Wing Tips (Part 3)

After hot wire cutting, I did some sanding to smooth it out, as well as making a radius on the bottom edge. I also trimmed away 3/4" of the trailing edge foam and filled it with micro, same as with the wing. After I was happy with the shape, I put two layers of 5.8 oz cloth on the top, two layers on the bottom, and two layers wrapped around the thin edge. The top and bottom layers extend about 6 inches onto the existing wing, and the whole thing is quite stiff and strong. I don't think I'll make any changes to the process for the other side, so I won't bother uploading any new pictures unless something unexpected happens. Here's the (semi) finished product.

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)