It's been a while since I posted, but progress continues on both wing spars. As I stated previously, I want both main spars complete and ready for ribs before starting to add the other wing structures. The wing takes up a substantial part of the work area, so the left spar when complete will go to storage while I build up the right wing. Once the right wing is skinned, it will swap into storage and I'll build up the left.
All the bucked rivets in the right wing spar are done, so it was time to final hole drill the left wing spar in prep for debur, prime and final reassembly.
This picture was taken around Halloween. The black A5 clecos and copper (orange) A4 clecos reminded me of the season!
Unfortunately, one of the holes in the spar web doubler came through very close to nicking the spar cap. Thankfully it didn't, but this will make it impossible to have enough clearance to drive the rivet with the bucking gun. Solution was to drive the rivet from the other side - not ideal, but perfectly acceptable according to the build standards.
With all the holes now complete, it comes all apart for debur, prime and reassembly.
When I was reviewing the drawings, the question of dissimilar metal corrosion (called a galvanic reaction) came to mind. Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. The electrolyte in this case can simply be environmental humidity. Salt water exposure would be worst case scenario.
The aluminum spar attachments are made of aluminum. The connection points on the struts and front of the cabin frame are 4130 steel. I did some research on the forums and it seems most people just ensure they have good prime/paint on both parts and/or powder coating on the steel parts.
So what does the aerospace industry do? They anodize their parts.
Anodizing in the simplest of terms is a process of increasing the thickness of the natural oxide layer on the surface of a metal part. This thickened oxide layer renders the part non-conductive electrically, thereby preventing galvanic reaction with other metals. The anodized aluminium layer is grown by passing a direct current through an electrolytic solution, with the aluminium part serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode, either lead or aluminum) and oxygen at the surface of the aluminium anode, creating a build-up of aluminium oxide on the part.
This doesn't however change the dimensions of the part as the layer is nano-metres thin.
Sounds complicated, but is actually simple enough to do in the shop. There are literally dozens of YouTube videos online showing different methods for doing this. Many home hobbyists do this when making parts for their car, computer cases, flashlights etc.
I managed to find a good article in KitPlanes magazine which was simple enough I thought I'd give it a go for my strut pick-ups and spar attach brackets. In the end, if it it didn't work, I can always just prime and powder coat where necessary. The over-riding mantra of my build is to learn, so this is something worth trying.
There are several components needed for the home anodizing process, which I'll try and detail here in pictures.
I scored a small aquarium air pump at the thrift store - 2 bucks. A new piece of air tubing for $3 and a air-stone for making bubbles in the electrolyte $5 from the local pet store. The purpose is to agitate the electrolyte, essentially circulating the solution as the process happens.
Distilled water which makes up the majority of the electrolyte solution. A thrift store kettle for $5 - I should have bought a larger one as the parts once complete need to be boiled in distilled water for an hour to seal the anodizing coat (more on this later). Baking soda to neutralize the electrolyte bath acid and also to contain any spills.
The main electrolyte bath tub.
The main rinse/neutrailizing bath - warm distilled water/baking soda solution:
My electolyte bath is a 10% solution of Muriatic acid and distilled water. I followed the measurements closely and all the warnings of adding the acid to the water slowly. Never add water to acid - always acid to water. The thermal reaction is easier to control by adding acid a little at a time - it can be explosive if you do it the other way around! Pay attention closely to the instructions. Muriatic acid is nasty stuff, so wear gloves, goggles and breathing mask. Make sure to use a well ventilated area.
Before the parts can be anodized, all residual oils, markings and natural corrosion must be removed. The easiest was to do this is using household amonia. Again, proper gloves, googles and respirator mask - this stuff is hard on the eyes.
I proped everything up on some bent metal strips to allow full circulation around the parts. The parts sit in the bath for a while.
For the negative plate in the anodzing bath, I used a piece of scrap aluminum sheet that had too many dings/creases in it to be useful for anything else. I cleaned it with lacquer thinner and made sure it was completely dry before bending it to shape inside the acid tub. The more surface you can expose to the bath and parts the better, so it's bent up both sides of the bath.
Checking back on the ammonia bath, the parts are starting to bubble - a good sign that any contaminants on the surface are being lifted away. I don't have any pictures, but once I was satisfied they had soaked long enough (about 30 mins), I used a clean 3M scrub sponge to wipe them down, then a spray bottle of distilled water to rinse them off. A good indication that the part is completely clean is when the water spray refuses to stick to the aluminum and jut flows off. Water beading on the part means contaminants remain. Mine parts just flowed the water freely. It's important to not touch the parts at this point without clean gloves as any natural oils on your skin will contaminate the part again.
As the parts are left to air-dry, I prepared the anodizing tub. I connected the negative lead from the power supply to the aluminum plate in the tub (the cathode).
The process is hard to capture, but the next steps are to hang the parts in the electrolyte acid bath from aluminum wire. The parts need to hang freely, not touching the other parts or the cathode. The positive lead from the power supply is connected to the parts via the hanging wire. The circuit is now complete and the power supply is engergized. The airpump is also turned on. The following pictures are of the anodizing bath well underway. The electrolyte solution is fairly cloudy by this point, making it hard to see the parts.
The voltage and current applied is calculated with an online tool, using the total square area of the parts to be anodized and what thickness you want the anodized layer to be. This gives you the starting voltage and current and how long the circuit needs to run.
As the anodizing takes place and the oxide layer builds up, the current slowly diminishes to almost zero as the parts no longer can conduct the current. A good indicator other than the readings on the power supply is the distinct reduction of bubbles coming off the parts.
Very close to the time to shut off the circuit, the bubbles coming off the parts took a dramatic downturn as expected. I waited for the time to run out and stopped the power to the circuit and the air pump. The parts are carefully lifted out off the acid bath and immediately dunked in the soda bath to neutralize the acid on the parts. With gloves on, sloshing the parts around makes sure the acid is fully removed from the parts. Once satisfied the parts are "neutral" they get boiled in fresh distilled water to seal the oxide layer. It's at this step that some people add dye powders to the boil to colour their parts. Iwas thinking of doing this, but decided colour wasn't import as I was going to prime/paint the parts as well.
Overall the process of anodizing went ok and I learned a lot. In hindsight, I'm not sure it's worth all the effort when prime and paint will suffice. There is also anti-galvanic paint on coatings that they use in marine applications. I'll look into those as well. The complexity of the system and process turned out to be too big a distraction from acutally building. It would have probably been easier to send them out to an anodizing shop, so my foray into anodizing is over, but in the end I'm smarter about it now than I was.
With the left spar primed and reassembled, I finished bucking the last of the rivets in the spars.
The spar pick-ups have been primed at this point on top of the anodizing I did. I need to order some AN bolts to go with them and the strut pickups (should be here next week). They look fantastic!
With the spar now essentially complete, it is time to start lining things up to ensure the spacing of the ribs match the spacing of the slat and flapperon pickup brackets.
I stood the spar up and anchored it to the bench. I started to add the ribs temporarily using small clamps at the spar and masking tape:
With the ribs in place temporarily, I clamped the nose rib slat attach brackets. These are only in the lateral position for now to allow for lateral measurement. They actually sit up higher on the nose ribs when mounted.
I did the same for the flapperon brackets at the tail of the rear ribs. Preliminary measurements show that the slat and flapperon bracket positions are perfectly matched to the slats and flapperons. This allowed me to drill the pilot holes in the spar for the ribs.
Next up for assembly is the rear wing channels - an inboard and outboard.
I previously had these channels bent by a professional shop as we don't have a suitable bender available at this length. The inboard channel has a support angle across the top. I laid out the angle rivet spacing and drilled out to A3:
The inboard and outboard channels are joined by a splice channel at the rear strut pickup point.
I laid out the splice channel, drilled out the holes to A3 as per the plans.
With the holes complete through the splice channel and the rear channel, I laid out the rear strut pick up. The plans aren't completely clear on the placement, but with a little figuring I was able to confirm the placement.
I clamped the strut pickup in place to the splice channel and backdrilled through the splice channel, ensuring accurate line up of the holes for the entire assembly:
The rivet spacing is tight here and one of the holes is actually for an AN3 bolt. Before drilling the holes out to A5, I finished adding the strut pick-up AN6 bolt hole:
Moving the clecos to the underside, clears room to work with the drill to brig the holes up to A5 size. The far right row of rivets is where the tail end of a wing rib attaches through the channel and splice channel. I'm leaving them as A3 until I fit the rib. I'll have to decide if I want to debur, prime and rivet this section first, or wait until final fit up of the wing ribs.
I did the same at the root end of the rear channel. Lay out the rivet pattern, then back drill through the root plate (a .125 plate inside the channel) that supports the rear channel attachment to the cabin frame.
With the placement of the nose and rear ribs confirmed earlier, I could drill the remaining pilot holes in the rear channel for for the rear rib attach points. Until the wing ribs are in place, I'll wait to debur and prime everything before riveting. After this picture was taken, I drilled everything out to A5 size and trimmed the outboard end of the channel to length and taper (more on this later).
Before final layout of the wings, I decided it was probably best to confirm the work table was completely flat, so I cleaned it off completely. It still was very flat and required almost no adjustment. It was weird to see it so empty!
Making sure the spar is completely straight laterally, vertically and no twist is critical. This is accomplished by using the flat table as a reference. The right angle towers are placed at each end across the rear face of the spar and secured to the table. A tight string line between the outer uprights gives a straight line reference for the other uprights. My spar is straight in all dimensions.
The camera shot give the impression this is far from vertical. It is, confirmed by inclinometer - the view is an optical illusion.
With the spar completely vertical and straight, I started to add the rear ribs. I back drilled through the spar web and into the rear rib flange, using an upright bubble level to ensure the rib was square up and down.
Very happy how this is going. The ribs are a perfect match to the spar. Once I have all the rear ribs in place, I'll remove them one at a time and repeat the process for the nose ribs one at a time, back drilling them through the rear of the web.. This will ensure they too are lined up exactly correct.
Lots to go on the wings, but they are starting to come together. I'm back to Monday to Friday schedule at work, so I should be able to get to the shop more regularly, perhaps one or two nights a week and a full day on the weekend. Hopefully my blog will keep up!
Thanks for following along.
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.