Chapter 12

 

Chapter 12

Designing your own

After you've built a few ships from this book, kits or magazine plans, you'll probably get the urge to design a model of your own; to see an original creation floating overhead or cutting circles on the control lines.

Maybe you want to build a scale model of a favorite ship and can't find a plan that suits you, or maybe you have a mental picture of something that will make those other crates look clumsy and out-of-date. But perhaps you haven't tried putting your ideas to work because you didn't have a degree in aeronautical engineering to help you master the intricacies of building an airplane without a printed plan to refer to. The fact is, by observing a few basic rules, you can turn out a model that will fly every time. Although some of the top professional designers use the slide-rule approach to setting up a new design, some of the best ships ever built were worked out empirically, by rule of thumb.

Don't get the idea that designing a model airplane is a haphazard process. It's not enough to assemble wings, fuselage and tail, add an engine, and fly. For each type of model, there are basic proportions and relationships which determine the flying characteristics and abilities of the ship. These are not precise and unalterable rules; you have quite a bit of leeway, varying one factor against another. The skill with which you do this determines your skill as a designer.

Before starting, first decide exactly what you want to accomplish with the model you are about to undertake. Your objective in this first design should be a limited one; to conceive and engineer a ship which will get off the ground, perform in the air, and land in one piece. Your design may be either a completely original conception, or a scale model of a full-sized plane. It is advisable, in any event, to start with a relatively simple straightforward design of conventional layout. After your first success, there will be plenty of time to develop competition jobs or experiment with helicopters, canards, Delands, pushers, and other mutated species.

If you're planning a scale model, you should have on hand the most accurate three-view drawings available. If you can get these photostatically enlarged, you'll save the labor of enlarging them by hand. If your project is an original, now is the time to sketch out some reasonably precise three-views at a small scale, to establish the basic shape of your model. If you want a highly maneuverable ship, keep the nose and tail moment arms short; for a smooth-riding stable job, lengthen them. In either case, the distance from center of wing to tail should be about 2-1/2 times the wing-to-nose distance. A deep fuselage helps give stability (except with a low-wing job), as does a wing of high aspect ratio, but at the cost of decreased speed and added drag. Low-wing F/F ships are less stable than high-wing designs as a rule; biplanes are frequently hard to adjust. For free-flight, keep the engine thrust line fairly high, and establish a positive angle of incidence between wing and elevator. With C/L, try to keep engine, wing, and elevator nearly on a line for minimum drag, set the wing well back, and provide for a generous elevator area.

You can save yourself later trouble at this point, by avoiding unduly complex details - remember, you'll eventually have to build them. Try to decide on a simple engine and wing mounting setup, and uncomplicated landing gear, a minimum of complex cabin structure or needless curved sections. More elaborate ideas can all come later, with experience.

Sometimes the nature of the model you wish to build will dictate the type of construction; for example, if you want to do a scale model of a specific plane, its configuration will impose certain limitations. But let's say you're starting with nothing more than an idea as to the lines of the finished model and that the possibilities are wide open. In that case, let's consider some of the alternatives you'll have to choose from before you do any detailed planning.


A twin-engined scale model.
A twin-engined scale model.
 

Scale models are, as a rule, more difficult to build, require more fine detail work, and depend more on their finished appearance for effect than other models. Frequently, though not always, their flying ability is cut down by the need to stick to prototype configuration and proportions. Still, for the builder who wants to see the real thing, there is nothing to equal a fine scale model going through its paces.

There are many airplanes which lend themselves to scale free-flight modeling, particularly light planes, military liaison jobs, WW I fighters, home-builts, etc. Good free-flight prototypes almost always have high wings and one engine, although a low-wing type with plenty of dihedral will often work out well. A multiengined airplane such as the Ford Trimotor or the Northrup Pioneer can be free-flown successfully with only the center engine functional, dummies with freewheeling props being used in the other nacelles. It is sometimes necessary to make minor changes such as enlarged tail area, increased dihedral, etc., to achieve a stable scale F/F model.


Hand-launching a rubber powered endurance model.
Hand-launching a rubber powered endurance model.
 

For control-line flying, there are few limitations on the full-scale designs which can be accurately modeled. It is a good idea in selecting a scale C/L project to stay away from ships adorned with fragile projecting antennae, Pilot tubes, exhaust pipes, rigging, etc. These don't last long under flight conditions, and there's not much point in building a scale model and leaving them off. Cumbersome cowlings, wheel pants, and canopies can become more trouble than they're worth if they mean inaccessible engines, or inadequate clearance for the propeller. It's no use having a pair of high-powered .15's sitting on the wing, and only space enough to mount 6-inch props. Also, think about installation of the bellcrank and lines. On a scale model it's desirable, when possible, to completely enclose the control system, or at least, to arrange a neat and inconspicuous installation.

The landing gear is an important item to check. Many a handsome model has had its looks spoiled by the alteration of the landing gear for practical reasons. A beautifully scaled fuselage is wasted if the landing gear legs have to be doubled in length, moved forward halfway to the nose, and fitted with oversized wheels. It should be noted, however, that such measures may be completely unnecessary, the result of excessive caution on the part of the builder. Good examples were the early gas models which used huge air wheels set out in front of the prop to protect the "delicate" engine; this wasn't really necessary. If the L.G. is a little ahead of the center of gravity of the plane, provides clearance for the prop in a take-off attitude, and is not unusually elaborate in design, it is O.K. for exact reproduction.

Make a final check before deciding to go ahead with a scale project, to make sure there are no insurmountable structural problems inherent in the design. Why get involved with an elaborate greenhouse, or tricky gull wings, unless you can visualize a method of building them? There are plenty of planes available which have beautiful lines, and are easily built; and for now, stay away from pushers and seaplanes. A compromise solution is a semi-scale model, essentially a sport model inspired by an actual airplane. These can be a lot of fun, are easy to build, and provide most of the scale effect of a true scale model. The use of profiles, painted-on details, and simplified lines makes it possible to get some very good effects easily.

When you've got a couple of good conventional craft to your credit, you can branch out into the unorthodox. For a few basic hints on how to lay out an unusual type, study kit and magazine plans of similar designs - they'll show you basic dimensions and relations for pontoojis, seaplane hulls, pusher engine mountings, etc.


A pendulum-controlled F/F ship heads upstairs.
A pendulum-controlled F/F ship heads upstairs.
 

After you've tested a model with wheels, you can convert it to floats - just build a set along the lines shown in Fig. 8 (Chapter 1) and fit them on the L.G. struts in place of wheels. They're aerodynamically balanced, so they won't affect trim much.

Whether you build F/F or C/L is mostly a matter of taste, with the added factors of flying facilities and model specifications thrown in. First, let's eliminate the obvious. If you love free-flight and don't want anything to do with yo-yo's (or vice versa), the problem is simple. If you prefer F/F but don't have room to fly the loose ones, or like C/L but never learned to control 'em, or can't find a teammate to help you get them aloft, the matter is settled. But maybe you're lucky, and can take your choice. If that's the case, let's take a look at your dream ship. Do you picture a long, low-slung job with plenty of power up front, stubby wings, and a trim little tail assembly? A job like that will fly with lines on it; not otherwise. But maybe you have visions of a spreading high-camber wing, a taut tissue-covered fuselage, something that takes off and lands like a feather. Don't put any tethers on that one!

Basically, C/L jobs are smaller, heavier, sturdier; they have big engines, lots of solid balsa construction, small tails without airfoil, rugged fixed landing gears, plenty of paint, spinners, wheel pants, scale details, and realism when desired. Free-flight jobs, on the other hand, are light in weight, bigger, more fragile, and more critical in weight and area distribution. For the sake of simplicity, we'll assume that your ship will employ a glow or diesel engine. Most of the principles discussed herein are equally applicable to rubber or jet design, but your choice of plans should take into consideration the power you will be using.

If you prefer rubber power, you can make up a motor of any desired size to fit your plane. It's not a bad idea to test the model with only a few strands, then add more for higher power. An average sport flyer of 24-inch span might carry from 4 to 10 strands of 1/8" flat rubber; a small ROG flies well on 2 to 4 strands; a big endurance model may use 24 or more strands of 1/8" rubber. The more rubber, the more power - and the more weight, not to mention more prop area required.

If you have a good dependable engine on hand, one that starts and runs consistently without excessive cranking and delicate adjustment, it is a good idea to plan your first original design around it; it is really simpler than using rubber power. There will be plenty to demand your attention when you go out for that maiden flight, without a cranky engine to contend with. Of course, if the whole project is based on a desire to see what you can do with a hot new engine, that's another matter; but be sure to test-run the motor until you know its characteristics thoroughly before you base any planning on it. If you're buying an engine especially for your new design, you'll find a wide selection available, of a quality which would have been unbelievable a few years ago. Some high-performance contest engines achieve their hot performance at the expense of easy starting and wide adjustment range; so it's best to pick a sport-type engine, such as the Cox Sportsman, with dimensions which lend themselves to the plane you plan to build.

Full information on many engines can be gotten from manufacturers' advertising, or from engine-analysis articles in the magazines. Some good hints can also be gotten from magazine plans, which usually indicate engines to be used; find a plan something like what you have in mind, and see what the designer used in his.

If your plane calls for an inverted engine for maximum good looks (warning: they're harder to operate upside down), don't pick a motor with fixed tank which can't be inverted. For a ship with a long nose, select an engine with a long shaft; and keep in mind the location of the needle valve or compression lever, and its accessibility when mounted in your plane. In other words, try to reduce your problems by giving some thought to picking the right engine.

You'll also have to decide on the size ship to build. It's always a kick to launch a monster model, and frequently easier to adjust a big ship, but they're more expensive to build, and harder to transport and store. Big models require big engines, which cost more and eat more fuel. You'll need bigger wheels, a bigger fuel tank, heavier-gauge wire for landing gear or controls, and lots more balsa, thicker plywood, more dope. Big F/F ships need lots of room to fly in, and have a tendency to float away over the trees into never-never land (which must be getting full of lost models by now). But, on the other hand, large ships are impressive, and there is plenty of room in them to install tanks, controls, etc., without crowding. Big ships are less susceptible to breezes and rough ground, and they're fun to fly if you have plenty of space and you're willing to invest the extra money for the extra thrill of a large plane. However, if you want to start off modestly, or try out something unusual, it isn't a bad idea to start with a small ship. If the idea works out well, you can enlarge your plans and try a big one. Big or little, decide on a fuselage length and wing span now.

Before you can draw the first line on brown wrapping paper using a yard stick as a straightedge, you will have to decide on your approach to construction, the engineering of the model. It is up to you to select the best structural method for your particular plane and your own abilities. The simpler the structure, the better it is likely to be; simplicity means fewer members, fewer joints, lighter weight, and greater strength. Not that it can't be overdone; remember the structure has to be right for your ship. So spend some time studying the outlines of the plane, whether sketches of an original or three-views of a prototype; and note the form of the wing or wings, the fuselage, the rudder, and the elevator, the configuration of the landing gear, and of wing struts if any. Since by comparison with the fuselage the other parts of the plane are usually simpler, let's consider the body first. In studying the outline drawings, the cross-sections are of more importance than the side and top views. If the fuselage is square or flat-sided, as are many WW I jobs, for example, a simple box is probably the best choice.


fig.62
fig.62
 

If the cross-section is unusually complex, it may be easier to use crutch or half-shell construction; but remember, it is not necessary to resort to complex built-up structures as long as soft balsa is available (see Fig. 62). Unless you are already an expert builder (in which case you may step to the head of the class), you'd better stick to a simple method for your first few designs.

Next the tail assembly; for free-flight, elevators should be about 1/3 to 1/5 the area of the wing, with the rudder having about 1/2 the elevator area. Fig. 63 shows basic F/F sport proportions. For C/L design, the elevator should be about 1/5 the wing area, of which the hinged portion should constitute one-half. Rudder area is not critical on C/L models.


fig.63
fig.63
 

Scale-model designers, especially for F/F, have a habit of enlarging the tail assemblies; this is rarely necessary. One-third of wing areas is a standard elevator size for models, but an elevator with 1/6 to 1/8 the area of the wing is perfectly O.K.

This is also a good time to give a thought to the method of attaching the tail members and hinging the control surfaces.

The area of a F/F wing should be about twice that of the side elevation of the fuselage, although you have wide latitude on this point. In terms of engine size, the average sport F/F wing for an engine of .049 cubic inches displacement has an area of about 100-200 square inches; for an .099, 200-350 square inches, and so on. Check magazine or kit plans for typical wing area-engine combinations. C/L wings are relatively much smaller - about 1/2 to 1/3 as large as a F/F wing for the same engine.

F/F wings should usually be of the knock-off variety, so consider how you'll attach them to the fuselage, and also how the cabin, wing struts, etc., will be affected.

Plan the mounting of the engine, landing gear, struts, etc., at this point, to obtain maximum strength with greatest simplicity; there is always a simplest way to solve any structural problem. While it isn't expected that all these points will be worked out before rough drawings are made, it saves work and complications later on if you can start drawing with some preliminary ideas.

Now you need a sheet of paper big enough to draw up the ship full-sized, a ruler and a long straightedge (a T square and a triangle if possible), a big eraser (who's perfect?), and a flat place such as a drawing board or a kitchen table to lay it out on. If you're doing a C/L job, have your fuel tank, bellcrank, etc., on hand to enable you to provide proper installation space for them.

To start the side view of the fuselage, draw a long straight line horizontally across the paper, parallel to the bottom, and high enough so that you can fit in the fuselage and landing gear under it. This is the datum line of the fuselage, and all measurements and angles will be set up in relation to it.

You have already determined the length of your fuselage; lay it off on the datum line. Now find the datum line on your outline drawings, if you have scale views. If it's not indicated, or if you are using your own design sketches, assume a datum line through the prop hub, running back to the rear, parallel to the stabilizer.

If you are lucky enough to have enlarged three-views of your scale job, of course you are spared the next step, the construction of the full-sized outline, including wing and elevator cross-sections, around your datum line. If you are enlarging your small three-views, it is easiest to draw a grid over them, and then transfer the points to the large sheet, on which a correctly enlarged grid has been drawn (Fig. 64).


fig.64
fig.64
 

The finished fuselage outline should show about twice as much area below the engine thrust line as above it, for stability. Sketch the engine, tank, wheels, etc., in position, and check clearances. Now look over the layout, and check a few general points. Is the wing positioned about 1/3 of the way back to the rear? If a F/F model, is the rudder about as big as the area of the side in front of the wing? If you're using a two-wheel landing gear, are the wheels a little ahead of the 1/3 point of the wing? (See Fig. 63.) If not, fudge the lines around until they are - this is where the eraser comes in.

When you have the side view outline done, draw the top view outline directly above it, with the front and back ends right in line. If you have cross-sections for a scale job, indicate their positions very carefully, and check to make sure their dimensions match the outlines. If they don't, stretch them around to fit.

Now you can start the engineering process by determining the location of your bulkheads, or former stations. Placing the engine on the plan, you can quickly locate the firewall. If the engine is radially mounted, the firewall must be placed so that the shaft protrudes the proper amount from the front of the fuselage. If the engine is beam-mounted, you have a little more leeway; but the closer the firewall is to the engine, and thus the shorter the unsupported projection of the beams, the better. This limited freedom of choice may allow you to place the firewall so as to attach landing gear or wing struts to it. You should keep in mind that holes may be cut in the firewall to accommodate rear-projecting tanks, air intakes, etc., thus shortening the beams. Don't forget to slant this section forward slightly to provide down-thrust, if the model is to be a free-flight job.

Next, you'll have to provide a solid mounting for the landing gear. Assuming you're using a fixed gear on this first model, the simplest method is to bend a wire shape which has an upward extension which can be laced to a plywood bulkhead, with the legs projecting from the lower side of the plane (Fig. 65). This bulkhead will establish another station on your side view. If you're planning a low-or shoulder-wing model (not advisable on your first job), you may have to place the plywood piece horizontally or shorten it. The placement of the elevator should be decided now, too. If you plan a knock-off tail, which is a good idea on all free-flight models for ease of storage and crash protection, there will have to be a platform wide enough to provide a good base. If your top view is very narrow at the tail, you may have to add a balsa or plywood platform. Don't let any obstruction interfere with the pop-off of the tail. It is not a good idea to have a removable elevator in a slot; it's better to have the portion of the fuselage above the elevator, including the rudder, attached to the elevator and removable with it.


fig.65
fig.65
 

If this is a control-line model, decide where the bell-crank will be placed, and how the pushrod is going to attach to the elevator, and where it is going to be located, whether inside or outside. Locate the holes in the bulkheads to accommodate it. If the ship is a profile job, or a simple sport flyer where appearance is secondary, it is often much simpler to mount the entire control system on the outside of the ship; but for good looks, the system can almost always be completely enclosed, if you have provided space for it.

The wing mounting is next. The width of the wing (chord) should be somewhere in the general vicinity of 1/5 the length of the fuselage for free-flight, more for a stunt C/L job. The angle of incidence should be about 3-5° for the average F/F job, 0° for C/L. (The frequent use of the words "about" and "approximately" indicates the leeway you have as a designer.) A knock-off wing is a must for a free-flight job, except perhaps for the smallest all-balsa models. Dowels or rods for hooking the hold-down rubber bands should project about 1/2 inch, and should be very well anchored. Be sure they are evenly spaced at least as far in front of the wing and behind it as they are below it, to provide the proper fore-and-aft grip. If the wing is mounted atop a cabin with a curved front windshield, you can use a single center dowel for the front. A high-shoulder wing can be handled like a top-mounted wing, by having the portion of the fuselage above the wing removable with the wing. Cut the removable section at an angle of 45° to permit easy release in a crash (Fig. 66).


fig.66
fig.66
 

Now, with these details worked out, set up the remaining stations, each of which will require a cross-section. You won't need more than eight stations altogether, including the firewall and rear post, unless you have something extra long or large in mind. The remaining stations should be spaced between the ones already established. Try to arrange things so that you have a vertical member (either a bulkhead or a side spacer at a station) to support the trailing edge of the wing and the wing mounting dowels, and another at the front of the elevator. A little juggling of the outline of an original should enable you to make everything come out about even. If you are building a scale model, the stations shown on your source three-views will probably be quite satisfactory, and should be used, especially if you have the cross-sections to match them.

Now decide how you will handle such items as the window outlines, strut mountings, and other features. Frequently, the window-sill line on a cabin model will give you a good starting line for a side frame or a crutch. Consider in advance how you're going to anchor load-bearing struts - don't try to fit them into a finished design.

If the design is an original, now is the time to work out the cross-sections. You know about what you want; just be sure you follow the widths and heights established by your side and top views, and lay out a firewall, a section near the center, and one to the rear. Keep the placement relative to the datum line the same as on the side view.

Looking at the cross-sections of the fuselage, pick out any flat side area which can be built directly on the plan; if there is no such area, look for a basic rectangle to which you can add formers to build out the outline. If the cross-section is curved all the way, it is still quite easy to build it up on a basic box. Start by drawing a rectangle on the plan of the firewall (of the section nearest the front). Try to determine the largest rectangle you can fit into the curved shape, touching at all four corners. Now measure down from the top of the curved cross-section to the top of the inscribed rectangle, and lay this distance off on the side view of that same section, which should be station No. 1. Next do the same with a section about halfway back; if there is a window sill to consider, use this as the location of the top of the rectangle, at the appropriate section. Repeat with a section near the rear. Through the three points thus established on the side view, construct a smooth curve. This can easily be done by placing pins in the points and bending a balsa strip along them. If you are doing a cabin model, the line should probably be interrupted at the rear of the cabin and continued higher up. Then lay off the bottom, spacing in the same way. Voila! You have the side frames laid out.

Now, on cross-section No. 1, measure the distance from the side curve in to the side of the rectangle and lay it off on both sides of the top view, repeating for the other two stations, again constructing a curve through the points.

Now, at each station on top and side views, measure the distances from the outline in to the curves you have just drawn, and lay these distances off on the remaining cross-sections to get the exact shape of your formers. If you have yet to draw these sections, construct them using these points and the heights and widths from your side and top views. If you need more than the four points of a rectangle to sketch in the required curves, draw a second set of rectangles, with corners falling between the corners of the first set and top, bottom, and side center points, and lay out a second series of points. By drawing the smooth curves on the side views, you will be assured of a smooth contour on the finished fuselage (Fig. 67). Of course, if you are doing a scale job and have all the sections, you can skip this step.


fig.67
fig.67
 

With the outlines of your side frames set up, you can select a size for the longerons and spacers, and draw them in. For a fuselage up to about 20 inches long, 3/32" square medium-hard balsa is generally large enough. 1/8" square will make a sturdy fuselage 30 inches long, and 3/16" will handle anything up to man-carrying size. Certain members, such as the window sills, the vertical at the rear of the wing, and the rear post, should be about double the size of the rest of the frame. It is a good idea to cut curved formers from sheet balsa of the same thickness as the frame members, to avoid having to bend any deep curves (Fig. 68).


fig.68, fig.69
fig.68, fig.69
 

Cabins can be laid out as a part of the side frame or they can be built up and added to the box after the sides are joined, as shown in Fig. 69. This is a good idea when the sides are curved at the position of the cabin. Keep wing mounting in mind, if you have a high-wing job, and plan a cabin roof to receive the wing. If you prefer, you can use another structural method to build up the body. If you use a crutch, place it so that it doesn't interfere with window sills on a cabin job, and so that beam motor-mounts, if used, can be attached to it. If your cross-sections have been laid out on a single line representing the datum line, you will easily be able to indicate the position of the crutch on both side and top views.

The crutch is built on the top view, with cross-pieces placed at each station. The crutch members should be about twice as large as box members for the same size ship. Cut the bulkheads apart at the line where the crutch divides them, and notch them to fit over the crutch. A bottom keel should be designed to give rigidity to the structure, and notches provided in the bulkheads to receive it. Curved formers should be used rather than deep bends, which will draw the frame out of line.

If the half-shell method is used, both top and bottom keels will be necessary. Draw them on the side view, and notch the bulkheads for them.

For these two methods, plywood bulkheads should be used where the greatest loads will be applied; e.g., firewall, landing gear mounting, etc. These should not be cut; the framework should be assembled without them on the plan; after the framework is complete, they can be installed with any required landing gears, wing struts, etc., already attached.

With the basic framework designed, it is time to give some thought to covering. If the framework is to be tissue- or fabric-covered, you don't need to do anything further at this point, but if you plan to use sheet balsa covering or strip balsa planking, the thickness of this should be indicated now on all views, including bulkheads, so that the formers can be cut undersized. The planking will build the section out to the required finished contours. In some places, as for example the nose section, it is advisable to either plank or fill the framework to give greater strength. Scale models representing other than fabric-covered prototypes should be balsa-covered if possible, for better appearance. If you are faced now with any awkward situations, such as compound curves, hard-to-manage fairings, tricky cowlings, etc., which present construction or covering difficulties, remember those soft balsa blocks.

Before going any farther with the fuselage design, you'd better give detailed consideration to wings and tail. Tail assemblies are easiest, so let's get the rudder and elevator out of the way first. Solid balsa, or sheet balsa tails can be used in almost every instance, even on contest-type models. The only exceptions are large free-flight or control-line scale jobs, where the weight might prove excessive. Small free-flight tails may also be cut from sheet balsa. For small models up to about 20 inches in span, 1/16" medium balsa is fine, 3/32" will do for jobs up to 36 inches; above that, use medium-hard 1/8" for C/L, and built-up construction for F/F.

Tail assemblies for scale C/L models having thick sections can be carved from soft balsa, for high-powered ships up to about four feet in span. Beyond that it is advisable to consider building up tail surfaces using thick soft balsa squares for leading edges, deep spars, solid tips, and sheet balsa covering.

Check the finished elevator and rudder outlines against the fuselage drawings to be sure everything is going to fit. Start the wing by drawing up an outline and a front view. The wing, with few exceptions, should be built in one piece. If you are working on an F/F original, you can choose simple dihedral, polyhedral, or tip dihedral; just be sure to get the tips about 1/15 of the span above the center section. The outline can be rectangular with square or curved tips, tapered, or elliptical, in order of difficulty. Fig. 70 shows several dihedral schemes and outlines. You can vary these to suit your taste. For your first job, a constant-chord wing is recommended, as all the ribs will be identical and you will be saved the labor of working out a dozen different rib outlines. By varying the tips, a great deal of variety can be obtained with such a wing, and in both design and construction it is a timesaver.


fig.70, fig.71
fig.70, fig.71
 

Don't spend too much effort on the creation of your airfoil; on sport model planes, airfoil is not critical. For all practical purposes, in sport flying, an approximation of one of the widely used airfoils such as the Clark Y will work very well. Fig. 71 shows some basic types, and you can easily pick up a good airfoil from a kit or magazine plan. Later, when you have graduated from the primary designers' class, you will have plenty of time to experiment with the fine points of airfoil design.

If you plan a tapered or elliptical wing in which all the ribs are different, start by drawing up a root and a tip rib and making a pattern for each from aluminum. Decide on your rib spacing and wing span, and count the number of ribs you'll need. Next, cut a set of blanks from 1/16" balsa, graduated in length and width to conform to the dimensions already established in your front and top views. Stack the whole set of rib blanks in the correct order, with the metal patterns on the outside, and drill a hole through the stack, preferably where the spar will be located; then run a bolt through the hole, and tighten it up. With a sanding block you can now quickly shape all the ribs as a unit; trim the front and rear ends and notch (Fig. 72).


fig.72
fig.72
 

Separate the ribs and use them as patterns for the ribs for the other half of the wing. If the wing is balsa-covered, or if cap strips are used on the ribs, the slight bevel of the outlines will cause no trouble, although the ribs from the two sets should be alternated in each wing panel to equalize differences. If the wing is to be tissue-covered without caps, two new sets of ribs should be cut, using the first set merely as patterns. An alternate method is to construct each rib by a method similar to that employed to lay out the fuselage cross-sections (Fig. 73).


fig.73
fig.73
 

In drawing up a scale wing, particularly a deep-sectioned wing for a scale model of a heavy airplane, it is important that the front view show the correct datum line on which the spars are based; otherwise, you'll end with a wing having strange and mysterious warps which can't be corrected, usually the result of assuming that the angle of the bottom of the wing is constant from leading edge to trailing edge. This is definitely not true; the angle of dihedral or taper is constant from edge to edge only along the plane of the datum line. This line should be indicated on three views; if it isn't, it will be necessary to determine it before designing spars. For practical purposes, it can be assumed to run through the foremost point of the leading edge and the center of the trailing edge. This line should be carefully located on the front view, and the spars constructed on it, referring to the rib patterns for correct depth at each rib station. If the wing has a constant taper, the spar can be constructed from root and tip ribs only.

For non-scale wings, spars should be located at the deepest point on the rib outline, usually about 1/3 of the way back from the leading edge. The total cross-section area of the spar or spars, which should be made from hard balsa or yellow pine, or laminated balsa-celluloid-balsa for heavy ships, should be about 1/50 of the chord area. More simply, for a rib 5 inches long a spar 1/8" x 1/4" or 1/16" x 1/2" should be adequate. Unless you're using a one-piece or reinforced spar, reinforce dihedral joints with a piece of thin plywood or thick celluloid cut to follow the dihedral angle, and extending to the ribs on either side of the dihedral joint, as shown in Fig. 74. The trailing edge should be, in width, about 1/6 the length of the rib; the leading edge should extend back about 1/10 of the length of the rib, spliced in the same way as a spar, with reinforcement.

Free-flight wings are usually attached after everything is completed, but fixed wings are frequently installed midway through the building process. Sometimes a spar and leading and trailing edges are slipped through the finished fuselage, and the ribs and remaining structure added later (Fig. 75). Or it may be necessary to install an almost completed wing in a half-built fuselage, in order to get the greatest ease of assembly.


fig.74, fig.75
fig.74, fig.75
 

Before going further with the design process, the fuselage should now be built up to the point discussed. Note on the drawings any changes that become necessary or advisable during construction. With the basic frame complete, it is easy to visualize construction details, and plan the next steps, as well as to criticize what's already been done. Don't hesitate to chop, splice, alter, and improve your framework now and reflect the improvements in the plan.

Install the hardware, such as landing gear, motor mounts, tank, bellcrank, etc. Next, go ahead with the wing framework (or the portion of it which will be installed as a unit in the fuselage) and the tail surfaces.

Check the installation of these to make sure everything is going to fit. Install any assemblies which can now be added without interfering with the development of the fuselage, but don't rush things. If you are satisfied the wing and tail will drop into place when ready, you can go ahead with the fuselage. Decide now where you'll need stringers, fairing strips to even up surfaces for smooth covering, reinforcing gussets and filling. Make any attachment points which may be required for cabane struts or other fittings. Don't wait until the framework is complete, and then try to put the works inside with tweezers - it won't do. Study the contours and decide what areas to cover with paper, which ones to plank, where to use blocks - if you still have any doubts on these points. Plan the method of building up the cowling, if any, and use direct measurements to fill in any doubtful points on your plan.

Now go ahead to complete the model, following your plan, amending it when necessary. Don't add any details to the ship at this point, or apply a fancy paint job. Complete only the part of the work that's necessary to enable you to make flight tests. Wheel pants, cowlings, rigging wires, etc., affect the flight very little, and will merely be liable to damage.


A twin-engined C/L job with clockwork retracting gear.
A twin-engined C/L job with clockwork retracting gear.
 

With other flyers, spectators, small boys and cows in the area alerted, you are now ready for the first flight of the X-l. Make your first tests even more cautiously than you would with a kit model, but following the same procedure of careful test glides, low-power flights, and constant adjustment. If you have adhered to the rules of thumb discussed above, you should have a flyable ship, but only after proper trimming. After you have gotten the ship trimmed, indicate the balance point, final angle of incidence, etc., on the plan. It may even be necessary to alter the ship by moving the wing, shortening the nose, shifting the position of the L.G. Don't be bashful about operating on the model to make these corrections; very few ships fly perfectly, right off the drawing board.

You may, of course, have the unhappy experience of discovering that your plane will not fly at all, even with every possible adjustment of incidence, thrust, trim, power, and balance. That means that you have another job ahead; finding the flaw in your design and correcting the trouble. After all, that's part of being a designer!

Let's picture a few symptoms of design error, and see if we can decide what to do about them. Suppose your F/F ship heads out fine under power, but begins to swing from side to side in a rocking motion, or flips over and spins in. Chances are, you need more rudder area. Sometimes this shows up only in the glide, or in a high wind, but that telltale rocking is a giveaway. Increase the rudder area by half for a start, and try again.

On the other hand, perhaps your job sails out in a nice climbing turn that keeps on turning - right into a spiral dive. That usually means too much rudder area, or not enough area down low, or inadequate dihedral. Or perhaps your center of lift is behind the center of gravity. In that case, reduce incidence and load the tail.

If the plane is very mushy in the glide, you probably have a built-in head wind, such as too much incidence to compensate for excessive nose heaviness, or possibly too thick a wing section, or in a biplane, wings placed at different angles of incidence.

If the plane upsets easily in a wind, while flying well in calm air, this could mean insufficient dihedral of rudder. If the ship glides, but bores in under power, perhaps you have a negative angle of incidence in the wing, in relation to the elevator, or too much down-thrust.

If none of these diagnoses seem to be applicable, ask an experienced modeler to take a look, or compare your ship with kit or magazine plans of a generally similar design, looking for the difference which might be causing the trouble. Unless you've wandered far from the general restrictions set forth above, the ship will eventually fly.

C/L trouble-shooting is considerably simpler, since the lines restrict the action of the ship, and inherent stability is not a necessity. If your ship complies with the usual balance criteria and has enough power to lift off the ground, it is sure to fly. If it doesn't, re-check balance, rudder, and engine offset.

Having seen your ship do its successful solo, you can go ahead and finish up the details and add the extras which give it the finished look you want. The experience you gained in flying the model may lead you to change some of your ideas along these lines. Items which look good on paper can sometimes be a nuisance on the flying field. But note it all on the plan; after all, the experimental ship was merely a device to assist you in drawing the plans. You may wish to make structural changes now, consolidate and smooth out some of the on-the-job corrections you made on the model. When you're satisfied that you have all the corrections, additions, deletions, and alterations done, redraw the plan carefully on plain white paper. If you make your drawing on draftsman's tracing paper, using a No. 3 (or 3H) pencil, you can have it reproduced cheaply at your local blueprint shop, thus enabling you to pass out copies to your friends and see your job mass-produced. This is the acid test; the flying ability of a plane built by another modeler from your plans is the measure of your success.

Keith Laumer Plans

Full-size plans for Keith Laumer designs are available from Aeromodeller Plans Service, Box 35, Hemel Hempstead, Herts, England, HPI 1EE.

Whizzler - 24" all balsa glider (order code number G 791)

Flutterbus - 18" simple rubber-driven model (order code number D 797X)

Sure Flyer - 30" Rubber-driven duration (P. 185) (order code number D 800)

Sharp Scooter - 29" Free-flight power model (order code number PET 804)

Sharpoon - 36" Control-line acrobatic (order code number C/L 706)

Price of each plan approximately 45p.

the end

 
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