MikeFreas

Just like the title says I'm considering making this change. I have seen it done with the kit version though I don't remember if he modified the horizontal stab so it was flat blasa vs built.

My question is if I do this will or should I induce some negative incidence? Also, the vertical stab is going to have to be fairly thick to support the weight and flight loads. Will this cause a problem with airflow over the rudder?


Doesn anyone have any pictures of any tails during construction with this configuration?

BMatthews

If you do try this then just keep the same incidence angles. But you'll need to do some stuff structurally. The fin will take a lot of bending loads from the inertia of the high mounted weight of the stabilizer. Build the fin up like a proper wing with spars and a thick airfoil in the 10% thick range. The fin's spars shoudl also run down and bond securely to the sides of the fuselage and the fin's trailing edge where the hinges go should also be extended down between the sides. Fully sheeting the fin with some 1/16 sheet would not be a bad idea either. Meanwhile the rudder can be built up to match but left really light with ribs that are just covered.

Also do not over build the stabilizer. Keep it light. The lighter the stabilizer the less bending and twisting forces the fin will need to withstand.

If you downplay the loading in the fin and just build a light flat fin I can guarantee you'll be constantly gluing it back together and back onto the fuselage.

I'll leave this here for the moment but really there won't be any aerodynamic considerations worth mentioning for such a mod. At most you would need a click or two of trim compared to the normally positioned stabilizer. But just in case anyone wants to add to this I'll let it sit for a day or two and then move it to Scratch BUilding.

da Rock

BMatthews got this one covered like a blanket.

But I will throw in that I've done a BUNCH of T-tail gliders. And they got a steady beating from glider contest landings. The glider very often whips around when you stick the nose into the spot-landing circle center. So listen carefully to what BM said about the structure of the fin. What I did find was that sheeting the fin is the magic trick for this deal. The sheeting distributes the stress support along the entire chord of the stabilizer and gives a much more durable joint. Not only the joint lasts longer, but the stab won't fold as easily.

da Rock

Oh yeah, on the T-tail gliders......

I converted a couple and found that they required a bit of unexpected elevator trim. And it was the same way for those airplanes. So when I started designing my own, I took a WAG and used less negative with the first T-tail. Although, in truth there wasn't much at all with any that I'd measured. And my first one didn't need any elevator trim. Better to be lucky than smart. But in this case, I got off my butt and pulled out a couple of the aerodynamics books I'd bought and started re-reading them and TAAA DAAAA.........

Basically, the h.tail incidence is set based on the downwash of the wing. Move the h.tail back or up or down, and the downwash angle is different. Never in my wildest dreams would I have thought it'd matter to a h.tail that was as far back as a glider's is, but it really looked like that's what it was.

I had been "designing" by copying up until then. After that, I continued to design, but with a little less copy-and-change in it. The lesson on setting the h.tail incidence had been learned.

MikeFreas

Thanks for all the input guys. It shouldn't be that hard to cut some ribs and fab up a new thicker vertical stab and rudder.

So to consensus is I really shouldn't need negative incidence?

Tall Paul

Considering the structural needs, I'd not do this at all.

MikeFreas

Please explian your reasoning.

I know the ARF version of the STM is built like crap and I plan on beefing up the tail like I did on my other one. If built correctly what's the difference in loading on the fusalage vice the stock location?

Don41

Go for it Mike. I have a Senior Telemaster flying and I am about to start the 6' version. Just to be different I intend to modify it extensively (to the point it will be hard to recognize).

Your T-tail intrigues me so I think I'll at least consider the mod myself. I think the push rod can be hidden in the built up fin.

Should be a fun mod.

MikeFreas

Yes a heavy-duty pushrod will be hidden in the tail if I do this. I still want more input from those who think I shouldn't.

There are three critical points that I see.

1. The attachment point of the vertical and horizontal tail sections
2. The attachment point of the vertical tail to the fuselage.
3. The structural integrity of the tail center section without the support of the aft end of the fuselage.


I plan on solving these several ways but it's going to take some thinking on my part.

1st. The stock built tail is just ok structure wise. There is a fair amount of flex that I believe can be fixed without adding to much weight by sheeting it with some balsa.

2nd The attachment point between the two is the most critical in my book. The plan would be to build a new rudder assembly with an actual airfoil (built tail) like the horizontal stab. This would have ribs and two or three hardwood square pieces for which the ribs would attach. Extending these so they could fint into a "receiver" plate built into the tail should provide any and all structural strength I need. I would also add some gussets to the fuselage and blend everything in so it was one.

Honestly now that I start really thinking about it why the hell not do this?

What is the absolute worst thing that could happen? Well, My built tail could...

1. Be to heavy preventing me from getting the damn thing to balance correctly

2. Have very bad flight characteristics and be unstable

3. Complete tail structural failure in flight.

Any of these would ruin my day for sure but I'll never know if I don't try. I was given this airframe and a ton of other gear. I don't have much to loose other then my time and a few hundred bucks. Doing things different is fun in my book and trying is what I will do.

Don41

Build the fin and stab as strongly as you wish, balance will not be an issue. Wing loading is very low and I have been seriously considering adding anywhere from one to two pounds of cargo to my Senior because I believe it will fly better (more to my liking anyway). With that in mind it would be a simple task to just move the added weight further forward.

BTW: Mine is built from a kit but I was initially a little concerned about the flexibility of the aft fuselage. The concern turned out to be unjustified. Even so, with this mod I'd lean towards sheeting the aft area.


My Senior is glow powered (Saito 82 four stroke) and weighs in at 9 pounds, 2 ounces.

MikeFreas

I'm well versed in the STM mods. I also built a kit that met it's demise after my wing spar snapped in half. My second has a very large bomb bay and is powered by an AXI 4120/18 turning a 14X10 E prop. With all my mods and reinforcements it weighs 11 lbs. I added very large 6" tundra wheels that send the weight right under 14 lbs. While it flew fine I needed more power.

I'm going to be doing a twin on this also hence the concern for the CG. I'll get it figured out some how. Oh, the ARF version is built really bad. This new one has cracks all through the skin which is what the tail structure is constructed of.

Don41

Keep us posted Mike.

I haven't started my Tele40 just yet because I'm trying to finish up the Elder 40 I'm working on. I don't need still another "almost" finished plane (:-)

BMatthews

Reminders and tips. On a T tail setup you want to keep the stabilizer and elevator as light as practical. Weight in the stabilizer acts like a hammer on the fin. A lighter stabilizer greatly reduces the loads on the fin. Hold a hammer and swing it around. Now turn it around so you're holding the head and swing it around. BIG difference. Keep the stabilizer light ! ! ! ! And besides, the stab is now up out of harm's way for the most part. Light doesn't have to mean weak. Good design and wood selection will let you build a very light stabilizer and stabilizer mount that is still plenty strong but is light at the same time. Again making the fin and rudder airfoiled and using a fairly thick % section will also help it to be strong enough to take the loads but still reasonably light.

Do NOT use an S shaped bend in an plastic pushrod to go to the elevator. I did that on one T tail glider and was amazed at the amount of lost travel due to the slop between the inner and outer components. It generated a really large "wandering neutral" due to this lost motion. My last two T tails used an internal bellcrank and vertical pushrod build into the fin and the extra precision in the elevator control was head and shoulders better then the Nyrod method. Yes it's more work but I'll take a bellcrank or transfer lever over tube in tube for a T tail any day of the week. If you MUST use a nyrod like setup then at least miniminse this slop loss by using a very long control horn and servo arm so the nyrod slop is a smaller % of the total motion. I still say that it's better to use a nice pushrod and bellcrank setup as long as you can engineer it to have tight non sloppy connections.

Tall Paul

.
The stresses on the aft fuselage and the vertical require more structure because of the horizontal's position on top of the vertical, and any landing which results in a ground-loop can snap off the vertical due to inertia forces.
There's the monkey-motion connection to the elevator.
Other than looking spiffy, it's more sensible to stay conventional, or go to a vee-tail.
This is a 100" span electric with a flying vee-tail.

MikeFreas

Well I started the tail last night. I really have a hard time believing that some of these ARF’s haven’t come apart in the air with the way it’s built. I ended up cutting more then I thought I was going to have to but there were cracks in all of the balsa sheeting and I also quickly realized that it simply wasn’t strong enough without modifications.

As you see in the pictures it looks bad right now. My plan is to use 3/8 X 3/8 square balsa and run them through the existing hardwood ribs all the way to the landing gear block. I was just going to run them to the second rib in but then the weak point would be where the empennage starts and that’s not good. It will not add much weight but lots of strength and all me a solid place to start. Of course the balsa sheeting will be replaced but no it will not be the only thing holding the rear of the bird together.

The biggest challenge I have yet to solve is the control of the elevator. Someone on RCG warned me against using a flexible pushrod because of the slop. While it would save me a ton of time and heartache I have to agree with him. I’m thinking of two solutions.

1. Using two 90 degree bell cranks I can get the control I need and be able to place the servo in the stock location saving weight on the tail again. Remember, this is going to be a twin engine bird also so I need to make sure I can get it to balance. The only issue I see with this is the space for the second 90 degree bell crank which must go either in the horizontal stab or the rudder, neither have much space.

2. I could use two smaller servos like the HiTec HS-65’s with the carbonite gears. I have to check the specs on these though and make sure they have enough torque to handle half of the elevator each. While this would be easy to do it will add weight to the tail which I don’t want to do if I can help it.

Please keep the ideas and suggestions coming. I’m in completely uncharted territory for my skill level and available tools. I do however like a challenge and love working in the garage.

MikeFreas

After several chimed in here and over at Watt Flyer warning me against this modification I'm re-thinking things. I still want it to look different and not make me loose more hair trying to figure out how to control the surfaces. The V tail option is looking better to me right now. I don't have any experiance with these and would most likely have to completly rebuild the horizontal stab. How do you go about attaching the half's to the fusalage? Should the main spars extened past the last rib to fit into some kind of fitting in the fusalage?

vmsguy

Mike,

I sent you a PM. But while I was typing it, I see you posted.

Man, you're really taking the fuse down....

I still like the twin tail idea myself. With your plane I would.

1. Cut the tail of the fuse completely off after the F3 former. Retain one side for a pattern (more on that later)
2. Since you're planning on using square stock up to the landing gear continue to do so. But also add two on the top of the fuse. These 4 pieces will form the corners of the fuse. Use retained fuselage side to cut them to appropriate lengths. Essentially you are making the back of the fuse look like the kit version, while the front is the ARF.
3. DO NOT join the rear end of the fuse to form a point. Keep them about a inch and a half apart. Why? This will provide a wider base for the stabilizer to sit on. No longer needing the tri-stock on the outside of the fuse. If a twin tail, you can use this square to run the elevator control rod inside. (Look at the Kadet LT-40s. There's a thread where a guy made an LT-40 a taildragger. He also ran a control rod for tailwheel steering.)
4. Now reinforce back of fuse for twist strength as needed.

I think this'll keep your weight down, as well as provide a strong base for the tail.

Good luck...

P.S. I did some searching and found a picture.

http://www.rcuniverse.com/forum/m_99...ilwheel/tm.htm

Post #32. Not exactly the one I remember, but it'll hopefully help with my idea of an open ended fuse.

Don41

Warning you against it?? All the more reason to make the mod and succeed (:-).

I've been playing with the design ever since you brought it up. At the moment I'm looking at rear mounted servos using a pull-pull setup on the rudder and elevator.

I might be dead wrong but my experience with the Senior has lead me to believe there won't be any problem caused by the addition of weight or structure. What DOES concern me is the aerodynamics involved. That is something I'm working on (not very much at the moment).

Don

MikeFreas

Wow...

Yeah I'm changing my mind more then my wife on this.

Bottom line is the tail needed to be redone anyway reguardless of actual configuration.

Unfortunetly the balsa sheeting past the last rib was cracked and then busted by me when I removed the stab. I still have the stock bottom sheeting so I can tell how long I need to make things.

The idea about making the point of the tail slightly wider is a good one and I didn't even think about that.

Yes I plan on running 4 3/8 X 3/8 pieces but I had only cut 2 holes when I took the picture. Combine those with some sheeting in places and it will be very strong.

I have just about given up on the T tail modification only because everyone makes a good point and I'd like to fly this before I move in June. I'm not completly sure what direction I'm going now but I'll get her done.

I did look at the Rimfire line of motors you mentioned in your PM but not close enough. I'll take another look and see if they have one in the "46" glow class.

Yes CG may be an issue with the twin. With my old STM two servos were mounted in the tail and I didn't have any problem getting the CG in with my pack right under the forward wing attach point. If I could get away with not having those servos back there then I should be able to get things in where I need them. Again, I'll wait to glue and mount servos until I know how bad it's going to be. One way I think I could do it is a pull pull system for at least the rudder but maybe both.

dreadnaut

My Feedback: (5)

T- tails were used in commercial jets in the 50's and 60's (B-727, DC-9) to keep the horizontal stab out of the wing wake a high angle of attack situations, such as landing aproach. The number of jets that still use t-tails is dwindling because of the added weight. Notice that most modern commercial jets have substantial dihedral in the horizontal stab as a trade off.

Do the t-tail if you must, but a conventional tail will be lighter.

flyinbr

How are you doing on the T tail?? I did a V tail on mine and it flies very good. Set mine up to tow gliders.

MikeFreas

I finally got time and money and over the past two nights have scratch built another tail this time in two pices. I have the rough sanding done and now it's time to decide at what angle I want the tail to be. I printed a protractor off the net and cut it out to look at some angles. 120 seems a bit steep to me so I went with 130 and cut one of the two braces I need out of ply. Before I get to the point of no return I wanted to get the advise of some folks here and on Wattflyer. If I screw this up I have to start over and I don't want to do that! Here are some pictues of progress so far and my proposed angle.

BMatthews

100 to 120 are popular angles in the sailplane world. 110 is very common.

But it's not so much the angle that is important as the effective areas that the angle produces. But with the tails on the Telemaster things are so big that you don't have much to worry about and "looks about right" will be right enough.

pjwright

I'm in the process of converting a Senior Telemaster from electric to glow, Mike ... which always leads to all sorts of "opportunities" to make other mods. As I grazed the threads on Telemaster mods, one overwhelming point came out: increase the effective rudder area. One point that didn't get much coverage that relates to monkeying with the area of the vertical stab/rudder is covered in Andy Lennon's book on R/C design - keep track of the "center of lateral area". I followed Mr. Lennon's low-tech method of calculating the CLA, and found that the stock design has it about right ... and adding significant square inches with such a long tail moment potentially throws the CLA too far aft ... which in turn increases the potential for unwanted/unpredictable spin entry. I did enlarge the rudder about 75%, but backed off from my sketch plan that added substantially to the vertical stab for purely aesthetic reasons.
Good luck ...
[sm=wink_smile.gif]
PJ

MikeFreas

It's been to long since I updated this so here it goes.


As you can see in the pictures much progress has been made and a solution has been found for controlling the tail surfaces without moving the servos aft. I had a major challange mounting the tail to the fusalage. I only have basic tools (old bent saw, excto knife, and a big jig saw) to cut anything so it makes my cuts look like crap. If you look back in this thread you will see how far the tail was torn apart when I started.

First step was to make the enitre tail stronger by using 3/8 X 3/8 square balsa. I cut holes in each bulkhead to allow these to go from the tail all the way to the fusalage and glued them in. Now that I had a somewhat solid foundation I used some ply to build a blukhead where the forward edge of the tail would sit in the fuslage. From the forward point I have three more V angled bulkheads to support and to bind the tail to the fuslage.

At this point I had the thing glued (30 min epoxy) to the fuslage but it really looked bad so I added some doublers to make sure it didn't depart in flight. I managed to get it less then an 8th off from straight. Yeah, I know it's not perfect but it's the best I could do and I doubt it will make a difference in flight. I did get it to level so that is good.

The tail still had just a slight amount of weakness in the twist department but I knew that would be fixed with the sheeting on the tail. Remeber folks the stock ARF gets all of it's strength from the sheeting itself and it's weak as hell!

Last night I finally got all the sheeting done and rough sanded. I think it looks fairly good right now and is at least 50 times stronger then the stock setup but only slightly heavier.

My main consern was not being able to balance this without the motor on the front. Remember, this is also going to be a twin! Based on experiance with my other ARF's having two heavy digital servos mounted in the tail my battery sits about 2" away from the firewall. Remove the weight of the servos and move that pack forward and I should be good. Well I wanted to see to I strapped my spare wing on to get a rough idea.

Without the motor, ESC, prop, and spinner she balanced about right with my pack all the way to the firewall. Now adding two motors 3-4"s forward will help with this. I also sent my back today back to TrueR/C for him to add a cell. I'm going to run each motor off of a 3s 8000 mah pack.

How to control the tail surfaces was a problem I had been thinking about for weeks. I didn't want to have to, nor could I, mount two servos in the tail because it would never balance. I don't like flexable control rods for big birds so that was out. I was just about to go the way of a pull pull system when I found some high grade bellcranks at the AMA convention. Usually these are used on big 35% birds but will work perfect for my application. All I really need to do is "raise" the output of the servo between 2-3"'s and this will do just that. Pictures will hopefully give you an idea on what I'm talking about. I'm using carbon fiber rods as the input/output for a perfect slop free connection.