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Building Tips and technical articles. => Building techniques => Topic started by: Motorman on January 13, 2016, 07:02:05 PM
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I'm building this plane from plans and I've come to the trailing edge. The plans show the trailing edge sheeting ends at the end of the ribs then there's a 1/4 x 3/8 stick that gets glued on the back for the flap hinges to mount in. Does that sound right because, it seems it would be stronger if the sheeting went all the way back and covered that stick. How is it usually done?
Thanks,
MM
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I do not have very much experience with flapped designs, but I have seen that before. In fact 2 of mine that are next up are the same as you describe. I agree with you in thinking that it may be weak, but my 2 are old designs with many built by others over the years and have not heard any negative about it. So, in my opinion, I did not design it and will trust the designer here. Just my opinion.
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I trust the designer I just don't trust my glue joints lol. It would be easier to do it that way, just wondering if it's normal on a modern plane.
MM
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That seems relatively normal to me. I would let the ribs into slots in the TE stick, though.
I suspect that making the TE thin enough to sheet over might make it too flexible to keep reliably straight during construction without taking other steps to mitigate it.
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MM, I agree with you regarding joint strength. However, the problem is cutting the taper on the TE piece. While I have access to a reasonable number of wood working machines—some of them my own—I have not been able to work out how to cut the taper for the first side, at least. The two wings that I have in progress at present require a taper of 13° and 14° respectively on each side and until there is a TE sheet bracing the whole assembly, I would not trust sanding the taper into the strip on otherwise unsupported ribs.
I've already broken some ribs on one wing attempting to do that.
With the method shown on the plan, the whole trailing edge is braced by the TE strips. You can then true the whole wing TE with a long sanding bar before fitting the TE strip on the back. After that, you sand the wing section into the fully supported TE strip.
Remember that if you use a glue that wets and soaks into the balsa, the joint is stronger than the surrounding balsa after the glue is cured.
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If you search you may find that is the excepted method of glueing the trailing edge strip to the ribs. Put one sheet on and then put pieces between the ribs at the front edge of the sheet so the second sheet will cover them, basically making a box. Punch small holes in those pieces so air can escape. Makes for a very rigid trailing edge. I think Bobby Hunt has posts on doing this.
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Some glue in blocks to give the nylon/pin hinges more to anchor into, while others make the TE strip "wide" (chordwise) enough that they can cut or laminate the strip for hinge slots. I like the laminectomy plot better than cutting slots. Do yourself a huge favor and make your moveable surfaces removable. y1 Steve
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Why would you want to make the control surfaces removable, I've never heard of that.
Thanks,
MM
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Easier to put finish on.
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Why would you want to make the control surfaces removable, I've never heard of that.
That's very common (after Paul Walker showed us the way). It allows you to finish around the hinges, it permit you to remove the control surfaces for maintenance, allows you to replace the parts entirely, even use different size or configuration of flaps (or elevators, since those are usually removable, too). Also, the hinge seals can cross the gap stuck to the hinge pin.
I have used them to good effect for many years now. I wouldn't have won the '06 NATs if it was impossible to remove my flaps and repair the water damage they got during Wednesday qualifying. Everything swelled up and we had to pop the flaps off at about midnight, dry them out, and perform some reinforcement where the horns go in.
Brett
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Ok I did a search and more confused.
You use one long piece of music wire for the hinges? how do you keep the music wire from sliding out?
If your hinges are recessed in the flaps how does the wire go straight through all the hinges?
What if your flaps don't go all the way to the wing tip?
MM
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OK, I was going to stay away but, here you go. Honestly, I can see no reason not to make control surfaces removable, for the reasons Brett described.
http://stunthanger.com/smf/building-techniques/hinges/
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If your hinges are recessed in the flaps how does the wire go straight through all the hinges?
Don't recess your hinges into the flaps, whether or not you are using a single hinge pin. If you use a conventional hinge line shape, the center of the hinge pins should be aligned with the gap, not recessed into the flap. If you just recess it into the flap, the "point" of the wedge translates up and down and that is both incorrect from an aerodynamic standpoint (since the gap changes as you move the flap), it also makes it very difficult to seal the hinge line later, since the tape ends up having move up and down.
If you want to use multiple pins instead of a continuous hinge, and want a "zero" gap (also a big mistake, but lots of people think they want to) you have to recess it into both the trailing edge and the flap, so they line of the hinge pins line up with the point of the wedge.
The alternative is a recessed hinge with a *rounded* flap LE, in which case the hinge should be recessed by exactly the radius of the rounded part. This is sort of like the Yatsenko models. Note that I have had my first chance to examine a Shark close up (rather than taking the Top 5 picture at the NATs) and it is seemingly intended to work that way. Unfortunately, it was not properly centered and the gap closed up in one direction and opened up in the other, meaning the hinge pin was not properly located. I strongly discourage anyone from building them this way, as it makes it effectively impossible to seal properly. If you must, "lap" them by sticking 220 grit sandpaper to the TE of the wing, then move the flaps up and down as you push the flap into the TE, so it makes absolutely certain the radius on the radius on LE of the flap is perfectly centered on the hinges, so the gap is perfectly constant throughout the travel.
This stuff is FAR more important that the usual aerodynamic "numbers" nonsense we discuss in the winter months.
Brett
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Thanks guys you make it look easy.
MM