PORTPLANE BUILDER By Tony Bingelis

Transcription

1 PORTPLANE BUILDER By Tony Bingelis MAINTAINING ALIGNMENT DURING CONSTRUCTION It doesn't take long to learn that building an airplane and assembling its various components entails an endless series of alignment checks. Naturally, you would expect to maintain the symmetry of the fuselage and strive to get both wings the same size, but the alignment needs go beyond that. Even the holes you drill must be accurately aligned or the parts will not fit. I think it is reasonable to believe that if you keep a close check on the alignment of every single part and component you construct and install, you will be rewarded with an airplane that tracks and flys straight. The number of alignment checks that have to be made during the course of construction is truly surprising. However, the methods generally used to maintain structural alignment are but few in number. Each is easy to use by itself, or in concert with the others. I believe the secret to good alignment lies in the early establishment and use of centerlines, reference lines and reference points. Methods of Alignment You Can Use Sight (Eyeball) Alignment Checks Amazingly, this is a fairly accurate way to determine the degree of alignment. Your eyes should be able to detect a tilt, or a twisted component, simply by standing back and viewing the overall image. A Steel Tape Alignment Check A 50 foot steel tape will handle just about any measurement you need to make unless you are building a B-17 replica. Stretch the tape snugly while applying an approximate 5 pound pull to obtain uniformity in your readings. A Bevel Protractor Use it to verify duplicate angles such as the incidence at each wing tip (see Figure 1). Straight Edges These are best made of wood provided both edges are perfectly straight. You can use these lightweight alignment aids for many applications. A Spirit Level One at least 24 inches long is just right. Get one that has the glass spirit level visible from either side. Handle this instrument carefully but use it often. A Trammel If you need one, a trammel about 3 to 4 feet long will do. This gadget is mostly used to adjust drag and anti-drag wires. Expensive metal ones have sliding heads at both ends for easy adjustment. A simple poor boy version can be made of a strip of wood with a pointed nail driven through one end to serve as one point. The other end (adjustable) could consist of a clamped short piece of wood with a pointed nail sticking out. That movable piece, being clamped with a spring clamp provides an easy, although crude means of adjustment. A Plumb Bob Suspend it from the top of a frame, or jig, or wherever vertical alignment has to be verified. Here's a handy tip. Suspend the point of the BEVEL PROTRACTOR CHECKING WING TIP INCIDENCE FIGURE I. plumb bob in a small clear plastic container filled with water to dampen its oscillations (see Figure 2). Surveyor's Level This and similar peek-a-boo instruments can be used to provide accurate elevation angles to determine, for example, that both wing tips are at the same level. Water Level Device As the name implies, the gadget makes use of the water level to help establish widely separated level points. If your shop floor is uneven and you have to find if your wing jig is level, this is the device to use. As you know, water seeks its own level no matter what so, therein lies the principle. Use a length of clear plastic hose (30 feet more or less, as needed) with an I. D. of at least 1/2", filled with water (colored water is easier to see so add a bit of food coloring... sure, any color). After filling the tubing, raise and hold both ends together so that the water level is about 10" from the open tubing ends. Mark the exact water level on each tube with a black felt marker. To use this water level, tape one end of the tubing so that the mark is right on one of the reference points to be checked. Then, walk the other end of the tubing over to the other reference point and raise or lower its open end until the water line is on the mark. The amount of misalignment will immediately become evident. You have to keep both ends of this device up when it is not in use or the water will drain out. PLYWOOD TEMPLATE (INCIDENCE BOARD) f., r r-k. SPORT AVIATION 29

2 CARPENTER SQUARE (CHECKING VERTICAL ALIGNMENT) TYPICAL ALIGNMENT CHECKS FIGURE 2. Sources of Alignment Difficulties Common alignment difficulties generally stem from misreading the plans, from inaccurate layouts, a reversal of fittings and parts, a failure to make a left and a right, and, in general, a lack of attention to details. Sometimes a part may appear to be symmetrical but it is not. Too often the builder will try to force bolts into an inadvertently reversed fitting. This is wrong, wrong. If a part fits correctly when you first made it but it won't now, stop and regroup. You are doing something wrong! Don't forget, too, that some pairs of parts are made to fit opposite sides a left and a right as it were. It is essential that you mark every part you make. Also, mark a centerline, chordline, or reference point (as the case may be) clearly on both sides for easy reference. I guess you must be getting tired of hearing it, but here it is again. Always drill your holes undersized initially. This will provide you with some margin for error and will result in better fitting holes when you finally do line-drill them to size later. 30 SEPTEMBER 1986 PLUMB BOB SUPPORT (FOR VERTICAL ALIGNMENT) MARK CENTERLINES ON ALL SYMMETRICAL COMPONENTS. CARPENTER SQUARE (HORIZONTAL ALIGNMENT) CENTERLINE MARKED ON FLOOR (OR ASSEMBLY TABLE) Maintaining Fuselage Symmetry and Alignment Since your fuselage will become the foundation piece for the airplane, it is important that you maintain its symmetry and alignment during construction. Prior to assembling the typical fuselage, you should lay down a centerline on your assembly bench, or on the floor if a jig instead of a work table is used. Every measurement thereafter must be made from this basic reference. I think it is a good idea to scribe the centerline on the work bench as an ink or pencil line can fade or rub out. Of course, it goes without saying your work bench should be leveled. This may be difficult to do on an uneven shop floor so why not try this method to level the bench. Screw a lag screw into the bottom of each leg. Screw them in all the way. After you move the work table (bench) to the shop area in which you want it to be located, check the table's level. Make the necessary adjustments with a wrench by screwing out whichever lag screws will effect the adjustment you need to level the bench. A level work surface is important because you will undoubtedly need to make frequent use of a spirit level and plumb bob. Be sure you get your firewall square with the centerline. Verify this frequently with the carpenter square checking it for both horizontal and vertical alignment. Should you accidentally allow one side of the firewall to become skewed, it will trigger a domino effect with multiplying alignment problems. Want some examples? If one side of the fuselage at the firewall accidentally slips during construction and becomes fixed somewhat ahead of the other side, your engine mount and engine, when installed, will likewise be offset to the same degree. With the engine thrust line offset due to a misaligned firewall, torque problems could increase. Also, due to an offset engine, the symmetry of your cowling may be affected because the front cylinder might be too close to fair into the cowling profile. The misalignment at the firewall is not an end to the problem. All of the fuselage fittings on that side will also be offset or misaligned. These include the landing gear fittings, wing attachment fittings and the tail attachment fittings. Since these fittings all affect the alignment of the landing gear, wing and tail, you cannot afford to allow such a misalignment to occur. Frequent checks with the carpenter square can assure you that your fuselage assembly is proceeding as it should (see Figure 2). Incidentally, soon after you start the assembly of the fuselage, you should select and identify an area on the top longerons that can henceforth be used for leveling the aircraft. If your aircraft has no longerons, you should affix some permanently installed blocks, or at least screw heads on which a spirit level may be rested for checking the level of the aircraft. Remember that when leveling the aircraft, it must be leveled fore and aft as well as laterally your permanent level points should provide for this capability. Wing Alignment Checks The alignment checks you must make as the various wing components are assembled vary considerably with the design of the wing and the type of construction employed. First, let's look at the typical externally braced wing. The Wood and Fabric Wing A wing of this type is ordinarily built up around two wood spars over which a number of one-piece ribs are slipped, and glued and nailed at designated intervals along the wing span. This forms the basic wing skeleton. Since each rib is built in one piece, they are easy to glue and nail to the spar. While their attachment is quite easy accurate alignment is another matter.

3 LEADING EDGE STRIP. 4 SMOOTH PROFILE OVERAlNOER SPAR FLANGES IS ESSENTIAL PROPERLY ALIGNED WOOD RIB SECTION BEVEL DIMENSION AND BEVEL SPARS BEFORE ATTEMPTING RIB INSTALLATION CORRECT ADJUSTMENT OF RIB SECTION ENDS IS ESSENTIAL FOR ALIGNMENT. ALIGNMENT STRIP WOOD ALIGNMENT STRIPS SIMPLIFY RIB INSTALLATION SPAR/RIB ALIGNMENT PROCEDURE (WOOD CANTILEVER WINGS) (TYPICAL) FIGURE 3. If the spar depth (height) is such that the spar cut out in the ribs is not completely taken up by the spar, it is entirely possible to inadvertently glue and nail some of the ribs too high or too low. If you are aware of this, you can cope with the problem. The preventative measure for this type of misalignment is simple. Lay a couple of straight edged wood strips (approximately 3/4" x 2" x 5') spanwise across three or more ribs. Clamp each rib to the straight edges so that the ribs will be correctly aligned vertically. You can then glue and nail them to the spars knowing their external alignment is assured. The next order of business is to carefully trim the leading and trailing edges of the ribs checking frequently with a long straight edge until you obtain contact with each rib. Be sure to make this alignment check before you affix the leading and trailing edges. CORNER BLOCKS S TRAILING EDGE STRIP SPRING CLAMP This type of wing calls for a rather tricky alignment chore which includes the precise tightening of the drag and anti-drag wires. The job can best be done with the aid of a tramel to check that each compression bay is perfectly square. If the wing drag and anti-drag wires are not accurately adjusted, you will end up with a wing panel that could be swept forward, swept aft, or endowed with unwanted curves in the spars. There is little need to check for wing warp (twist) until you are ready to cover the wing with fabric. Even then, it would not be as serious a matter as it would be in a plywood or metal skinned cantilever wing. The reason being, of course, that a fabric covered wing can be twisted slightly even after it has been installed... a cantilever wing cannot. However, it is well to remember that if a fabric covered wing has to be warped more than just a tiny bit after it is covered and doped, the fabric will reveal your goof with a most noticeable diagonal ripple for all the world to see. The Cantilever Wood or Metal Wing This type of wing requires several critical alignment checks during construction. First, although the wood wing ribs are usually built in one piece, the metal ribs are not. For installation, however, even the wood ribs must be cut apart because a full depth cantilever spar doesn't leave space for any rib capstrips over or under it. This means, my friend, that you have a wing rib that is in two or three separate pieces. Aligning each of these rib parts can tax your patience. The objective is to secure these separate rib parts to the wing spars, and do it so that the end result is a perfect airfoil shape at that rib station. Figure 3 illustrates the terrible results you can expect if you slight the rib alignment problem for this type of wing. One handy assembly aid is also shown in Figure 3. Use a couple of capstrip-like wood strips to entrap the rib parts. Clamp the strips at each end with rubber bands or with spring clamps. You can rely on the natural curves in the bent wood strips to correctly position the rib sections for gluing and nailing (or riveting) to the spars. Caution: Be sure that the top and bottom spar flanges have been correctly beveled and dimensioned before you attempt to fit the ribs. You might, otherwise, still end up with a faulted rib profile section. Your ribs, if correctly lofted from the plans, should already have the correct end angles so that the exact amount of wash-out (wing twist) will be automatically built-in when attached to the spars... assuming, of course, that wash-out is required at any particular rib station. However, don't count on the ribs being exactly trimmed. They might be off slightly, and if you don't stay right on top of your alignment responsibility, one wing tip might develop a bit more twist than the other. Remember, no cantilever wing can be twisted after its top skin (closing skin) is installed. This means you must jig both wing tips at the same incidence angle, and do that just before you close the wing. One way to verify that both tips have the same amount of twist or incidence is with the help of a plywood half template. Make one for each wing. Make the template with a straight top surface on which you can place a protractor level for verifying alignment. Alternatively, if your fuselage doesn't get in your line of sight you could lay a straight edge on top of the half templates and squint from one wing tip to the other to see if both straight edges parallel each other. If they do, both tips have the same incidence (see Figure 1). SPORT AVIATION 31

4 RIB ALIGNMENT STRIPS (ALUMINUM 2024 T3.040" x f ) C-CLAMPS OR CLECO CLAMPS SFfcR/RIB AUGNMENT METHOD (METAL RIBS) FIGURE 4. SELECT EASY TO IDENTIFY REFERENCE POINTS FROM WHICH TO MEASURE. MEASUREMENTS MADE WITH A 30' STEEL TAPE. IDEALY DISTANCES ON BOT1 SIDES SHOULD MATCH. IA,-A2/BI -83, ETC.) I, FIREWALL CENTERLINE I hate to dwell on it but if you don't jig that wing perfectly before installing that closing skin, there is no correcting the wing twist afterwards short of a major rebuilding. A cantilever wing with differing tip angles (incidence) is guaranteed to be unruly in flight. On final assembly of the wing to the fuselage you should check the installation for symmetry. That is, measure the distance from each wing tip to the tail post with a steel tape pulled snug (always apply a uniform pull of about 5 pounds to tension the tape). Measure the distance first on one side, and then on the other. Both sides should have exactly the same measurement... however, a plus or minus 1/4" or so is as good as some builders achieve. Nobody but you will know it and the flight characteristics should not be affected (see Figure 5). 32 SEPTEMBER 1986 CHECKING AIRCRAFT SYMMETRY (TYPICAL) Alignment of Tail Surfaces The establishment of the correct incidence angle for the stabilizer is the most important of the several alignment checks you will make for the tail surfaces. Obtaining the correct geometrical alignment of the complete tail assembly is of somewhat lesser impor-

5 FORWARD ALTERNATE ALIGNMENT METHOD - REMOVE WHEELS AND CLAMP A STRAIGHT EDGE TO BOTH AXLES. RUBBER BANDS (SNUG-NOT TIGHT) ALIGNMENT STRIP (3/4" x 2" x 6') MARK CENTERLINE ON FLOOR ZERO TOE-IN RECOMMENDED tance. More often than not, this particular alignment is more important to the eye than it is to aerodynamic necessity. For example, when viewing the tail from behind, if the angle formed between the centerline of the vertical tail post and the horizontal stabilizer spar is not 90 degrees but is, actually, a couple of degrees off, flight effectiveness is not likely to be impaired. On the other hand, a few degrees of error in setting the angle of incidence for the stabilizer can produce serious trim problems. Besides assuring yourself that the tail surfaces are perpendicular to each other you should check them for symmetry with the fuselage as you do the wings. Measure from each stabilizer tip WHEEL ALIGNMENT PROCEDURE to the top center of the firewall. Naturally, you want the measurement on each side to be the same. Some builders refer to the method as measuring by triangulation. Wheel Alignment Wheel misalignment can cause a lot of grief... literally. Strive to get your wheels lined up so that each is parallel with the other that is, no toe-in and no toe-out. Certainly, don't err in the direction of toe-out. Figure 6 shows a simple way to check your wheel alignment. All alignment checks should be made with the aircraft in its normal level flight attitude for accuracy. In Summary FIGURE 6. A tiny alignment error at the inboard end of one wing, or at the fuselage attachment, can cause that wing tip to be swept forward, or aft, an inch or more. Errors in alignment have a way of compounding because they are passed on to the joining component. Guard against the initial error by double checking the dimensions and then follow that up with an alignment check using two different methods if possible. If you wish to contact the author for additional information, please write to Tony Bingelis, 8509 Greenflint Lane, Austin, TX SPORT AVIATION 33