ORIGINAL: doxilia
your post and reply to my question is actually very timely. Thanks for the info. I know you AMA pattern guys (especially you) are very familiar with CF wing tubes, so if I may, I have a question:
I'm working on the re-structuring of a 1980's 60 2-stroke classic - for levity, convenience and ease of build. For that period, the model is rather large (60.5" length, 66.5" span) and weighed over 9 lbs!. I'm aiming for a model at 8 lbs all up wet or less. The original has a one piece wing but as I was studying it and with the modifications I have in mind, a two piece plug-in wing makes much more sense. Access to the model will be primarily from an underside hatch which is flat the length of the fuse (cowl to rudder TE). I'm setting up the model for a vertically mounted rear exhaust engine with pipe running through a glass canopy section and continuing into what will be a sheeted built up turtle deck (i.e., no foam). The deck will have a half round recessed glassed section to cradle the pipe. If you are familiar with the Bootlegger, the setup is similar but there is an angled floor (about 9 degrees) from firewall to exit at top. I plan to also make part of the pipe floor (the front part under the canopy) removable (1/8" lite ply) to access the tank (there is no other sensible way to reach it otherwise) and possibly battery pack (if installed behind the FW - that will depend on balance).
Anyhow, that's really a prelude to my question. The model spans 66.5" and each plug-in panel would be 31.7" in span (fuse side to tip). I was thinking of using a 26" long 7/8" OD CF wing tube. The tube runs just forward of the CG and the main landing gear (the mains wells just graze the tube sleeve). The wing has strong sweep (both LE and TE taper) so the tube runs at about 30% of the root chord at the center of the model to about 20% of the chord where it ends.
Do you think a 13" CF 7/8" tube section into each 31.7" panel is sufficient for the flying stresses as well as landing force on retracts? That's a little over 33% of span support by the tube. Longer? Shorter?
Also, other than a 1/8" lite ply rib section at the end of the tube and some CF reinforcement above and below it, do I need any other structural support? I am planing on running CF flat spars recessed into the core in two spots on the underside and along the strut line on the top. This all under 1/32" (note not 3/32"!) balsa skins (literally).
The tube currently ends about 2.5" outboard of the edge of the retract mount plates (which I actually moved outboard a tad since the stance struck me as very narrow originally).
Apologies in advance for the detour on the thread but since it seemed we were already discussing wing tubes and composite failures, I thought it might be timely. A reply via PM (or here - whichever seems preferable) would be much appreciated.
Thanks, David
Dave,
It's a good set of general questions and the answers should be of interest to others so I decided to post it rather than PM. My alswers are not the only possibilities of course. These have worked well for me for decades so I always use them with an eye open for lightness, noise suppressionand strength, always.....
First, on the length of the wing tube....The 13" x 7/8" CF tube into each panel will be more than enough. It will handle whatever landing loadsyou mayplace on it, eventhough the skins are 1/32". I assume that all cut outs will be inside the 13" socket length. If not, either strips of .007" carbon laminate, or subspars (explained below) should be used out beyond the furthest cut out, likely the servo. BTW, for what it's worth, the 7/8" wing joiner is plenty strong enough even for a piped DLE55 hauling the model. My latest design is set-up with a 7/8 wing joiner and sports a piped DLE55 up front (around 6 HP) some 1150 squares of wing area. Wings are not heavy by any standard...around 15 ounces per panel RTF, all paint. Except skins also included carbon veil inside for strength, stiffness and lightness.
On joiner position, the swept wing is more of a challenge. Can you re-cut the socket holes? 30% back of the LE at the root might be okay but it might be marginally okay, because of the sweep. Your CG will more than likely be well behind of the socket. Try to get the sockets centered on the CG, even if they are located aft of the wing high point. Your CG will most likely be located at 25%-30% of the MAC, which, on a swept wing locates it well aft of the wing LE at the root
Let me suggest that you use 1/32" 6-8 lb density stock rather than contest balsa. Each sheet shouldn't weigh more than25 grams for the size wing you are making. This balsa size is fine for full blown 2meter pattern wings also, but we generally don't use retracts in the wings any longer. Plenty of strength for filght loads as long as some judicious glass or carbon is used on the end of the socket as per Shawn's explanation.
If you want to make the CF wing joiner shorter, you can. The way I build foam wings is a little different than most. I shorten the sockets on 2meter stuff to include around 10-11" of CF wing joiner. BUT, I also add 1/8" balsa subspars centered on each socket and extending beyond the end of the socket 4" or so.A 3" square of 2ounce glassis added on bias stradling the socket's end and the sub spar extensions. The stress riser is brought that much further out and beyond the servo cutout.
You can also make a two-piece plug together one piece wing. Years ago Imodified a full 2 meter wing that was normally a one-piece into a two piece plug together type. The amount of CF wing joiner I used was a total of 15" which meant only 71/2" of CF tube into each panel. But the sub spars were still extended out beyond the servo cut out. The subspars also extend from the socket surface up to the skins, forming "I" beam structurewith the skins. The min depth that I've used is 1/4"; smaller than that probably defeats the purpose so on your wings, you need to have enough room in your foam
I am not a fanand do not use the false ribs at the socket ends. It isn't as mechanically sound to do it that way and I've seen sockets loosen over time from vibes. But on a smaller, lighter model, it's probably okay, just be forewarned. Also, as mentioned above, you might not have enough foam thickness to accomodate anything else but a false rib
On pipe supports,I always soft mount my pipes at their approximate CG, even for hard mounted engines. I use a single lord mount like a Dave Brown, and a tie wrap. My pipes are not allowed to touch any part of the airframe excepting the soft mount. This treatment minimizes any other structure surrounding the pipe. In your set-up, I think your lite ply pipe half round might be noisy even if lined with rubber.
If there are specific details, maybe we can take that off line with PMs