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From a letter to my friend Gene Christensen who asked about balsa, October 14, 2006:
Gene:
You're getting wood from Lone Star; That's great! Lone Star is Riley Wooten, a great U/C Combat flyer. Don't think he's been an active flyer for quite some time, but a great flyer in any case. BTW, if you aren't picky about getting wood in odd lengths, he many times may have great wood available for a better price. For ribs, for example, a 19" sheet 2½" wide is just fine. Certainly ribs and other parts rarely need a 'perfect' 3"X36" piece to start. See LoneStar here: http://www.lonestar-models.com/
A note about wood grain: Balsa is typically cut from the logs so it yields the most sheets of 'standard' sizes from the log, which yields more or less random grain sheets. Sig, for one, and Lone Star for another, cut lots of random grain wood as do others, but they also cut wood that's marked/sold as 'Contest' and C grain, or Very Light, C grain, 4-6 pound, etc. C grain is also called quarter-grain. C-grain is easily seen as the sheet will have a splotchy or mottled appearance. 'A grain' balsa is the most flexible across the sheet, and 'C grain' is the stoutest across the sheet. B grain in balsa is anything that isn't C or A grain. This site has a good deal of information about balsa: http://www.uoguelph.ca/~antoon/hobby/sigbal.htm
I'm really picky about wood myself, and so I may complicate things a little. Anyway, your questions as pertains to a Lucky Lindy (and really, most other wooden airplanes, too). The weights I laid out are just rough, and certainly others may call weights out differently. All of it's just one man's opinion anyway. Each in pounds per cubic foot.
Very light (VL) = 4-5
Light (L) = 6-7
Medium (M) = 8-12
Hard (H) = 13-16
Very hard (VH) = 17 up
_______________________________________________________________And to answer your question about which weight and grain to look for:
Center section ribs = H
Main panel ribs = M
If ribs are cut from different sheets, I weigh each on a tenth gram scale, then lay them out from closest to the CC to the tip panel in order of weight, one set for the left and one set for the right.
Tip ribs = L
Tip end ribs = M or H...but on a Lindy you have a carved end, so a light rib on the end is fine. Use L on the carved end, hollow it out, and cover it with light glass cloth to harden it up.
Dihedral break ribs = H or VH Here I'll use thicker ribs; say, 3/32 where others are 1/16, 1/8 where others are 3/32, etc.
L.E. = Choose wood that matches the panel ribs, quarter-grain first choice.
T.E. = Choose wood that matches the panel ribs, A-grain first choice.
Spars =
I almost always use built-up spars, even in stabilizers.* For topside 'multi-spars' I'll use wood that's pretty firm as they tend to get damaged easily. They don't do much for the wing structurally and really serve mostly to hold the covering up on the topside forward of the high-point and may serve some aerodynamic purpose to 'turbulate' the airflow, hence the term 'turbulator spar'. That's another topic all on its own. *Main and center-section built-up spars are strongest if the webbing is X-grain, rather than vertical grain. X-grain means lightweight sheet balsa that is laminated together first, the laminations set at opposing 45° angles. The sheetwood can be lamintated with nothing more than a light spray of '77 first. The laminated sheet is then cut to size to fit between the spar caps, and installed between them with your favorite adhesive. I still think that Titebond II is the best thing for most wood construction. For the best adhesion to extruded carbon fiber, Titebond isn't a good choice; Medium CyA or one of the great epoxies like 3m's DP460, applied in a very thin layer gives the best adhesion to carbon fiber. A good source for epoxies, carbon fiber of all kinds, CyA adhesives, and most other high-tech materials is CST; http://www.cstsales.com/index.html On tip spars, I most often use very light balsa run longitudinally, with a thin cap of basswood. Plenty strong enough for a lightly loaded tip panel. Stab spars work well done similarly, but in the interest of lightness and simplicity, I usually use 1/32" sheet installed vertically, after the stab is otherwise built. Pylons =
I'll choose wood and method of construction depending on the model and type of pylon. On a fat built-up Lindy, I use L wood on the sides, H or VH for the LE and TE posts inside, and M for the top and bottom.
I always glass a Lindy pylon. Two or three ounce cloth is good here. I apply it to the pylon with thinned epoxy before it's attached to the model.
On other pylon models with the typical solid sheet wood, I may lay them up for a 1/4" or thicker; Use light in the middle 1/8, and firmer 1/16 sides, for example. Most times the LE and TE of a pylon I use some H wood.
Fuselage =
Sides:
Front end to a point a little behind the pylon, about the front third, H.
From behind the pylon to a little ahead of the stab, M 8 or 9 pound stock.
And lighter yet to the very tip of the tail.
Top and Bottom:
Bottom is two piece. H or VH on the front section and medium from there to the tail end.
Top is two piece. M or the light end of H to behind the pylon, and M for the remainder.
Bigger models, tend towards the harder end of the spectrum, and smaller models,
tend towards the lighter. And to answer your question about which weight and grain to look for:
Center section ribs = H
Main panel ribs = M
If ribs are cut from different sheets, I weigh each on a tenth gram scale, then lay them out from closest to the CC to the tip panel in order of weight, one set for the left and one set for the right.
Tip ribs = L
Tip end ribs = M or H...but on a Lindy you have a carved end, so a light rib on the end is fine. Use L on the carved end, hollow it out, and cover it with light glass cloth to harden it up.
Dihedral break ribs = H or VH Here I'll use thicker ribs; say, 3/32 where others are 1/16, 1/8 where others are 3/32, etc.
L.E. = Choose wood that matches the panel ribs, quarter-grain first choice.
T.E. = Choose wood that matches the panel ribs, A-grain first choice.
Spars =
I almost always use built-up spars, even in stabilizers.* For topside 'multi-spars' I'll use wood that's pretty firm as they tend to get damaged easily. They don't do much for the wing structurally and really serve mostly to hold the covering up on the topside forward of the high-point and may serve some aerodynamic purpose to 'turbulate' the airflow, hence the term 'turbulator spar'. That's another topic all on its own. *Main and center-section built-up spars are strongest if the webbing is X-grain, rather than vertical grain. X-grain means lightweight sheet balsa that is laminated together first, the laminations set at opposing 45° angles. The sheetwood can be lamintated with nothing more than a light spray of '77 first. The laminated sheet is then cut to size to fit between the spar caps, and installed between them with your favorite adhesive. I still think that Titebond II is the best thing for most wood construction. For the best adhesion to extruded carbon fiber, Titebond isn't a good choice; Medium CyA or one of the great epoxies like 3m's DP460, applied in a very thin layer gives the best adhesion to carbon fiber. A good source for epoxies, carbon fiber of all kinds, CyA adhesives, and most other high-tech materials is CST; http://www.cstsales.com/index.html On tip spars, I most often use very light balsa run longitudinally, with a thin cap of basswood. Plenty strong enough for a lightly loaded tip panel. Stab spars work well done similarly, but in the interest of lightness and simplicity, I usually use 1/32" sheet installed vertically, after the stab is otherwise built. Pylons =
I'll choose wood and method of construction depending on the model and type of pylon. On a fat built-up Lindy, I use L wood on the sides, H or VH for the LE and TE posts inside, and M for the top and bottom.
I always glass a Lindy pylon. Two or three ounce cloth is good here. I apply it to the pylon with thinned epoxy before it's attached to the model.
On other pylon models with the typical solid sheet wood, I may lay them up for a 1/4" or thicker; Use light in the middle 1/8, and firmer 1/16 sides, for example. Most times the LE and TE of a pylon I use some H wood.
Fuselage =
Sides:
Front end to a point a little behind the pylon, about the front third, H.
From behind the pylon to a little ahead of the stab, M 8 or 9 pound stock.
And lighter yet to the very tip of the tail.
Top and Bottom:
Bottom is two piece. H or VH on the front section and medium from there to the tail end.
Top is two piece. M or the light end of H to behind the pylon, and M for the remainder.
Bigger models, tend towards the harder end of the spectrum, and smaller models,
tend towards the lighter.
NOTE: I make long scarf joints. For an FAI size LL for example, the scarf is about 8" long, the joint direction opposed side to side, and top to bottom. Extra work, but it's one of the little things that makes a model last for a very long time, along with judicious use of glass cloth and epoxy. Bigger and smaller models, size the joints accordingly.
Rudders/Fins =
On models that have fins on the underside I'll use L to M stock, and outline the LE, TE and bottom with some hard stock. On an FAI size Lindy Mk I or II with the bottom tip fins, I use fairly light stock and edge each with H or VH 1/8" stock. Mine are covered with carbon
fiber random grain paper, too. Half-ounce 'glass is a good choice here, too.
You asked how I choose spruce for the spars:
To be honest, I've never weighed spruce. It surely can be lighter or heavier, just like balsa, but I don't think it varies to the degree balsa does. Anyway, I choose spruce, most always for spar caps top and bottom, thus: Straight as I can find to start. When choosing a piece, bow it some in both planes; side to side and top to bottom. In a shop, you may have to buy some that breaks, but that's minor. What you are looking for here is to choose spruce that has the grain running straight as well. That which breaks or bows strangely will have grain that runs to the edges...not good. Here's more about spruce from Sig - Look under the tab 'Building Materials' and select Spruce:
http://www.sigmfg.com This took a while to write up, but I think it's worthwhile information, at least in my opinion. I'm open to suggestions, corrections and changes. In any case, I think I'll add this to my Web site, and note how it originated. You'll be famous! Oh, and I added some photos to the engines page; your first Veco is there already. These photos are on the right hand side of the screen; you may have to scroll over to see them. Let's go fly!!!
Bob
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