Outer Wings (Part 6)

Continued from Outer Wings (Part 5)
Today I finished sanding the wing to shape. My micro trailing edge is nice and solid, and I sanded the overlapping fiberglass on the leading edge down nice and smooth for the next couple of layers. I hand planed the spars down to slightly below the airfoil contour, which was much easier after I took five minutes and sharpened the blade. I didn't take many pictures, but I did document how I'm prepping the sump drain as well as the WAF cut-outs in the foam. The cut-outs each get some one-on-one time with a Dremel, so that the foam is slightly recessed. Next I take a piece of pre-layed-up fiberglass and cut it to fit the cut-out.

Once I've got all four cut to fit, I mix up some micro and apply it to the foam, then set the fiberglass in the cut-outs. I'm not too picky about how it lays, but generally I try to make it as close to the airfoil contour as possible, erring below rather than above.

For the sump drain, I start by attaching a circular piece of duct tape over the hole. Then I use a really thick micro to build up from the aluminum block to the airfoil contour. While the micro is in the "green" stage, I use a Forstner bit to generate a nice circular hole down to the aluminum block where the duct tape is, which gets removed (because it's now all gummy and may not keep the resin out during the big layup.)

I'll replace the duct tape before I do my layup. I'm prepping for that by cutting templates out of plastic 2 mil drop cloth, then cutting the fiberglass from that. If everything goes well, by this time tomorrow the wing will be finished and off the plane.

Outer Wings (Part 5)

Continued from Outer Wings (Part 4)

I started by sanding down the foam I glued on yesterday to fix the low spot. Once that was sanded to contour, I went ahead and sanded the trailing edge to contour as well. I was a little pressed for time, otherwise I would have finished sanding and gotten ready to glass. I did have time to mix up some more micro, though, so I poured/squeegeed/scraped it into place to finish filling in the low spot. After sanding:

And after adding micro, it's nice and smooth and follows the airfoil perfectly.

I also had time to use a wire wheel in a power drill to scrape away the foam within about 3/4" of the trailing edge, and filled that with micro for strength. I did this on the other wing as well, and it's crazy stiff and crazy strong now.

I have to hand plane the spars down a little so they don't stick out, then sand the leading edge and prep it for two more overlapping layers of fiberglass. I also need to cut and attach some prepreg FG strips over the WAFs so I don't lose a ton of vinylester down those cut-outs. After that, I'll be ready to glass the bottom of the wing, then move on to the wing tips.
Continued in Outer Wings (Part 6)

Pitot (Prandtl) Tube

Let me start off by saying, it's probably much easier to just buy a pitot tube. But one of my major motivations for a wide variety of my hobbies is the voice in my head that says, "I can do that, and I can probably do it better*." So in the spirit of not accepting the status quo, I decided to see if I could make my own without buying any new tools.

Let me also explain that a wide variety of homebuilt aircraft builders find inexpensive ways around all sorts of problems, and this is one I've seen a lot of creativity with. From just soft aluminum tubing to VW pushrods to all manner of "I had this laying around the house" DIY, it's been proven that with a little trial and error, you can make almost anything work if you try hard enough. However, due to the speed at which this plane travels, and the amount of trouble I would be in if my airspeed indicator decided to stop working due to a bent/broken pitot tube, I felt it a worthwhile pursuit to build a very sturdy, very likely to be accurate structure. I didn't design anything new, just copied other designs that made sense to me, focusing first on strength and second on aerodynamics. With that out of the way, here's what I did.

I determined that 2024 aluminum was a good material to use, as it's quite strong and still quite light. Also, because it's "just aluminum," I was willing to try turning it on my wood lathe. I've turned wood for about 7 years, which certainly doesn't make me an expert, but I've had the pleasure of playing with a lot of different species. Aluminum is much easier to turn than many of them. It takes much longer to shape, but there's no grain to it, so I had no troubles turning it with even my cheap lathe chisels. I started with a 3/8" rod, cut it to length, trued up the ends in the lathe, then drilled a 3/16" hole through it, and finally turned one end to a point. This would have worked fine, but...

One of the difficulties with an airplane is, you're moving through the air. The difficulty comes in figuring out just how far away the ground is. We know that, as a general rule, air is less dense the higher we are. The altimeter in an airplane counts on that fact to display its altitude. An altimeter measures air density by measuring air pressure, but where do you take that measurement? Inside the airplane isn't a safe bet, as air currents can vary wildly and cause massive pressure shifts depending on airspeed, air vents, and where you put your flight bag. Outside the airplane, the air is moving very rapidly, and if the static air source is tipped slightly into *or* slightly away from the direction of airflow, your reading will be very very wrong. One of the more common methods is to place your static source, or port, on the side of the fuselage, where the port will be perfectly perpendicular to the airflow. If you fly the airplane in a slip or skid, however, your measurement will be off again, so you need another port on the other side of the plane so the measurements can even out. One of the more elegant solutions to this is called a Prandtl tube.

A Prandtl tube combines the functions of a pitot tube and a static port, and does so in a way that, theoretically, is less prone to error under normal conditions. Do other methods work? Just fine, as a matter of fact. However, for this airframe, a number of people have used a number of methods, and it seems that what works for this guy causes major problems for the next guy. A Prandtl tube should work on any airframe, as long as it's placed in a free airstream. I stole a diagram from Wikipedia:

Pitot and static, measured from the same spot. How to accomplish this with minimal effort?

First, I got a 5/8" 2024 aluminum rod and turned the shape. Next, I drilled it down the center with three drill bits: 1/8" bit from the front straight back about an inch, 3/16" bit from the back all the way to the start of the 1/8" hole, and 3/8" from the back about an inch shallower than the 3/16" bit.

Next, I drilled 4 1/16" holes about 2 inches back from the tip, each 90* from each other, so I ended up with a hole in the top, bottom, and each side of the tube. I then took some 3/16" OD aluminum 3003 soft tubing I had laying around (after quite accidentally ordering 25 feet of the wrong size) and floxed a piece all the way down into the 3/16" hole, and then floxed another piece about an inch inside the large hole, then used more flox to seal the large chamber. What I ended up with looks about like this:

The red is the tubing, the green is the flox. Only two of the 4 holes are shown. Once that all cured, I hooked it up to my airspeed indicator and altimeter and took a drive, holding the Prandtl tube out the window. It seemed to work poorly, until i disconnected the altimeter. Turns out the altimeter leaks like a sieve, but the Prandtl tube works like a charm. I was getting very consistent readings, even angling the tube 15 degrees or so from straight on into the wind.

Ok, so it works. Now all I had to do was build a strut to hold it ~4" below the leading edge of the wing. To accomplish that, and because I was feeling picky, I printed off a low drag NACA airfoil (NACA 0021, I believe) and cut out two wood templates, glued them onto a piece of foam, and sanded that foam to shape. Then I glassed the foam, drilled a few holes to attach the Prandtl tube and to allow the two 3/16" tubes to run up the center of the foam, and glassed that all together.

After it had all cured, I put on a couple more layers of micro, building up the trailing edge, fairing in the transition between strut and Prandtl, and then sanded it nice and smooth. It'll stay that way until I'm ready to attach it to the plane. (Obviously the last picture is not sanded smooth. I'll update this with another picture once it's finished and attached.)

*Better, more often than not, means good enough for less money, or better for my specific application. If I really thought everything I did was better, I'd be a businessman (and probably a very, very broke one at that.)

Outer Wings (Part 3)

Continued from Outer Wings (Part 2)

On the left wing, I tried to modify the plans method to make it easier on myself. I cut out foam nose ribs that, rather than stick all the way out to the leading edge, only stuck out far enough to support the foam planks. Unfortunately, they weren't quite right, and it caused a lot of headache and frustration trying to rip out and replace foam. So this time, I did it a little closer to the plans method. Glued foam nose ribs in place, but then sanded them to the airfoil contour before adding the rest of the foam. It worked quite well, and was much easier than trying to work out the geometry of the nose supports changing size, length from the spar, and angle changes for washout.

Foam planks being cut and placed:

Once in place, I began the arduous sanding process yet again.

Honestly, it only took a couple hours, including frequent hydration (read: lost motivation) breaks and several "Is there *any* other way that would be *slightly* better?" pauses. One of those gave me the idea to cut slits along the entire length of my aluminum angle sanding block, and use that edge as a saw. It worked fairly well for large chunk removal, as long as you were careful not to cut too close to the final shape.

Eventually, I did manage to finish sanding the top of the wing (although not perfect - thank goodness for micro!) and got ready for another round of glassing. It's been so hot, I've been getting all of my supplies ready and all of the prep work done in the evening so I can get up early and finish glassing before 9 or 10 AM. Any later and the vinylester just sets up too fast. One interesting thing with vinylester that seems different than any other two-part system I've used - if the vinylester is gelling, and I'm not quite ready for it to set, I can actually add a little extra time by adding a little more vinylester (with hardener added, of course.) Not sure how or why, but it's saved me from a lot of repair work a couple of times.

I didn't get any pictures of the process, but it went like this:
Step one: micro from the front spar back to the trailing edge (skipping the spars)
Step two: lay one layer of 5.8 oz cloth and one layer of 1.5 oz cloth on the micro'd area, wet out completely
Step three: micro from the front spar forward and underneath, two inches past the chord line
Step four: wet out two layers of 5.8 oz cloth on a piece of 2 mil plastic, then lay the cloth in place on the leading edge of the wing. Much, much easier to do one layer at a time, otherwise you risk wrinkles in the bottom layer that are pretty impossible to get out once the resin begins to set.

The result is quite good, with very very few bubbles and only one small wrinkle. If the thunderstorms stay away long enough tomorrow,  I'll pull the plane out of the garage and flip it, then begin gluing foam around the bottom leading and trailing edges. Beginning to get excited about my progress, and I think I've figured out how I'll do my wing tips next.
Continued in Outer Wings (Part 4)

Outer Wings (Part 2)

Continued from Outer Wings (Part 1)
It's been a month, and I've been making progress. I had some foul-ups, which required tearing foam out and replacing it, but nothing too major. The following pictures show me finishing the main portion of the left wing.

Adding micro and foam to raise a low spot:

Glassing the top:

Flip the plane and repeat. Notice the two holes in the bottom - these were cut to address some slight leaking issues with the fuel tank.

I used a hand plane to shave a little bit of the spars off, so they were at or below the airfoil contour. You can see the wires poking out that I ran for wingtip lighting.

Originally I planned to build a tricycle gear, but decided to go with tailwheel instead. As a result, I needed to change the location of my sump drain on this wing. The original drain will remain plugged rather than torn out and re-glassed, because its proximity to the tank wall makes me unsure I could do so without causing more leaks.

One down, one to go. I learned a few things this time that will make the second wing easier... I hope.

Continued in Outer Wings (Part 3)

Fuel Tanks (Part 3)

Continued from Fuel Tanks (Part 2)

I followed the same basic process for the other side, but I sacrificed about 2.5 gallons volume in exchange for a good sump. I used a router to get close, then sanded the bottom of the tank into the shape I needed. The design I drew up will work equally well whether the plane sits on conventional gear or tricycle, because I like to keep my options open.

After sanding to shape, I laid up the fiberglass, allowing the first layer to cure before adding the second and hot coating. I had to make several incisions to get the cloth to lay correctly, as well as add a few patches of cloth in trouble spots. There were still a few voids left, so I used a syringe to fill those with micro. Hopefully the pictures below give you a good idea of the geometry involved, keeping in mind that the entire tank will be tilted 5" up at the outboard end. Therefore, the low area is at the inboard rear end of the tank area.

I added the (previously drilled and tapped) aluminum plate for the drain valve, as well as a thin aluminum plate where fuel from the transfer tank will be directed. The theory is that pressurized fuel could, over time, wear a hole through the fuel tank. Not sure how worried I really need to be about that, but it's an easy fix to a "what if."

After the spars were in place, I lifted the bottom in place and began cutting and fitting my baffles and braces. This time around, I managed to seal the corners without much difficulty. I taped the corners as pictured below, then used a long needle on a syringe to fill the resulting area from the bottom, which kept air bubbles from becoming trapped and leaving voids.

I fiberglassed a small door onto an aluminum hinge to hang over the opening to my sump area, which will allow fuel in but not out. Another copy from Mark Langford.

With the baffles in place, I made the ledges for the top the same as before.

Bottom went on, then leak tested. After the bottom was in place, I shaped the one-way door so it would cover the hole to the sump area.

Because of the angle the baffle is at, I used some flox and micro to build up the area and get a good flush fit between the hole and the door. To do this, I attached the door, used plastic and tape to keep the vinylester from sticking to the door, and then started filling the gaps on the sides and top with flox and letting that set. I used runny micro for the bottom, so I wouldn't end up with a lip that could trap contaminants until inopportune moments.

The result is a gravity powered one-way valve. I considered using actual fuel valves, but they would have become clogged if any debris made it into the tank. This design will allow debris to flow through, and then collect at the drain valve and be removed during preflight.

Notice that the top of the baffle that holds the one-way valve is open to the rest of the tank. That's because the line from the transfer tank will feed directly into the sump area. This way the sump/pickup area will always be the first place that gets fuel (which is really the only place that ever needs fuel.) When the sump area is full, fuel will overflow into the rest of the tank. If I leave the transfer pump on and the tank fills completely, the vent line will allow fuel to overflow back to the transfer tank instead of building pressure and causing damage.

Next I set my fuel filler neck in the top of the tank. I drilled holes in the sides of it to let the micro through in an attempt to keep the twisting force exerted on the fuel cap from twisting the filler neck free.

Lines in place:

Fuel level sender:

I attached the top of the tank, leaving the outboard end uncovered so any leaks will be easy to find and fix.

Upon pressure testing the tank, I discovered I had a major leak. So bad that no matter how much air I allowed into the tank, I couldn't get anything to register on my pressure gauge. I spent about an hour going over every inch of the tank, and I couldn't find a single spot that had even a tiny bit of air coming through. Well... *almost* every inch of the tank. It turns out the drain valve was in a spot that was difficult to reach in my crowded work space. Pro tip: if you're going to pressure test a vessel, make sure all of the holes you intended to make are plugged. Once I put the drain valve in its proper place, I had zero leaks, so this wing gets removed and put away for the winter.

After passing the pressure test, I attached the rest of the foam to the top. Wing tanks are finally finished, so I'll lay off the vinylester until the weather gets nicer. To give you an idea of the time penalties cooler temperatures incur, I'll say this: In October, the temperature outside was around 70 degrees, and if the sun was out I could expect a layup to cure in under an hour. With the heater on the fritz in the shop, the temperatures fluctuated from as high as 60 to as low as 30 overnight (although I made use of some stinking bright, stinking hot work lights to help.) I routinely leave a little extra vinylester from each batch in its mixing cup, to verify that it cured correctly, and there have been several occasions where that stuff didn't fully set for two whole days. If I had warmer temperatures, this process would have taken much less time. Henceforth, I'll be using this winter to accomplish anything and everything that isn't fiberglass related, and get that stuff done once it warms up outside.