This comes in multiple pages 'cause they just get so long...
This is page 1, on general considerations and wing building
Or, go to: Page two, rough fuselage and rough tail
Or, go to: Page three, spin discussion, tail details
The Flying Machine Project 08/12/2009
Although I only have so much spare time, it seems like it would be fun to design and build an airplane. If you have had this inclination, then perhaps you can learn from my examples and mistakes. I've learned so much from other's sites and wish to return the favor by documenting my progress in considerable detail for others. If there are any questions, please email me here.
My objective is to get into the air as cheaply as possible and without government restrictions. This requires design, construction and operation under the rules of FAA part 103. This FAA rule allows anyone to build and/or fly an airplane without a license, at any age, with no required FAA inspection. Of course, if an FAA representative challenges your claim that your aircraft really is within part 103 limits, you may have to provide proof. 254 pounds empty weight, 27MPH power-off stall speed, 63MPH maximum straight and level full powered flight speed, 5 gallons of gas max. -And a little kid could legally fly one!
Inspired by this island of freedom in an otherwise sea of restrictions, I would like to add a few other requirements for my project: Design by myself without any help but photos of other's planes and some free tools from the internet, and built from the cheapest available materials. That last one may be impossible, because you can't get high quality 0.032" aircraft grade plywood for cheap at the local hardware store!
There are some really neat designs out there, one of which I really like: Cri-Cri. This amazing aircraft is extremely efficient, with tiny wings and two little 10HP engines. The problem with the Cri-Cri is that it can't fly slow enough to meet part 103 rules. The wings have a very small area, and although they are efficient at high speed, they will stall quickly, no where near the 27MPH stall speed required by part 103. The Flying Machine will need much larger wings, and it will fly much slower.
I'm really impressed with the work of Chris Heintz, who developed the Zenith CH-701 bush plane. This is quite an airplane, with lots of interesting design features, but built from aluminum. I just can't imagine scaling it down to 103 rules and actually building it though, what, out of 0.01" sheet stock or cut open and flattened Coke cans? So the current direction is a chrome-moly frame and wooden wings and tail section, with fabric covering. -For now.
I am not an airplane designer, never having taken a class in aeronautics, but I have seen many airplanes, taken a ride in a few, and actually flown one or two for fun. I built a few models as a kid, and do know about balsa wood ribs and paper covering, but somehow I don't think paper will do for this one. You gotta admit though, a paper and epoxy airplane that you could really fly would be pretty sweet!
As for airfoils, one can get a good idea of how things work with the FoilSim program which shows how lift changes with shape, but doesn't seem to offer drag values. I figure that there are so many airfoils out there that are all pretty similar, and it can't be that if the design is off by a few percent from ideal (what is ideal anyhow?) the plane would fall out of the sky, so I'll draw one freehand in AutoCad using the spline curve drawing tool. Kinda' flat on the bottom, thick so that the spars will be strong in bending, a low-slung blunt-nose in the front and all of it tapering to a point in the rear:
However, now looking at it on the webpage, its fatness should maybe be moved a bit aft, but I intend to put a 'D-box' in the front, and this means that I can't have the fattest portion (where the main spar fits), too far aft, or the thin plywood nose-covering will be too big and heavy. The curve fitting spline tool left the bottom surface a bit curved, so I'll have to fix that too. I would think that, although I'd like a slightly concave bottom surface for increased lift, fabric covering could become difficult, so I'll leave it flat. So far, I'm looking at a 48" chord and an 8" thickness.
My first thought was to build the wings from hot-wire cut Styrofoam and cover them with epoxy-fabric, but the weight of two 8' long such constructions made of 2 pound/cu ft foam would weigh 53 pounds, before the fabric and epoxy go on. I'm not confident that 1 lb/cu ft foam would be strong enough. Anyhow, how would you connect the wings to the fuselage? Since wooden ribs and spars seem popular, I'll expect that this is the best approach, no matter how much work is required. I'll opt for going down the same path as others, not too keen on risking my life for a cute idea...
From researching such stuff on the internet, it looks like ribs can be made from 1/4" square wood sticks, carefully selected and bent to shape in a jig, epoxyed in place with junction plates (gussets) cut from very thin plywood. The quick calculations show very high strength and very little weight, and I get spars to grab onto for fixing to the fuse.
This view shows nose plates cut from thin plywood, and a possible 'stick' layout for bracing the whole rib, but doesn't show the gussets. There are three spars in this view, but I probably only need two. The spars are 1/4" plywood, maybe only 1/8", and upper and a lower solid wood beams glued to the ply on one side. These two spars can be reinforced wherever stresses arise, such as at the root and in the middle where I'll have exterior supports back to the fuselage. So far so good, but not detailed enough, and hey, this is just the wing...
Yesterday I was getting toasted with a buddy and noticed how he was sitting in a chair... Legs were just about right to be working rudder pedals, and instantly a wing popped into view above his head. This lead to the latest rough drawing:
This one puts the lower surface of the wing at about 48" off the ground, so one would have to crawl under the wing to get inside. It is cute though! The wings are attached to each side of the cabin, and a clear plastic sheet covers the cabin following the upper surface of the wings. This gives the pilot the ability to look for air traffic upward and maybe back toward the tail. Notice the leading edge detail, 'slots' to increase the lift at a high angle of attack. This is a great story, which also describes the full width 'junker-flaperons' and can be found here. Yes, I think Chris is onto something.
I figure that if the slots allow a higher angle of attack before stall, I can get away with a shorter wingspan (lighter, stiffer) and still support the airplane (250lbs) and the pilot (190lbs) and the fuel (30lbs) at a stall speed of 27MPH. Problem is, there's no clear formula for determining stall speed, and as far as testing goes, how do you determine the power-off stall speed of an aircraft? I would think the plane would be dropping like a rock, but if its still flyable, how do you determine if the wing has stalled? The FAA seems to have formulas, but from what I can tell, they put odd cases up to a panel of FAA specialists to make such decisions. This may be trouble, but I really like the basic concepts. -Can't wait to see how effective these strange details are.
As for an engine, there are several possibilities, the DL-100 looks promising, but a bit light on the power side, but really cheap at about $700 and only about 6 lbs. This engine needs a battery for the ignition, but doesn't have a starter option or an alternator. Then again, the Hirth F-33 has both as options and three times the power, but is 5 times the expense. I like the idea of having a small motorcycle battery and electric start, despite the weight. If this was a Cri-Cri, that you just jump into after pull-starting the engines, then that would be alright I suppose, but this one you would have to crawl into, which might be tough with a rattling engine, and I like the idea of a single engine anyway. Further, if the engine quits in flight, you can't get out to get 'em going again, although I suppose a pull start could be brought into the cabin...
Hirth, 28 HP for now.
I've been on an extended vacation, which inspired this mess, and am itching to get back home to the shop. I figure the ribs can be made from carefully selected fir 2x4s, run through my table saw to produce the rib material. This would be cheap, but I'm not sure I can get long lengths of 1/4" square Douglas fir that meet the requirements. The story is that the grain of the wood must be parallel to the strip for the best strength, with only one grain exiting the side of the strip in every 10" or so. My memory of construction grade 2x4 is that some will meet this spec, some won't, but a single 8 foot 2x4 can make one heck of a lot of sticks! In fact, so many sticks that I just might go for the expensive stuff, maybe clear and straight, 'choice' lumber. Also, I need to source really thin plywood, maybe 3/16ths for the spars, where I need long strips or a method of adequately splicing them, and some really thin material for the leading edge D-box, and of course, some stuff that I can cut up for the front, curved sections of the ribs and of course, the gussets.
In the mean time, I guess I can lay out the wing cross section and display it on this page on a 1/2" grid so that I can transfer it to a jig (and anyone else can copy it). For the not-so-computer-literate, on a PC, right click on the image and select "save picture as..", save it to "My Pictures", then navigate to that directory with Windows explorer, double click on it to open the saved picture and print it. Otherwise, you can just print it directly from the webpage, but you may not be able to control how.
The ribs sticks are 1/4" square fir with straight grain, the nose pieces are arches cut from 1/4" plywood. The nose pieces could have holes drilled in them to lighten them, and the internal shape is not important: We need something light, but only strong enough to support the 1/32" nose sheet to complete the 'D-box'. The two spars are 1/4" plywood, each laminated to an upper and a lower 1" x 0.75" spar element of clear and continuous fir. I decided on 1/4" ply for the spars because I can buy that in 8 foot lengths, although it is construction grade, not aircraft grade. So which would be better, 3/16ths aircraft grade or 1/4" construction grade? -Probably the aircraft grade, 'cause its lighter too, but where do I get 8 foot lengths? -I'll select the best parts from the best sheets I can find at the lumber yard. Notice the plywood sections of the spars are spaced 24" between centers, an easily remembered number for other planning. Notice the forward spar is attached to the nose ribs on the front, to the plywood side, and the stick rib structure on the back against the 0.75x1 strips. This spar extends right out to the upper and lower surface, and the upper 3/4"x1" beam should be shaped to the airfoil contour. The rear spar is encased by the stick ribs, so the ribs can be built on a fixture and then the rear spar can be inserted into the ribs and then the ribs can be glued to the front and rear spars during final wing assembly. The layout can be printed and you can use a ruler along the provided lines to determine any detail's location.
In any case, I've drawn this so that it looks good, at least similar to photos I've seen, and precise placement of rib sticks is probably not important. I've tried to make sure that the longest surface stick-span is no more than 6". Gussets will be applied to both sides of the rib, at every spot where sticks join. These will be triangles and squares of 1/16" plywood, epoxyed into place and clamped until cured. Gussets should be large enough to cover about 1" along the sticks, and sanded after gluing to match the airfoil contour. After gluing and sanding to shape, the final rib must be checked to be like all of the others in overall shape and the opening for the rear spar must be checked so that the spar can be slid through the ribs with a tight (but not interfering) fit. I don't think that the sticks need to be carefully cut with precise angles to fit perfectly, as the gussets are actually making the mechanical connections.
Also, although 5 minute epoxy is attractive, and I'm sure strong enough for this application (wood will break before epoxy), I would guess that 30 minute is better. Gives time to go get a cup of coffee and still allows a few ribs to be produced each day. 5 minute epoxy doesn't allow enough time to mix and apply and assemble for an entire rib. Aircraft epoxy? Will such stuff be any stronger? I'll bet it costs a lot more, and will take a week to get through the mail.
The nose pieces are held in location by epoxy to the front spar with 1/4" square reinforcement sticks, and a 1/2 x 3/4 nose spar; this is primarily to give the nose ribs support before the nose plywood is glued in place. The nose covering will be 0.032" plywood, so it might be good to make the forward spar 1/16" smaller in height and the nose outer contour 1/32" inward, to allow for the nose covering ply. This detail is too fine for the above plot, but we'll use a little give-and-take in the whole construction, to make up for the initially crude design. In fact, if you want to add sticks to better support the ribs, it probably will not add much to the wing weight, but will surely improve strength. -You could easy break this wing by punching it with your fist (although you might get some bad splinters!), but it will probably have very good strength in the air...
The trailing edge is designed to glue to a triangular piece that runs the length of the wing, to which small pieces will be attached later, during final wing assembly. I figure we will need a nice surface to which the fabric covering can attach, particularly at the rear. My guess is that covering will begin at the front edge where the nose ply is a nice surface and end at the trailing edge where we need solid attachment.
Each wing will be 6 feet long, so they can be built on an 8' bench, specifically constructed as a layout table for wings. To the wings will be attached the Horner wingtips, details of why can be found here. Once again, Chris comes to the rescue! -If Mr. Heintz is right, that this wingtip detail actually does increase the effective wingspan of the aircraft, then I'll take it. I don't know how it will be done, but perhaps a wood-stick construction can be made that glues on, or maybe a thin fiberglass construction, despite my reluctance to mix materials that could develop stress under temperature or humidity conditions. -That will come later though, and may require additional ribs stacked up at the tips for reinforcement. The ribs should be very light, adding a few more at stress points shouldn't hurt.
At the wing root we will surely need reinforcement, at least making a good connection to the spars with extra blocks of wood and a few aluminum (or steel) plates. We will need attachment at the wing root to the fuselage and also somewhere around the 60% point toward the wingtip we will need to attach struts that connect down to the bottom of the fuselage. For now, our rib should be OK though, we will add the attachment details for the final wing plan later.
The wings can't be detailed further without an understanding of the slot and flaperon constructions, because we need a solid means of attaching them to the wing. We can probably expect though that these can be supported every 24", allowing 4 attachment points for each, along a 6 foot wing, with ribs on 12" centers... This is perfect. Ok, then we can outline the whole wing, which needs details of the spars too:
With ribs on precise 12" centers, our wing becomes 72.25" long, which is acceptable, considering the simplicity of 12" rib centers. We will make the end ribs and every other rib slightly different from the others, a bit more reinforced, so that these spots can be used for attachment to the slots in the front and the flaperons in the rear. These attachment points will be added plates of 1/4" plywood (at the trailing rib ends) to which aluminum plates can be attached with screws, and slightly different nose ribs (at the leading edge) for slot attachment. -Don't know yet how the nose ply will wrap around the slot supports (cut slots in the ply before wrapping?) or how the wing fabric will get around the flaperon supports... I'm really attracted to placing aluminum plates at the bottom of every other rib for flaperon attachment, with two press-nuts in each plate, so that the skin can be applied flat on the wing bottom and external aluminum plates can be screwed through poked holes in the skin into the press-nuts afterwards. If such a scheme can be applied to the more complicated leading edge for the slots, then all is well for now...
By the way, these wings are stubby, which is probably the most striking deviation from traditional design. The addition of the Horner wingtips will make the aspect ratio look a bit more normal, and when fitted to the 24" wide cabin of the fuselage, the total wingspan will come out to about 16 feet. I really don't know how the aircraft will feel when rolling, without some mass out on long moment arms, but we'll see! At least the center of mass will be below the center of lift, which is what I'm counting on for roll stability. This is, nonetheless, one very high wing loading for a 27MPH stall speed, and frankly, I'm worried. I'm hoping that the slots are effective, and that wingtips and flaperons can be modified later to increase slow flight lift, if needed.
Because I'm concerned about the metalwork that attaches the wing accoutrements, so let's do that now:
This is the flaperon support at the trailing edge, which is easy to fabricate with tin-snips and a drill press, but the bends could be a problem. The bends can be assisted by drilling small holes along the bend lines, then bending becomes much easier, perhaps around a wooden form in a vise. Be sure the material is something strong like 6061-T6 sheet, and be sure the material does not crack at the bends! If it does, anneal the material by heating with a torch and quenching before bending, then re-harden by putting the part in a 440 degree F oven for a day, with slow cooling after. The bracket is attached with two screws and nuts, and epoxy between. We don't want to ever have to tear the wing apart to fix this... The rib will need some Xacto knife modifications to allow the bracket to fit flush with the bottom edge, some space for the external bracket attachment screws and some fill ply will need to be inserted so that the through-screws have some meat to grab onto. The press nuts should be 10-32. The attachment holes will be on 24" centers along the bottom edge of the wing, right down the centers of the ribs. I like this so far, but the leading edge slot supports will be more complicated:
But maybe not so complicated... These brackets are made just like the other one, and carefully positioned so you can find the holes after the nose has been covered. This is great, because so far we've got what looks like pretty strong slot and flap attachments, and the scheme doesn't mess with the covering process. -It does leave to 'tomorrow' what we could do today, namely figuring out the external brackets that actually fix to these points, but at least we have a start on the problem. We can always make the brackets differently later, if we discover that they need changing, provided we do so before actual building. In any case, we will need to epoxy the brackets into place onto the plywood plates, and notch out the nose ribs to allow for the screws...
The addition of the plates to the end ribs will make the wing slightly longer, at least at these points, but we still haven't firmly established how the wingtips go on or how we fix the wings to the fuselage. I'm leaving that 'till later.
We need to establish the design of the spars now, especially the points where the external struts attach. The spars will not be universal, that is, there will be a left set of spars and a right set. We'll just draw one each and assume mirroring in fabrication. First, we need a rough look at the angle of the struts:
The angle of the struts is about 38 degrees, and if we're off a bit when the final carriage is drawn, I don't think the off-axis stresses will tear anything up. Or, I certainly hope not. Sure, we can fly this thing on hope. We run entire countries on hope, why not the Flying Machine?
The strut attachment grabs onto the spars between the second and third rib, counting from the wingtip. The spars will have vertical braces at every rib location to strengthen the 1/4" ply from buckling, and we will have to put an angled brace between ribs 2 and 3 for the strut attachment anchoring.
The spars are not precisely defined, but the idea should be clear. We're going to do the same thing as with the slot/flaperon attachment, with brackets that are contained within the wing. However, in this case, we will not wrap the metal around the spar bottom, because we would have to weaken the spar's lower edge. Instead, we'll fabricate metal brackets that can go on both sides of the spars, bolted through angled braces. The braces are the same thickness as the top and bottom spar elements, 3/4 wood sticks. There are 1" wide vertical spar stiffeners on 12" centers (except for the ends, which are somewhat inside the 72.25" plywood), which are not visible on the rear spar because they are pointed away from the rear view. The brackets are fabricated from 1/8" aluminum, which will either need to be welded or folded carefully from annealed stock, then re-hardened. The brackets are made in mirrored pairs, one on each side of the spar. The exterior plates that attach to these brackets should engage both screws with a plate for strength. I would use 1/4-20 screws and press nuts for this application.
The root attachments are made to a solid 3/4" thick wood block, 4" into the wing, that give us attachment points at the fuselage. The brackets are from both sides of the spar, in 1/8" aluminum with 1/4" holes for attachment bolts. "1/8" channel might be stronger. This detail can be finalized just prior to construction.
Wow, so far we have all connections within the wing's covering, which makes me feel better about my first wing covering experience! We do have some metal work to do, but that's nothin' compared to the Flying Machine's frame.
I'm pooped for the day. Whew! I think the wing is done for now, we should look at the chassis tomorrow?
Actually, we need to detail the flaperons and the slot first, to be completely done with the preliminary wing. Oh, and the Horner wingtips... This is going to be difficult, because I need to actually wrap some 1/32" ply to get a feel for the process first. Not possible here in the jungle, and I doubt tree bark is a good model.
The kids are screaming about how they now prohibit me from building an airplane; they're worried that I'll crash. Nothing I say will get them to change their minds. Teenagers without a spirit of adventure, can you imagine that? At least the woman is indifferent.. I push forward, despite the noise.
The flaperons need to be big, because we have such a small wing area; the flaperons will give us the slow stall speed we need for part 103. -That, and of course the leading edge slots for a higher stall-free angle of attack.
I'm a bit worried about the large flaperons, because I've only flown a Piper Cherokee 140, which had half flaps and outer ailerons. I'm worried that the full width flaperons, although excellent for the extra lift, will be really touchy at the controls, particularly in roll. I don't want to have to go through an extended learning period trying to figure out how to keep the plane in the air. Maybe the stick can have a wide side-to-side throw, so that extreme roll is possible, but most operations are within a comfortable middle range. That, or I suppose the stick affect on the flaperons can be slight, compared to the flap control. We'll see, but this is one that makes me uncomfortable leaving to later, and its a serious issue that I can't solve now. Maybe this is why folks don't just 'build it', because there are so many 'but what ifs' involved. I'll expect that careful attention to getting slowly into the air, long experiments on the ground, a lot of slow flight just above the runway, slowly getting into the air with slight modifications along the way. We'll get there, but not if we're in a hurry.
Let's tackle the flaperons first, which I would like to make a camberless shape (no overall curvature) which will be very different from the slots which have extreme camber and therefore will be more difficult to cover. Because these are much smaller than the wing, I figure I can use 1/32 ply to cover them and waste some weight, hopefully getting strength and durability in the process. By the way, I have an empty aluminum beer can here that is fascinating. I'm guessing the wall thickness to be about 0.006", since a few months ago I had torn apart an American Coke can and measured its wall at 0.004" (for a different project), and this one seems a bit stiffer, maybe made with an older technology. This can is of the tall type, maybe 7.5", and it can be twisted by grabbing onto the top and bottom with no noticeable deflection and it only buckles when the spots where you're grabbing it buckle from holding force. Amazing. Our constructions won't be nice round tubes, but it gives me confidence that really thin skins, properly supported in spots, can withstand severe torque.
Since our wing attachment points for the flaperons and the slots are on rib centers exactly, we should be able to build these wing add-ons with the same technique, attachments and all. We'll solve the problem of actual attachment with brackets that we'll make later. Heck, 8 flaperon brackets and 8 slot brackets should be easy to chew out of aluminum sheet, and we can change the positions of these things at will, simply by changing the brackets. I really like this; we're designing an airplane from pictures after all, and it provides some comfort that the design is now somewhat variable when we head to the air tests.
BTW, I heard back from Tom Benson from FoilSim who says that they may show drag in the future, it's just a very difficult set of calculations. Wind tunnel tests can hopefully provide some values that can be applied to the program. Hopefully this will be in place when you attempt your own plane. In fact, I've been attracted to the notion of a big fan that can blow past a small model, which may be helpful in placing the slot and flap bracket; we'll see about this too.
Oh, the flaperons...
This should do, and since the shape is so simple and there are a few dimensions to work from, the final layout should be easy to copy. Small fir spars extend through the ribs from end to end. The ribs are made from 1/4" plywood. The attachment, only required on every other rib, is of the 'U' type used in the wing, with 1/4" ply pieces added to the ribs to increase the rib thickness to 3/4" at the bracket only. The rear spar is to provide an attachment for the covering along the rear edge. Since the attachment points match those in the wing, figuring out connection brackets should be straightforward.
The 1/32" plywood covering will begin at the rear edge, gluing the bottom of the airfoil to the ribs up to a spot near the brackets, but not all the way to the nose. After curing, the excess ply is trimmed to the rear spar. If the brackets are carefully positioned, the precise location of the screw holes can be determined by measurement along the finished part after assembly; don't bother trying to poke holes in the covering that match the mountings.. yet. The second wrapping of the covering will require a fixture that holds against the rear edge (which is now quite solid), by applying epoxy to the ribs and pulling the cover around the ribs to at least the upper spar. This is the toughest part, because of the rather sharp leading edge. A fixture that is composed of a long stop attached to a flat plate (plywood tooling) allows strong rubber bands to pull the covering around the ribs, matching their shape. Finally, the last of the covering is applied to attach the ply to the trailing edge and it is trimmed again to the lower, already trimmed edge. The remaining 1/16th" thick trailing edge is OK. -Or so says the guy that isn't an airplane designer!
We won't detail any end caps for the flaperon, because that is pretty much cosmetic, and we may need to provide an attachment point for the control rods that will allow pilot control of the surface. Later, when these issues are resolved, we can make up something that looks nice.
Ah, but the all-important leading edge design of the.. leading edge, the slots...
This shows a reasonable position for the flaperons, pivoted about the tiny circle below the flaperon leading edge. This spot is 1" below the lower flaperon spar. The view shows 30 degrees down and 15 degrees up. At greater flap-down angles, the flaperon clears the trailing edge of the wing.
The slot is an extreme airfoil, designed to trap air at high angles of attack and pass the air at high velocity across the upper surface of the wing. Here is where my intuition is starting to wane. I really don't know how to calculate anything like this, and it seems a wind tunnel is the only way to acquire an empirical understanding. Build a model and hold it out of the sunroof of a moving car? -Maybe. For now, it seems like this looks like Chris' design, and maybe the connecting plates can be adjusted to vary the angle of attack as more data becomes available. Anyhow, it looks like a good shape, let's build it.
Because of the extreme concave bottom surface and the sharp nose, I expect that this will require some serious thought about a gluing jig. Right now I would expect that the lower edge should be attacked first, then wrapping the skin around the upper edge. The construction is just like the flaps, but the trailing edge is a 1/16" thick strip of hardwood, maybe maple or hickory, but you can always change this, the idea is strength and a good attachment point for the start and end of the cover wrap. I thought about an epoxy-glass strip, but despite it's strength, it will expand and contract at a very different rate from the wood, and place significant stress on the assembly. Because the shape is so complicated, attachment points may have to be adjusted so that they can be easily located after covering. It might be nice to build a template that shows press nut positions from the outside of the shape, so that after covering the locations can be more easily found.
Then there's the wingtips, which I'm not lovin', because they are so 3D and I hate working 3D in AutoCad. What I would like to do is continue the upper surface of the wing outward, then bring up a flat surface from the bottom edge of the wing to meet the upper curve, at maybe a 33 degree angle. This should give the outer end of the wing a curved shape, extending as much as 12" from the last rib of the wing.
What I propose is a 'fitting' process, where the assembled wing, prior to covering, is set up on a flat surface, and a thin material, maybe cardboard, is set at the desired angle from the lower edge of the wing outward. Then we lay a thin stick along the wing's rib structure out to the cardboard and carefully draw a line on the cardboard, all around the wing's contour, including the rise around the lower nose. Maybe we could shine a flashlight down the wing, casting a shadow from the ribs onto the cardboard. The cardboard drawing now becomes a template we can use to cut a piece of 1/4" plywood, or construct tip ribs from. If the end detail is made from plywood, many holes can be cut into it to lighten it and still retain strength. The main spars will not extend to this area, but many sticks, cut from 1/4" stick-rib stock can be used to finish the shape. This is what we will grab onto when we pre-flight our aircraft before heading into the blue, so it needs to be able to be very roughly handled and very well attached to the main structure.
This is a jpg of the wing airfoil that I have stretched vertically after inserting into the webpage. Since the lower surface of the wingtip is angled and flat, joining at the bottom of the wing, this should be a reasonable template for an angled end piece. I'll probably do exactly this when cutting an angled end cap for the wing.
Since these pages are getting long, I'll do this in multiple pages for convenience.