|Dec 23, 2012, 01:13 PM|
Boeing 717 - Scratch Foam Build
Last Update : 02/17/13
Okay, so here we go again. I've decided to try something I've never done before that being a scratch build. Forty-two years ago when I started up in the hobby, scratch builders were common place. You either bought a kit or drew up your own and in truth the plans were the only significant difference since a kit consisted mainly of a set of plans and a box of wood. If you were lucky, the wood might have a pattern printed on it but there was just no such thing as die-cut plans or if there were, I didn't know about them and never saw any. At that time I knew virtually nothing about drafting of design or flying for that matter so I began like many of my generation, building a high-winged trainer from a kit and progressing from there.
Fast forward forty some years and I know a bit more but still have never ventured into this venue until now and again, I do so more out of necessity than desire. Yes I want the model, and if it were available commercially I would likely buy the ARF and do my usual thing which is modify it to meet my requirements, but with Airliners as a rule that isn't yet an option. The choices are few, and although I do own a couple, the rest of what is out there I'm not feelin at the moment. So enough of the why.
Surprisingly, hours of searching the net and my local library yielded very little in the way of blueprints for this first generation new kid. I guess it's short production run and the limited numbers made it less than desirous author fodder necessitating that I dive into this less familiar end of the pool. So, after reading hundreds of posts by the folks kind enough to share their experiences I found what I judged to be a suitable three-view, scaled it up to a size I could live with, 70" fuse and a 50" wingspan and broke out my old drafting tools. I could have taken it to Kinko and had them scale it up and print it on the plotter, but I'm too much of a control freak and wanted the ability to change whatever I wanted on the fly (sorry about the pun) so I did it myself.
After posting a request for help with the airfoils for this AC on the forums, Keenan Smith was kind enough to answer and provide me with an entire set of Auto-Cad drawings. Special thanks to him for this kindness.
The plan is to go with twin 70mm CS10s which are going to be disproportionately large but here again, that's one of the changes I referred to previously. I'll initially run these on 3s motors in the 3200kv range. At this size the power/weight ratio should be somewhat less over the top but I want the option to run 4,5 or even 6s as well. The sound and displacement of a 10 blade is only offered in a 70+mm format and, as it is a size I like and have done many mods with, I feel confident that I will be able to make them work properly and still look acceptable to all but the most hard-core.
This will be a full function build with all the usual bells and whistles (fowler flaps, leading edge slats, spoilers, retracts, lights etc.) as well as the addition of thrust reversers, something I've only seen Mike Neff do successfully, but once seen, I knew I had to have a plane with them as well
This is a dollar store foam build. The foam board is 5mm thick including it's two sided paper backing and just over 3mm thick with it removed. I decided to use a tubular inner support structure, and card model type adaptation for the flight deck and empannage.
I began by cutting out the support bulkheads (ribs) which will each be two sheets thick. I then stripped the paper backing off of both sides of what became the central tube, cut it to size and cold rolled it using a method I learned some time ago on YouTube.
Next I sized, cut and glued the fight deck central spar framework together. This part caused my first real headache. I had forgotten about the below mid-line offset of the horizontal center former. After puzzling this out (it took three tries) I was pretty happy with the results.
Next the paper backing was stripped from both sides of a piece of foam board, and covering sections were sized, fitted from former to former and finally secured with foam-safe CA. The nose is a veritable jigsaw puzzle but I took my time and ended up with a better than expected result. A little sanding, spackel and finish and it should be flawless.
Next I moved on to the empennage. The center spar was cut and laid out on the plan, and the tail section ribs were cut and dry fit. (the V-Stab itself will be a 3 ply lamination. Then the H-Stab & Elevator cores were cut and laminated. For the lamination process I chose to use Titebond III wood glue for it's added strength and sandability. The process is described in more detail in the picture captions.
While waiting for the laminations to cure, I moved on to the wings. Here I chose to use the same technique as I used on the fuse, a central structure (in this case the entire flat wing form) and airfoil formers, cut longitudinally and glued top and bottom to that form. This will be modified on the fly as I figure out optimum servo placement. A lot of control surfaces means a lot of servos so placement will be critical in order to insure that they work properly and can be easily maintained.
Wing scratch building......what was I thinking?..... The simplest looking things have so many little hidden nuances and my own carelessness (I actually made two right wings....)complicates matters even more!... Okay, enough whining. The wing box, and hard point formers and braces were constructed today. I used a different laminating process on joiner box and the wing roots. I used 3M Super 77 spray adhesive to initially join the foam and wooden parts, allowed these to cure for a couple of hours and followed it up with thin foam-safe CA all along the perimeters.
I epoxied the wings to the joiner box and used a wedge between the wing root plate and the wing joiner box to set in the 3/4" dihedral. A spar brace (a 2" long hard balsa wedge) ,applied where the spars meet inside the joiner box, helps reinforce the dihedral as well as helping to distribute the wing load.
A dry fit and weigh in has the assembled parts so far in at just under 1 lb. ( target weight is 4lbs total )
Next I removed the wing control surfaces and lined the control surface roots with balsa and foam after which I cut, sized and glued the top wing skins in place. The wing skins are a single sheet of material with the paper removed from the underside. I then used spread contact cement onto the wing ribs and any surface that would come in contact with the skin after which I laid the skin in place and pressed down firmly against all of those contact points to insure that the glue contacted the underside of the skin. Then I removed the skin and allowed the glue to become tacky which took about five minutes. Once the glue was very tacky I reapplied the upper skin, pressed it back into place and used strips of blue painters tape to hold it in place for two days while the contact cement cured completely.
Today I checked on the wing skins, they are cured and applied two coats of filler to the flight deck section of the fuse. For this I used a 50/50 mixture of DAP's lightweight spackling compound and Durhams Rock Hard Water Putty. I use this like auto body guys use Bondo on a car body, to fill low spots and seams, divots and dents. This mixture whips up like a light mousse (I add a couple of squirts of water to make it slightly less stiff than the spackle is naturally.) but dries much harder while remaining very light.
While the nose and skins were curing, I moved on to filling, shaping and finishing up the H stabs & elevators. Below you will see a picture of the dry fit of these pieces. The next step is to determine where the elevator servos will be placed. Traditionally I use the underside of the H. stab and try never to rely on U-bars or long push rods as I find both a constant source of aggravation. In the case of a split elevator, rather than a connecting U-bar I prefer to use two independently adjustable servos. The added weight is a very small price to pay for the adjustability and accuracy afforded by by this method. As on any mission critical control surface I will always use an analog metal geared servo. On this type of AC digital servos are an unnecessary power drain (constantly hunting center) and the metal gears ad security and longevity. Once buried in place, replacing a servo can be a real P.I.T.A. so the extra grams in trade for these benefits is a real bargain.
Next I got to work on the leading and trailing edges of the V-stab, I filled and sanded it and applied the third coat of filler to the flight deck area of the fuse. Both areas have been sanded to a pre-finish smoothness. The fight deck is ready for a finish and the tail is ready for some formers and a skin.
I then fabricated and fitted two different versions of a "leading edge slats", one from pink foam the other using the more traditional rib method to see which I liked better. Either of these would do the job but I think I'm going with the later method.
Completed the fabrication of the L.E.S. and moved on to the Nose Wheel retract unit. I adapted an oleo strut from an old LS B2 by boring out the pin to 4mm, removing the servo halter, loosening the spring a bit and attaching some clevises for the pull/pull steering. This strut mod worked out perfectly although I will need to get some better tires eventually. For now the foam tires from the B2 will work.
I used one of Demondrivers designs for the nose wheel containment unit which houses the E-Track and the steering servo then dug out a home for it in the flight deck. I decided to use a tad more strut length than scale in an effort to insure T.O. rolls don't go excessively long.
It's a good thing I don't do this for a living as I work way too slowly to ever make anything over Chinese slave wages but that's the price you pay for wanting to get it right I guess.
Took the entire day to frame in, skin and spackle the underside of the flight deck. I sure hope slow and steady wins this race. It was pretty much like cutting out and fitting pieces of a jigsaw puzzle. The upper surface went on very quickly, but along with the many little angles and bends in the underside there was the added headache of figuring out how I was going to attach gear doors and how much clearance was going to be needed for the nose wheel. All in all not the most fun day I've had on this project. I'll be glad when the doors go on and worried until then.
The last couple of days have been a little slow. Most of the work done has been electrical in nature. I rebuilt and rewired a few servos that I will be using in the wings, ran servo extension leads to the various servo mounts, mounted the servos and finally skinned the underside of the both wings.
On this project I chose to work with the servos mounted and exposed as opposed to buried. When building on the cheap the use of less than top quality equipment is a necessity. This means that there's a far greater probability that one or more of the more fragile parts (in this case servos) may fail. If the servos were buried away in the wing or the tail they would present a far greater challenge to swap out. Furthermore the L.E.S. system that I'm attempting on this AC will, I'm sure require many adjustments in order to get right. Having the servos buried would simply serve to complicate the already complex task of adjusting/replacing them therefore they've been left 50% exposed.
The empennage has been readied for skinning on one side and a piece of that skin has been dry-fitted and appears to be ready for glue. Before skinning however I needed to get the motor mount fabricated and fitted. For the mount I used two lengths of 1/8" ply laminated together and supported and secured by foam below and pine blocks above. These blocks are glued to the central spar and the motor mount, which passes through the spar, is screwed to the pine; simple but very secure. (pictures below)
The last couple of days I've been exploring/experimenting with different nacelle options. The modification of commercially available foam nacelles was one option I experimented with. The results are satisfactory but I'm nothing special. The other options were both fabrications, one based on laminating layers in order to develop enough depth for sanding the contours, the other is a ring build up, using angles and increasing/decreasing size rings to build-up the contours. At the moment I'm leaning toward the lamination product, both it and the modified commercial product are pictured and explained below. (pictures below)
Added two pictures (64-65) of the third (ring consturction) nacelle prototype, one with and the other without the motor dry fitted. Although this is the lightest of the three prototypes, it's also the flimsiest and I believe would require glassing in order to be appropriate for outdoor use and therefore might not be the lightest in the end.
I also added a couple of pictures of the laminated construction nacelle ready to seal and also one with motor fitted. In both cases a styrofoam cup is being used as a stand-in for the yet to be fabricated thrust reverse nozzle. I believe that this is the most appropriate type for this particular build. It's rugged, will require only sealing (no glassing) and should look very scale when completed.
Began fabricating the clam shell thrust reversers for installation on the thrust tube of the nacelles. The units are comprised of a thrust tube and two clam shell halves. Dap light spackling containers with their bottoms and upper most lip removed are used as thrust tubes and will be epoxied directly to the end of the nacelles. Each thrust tube is partially covered with a ring of foam. The clam shells are fabricated from 64mm EDF housings which have the stators removed and are then cut in half. Pictures 68 and 69 show the materials and the modified parts dry fit. The hard points (popscile sticks) are epoxied into place, and the clam shells are fitted, marked and drilled. Straight pins are used as temporary hinge points to insure the geometry is correct to allow free and full opening and closing.
The pictures show the finished and assembled thrust reverser as it will sit.
The pictures show the nacelles with the Change Sun EDF housings in place dry fit on the empennage without and then with the thrust reverser in place.
Although the proportions of the nacelle are about 15% larger than actual scale the proportions are acceptable.
The foam clam shell covers were laminated in order to allow 90% closure of the TR when deployed. This took many hours of cutting card stock and fitting, then trimming to finally get right but it should be worth the effort. The foam covers were laminated to the plastic clam shells using contact cement, then sealed and painted.
The foam clam shell covers were laminated to the plastic TR bucket halves using contact cement and reinforced along their inside edges with fiber woven duct tape. Next the TR units were dry fitted, again with tape, so that the proper geometry for the linkages could be found. The linkages themselves are constructed of three sections of push rod which form a Y. The ends are joined at the Y juncture with copper wire which is tightly wrapped around all three rods and soldered to form the Y. The ends which attach to the bucket halves are given Z bends and the servo end is left straight and attaches with an easy connector. The servo itself is housed in a plywood brace that is screwed directly to the EDF boom between the nacelle and fuselage. Check this video out for clarification.
The next item to be addressed were the fowler flaps. These I initially made out of 3 layers of laminated foam board, however this lacked the necessary rigidity and the flap would flex before moving. As well as the flexing problem I was temporarily stumped by the geometry required to move the flap through it's full range and maintain enough tension in the closed position to keep the surface firmly closed. Once the geometry was sorted out I re-fabricated the flaps themselves using 1/4" hard balsa as a base, balsa wedges to create the proper plane and a foam top cap.
Below pictures show the completed control surface installation.
The rest of day was spent making the canoes and servo covers which took surprisingly longer than I anticipated although it wasn't anything difficult, I just wanted them to look a certain way so I had to remake the first few until I got the look I wanted.
Got up today, looked at the canoes and hated them so I redesigned them and now I'm much happier. I finished up the fabrication of these details and am ready to seal, fill and sand the wings and the main fuse.
The last few sessions have all been about sanding, spackling and preparing to join the three fuse sections. During this process I became unhappy with the prospect of videoing the maiden in such an "unfinished" (albeit cosmetically only) condition so I decided to put on a temporary finish until such time as the AC proved itself to be a keeper. My normal finishing medium is paint, something to which I normally dedicate many hours and a great deal of effort to get right. However for this "temporary" finish I decided to use vinyl hobby-craft covering. Having done virtually NO covering in the past 25 years my skills in this arena are woefully few, but I've managed a marginally satisfactory job given the temporary nature of the finish. To simplify the matter I chose the QANTASLink Livery scheme. Hopefully, if the AC preforms up to expectations I will be changing it over to the old school TWA livery pictured at the head of the build log.
The last two sessions I've spent cutting out and affixing passenger windows and various other innocuous details (something I said I wasn't going to do until after the maiden). Although I'm certainly no pro at covering, I must admit that it looks a lot better than it did before I began and much better than I anticipated using this material.
I beginning to get into the end game here and already planning my next project... LoL....
Was forced to order some new retracts as one gave up the ghost during testing. The only place around that had the proper size was HK so I must now endure the interminable wait for them to arrive.
So today I decided to tackle the wing fairings, something that I didn't anticipate would be as big a P.I.T.A. as it turned out to be. This project, although a great deal of fun, has been very humbling. Having done in the neighborhood of 100+ mods I really didn't think I would have much of a problem applying those skills to the full scratch build process. Boy was I dreaming! Every turn has held a new set of challenges, adjustments and design changes and none greater than these fairings. Early into the wing build it became apparent me that I wouldn't have room to install the entire main gearbox/door assembly I had originally planned. The model is just too small, the wing too thin and the gear mechanisms too large and any attempt to mount doors and a fully enclosed gear mechanism was going to leave me with a "Pregnant 717".
But the fairings proved to be almost as problematic. Having designed the wing box as part of the wing structure I had originally planed to carve and glue them directly onto the fuse (don't ask me how, but it seemed like a good idea at the time) but once I realized that wasn't going to work, I had to figure out a way to "skin" the fairings on. First I attempted to learn enough sketch-up to allow me to try to flatten the part so I could cut it out, bend it and have it slip right on... Yeah.. right...That was a miserable failure. I then began searching for some card models with similar fairings in order to get the basic shape. I ended up modifying a V-stab fairing which surprisingly worked pretty well for all four corners. Then the problem of how and what to attach the fairings to raised it's ugly head. Since I wanted the wing to remain at least somewhat removable, I ended up using the vinyl covering material to secure the 10 pieces (like a jig saw puzzle) in place making the wing a semi-permanently attached feature. The wing remains removable but only at the cost of all of the fairings.
Covering the fairings was another debacle involving a myriad of complex curves which made me want to throw the entire thing in the bin on several occasions....... But I finally got it done and although I'm certainly not going to take any close up beauty shots of it, from 10 feet off it will serve.
What a learning experience.... I certainly hope the next one is easier and looks better.
Short day with no drama. Nacelle fairings cut and installed, a piece of cake by comparison to the wing fairings. Now I'm just awaiting HK to deliver my retract order and I'll be ready for a taxi test.
That's all the progress so far.
I'm always looking for advice and helpful suggestions so feel free to chyme in.
Comments and suggestions are encouraged. Hey, I'm winging it too!
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|Feb 17, 2013, 06:06 PM|
The secret to being able to 'flatten' curved parts in CAD is to avoid complex curves. If you can limit the curvature to a single plane, the parts will flatten without too much difficulty. It helps a lot if you can break the part your are designing into smaller components that are keyed to specific features on the aircraft. Sketchup isn't the ideal software for this kind of work, but it can do a reasonable job if you know what you are doing. The problem is that the learning curve is pretty steep. (I have to admit that I tried it and couldn't get the hang of it for the life of me, and I've been doing CAD for years) The fairing you have is actually not too bad, it just needs to be a little longer so that the transition between the fuselage and the wing is a little smoother.
Another method you might try is to use paper. Get some construction paper and bent it so that it approximates what you want the front part of the fairing to look like. put it in place on the fuselage and when it looks right, use a pen or a pencil to trace where the paper touches the fuselage and you will end up with a rough part template that you can refine.
|Feb 17, 2013, 06:30 PM|
Thanks to both Chris's, DD for the encouragement and KFC for the advice.
I took a run at Sketchup when it first came out years ago for some woodworking projects. I had limited success and eventually went back to pencil and paper. Sketchup 8 is said to be far different than the early 2.??? I used but I didn't find it any more intuitive and may examine it again, but am not holding out too much hope for it to become a staple in my design process ..LoL.
On the actual AC the fairings are quiet small and shallow suggesting that my wing placement is incorrect (too low). When I tried the more scale longer gently sloping fairing, it distorted the contour so dramatically (damn thing started to look like a G5 with those huge nacelles) that I abandoned any thought of trying to keep the fairings looking scale.
The whole thing has been a learning process and I'll take a great deal more care to consider the neuances and consequences of all the little design features before I run around changing them willy nilly on the next one.
|Apr 23, 2013, 06:28 AM|
I have been using sketchup 8 on my 319 and the advice you been giving and watching your build I think I might be able to help you with your next design if you want. It will be free no charge and i can make pdf off of the sketchup and send them to you as we complete each section.I am also making some template parts of rc lander gear and some servos so that accurate placement can be done. I will let you after I get the 319 done.
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