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Old Sep 13, 2012, 11:21 PM
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Carbon caps in place of spruce?

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Originally Posted by fnev View Post
A carbon tube spar system, or any type of tube spar system, will put a lot of stress on the ribs and the external structure as the bracing between the tube and the remaining structure becomes critical.

I don't see a lot of effort involved to just put a normal laminate pine spar as mentioned earlier. Brace the wing properly on a building board (well supported: critical). With a small cutting disc cut the structure along the outer line of the spar you want to put in. If you want a three laminate spar you will need to cut at the three different depths along the span. Then use a router bit and clean the channel for the spar. Build your spar from the bottom up: first the shorter root element, than the medium one and the full span one last. One evening. Let cure.

"De-mold" the wing and repeat operation on the other side. Before you bond in place the spar you will need to work on the shear web. Nothing fancy, but make sure that you tie up the two spars and the wing joiner system. One evening (maybe two if it is pretty involved). Put in the spar system. One evening. Let cure. General cleaning and sanding: ready for covering.

Repeat for the other wing... In this process you can decide if you want each wing in one piece (easier) or two pieces (more involved)... For ease of process I would start with working on the wing bottom: better control of alignment with tabs under the TE and better control of the wash out.
Sounds great, nice solution.
The Carbon tube approach will have a square ply peice with a hole in it This glues to the ribs top anf bottom and buts up on the rear face of the existing front spar web. The tube is glued into these squares and glued to the webs along its length. It should be less disruptive to the existing structure I'm thinking.

By the way, I could come up with a uni carbon spar cap in place of spruce if anyone wishes to go this way. If you reduce the spruce cross sectional area which you can get by mutliplying the width by the depth in the table above then dividing by 15.6 you come up with the carbon cross sections you need. Way way smaller. Make these 3 times as wide as they are deep and web them full width with epoxied end grain balsa and that will work fine and be much more compact. The root joiner receiving tube needs to be well tied in though. Box it in with ply and wrap it with Kevlar thread or a couple of wraps of 2 oz kevalr cloth. I'm very confident about this sort of structure as I have used it a lot on F3x type models.
You can either lay up tapered caps with epoxy and carbon uni (6 to 9oz) or buy the commercial strips and laminate them up.

So there is another option for you.

Allan
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Old Sep 13, 2012, 11:34 PM
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Joiner size for the outer joint

By the way you would need a 0.336 inch diameter solid round carbon joiner at the tip panel to inner panel joint to match that 5G spare if you wanted to keep it a 4 peice wing. Make it at least 4 inches into each panel min. 8 inch long joiner.
Box those receiving tubes in well in the spar webs between the caps and all will be well.
Commercial pull trudes round solid carbon rods whould be available in this sort of size or you can make your own by pulling wetted carbon tows through a suitable aluminium tube then disolving the tube in caustic sodar leaving you with a nice carbon joiner. The same sze al tube can then be used as the receiver in the wing. These are easy really and I have made and used many of them in lots of sizes.

Enough for now,

Allan
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Old Sep 13, 2012, 11:50 PM
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[QUOTE=AllanK1;22729168]Sounds great, nice solution.
The Carbon tube approach will have a square ply peice with a hole in it This glues to the ribs top anf bottom and buts up on the rear face of the existing front spar web. The tube is glued into these squares and glued to the webs along its length. It should be less disruptive to the existing structure I'm thinking.

This approach is really appealing, especially since it would eliminate the need to cut into the wing surfaces for the spar caps, and maybe be done all from the bottom.. If the cf tube will be in the approximate location as the existing 1 1/4" front joiner, will the existing front joiner receiver tube be removed along with the aluminum joiner to allow the new cf tube to go the #1 (root) rib?
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Old Sep 13, 2012, 11:59 PM
Vintage wood is the best!
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I kinda like the carbon tow approach......because.....that wood spar is one big mammber-jammber! Also.....what about using the existing joiner but sleeveing it inside with an additional aluminum tube? You would need to box in the receiver tubes real good and tie them to the new spar.....
Allan....I'm Tom .....the guy from the email conversations.
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Old Sep 14, 2012, 12:16 AM
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I still don't see how the torsional loads are transferred with your tube spar system as you mainly rely on the tube/rib bonding to address this potential problem. Every time you want to alter a structure for strengthening, you need to have a look at the new stress/loads paths that might be created and their consequences. With such design I would be worried of potential wing flutter under aileron deflection as the "D" tube front section and the covering do not look up to the task for proper damping.

As Allan said: this is a fairly large model and the flight loads can be (are) substantial. On a glider of this size and remotely controlled it is very easy to get over 3 to 4 G's add on a 1.5 safety factor and you do need a minimum 6 G's design requirement for positive loads.
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Old Sep 14, 2012, 12:27 AM
Vintage wood is the best!
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One last question......how in the hell are all the existing planes out there still flying? A bunch of these are in Europe and I haven't read of any failures on the European fourms.....
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Old Sep 14, 2012, 01:44 AM
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Quote:
Originally Posted by SZD16 View Post
One last question......how in the hell are all the existing planes out there still flying? A bunch of these are in Europe and I haven't read of any failures on the European fourms.....
Maybe different batches with different manufacturing standards/QA/QC.

As long as you don't go over the ultimate flight load nothing will happen: gentle flying in smooth weather conditions...

BTW a load testing is not "that" difficult to do and will give the necessary "peace" of mind... Or will point out if something might go wrong... This should be mandatory when above a certain size and/or mass aircraft.

This is an other subject all together, but as our models become more sophisticated and with potential much higher performances a minimum should be required to insure the integrity of the structure and flight controls. What's the point to have reliable radios, servos, battery systems, telemetry, etc. if the structure and airframe is not up to the intended task?
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Old Sep 14, 2012, 01:46 AM
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Quote:
Originally Posted by SZD16 View Post
One last question......how in the hell are all the existing planes out there still flying? A bunch of these are in Europe and I haven't read of any failures on the European fourms.....

Just gentle flying and good luck I guess. They may even have different structure I suppose. Once I have some more data on the existing wing I will try and calc the failure load. It will almost certainly be less than 2G. Flown gently with no aeros thay could well survive. Straight and level flight with no turbulence is 1 G.
Allan
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Old Sep 14, 2012, 02:19 AM
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For making a loop in a 1:1 glider you have to dive to 2,5 time the stall speed, and pull 4 G, then it will do a nice loop.
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Old Sep 14, 2012, 04:11 AM
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existing structure

I had a good talk to Woo this evening on the phone. As you have seen, he has his wing stripped too and getting ready to add a tubular carbon spar. Woo like many of you has a natural feel for what is right having designed and scratch built many fine large gliders. His solution to this problem will be good to watch.
Anyway...I do need to allow for the Al tube running spanwise inside the D box. I need some info on its size and material. It is likely doing more for the wing bending strenght than the so called spars.
At worst it will be pure Aluminium which is soft. At best it will be an Aluminium Alloy with a bit of cold work added in the tube forming process which will be twice as strong as the pure AL. Short of a hardness test or chemical analysis we won't know so probably have to assume the worst.

Tom can you measure up this tube in the broken wing? ID and thinkness?

Does anyone know if the there is a tube in the Tip wing panel D Box?

If yes and we can transfer load to it, then it may mean the tip panel is OK.

Allan
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Old Sep 14, 2012, 04:24 AM
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[QUOTE=flyingfever;22729315]
Quote:
Originally Posted by AllanK1 View Post
Sounds great, nice solution.
The Carbon tube approach will have a square ply peice with a hole in it This glues to the ribs top anf bottom and buts up on the rear face of the existing front spar web. The tube is glued into these squares and glued to the webs along its length. It should be less disruptive to the existing structure I'm thinking.

This approach is really appealing, especially since it would eliminate the need to cut into the wing surfaces for the spar caps, and maybe be done all from the bottom.. If the cf tube will be in the approximate location as the existing 1 1/4" front joiner, will the existing front joiner receiver tube be removed along with the aluminum joiner to allow the new cf tube to go the #1 (root) rib?
Reply: I see the existing front AL tube staying. The carbon tube would be added to the rear face of the front spar web and have its own new joiner.
Have I answered your question?
Allan
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Old Sep 14, 2012, 04:35 AM
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Quote:
Originally Posted by fnev View Post
I still don't see how the torsional loads are transferred with your tube spar system as you mainly rely on the tube/rib bonding to address this potential problem. Every time you want to alter a structure for strengthening, you need to have a look at the new stress/loads paths that might be created and their consequences. With such design I would be worried of potential wing flutter under aileron deflection as the "D" tube front section and the covering do not look up to the task for proper damping.

As Allan said: this is a fairly large model and the flight loads can be (are) substantial. On a glider of this size and remotely controlled it is very easy to get over 3 to 4 G's add on a 1.5 safety factor and you do need a minimum 6 G's design requirement for positive loads.
Reply: you are right to be concerned about torsional rigidity. High speed flight can excite flutter if the wing is not torsionally stiff enough. This wing looks like it would be OK to me by gut feel though. It is thick and realatively low aspect ratio. What I do know is that it will be much much stiffer in torsion with the addition of the carbon tube we have been talking about. Even that big wooden spar will help but nothing like the tube.
I need to do some more research on comercially available tubes and their properties. Anyone know a good source? I'll likely not get this sorted untill next week. Woo's tapered wind surfer masts would be great but I suspect they will be expensive.

Allan
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Old Sep 14, 2012, 04:36 AM
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Quote:
Originally Posted by SpeedsterDEN View Post
For making a loop in a 1:1 glider you have to dive to 2,5 time the stall speed, and pull 4 G, then it will do a nice loop.
Yip, would apply to our models too.

Allan
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Old Sep 14, 2012, 07:33 AM
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HMM. A lot of creative thinking going on here. But after reading I have some questions. I read things like "high speed" and "6 g loads" Isn't this a vintage ship designed for low speed and more like 3-4 g loads? And if you need to pull 4 g's to do a loop, by inference there are many examples already doing that all over the world. Might not the one that broke be the exception? An MDM Fox with two up is only rated at +7 and -5.
I think one has to put this in perspective, this is an under engineered product offered a a ridiculously low price considering the shipping costs added to the airframe costs. I think your putting a silver dollar fix on a nickel plane, but none the less its interesting reading, and its nice to see so much interest in helping with a solution.
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Old Sep 14, 2012, 08:07 AM
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Are we now redesigning a slow flying, scale vintage model into an aerobatic Fox? Is the expectation that after the rework to put the plane into speed dives and then execute tight loops, rolls, etc?

If not, I think it will be helpful to agree on what our general expectations are up front before we break out our slide rules and carbon tow.

My own expectation with the K8 is to have a slow flying scale ship that safely tows and flies and comfortably withstands the rigors of normal RC sailplane flight which include all the usual conditions we encounter e.g. the less than perfect tow, landing, etc. I don't expect anywhere near the performance and strength of a fully molded glass slipper.

Do the rest of the K8 owners expect radically different performance?

Regards,

Steve
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