Wing Tips (Part 2)

Continued from Wing Tips (Part 1)

Once the foam was all glued up, I sanded it down to shape just like the rest of the wing. I ended up with 15" of extra length, and my final airfoil chord at 33.75 inches and right around 0* incidence. Time to make the actual tip.

I'm going to be using a modified Hoerner tip. I used a paper written by Hoerner, as well as the video I linked to in the previous post, to come up with my own hybrid. Hoerner describes and promotes a wingtip that is simply cut at about a 45* angle from bottom to top, which was my original plan. However, in the video uploaded by YouTuber schmleff, his tip changes angles as the wing thickness changes. I'll be using aspects of both, and hopefully not eliminating the good effects of both in the process.

Hoerner tip:

Schmleff tip: (before additional nose shaping)

My adaptation:

Hoerner tip pros: highest L/D ratio without using "tip devices" (i.e. winglets), as a result of vortices around the outside of the wing being pushed out past the wing.

Schmleff tip pros: Higher lift at tips, causing greater roll stability and lowering stall speed

My tips: (in theory?) push vortices past the wing, and give smooth airflow transition to rear of tip where greater lift can be achieved with minimal decrease in top speed, allowing a potentially lower stall speed and therefore slower landing speed.

Enough about theory, here's how it went.

I started by following the schmleff video, taping about 3/8" below the top of the airfoil and 6" inboard of the tip on the bottom of the airfoil. (Wing is upside down in picture.)

I went ahead and cut the foam with the hot wire cutter at this stage, giving me the start of the schmleff tip.

I had a fancy jig planned that would hold the hot wire cutter at the right angle as I moved it over the front of the wing, but I realized since I had this modeled in CAD already, I might as well just plot it on my wing, tape it off, and cut it. Could have done that all at once, but I didn't, so I had to glue back some of the foam I just cut off. The nice thing about hot wire cutting is the pieces fit back on really nice.

Once I had that all taped up, I used the hot wire cutter again, and the general shape was achieved.

I like the way that turned out. All that's left is about 10 minutes of touch-up sanding, and to round the leading edge a little bit, and then I'll be ready to glass it.

Wing Tips (Part 1)

With both wings glassed, it's time for wing tips. I'm doing a version of Hoerner tips, which ought to reduce drag and increase lift by moving the inherent turbulence out past the wing. I did a fair amount of research, and came up with a couple of studies that promoted the Hoerner tip, and a few studies that promoted simple round or rhombus end plates as much better options. I don't like the idea of those end plates, because 1. if they aren't lined up right they'll increase drag, and 2. I don't like how they look. I'm basically following a great Youtube video found here, which details the entire process.

The first step I needed to take was making my wing straight and square on the end. This should have been done by shaping the spars accordingly, but I wasn't smart enough to think about that at the time. As a result, the ends of my wings are pretty wavy.

In order to fix this, I started by cutting an oversized airfoil from a piece of 2" foam, and cut it into a bunch of sections that would follow the angles on the wing endplate.

These were glued on and left to cure. Once they were set, I sanded them to match the rest of the wing.
Next I laid a framing square along my front spar, which is perpendicular to the aircraft's centerline, and used that to draw a line on the top and bottom of the foam 1" outboard of the endplate. After the lines were drawn, I laid some 2" aluminum foil HVAC tape along the lines all the way around the foam.

Once that was lined up and pressed on really well, I fired up my hot wire foam cutter and used the tape as a template. When I was finished, I had a really nice straight, square, flat end plate to glue my wing tip onto.

I'll be extending my wing 15" outboard of the 3/32" plywood endplate, in order to try and squeeze a little better glide rate out of the plane. From what I've been reading, it doesn't seem to have much of a speed penalty, and a number of folks have extended their wing 18" or more this way without adding to the length of the spar. Since the foam I used to fix the wavy endplate issue is 1" outboard of the main spar, I'll add 14" of foam by cutting 7 pieces of 2" stuff and gluing it all together. It's cut to a slightly large airfoil shape to help me get it to the final shape faster. I'm using 3M spray adhesive so that my hot wire foam cutter will be able to slice through it once it's ready, although I'm slightly skeptical about how well that'll hold the foam for sanding.

Continued in Wing Tips (Part 2)

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)

Outer Wings (Part 4)

Continued from Outer Wings (Part 3)
With the plane upside down and wing reattached, I began by cutting the rest of my foam pieces to shape. It was during this process I realized that, during my fuel tank construction, I glued the bottom center section of foam incorrectly. By incorrectly, I mean that the foam was actually recessed below the airfoil shape for about half a square foot. I had problems fixing similar trouble spots before, but I've decided to try a different method with this area. I didn't take a before pic, but the after pic gives the general idea. The darker area is the low spot.

I decided to go ahead and sand the center section to the airfoil shape so I would have a solid understanding of where the foam was low. After that was accomplished, I traced and cut out a piece of foam 3/8" thick that was 3" smaller around the edges than the space that was low, and I glued that on. My next step will be to sand that to shape, then use micro to fill the outer band of the low spot, utilizing a straightedge while the micro is still wet to get a nice flat surface. This should work much better than my previous attempts, which were to glue foam over all of the low spot and sand it away. The problem was that the micro didn't sand as readily as the foam, and created high spots. 

With that figured out, I turned my attention back to adding foam over the rest of the bottom of the wing. Pretty straightforward, just make sure there's enough foam sticking up to sand away, and enough foam left after sanding that it won't flex after fiberglassing.

I came up with a simple (sort of) way to help hold long pieces of foam in place when they want to sag. First I cut off a piece of string about twice the length of the foam. Next I poked holes every foot or so in the foam I want to attach. The string gets secured at each end, by whatever means necessary. Finally, the string gets pushed up through each of the holes, then pulled until there's no slack. When you're finished, you've got plenty of string loops to help rearrange or secure your foam. I didn't end up using them to hold the foam in place, but they were very helpful for making small adjustments while the micro cured.

Top side:

Bottom side:

Continued in Outer Wings (Part 5)

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)

Outer Wings (Part 1)

I decided I couldn't wait on my landing gear legs to start my wings, so I bolted them up and started gluing foam in place for the leading and trailing edges.

Once it was all glued in place, I started sanding. I sanded grooves in the top, then used a hand power planer to get close to the final shape. I began sanding with a long piece of aluminum angle and some sandpaper attached to it, but decided I needed coarser stuff. I'll be picking up some more paper, then going back at it.

Continued in Outer Wings (Part 2)

Wing Walks!?

I've ordered and received a whole bunch of foam from Dynamic Systems (aka SunMate), which I chose as a crash safety measure as much as I did as a comfort aid. I've been cutting away and shaping, but I've gotten to a point where I need to be doing a lot of sitting in the seat and seeing if it's comfortable yet. Only problem is, the plane has no wing walks.... and as challenging as it was to get in the plane standing just on the rear spar, that was preferable to setting up a makeshift seating setup outside the plane. I decided, since I need wing walks eventually, I might as well get them knocked out now so I can get this seating stuff taken care of.

The previous builder had glued and shaped the foam for the top of the inboard wing, and laid what appeared to be one layer of FG on it. I didn't want to disrupt the shape of the wing, and I didn't want to completely start over, so I decided to try and add a wing walk structure without disrupting anything I didn't have to. First step was to flip the plane over and get a good look at the underside.

The previous builder had used a single sheet of 1" foam, and cut slits halfway through it about every 2 inches in order to allow an easy curve to the shape of the wing. This was about how far apart I wanted to put supports, so I started by using a razor blade to extend the slits all the way to the fiberglass skin. I made my wing walk 12" wide, and it runs from the front spar to the rear spar. I dug out the foam in the slots, then cut another slot for a support to run lengthwise from spar to spar. I used ⅜" a/c plywood for the long support, and 3/32" a/c plywood for the rest of the supports. All the supports are 1" wide.

The last step before epoxy(or so I thought...) was to cut slots in the foam for two ½" spruce blocks to (drastically) increase the gluing area for the ⅜" main support.

When I test fitted the supports, I noticed that my long stringer (which was cut very, very precisely to the airfoil contour) didn't touch the skin in a lot of places. As in, ¼" to ⅜" gap. A closer inspection of the wing showed the problem; the foam and fiberglass had sunk, or had been attached too low at the rear spar, and the wing shape was not preserved except at the two wooden templates. I had to think for a couple days to come up with a solution to that one, but I think I came up with a good one. 

I used my hot wire foam cutter and two wooden templates to cut two 12" wide foam templates, one for each side of the wing skin. The outside template I left long enough to rest on the spars, while the inside template I cut about 1" too short so it'd be able to easily drop in place once epoxy was curing. I used some highly sophisticated and carefully calibrated bracing structures (read: spare tires) to support the outside template and, once it was in place, I mixed up my epoxy with some micro to fill in gaps and started inserting all of the supports. Once the supports were all in place, I placed the inside template (with plastic trash bag to isolate it from the epoxy) on the wing, and added a good amount of weight.

When I came back the next day, I was *very* happy to find that the setup had worked perfectly. My wing was now the shape it should've been (or, much closer to it) and the supports were all in place and holding. I mixed up more micro/epoxy and filled more gaps, then laid up two layers of FG and placed them over the structure. Although I'm not sure it was entirely necessary, I repeated the process of weighting the template over this to make sure it all cured with the right shape.

I did the other side the same way, and although the previous builder cut a few more slots in the other side, the results were similar:

The final result was solid. I'll be adding one or two more layers of FG to the top of the inboard wing, but it's already strong enough for me to stand on unsupported with no perceptible give. I'd call this a success.