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Balsa selection

The first step in building a good glider is to use good materials.This is an all-balsa glider, and the balsa you pick will determinehow light and strong your glider becomes. You can take good balsa andmake a bad glider out of it if you aren't careful with yourcraftsmanship, but you can't take bad balsa and make anything otherthan a bad glider out of it.

Generally speaking, light balsa is good balsa and good balsa israre. Some specialty companies (Sig Manufacturing being the biggestof these) sell weight-graded balsa, but most hobby shops carrymass-market balsa lines like Midwest. Even Midwest accidentallyincludes the occasional good piece in the bulk packs of generic balsathey send stores, so every hobby store I go to, I sort through everypiece of balsa sheet stock they have, looking for the one "perfect"piece. What is the "perfect" piece? Well, for glider work it islight-density C-grain.

Balsa wood grows in different densities, but generally isavailable in densities ranging from 6 pounds per cubic foot to about20. Most hobby store balsa is in the 10 to 18 pound range, which isfine for boats and for model airplane structural members (and modelrocket glider fuselage booms like the D-Light), but when used onwings it makes for heavy boost-gliders that glide poorly.

"Contest" grade balsa is 5-8 pound density, and for model rocketglider wings and stabilizers or rudders 5-7 pound stock is the ideal.Unfortunately, to measure balsa density exactly you need a scale,which is kind of obvious and not always welcome when you walk into ahobby store! You may just have to buy the lightest piece you canfind, hope for the best, and weigh it when you get home.

For the D-Light wing (which is made from 3/16-inch thick, 4-inchwidth balsa), if your 36-inch-length balsa sheet weighs 50 grams orless (7 pound density), you have picked well. For the rudder andstabilizer, which are 1/8-inch balsa, if your 3x36 inch balsa sheetweighs 25 grams or less (7 pound density), it's good. The fuselageshould be 12-16 pound density hard balsa, which I find to be stifferand lighter than the spruce that others use for booms.

There is one other factor to consider for your balsa besidesdensity, and that is strength. Heavy balsa is usually strong, butlightweight balsa can be too weak for use on wings if it was cut thewrong way from the balsa tree. "C grain" balsa sheets are thestrongest and most rigid for a given density and should be what youuse for the D-Light wing and tail; these sheets are cut in a radialdirection from the balsa tree trunk, perpendicular to the age rings.The surface of C-grain wood has a distinctive mottled, almost shinyfish-scale like appearance. Experienced modelers picking throughbalsa displays have been known to come to blows over the right to buya rare sheet of light C-grain!

Building a Wing

The wing makes or breaks a glider's performance. It needs to bethe right size to provide enough lifting surface for the glider'sweight and visibility for timers, the right strength to withstandboost, the right shape to provide stability in glide, light enough toensure low wing loading and good glide ratio, and the right airfoiland surface finish to ensure low drag and high lift coefficients.Wing design and construction is a trade off among all thesefactors.

The D-Light wing is about 60 square inches of area with a 17.5inch span and an aspect ratio of 5.6. It uses a standard low-camberasymmetric airfoil with the high point at 25 percent of the wing'schord. This is fairly small and low-aspect-ratio for a D boostglider; this bird is designed for high-altitude, high-speed boosts onan Estes D12 motor.

The D-Light wing is made from 3/16-inch thick, 4-inch width, 6pound density C-grain balsa. (If you do not plan to tissue the wing,use 1/4-inch thick balsa of the same quality instead).

Use the half-wing template in the plan and lay out a completetwo-sided wing on the balsa, then cut out the whole wing as a singlepiece. Do all of the sanding, surface preparation, and tissuing tothe wing as a single flat piece, then cut it in half at the very endfor installation on the glider.

Step one for making any wing is to sand in the airfoil. Doing this is a multi-step process, as shown in Figure 1. First, put a felt-tip pen line across the wing at the point 25 percent of the way back from the leading edge toward the trailing edge. This is where the highest point in the airfoil will be at the end of your sanding, and is the only place where the finished wing will remain the full thickness of the original balsa. When you are done sanding, this line should still be there, at least at the center part of the wing.

Next, use a balsa planer or a sanding block with coarse paper (Iuse 60 grit cut from a belt-sander belt and placed in a hand sandingblock) to rough-cut the airfoil shape as shown in Figure 1. Also,taper the wing's thickness somewhat out toward the tips so that theairfoil's thickness is constant as a percentage of the wing chord ateach point along the span.

Finally, use progressively finer sandpaper (ending with 400 grit)in a sanding block to bring the wing to the shape of the airfoil. Isight down the length of the wing frequently during this process, toensure that I am doing the sanding evenly along the whole span, andat the end I hold the wing up to the light to ensure that the sameamount of light passes through at each point along the trailing edge,indicating that it is thin enough and evenly thin all along thewing.

If you are going to tissue your wing, this is the point in wingconstruction where you do that. More about this later. If you do nottissue the wing, at least put one coat of clear dope on it to improveits resistance to moisture and warpage, and to make its surfacefinish smoother and less draggy.

Once the tissuing or doping is done, it's time to cut the wing.The D-Light uses simple dihedral (two-piece wing); other designs mayuse a three-panel or four-panel polyhedral. Regardless, the wingcannot stay flat or it will have no capability to provide correctivestability and bring the glider back to level flight in the event ofgusts or other disturbances in glide. Normally, the wingtips are eachraised about 1/8 of the total wing span by use of either dihedral orpolyhedral. For the D-Light, this is 2 1/2 inches per wingtip.

Cut the wing along the line where the two half-wing patterns met.(It's OK to stop here for a minute and admire the cross- section viewyou just created of the airfoil you worked so hard to sand in.)

Now you need to bevel an angle into this fresh-cut edge of each ofthe two wing halves. Place each wing half with this edge lined upalong the edge of your workbench, with the wingtip propped up by a 21/2-inch block on the bench. Use a sanding block along the edge ofthe bench to put in a bevelled root edge so that when the two halvesare joined with the wingtips raised, the beveled edge on each halfwill be in 100% contact with the other for strength.

Once the bevels are done, use the "double gluing" technique toattach the two wing halves: put a thin layer of aliphatic resin(yellow) glue on each edge, push the two together, then separate andlet dry. This ensures complete penetration of the glue into the wood,greatly improving subsequent strength of the bond. Then apply glueagain, and push the two halves together. Prop each wingtip up 2 1/2inches with a block and put a weight on the wing joint to hold itflat on the workbench (with a piece of wax paper under it so it doesnot get glued to the bench). Let dry overnight, then reinforce thetop of the joint with 30-minute or slower epoxy that can soak intothe wood before it sets.

Tissuing

Adding a surface finish of model airplane tissue to a balsa winggreatly enhances the stiffness and smoothness of the wing, improvesvisibility to timers due to its color, and distinguishes the user asone of the true elite of glider people. If you do not use tissue onthe D-Light, be sure you use 1/4-inch balsa for the wing rather than3/16-inch and use Magic Marker or other lightweight pigmentation tomake the wings a more visible color than bare wood.

Standard wrapping tissue is not what you use for gliders; use onlythe strong and light "Japanese" model airplane tissue, imported byPeck Polymers and sold by some hobby shops and model airplanesuppliers. My personal preference is black tissue, but red is also agood visible color. Blue is counter-productive, but I've seen peopleuse it!

Applying tissue starts with prepping the balsa surface. Thesurface needs a 400-grit or finer surface finish, and is then given asingle coat of thinned clear dope which is sanded smooth after itdries. I prefer to use the Sig "Litecoat" brand of clear dope fortissuing, because it has a greater fraction of volatiles than mostother dopes and dries with a much lighter weight, but any clear dopewill work.

Next step is to cut out the tissue pieces that will be applied.Always apply tissue so that its dull side is down in contact with thewood, and always tissue both sides of a wood part to avoid warpage asthe tissue dries and shrinks down tight. Cut the tissue pieces to fitthe wing using a fresh, very sharp single-edge razor blade, and cutthem so there is substantial excess tissue around every edge.

You may wish to make two half-pieces for the top of the wing,leaving a 1/2-inch gap at the center of the wing. This will be thejoint for the wing's dihedral, and also the area that the rocketmotor's exhaust will toast during boost, so this strip of the wingshould be coated with epoxy eventually, which does not stick well totissue.

Tissue the top of the wing first. Apply clear dope (thinned 50%with thinner) starting at one end of the wing, and lay the tissue(dull side down) on the fresh wet dope progressively as you workacross to the other wingtip. When the entire top has beentissue-covered, rub the whole thing down with a small pad of toiletpaper soaked in thinner, ensuring that there are no wrinkles.

Once the tissue has dried, trim carefully around the edge of thewing with a sharp blade, leaving a 1/16-inch excess. Wet this withundiluted clear dope and tack it down over the leading and trailingedges onto the bottom side of the wing one section at a time.

After the top and edges are dry, repeat the tissuing process onthe underside of the wing. When this side dries, trim or sand allexcess tissue from the underside tissue all the way around the edgesof the wing. Apply one coat of thinned dope to the entire wing, thensand completely smooth with 400 grit or finer when this dries. Thewing is now tissued!

Building the Rest of the Glider

The stabilizer and rudder of the D-Light are very straightforwardto build. Simply cut them out of light (6-7 pound C-grain) 1/8-inchbalsa and sand in a purely symmetric airfoil (rounded leading edge,sharp trailing edge) just like a fin. They can be tissued if you wishfor appearance purposes; the added strength is not required. Like thewing, if you do not tissue then give them one coat of clear dope.

The fuselage of the D-Light is made of 3/16-inch hard, stiffbalsa; 12-16 pound density is best. It should be sanded such that anyportion that will not have other parts (wing, pod, or stabilizer)attached to it has rounded, smooth edges. The whole thing should begiven a 400-grit or better sanded finish and one coat of clear dope.The cutout where the pod's "piece-X" tab fits in should be cut outafter the pod is made, by tracing the outline of the pod's tab on thefuselage and cutting out this exact shape. Glue 1/16-inch hard balsaside pieces on both sides of the fuselage over this tab cutout.

When gluing the glider together, first put the stabilizer on thefuselage. Use yellow glue for this joint (reinforced later withepoxy) and use blocks to support the fuselage exactly vertical whilethe stabilizer glue is setting. If you tissued the stabilizer, use aneedle to poke dozens of tiny holes through the tissue along the linewhere the glue will go.

Put a 1/64-inch thick shim between the leading edge of thestabilizer and the fuselage, so that the stabilizer's aerodynamicforce will tend to kick the glider tail downward. While this has tobe counteracted for glide trim by adding some nose weight later, itensures that the glider transitions quickly from boost, without a"death dive" type of failure.

Put the wing on the fuselage next, while the fuselage is beingheld nicely vertical on your workbench by the already-attachedstabilizer. First sand a flat gluing surface in the sharp V edge onthe bottom of the wing where the two halves are joined. Glue the wingon once again with yellow glue, followed later by epoxy fillets alongthe underwing wing-body joint.

Put blocks under each wingtip, but don't make the two wingtips thesame height above the workbench unless you want your glider to sailoff into the distance in a straight line when you fly it. Make theleft tip about an inch higher than the right tip, so the glider willdo nice lazy clockwise (right) turns in flight. Reinforce this turnby gluing the rudder to the fuselage very slightly out of straightalignment and toward the right.

Dethermalizer

Once the glider is assembled, you may wish to add a"dethermalizer" or "D/T". This is an aerodynamic control device tolimit how long the glider stays up by breaking up its glide at apredetermined time so that it does not fly off into the distance ifit catches a thermal; hence the name. It is useful for thosecompetition flights where you absolutely must get a return on theglider (e.g. you flew away your first one already in a contest). Ifit is well-designed, it adds little performance penalty when it istaped in the not-enabled position during those flights where it isnot needed. There are many different designs, but I prefer a the wingflap type shown in the D-Light plans for my larger gliders.

The timing element of a dethermalizer is a length of cotton cord(sold by model airplane suppliers such as Sig Manufacturing as"dethermalizer fuse") that burns at about 1/2 inch per minute. Apiece of the appropriate length is cut and installed, ignited justbefore launch, and at the end of its length it burns through a threadthat had been holding down a pop-up aerodynamic surface&emdash;in theD-Light, a wing flap on the wing panel on the right wing (the inboardside of its turn direction.) When the flap pops up, it converts theglider's slow turn into a rapid dive to the ground.

Because of the potential fire hazard, the end of a D/T fuse isalways held in a "snuffer tube," an aluminum tube that shields andcontains the glowing fuse remnants when the glider lands. And thearea along the fuselage that the fuse contacts while it is burningduring glide is coated with epoxy to avoid setting the glider onfire.

The D-Light's D/T design is shown in Figure 2 above. The flap iscut out after the wing is installed on the glider, then a piece offine (.015 inch) music wire is bent in a Z-shape, with one of theends then being glued (with cyanoacrylate glue) to the main wing andthe other (which is bent upward by 30 degrees or so) to the wing flapwhile it is in the raised position. The center part of the Z providestorsion to lift the flap and is not glued at all, but is held inplace by a mylar tape hinge that secures the flap to the wing.

As shown in the figure, a thread runs from a music wire loop onthe underside of the flap, across the bottom of the fuselage, overthe D/T fuse where it enters the snuffer tube on the opposite side ofthe fuselage from the flap, and to a wire staple just above thesnuffer tube. When no D/T action is needed, the flap is just taped inthe flush position and no fuse is installed.

Be sure to check the D/T fuse burn rate with a ground test beforecutting to the flight length you want. This type fuse generally burns25% or so faster in glide with air going by than it does in a staticground test.

Building the Pod The pod is pretty standard in appearance and function, as shown in Figure 3 at right. The standoff is made of a "sandwich" of 3/16-inch hard balsa (as the core) with 1/16-inch hard balsa sides laminated on using yellow glue. The "piece-X," or tab that holds the pod onto the glider, is made from spruce and is placed in a cutout space from the core balsa during assembly of the sandwich. The pod is held straight on the glider with a small piece of .030-inch music wire several inches forward of the "piece-X" that fits into a small hole on the glider fuselage. Fit of the pod is critical to preventing the dreaded "Red Baron"; when you hold up the glider-pod assembly by the glider in ready-to-fly configuration, if the pod does not fall off, it's too tight and you need to sand the tab! I recommend that you use an external anchor line (as shown in the drawing) to attach the recovery system to the pod, so that the pod does not hang vertically with its balsa standoff downward during recovery and risk breaking off the balsa on landing. I also have had much better success in avoiding Red Baron entanglements between pod and glider when I use a parachute rather than a streamer on the pod.

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Flying the Glider

The D-Light should be trimmed carefully before flight. It willneed nose weight; start by adding enough to put the glide balancepoint 3/4 of the wing's chord back from the wing leading edge asshown in the drawing, then work from there.

Toss the glider with a straight-ahead, wings-level "push" throwfrom shoulder height at first, adding or removing nose weight asrequired to get a good glide. Once this is working, do somestraight-up hard throws to simulate boosts and verify that the gliderpulls out of any post-boost dive and transitions to glide with anappropriate turn. Do this in soft ground, so you don't shatter theglider if it does not pull out!

Use a little weight on the right wingtip if the turn is too weakto suit; but do not trim for a turn tighter than about a 100-footradius.

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This glider needs to travel three feet up a 3/16-inch rod to flywell on an Estes D12. If your rod is only three feet long, tape therod to a tall stake in the ground so that the glider hangs down belowthe base of the rod and the glider pod has a full three feet oftravel. If you're using a D/T, remember to light it just beforelaunch and launch as soon as you light it!

I have flown this glider with great success and a perfectlystraight, very high boost on Estes D12-3 motors and on the oldAerotech single-use D7 motors. The Apogee D3 motor does not haveenough thrust to provide a glider this big and heavy with a straightboost except in dead calm conditions. The Aerotech D9 reloadablewould probably be a good choice once it gets NAR certified.

Build light, fly high, and glide long!