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Old Dec 21, 2015, 03:03 AM
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Wobbly T-Tail

Hi fellow rc flyers, I just finished putting the airframe, the wing and the tail section (except for the h-stab, v-stab, and the wing, it's all bamboos). I didn't stick them permanently until all the trouble has been dealt with. So, the bottleneck is that I designed the plane to be pusher and the motor is mounted behind the wing. to get the prop to blast the tail, I heightened the horizontal stab to the top of the fin, effectifely making it a T- Tail.

Now, all is good and fine, the tail boom is strong enough (made of bamboo, it's firm enough, wiggle a bit if you nudge it strongly, but it's fine I guess). next I installed the fin and the horizontal stabilizers. I use hot glue for that. It sticked hard and firm, but you see, when the motor is on, the fin would wiggle. the harder the motor spin (while holding the plane in place), the more violent the wiggle. I don't like it.

So what can I do to make it less wiggly? I need suggestions. Oh and I only have bamboo for material. I think I could order a carbon fiber, but it's gonna take very long to get here. Plus I'm affraid to cut the CF Rod for I heard the dust is rather sharp and dangerous. I'm thinking of making it double boom instead of single one. But any other complex structure would make it heavier, which is bad. Btw the current AUW is 350 g (including 3s lipo 1100mah + servos + esc + rx), I got surprised that it's HEAVY!! make me unsure if it could fly or not
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Old Dec 21, 2015, 06:06 AM
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Does it Fly ? A little wiggle can be expected ...
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Old Dec 21, 2015, 06:44 AM
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I haven't flown it yet, cause I'm heavily concerned by the wiggle. Currently the tail part is disassembled, and is being redesigned. Would update the progress, but I'm curious, perhaps it's the T-Shape of the tail that cause it to wiggle, since it put the weight to the top of the v-stab?
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Old Dec 21, 2015, 06:52 AM
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Try bracing it similar to this
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Old Dec 21, 2015, 07:24 AM
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Or, using dental floss, go from the front of the stab/rudder joint to the outer skid ends.

Gord.
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Old Dec 21, 2015, 07:41 AM
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From what I can see, the "reason" your tail wobbles is that it is directly in the prop wash, so lots of swirling, turbulent air going over it; your boom is a single bamboo stick, which may be strong in bending, but probably not in torsion, so it twists readily as the air swirls past. If you think your h stab to v stab joint is stiff enough (the tie wires mentioned above will insure that) then the next step is to improve torsional stiffness of the boom. Since your fwd fuse is a bamboo cage, I suggest you do the same with the aft fuse, perhaps three sticks arranged to miss the prop. You can perhaps split one of your sticks in three, and have no weight gain. (I suspect you can reduce overall weight as well by splitting the sticks ... On the next one!). Google early DeHaviland planes like the DB2 for how they did the tail structures.

By the way, I think the cage fuse looks very cool!
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Old Dec 21, 2015, 07:57 AM
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bracing!! ok I'm gonna try that. looks like it should work, if the tail boom is torsionally stiff, which as springer said, could be the main problem (my h-stab to v-stab joint is pretty stiff). Turns out he's right, I think I'd solve that first. Btw I just did some browsing in a hobby shop in my country, and it turns out that CF tube isn't as light as I thought it was. So I'd still go with bamboo. Thanks guys, I'm gonna try your suggestions, but first I'm gonna make the tail boom not easily twistable
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Old Dec 21, 2015, 08:24 AM
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Bamboo is pretty tough to beat as a model building material. (CF may give you better torsion stiffness for a given diameter, but for more money). And you live in an area where you can get premium stock, as opposed to us in the northland's who have to scrounge for straight sticks in BBQ skewer bags!
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Old Dec 21, 2015, 10:06 AM
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Quote:
Originally Posted by springer View Post
From what I can see, the "reason" your tail wobbles is that it is directly in the prop wash, so lots of swirling, turbulent air going over it; your boom is a single bamboo stick, which may be strong in bending, but probably not in torsion, so it twists readily as the air swirls past. If you think your h stab to v stab joint is stiff enough (the tie wires mentioned above will insure that) then the next step is to improve torsional stiffness of the boom. Since your fwd fuse is a bamboo cage, I suggest you do the same with the aft fuse, perhaps three sticks arranged to miss the prop. You can perhaps split one of your sticks in three, and have no weight gain. (I suspect you can reduce overall weight as well by splitting the sticks ... On the next one!). Google early DeHaviland planes like the DB2 for how they did the tail structures.

By the way, I think the cage fuse looks very cool!
hehe I think I'm gonna make the tail part a lil bit like that, and with smaller diameter. I thought the sticks I used was quite thick, but I can't disassemble the fuse because I'm not sure if I could assemble it right back again but perhaps when I'm finished with the tail part I'd do a full reassemble. Gonna be a lot of time spent there
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Old Dec 22, 2015, 06:51 AM
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As Flypaper mentioned, dental floss is very very strong, if you string it from the outer edges of the H Stab, to the boom, each side will act against each other, making the H stab rigid, dont do it too tight, or you may inadvertently bend the H stab.

The advantage to using dental floss, is that it is light, and strong. It is also hard to notice.
This cab be used for bracing wings, and stabs too, so if your tail fishes, you can go from the bottom corners of the V stab, to the boom, or to the corners of the main wing.

this will not add much weight at all, but again can be done in minutes.
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Old Dec 22, 2015, 02:03 PM
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Yup, I'm with springer on this one. A cage-like tail section can be incredibly stiff, yet light. It's a common approach on many ultra-light airplanes like the Quicksilver MX Sprint and others.
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Old Dec 23, 2015, 05:00 AM
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You really need to try flying it ... the air loads are usually a lot LESS than wiggling it with your hands ... What can seem fragile on the ground is usually strong enough in the air ... save any really stressful flying ...
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Old Dec 23, 2015, 08:19 AM
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Good point, Glen. That's the only way to know for sure if the flying characteristics are affected.
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Old Dec 23, 2015, 01:56 PM
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I totally agree also.
However when I had the identical problem with the cobbled up FPV plane in Post #4 I was too chicken to try.
4 bits of bamboo and 6 dobs of hot glue fixed it just fine.
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