Author Topic: LAB (Life After Balsa) (Read 9945 times)

Steve Berry · « **Reply #50 on:** November 07, 2021, 07:04:47 AM »

Quote from: Mike Alimov on November 06, 2021, 10:16:05 PM

Steve, the completed airframe is shown in Reply #3 of this thread. A fully painted version is yet to be built.

Yeah, apparently I skipped right over that. My bad. It is a good looking aircraft.

I, too, would like to see how the blue foam gets prepped and such. This is such an informative thread, with clear concise documentation. Thank you for doing it.

Mike Alimov · « **Reply #51 on:** November 07, 2021, 08:07:16 AM »

OK, let's talk about foam.
First, a bit of theory. The purpose of the layer of core material in a composite structure is to create spatial separation between the outer and inner fibrous layers, and to take up the compressive load when the finished structure is being stressed. The thicker the core layer, and the higher compressive rating (measured in psi) it has, the stronger the whole composite sandwich will be.
However, for our applications, there are limitations, and some compromises have to be struck.
First, the thickness: the thicker the core, the more it will weigh, no matter the material (foam, balsa, etc). Also, thicker material is harder to bend to small radii which are found in places like the trailing edge, some wingtips, etc. If the core can't bend, it will crack, and we generally don't want that. I empirically determined that the foam thickness of about 1/16" is about right for my choice of material and my layup schedule (i.e. the number and gage of fiberglass layers).
As it turns out, not all foams are created equal. There are different types of foams (XPS - eXtruded PolyStyrene, EPS - Expended PolyStyrene, polyurethane foams, expanded polymethacrylimide (PMI), and many others), and then there are different densities within the same type. Everyone knows the large pink foam sheets made by Corning and sold at Home Depot, but did you know that they come in different grades? For most people the R-value (thermal insulating value) of the foam is most important, but for us it is density and compressive strength. Here's a table detailing different grades of Corning Foamular pink XPS foam:
https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/EIS/10015703/10015703-FOAMULAR-SI-and-I-P-Units-for-Selected-Properties-Tech.-Bulletin.pdf
I had a sheet of blue XPS foam from Lowes that I bought long time ago, and it is similar in properties to the Foamular 250, which means it is a 25 psi foam with a roughly 1.5 lb/cu.ft density. Read that again: 1.5 lb foam - can you find balsa this light? And if you did, how much would you need and how much would it cost?
When I sliced half of this 4'x8'x2" board into 3/32" slices the other day, I got over 60 wing panel -sized sheets. The entire board costs about $35 plus tax. Not bad.
Here's another interesting fact that I've learned from the Corning foam table. While the foam density (lb/cu.ft) increases by only 16% when going from Foamular 250 to Foamular 400, the compressive strength goes up by 60%! Which means it is more benefitial to use stronger foam. If you can find it, that is. Regular home improvement stores don't carry those foams, they are considered specialty material and are sold by specialty stores, and cost quite a bit more.
Don't even get me started on specialized brand name foams like Rohacell, Herex, Airex, etc. A 2'x4' sheet of 3mm (~1/8") Rohacell is listed at $99 in small quantities. Some of these are difficult to find, too, and I've never seen them in thicknesses that would work for us (1 - 1.5 mm). If you have a source, let me know.
Anyway, once you hotwire your blue or pink foam to about 3/32" thickness, it comes out with a layer of crust (melted foam on the surface) covered with long whiskers. Both the whiskers and the crust need to be sanded off.
Here Gabe is sanding away using a cheap Harbor Freight sander.
I highly recommend doing both the hotwire cutting and the sanding outside (pick a nice day), if you don't want to be evicted by your wife or develop some disease as a result of exposure to toxic fumes and foam dust.

I will continue when I have more pictures.

Mark wood · « **Reply #52 on:** November 07, 2021, 08:27:57 AM »

Quote from: Mike Alimov on November 07, 2021, 08:07:16 AM

OK, let's talk about foam.
First, a bit of theory. The purpose of the layer of core material in a composite structure is to create spatial separation between the outer and inner fibrous layers, and to take up the compressive load when the finished structure is being stressed. The thicker the core layer, and the higher compressive rating (measured in psi) it has, the stronger the whole composite sandwich will be.

Nice job explaining the purpose of the core. Balsa simply would be a waste used in a core. The reality is that by the time you bulk up the section to handle the compression load using balsa for our models, the balsa will handle the load negating the need for the carbon. Well, up until you scale up to wind turbine blade size that is.

pmackenzie · « **Reply #53 on:** November 07, 2021, 08:29:10 AM »

CST has Rohacell in 1mm thickness

https://www.cstsales.com/a-rohacell-ig-51.html

pmackenzie · « **Reply #54 on:** November 07, 2021, 08:31:33 AM »

Quote from: Mark wood on November 07, 2021, 08:27:57 AM

Nice job explaining the purpose of the core. Balsa simply would be a waste used in a core. The reality is that by the time you bulk up the section to handle the compression load using balsa for our models, the balsa will handle the load negating the need for the carbon. Well, up until you scale up to wind turbine blade size that is.

Pretty sure the Yatsenko models use balsa cores.
Balsa is a very hard material to replace

Mark wood · « **Reply #55 on:** November 07, 2021, 09:00:01 AM »

Quote from: pmackenzie on November 07, 2021, 08:31:33 AM

Pretty sure the Yatsenko models use balsa cores.
Balsa is a very hard material to replace

That simply adds weight as the carbon carries all of the load. All the core does is keep the carbon in column. Without a core the carbon would buckle under compressive loads. It takes a working knowledge of the Euler buckling equations to fully understand.

pmackenzie · « **Reply #56 on:** November 07, 2021, 09:11:26 AM »

From what I have seen Yatsenkos are glass/balsa/glass, not a lot of carbon in them.
The main thing that balsa will provide over foam is ding resistance, since that mainly comes from the core material compressive strength due to the thin composite layer.
Rohacell is better than the extruded PS foams for this.

Avaiojet · « **Reply #57 on:** November 07, 2021, 04:33:07 PM »

If I was going to build a model using no balsa, I would have a conversation with this guy. Istvan Travnik, and from 2012!! Wow! does he ever have experience.

https://stunthanger.com/smf/building-techniques/the-bluefoam-model-or-how-to-build-a-stunt-plane-without-a-bit-of-balsa/

Remember him? I'm sure many do.

Nothing like being ahead of the wave with experience, and this guy has plenty of it.

No secrets, BS, dead ends, or searching for articles either, he spells it all out in a great photo build.

And for those two individuals who will attempt this masterful craft, he can be PM'ed.

Mike Alimov · « **Reply #58 on:** November 07, 2021, 07:34:10 PM »

Quote from: pmackenzie on November 07, 2021, 09:11:26 AM

...
The main thing that balsa will provide over foam is ding resistance, since that mainly comes from the core material compressive strength due to the thin composite layer.
...

Bingo! Exactly! Balsa core is used to increase dent resistance, which is a purely utilitarian consideration, not a structural requirement. Stunt people are obsessed with shine and good looks, always have, always will. Sometimes to the detriment of the plane's flying ability (16 oz shiny paint jobs, anyone? I'm not making this up).

The beauty of our class (F2B/stunt) is that we are not competing for the fastest / biggest / strongest / highest / longest-flying airplane (insert any other superlatives here). If we were, then all stops would come out, and I would be using the highest grade of materials possible - carbon, honeycomb or Rohacell, etc, and so would everyone else. Look at all free flight classes, F2C team race, and RC pylon racing - there is not much balsa wood being used there (that is to say, effectively none). But for stunt we don't need the plane to be the most-est in anything (strength, weight, etc) - we just need it to be light ENOUGH and strong ENOUGH to take us through a pattern, and dent resistant ENOUGH to handle a few seasons' worth of competitions and practice and still look pretty good.

As a result, we have the luxury to choose from several technologies available to us. The most established one, of course, is a built up balsa frame finished in tissue and dope. Then there is the sheeted foam, now with epoxy finish. And the composites of different kinds.
The esteemed brothers Yatsenko have set a pretty high bar when it comes to quality and durability of their equipment. But their method and choice of materials can hardly be considered accessible /attainable by an average home modeler, or else everyone would be copying this formula.
When I was developing our composite methods, one of may goals was to keep the entire process attainable as much as possible. That's why carbon and Rohacell were not in consideration. My foam sheets end up costing pennies per sheet (not counting time/labor, of course), and 1.4 oz E-glass ended up being pretty close to a $1/yard, not much more than toilet paper.
To compensate for the lack of compressive strength in XPS foam, I simply doubled up the outer layer of glass - again, not because a single layer would not be strong enough for typical aerodynamic loads (it is), but purely to increase dent resistance.
So, there you go - it is possible to achieve about the same result using a combination of thin glass (3/4 oz?) with balsa or Rohacell, or thicker glass (1.5 - 3 oz) with XPS foam. I hope this does not devolve into another "what's better- pipe or 4-2 break engine" argument - as it is apparent that more than one composite layup formula can be viable for our application.

Lauri has mentioned something that I also wanted to address - that balsa core composite wings are more rigid, and how apparently it is better. Is it, though? Yes, when I finished the first composite wing (with only a single spar at the main location), the rear part of the wing felt like it would have a little play if subjected to mild vibration (and vibrations are present in all airplanes, including electric). Then I remembered that back in the ole days when everyone flew combustion powered airplanes with soft wing covering (tissue, Monokote, etc), we didn't have to worry about installing vortex generators, trip strips and the like. The slight vibration of the wing surface was doing a good job shaking the stagnant boundary layer off our wings and keeping the plane tracking very predictably (Mark Wood had a separate thread going for that effect). When the rigid foam wings came, and people learned how to create those stunning 20-point mirror finishes, combined with a smooth electric powertrain, we started seeing more and more of these devices on wings and tails.
So I decided to skip ribs and secondary spars of all kinds, and let the back of the wing vibrate a little bit.
We have not had a need to install any vortex-generating devices on the Ripslinger, and it is tracking pretty well. Coincidence?

Mike Alimov · « **Reply #59 on:** November 09, 2021, 09:36:56 AM »

It is time to put some controls in that new shiny composite plane. As has been discussed here in great detail (and pretty much agreed on - a rare phenomenon on this forum), electric-powered planes have an even higher need to have smooth controls, as there is no vibration to shake them loose.
Here's how I make controls these days - maybe not the best way (I welcome any suggestions on how to make it better), but accessible to everyone, and using widely available materials and tools.
Start out by buying an electrical connection block at Lowes. Using side cutters, detach one segment, remove two retaining screws, and free up the metal insert.

Mike Alimov · « **Reply #60 on:** November 09, 2021, 09:40:18 AM »

Secure the metal part in Vice-grips and cut in half with Dremel tool. Additionally, I mount the newly formed piece on a long M3 bolt (that's metric 3mm thread, folks) and grind a little more to remove burr and give it a more slender and noble shape.

Mike Alimov · « **Reply #61 on:** November 09, 2021, 09:50:05 AM »

Pre-cut 1/8" music wire to appropriate lengths as needed for your design. I chuck each piece into my little lathe (a drill press will also work) and polish the surface starting with 600 paper and progressing to a polishing compound. This surface will eventually ride inside nylon bearings. Do this step for both the flap and elevator horns.

Obtain the following supplies:
- long (40-50 mm) M3 screws; I recommend plated, not black oxide, as they solder much easier;
- 3 mm horn bearings from Hobby King (they call them Unilateral Hinge 5x3x20);
- a block of nylon. Scraps from machine shop.

The horn bearings will need to be drilled out to accept 1/8" music wire. Use index drills and pick one that provides for free rotation but no slop.

Mike Alimov · « **Reply #62 on:** November 09, 2021, 09:59:43 AM »

Make the first 90 degree bend in the horn wire, using your favorite wire bender. Then slide the metal joiner and the nylon hinge(s) in whatever order makes sense for your airplane. My pictures reflect a horn being made for a profile plane, hence the horn will be offset. In a typical full body stunter, the horn will be centered and flanked by the two nylon hinges.

Using the same cut-off wheel on Dremel tool, grind a little flat on the horn wire in such a way that the screw, when driven into the brass joiner and positioned over the flat, locks in a vertical position. If a little off, re-grind the flat at a corrected angle. Be careful not to grind too deep into the wire, no more than 1/4 of the wire diameter.
When satisfied, apply soldering flux designed for steel alloys and flow some solder, locking the screw in place. A little solder should flow out of the brass joiner and along the music wire.
Let cool and clean the flux off with baking soda and water.

Then mark and make the second bend (making sure that all your bearings are installed first!)

Mike Alimov · « **Reply #63 on:** November 09, 2021, 10:23:43 AM »

The last step is to make a small nylon block with three holes: one tapped for M3, and two others drilled for the diameter of your pushrod wire (I recommend 3/32" music wire, with polished ends of course).

The elevator horn can use a piece widely known in the RC community.

The best feature of these horns is that they provide a very fine adjustment of the arm length: unlock the pushrods, turn the piece 180 degrees (half a turn) and get a 0.25mm (approx. 0.01") height adjustment. Fly and try - if you want more, turn more; if you want to go back, simply go back 1/2 turn. Predictable and reliable.
Here are some benefits compared to traditional horn construction:
- does not require brazing;
- no metal-on-metal friction points that require lubrication, all pivots are metal on nylon;
- very fine and repeatable adjustability.

Mike Alimov · « **Reply #64 on:** November 09, 2021, 10:38:25 AM »

Our bellcranks are made out of 1/8" phenolic board (bakelite? garolite?). Earlier versions had a simple hole for the axle (1/8" music wire), while the Ripslinger was the first plane to use a bellcrank suspended in ball bearings.

That's pretty much all there is to the controls. Let me know if you have any questions / comments.

Brent Williams · « **Reply #65 on:** November 09, 2021, 09:57:52 PM »

Have you had any fear of failures with those dubro clevises?

Monty Sumach recently lost his beautiful ship to the elevator clevis opening up.

Mike Alimov · « **Reply #66 on:** November 10, 2021, 08:46:31 AM »

Quote from: Brent Williams on November 09, 2021, 09:57:52 PM

Have you had any fear of failures with those dubro clevises?

Monty Sumach recently lost his beautiful ship to the elevator clevis opening up.

Brent, no, I'm not familiar with Monty Sumach, his airplane, or the circumstances of this accident. So, after hundreds of trouble-free flights using these Dubro kwik-link clevises, I was living without fear...
Please tell us more details: was it a control line or RC airplane, combustion or electric powered (I think I see a pipe brace in the fuselage belly?), was the brass locking plate present in the clevis when first discovered? Etc...

And even if the community agrees that this is not the best clevis to use, we can switch to an all-metal Sullivan version, or a single-sided nylon block similar to the one we use on the flap horn...

Brent Williams · « **Reply #67 on:** November 10, 2021, 09:07:26 AM »

If you're on facebook, the Alberta control line page has the details of the crash. Monty's plane was a large, piped, control line stunt scale ship. From his report, it had a persistent vibration which possibly led to the pin backing out.

Perhaps the reduction in overall vibration in electric planes would make clevis type connections more trustworthy.

Mike Alimov · « **Reply #68 on:** November 10, 2021, 03:03:21 PM »

Brent - I would never recommend using RC clevises in internal combustion planes; that's a disaster waiting to happen.

Lauri - very interesting and valuable input. I could simply insert a partial-span foam spar about halfway between the main structural spar and the trailing edge, and that would stiffen up the back of the wing. Otherwise, as you noted, the thicker foam core makes up for the lack of compressive strength, and the wing is as strong as anything else out there.

Great discussion, guys.

Howard Rush · « **Reply #69 on:** November 12, 2021, 07:38:47 PM »

Quote from: Mike Alimov on November 07, 2021, 07:34:10 PM

Bingo! Exactly! Balsa core is used to increase dent resistance, which is a purely utilitarian consideration, not a structural requirement.

The core might be taking some shear. I'd have to check with the stress man.

Quote from: Mike Alimov on November 07, 2021, 07:34:10 PM

To compensate for the lack of compressive strength in XPS foam, I simply doubled up the outer layer of glass - again, not because a single layer would not be strong enough for typical aerodynamic loads (it is), but purely to increase dent resistance.

I made some Kevlar-Foamular-Kevlar shells that were kinda dent prone. I put some .5 oz. carbon mat on the outside surface. It made a nice surface finish and helped the dent issue. I used Epon 815 epoxy. Some higher modulus epoxy like that in prepreg might also have kept it from denting for less weight.

Quote from: Mike Alimov on November 07, 2021, 07:34:10 PM

Lauri has mentioned something that I also wanted to address - that balsa core composite wings are more rigid, and how apparently it is better. Is it, though? Yes, when I finished the first composite wing (with only a single spar at the main location), the rear part of the wing felt like it would have a little play if subjected to mild vibration (and vibrations are present in all airplanes, including electric). Then I remembered that back in the ole days when everyone flew combustion powered airplanes with soft wing covering (tissue, Monokote, etc), we didn't have to worry about installing vortex generators, trip strips and the like. The slight vibration of the wing surface was doing a good job shaking the stagnant boundary layer off our wings and keeping the plane tracking very predictably (Mark Wood had a separate thread going for that effect). When the rigid foam wings came, and people learned how to create those stunning 20-point mirror finishes, combined with a smooth electric powertrain, we started seeing more and more of these devices on wings and tails.
So I decided to skip ribs and secondary spars of all kinds, and let the back of the wing vibrate a little bit.
We have not had a need to install any vortex-generating devices on the Ripslinger, and it is tracking pretty well. Coincidence?

A local guy puts more VGs on his open-bay Flaming Dog Head airplanes than on his all sheeted ones. I'll set him straight.

Doug Moon · « **Reply #70 on:** November 15, 2021, 02:48:55 PM »

Can you show the inside of the wing and how you set that up? Can you show how you joined the two shells together?

Mike Alimov · « **Reply #71 on:** November 15, 2021, 04:36:57 PM »

Quote from: Doug Moon on November 15, 2021, 02:48:55 PM

Can you show the inside of the wing and how you set that up? Can you show how you joined the two shells together?

Hi Doug,
here are a couple of shots of the inside of the first wing prototype (the plane is in storage now, hard to get to), which is pretty close to the final version. Not much inside the wing - primarily a spar made out of 1/8" balsa - we may be replacing that with a composite sandwich in the future. Besides that, there are 3 balsa inserts about 1"x1" in the trailing edge for hinges, and the take apart hardware (the root rib has cut outs for that), both not seen in this photo. The take apart hardware will be specific to your chosen system (if you go that route); otherwise just glue a couple of basswood or plywood joiners and glue the wing halves together.
As you can see, there is only one spar - no further structural elements. I didn't see a need for those. I'm being told otherwise (not for structural, but for aerodynamic benefit). I have a feeling that only those who consistently fly 580+ point patterns will feel the difference.

I don't have pictures of how two shells are being glued together. You will need to wait until our winter building cycle begins, which hopefully won't be long now. Gabe might even upload a few videos to his YouTube channel, since some things are easier to show/explain live rather than in a pictorial.

Doug Moon · « **Reply #72 on:** November 16, 2021, 11:34:02 AM »

Looks great!

Thank you

Donald Main · « **Reply #73 on:** November 21, 2021, 06:38:17 PM »

I just read thru the whole thing and enjoy everyone's hard work. Your dedication is awesome and your kindness to share shows how humble you are. I know Gabe and Luca will keep excelling for a long time into the future. All three of you are ambassadors for our sport. Thank you.

Dennis Toth · « **Reply #74 on:** November 22, 2021, 06:12:43 AM »

There's a material call Depron foam that is a rigid sheet, sandable, light and in 1 - 4mm thickness. Midwest use to carry it, It is the material that the foam dinner plates are made of. I have used it for small catapult gliders, it glues well with Elmer's and takes waterbased acrylic paint well. This type of foam could be used for ribs and wing sheeting with a good I beam type spare. Problem is I can't find a supplier who has any in stock.

Best, DennisT

Mike Alimov · « **Reply #75 on:** November 22, 2021, 09:25:35 AM »

Don - thank you for the kind words. We always enjoy your company at the contests, and look forward to flying with you in the future. It is my intention to share whatever I know with the community, as a way to give back for all the years that I've been on the receiving end of mentorship and help.

Dennis - yes, I was aware of Depron, and have a few sheets. However, I did not pursue this material for the same reason you mentioned: it is simply not available in the US. I just checked, and the two top sites (RCfoam.com and RCdepron.com) do not have 1mm foam. Other thicknesses are not acceptable due to their increased weight and stiffness.
So, until/unless someone decides to sponsor us by shipping a few dozen sheets of 1mm Rohacell or Airex, we will continue to use the blue (pink?) insulation foam.

Mark wood · « **Reply #76 on:** November 22, 2021, 12:19:16 PM »

Quote from: Lauri Malila on November 22, 2021, 09:45:36 AM

Mike,
While I think that nothing serious can be made from Depron, its original use is an insulation layer under those cheap laminate parquets. So maybe a good source would be someone selling/installing floors.
As main spar I'd like to see something better, a proper I-beam. So, 3-4mm vertical grain balsa core, perhaps secured with +/-45 degree thin glass on both sides and good hardwood or carbon strips as spars. L

Well, I would debate that some. The trouble with this structure, which is actually holding up, is from buckling and not shear. While vertical grain wood would be good the horizontal grain is likewise adequate provided the intergranular shear strength isn't exceeded. Since there has not been a failure indicating a short fall, increasing the difficulty of manufacture and weight isn't directly warranted. From reading the only "problem" is some buckling causing some profile deformation. It doesn't take much to prevent buckling provided the section modulus is great enough. Thin carbon won't do that. It will carry lots of tension but by the time you fixate it for the buckling the need for the carbon no longer exists or there is far more carbon and weight than necessary. Wood and fiberglass have similar stiffness so if I were going to add a spar cap, it'd be spruce but the if part of this is that I probably wouldn't, that balsa shear web spar seems be enough and if'n I was worried about it, I'd simply switch from 1/16 to 3/32 and head down the road. Think of it in terms of a stress skin structure. Keep the skin in alignment and all is good.

Of course there is always the coolness factor and carbon fiber certainly fills that requirement.

Mark wood · « **Reply #77 on:** November 22, 2021, 01:41:57 PM »

Quote from: Lauri Malila on November 22, 2021, 12:53:46 PM

Mark,
I’ve gone through that development from the very first Shark versions (foam or lenghtwise grain balsa spar, foam ribs) to the latest versions (as I explained in previous message).
The first ones were really annoying, a cracked rib or spar leads to a structure that does not work correctly, ALSO with this kind of monoqoque structures.
But I must add that my experience is with IC engines, with electric it may be less sensitive.
Anyway, they are details that are difficult to fix afterwards, so
it’s better to make it right in the first place. And it’s really not that much more work.
Another thing is that in a take-apart model, the take-apart components in wing are easier to anchor to a good spar(cap) than just to skin. L

Well, Lauri, I can't disagree with that. I went away from vertical grain webs in almost all of my designs as the shear in the web isn't all that high. I also don't do monocoque structures as my stress analysis skills and patience aren't up to the task. I prefer a D Tube I beam spar design as it is easy for me to analyze. Of course I don't do that on models as they are kind of a known quantity to plagiarize. For models my preferred wing on CL models is a hybrid between the Nobler style and how Yuriy builds the classic wing although I wasn't aware of his construction before making mine. The web I use is more for a construction alignment aid than a full structural member. Being a long time free flight builder I am hyper aware of weight and challenge every decision when adding structure. Carbon fiber is great, however it is 15-20 times stiffer than fiber glass or balsa wood and much more dense than balsa which means that, in order to satisfy Euler enough of balsa or foam needs to be used or a section of carbon weighing more than the balsa needs to be used. Generally by the time you satisfy Euler, Poisson is no longer in the neighborhood when using balsa.

I'm not saying it is a bad idea as there are plenty of examples of it working and most of my FF models include CF hybrid spars which are joined to CF D tubes and CF capped ribs. As you know from your engineering background that the trouble begins when you mix materials of differing stiffness. That's my point primarily.

Howard Rush · « **Reply #78 on:** November 23, 2021, 04:19:34 PM »

Quote from: Mark wood on November 22, 2021, 01:41:57 PM

Carbon fiber is great, however it is 15-20 times stiffer than fiber glass or balsa wood and much more dense than balsa which means that, in order to satisfy Euler enough of balsa or foam needs to be used or a section of carbon weighing more than the balsa needs to be used.

Another story: I was building Snorts awhile back and wondering if a 1/4" carbon spar cap with staggered plies would suffice. A structures guy (nobody you know) was at the house, and I asked him. I knew that 1/4" square spruce tapering to 1/4" x 0 worked, so we looked up some parameter of spruce (I forget what) and decided that the carbon strip had the same amount of it. I made the spar and something seemed wrong. I went back and looked at the book and noticed the "x 10³" at the top of the spruce column. I ended up with carbon-honeycomb-carbon spar caps. This was far from the goofiest Snort issue.

Mark wood · « **Reply #79 on:** November 23, 2021, 06:33:07 PM »

Quote from: Howard Rush on November 23, 2021, 04:19:34 PM

Another story: I was building Snorts awhile back and wondering if a 1/4" carbon spar cap with staggered plies would suffice. A structures guy (nobody you know) was at the house, and I asked him. I knew that 1/4" square spruce tapering to 1/4" x 0 worked, so we looked up some parameter of spruce (I forget what) and decided that the carbon strip had the same amount of it. I made the spar and something seemed wrong. I went back and looked at the book and noticed the "x 10³" at the top of the spruce column. I ended up with carbon-honeycomb-carbon spar caps. This was far from the goofiest Snort issue.

There have been some bad lessons learned in the 4/4 aerobatic world trying to use carbon with spruce in spars. There are a few that have succeeded and a few that haven't. The problem is that carbon is so much stiffer than spruce that it will carry all of the load and the spruce won't carry any. When a load is applied to the wing it bends and the stiff carbon doesn't stretch much which creates a large shear force along the bond interface and eventually tears loose and the wing fails. It's kind of one of those get all the way in kinds of things. By the time you get the carbon to where it can manage the buckling spruce becomes far to heavy of a core as it doesn't carry any of the load. Foam and honeycomb are much better core materials than spruce for carbon.

Howard Rush · « **Reply #80 on:** November 23, 2021, 06:58:35 PM »

I guess I wasn’t clear about the spruce. Some guys just can’t tell a joke.

Mark wood · « **Reply #81 on:** November 23, 2021, 08:06:50 PM »

Quote from: Howard Rush on November 23, 2021, 06:58:35 PM

I guess I wasn’t clear about the spruce. Some guys just can’t tell a joke.

Yeah, it doesn't come through on the typewriter...

Cool spar cap though

Mike Alimov · « **Reply #82 on:** November 24, 2021, 11:04:40 AM »

I think we should be guided by theoretical engineering, but the limitations of practical model building sometimes alter the choice of materials, as I illustrated with foam. Maybe Euler and Poisson would demand that I use Rohacell at $50 per sheet, but my wife thinks that the 25 cent blue foam is better for the family budget.
With regards to the spar: what you saw in the pictures was a prototype wing. The real wing used three staggered layers of 2oz/yd glass about 2" wide at root and tapering to 1" and tip: first layer full span, second layer 2/3 span, third layer 1/3 span. So, the root of the wing has an equivalent of 6 oz glass spar caps, and also 1/8" vertical grain balsa about 1/3 span, laminated to the lengthwise 1/8" spar shown on the photos.
It wasn't too heavy and the wings didn't fold, while being easy to build.

What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

Mark wood · « **Reply #83 on:** November 24, 2021, 11:41:48 AM »

Quote from: Mike Alimov on November 24, 2021, 11:04:40 AM

I think we should be guided by theoretical engineering, but the limitations of practical model building sometimes alter the choice of materials, as I illustrated with foam. Maybe Euler and Poisson would demand that I use Rohacell at $50 per sheet, but my wife thinks that the 25 cent blue foam is better for the family budget.
With regards to the spar: what you saw in the pictures was a prototype wing. The real wing used three staggered layers of 2oz/yd glass about 2" wide at root and tapering to 1" and tip: first layer full span, second layer 2/3 span, third layer 1/3 span. So, the root of the wing has an equivalent of 6 oz glass spar caps, and also 1/8" vertical grain balsa about 1/3 span, laminated to the lengthwise 1/8" spar shown on the photos.
It wasn't too heavy and the wings didn't fold, while being easy to build.

What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

No Euler is the buckling trouble and the separation created between the layers by the foam is what is necessary. In essence the Euler buckling equation take in to account the section modulus under compression. The foam core helps keep the load bearing material in column. Buckling occurs when the load bearing member deflects a little bit and then departs catastrophically. That is what the foam or balsa or what ever core material being used does. In lightly loaded structures it doesn't take much to do that and the very lightest foam cores should be sufficient. Of course cheap is generally pretty light. I think your wing lay up schedule is nearly perfect for a composite structure.

Balsa is super hard to beat in small light structures. Since the loads are low in the stab, foam spar webs and ribs could work. There are several full size aircraft that use a method where the wing is made in two half molds, the same as your molds, with ribs and spars in mirror image. he spars and ribs are basically split in half making a top and bottom pieces along the for aft center line. The spars have caps and webs on both sides made with composite material. The wing two halves are then simply glued together same as you are currently doing. Since the spilt line is basically the neutral axis of the spar the butt glue joint is sufficient. For the stab I would probably forego the thin foam and use a single biased layer over the entire planform, some tow as a spar cap and maybe another layer.

The construction schedule would be to first form the skin in the mold. The spar pieces could be made at the same time in a separate mold maybe. They could even just be balsa. The foam ribs and spar halves would be glued in to the skin while still in the mold. The spars could be reinforced with glass at this time if needed. After curing the halves sanded flat and glued together.

Crist Rigotti · « **Reply #84 on:** November 24, 2021, 12:12:38 PM »

Quote from: Mike Alimov on November 24, 2021, 11:04:40 AM

<snip>
What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

Helium?

Ken Culbertson · « **Reply #85 on:** November 24, 2021, 12:32:36 PM »

Quote from: Mike Alimov on November 24, 2021, 11:04:40 AM

What presented a greater challenge than the wings was stabilizer. We used the same layup schedule as everywhere else, but weight did not scale favorably to the smaller parts: the stabilizer alone came in at 3oz, whereas it should be more like 1.5oz. Any ideas would be welcome.

Maybe that is not a *really* bad thing depending on other factors. When I switched to electric I basically built like it was for IC but a different nose. Everything came out nose heave and I found I was putting lead or clay under the stab of just about everything. So I started building a more robust stab which was naturally heavier and I haven't had to add weight to regain the balance since. Just a thought.

Ken

Mike Alimov · « **Reply #86 on:** November 24, 2021, 02:35:29 PM »

Quote from: Ken Culbertson on November 24, 2021, 12:32:36 PM

Maybe that is not a *really* bad thing depending on other factors. When I switched to electric I basically built like it was for IC but a different nose. Everything came out nose heave

Ken

Well, we have 3 oz(!) of extra weight in the nose. Ended up swapping an originally planned 2826 motor for a 3525, and replacing a 5S2800 with a 6S2800. Otherwise the plane would have been at 59oz flying weight.

Ken Culbertson · « **Reply #87 on:** November 24, 2021, 02:43:38 PM »

Quote from: Mike Alimov on November 24, 2021, 02:35:29 PM

Well, we have 3 oz(!) of extra weight in the nose. Ended up swapping an originally planned 2826 motor for a 3525, and replacing a 5S2800 with a 6S2800. Otherwise the plane would have been at 59oz flying weight.

Good for you! Most just slap in lead. I like to slap in more capability. With electric you can always add a bigger battery!
With IC you can always slap in more lead. wasn't implying that the extra tail weight was Good, only that it may not be a problem if you build nose heavy.

Ken

Mike Alimov · « **Reply #88 on:** November 24, 2021, 04:01:21 PM »

I guess it's a difference in design philosophies, but I don't like the weight penalty of the more powerful systems if I can't take advantage of the extra capability. I've flown the prototype with both power systems described above, taping 3oz of lead when necessary, and didn't feel any difference. Same prop, same speed, same line tension, airplane behavior, etc. I think it's a case where enough power is good, but more is not better.

Mark wood · « **Reply #89 on:** November 24, 2021, 04:44:54 PM »

Quote from: Mike Alimov on November 24, 2021, 04:01:21 PM

I guess it's a difference in design philosophies, but I don't like the weight penalty of the more powerful systems if I can't take advantage of the extra capability. I've flown the prototype with both power systems described above, taping 3oz of lead when necessary, and didn't feel any difference. Same prop, same speed, same line tension, airplane behavior, etc. I think it's a case where enough power is good, but more is not better.

You and me both on that one. However the larger motor is actually a little more power efficient for not too much more weight. Lead on a plane just scratches me in the wrong way.

Howard Rush · « **Reply #90 on:** November 24, 2021, 05:12:10 PM »

Quote from: Mark wood on November 24, 2021, 04:44:54 PM

However the larger motor is actually a little more power efficient for not too much more weight.

Then you can get away with a lighter battery. Of course, then you'd need to add more lead. Mind you, this is coming from a person who made his airplane's nose so light that he repeatedly broke it off while building it, then has added so much ballast that it's hard to find room for more. The lead doublers are strengthening the nose, though.

Mark wood · « **Reply #91 on:** November 24, 2021, 05:22:10 PM »

Quote from: Howard Rush on November 24, 2021, 05:12:10 PM

Then you can get away with a lighter battery. Of course, then you'd need to add more lead. Mind you, this is coming from a person who made his airplane's nose so light that he repeatedly broke it off while building it, then has added so much ballast that it's hard to find room for more. The lead doublers are strengthening the nose, though.

Yeah, I thought of that when I was writing the words. It's a consolation thing. And Mike is right, better to have more battery and capacity than need than lead. Lead just scratches all the wrong places. Especially when it's in the front. Then it's not only scratching in the wrong places, it's drawing blood.