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Old Jul 08, 2010, 09:19 PM
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Question
T-Tail construction

Greetings all

I was wondering if someone would happen to have some pics of a good way to build a T-tail.
I have been playing with the idea to build one, but cant seem to get a concepts worked out in my head as to how to build one light, yet strong enough to take some serious speed.
I was also thinking of making the horiz and vert tail foam core with balsa and glass over it. Would this work for a t?

Any hints/tips/ suggestions?
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Old Jul 08, 2010, 09:46 PM
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Did you want an elevator tail or a full flying stab? It'll make a big difference to the construction
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Old Jul 08, 2010, 10:14 PM
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Did you want an elevator tail or a full flying stab? It'll make a big difference to the construction

horiz stab, and elev.
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Old Jul 08, 2010, 10:48 PM
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Then it's pretty easy.

When drawing your tail, simply set up your fin so that the top of it has the right incidence.

WHen I do mine I laminate the fin out of a central core with a control rod snake (coming out the top is easiest but doesn't look as good) cutout in it, the thickness of your outer sleeve, i.e.if the outer sleeve is 1/8" diameter, your inner core should also be 1/8" if it's 3/16", the core should be 3/16" etc. Add lightening holes should you chose to do it, and then I sheet the outsides with a minimum of 1/16" sheet at an angle to give you a balsa ply fin. I fit triangle stock, to the top of the fin to give you a good gluing area, then fit your stab and elevator, and hook it all up.

Works well on sailplanes up to around 100".
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Old Jul 09, 2010, 02:42 AM
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The attachment between fuselage and fin is critical. Extend the fin so that it connects to both top and bottom surfaces of fuselage. Also the fuselage right in front of the fin is a weak spot. Add a bulkhead to prevent the fuselage from buckling/collapsing and if possible, use an extra layer of thin glass cloth at 45 deg bias to add torsional stiffness.
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Old Jul 10, 2010, 01:22 PM
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As jkettu suggests it's important to make the fin quite strong unless you plan on ALL your landings being feather smooth. The horizontal surfaces being up there have a lot of leverage to work against the fin so it's almost impossible to make the fin TOO strong. However it is very easy to make the stabilizer too heavy and needlessly increase the work load of the fin during a bad landing or during any possible flutter moments while in flight. Do what you can to make the stabilizer and elevator light without sacrificing too much rigidity. The stab is up out of the way so it can afford to be fairly light without risk unless you plan on a lot of flip over landings. And keeping the stabl light is the best way to ensure that the fin is adequitely strong.

On a couple of models I've done with T tails the fin used a regular built up rib and spar setup skinned with 1/64 plywood. I did it that way so that the elevator control system was easy to enclose. The skin extended down past the last rib so that I was able to glue it to the fuselage sides over much of the length. This really tied the fin to the fuselage well. Then I feathered it in with some lightweight filler and finally 1/2 oz glass cloth was doped on for a little extra strength in a sort of lightweight composite skin to toughen the outside of the fuselage and give it a little extra torsional strength.

If this is for a sailplane I would strongly suggest you do not use a flex cable or flex tube (ie: Nyrod) drive in a big S curve to run the elevator. The one model I used that on had a LOT of lost motion around neutral due to the slop in the S bend. On the next one I used a rigid pushrod and bellcrank arrangement that has proven to be far more accurate. And it's trouble free too having lasted for a few hundred flights over the years I've had it. So take the extra time and do it right.
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Old Jul 10, 2010, 09:11 PM
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Bruce, thanks for the very imformative post. The plane I am working on is a wannabe built up hotliner. I was thinking of a foam core horiz and vert. As that was the case I was planning on using a nyrod. However the nyrod sleeve would be epoxied in place in the foam so there would be very minimal slop. At least in theory.
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Old Jul 10, 2010, 09:25 PM
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Don't you bet on it. My own S curve job had the outer guide epoxied into an S curve cut into the sheet core of the fin. No movement at all there. And it's not the movement of the guide tube but the slop between the inner and outer tube. For some reason an S bend really accentuates this issue far more than a 90 degree curve. The mechanical deadband I had due to this slop was enough that if I let the elevator return to neutral from using up it would fly at a slow almost "minimum sink" speed. If I used a touch of down for whatever reason and let it return to neutral it would stop a few degrees below the "other neutral" and the model was in a high speed cruise mode. At first I suspected servo deadband and all manner of odd things. Once I found that the culprit was the S bend I did two things. First was to swear off nyrods for any model where controls are critical such as sailplanes (and that would certianly include a hotliner), racers or most any other model that I actually installed a part of my soul into...... Hmmmmm... guess what, I've only used Nyrod on ONE model since that time almost 25 years ago. It wasn't just this. I found out that Nyrod is also temperature sensitive. A rudder elevator glider would require almost all the left trim to fly straight on a cold day and almost full right trim to compensate for the nyrod growth on a hot summer's day. The elevator debacle was just strike 2. I didn't wait for a strike 3.

The second thing I had to do with that model was to get used to using a small blip of up elevator and let the stick come back smoothly. That way it always returned to a consistent neutral. This worked well because when coming out of a turn I didn't need to do anything as the last of the up was removed. If for some reason I had to use some down elevator or after a run with some down trim I'd just kill the speed with a blip of up. Actually truth be told I also used this as an instant access speed control. If I needed to punch out I would just nudge in some down and it would return to the lower end of the lost travel and I'd get a nice speedy transit. At the end I'd nudge a little up and it would be back in trim for a nice float (well, float for THAT model, it was a bit of a speeder).

But all in all I would have preffered to not have to deal with this Nyrod issue. I'd suggest you go the extra and engineer up a suitable bellcrank system with zero clearance light interference fits. With the nylon bellcranks we have you'll get years of slop free life from such an installation.
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Old Jul 11, 2010, 07:04 PM
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Ok, so after reading and thinking and, thinking and reading and pondering... this is what I have come up with so far.
Nyrod -yes
S-bend - no
Strong and light-yes, hopefully

The part shown would be cut out of 1/8" or 3/16" balsa sanded to a sym airfoil. Both sides would then be sheeting with 1/32" or 1/16" balsa.
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Old Jul 12, 2010, 05:27 PM
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That would certainly be better than an S bend. I'm still not a fan of Nyrods due to their strong size change due to temperature. On most NORMAL planes it's not a big deal. A click here or there and all is close enough. But on a hotliner where you are presumably running very close to the neutral point and even small elevator trim changes have a big effect I think you'll find that you're hunting around for a consistent neutral.

Another thought would be a small and light metal gear servo installed on an angle in the base of the fin with an angled pushrod extending up to the same angled coltrol horn you show in your drawing and keeping in mind that the pushrod and control horn should meet at 90 degrees to each other to avoid any differential throw in the surface.
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Old Jul 12, 2010, 10:07 PM
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That's definitely another option that I had not thought of. Right now I am stuck trying to cut a very thin wing core... something just is not working right.

I am having a hard time dealing with a design that is past my current skill level of building and it is hard not to get frustrated.
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Old Jul 14, 2010, 07:33 AM
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Have you looked into Dubro push rods? an example at http://www3.towerhobbies.com/cgi-bin...?&I=LXPJX9&P=7 and http://www3.towerhobbies.com/cgi-bin...?&I=LXJGT6&P=M They come in three sizes: 0.062, 0.047, 0.032 and don't seem to suffer from expansion and contraction of the Nilinks. I've used them with a mild S turns and have not experienced slop nor binding. I do support them in three (or more) places along their length so the sleeve cannot move (laterally).

Your idea of using a simple bend is used on the Guppy: http://www.rcgroups.com/forums/showp...9&postcount=14 picture near the bottom

Alan
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Old Jul 14, 2010, 09:02 AM
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Quote:
Originally Posted by Al Offt View Post
Your idea of using a simple bend is used on the Guppy: http://www.rcgroups.com/forums/showp...9&postcount=14 picture near the bottom

Alan
That is exactly were I saw it as well. And that is why I was thinking it would work. However, BMatthews brought up a good point. The intent of this plane is a mild hotliner, and any slop at or around neutral will adversely effect the trim of the plane.
The Dubro pushrods I have used a lot before on other planes. The do work well, however, I was concerned putting them in with such a tight radius that they would bind in the sleeves. The third option i am going to try is :
http://www3.towerhobbies.com/cgi-bin...&I=LXFU95&P=ML

where the last inch or so before the control horn would be soldered so that it could not flex, but where it goes thru the bend, it would remain flexible. I am not sure if this will work, but I am open for suggestions.

I have a drawing over at the laser cutters waiting to get cut of the tail drawing I did so I can do some testing and see what might give me the best result before I commit to one idea during the final build.
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Old Jul 14, 2010, 09:24 AM
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A single connecting rod from the control surface down through the fin to a bellcrank fitted with two tension wires to the servo (a closed loop system) is a far more precise control layout. No matter how "Fixed" a snake outer is, there will always be lost movement when negotiating curves.

Regards Ian.
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Old Jul 14, 2010, 10:01 AM
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Quote:
Originally Posted by Circlip View Post
A single connecting rod from the control surface down through the fin to a bellcrank fitted with two tension wires to the servo (a closed loop system) is a far more precise control layout. No matter how "Fixed" a snake outer is, there will always be lost movement when negotiating curves.

Regards Ian.
Quote:
Originally Posted by BMatthews View Post

If this is for a sailplane I would strongly suggest you do not use a flex cable or flex tube (ie: Nyrod) drive in a big S curve to run the elevator. The one model I used that on had a LOT of lost motion around neutral due to the slop in the S bend. On the next one I used a rigid pushrod and bellcrank arrangement that has proven to be far more accurate. And it's trouble free too having lasted for a few hundred flights over the years I've had it. So take the extra time and do it right.


So how about something like this?
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