vmsguy

Hello all.

I'm currently building a Senior Telemaster from scratch. I intend to build it with a two-piece wing. I have the wing halves done to the point where I need to build in the joining area.

The first three ribs in each wing half are plywood, and there is a hole for the joint tube ahead of the spars. (Picture 1.)

The joint tube will span from inside of rib #4 through the center and out to rib #4 on the other wing half. (Picture 2, at 16-1/2". )

My idea is to cut some plywood pieces to glue to the side of each rib 1 thru rib 4. The 4th rib will have 1/4" thick piece.

I was planning on putting a paper tube in place, and the aluminum tube inside the paper. The paper is only there to act as guide, it really doesn't have any structural purpose.

I did a test drill (Picture 3) And the 1/2" drill bit I used was only large enough to let the aluminum tube pass through, not the paper.

I have a couple questions...

1. Is a 1/2" diameter tube going to be large/strong enough? Or should I use something larger?

2. Should I build it with a paper tube? Or forgo the paper tube all together?


TIA

Lyndon

BMatthews

You're right in that the tube is just a guide. I'd suggest something a hair tougher though so it won't tend to break if you should accidentally lever the rod as you're inserting it. What about a thin wall plastic tube of some sort? Or even K&S brass or aluminium tubing for the guide?

I also have some structural doubts about how you're doing this. Note that the outer ribs are very much cut away on the lower side of the hole while the inner face ribs are cut away a lot at the upper surface. The issue being that these two points where the ribs are the most cut away are the spots that will have the most loading on them. But you can get around this with some suitable beefing up. I would suggest that you pack the area not only with your plywood "washer" discs but also cut up some medium weight fiberglass cloth into short strands of about 1/4 to 1/2 inch long. Mix up the cut stranding with some epoxy to make a very thick glass paste and generously putty it into the root area to further join the tube assembly to the main spar. Coupling the loads from the tube to the main spar as well as possible will greatly remove the load carrying duty from the ribs.

And another hint. Leave or make up the guide tube as one long tube. Slip all the bits into place and block up the wing halves to the right angle and then glue it all together and do the upper side glass fiber paste packing. But leave the root face ribs about an 1/8 inch apart using balsa or ply packing for a cutting gap. This setup will ensure the tube lines up and that the wing panels will slide together with no gap. Saw down through the gap and presto, two wing panels that will fit like they are one.

WHat sort of joiner rod or tube are you planning on using? I sure hope it's an alloy steel rod or tube like chrome moly. Don't even think about any sort of thin wall aluminium or steel options on this size of model. Some years back a buddy used some 3/8 drill rod for a joiner on a glider but didn't heat treat it so it was the same as mild steel. He soon bent it during a fairly gentle winch launch. We got some new drill rod and got it heat treated and that same rod heat treated to a spring temper was able to take full power launches. Your Telemaster will easily have enough G load in a fairly sharp dive pullout to bend any sort of mild steel rod or tube even at 1/2 inch diameter unless it's got a proper heat treating to it. Basically if you can bend it at all by hand or even with the help of a vise it's likely not strong enough. But with a spring temper it'll flex but return to straight.

Deadeye

How big is the plane? For reference, my SIG Somethin Extra uses at least a 3/4 inch tube for a less than 4 foot wingspan. A friend here in my town that kits an Ag Tractor with a 70" span uses 1 1/4 inch aluminum tube.

BMatthews

The Somethin Extra uses aircraft aluminium tubing of large size on a more compact and lighter model. The Telemaster is more about 84'ish wingspan and the wing isn't as thick.

vmsguy

The Senior Telemaster has a wingspan just under 8 feet. All Up weight, I've heard, is anywhere from 8 to 14 pounds.

My original plan is to use 1/2" aluminum tubing. I haven't had a lot of luck finding anything larger at the LHS.

This airplane will also have function wing struts. That should help with any wing stress.

The top of the wing from rib 1 to rib 3 will be sheeted. That should help the strength a little.

There is sheer webbing in first three rib bays. Currently sheer webbing on the back of the spars, but once the tube is in place, the front will be done as well...

I added a couple pictures to explain how I plan to secure the tubing/joint rod in place. I took a piece of scrap plywood, and hand drew a circle on it. I clamped it to the side of the rib. The actual piece will be cut flush with the top and bottom of the rib. Each rib will have one of these on glued to the side of it. I hope this make sense...

I hope this configuration will be strong enough.

BMatthews

Ok, there's a heap of issues here.

First off your use of FUNCTIONAL wing struts totally changes the picture. With wing struts being used you just completely removed ALL bending loads from the center section. So that means all your work as shown is now gross overkill. You could use a bit of 1/4 dowel as a joiner and it would all be fine. That's because the wing struts will be transferring the load down through the struts and putting all the load on the joiner between the two strut mounts on the belly of the fuselage. So I hope you made those strut mounts strong enough to handle a good 1/2 x 6G's worth of pull on each side.

Here's the force diagram of a wing using struts. Note that by forming a triangle with the wing, fuselage and struts you've taken away all but a small remnant of the bending loads in the wing root joint. The wing from the strut point out to the tips will still be a "cantilevered beam" and bend in the normal way. On the part between the strut mount and the fuselage the spar will try to arc upwards but won't bend like a normal wing would bend. However because the strut is on an angle it has to resist FAR more than the simple lift load. If the plane comes in at 10 lbs and you want it to resist 8G's without destroying itself then that's 80 lbs of lift. Call it 100 so there's a 20% margin That means each wing is holding up 50 lbs in a sharp pullup at max G. That's 40 lbs at the strut. But the tensile pull on the struts will be 40/sin of the strut angle to horizontal. If the strut is angled up at 20 degrees from horizontal that means it has to resist 120 lbs of tensile load. So that means the struts need to be made from something suitable and the strut mounts need to be made from something far more resistant than just glueing something to the balsa spars. You'll need plywood spreader plates and then some metal strut mounts that use multiple screws to hold them in proper shear to these plywood spreader plates. Quicklinks for strut ends just ain't going to cut it for this size of model if you find the need to pull up sharply. There's also going to be 240 lbs across the base of the fuselage so the link you use there had better be up to the job as well.

And don't think you can just make the root stronger and assume that it'll take some of the load off the struts. This is an all or nothing deal once you put struts onto the model. There's no middle ground.

So the good news is that your center section joint will be fine as it sits and is actually now overdone. The bad news is that you had better review the material you're using for the stuts and how they mount to the spar and what you're using for ends so you can remove the wing easily. Some more good news is that something like 1/16 music wire will easily resist the tensile loads but you need to ensure that the ends are properly done if soldered and that the screws you use to connect them are something like 10-32 grade 5 or better. Think allen cap here and you'll be fine.

Those numbers sure add up in a hurry don't they? And 6 to 8 G's isn't hard to do at all if you get into a bad dive or spiral dive and suddenly yank back on the stick. Even steep banked turns can generate 3 or more G's so using 8 G's as a max figure isn't out to lunch at all.

Going back to your last post... IF you should decide to switch and go without struts then your 1/2 inch aluminium tubing will not be a suitable joiner tube. Or do you mean as a guide tube? Since you're talking hobby shop tubing I'm going to assume that this is just for the slip guide and you'll get something far more structural for the joiner. For no struts the sheeting on the wing won't do diddly. The sheeting means nothing. Shear webbing helps out where the bending is taking place and that includes the root but it won't help with the joint. And I would suggest you add shear webbing for the whole span and not just the first three bays. The strength the webbing adds is lightyears above the small weight penalty it has. Your ply addons will help but you're still going to need to bond that tube to the spar and since it's out in front of the spar instead of between the top and bottom caps like sailplaners do I suggested the glass fiber "putty". I used this same method on a wing that I retrofitted joiners to and it worked pretty darn good.... at least it hasn't broken yet some 20 years later.... Mind you that one is a 5.5 lb old timer and I fly it in a gentle manner with a high respect for minimizing the G loads.

I hate to sound like the voice of doom and gloome but with bigger models like these we often don't appreciate the numbers involved. The old model methods often need to give way to more full sized techniques. And I honestly feel that this is one of those times. I would suggest that since you have the wings made up already that you make up some extra spar sections WITH shear webs and then mock up some ideas for ply load spreaders and strut attachment points and mount these so that you can make up a stirrup and use your own body weight to load test them to success or desctruction. Find someone that's around 100 to 120 lbs and if your strut mounts and strut designs can hold them up then use it on the model. If you choose to go with a lesser load factor of around 5 (I strongly suggest you don't go any lower or it'll end up raining parts at some point) then do so but fly the model with a light touch and respect for speeds and elevator inputs. No high speed stuff at that design limit.

vmsguy

Thanks for the advice on the struts.

My current strut design idea is as follows.

Both the upper and lower wing spar is 1/2" square material. The lower spars are made from glueing two 1/4" x 1/2" spars together, making for a stronger spar. The wing half has 15 rib bays. In the 8th rib bay there are strut reinforcements in the lower spar. I inlayed a 1/8" ply into the bottom of the lower spar. I haven't yet, but I was also planning on adding another 1/8" ply on top of the spar in that bay. Both pieces of ply span the entire width of the bay. The lower plywood piece would be to prevent crushing of anything I mount/bolt to it. I would drill holes through the ply-spar-ply structure, and 1 or more t-nuts would be secured in the upper ply. The upper ply was also intended to prevent crushing, and to spread the stress to the width of the bay.

I can easily extend sheer webbing out to and beyond the bay where the strut mounts. The first three bays, the sheer webbing is 1/16" ply.

I haven't figured out exactly what material I would use to secure the strut to the wing, but something would be bolted on.

The strut itself would be a harder wood material, maybe birch, probably a thin oval shaped, about 1/2" thick front-to-back, and 1/4" thick top-to-bottom.

The strut mount on the fuselage would be a band of steel or brass going across the width of the fuselage and extending out on each side slightely. In this way, the struts are pulling against each other, and not trying to pull the fuselage apart. See attached picture. This was taken from another Senior Telemaster builder/flyer. It is almost exactly what I plan on doing for my strut/fuselage mounting.

It would seam to me that the struts under the most stress during "lift" operations. Which is good, for under lift operations, the strut will want to be stretched. Which it is the best at resisting. Struts are not as good at resisting compression forces. So any "down" operation on the wing would want to compress the strut. So while a well constructed/mounted strut will be able to withstand the forces of severe "lift" operations, I need to make sure the wing joint will be able to withstand a reasonable amount of compression or "down" scenarios. Like and outside loop, or maybe inverted flight.

BMatthews

Sounds like you're well on the way to getting it right.

While the wooden strut may take the tension if the grain is all running the right way it's so easy to fit some music wire into the wood and run it the whole way that I'd suggest you consider it. Again the wood may take the tension OK but how would you join the connectors at the ends? With a wire core you can easily bend the wire around some sort of bobbin or pass it through a plate at the end and not have to rely on glue joints or pins through the wood.

I'd be a bit worried about trying to inlet plywood into the spar material. Doing so will add in stress riser points at the ends of the inlay. You'd be better off to use something like plywood webbing in those bays and then glue and thru bolt the strut attachment plates or blocks to that webbing and leave the spars alone. But if you've done the inlay already on the lower side then I'd suggest perhaps you epoxy long strands of carbon fiber or kevlar spanwise over the inlay and a good 2 to 3 inches to either side to tie in the spar on either side of the inlay. It's not a good practice to disturb the outer surface of a spar that will be under tension like your lower spar will be. The most highly loaded fibers of the wood in this case are the ones on the very lower and upper surfaces and if you inlay something into them then you've just cut them where it'll do the most damage.

Sorry if it seems like I'm being an obsessive pain and perhaps I'm just not imagining what you did quite right. But it would be a shame to see this thing fold up in the air if it's avoidable

John 38

VMSGUY,
tis a pity you are so far down the line on construction, as I have built a scaled up Mamselle vintage model to 108" span and I have used 1/4" and 3/16" steel rods into brass tubes to join my wings. Tubes and rods extend only to 3rd rib ( about 6" out ). since the tube is only just over 1/4" diameter, it leaves plenty of rib depth to reinforce around tube.

With exception of root rib (1/8" lite ply onto 7/8 white foam ribs ) all ribs are 7/8" white foam with 1/16 ply facing both side of next 2 ribs to strengthen foam around brass tube.

The key part of design is that tubes go thru ribs beteen top and botton wing spars with space infilled between spars aand tube and then vertical webbing either side to close the tube box. this has proved very strong and the fishing trace wire "struts" that I built just in case proved unnecessary.

On a smaller model, I would use carbon rods to keep weight down.

Wings are coming up on 20 years old and still stand continuous looping.

photo of model in my gallery

John