Thursday, August 25, 2016

Wing Tips (Part 2)

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.

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