving when looking at the them when scaled down from full size. Close attention shows that the four corners of the fuselage aren't actually directly straight - the fuselage tapers very gently from the back of the cabin to the tail along all four longeron corners.
I've been pondering this for a while and when I was at the Zenair factory in Midland buying my longerons last month, I asked Nicholas Heintz (Chris' son) what would be an acceptable way for a scratchbuilder like me to make the correct tapered curve of the skins (the longerons curve to match the skins and are pre-cut in the kit versions).
Nicholas said the taper is very subtle and I should just connect the measured points out from the centre-line as shown in the plans. As long as the taper is equal on both sides of each of the four fuselage skins, all would be acceptable, but the taper is important as it provides some longitudinal rigidity to the fuselage..
Apparently that's not good enough for me! I wasn't confident that each 500mm point would give me enough points close enough together to draw through to make the gentle curve. So I grabbed the plans and entered them into CAD, like a lot of the templates I've made.
CAD has a great "spline" tool that is like using a draughtsman's French curve ruler to average out the points in the plans to establish a smooth curve. Then I used tools within CAD to section each of the 500mm sections on the plans into 100mm smaller sections, then measured where the points along the curve so I could transfer this onto the aluminum. Enough points along this curve means I can connect them with a real French curve ruler and have my taper drawn correctly on the aluminum sheet.
Here is a snapshot of the fuselage bottom skin, half way through the sectioning process in CAD:
The fuselage skins are all 0.020 thick, with several doublers between the longerons laterally and diagonally to stiffen everything up. The bottom skin is laid out on the table and I used the long straight edge to draw the centreline from which the edge points will be measured:
I measured out each of the sections lines along the centre-line, then used a square aligned with the long straightedge to plot the section lines out the edge of where the skin will be. I soon realized that Nicholas was probably right, curves based on the 500mm sections would probably be enough, so I decided to divide the sections into 250mm sections instead. This is a good compromise and accurate enough for me to make the taper correct using the French curve ruler we have.
This picture shows the lateral lines drawn out from the centre - the circles are where the curve of the edge of the skin will be and the lines from circle to circle are the skin edges which will be trimmed later. From there, I laid out the lower hatch door opening (often called the hell-hole) and some of the other lateral lines for the stiffeners, diagonal "L's" and torque tube bearing channel (supports the control torque tube):
A 10mm line scribed inboard from the edge skin is the rivet line where the skin will be fastened to the longeron once the skin sides are trimmed. This picture is the tail end of the lower fuse skin.
The lower fuse skin is actually too long to fit on a full 12 foot sheet of aluminum, even if I "tilt" the lower skin outline on the sheet, so a lower rear skin is added - it becomes part of the horizontal tail "box" and further strengthens this part of the fuselage to support the tail structure. Aligning it on the centre-line at the proper location ensures the lower skin is the correct length from the cabin to the rudder supports. Here it is temporarily clecoed in place:
Z-channels are added around the sides and rear of the lower hatch. These still need to be trimmed, I just wanted a rough idea where/how they interact with each other and how the rivet spacing should lay out. The square of cardstock is a quick cut out for me to use to visuallize where the flap motor will mount - again for rivet spacing, etc.:
Cutting out the access hatch was fairly easy. I used the fly-cutter to shape out the corners:
Then connected the edges of the circles and cut out the hatch. Some filing and sanding to take care of some rough edges and the access hole is done. I'll make the hatch cover from some of the left over 020 of the top skin.
I finished (for now) the bottom skin. All the rivet lines are laid out and drilled. Before I cut the bottom skin out, I've rolled it back up as as full 12x4 full sheet. Much easier to store.
Returning to CAD, I sketched out the two side skins and the top skin - this time with 250mm sections. It worked so well with the bottom skin, this will be the path to obtain the tapered curves for the other 3 sides of the fuselage (these are snapshots from CAD, not scaled together):
The top skin is somewhat shorter in length than the bottom skin, so I can comfortably "tilt" it on the aluminum sheet to save wasting some of the sheet.
Same layout method as the bottom skin. This skin has a flanged hole near the tail - this is where the elevator control cables will pass through the fuselage into the vertical tail assembly. Cut it out using the fly-cutter. I'll flange it later once the skin out cut out form the sheet so I can clamp the flanging dies easier (can't reach the hole now):
A second hole is cut near the tail end. This will form the round end of the channel as laid out by the lines drawn rearward from the circle to the rear skin edge:
Now onto the part that has been keeping me up at night.
We don't have the ability to bend complex shapes such as the upper top channel shown below. It's a slightly leaning C shaped channel, tapered at the ends that forms part of the wing spar carry-through on the top rear of the cabin. It is made of understandably stout 063 aluminum and both the shape and dimensions are critical to ensuring the wings mate to the fuselage at the right angle and location. So I ordered this from Zenith and picked it up the same day as the longerons.
Problem is, I hadn't thought it through and asked them to provide the channel pre-drilled, as they would in a kit. What I didn't consider is how to transfer the holes to the cover channel and doubler that make up the other sides of the top channel to form the spar carry though box.
I could order those pieces too, but when I spoke with them, they couldn't guarantee they the holes in the other 2 parts would be an exact match to mine as they drill them together at the time of manufacture, and mine was a one-of ordered part.
What to do. Start looking at order of operations and see if I can match drill the holes somehow, while respecting the bends yet to be made in the other two parts.
I can access some of the holes at the end of the top channel where it is cut diagonally to match the cabin uprights, so I placed the top channel in the proper position of on the top skin, drilled/clecoed the accessible holes form above into the skin. With that done and the channel secured to the skin in the right location I then duplicated the remainng holes along the bottom of the channel (actually the top when the skin is in place) onto the top skin because I can use the strap duplicator before the other pieces are added and drill from below the table level:
Unfortunately, as well as that worked, several of the holes on the channel are the bigger A6 river size rather than the standard A5 in the rest of the channel. Here is the channel lying on it's back. The 5 holes grouped close together are the ones I'm taking about. I don't know if an A6 duplicator is available, but we don't have one. These A6 holes are not accessible from inside the channel either.
Time to get creative.
I bent a matching piece of 016 to fit inside the channel and long enough to cover both the A6 holes, two inboard A5 and the A5 hole outboard holes and clamped it in place:
Back drilling through the channel into the 016 (using clecos in the three A5 holes to hold it together straight) and A6 size in the other holes gave me an excellent template to transfer to the skin that will match the channel holes:
Template added to skin, secured with clecos in the existing A5 holes, then A6 holes drilled through the skin:
I added more holes in the template from the forward facing A6 holes so once I have the doubler and channel angle bent, it too can be duplicated without having the access from inside the channel:
Fabricated and drilled the upper baggage area rear panel support angle to the upper skin:
This angle won't change once needed, so I finished all the river holes, deburred it and put it back into storage.
Next up was fitting and predrilling the upper fuselage top doublers. These doublers are almost like the shoulder blades of the wing/fuselage junction which transfers the loads back over the fuselage longerons and upper skin.
First I lined them up on the top skin and used extended lines from the top channel holes and the longeron rivet lines. Once in place, I traced those same river lines onto the doubler and laid out the rivet locations as per the plan. The doubler actually fits onto the outside of the fuselage, but laying it out this way allows me to see the lines before drilling through the doubler and the skin.
Once I was satisfied with the layout, I drilled pilot holes in the doubler on the drill press in the require locations, but not where the top channel lays. Then I back drilled through the doubler in 5 locations to secure it to the skin. The rest I'll drill once the longerons are in place:
Both doublers in place, secured enough for the next steps:
Shifted the doublers to their proper position on the outside of the fuselage skin (remember, I'm working on the inside of the skin and it's actually upside down on the table) and clecoede them in place from above. Then I placed the top channel back in place and secured it from below using the holes duplicated earlier in the skin:
I drilled the forst A5 hole and the two A6 holes I can access at the end of the top channel, then removed the channel and marked the others through the skin and into the top skin doubler:
Removing the top doubler and finishing the holes on the drill press, proves the matching worked (for some reason the picture below seems to show the holes are out of round, but it must be a shadow from the flash as they are actually perfectly matched):
I've also been scrounging a bit online on various marketplaces, classified listings and forums. Scratch building makes you keen to grab deals when they come up and I've been scoring well lately.
Both Ron and I want to make epoxy resin castings for our navigation light lenses and strobes. One of the tools needed to cast clear lenses is a vacuum chamber which is used to de-gas both the silicone molding mixture and the epoxy itself.
I spotted this complete set on Facebook from a seller not to far away. She wanted $125 for everything but I managed to get it for $100 just by asking. It's really brand new, she told me she'd only used it a couple of times but couldn't handle the fumes, so she was looking to sell it to someone who could use it. I looked it up after I bought it and there is easily $400 worth of stiff here, so I was a bit surpised she was willing to sell it for so little.
I brought it to the shop and tried it. Ah. It's not working, that's why.
The vacuum pump appeared to be pulling lots of pressure at the end of the vacuum line, but nothing was jhappening in the pot. Originally I thought maybe the valves were bad or something and I would have to replace them.
When I looked closer however, it looks like someone let the some casting epoxy get into the vacuum port on the pot lid, sealing it completely over! A quick drill and clean out of the port, freed up everything and it is working like new again. Score! We can also use this to de-gas paint for the planes too.
On the Zenith website classifieds, I spotted a suitable airspeed and altitude indicator. Used, but in excellent used condition, clearly taken care of by the previous owner. $200 USD for the pair - Score!
Another Facebook marketplace find were these cable turnbuckles, cable swivels, and throttle cable. Less than $200USD for everything - Score!
Ron continues to look for some parts for his Continental O-200 conversion for the Aeronca Scout rebuild. He bought a Cessna 140 for the engine, but it needed a new intake spider which was cracked.
These are getting very VERY hard to find as they often get damaged during prop strikes and need replacing. New ones are available, but cost north of $800USD!! I spotted one on Facebook marketplace, contacted the seller and managed to get this good used one and a box of other intake parts for $100USD shipped! - Score! I've given it to Ron as thanks for all his help with my build and his kindness in letting me use his shop. That's the kind of karma that I believe we need more of in this world :)
I'm getting close enough now that I need to consider what I need for my Corvair engine install with regards to firewall forward stuff.
I ordered my Corvair/Zenith installation manual as well as the MOP (Maintenance and Operations Procedures) manual.
They arrived in my hands and I sat on the dock the following morning looking through them. This build is always on my mind, even in the quiet times :)
More to come, thanks for reading along!
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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.