DC-3 Display Project
Click Here for Construction Part 1
5-31-2002
I figured it was probably time to get a few motivation pictures, so I assembled the major components of the airplane. This involved rearanging at least one room in my house; the assembled model actually used space in three separate rooms. Here are a few pictures; click on a thumbnail for a bigger picture.
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I've secured another 35 sheets of balsa, so I am ready to start sheeting the top of the right wing panel. That will probably be the next step that I will finish.
9-5-2002
I've certainly gone too long without an update, by any stretch of the imagination. While I haven't done much with the web page since May, I have been working on the airplane. At this point, both wing tip panels and the respective ailerons are ready for finish sanding.
I've started work on the center section of the wing, and have encountered a few problems. The key issue is that the center section of the wing must support the weight of the entire model. This seems like a modest proposition, since a flying version would have to support well over 10 times this force. But of course, since I'm on a non-profit budget, I don't really see the need for a full-strength wing and the expense that would come with replacing balsa for spruce, etc. On the other hand, I certainly don't want for the model to be fragile when it comes to supporting its own weight. When I framed up the section, I used 3/32" balsa for the ribs, 1/4"x1/2" balsa for the top and bottom main spars, and 1/4" square balsa for the top and bottom aft spars. I did use 1/8" balsa for the ribs that form the nacelles, and 3/32" lite-ply for the end ribs. My current plan is to hang the model from the metal screw-eyes that hold the tip panels on. This places the supports well forward of the static model's CG, which will allow for a stable 3 point harness with the third point on the tail. It also makes for a very simple and clean design, both on the inside and the outside. The eyes actually screw into a 1" hardwood dowel that protrudes into the tip section, so the threads have plenty to hold on to. Inside the center section, the dowel passes through the lite ply tip rib and the two ribs that form the nacelles.
In an attempt to beef up this area and to keep the ribs vertical under stress, I added 3/32" balsa shear webs between the two main spars for the outboard bays. Since I'm not any kind of structural engineer, I don't really have any way of knowing whether the section will be strong enough or not. My strategy is to reinforce the structure until it appears to be rigid enough before I install the top sheeting. The top sheeting should provide significant contributions of its own, so that will allow for an extra margin. This philosophy stinks for building flying machines, since weight is so important. But I figure since this one is a static model, it will do.
The first reinforcement technique (and what I hoped would be the last) was to fill the bays between the ribs with foam. Here is how well that brilliant plan worked.
I planned to fill the foam over the top of the ribs, and then trim it down and sand it flush. Instead, the uneven expansion of the foam relocated the ribs. This wasn't too big of a deal, but after about 2 weeks of sitting, the foam appears to have receded and left vacancies by the ribs. So now I have a bunch of useless dead weight in the middle of the wing. I haven't decided whether I should try to salvage it by filling in the space between the ribs and the foam, or just dig it all out and try another plan. I'm sure I'll come up with something.
On the subject of planning, I've also changed the finish-sanding strategy a bit. My previous schedule was to finish the framing and sheeting of the entire airplane before I started any finish-sanding and finishing. This strategy arose from the somewhat frequent housing relocations associated with my college student lifestyle. Every time I move I end up breaking or denting some other part of the airplane, and it just seems silly to work for an immaculate surface if I am just going to mess it up later. The problem with this idea is that I have very little experience with the aluminum material that I am going to use on the finish. I have already applied it to the engine cowlings in an effort to understand the preparation requirements, but they are made of fiberglass, and the structure is sheeted with balsa. If the aluminum tape has special requirements, I would prefer to discover them early on in the process, not after the entire plane is sanded, hardened, and ready to go. As a result, when I finish the structure and sheeting of the wing, I am going to go ahead and take it all the way through the process. The only problem with this idea is that the wing may need further modification where it joins the fuselage, but I think the risk of inadequate fuselage interaction is very small. If there are any issues, I should be able to resolve them with the fuselage alone.
Here is the new picture of the foam. I filled it in with a few more applications, and then carved and sanded to get what you see here. When I started this little fiasco I really expected to have foam left over from the first can. Little did I know, it was actually going to take four. On can number three I finally read the label to find that I could expect it to fill 300 cubic inches. I started thinking about the 36 inch span of the center section, and the 17 inch chord, and the 4 inch thickness, and it all started making sense. Anyway, I have most of the top section sheeted now, so it is on to the nacelles. I've really been inspired to finish this project up, so hopefully I'll be able to make more progress soon.
Click Here for Construction Part 3
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