Reference materials for balsa structural engineering - RC Groups
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Nov 08, 2017, 12:11 PM
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surfimp's Avatar
Question

Reference materials for balsa structural engineering


Hi all - I've looked through the stickied post at the top of this forum pretty carefully, but didn't quite find what I was looking for.

I'm interested in learning more about how balsa airframes are engineered... in other words, general principles for things like wing rib spacing, the appropriate number and layout of fuselage trusses, and so forth. When do you sheet a D-box or full wing, and when is it appropriate to leave the structure open? How do you determine when you need to do one or the other? And so forth.

I know quite well that many of these things don't have a finite answer - a lot depends on the desired application. A lightweight freeflight plane is going to require a different approach than a 1/4 scale gasser, obviously. But beyond rote copying of existing designs, surely there have to be some reference materials - books and otherwise - that help speak to these questions?

I have a good amount of background in general model aircraft theory, having designed and flown a number of successful models at this point, including one that has gone on to prove a commercial success. But almost all of this experience has been acquired in the realm of foam and/or composite aircraft - I've got very little balsa building experience! And I'd like to change that

Thanks in advance for any guidance you can share.

Steve Lange
Latest blog entry: Of Fish, France, FPV, and Fun!
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Nov 08, 2017, 01:29 PM
Registered User
Balsa structural engineering is more empirical than quantifiable. EXPERIENCE and "that looks about right" is the standing order of the day.
Nov 08, 2017, 04:06 PM
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surfimp's Avatar
I'd be happy to settle for "rules of thumb"... I'm not asking for anything like a comprehensive structural analysis.
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Nov 08, 2017, 04:18 PM
paddockus unyeildapus
Azoic's Avatar
This might be the sort of thing you looking for, I didn't read much of it, just skimmed the front page, it might be of some help.

http://www.airfieldmodels.com/inform...sign/index.htm
Nov 08, 2017, 08:15 PM
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surfimp's Avatar
Yes, I've been all over that site, it's quite good but doesn't really address my questions around how to layout structures for scratchbuilding one's own design.

Anyways, I figured it was worth a shot. I guess I'll need to look elsewhere.
Latest blog entry: Of Fish, France, FPV, and Fun!
Nov 08, 2017, 10:35 PM
B for Bruce
BMatthews's Avatar
I'm not even sure that there are generally accepted "rules of thumb".

I can say that when I design a model of my own it never has rib spacing of more than 2" because regardless of if it's tissue and dope or some sort of heat applied fabric the resulting scalloping between the ribs is too much for my general sense of what is right. Or if I do use a greater degree of spacing then it is related to the use of sheeting or multiple turbulator spars to support the covering.

This same need to support the structure against the tension found in well tightened covering applies to stick built fuselages or fuselages that have at least some portion which supports the covering with stringers. In the case of small 1/16 and 3/32 square main longerons on a square built up fuselage I'll position uprights and cross pieces at around 1 to 1.25". With an increase to 1/8 square hard balsa longerons in old time rubber models I go with what is on the plans. But I'll avoid plans that have uprights and cross pieces spaced any wider than around 1.5". Almost inevitably if I ignore this I end up with the main longerons pulling inwards and giving the model a "starved dog" look. Or if need be I"ll "cheat" and up the main longerons to 5/32 square and make the uprights and cross pieces from 1/8x5/32 if I just MUST build some model or other.

Wing spars are a whole other area. Again it's mostly by a look and feel situation. But without a doubt the strongest wing spars are those which use upper and lower caps linked with vertical grain webbing that is glued in place using a good high strength glue and wood which is hard enough to take the loads and for the webbing to not rib out the surface grain under load.

Now there are lots of successful and popular designs that do not follow my own preferences. And in the end you come up with your own rules.

Much of my own rules came about as a combination of lots of free flight model building which demands a strict balance of good building and good strength but matched against very light weight. And a lot of my "training" came from closely inspecting the various model magazine plans presented each month. The model magazines are pretty much a thing of the past. But you can still learn a lot by studying the plans that are available on such web sites as Outerzone and other sites dedicated to hosting old model airplane plans. And yes, it very much is a case of "self study"....
Nov 08, 2017, 10:50 PM
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Zipman's Avatar
Steve,
My suggestion is to go on to the Outerzone site (Google it).
They have almost 9000 model plans free for download. About 99 % are of Balsa construction. Along with the plans is a model publication library. There you will find some detailed design books for model aircraft. One in particular is
RCL#1433
Basics of R/C Model Aircraft Design by
Andy Lennon
Air Age Media Inc., Ridgefield, CT
1996. Book, 136 pages.
Air Age Media - Model Airplane News Mag also has books for sale on many model topics.
By downloading and studying some model plans of the type you are interested in (IE, Scale, glider, sport etc) you can get a design education in balsa modeling. The site has plans for tiny rubber band powered models to 1/3 scale monsters. Different size models (depending on the power source) will be constructed super light up to very robust structures.
Another source is the local library (remember those). If you have technical colleges near by I bet there libraries will have the materials you seek. Just remember the models came first, then the full scale.
Hope this helps.
Stan
Nov 09, 2017, 12:05 AM
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surfimp's Avatar
Quote:
Originally Posted by BMatthews
I'm not even sure that there are generally accepted "rules of thumb".

I can say that when I design a model of my own it never has rib spacing of more than 2" because regardless of if it's tissue and dope or some sort of heat applied fabric the resulting scalloping between the ribs is too much for my general sense of what is right. Or if I do use a greater degree of spacing then it is related to the use of sheeting or multiple turbulator spars to support the covering.

This same need to support the structure against the tension found in well tightened covering applies to stick built fuselages or fuselages that have at least some portion which supports the covering with stringers. In the case of small 1/16 and 3/32 square main longerons on a square built up fuselage I'll position uprights and cross pieces at around 1 to 1.25". With an increase to 1/8 square hard balsa longerons in old time rubber models I go with what is on the plans. But I'll avoid plans that have uprights and cross pieces spaced any wider than around 1.5". Almost inevitably if I ignore this I end up with the main longerons pulling inwards and giving the model a "starved dog" look. Or if need be I"ll "cheat" and up the main longerons to 5/32 square and make the uprights and cross pieces from 1/8x5/32 if I just MUST build some model or other.

Wing spars are a whole other area. Again it's mostly by a look and feel situation. But without a doubt the strongest wing spars are those which use upper and lower caps linked with vertical grain webbing that is glued in place using a good high strength glue and wood which is hard enough to take the loads and for the webbing to not rib out the surface grain under load.

Now there are lots of successful and popular designs that do not follow my own preferences. And in the end you come up with your own rules.

Much of my own rules came about as a combination of lots of free flight model building which demands a strict balance of good building and good strength but matched against very light weight. And a lot of my "training" came from closely inspecting the various model magazine plans presented each month. The model magazines are pretty much a thing of the past. But you can still learn a lot by studying the plans that are available on such web sites as Outerzone and other sites dedicated to hosting old model airplane plans. And yes, it very much is a case of "self study"....
Thank you, sir! This was an extremely helpful post. If RCG had a reputation system like other forums, I'd be smacking the +1 button like crazy!

I have been studying plans on Outerzone quite a bit, and I see - with respect to fuselages - that many seem to keep to a more-or-less equilateral triangle dimension when using truss-type construction. But I hadn't considered (due to lack of experience) how the covering material could play such a role in "starving the dog" once applied, though it immediately makes sense when reading your description. Very helpful!

I am mainly a glider pilot, and in the past 5 years have had the good fortune to know a really ingenious builder who showed me how to build my own plane to much lower weight than I'd been able to achieve on my own. It took some "outside the box" thinking, for sure, but it really was cool. Now that I'm getting interested in balsa - mainly for aesthetic reasons - I want to try to keep this focus on lightweight yet strong construction. But you can't go outside the box till you know what the box looks like!

Thanks again for the help. The self-study thing is no worries, it's something I do all the time and which I enjoy quite a bit.

Steve
Last edited by surfimp; Nov 09, 2017 at 01:44 PM.
Nov 09, 2017, 12:11 AM
Piscine Promulgator
surfimp's Avatar
Quote:
Originally Posted by Zipman
Steve,
My suggestion is to go on to the Outerzone site (Google it).
They have almost 9000 model plans free for download. About 99 % are of Balsa construction. Along with the plans is a model publication library. There you will find some detailed design books for model aircraft. One in particular is
RCL#1433
Basics of R/C Model Aircraft Design by
Andy Lennon
Air Age Media Inc., Ridgefield, CT
1996. Book, 136 pages.
Air Age Media - Model Airplane News Mag also has books for sale on many model topics.
By downloading and studying some model plans of the type you are interested in (IE, Scale, glider, sport etc) you can get a design education in balsa modeling. The site has plans for tiny rubber band powered models to 1/3 scale monsters. Different size models (depending on the power source) will be constructed super light up to very robust structures.
Another source is the local library (remember those). If you have technical colleges near by I bet there libraries will have the materials you seek. Just remember the models came first, then the full scale.
Hope this helps.
Stan
Thanks Stan! That Lennon book was one I've browsed previously (a friend has a copy) and your other suggestions are most welcome, too. As mentioned in my previous reply, I have "found" the Outerzone and been really enjoying looking at the plans there. I feel like I have a pretty good basic sense of the materials and am just trying to get in the ballpark for any structures I might try to come up with. These suggestions are very welcome and most helpful.

Thank you again,
Steve
Latest blog entry: Of Fish, France, FPV, and Fun!
Nov 09, 2017, 12:40 AM
Registered User
I should have indicated in my earlier post, I feel most model structures are based on full size building practices. A scale model built using scale like structure (either balsa or spruce, etc) will generally yield model appropriate design elements. Even non-scale models use scale like building practices. Early aircraft used wire or diagonal braced fuselage trusses, made into a box like structure. Modelers did and still do. However, some soon substituted sheet balsa sides in place of the trusses. Saves time and gained strength, etc.

A good education can be had by looking at various model plans, especially those similar to what you want to build. I've been designing model aircraft since 1955. I still get a real kick perusing outerzone.com, looking at why and how others did what they did.
Nov 09, 2017, 01:08 AM
B for Bruce
BMatthews's Avatar
I don't think I exaggerate when I say that the two key styles of models you will learn the most from by studying the plans are contest free flight models and contest rated old school RC sailplanes. Both of these categories put the focus on the most from the least.

If you're interested in a design that utilizes a full truss work look at pictures of truss style lift crane booms as well. Yes, they are primarily equilateral triangles. But the way the truss cross pieces all intersect at the joints from one side to the next is a key element. The diagonals in such a boom actually form a sort of spiral wrapped "lines" if you follow the ends around the boom.

That same design element is a common system in some of the most serious rubber models made. The early dawn event. Models that can have up to 50 or 60% of their flying weight in rubber that forms a thumb sized cross section of snarled and very unforgiving power. Yet it's all held in check by a couple of ounces of slender balsa sticks.... It's very humbling. For this style it looks like most of the builders go with the diagonals forming a 30-60-30 triangle so the leg between the two 30's is a bit shorter and doesn't suck in from the covering.

Here's a video of Bud Romak with this style of model. In particular note the light shining through the tissue on the fuselage as he prepares for winding about half way through the video. The still in the pane below is a hint of that. And you can see that they are not quite equilateral triangles in this vid.

Bud Romak and his Dawn Patrol (11 min 3 sec)


This same method shows up at least partially in some old timer sparkie models from the 30's. A lot of those models used diagonals along the sides but then went with regular cross pieces from side to side across the top and bottom. Not sure why other than I guess they figured that they "closed the triangle" at the tail of the model?

Anyway, enjoy the learning. Plagarize like hell and then tell everyone that the combination is all your own idea...

If there's anything I can help with feel free to contact me.
Nov 09, 2017, 10:28 AM
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surfimp's Avatar
Quote:
Originally Posted by BMatthews
Plagarize like hell and then tell everyone that the combination is all your own idea...
So you HAVE seen my Le Fish design!

(In fairness, I cite my sources )

Thanks again for another really informative post! It is really crazy, thinking about FF planes and trying to make them as light as possible while also strong enough to withstand crash landing _every_time_ as well as that beastly wound up rubber motor.

This is going to be fun
Latest blog entry: Of Fish, France, FPV, and Fun!
Nov 09, 2017, 12:55 PM
Registered User
Ditto what others have said about looking at a wide range of plans, build threads, etc.

There is in the general aviation category a thread hosted by Dave Terrell that provides free access to the complete inventory of RCMagazine plans - from prior to 1965 to about 2004/05 when RCM stopped publication, along with links to many of the accompanying construction articles. Check post 1784 and the next several posts for a chronological picture catalog of the planes:
https://www.rcgroups.com/forums/show...1%21%21/page18

Check out threads by TomClark in the scratch builders subcategory for building larger sized glow/gas funflyers/pattern models, using lightweight construction techniques.

With the advent of electric flight and higher quality ARF (eg. Precision Aerobatics), a lot of older glow designs are now borrowing from these areas to "re-build" these models, using the legacy plans, but building lighter.

Michael in Ontario, Canada
Nov 09, 2017, 01:47 PM
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surfimp's Avatar
Quote:
Originally Posted by packardpursuit
However, some soon substituted sheet balsa sides in place of the trusses. Saves time and gained strength, etc.
Thanks for a good post. Your statement above draws my attention - is a slab sided fuselage in fact stronger, assuming we're talking about a typical tapered rectangle, than a truss fuselage? Or just significantly easier to build?

I could see where a slab sided fuselage would get heavy quickly, unless you scale down the thickness of the balsa and/or made cut-outs. But with the wide variety in wood available to work with, I could also see that this would really depend, and (without knowing for sure one way or the other) I suspect this might be an area of some amount of debate within the balsa building community?
Latest blog entry: Of Fish, France, FPV, and Fun!
Nov 09, 2017, 02:45 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by surfimp
So you HAVE seen my Le Fish design!

(In fairness, I cite my sources )

Thanks again for another really informative post! It is really crazy, thinking about FF planes and trying to make them as light as possible while also strong enough to withstand crash landing _every_time_ as well as that beastly wound up rubber motor.

This is going to be fun
YOU DID LE FISH! ? ! ? ! ?

I doff my cap to your efforts. That is a WONDERFUL design. I especially love the super light version doing the close in "nap of the cliff edge" flights. Hell, not just a great plane but superb piloting. I love how the maneuvers just "flow". It's a really great You Tube video and I end up watching it through about ever 4 or 5 months.....

We Free Flighters really do commit ourselves to lightness. A few years back I designed and built a GeeBee inspired Bostonian rubber model. Didn't fly worth a darn.... but that's another story.... Anyway I was wondering about how to do the profile wheel spats and fairing in a manner that would be light enough yet strong enough. I did one version in laminated 1/16 balsa and another that I did all built up from laminated outlines for the spats and built up leg fairing. The built up version was going to be 1.5 grams lighter so I did the 5 hour option and built them up vs the easy 15 minute heavy option. If you want light you really need to commit yourself.


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