De positieve mal vordert slechts langzaam. Geeft niet.
Ik heb al wel voldoende materialen besteld bij EMC. De materialen voor de rompmal haal ik bij Ruud Zandvliet. Maar daar heb ik nog geen tijd voor gehad.
Ik kreeg van
Christian Baron zo'n beetje alle relevante informatie die er te vinden is. Ik plak dit 1:1 hieronder:
Supra general
I created a new Supra folder in the Files section. This has PDF drawings of my new 3.4m Supra TD/F3J ship. Preliminary photos and some videos are at Jeff Newcum's site:
www.charlesriverrcpictures.org . Click on "DBSF Contest".
The Supra wing is a slight modification of the Aegea wing. The sweep has been eliminated, mainly to reduce the flaps-down launch torsional loads by a factor of 3. This greatly reduces the amount of washout twist during launch, giving a better spanwise load distribution. The wing will still bend mightily, but this has relatively little effect, since the twist is almost nil. The 2.4 oz CF carbon in the wing layup could be replaced with 1.7 oz Kevlar for TD flying, since this is somewhat less violent than F3J. Tom Kiesling will be flying nearly the same gliders at the F3J WC, the main difference being that his ships are stronger and heavier to withstand monster 2-man tows without too much bending.
Besides the unswept planform, I've tried a number of additional new ideas on this ship. Following the SuperGee philosophy, the fuselage pod is minimal, and the wing is on a pylon mount. I think it works, because the glider (Tom's also) is amazingly quiet in a fast flyby.
Surely a good sign. I may have overdone it on the pod size – the radio installation was rather difficult. A 5% larger pod would be much more practical.
The Supra's EDA of 6 degrees is typical on DLGs, but is quite large for this type of glider. The idea was to give it approximately neutral spiral stability at moderate glide speeds. This makes it extremely easy to fly precise thermal circles with little pilot workload. Once a moderate-bank circle was established, and I fed in the usual up-elevator trim, I could take my hands off the TX for almost a minute with the glider holding the circle nicely. Like with a DLG, using mainly the rudder for thermal circle adjustments works very well. It remains to be seen whether the large dihedral impairs landing precision in gusts.
I also tried hard to get the extremities light. I tapered the tip spars in width and used 1.0 oz Kevlar on the tails and wingtips. Seems plenty sturdy on the Hi-load core.
I also have RDS drive on all four wing servos. I used large-diameter hypo tubing for the shafts rather than the commercial solid wire shafts. The linkage is amazingly tight -- tighter than is possible with horns and pushrods I think. It also eliminates the need for electrical connectors across the outer wing joint. The drawback is that it's considerably more work than a horn setup, but not too bad I think.
Les Horvath of
www.compufoamcore.com generously provided the cores for this glider. They were flawless as usual.
The drawings I posted already have a few modifications which I think are appropriate. Specifically, I've enlarged the rudder area by about 25%, for better yaw damping to reduce ruddering workload in rough thermals. I also made a few simplifications to the wing layup.
One other new thing I tried and forgot to mention is the integral-bagged hinges. This is an adaptation of a hinge technique that Ib Jensen posted on Ezone a while back.
The cores have the hingelines cut, faced, and wrapped with light glass before bagging. The main advantage is that the hinge is incredibly strong, since it no longer relies on the poor peel strength of the skin on the foam. Instead, the skin at the hinge ends up bonded to
the light glass wrap after bagging. See the hinges.pdf sheet for details. If you want to try this technique, I suggest practicing on the vertical tail first.
I added a file showing the spar building method I used. It's similar to how Tom Kiesling does his spars.
I used the foam piece cut from the wing core as the spar core. The numbers indicate that Hi-load 60 is rather marginal in compressive strength in this application, but only if we assume that the spar wrap has no compressive load capability. But the wrap surely will carry some compressive load, as will the surrounding wing foam, so the foam spar core is probably OK. Hasn't failed yet in Tom's wings. It's certainly easier and lighter than endgrain balsa. I wouldn't try this with any softer foam, though. It's prudent to use endgrain balsa in the spar center over the wing saddle.
The joiners are carbon rods in Kevlar tubes, buried in endgrain basswood and hard endgrain balsa between the spacaps. There's also light endgrain balsa for a few inches on each side of the centerline, to withstand potentially large pressure from the wing saddle. Foam everywhere else. I should point out that Hi-load 60 or Spyder are barely adequate for a spar core. Softer foams are not. If using lighter foam, it's also sufficient to use balsa only on the spar sides, since that's where the glass wrap is applying its load. For example, a 3/4" wide spar could have a light foam core 1/2" wide, with 1/8" endgrain balsa sides. The bond between the foam and balsa is unstressed, and could be made with 3M-77. Seems easier than a solid balsa spar core.
I also made a new V-mount mold for this larger boom, which is 5/8" o.d. at the stab location.
Wing
Curved wingtip outlines : I sand the cores at the tip. On the Supra I also sanded away a little bit of the corner in the LE at the mid/tip section joint, so the whole outer panel looks like it has a moldie-like curved planform. I removed less than 1/16" of the corner, which is enough to give the curved-LE illusion. Restoring the airfoil is easy. You just match the shape of the core on both sides of the modification. Viewing the surface with a compact shallow light clearly shows the shape.
Achieving an accurate leading edge : Basically the same method as Phil Barnes. The main difference is that I feathered the edge of the Mylar so that it follows the airfoil
closer to the LE. I also iron the LE strip after application with a 170F iron. This reactivates and very firmly bonds the dried 3M-77 so there's no risk of detachment.
RDS pocket : A place for the pocket is cut out before the core is bagged, but the foam is left in it's place until the bagging is complete. Then the foam is removed, and the pocket is potted in place..
Question:
Did you overlap the 2.4 oz (73g/m²) carbon to the 1.7 oz (51g/m²) kevlar or just matched the edges? The same question applies to the 1.7 oz to 1.0 oz (32g/m²) kevlar joint.
Answer:
There is roughly a 1/8" (3mm) to 3/16" (5mm) overlap at each skin changeover. On the heavier fabric side I removed the fibers which are on the overlap and parallel to the seam. So the overlap consists of only little "broom" fibers sticking out from one side. The thickness
transition bump is then almost unnoticable.
BTW, instead of the 2.4 oz carbon you can just use a 1.7 oz kevlar doubler.
It's a bit heavier but a lot cheaper that way.
Question:
>I was just wondering why the thicker air foils are being replaced by
>the thinner ones, especially the 2 meter on up to unlimited class of
>sailplanes.
Several reasons:
1) Wing loadings and Reynolds number have decreased.
2) Structural technology has improved.
3) New airfoils use camber better, so thickness is less important for float.
The "older" sections like the MH32, SA7035, etc, are best suited for 100 oz gliders. In TD we don't fly those anymore. The main reason is speed range. It has gradually become apparent that the best speed range is obtained with thinner airfoils and lower wing loadings than what has been used previously. Lower landing speeds is a nice side benefit.
>Is it easier to maintain laminar flow on the thinner sections?
Yes. Laminar ATTACHED flow, to be exact. The AG455ct is nearly 100% laminar in reflex and at low CL (high speed), so its drag is nearly the lowest which is physically possible. If this airfoil was thicker than about 6.5%, it would have laminar separation and much larger drag.
The AG40d is roughly 90-95% laminar (reflex, low CL), which is still about as clean as one can expect at its larger design Re. Again, significant laminar separation would result if its thickness was more than about 8.0%.
As Re is increased, the laminar flow fraction which can be expected gets smaller, and the tolerable airfoil thickness gets larger.
Spar
I used the foam piece cut from the wing core as the spar core. The numbers indicate that Hi-load 60 is rather marginal in compressive strength in this application, but only if we assume that the spar wrap has no compressive load capability. But the wrap surely will carry some compressive load, as will the surrounding wing foam, so the foam spar core is probably OK. Hasn't failed yet in Tom's wings. It's certainly easier and lighter than endgrain balsa. I wouldn't try this with any softer foam, though. It's prudent to use endgrain balsa in the spar center over the wing saddle.
From an Allegro-lite post (Warren M.) : "...Carbon sleeves (sic) also works well. I have been using them for years and results in a stiffer spar. At joiner boxes, I still use additional kevlar tow wrap as insurance. The sleeve is easier to apply, note that it necks down well but does not expand when choosing a size. The 3K sleeve is the equivalent of 8 oz fabric so it is heavier than two wraps of 3 oz fabric..."
The CF sleeve is indeed a good alternative to the Fiber Glass wrap. It is somewhat simpler to use than wrapping with glass. Also, the large 3K CF tows have considerable compression load capability, and probably offload the hard-pushed foam spar core better than the glass, but that's just a guess. Tom Kiesling used CF sleeve on his Supra spar.
The CF sleeve is best suited for the center panel, since it's relatively heavy and would be massive overkill for the tip panels. So if you wish to use CF sleeve, I recommend using it only for the center panel spar. For the tip spars, I would stick with the 1.5oz glass, and add the
CF sleeve only over the short joiner portion.
The materials required for the Supra spar (Mark Drela):
All strips are double-tapered type, from ACP.
Strips and joiner rods add up to about $80.
Center spar:
CLT8-61A
0.014 to 0.084 to 0.014, 72" long, 3/8" wide.
4 pieces (two packages of 2 each)
CA together on non-stick surface to make 3/4" wide strips.
trim to length at the ends
Tip spars:
CLT7-62
0.007 to 0.042 to 0.007, 72" long, 1/2" wide
2 pieces (one package of 2 each)
cut in half and trim to length at thin ends
Top fillers and center bend laminations:
CL1-45
0.007, 48" long, 4" wide
strip as needed, lots will be left over (useful stock to have)
Joiners:
R-24
0.375 dia, 5" long
Front alignment pins:
0.125 dia, 1.25" long
rod, or hollow rod
5/32" Al tubes on each side
Rear alignment pins:
0.098 dia, 1" long
Glued into center panel (just barely clears RDS shaft)
1/8" Al tube on tip panel (drilled out slightly to clear rod)
Other alignment pins could be substituted, e.g. smaller piano wire.
This spar is probably too
>From the drawing it does not appear that the top sparcap
>is double the thickness of the bottom cap.
Correct. The Supra spar is sized entirely for stiffness. In this
situation the two sparcaps must have equal thicknesses for maximum
stiffness/weight ratio. You want a thicker top cap only if the spar
is sized for strength.
Question:
On the top left, the top cap is shown as having three layers of something attached to it that are feathered so as not to emerge from the wing. Is this the balsa shim? Or is the threeness significant?
Answer:
They are 0.007" CF laminations. See the center airfoil section on the supra_wing.pdf drawing. I added these laminations instead of balsa filler to get just a little bit more bending stiffness.
Question:
The emergence of the spar from the top foam surface will cause a bubble under the skin that could be nearly 0.05" deep in the middle. Does this need filling? Maybe a blob of lightweight fairing compound trowelled on just before bagging?
Answer:
I did a combination of things to deal with it. I wet-sanded about 0.030", or 4 plies, off the top sparcap's rear edge in the center. The plies are visible as "iso-contour" lines if the carbon is wiped clean, so accurate sanding is relatively easy to gauge. The spar is sized for stiffness and doesn't come anywhere close to its failure stress, so "weakening" it in the center like this
is not a concern.
When gluing the foam to the spar, I left off the bottom masking tape off the spar for about 4" in the center. This made the spar sit a tiny bit lower in the very center. The spar then protruded very slightly, which I faired in with a little bit of microfill. You can't see a bump in the finished panel.
Vertical tail (fin)
The vertical tail has no sub-fin. This is an F3J requirement that the only thing protruding from the bottom of the fuselage is the tow hook. The boom has been designed to take the twisting moments generated by such a tail.
Core Specs
Use "Hi-load 60" for all cores.
Wing cores
The 2.4 oz CF carbon in the wing layup, could be replaced with 1.7 oz Kevlar for TD flying, since this is somewhat less violent than F3J.
Wing cores:
Pannel Root airfoil Root chord mm Twist Tip airfoil Tip chord Twist Span mm sweep (dxLE) mm skin thickness mm
1 AG40d(-2) 247,7 0 AG41d(-2) 222,3 0 800,1 6,4 0,08
2 AG41d(-2) 222,3 0 AG42d(-2) 158,8 -0,5 596,9 25,4 0,08
3 AG42d(-2) 158,8 -0,5 AG43d(-2) 95,3 -0,5 304,8 36,5 0,08
Horizontal tail cores :
Root airfoil Root chord Tip airfoil Tip chord span sweep (dxLE) skin thickness
HT14t* 114.3 mm HT12 50.8 mm 330.2 mm 25.4 mm 0.05 mm
* HT14t is HT14 thickened to 8.0%
Vertical tail cores:
Root airfoil Root chord Tip airfoil Tip chord span sweep (dxLE) skin thickness
HT13* 203.2 mm HT12 88.9 mm 330.2 mm 44.5 mm 0.05 mm
*HT13 is HT14 thinned to 6.5%
Boom
A very useful addition is 5/16-16 threaded hole in the big end. This
lets me pop the boom off the mandrel with bolt and collar. Aluminum mandrel. Diameters are:
x D
0" 0.875"
2" 0.875"
48" 0.475"
The part from 2" to 48" is a straight taper. The Supra boom runs from 1" to 42". It's made from 150 g/m^2 prepreg I got from CST. 5 layers at big end, dropping to 4 layers at 22", and then 3 layers at 32". The two innermost full-length layers are +/-20 at the big end, and steepen to +/-35 at the small end. These give hoop and torsional stiffness. Prepreg is flat CF about 0.005" thick which already has heat-curing epoxy in it. Cure is either at 250F or 350F, depending on type of epoxy. The 250F is much more convenient for hobbyists, since a 250F oven box can be made from almost anything. It must be stored in a deep freezer, since the epoxy slowly hardens at room temperature. It comes with a nonstick paper backing. At room temperature the epoxy is the consistency of taffy. It makes the prepreg tacky at room temperature, sort of like Post-It adhesive, which makes it very easy to work with. There no wet-layup mess. You just apply it and cook it. But it absolutely must be compacted somehow during cure. I roll it onto the mandrel one layer at a time. Then I do a spiral wrap of peel-ply, then absorbent paper, then I wrap tightly with heatshrink tape. Then I place it in a 250F oven. First the outside heatshrink tape tightens, then the epoxy liquifies and the excess soaks into the paper as the carbon fibers get compacted, then the
epoxy gels and finally solidifies. It's important not to shrink the tape with a heat gun, because the epoxy will gel too early, so the excess will not bleed out.
Fuse:
The pylon is just part of the fuselage layup, it's not a separate piece. The fuselage was made by Terry Luckenbach. It's a sorta proprietary design, so don't ask him for one.
The rear of the pod and the pylon need a considerably heavier wall than the front of the pod. The rear area has to transfer the loads from the boom and the towhook into the wing.
Mark Drela, 13.8.2004:
I've now racked up a significant amount of flying time on my Supra, including four contest days. I now have a good sense of its hefty 6 degrees of EDA. Bottom line: Thumbs up. Some additional comments...
As I mentioned earlier, 6 degrees is typical for a DLG, but it's a lot for a TD ship. I've already tried this out on my 2m Aegea, as did Mike Garton on his 2m ship which he wrote up in his Model Aviation RC Soaring column. I can now say that it definitely also works well on the big Supra. The Dodgson gliders like the Windsong had lots of dihedral, but for some reason it disappeared on later gliders. I don't know why, because I really can't see any drawbacks to it. I don't buy the landing-precision argument (neither does Mike Garton). We had lots of gusty wind at the CRRC contest this weekend, and I don't think I was knocked around on landing any more than other people with flatter wings.
Getting to 6 deg EDA with a 3-piece wing with a flat center panel is not so easy, since the tips must be at about 8.5 degrees. This is a bit extreme, and may promote tip stall. The Supra moldie that Barry Kennedy announced will be like my ship, with the dihedralled center panel and gentle tip angles.
If anyone out there has an old beat-up Mantis wing, you might consider putting some dihedral into the center panel to see how you like it. Assuming the tips are already at 3 degrees, then about 3 degrees under each center-panel half (6 deg center bend) should be about right.
This isn't as difficult as it sounds. Saw the panel in half, sand the correct bevels, and re-join much like a DLG wing. For the seam tape you must use something at least as strong as the carbon skin. The best material would be uni carbon, fibers running spanwise of course.
I'd use 2 strips on the bottom and 3 strips on top. Bottom strips are 2.0" and 1.5" wide, and top strips are 2.0", 1.5", 1.0" wide. Use laminating resin (not CA), and sand the existing skin fully for adequate adhesion. The carbon tube does not carry much load, and can be left unjoined.
The joint will also need some vertical compression strength, to withstand the ~250 lb crushing load it will see from the "bent" skin. This can be provided by first gluing uni carbon to both foam faces with the fibers vertical. When the epoxy is green, shave the carbon flush with the wing surface and then glue the halves together.
The bolt support block will need to be incorporated into the joint somehow. The DLG technique using microballoons is probably not adequate.
BTW, The Supra moldie that Barry Kennedy announced will have the dihedralled center panel. Outer joiners in several angles may be offered, to allow adjusting the EDA to suit taste.
RDS:
The set screw in the thin steel tube will not be reliable. You also don't want the set screw sticking out from the shaft at that location. It will gouge the foam channel in the wing. Better to just solder the wiper arm into the tube. Use soft solder with acid flux for stainless steel. Silver solder is overkill, and requires a torch which will turn the hard steel to soft mush.
If you try the epoxy spline, wrap a bit of carbon tow around the servo shaft before potting it into the tube. Epoxy alone will shrink during cure, and put the bondline into tension. The tow will resist the shrinkage.
When I originally said that making the SG's RDS bits was difficult, I really meant time-consuming, at least relative to the usual horn and pushrod system. If you have the time, it's quite doable.
I don't think the torque shaft needs to come apart. Yes, it's trapped in the wing, but I don't see any reason to take it out. If you use a carbon shaft, you can cut it if absolutely necessary. If you definitely want a removable shaft, I would use a pinned yoke like on the Supra. This may be a bit more tricky to make at the smaller scale. I used the pins not so much to make the shafts removable, but to give a positive connection between the carbon yoke and the steel shaft.
From Dr. Drela:
Deflected ailerons deform the load distribution away from the ideal near-elliptical shape, and hence increase induced drag. Partially slaving the flaps to the ailerons can alleviate this load distribution deformation, and thus mitigate the ailerons' CDi penalty. The question is what's the optimum amount of ail-> flap mixing. The lowest-drag aileron system is wing-warping as used by the Wright Brothers -- the wing is linearly twisted from tip to tip. When such a twisted wing reaches its steady roll rate, the load distribution returns to its optimum level-flight shape, and the drag penalty is zero.
With a finite number of hinged control surfaces such a linear twist cannot be achieved. But it can be approximated as close as possible if each surface's deflection is made proportional to its distance from the aircraft's centerline, measured at the surface midpoint.
If the four control surfaces have equal span, we then have:
surface mid_span_loc. deflection
------- ------------- ----------
L.aile. -3/4 -100%
L.flap -1/4 -33%
R.flap +1/4 +33%
R.aile. +3/4 +100%
So for this wing the flap motion should be 33% of the aileron motion. Using AVL I've verified that this mixing ratio produces very nearly the smallest induced drag penalty. If the
flap span differs from the aileron span, the table above can be adjusted accordingly.
Longer flaps will have larger travel and vice versa. BTW, this "distance-proportial deflection rule" strongly argues against stopping the ailerons short of the tip. The resulting unhinged tip portion should in fact have the largest deflection.
The "distance-proportial deflection rule" can be fudged if there is a tip stall problem in a sustained turn, where some opposite aileron must is held. By increasing flap travel over its "optimum" amount, the flaps can carry a greater share of the roll power, which reduces the required downward deflection of the inside aileron, and thus delays tip stall. So if your
TD glider has insufficient tip stall margin, I suggest increasing the flap mixing and you should see some improvement.
The extreme case would be 100% flap mixing, which mimics full-span flaperons. Flaperons
give excellent tip stall resistance, as is obvious to anyone who flies a DLG with a good
2-servo wing. A 4-servo TD wing with decent planform should not need to go to this extreme,
especially if it has some washout like the Aegea wing.
To prepare the Mylars...
Cut the Mylars accurately to the planform at the LE and curved tip.They should be cut a little bit back from the LE -- about 1/16" for a small chord, to 1/8" for a big chord. Extend at least 1/2" behind the TE. The rear edge must be straight across the entire panel.
I feather the 0.014" Mylars for about 3/8" at the LE and tip curve, on the outside surface obviously. This feather allows them to conform to the high curvature at the LE, and it's what allows you to bring the edge of the Mylar within 1/16" to 1/8" of the LE line. The Mylar is quite tough to feather. I use a cylindrical carbide cutting Dremel bit held at a shallow angle against a steel edge, and pass the Mylar edge under it. Sort of a tiny angled planer. Maybe you can devise a better method. Forget sanding -- Mylar is way too tough. Scraping with a razor blade works OK, but it's still a LOT of work. Think power tool.
Tape the top and bottom Mylars together at the rear edge with tape on the inside. I use 1/2" or 3/4" wide clear tape. I'll sometimes fold the seam and put another layer of tape on the outside, wrapped 180 deg around the folded seam. Mark lines for sparcaps, layer edges, etc., on the outside of the Mylars to help alignments during layup. All taping and marking must be done before Freekote application – nothing sticks afterwards.
Clean the hinged Mylars with alcohol, and wipe on Freekote. Paint the Mylars at this point if desired. Matte paint will stick to the wing epoxy much better, and seems to cover better (better light scattering?). Matte paint will of course still be glossy against the Mylar.
All the stuff above can be done well in advance of the layup. After the paint dries, store the Mylars folded to keep them clean.
To cut the glass (also applies to Kevlar)...
I very strongly recommend using a wax paper "carrier" for each piece of glass.
1) Accurately cut all the glass piece patterns out of wax paper. Make the patterns about 1/4" to 1/2" wider than the true chord, with this excess extending behind the TE. Also cut the wax paper patterns for the extra LE strips, usually 1" wide for a 10" chord, maybe 3/4" wide for a 6" chord. I also use a second LE strip about 1/4" narrower than the first.
2) Lay out the glass on a big smooth surface. Lay a big square on it and get the weave threads straight and perpendicular.
3) Lay all the wax paper patterns on the glass, orienting each one accurately at 45 degrees. It's especially important that the two patterns for the top and bottom of a panel are exactly at the same angle and the same orientation on the cloth. i.e. if the warp threads sweep back 45 deg on top, then they must also sweep back on the bottom (not sweep forward!). The latter point is important since glass is never the same in the warp and fill directions. If the orientations between top and bottom don't match, the panel will twist as the epoxy shrinks during long-term cure.
4) Lift off each wax paper piece, turn over, lay on the floor on newspapers, and very lightly fog with a trace of 3M-77. I spray a few feet above the piece, and let the mist settle down. Then place the wax paper piece sticky side down on the glass exactly where it came from. Repeat for each wax paper piece.
5) Cut around the wax pieces. The wax paper prevents the glass from deforming and makes it very easy to handle. If you sprayed the right minimal amount of 3M-77, it might tend to fall off in spots. This is not too much of a problem.
Preparation just before layup...
I strongly suggest using a breather layer which almost completely covers the outside of the Mylars on top and bottom, starting just behind the LE and extending to well behind the Mylar's TE. This breather gives a nice uniform compression over the whole wing even if you have a small leak somewhere. The extension of the breather behind the TE picks up the vacuum-propagating rope or felt strip.
The breather can be cut from a paper towel roll, the fluffier the better. Double-stick this breather sheet to the Mylar to make life easy when things get hectic when closing the bag at the end.
The layup steps are...
1) Spray the narrower LE glass strip with 3M-77 (still on the wax paper), more heavily this time. Stick it to the core, wrapping it around the LE. Remove wax paper carrier. Repeat with the second wider strip.
2) Mix epoxy, and apply very sparingly to Mylar with a narrow paint roller, about 1" to 2" wide is good. You don't need complete coverage at this point.
3) Place the glass/wax-paper accurately onto the wetted Mylar. The front edge of the glass should be very close to the front feathered Mylar edge, but not extend in front of it. Smooth it down and peel off the wax paper. Here's where you find out if you sprayed on too much 3M-77. If it's reluctant to separate, peel it straight back while pressing down on the whole peel line with a metal strip (keeps the glass down).
4) Roll the glass with the roller just enough to wet out all the dry spots. If things look too juicy, blot off excess resin with paper towels and a hard dry roller. Better to apply too little epoxy at first.
5) Wet out the LE strips on the core using the paint roller.
6) Position the core on the bottom Mylar, and fold the top Mylar over.Check to make sure feathered Mylar edges are not too close to the LE. Better to have them too far back than too far forward.
7) Place Mylar/core sandwich in bag, place on top of the bottom foam shuck to set the right twist, and apply vacuum level appropriate for your foam. Only Hi-load-60 and Spyder can take full vacuum. Other foams will crush, so a partial vacuum must be used.
8) I cure at a somewhat elevated temperature, 100-120F. This is optional, but it does seem to give a better end product, with a better skin/foam bond. Also, if the resin got very thick towards the end, the heating will liquefy it again.
That's about it. I also suggest looking at the Kahu page at CRRC.
If you decide to use the carbon doublers, I suggest using Phil Barnes's trick of lightly spraying the carbon with 3M-77 and letting it dry. Otherwise the loose weave tends to disintegrate on the narrowest parts of the doubler strips.
An even better trick is to just lay up a piece of the 2.4oz carbon between two mylars. Roll the wet layup with a hard roller to flatten the carbon tows and close up the weave. Allow to cure. Use this cured carbon material for the doublers.
Some handling tips for the cured carbon material; Stick some copy paper to the carbon with a light spray of 3M77. Lightly sand the face of the carbon with 100 grit to remove wax residue and promote good epoxy adhesion. Do both sides of the carbon material this way. Leave the paper on after the second sanding so that that you can draw shapes on it with a pencil (it's real hard to draw on carbon). Draw out your doublers and cut with a knife and stright edge. You may now remove the paper. When doing the wing layup you can place the carbon doublers on the foam core rather than on the mylar. It is easier to correctly locate the doublers on the core rather than on the mylar. To make the carbon doublers stay in place on the core you should roll a little epoxy on the core where the doublers sit. I also roll the doubler with epoxy to wet it out before putting it on the core. The wetter the doubler is, the better it stays put on the core.
The above method is what I did on my latest DLG wings. I was able to locate the doublers so accurately that the edge of the doubler was exactly on the hingeline on the bottom of the wing. The top doubler was placed exactly one half of the hinge gap back from the hinge line. You can't locate a doubler with such precision unless it is precured and stuck to the core. All I
needed were a couple hash marks drawn on the core with a pencil to locate the doublers. With my trailng edge methods, the hinge line is always known to be an exact distance ahead of the finished wing TE which is known to be exactly 1/16" aft of the foam core TE.
Several questions and answers:
Firstly, how do you create the curved wing-tips?
I presume you hand-sand the cores using your angled facet method, (is this the case?) but then the shucks won't match anymore.
I sand the cores at the tip. On the Supra I also sanded away a little bit of the corner in the LE at the mid/tip section joint, so the whole outer panel looks like it has a moldie-like curved planform. I removed less than 1/16" of the corner, which is enough to give the curved-LE illusion. Restoring the airfoil is easy. You just match the shape of the core on both sides of the modification. Viewing the surface with a compact shallow light clearly shows the shape.
Secondly, what is your method for achieving an accurate leading edge?
Basically the same method as Phil Barnes. The main difference is that I feathered the edge of the Mylar so that it follows the airfoil closer to the LE. I also iron the LE strip after application with a 170F iron. This reactivates and very firmly bonds the dried 3M-77 so there's no risk of detachment.
It could be that you had too much epoxy in the layup. This can cause air to be trapped in the layup
How does too much epoxy cause air to get trapped?
Here's a guess:
Kevlar which is not too wet is in effect a breather material, since there will always be pathways in between the fibers through which the air can get out. If the Kevlar is very wet, all the pathways can be "flooded" and the air can be blocked. Once a small air bubble grows when subjected to the vacuum, and it's fully surrounded by liquid, I don't see what would drive it to the edge of the mylar.
Helmut Lelke in the CRRC club once made a glass-skin wing which had numerous bubbles trapped under mylar. We couldn't figure out at that time what could have caused it, but it did look like a rather wet-looking layup. He used 3 mil mylar for this wing, and he said he hasn't had the bubble problem before with 14 mil mylar. The 3 mil mylar will follow the foam irregularities more closely, which may have aggravated blocking of the trapped air.
My approach with Kevlar layups is to wet out well, and then blot out as much as possible with smooth paper towels and a hard roller. I usually make two passes, rolling spanwise and then chordwise. I press down hard on the roller, and don't roll too fast, so the epoxy has time to wick into the paper towel.
I don't think it's possible to blot out too much epoxy out of Kevlar. When I laid up my SG2 wing I thought I overdid the blotting. The Kevlar was so dry that it was almost falling off the Mylar. But the wing hasn't suffered a single skin delamination after a full flying season. I cure the layup at 130F to 150F, which probably helps. I also blow all the sanding dust off the wing before bagging, which also helps with the skin adhesion.
i wanna "electrify" the supra so the total weight increases probably up to 2100gr. is it necessary to increase the diameter of the carbon-joiner to 12mm?
Probably not. The electric version will not see large winch loads. You might also be able to make the spar lighter for the same reason, and reduce the weight of the skin: No carbon in the center, the light Kevlar over the whole tip panel.
which material do you use for the ribs between the wings ("root-ribs") ?
plywood or balsa or something else?
I use "plywood" made of medium 1.5mm balsa, 3 layers. Vertical grain goes in the middle. I cut the ribs slightly oversize, glue them to the foam core while it sits in the foam bed overhanging the end slightly. Then I sand the rib flush with the foam surface. Very easy. The vertical grain layer connects the skins, and makes the end very crush-resistant.
Boom?
One can cut a 41.25" boom down to 36" easily from the tail. However, a short boom cannot be made longer,
The practical boom length constraint is pretty severe, as Rick points out.
One solution would be to make the pod longer in the back to make up the difference. Just make sure that the pod wall in this extension roughly matches the boom wall. Four layers of 2.9 oz Uniweb together with the usual Kevlar layup should be about right. The extra Uniweb layers can rapidly drop off as the pod diameter increases going forward.
Is the 2.4 oz. CF layer on the center section of the Supra wing a doubler or a layer in lieu of the 1.7 oz. Kevlar (I understand the CF
on the flaps is a doubler, but the center section CF is unclear)
Long story here...
My Supra actually has a doubled Kevlar skin on the entire center panel ahead of the hinge. I figured that since 1 kevlar layer on the center panel is not stiff enough, I went with 2 layers, and also a light pink foam core to take advantage of the heavier skin's larger dent resistance and to save some weight. I have since discovered that the dent resistance of this setup is OK, but it isn't as good as 1 layer on Hi-Load 60 foam. So I switched to Hi-Load 60 for the official design, and that's what I would use on the next wing. To take care of the required extra stiffness, I replaced the 1.7 oz kevlar with 2.4 oz carbon in the center. The carbon is about 2x stiffer, and hence could be replaced with just a Kevlar doubler there. This is cheaper, but a
bit heavier. But the extra weight is minor, so I'd certainly use the kevlar doubler approach if you don't want to mess with the carbon. There will also be a visible slight bump at the doubler edge, but that's life.
Kevlar doublers might also work OK on the flaps and ailerons, but here there will be some loss of stiffness compared to the carbon doublers. I don't know how significant this would be.
If you decide to use the carbon doublers, I suggest using Phil Barnes's trick of lightly spraying the carbon with 3M-77 and letting it dry. Otherwise the loose weave tends to disintegrate on the narrowest parts of the doubler strips.
On the top left, the top cap is shown as having three layers of something attached to it that are feathered so as not to emerge from the wing. Is this the balsa shim? Or is the threeness significant?
They are 0.007" CF laminations. See the center airfoil section on the supra_wing.pdf drawing. I added these laminations instead of balsa filler to get just a little bit more bending stiffness.
The emergence of the spar from the top foam surface will cause a bubble under the skin that could be nearly 0.05" deep in the middle. Does this need filling? Maybe a blob of lightweight fairing compound trowelled on just before bagging?
I did a combination of things to deal with it.
I wet-sanded about 0.030", or 4 plies, off the top sparcap's rear edge in the center. The plies are visible as "iso-contour" lines if the carbon is wiped clean, so accurate sanding is relatively easy to gauge. The spar is sized for stiffness and doesn't come anywhere close to its failure stress, so "weakening" it in the center like this is not a concern.
When gluing the foam to the spar, I left off the bottom masking tape off the spar for about 4" in the center. This made the spar sit a tiny bit lower in the very center. The spar then protruded very slightly, which I faired in with a little bit of microfill. You can't see a bump in the finished panel.
I hope to be able to come up with a structure with minimal weblets (maximum weblet spacing) and only one layer of skin fabric. This would be the ideal for production work and would be ideal from a cosmetic perspective if all skin doublers could be eliminated.
Keep in mind that such a wing will flex a lot more. This favors going to a planform with less sweep, more like the Supra's. Sweep + flex causes the tips to unload, which adversely affects launch performance. But with a straight wing, the flex has little aerodynamic effect.
It's probably prudent to make the weblets as hefty as possible, so they significantly contribute to the bending stiffness. Some sort of thick knife blade might work to make the foam cuts sufficiently wide for a thicker weblet material, like 0.014" or even 0.021".
Do you think that I could go with shorter beams since I don't plan to use skins that are three layers thick?
Going to a thinner skin will just increase the skin stress in inverse proportion. The foam shear stress doesn't change.
I'm having trouble visualizing a shear failure of the foam. Where and how is the foam core subjected to shear stresses?
Draw a square on the front foam face of the beam, say halfway between the center and one of the end load points. When the beam is loaded, the square becomes a slight diamond shape as the foam is subjected to shear by the sparcaps trying to slide past each other. This shear can also be visualized as tension on one diagonal and compression on the other diagonal of the square.
For a given sparcap load, the foam shear stress is inversely proportional to the beam length. So the test beam has to be long enough so that foam doesn't fail in shear before the skin does, like on an actual wing.
If you want to use shorter test beams, you have to reduce the shear load on the foam somehow. The only feasible way that I see is to taper the beam so that its height is maximum in the center, and drops linearly to near zero at each end. From the front, the beam looks like a shallow inverted tent. In such a beam, the sparcap load is nearly uniform along the beam span, and the foam shear is near zero. So a shorter beam can be used, maybe as short as 12" total. I don't know how well this will simulate the failure mechanism on an actual wing. Might be good enough for comparing different layups.
I might test some beams with small doublers at the center so let's say that my max skin thickness would be two layers or .010".
What is meant by "a tapered skin thickness"? I assume that this refers to the skin having more layers at the center than at the tip but then I don't know what is meant by having it not "taper faster than linearly towards the end.
Having asked the second question, I think I may have come up with the answer; For a skin with two layers at the center which tapers to one layer at the end, the center doubler layer would need to extend to at least 1/2 span on the beam. For a skin with three layers at the center and tapering to one layer at the end, the first doubler would need to go to at least 1/3 span and the second doubler would need to go to at least 2/3 span.
Did you overlap the 2.4 oz carbon to the 1.7 oz kevlar or just matched the edges? The same question applies to the 1.7 oz to 1.0 oz kevlar joint.
There is roughly a 1/8" to 3/16" overlap at each skin changeover. On the heavier fabric side I removed the fibers which are on the overlap and parallel to the seam. So the overlap consists of only little "broom" fibers sticking out from one side. The thickness transition bump is then almost unnoticable.
BTW, instead of the 2.4 oz carbon you can just use a 1.7 oz kevlar doubler.
It's a bit heavier but a lot cheaper that way.
Doesn't pre-preg need to be heat cured? Doesn't it come in the form of pre-cured carbon? I just can't in vision how you are working with semi stiff carbon on a tapered mandrel. Appreciate any more info you can provide.
Prepreg is flat CF about 0.005" thick which already has heat-curing epoxy in it. Cure is either at 250F or 350F, depending on type of epoxy. The 250F is much more convenient for hobbyists, since a 250F oven box can be made from almost anything. It must be stored in a deep freezer, since the epoxy slowly hardens at room temperature.
It comes with a nonstick paper backing. At room temperature the epoxy is the consistency of taffy. It makes the prepreg tacky at room temperature, sort of like Post-It adhesive, which makes it very easy to work with. There no wet-layup mess. You just apply it and cook it. But it absolutely must be compacted somehow during cure.
I roll it onto the mandrel one layer at a time. Then I do a spiral wrap of peel-ply, then absorbent paper, then I wrap tightly with heatshrink tape. Then I place it in a 250F oven. First the outside heatshrink tape tightens, then the epoxy liquifies and the excess soaks into the paper as the carbon fibers get compacted, then the epoxy gels and finally solidifies. It's important not to shrink the tape with a heat gun, because the epoxy will gel too early, so the excess will not bleed out.
Angled centerpanel:
The key is to first build the angled building boards, sketched in the bends.pdf file. A table saw makes it almost trivial. Still pretty easy if you have a bandsaw and a disk sander or belt sander.
After that, building the bent spar and center panel is not significantly more difficult than a flat spar and panel. The processes are the same, except that you're working on the angled boards rather than on the table. There is one extra step of laminating the short carbon "leaf spring" in the center of both sparcaps, but that's pretty straightforward, and doesn't require vacuum. Using tow would be much more time consuming I think.
Mike Lachowski builds his F3B spars from tow, but only because this lets him use high-modulus fiber which is available only as a dry tow. He painstakingly lays down each sparcap by taping down one tow at a time on the bench, and then wets out the stack. Then he uses it like precured CF. Lots of work.
Quote:
I agree that is brave. I'd be a little leary of sanding anything out of the carbon at the very middle of the wing.
The Supra and Aegea spars are sized entirely by stiffness. At the design load of 200 lb, the sparcap stress is only 70 ksi -- less than half of the max allowable. There no risk with sanding away a little bit of the sparcap. There's no point to using higher-strength resin for the same reason.
Zo, daarnaast is het handig om eens te kijken bij de Yahoo group. daar vindt je nog meer interesante zaken. Zoals hoe je de V-mount moet maken. Of een uitleg van Dr.Drela over de vleugelligger belasting.
Edit: op 9-8-2007 heb ik geprobeerd de tekst wat leesvriendelijker te maken.
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