This hobby we are involved in, is amazingly diverse and allows us all to participate at a level of complexity and expense that suits our skillset and pocketbook. At the same time, because we all have varying ideas of what makes a great model, the range of airframes out there is actually staggering if you think about it for a few minutes! Gone are the days of simple box fuselages and constant chord wings with high lift sections – now we can build or buy the finest scale models, power them with high-tech nitro or gasoline engines, electric motors, or even turbines! Those of us old enough to remember the “good old days” know that there wasn’t too much good about them, as we struggled with reliability of engines, structures and radio systems. More often than not, we’d leave home with a couple of decent models, only to return later that day with a bag or two of damaged parts! Yet we continued to strive, struggle, and finally triumph because that was our hobby and we were determined to make it work!
Now that I’ve established myself as one of the “old” crew, it’s time to take a look at what’s available today, and even with the fantastic range of designs, kits, ARFs etc. some of us find that our desired model hasn’t yet been designed, built or kitted. What are we to do? Well the simple answer could be just to wait until somebody does it, and then purchase his kit – but how much more fun would it be to design your own model from scratch! Then, when we go flying, we have a much greater sense of achievement, respect from other modellers for having gone the extra mile, and the knowledge that we’ve got a truly unique model that simply isn’t available in the stores. Go that one step further and offer it as a design or kit and you might even make some cash on the side, thereby allowing you to continue in this fine hobby. Wouldn’t it be great to make your hobby pay for itself – even partially!
Enter technical drawing…… with pencils, erasers, rulers, squares and the like. Lots of tracing vellum and those smelly purple-lined prints. Ah, the joy of it all…. but it’s now the 21st century isn’t it? Surely then, we should be using techniques that are more in keeping with modern technology an methods. I’m not knocking the old 2H pencil by the way – I’ve spent many an hour doing it the “hard” way, and had a great time while doing so. However, the “modern” way it to design your model on the computer, and I promise you that the great majority of current models are designed this way. Computers are much more accurate and quicker, and we can make changes with the click of a mouse, instead of erasing and redrawing it with that pencil. How about re-scaling it from 1/4 to 1/5 scale? If you’re still using pencil and vellum, you have to start over, but with a computer you can simply zoom the selected parts or design to any scale you wish, in just moments.
Have I whet your appetite? Are you interested in having a look at the techniques necessary and producing that one design you have yearned for, for years? Now hang on there a little while…. it isn’t just a matter of purchasing the software and magically arriving at the end design, and anyway, how much fun would that be really! There are some learning steps you need to go through, none of them difficult or particularly onerous, but steps nevertheless. Like learning any skill, it takes a little time and effort, and as they say – Anything worth doing, is worth doing WELL! So, if you’re up to the challenge, strap in and let’s get started.
I’m intending to work up several designs during this article and those that will follow. Initially, the design will be a simple, easily achievable one, but it will still give us the basic skills we need to be able to progress to that swept-wing, scale model with retracts, flaps etc.
There are a number of packages available, that will do the job for us. We need to look only at VECTOR-based graphics packages because they allow us to scale our drawings and still have fine, accurate lines in the finished plan. If you choose to use a raster-based program such as MS Paint or even Adobe Photoshop, you will be disappointed by the complexity, difficulty and poor results. Take my word for this.
I’m going to recommend CorelDraw (but other vector drawing packages are of course similar), and you can pick up an older version for less than $100 without difficulty. The latest version is not necessary, as even a 10-year old version will do everything we need. As long as your computer will run it, go ahead and use it. An alternative is to purchase an educational version, as these can be had for a similarly small amount, providing you qualify – or have a family member who does. Essentially, if you are a student or educator, you can get the latest version for less than $130, and as long as you’re only using it for your own purposes, nobody cares anyway.
Workflow and “Learning”
Before we jump into design, let’s get a couple of things in the open. Whilst the techniques I’m about to describe are in no way difficult to master, you will need to approach your design process with a certain workflow in mind. There’s no point starting the internal placement of formers, ribs and spars etc if you don’t have an accurate outline drawn. Similarly, don’t go too far on internal design unless you have an accurate three-view completed, with all the major components placed in the correct place on ALL THREE VIEWS! An error of 3mm when placing the leading edge of the wing while doing the profile view, will cause major headaches when working on the plan view. Ask me how I know this!
Also, like me, you will find that you are learning new techniques and short-cuts ALL THE TIME! I’ve been using CorelDraw since 1991, and I’m still learning new things about it. Every now and again, I’ll stumble on something that I’ve been trying to do for years, only to find that it was embedded in the software all the time and I just hadn’t noticed it yet. So take heart – if it seems difficult at first, it’s only going to get better with practice.
However, as with most skills, it’s best to start with “baby steps” and master the basics before continuing with the more complex techniques. With this in mind, I’ll be walking you through the basics of drawing and modifying shapes, cutting, grouping, changing line thickness etc. If you already have these skills, feel free to skip ahead to the next section or article. Otherwise, follow me through and I’ll try not to lose you in the fray!
On the left-hand toolbar, you will find the Rectangle, Ellipse and Polygon tools. Once selected, you can click and drag on the screen to make the shape. If you hold the CTRL key at the same time, the shape will be symmetrical – a square, circle, hexagon etc.
Holding the SHIFT key while drawing the shape will keep it centred on the point at which you started the shape. Combining the CTRL + SHIFT gives you a symmetrical shape, centred on the point. You’ll quickly get used to this.
Similar to other software, when an object is selected (using the PICK tool), a series of black boxes appears. This is called Marquee, and you can now stretch and size the shape you’ve drawn. While the object is marquee’d, any effect, colour, outline, texture etc. can be applied to it.
If the object is selected again, the marquee changes to the rotation handles, and you may rotate or skew the object to suit your purposes.
Convert to Curves
With the object marquee’d, go to the ARRANGE menu and select CONVERT TO CURVES. When you do this, the marquee disappears, and the outline shows a dotted centreline. While selected, the SHAPE tool can now be used to select line segments and change them to cures or add extra nodes to change the shape. In this way, it’s a simple process to make the shape conform to any desired form.
Choosing the design
I’m also a pilot of full-size aircraft, and one of my favourite aircraft has always been the Pilatus PC-6 Turbo-Porter. I flew one for a skydiving club back in the 90’s and found it to be a wonderful, honest, high-performance aircraft that provided hours of challenges and satisfaction, and was fun to fly too! Therefore, I intend to introduce that design to you now as my first example. As you will see from the drawings, it is a straight wing, tail-dragger with an essentially square and box-like design. Beauty is in the eye of the beholder of course, so I don’t expect everybody to love it, but that’s not the point of this article anyway. Meanwhile, if you close one eye and look at it “just so” it could be just another RC trainer!
Now, Pilatus is one of the few manufacturers who like to help modellers achieve their goals, and a simple search of their website will provide a set of accurate 1/15 scale drawings of the Porter on which we will base our design. If you want to download your own set, get them here http://www.pilatus-aircraft.com/#214
Import and Initial Trace
The Pilatus drawings are in PDF format, and can be imported directly into the drawing pane of CorelDraw. From here, we’re going to trace the factory drawing and produce our initial view. Note that the software generally has a “trace” function, but due to the vagaries of PDF and JPG files etc., the results will be poor if you use the automatic function. Far better to do it manually at this point, and this isn’t anything like as difficult as you might imagine.
Let’s get Tracing!
Well, the basics are in hand, so let’s FILE-IMPORT a three-view to trace. I like to start with the profile view, but it really makes little difference in the end. You may choose whichever view you prefer. Draw rectangles, convert to curves and adjust as necessary until you have the outline drawn. Work carefully, because your finished model depends on accurate drawing at this (and every) stage. Any error introduced at this point will certainly show up later when you try to build the model.
Following the tracing session, we are left with this drawing…..
Once you are satisfied the drawing is complete and accurate, select and remove the background drawing and the detail work may now begin. Notice that my drawing has the aeroplane in the “flying” attitude i.e. not with the tail down as it is on the ground. On the Pilatus drawings, I found that some of the measurement lines were parallel to the longitudinal axis of the aircraft. This made it easy to align, and I can now work with certainty to provide the correct downthrust angle for the engine as well as the correct incidence on the wing and tailplane.
You can see there is fairly significant downthrust in the engine (70 actually), and this is because of the high-lift wing section. Ultimately, we want the aircraft to fly essentially horizontal, with the wing set at a slightly positive angle (I’m going to select 20 in this case). At the same time, I’m going to set the horizontal stabilizer to 00 pitch angle.
Draw all three views before working on any internal detail. It is important that the views are all accurate, and checked against each other. Look for critical dimensions such as wing position, chord, overall length of wings and fuselage and any other major parts. You can probably appreciate the trouble you can cause yourself if you don’t complete this stage properly.
Once the three-view is complete, it’s now time to scale the drawings (all together) to suit the size you require. This can be a particular scale, or simply the correct final size of model to fit your vehicle – you choose. The Porter has a wingspan of 15.87 metres, so for my purposes I’m going to scale the drawing to 1/10 scale, giving us (of course) a 1587mm wingspan. This model will be electric powered, so this size is perfectly reasonable.
Once you have the scaled the basic drawing, you can then start planning for the wood thicknesses and rib and former spacing. A design can succeed or fail at this point, simply with the design of the internal structure. It is important that the structure be strong, yet light, being neither too heavy, nor too weak. Any unnecessary structure simply adds weight, but too little imparts weakness that may later lead to failure.
For the Porter, I planned for 3mm balsa sides as well as top and bottom sheeting. Having decided how thick the outer skin needs to be, it’s time to produce an inner contour of that thickness. This is best done automatically by using the contour tool.
Firewall and Formers
With the contour in place, now is the time to define where the firewall and other formers will go. In a model of this size, it is important not to add too much weight, so it definitely suits us to have a minimum number of formers and since we’re planning flat fuselage sides, this will be relatively easy. However, for a scale model we need to consider the location of things such as windows, undercarriage mounts and wing mounting structure, as well as batteries and radio gear of course. Refer to the next drawing and I’ll explain my rationale for former placement and any additional considerations.
Starting from the front, I’ve placed a 6mm ply firewall in position, with a 7 degree downward angle. Immediately back from that, is a 3mm ply former, and this one is intended to be open at the bottom so we can place a battery hatch there under the nose. Further back, we find another 6mm ply former. This one could actually be 3mm, but as it takes undercarriage loads I prefer to make it thicker. I’ve seen many ARF models lose their undercarriage on otherwise quite normal landings, but mine don’t leave the aircraft unless I’ve REALLY messed up the arrival! We’ll revisit this former when it comes to designing the undercarriage attachment, but for now we can move on.
Next come the two formers for the wing mounts, and in normal models both these could actually be deleted as there is plenty of strength in this area. However, I want to keep the windscreen and cabin area relatively scale, so these two formers will do the trick. We’ll come back to these also.
A natural position for a former is where the structure changes shape and the aft cabin area serves this purpose on the Porter. The remaining two formers are just placed at arbitrary positions wherever I felt the need, and their main purpose is to stop the fuselage sides from warping or “sucking in” like an anorexic chicken!
We’ll get to the individual former shapes next. For now, I’ve decided to make them all from plywood, although the last few could certainly be balsa if needed. I’m planning the parts to be cut by laser, so I can cut huge lightening holes in them anyway and the end result will be more strength for no extra weight penalty.
Ok, so let’s look at the former shapes. In this case, Pilatus have been kind enough to include most of them for us. In any case, the fuselage is essentially rectangular in shape, with rounded corners. If the formers need to be drawn “manually”, a simple measurement of vertical dimension, horizontal dimension, and a common curve on all corners will suffice.
The firewall may be left simply as a single, solid unit if desired. However, to make it easier to mount the motor, I’m going to have holes pre-cut. Additionally, there needs to be airflow into the nose section for cooling the ESC. Finally, I’m planning to place a couple of stringers along the sides and top of the nose to help with strength and shape. These need to be allowed for and drawn on the fuselage plan.
With the other formers, it’s best to keep them as light as possible, but of course structural strength is still the priority. My recommendation is to work on the formers with them positioned on the plan initially, so you can accurately place the stringers, wing and undercarriage mounting brackets etc. Where necessary, the formers can be made transparent, or the outline colour can be changed so you can easily recognise them while zoomed in etc.
By this time, I now have the formers pretty much complete, and I’ve included undercarriage and wing mounts. Note that main undercarriage mount is from 6mm ply, and fits into a notch in each side of former #3. The lower mount is formed by a central part that notches into the lower centre of formers #3 and 4.
Similarly, each wing mounting block is supported by a bracket each side of formers #4 and 5.
With this all completed, we have this structure designed.
A word or two about the fuselage sheeting. I’m planning on 3mm balsa sides, top and bottom for the fuselage. By considering the former corner cutoffs, I will now draw the outline for the balsa sheet, remembering to allow for the curved upper nose section. That will be a separate structure, so the lower nose section will look a little unusual initially. Nevertheless, this is what we eventually get. Note that I’ve cut out the lower aerofoil shape for the wing saddle. Obviously, the fuselage sheeting will need to be assembled from smaller sections as it is too large to fit on a single sheet of balsa, but we’ll deal with that later.
Now we turn our attention to the rear of the model. However, one thing I should point out is that I’ve cut the corners from each former, with the intention of using 6mm balsa sheet and sanding a radius. This provides more strength and will give a more simple (and cleaner) corner treatment.
Looking ahead to attachment of the vertical stabiliser, and I’ve incorporated a notch in the top of each of the last two formers. This will provide accurate and secure mounting for the fin. For the horizontal stabiliser, I needed to decide if I’m going for a scale aerofoil, or the easier sheet balsa method. Thinking about weight, it would probably be lighter if we had a scale, built-up tailplane, so that’s the way I’m going. Once the aerofoil for the tailplane and elevators are drawn, the correct tailplane saddle shape may be “cut” into the two plywood tailplane mounts I’ve designed into the rear structure.
For the moment, I’ve drawn in a semi-scale tailwheel, which on the Porter is attached to the rear of the fuselage, and in the real aircraft is probably the weakest part of the machine. If the pilot took off without having locked the tailwheel, there was a greater than even chance that he/she would tear the tailwheel completely from the aircraft on landing because once airborne, the tailwheel would move completely to one side and there was very little the pilot could do to fix it while airborne. For simplicity, the model will likely have a conventional “spring-type” tailwheel that will bolt under the tail and be steered via linkage to the rudder servo (separate to the rudder pushrod).
At this stage, the fuselage design is essentially complete, and all that remains now is to complete the plan and profile views, followed by a double check of the formers and stringers to ensure everything fits as planned. We’ll leave layout and detailing of the plans until later once all parts are designed. That’s one of the fun stages, so it’s only right that it should be about the last completed!
Planning Centre of Gravity
Most conventionally designed aeroplanes have the centre of gravity placed at around 25-35% of the mean aerodynamic chord. Basically, that means it’s a third back from the leading edge in the Porter. Coincidentally, this is normally the same place the mainspar will be placed, though there can be sub-spars, and other variations to be sure – it really depends on your preferences. In any case, the 30% position will normally be a safe starting point, and once you fly the prototype, you may prefer to move it one way or the other – aft for more manoeuvrability (and greater chance of tip-stalling/spinning), or forward for more stability. Let’s place the CG graphic on the profile view and we can now move on to the wings and empennage.
I’m planning on using the wing joiners to provide the dihedral for the wings. It isn’t much, but it’s there, and it will make the model more stable. With the end elevation view, measure the distance up from the lower surface of the wing at the root, to the lower surface at the tip. In this case it is 15mm for EACH tip. It is important we make allowance for this when we position the holes for the joiner tubes, but more of that next…
This is an area where suitable changes can and often should be made. Just because a full size aircraft flies well on a particular aerofoil, it doesn’t necessarily follow that the model will fly well on the same aerofoil. This is particularly important with high-speed aircraft that have small wings. In these situations, it is best to change to a thicker wing section and even increase the wing and tail areas. Early Spitfires would be a perfect example as they had quite small tailplanes, so typically designers will increase the area by perhaps 5 – 10% in order to obtain a well-mannered model. Additionally, it is sometimes difficult to fit hardware such as servos and retractable undercarriage into the otherwise thin, scale aerofoil sections, so in those cases, a thicker section is chosen to reduce headaches!
However, with the Porter having a thick, high-lift section already, and otherwise ample surface areas, I’m going to use the actual aerofoils throughout. Remember, I’ve already chosen to go and build up a scale-sectioned empennage too.
With the wing aerofoil completed, the next step is to scale it to fit the wing chord. This is a simple procedure, but care must be taken not to stretch the image in any way, but to SCALE it! Notice that I’m only going to work on one wing panel (the right in this case), and later, once I’m happy everything is complete, I’ll simply copy the right panel, and reflect it over to the left. We will therefore have two identical wing panels.
Before we go too far, we have to work out the rib details – how thick, what spacing, what material etc. In this case, I’m using 3mm balsa for the outboard ribs, 3mm ply for the inboard ribs, and as for spacing, that will be 70mm between each outboard, and 35mm between the inboards. It is important to note that in this case, I’m planning on two carbon or aluminium wing joiners, so it is important we have ample strength inboard to handle the stresses. At the same time, I’ve allowed for a 12mm balsa leading edge, 12mm trailing edge, and 6mm hardwood spars.
For now, a short diversion with the ribs. I’ve planned for 1.6mm balsa skins and capstrips, so the ribs need to be reduced in outside dimensions by 1.6mm top and bottom. For this we again use the contour tool. Note on the drawing, that I’ve made the aerofoil yellow, and the inner contour white. Once this contour group is broken apart, we will be left with the basic rib, and from here we remove leading and trailing edges, incorporate spar notches, holes for wing tubes and lightening in general.
Remember that the leading edge is going to be 12mm balsa (actually 2x 6mm balsa will do too), so that’s 12mm from the leading edge of the AEROFOIL.
Also, remember at this stage to work on a COPY of your rib, in case you make a mistake and wish to return to an earlier state. In fact it should go without saying that we save regularly, and occasionally change the name of the file so we have multiple stages of the same design. You never know when you might have an epiphany and want to redesign a component. If you have to begin again it will take longer, be more frustrating, and you will likely introduce inaccuracies. Far better to have “stages” that you can return to.
Most of the rib design work can be done with the rib superimposed on the wing plan. This gives us more accurate positioning of spars, wing tubes, trailing edges etc. Note on this drawing, I have placed the aerofoil to the rear of the drawing, while the rib was brought to the front. This is done by selecting the part, and then ARRANGE – ORDER – TO BACK OF PAGE etc. You should get the idea…
Also, to draw a 6mm square, you may start with any rectangle and simply size it.
It is important that the upper and lower wing spar are aligned in this case as I’m planning on incorporating shear webbing. However, if we keep the spars square with each other, there is a small misalignment with the upper and lower rib surfaces (as you can see in the expanded detail. Depending on the choice, there will either be a “step” between the spar and the rib, or we’ll need to sand the upper forward edge of the spar flush with the rib, prior to sheeting or capping. The choice is really up to the designer, but my preference is to sand down, rather than have a possible low spot in the final sheeting. Therefore, I’ve chosen to align the rear of the spars with the upper and lower rib surfaces.
Once both spars are positioned over the rib, it is best to GROUP them and keep them grouped for the balance of this process. This is done by selecting the first, holding down the SHIFT key and selecting the other. Then,
using the ARRANGE – GROUP – GROUP OBJECTS menu (there is a dedicated button on the toolbar as well).
Once the spars are grouped, they may then be selected, and again, holding down the shift key, select the rib. At this point we will trim the rib, but remember to select the spars FIRST as it determines which object is the “trimmer” and which is the “trimmee” (I just invented those words by the way…). For this function, we use the TRIM button on the main toolbar, as shown in the drawing.
We are then left with a rib, with the spar notches cut out as in the next diagram.
We’re beginning to get there!
Another note here: depending on how you intend to cut the parts, you might need to change the dimensions of these notches slightly. When a laser is used to cut the parts, the beam has a “dimension”, so if we want the parts to fit snugly, we need to reduce the dimensions of the spar notches accordingly. It is best to contact the laser company to find out their preferences, but we’re only talking about fractions of a millimeter, so I’m going to leave it at 6mm and accept a slight “sloppiness” in the ribs. The lasers used to cut my kits in the past have been sufficiently accurate that it isn’t a major problem.
Now, it’s time for another CorelDraw secret! One of the biggest and greatest features of this software is the inclusion of GUIDELINES and the SNAP TO GUIDELINES function. Guidelines are easy to bring into the drawing pane, simply by clicking on the ruler at the top and side, and dragging the curser (and guideline) to the desired place. Then, go to the VIEW-SNAP TO-GUIDELINES menu, and ensure the tick is shown at GUIDELINES.
Now, when you bring an object, such as the rib, to the guideline, it will snap to the line and allow even more accurate alignment of future parts as we shall see.
With the rib lower line “snapped” to the guideline, we will now draw two small (6mm wide) boxes, with the boxes snapped to the guideline. Make sure the boxes are entirely overlapping the rib, and then we will join them all up.
Select and group the two boxes, and then select the rib. Again, the box group should be selected first, followed by the rib. We then use the WELD function and they become one object.
We are now left with a rib that has tabs for building on a flat board, that will be “self-jigging” and accurate.
It is important to note at this stage however, that the lower spar is going to need a little packing during the build as it doesn’t coincide with the lowest part of the rib. This isn’t a major problem – unless we forget the packing of course!
Now we come to what is potentially the most difficult part of the design process, certainly for an otherwise very straightforward design as the Porter. We’re going to position the holes for the wing joiner tubes, and accuracy at this point will determine not only the dihedral and smoothness of the wing panel, but even if your inboard ribs are useable at all. I’ll speak slowly, so listen carefully…..
Remember that we’ve chosen to use 1.6mm balsa sheeting. We need to make allowance for that in this end-elevation drawing now, so we’ll go ahead and draw a 1.6mm contour inside the wing.
Then, placing the rib up against the wing, we see that the upper and lower surfaces of the rib touch the contour just aft of the spar slots. This is a little inconvenient, but we can make it work.
Now, we duplicate the rib (select the rib, CTRL+D) and position it on the next station. If you do it in one simple movement, when you next come to duplicate it, the new rib will be placed on the next station for you! Place all the ribs and you will have something like this.
At this point, we have five ribs, each aligned within the wing thickness, and essentially centred on each rib. I’ve separated a copy of all five in the above diagram, and now we come to another magic feature of CorelDraw….
With all five ribs selected, we go to the ARRANGE menu and ALIGN and DISTRIBUTE – ALIGN CENTERS VERTICALLY.
This aligns all the ribs, but leaves them spaced vertically like this. GROUP them for the next process.
On the wing plan, we left a rib profile, and it is now a simple process to draw a 10mm circle, place it on the rib, aligned with the wing joiner on the plan. Duplicate the circle, and position the duplicate in a similar manner for the aft wing tube. Note that because of the dihedral effect, the joiner enters the outboard ribs lower down, and the inboard ribs at a higher point – this is of course important as we need to ensure there is enough structure above and below the rib to retain structural integrity. On some models, one tube needs to be smaller diameter to ensure the best compromise between tube positioning and strength.
Selecting both the rib blank and the two circles, rotate them all through 90 degrees and position them over the uppermost rib on your stack.
Remove the rib blank. Now, GROUP the two circles, hold down the SHIFT key and select the rib group. Use the TRIM command, and you now have holes in each rib, spaced precisely. This process alone would take 30 minutes or more with a pencil and paper, and it took us about 30 seconds!
I know that heading sounds silly, but it is in fact, exactly what we are now about to do! If we’re having the parts laser cut, we can have extremely accurate holes and shapes cut into the ribs and other parts to reduce as much unnecessary material and therefore weight as possible. However, a caveat, we don’t want to mess too much with areas of high stress, such as the inboard ribs. Therefore, we’ll limit the holes in the inboard ribs to essentially those required to run servo wires etc. while outboard ribs can be more radically modified.
For servo-wire holes, it is best if they all line up so it makes equipment fit much easier. So, for the moment, we’ll look at the servo mounts themselves, and then plan the servo-wire holes. Notice on my plan, I have now drawn servo positions for both flaps and ailerons. Flaps are not really necessary on this model, and the builder might just as easily install full span ailerons if desired. However, I’m going “full house” and including them. In order to simplify the wing root detail, I’ve decided to put flap and aileron servos in ajoining bays. This means only two servo rails per wing and simplified hatches. In this case, I’m using Hitec wing servos, but the wing is so thick, a standard servo might also be used if desired.
This drawing shows the process. From the wing plan, we set the servo locations, along with mounts. In this case, 10mm x 3mm plywood will be used, and they will be full length across both bays, so they will support both servos.
Throughout this process, it is important to ensure the servo will fit, and the mounting holes will not interfere with any other structure or holes. I also superimpose an elevation view of the servo on the wing plan to assist in this process. If you were doing this by hand, a separate servo drawing on tracing paper would do the trick, but it still involves some planning.
The end result though, is that we now have positions for the mount holes in those three ribs (per wing).
At this point, go ahead and add the holes to the outboard ribs, remembering that they are to be made from 3mm balsa, and therefore will need around 5-6mm of “meat” around the holes to ensure the ribs don’t collapse with use.
Ribs are now complete, with the inboard five being 3mm ply. The three servo-mounting ribs will be balsa, as will be the remaining four outboard ribs. We’ve been fortunate that the wing section is constant throughout, as a tapered or swept wing causes a few headaches sometimes, especially if it tapers in thickness as well, but that’s a subject for next time…
In many ways, the tailplane is simply a smaller version of the wing. Given that it is also constant chord and straight, the design process is quite straightforward and the only tricky part is to position the mounting bolts and spars. There is no sheer webbing, but a block should be put in the centre section to ensure the bolts don’t crush the structure or pull out in those high-g pullouts! Note that I have only drawn one side. Again, the plan is to simply duplicate and mirror the drawing and in that way ensure symmetry, as well as being less work. Here again, computers make life so much easier for the designer, as we can move parts around until they are in the right place, don’t interfere with anything else, provide maximum strength and minimum weight etc.
In this case, I’ve not planned for any holes in the ribs, as the 1.6mm balsa will be sufficiently light. I am however, planning to sheet the entire surface for strength, though this is perhaps not strictly necessary. Again, my models don’t normally break under normal loads, and sometimes not even under very AB-normal loads!
Fin and Rudder
At this point, an interesting feature of the Porter comes to play, and that is – the tailplane and fin segments are interchangeable on the real aeroplane! This was done deliberately for ease of maintenance and repair in the field. Left and right tailplane halves are interchangeable with each other as well as the fin.
Having said that, I’ve chosen to construct the fin with a 6mm balsa core, suitably lightened of course, with half-ribs and 1.6mm sheeting. This is all to make the mounting cleaner and stronger. You will note that the 6mm core extends through the upper fuselage skin, and keys into the rear two formers. This should be quite strong for normal operations, and is very simple to engineer as you will see from the drawing.
About now, the structure is complete, and all that remains is to tidy up some finer details, and start cutting parts. If you plan on cutting by hand, ok – that will be fun! However, if you can’t be bothered – or like me, you more easily cut your hands and leak blood all over your parts – then the laser cutter beckons…..
Separate all your parts into groups of common material and thickness. For example, all the 3mm balsa parts will be in one file, laid out so they don’t touch or overlap, and are all CONVERTED TO CURVES. It is a good idea to include a reference square on the page also, and I normally make it 60mm square. This gives your kit-cutter the assurance that your parts are being imported to his cutting software in the correct scale, and makes it less likely you will get poorly fitting parts.
Once you have all your parts separated and processed, you can go ahead and add identification numbers and letters to them. Convention gives us ribs being identified from the root out to the tip, starting with W1. Formers go from nose to tail, starting with F1, and of course the tailplane parts are labelled with H for horizontal, V for vertical etc. You may choose any naming convention you wish of course – it’s your model after all, but if you want others to be able to build it, you will do just as well to stay with the conventions.
Now it’s time to export your parts to the desired format your kit-cutter needs – you have spoken with him/her haven’t you! Generally, this will be DXF or DWG format, and if you’re lucky they might even take your CDR files straight from CorelDraw (less likely). It’s best to leave it to the kit-cutter to lay out the parts for cutting. They will “nest” parts where possible, and rotate them so the correct grain orientation is achieved for best strength. Then it’s simply a matter of getting the files onto a disc or thumb drive and posting or delivering it. Then the wait begins…..
While you’re waiting for your parts to arrive, it is a good time to tidy up the plan, lay out all the parts and assemblies so they fit neatly on the sheet and can be printed. If you are expecting to distribute the plan, you also need to label the parts, show different grain and material types, and include dimensions, suggested electronics or accessories etc. This is an enjoyable part of the process, and can take as little or as much time as you think you need to give it. However, builders appreciate a clearly labelled and easily followed set of plans – don’t we?
You might like to contact the print shop at this time to see if they print plans, and what dimensions they can work with. Do they have stock sizes, rolls or single sheets etc. This will determine your layout. My print shop deals with rolls of paper so the width of the plan is essentially unlimited, but the height of the plan (width of the paper) is limited to 700mm. Therefore, I start with a drawing page that is 700mm high, and as wide as I think I need. I can always adjust it as necessary once the parts are laid out.
Then I lay out the fuselage (profile view) in the lower left corner, with the plan view above and lined up so as to have the formers and equipment positions correspond. If your model is not complicated enough or you think the plan view is unnecessary, of course you can leave it out. Generally, I’m going to have a separate sheet for the wings and perhaps the tail, so the main sheet can have parts arrayed as necessary. Don’t forget to put a box somewhere to list the wingspan, length, estimated weight etc. as other modellers will find this useful.
A word on line thickness is in order – generally we would prefer lines to be as thin as possible, but some printers just don’t pick them up too well, so you will need to think this one through a little. In any case, major assemblies (fuselage, wing, tailplane etc.) are generally outlined a little thicker than the internal structure. I use .75 pt for outlines, and .5 pt for internal structure.
The end….. or is it?
Well, by now the plans are pretty usable, and certainly accurate, given the precise nature of the computer, and CorelDraw in particular. It’s now time to get the parts together and build your new masterpiece. I’m not going to go into the construction sequence in this article, as I’m sure most builders can manage that anyway, and will have their own individual preferences. Most models of this nature will have common building processes, and the only difference in this case will likely be the scale details such as the undercarriage and undercambered wing/control surfaces.
For those of you wishing to build this particular plan, it is available on this site, along with a laser cut short kit, and a build article. It would be great to hear from builders about their experiences, and I’m happy to receive questions and comments too.
Next, we will look at tapered wings and other design features that might complicate an otherwise easy design. We’ll also look at drawing standard equipment such as engines, servos and the like. It is my intention that many of these assemblies will be available online for anybody wanting to download them for their own use.
Assuming your chosen design is a constant-chord wing and box fuselage, your life is very simple indeed. However, if all aeroplanes looked like trainers, then how many of us would have continued in the hobby for very long? So, eventually, you will want to build a model with a tapered, swept or even elliptical wing! Here is where life looks much more complicated, but really, with just a little forward planning, there is no reason why you should be afraid of it. No doubt about it, there is more work to do, but it isn’t rocket science, so there’s no need to be afraid of it, and if you’ve been following along with my previous works, you already have the knowledge and determination to do the job, so take heart.
We will deal now with tapered wings, though of course the same technique applies to tailplanes and swept wing shapes of all kinds. For tapered wings, we simply need the root and tip aerofoil shapes, and an understanding of another of CorelDraw’s features. This is called BLENDING.
Starting with the desired aerofoil, we overlay a correctly sized aerofoil on the wing plan, correctly sized of course. Then, using a COPY of this aerofoil, place and size the wingtip aerofoil so we now have not only the correct size of the wingtip section, but it’s position relative to the root rib as well.
But wait, there’s more to think about here…. Simply scaling the tip rib will provide a quite thin trailing edge. We’d prefer the trailing edge of the wing to be of constant thickness, for example 2mm. So, let’s take the tip rib and ensure the trailing edge is now 2mm deep.
Around this time, we might hear some alarm bells ringing in our ears, telling us that a thin wing section such as this will have a high potential for tip-stalling! We need to reduce the angle of incidence at the tip, and my suggestion would be by about 2 degrees at the nose. Let’s do it! CorelDraw makes this simple for us. We simply select the tip rib, and select ARRANGE – TRANSFORM – ROTATE, and set the rotation angle of 2 degrees, then APPLY.
And now – the magic! Place the ribs back on the wing plan in their correct position. On the left-hand toolbar, 6 up from the bottom, you will find a button for BLEND. You might need to click and hold the button, in order to see and select the blend option. Once this is done, click on one rib, and DRAG to the next rib.
The default number of blend elements is 20, which is far too many ribs for our current model, so we will change the number of objects to 8. When the original two ribs are included, we now have 10 ribs in total.
While still selected, select ARRANGE – BREAK BLEND GROUP APART. Select out, and then back to the new ribs, and you will discover they are grouped. I don’t know why CorelDraw does this, but I find it inconvenient, though simple enough to fix of course. Just UNGROUP them. You may now continue to develop your design as you’ve already seen previously.
The same technique may be used for the fuselage, as shown on this Macchi MB326H drawing I’m working on. You will note that the fuselage consists entirely of curves, and no two formers will be the same size or shape. This is a more advanced project, but it gives you the idea nevertheless.
Something to be aware of when designing your own models, is that laser cutting technology is now very advanced and you will be able to design very complex parts, and cut holes for running servo leads and antenna tubes, and just to make the design lighter. My Fokker DVII design is made predominantly of plywood, but large holes for removing weight. An example of a couple of parts is shown.
In this case, you will see one of the fuselage formers, with multiple tabs to key it into the plywood fuselage sides, as well as holes to mount the battery compartment floor. The trapezoid shape is the lower fuselage/tail plate, again with multiple lightening holes. Finally, the combination wing strut mount and servo hatch can be seen. In this case, I’ve left small gaps in the lines, so the servo hatch remains attached to the mount, and is therefore easy to find in the pile of wooden parts, but mainly a perfect fit for the outside frame. All that is needed is for the corner mounts to be glued inside the structure, and holes can then be drilled to mount the hatch – all very simple. This particular model incorporates so many self-aligning features, that you could literally build it in your lap! I’m not suggesting that you do that, but it is an example of the usefulness of laser-cutting your parts can be.
One of the best things about designing your own models in CorelDraw, is that you can also draw your equipment – servos, engines, fuel tanks etc. and in placing them on the plan, ensure that everything will fit in the right places, and your mounts are most efficiently positioned etc. This might seem like an onerous task, but really it isn’t too bad as you only have to draw these items once and save them to another file for later copy/paste into your latest design.
The equipment drawings need not be exquisitely detailed, and can even be just outlines if you wish. The dimensions and mounting holes are really all you need after all. See my examples and you will get the idea.
I went a bit overboard with my OS 1.6FX as seen here. Let’s just say I had a little more time to spare that night and couldn’t get to sleep.
As a bonus for readers, I will upload some standard equipment drawings to the website if you would like to use them in your plans. Feel free to take whatever you like, but I’d love to see your results. Feel free to email me at firstname.lastname@example.org I will treat your designs with confidentiality and will not pass on or use anything that isn’t mine. I’m also happy to assist where I can if you have any questions.
Let’s say for argument’s sake, that you have designed a model that suits your needs exactly and has the features you consider important with respect to your flying style and ability. Let’s also consider that one or more of your club mates would like to build one of these models too, but they don’t want all the same features as yours. Perhaps they want flaps, or retracts, or simply a tail-dragger, where yours was a tricycle fixed gear! Well all these changes can be easily incorporated into your plans at any stage, and depending on your own needs, may even be considered from the very initial concept drawings. The software capabilities are far in excess of our needs here, so let us just have a glimpse of its potential.
Shown in the accompanying figure, we see that the same basic design can be developed as a conventional, fixed-gear design, or indeed a tricycle retractable. Obviously, there are internal changes to be made, but the initial drawings serve to show the concept and assist your thinking process.
Similarly, let us assume you have a set of wings available, but the fuselage has been damaged beyond repair. In this case, you want to design your own fuselage to go with these wings. You may use software to develop the design in almost any direction you might be able to imagine.
These drawings need not be very detailed yet, just enough to help you make up your mind about which way you might like to go. Once decided, the plans can then be drawn, wood cut and glue applied. Then you can fly your old wings as a new model, and feel good in being able to design your own model while saving money, time and resources. Everybody wins!
Don’t forget that CorelDraw can also be used as a kind of sketch pad, where you can draw in simple form, and develop your ideas and thoughts into a workable model design. You might even like to work up a colourscheme so you can even better imagine your finished model.
Summary of Design Elements
I hope I’ve given you not only the urge to design your own model, but the basic tools with which to achieve your goals. However, I recognise there are design techniques I haven’t touched on, and this is deliberate as we have only a limited space and opportunity. However, it is my hope that I’ve given you inspiration to start. Once you hit a roadblock, you will be able to research the solution without problem. It’s either in the help files, or online, or even a simple email away.
In all, we simply need to approach the design process with a clear understanding of what we want, and a willingness to adjust our plans and theories to suit reality when we find we’ve been a little “ambitious”. There is little that cannot be achieved with the software we have available to us in this modern age – it just takes a little effort.