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Author Topic: New Limited Pennyplane (mostly)  (Read 13326 times)
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Olbill
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« Reply #100 on: December 07, 2016, 11:03:40 AM »

The solid spars referenced above are stiffer than .025 tubes but I'm not sure how they would compare to .028 tubes. My first line of investigation would be the attachment from wing posts to spars. If there's any flexibility there the whole wing can twist regardless of how strong the spars are.
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cglynn
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« Reply #101 on: December 07, 2016, 03:00:33 PM »

Thanks Bill.  I thought that too after the first flight, so checked the post to tube fit.  No slop in the joint or the fit so that was not the culprit. 

I am still thinking it is the spars.  When I lay the model upside down resting on its tip plates (not going to lie, it is a close version of your design) the wing deflects about .75" without a rubber motor.  Add ~2g of rubber to the overall weight and I just don't think they are sturdy enough.

I had thought about adding struts or more ribs, or both to help with wing twist.  Adding ribs wouldn't be an issue as the model ended up 400mg underweight, so could stand to gain a few mg's.  Struts would most likely stiffen the wing more, but in my mind would defeat the purpose of using the .028" carbon, and that is to keep the drag to a minimum. 

Also, in an attempt to reduce drag I tried a thinner foil section, which didn't fly for anything.  Using the same motor and prop I was flying on last season's model, same torque and turns, I was only doing half the time.  So need to rebuild the wing with the usual 4-5% camber, instead of 2.5%.  Bad choice, and I payed for it with time and OS film.

I still have the .028" tubes, and am going to try them in a 4.5% cambered wing.  I think the 2.5% wing just needs to fly too fast to make lift, which requires more torque, which is putting undue stress on the wing, and the whole thing ends up in a death spiral, literally and figuratively.

I am not keen to give up on the .028" tubes yet.  I do want to see if they actually are strong enough as they are very easy to build with and could possibly offer a slight advantage in drag reduction.  Even if they don't, I can definitely see why you like carbon for wing spars.

Chris
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Olbill
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« Reply #102 on: December 07, 2016, 03:43:21 PM »

It's the tube attachment to the spar that is critical. You don't want any bending in that joint.

I know I harp on this a lot these days. I just think it's a source for a lot of problems in a lot of different kinds of models. I think it may have been the culprit in a lot of flyability problems that I've had in the past. F1L in particular comes to mind.
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Tapio Linkosalo
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« Reply #103 on: December 08, 2016, 01:33:21 AM »


I suppose when Chris mentions putting the model upside down resting on the tip plates, the deflection he observes is indeed bending the spar, as in that position there should not be any load on the spar-to-post joint.

I wonder if making carbon-balsa-carbon sandwich spars would make a lighter and stiffer wing than solid carbon tubes or rods?

 
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Olbill
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« Reply #104 on: December 08, 2016, 09:26:29 AM »


I suppose when Chris mentions putting the model upside down resting on the tip plates, the deflection he observes is indeed bending the spar, as in that position there should not be any load on the spar-to-post joint.

I wonder if making carbon-balsa-carbon sandwich spars would make a lighter and stiffer wing than solid carbon tubes or rods?

 

Yes - I was referring to the flyability problem. As far as the stiffness of the spars goes, I use an .026" tube for the LE spar on my F1M's and have no problem with it.

Sandwiched spars could be stiffer but for LPP I like the solid carbon rectangles. For LPP the weight is not much of a problem and they are very convenient to use.
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mkirda
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« Reply #105 on: December 08, 2016, 09:35:51 AM »

I wonder if making carbon-balsa-carbon sandwich spars would make a lighter and stiffer wing than solid carbon tubes or rods?


I think the answer to this is "Of course". With the Prepreg carbon coming out of Europe, I think an ideal carbon/balsa/carbon LPP spar might be something like 0.065" thick center tapered to 0.045" or so, with one layer over the entire length and possibly one more on the top for the center 5-6". The layup under vacuum and heat would be simple. The hard part is cutting to the right width. Tapered width might also be attractive.
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Olbill
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« Reply #106 on: December 08, 2016, 10:41:51 AM »

But for LPP would this have any advantage over solid carbon spars? Other than learning some new skills I can't see any.
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Tapio Linkosalo
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« Reply #107 on: December 08, 2016, 12:23:42 PM »

I think the answer to this is "Of course". With the Prepreg carbon coming out of Europe, I think an ideal carbon/balsa/carbon LPP spar might be something like 0.065" thick center tapered to 0.045" or so, with one layer over the entire length and possibly one more on the top for the center 5-6". The layup under vacuum and heat would be simple. The hard part is cutting to the right width. Tapered width might also be attractive.

The CCT pre-preg - at 19g/m2 - would IMO be an overkill for PennyPlane. I molded some F1D spars from it, and the weight is only slightly higher than balsa-boron. And that was pre-cured carbon, which I glued on with some extra epoxy. Once I master using the pre-preg, I think the weight would be the same as for boron reinforced wings.

From some sources (at least R&G in Germany) you can also get 31g/m2 spread-tow tape (same material as used for the 60 gram cloth commonly used in light, hi-tech constructions), which to me seems ideal material for carbon-balsa-carbon spars for PennyPlane or F1M. So far I have only used it to mold wing pylons for F1D, and honestly feel that carbon is not needed for wings spars in F1M, but if I'd choose to build a carbon wing for one, that is the material I'd use.
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cglynn
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« Reply #108 on: December 08, 2016, 12:29:43 PM »

I would think that while super cool, the sandwiched balsa/carbon spar would add an unnecessary level of complexity to LPP.  

Bill, I will have to continue working with the .028" tubes.  It was suggested that I add a few inches worth of boron to the top and bottom of the tube to stiffen it.  I am thinking of going that route, or leaving the spars as they are, and trying a few other things.  One, I am going to make my tip plates taller.  When designing the model, I was trying to reduce drag to an absolute minimum.  I may have went too short on the tip plates, so no enough effective dihedral.  Also, I am wondering if the stability issues weren't because of CoG placement.  The model was balancing around 75% of the leading edge without rubber.  

Next one will get taller tip plates, more camber, and a slightly more forward CoG.  I will also build another wing with the .028" tubes and top and bottom borons to compare.

Thanks
Chris
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leop
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« Reply #109 on: December 08, 2016, 07:55:21 PM »

Bill is correct in suggesting using the 1mm (0.040") tall carbon rectangle shapes.  The 0.7mm OD carbon tubing has but 34% of the bending stiffness of the 1mm (x 0.4mm) carbon rectangle shapes.  Adding 0.004" boron on the top and bottom of the tube will not get one to the stiffness of the 1mm shape.  I use 0.004" boron on the 0.063" (1.5mm) thick balsa leading edge spar on many of my LPP's.  This makes for a spar that is about the same stiffness (or a bit less) as Bill's LPP carbon wing spars.  My latest design LPP's had a terrible dive problem when upset or steered (leading to much laughing by the spectating crowds).  Adding boron to only the leading edge wing spar solved the problem.

LeoP
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cglynn
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« Reply #110 on: December 12, 2016, 12:31:57 PM »

Time will tell if this works or not, but I put 6" lengths of boron on the upper and lower tangent of the .028 tube, LE and TE spar.  Weight gain was negligent.  Using the .028 tubes with the boron, covered with OS film, the finished model needs about 300mg of clay to make the minimum weight.  I would rather have the weight in the structure, but now I guess I will be able to really play around with CoG by moving the ballast.  I am flying again this weekend and will report back with my findings, then most likely hopping on the 'puter to order a bunch of .040" rectangles from CST.  Even if the .028 tube with the boron works, the .040 tubes while possibly heavier, have got to be easier to build with than mucking around with boron.

CG
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