Reference / Paper · 2012
Peak of Flight #328: Designing and Constructing Fins
Apogee Components' Peak of Flight newsletter, issue 328 (2012-12-18).
- peak-of-flight
- apogee-components
- model-rocketry
Reference / Paper · 2012
Apogee Components' Peak of Flight newsletter, issue 328 (2012-12-18).
Apogee Components' Peak of Flight newsletter, issue 328 (2012-12-18).
In This Issue Designing And Constructing Fins Cover Photo: North Coast Rocketry’s SA-14 Archer Rocket Kit. Get one at: www.ApogeeRockets.com/Rocket_Kits/Skill_Level_3_Kits/SA-14_Archer Apogee Components, Inc. — Your Source For Rocket Supplies That Will Take You To The “Peak-of-Flight” 3355 Fillmore Ridge Heights Colorado Springs, Colorado 80907-9024 USA www.ApogeeRockets.com e-mail: [email protected] Phone: 719-535-9335 Fax: 719-534-9050 ISSUE 328 DECEMBER 18, 2012 Designing And Constructing Fins By Tim Van Milligan {This article comes from the book: Model Rocket Design and Construction. You can get more information about the book at: www.ApogeeRockets.com/Rocket_Books_Videos/Books/Model_Rocket_Design_And_Construction} Fins should be constructed so that they can withstand the forces expected during flight. The materials and methods of construction determine how much force they can withstand. Very small models do not need very strong fins, and simple cardboard or balsa fins will suffice. Large models and those intended to fly at extremely high velocities will need more strength. The following list gives several materials and construction methods, in order of increasing strength: • • • • • • • • • Thick cardboard (not corrugated) Balsa wood fins Plastic fins Balsa wood with paper reinforcing Balsa with spruce wood reinforcing Built-up fins Foam core fins Fiberglass reinforced fins Plywood Cardboard is suitable for small rockets using up to a full A rocket motor. Straight balsa wood can be used for rockets of .91 m (3 feet) or less in height, using a maximum of a D engine. Larger rockets or higher-power models should use one of the other techniques or materials. Most of these methods will be described below. Since balsa is a predominant material, the chart below describes the uses for the various thicknesses of balsa available. If you plan to construct your fins from a solid-sheet material such as cardboard, plastic sheet, or balsa wood, first make a template of Fig 6-5 the fin shape so it can be transferred to the material. Before marking around the outside of the template with a pencil, orient the balsa sheet so the grain direction in the wood is parallel to the leading edge of the Snap Yes template. This will ensure fins NO maximum strength of the fin. If the fin grain is oriented parallel to the body Orient the grain on the tube, they will be very balsa wood with the leadweak and will snap easily ing edge of the fin. (Fig 6- 5). These fins can be fixed by laminating both sides with bond paper as shown in Fig 6-12. Sometimes it is necessary to make fins out of multiple pieces of wood. Such might be the case if the fin would be wider than a sheet of balsa wood (Fig. 6-6), or if there is an undercut in the fin that would result in a fin that is structurally weak because of the wood grain direction (Fig. 6-7). In either case, cut the wood so that the grain direction is oriented to create a fin with maximum strength. Fig 6-6 Fins with straight edges are the easiest to cut because you can use a ruler to help guide the knife. Lay the ruler on top If the fin is wider Uses for Different Thicknesses of Balsa of the fin to protect it if the than the width of 0.79 mm (1/32”) Hollow built-up fins knife blade should stray the balsa wood 1.59 mm (1/16”) Typical small rockets away from the ruler when sheet, you’ll have 2.38 mm (3/32”) Medium-size rockets you are cutting. Keep the to build it up from 3.18 mm (1/8”) Large rockets or clusters; “C” or larger engines blade at a 90-degree angle several pieces. 4.76 mm (3/16”) Large cluster engine or heavy models 6.35 mm (1/4”) Generally too thick; high drag Continued on page 3 About this Newsletter Newsletter Staff You can subscribe to receive this e-zine FREE at the Apogee Components web site (www.ApogeeRockets.com), or by sending an e-mail to: [email protected] with “SUBSCRIBE” as the subject line of the message. Page 2 ISSUE 328 Writer: Tim Van Milligan Layout / Cover Artist: Tim Van Milligan Proofreader: Michelle Mason DECEMBER 18, 2012 Continued from page 2 Designing And Constructing Fins Fig 6-7 Multiple pieces of wood glued together (called compound fins) should be made for fins that have an undercut to them. Note the wood grain direction. to the wood to avoid beveling the edges of the fin (Fig. 6-8). When making cuts, make several light passes instead of one heavy cut. {you can see the techniique in video format at: http://www. apogeerockets.com/ Advanced_Construction_Videos/Rocketry_ Video_1} Curved fin shapes are only slightly more difficult to cut. To ensure maximum uniformity between fins, make a template out of thick styrene sheet and use it to guide your knife blade. Thicker or harder materials such as plywood are more easily cut with a razor saw than with a knife (Fig. 6-9). Use care with a razor saw, since it is much harder to guide the blade and keep the edges straight. After you’ve cut all the fins, stack them and sand them all together to ensure uniformity of size and shape. Pay particular attention to the edge that will be glued against the body tube (the root edge). It should be perfectly straight to maximize the bond strength of the fin. Also at this time, sand the airfoil into the fin. You might just round the edges, but for minimum drag the fins should be streamlined. Using a small wood planer will drastically cut your sanding time on thick fins. Fig 6-8 1 2 3 4 5 Place ruler on top of fin to prevent it from being sliced if the blade should stray from the ruler’s edge Fig 6-9 If you want to have a definite high point on the fin, as would be needed on a scale model, lay a piece of masking tape along the dividing point (Fig. 6-7). If you sand into the tape, remove it and lay a new piece down. This will give a sharp line on the fin. Use a razor saw to cut thick balsa and plywood Finally, give the fin an all around surface sanding. Re- Sanding an airfoil shape will decrease the rocket’s drag Fig 6-10 Rounded edges Fully streamline Continued on page 4 Model Rocket Design and Construction By Timothy S. Van Milligan The Expanded 3rd Edition This massive, 328 page guidebook for serious rocket designers contains the most up-to-date information on creating unique and exciting models that really work. With 566 illustrations and 175 photos, it is the ultimate resource if you want to make rockets that will push the edge of the performance envelope. Because of the number of pictures, it is also a great gift to give to beginners to start them on their rocketry future. For more information, and to order this hefty book, visit the Apogee web site at: www.ApogeeRockets.com Apogee Components 3355 Fillmore Ridge Heights Colorado Springs, Colorado 80907 telephone: 719-535-9335 website: www.ApogeeRockets.com ISSUE 328 DECEMBER 18, 2012 Page 3 Continued from page 3 Designing And Constructing Fins wood glue, instant glue, or epoxy. You can make them from a variety of materials, including tissue paper, ordinary notebook paper, cardboard, plastic sheet, very thin plywood, and fiberglass. Fig 6-11 Masking tape on edge of high point Stiff skins are usually applied to flat surfaces, as they are difficult to apply to fins that have airfoils sanded into them. To prevent a balsa wood fin from warping while the glue is drying, place it between two sheets of wax paper and lay a book or other weight on top of it. The waxed paper will protect the table and the book from glue that might ooze out. To make a sharp edge on a fin, use a strip of masking tape as a guide move any lines you drew with ink pens, as they sometimes bleed through the paint on the rocket and ruin its appearance. You may apply sealer to the fins at this time, too. You’ll find it easier to sand the fins before they are attached to the body tube. Reinforced Balsa Fins: Balsa fins can be strengthened by providing reinforcement. The additional strength will increase their stiffness and make them more resilient to stress incurred Paper Skin Fig 6-12 during hard landings. To do this you may either apply skins or add stiffer structural members to the fins. Adding paper skins to fins is easy, and adds a lot of strength Skins are the easiest to apply; you usually bond them directly to the fins with Flexible skins that drape, such as tissue paper or fiberglass cloth, are better for curved airfoil sections. Apply tissue paper to fins or wings that need only a little reinforcing or to cover the grain of the wood, and use fiberglass cloth for applying maximum strength to any fin or wing. The methods of application are similar, but the adhesives differ. Tissue paper is commonly applied with model-aircraft dope or wood glue thinned with water. Fiberglass is almost exclusively applied with epoxy, polyester resin, or instant glues. You can also reinforce balsa fins by strategically adding stronger members to stiffen them. Members placed around the perimeter of Fig 6-13 the fin are called caps, while members placed inside the perimeter are called spars or ribs, depending on their orientation. Ribs lie parallel to Hardwood cap strips add the centerline of strength and keeps fin edges the rocket, while from being dinged spars extend nearly perpenContinued on page 5 • Soft, flexible foam padding provides superior protection from cracking • Conforms to the egg to eliminate pressure points • TARC style places the egg horizontally in the rocket • Lightweight & reusable Made by: Page 4 ISSUE 328 DECEMBER 18, 2012 www.ApogeeRockets.com Egg-Payload Protectors Continued from page 4 Designing And Constructing Fins Fig 6-16 dicular from the centerline (Figs. 6-13 to 6-15). Fig 6-14 Hardwood spar A simple hardwood spar helps prevent fins from snapping in two Fig 6-15 Hardwood rib Hardwood ribs prevents fins from splitting along the grain of the balsa wood reduction can be substantial. Built-Up Fins: A strong and lightweight fin, generally made hollow, with judicious placement of highstrength structural members inside an outer skin, is called a built-up fin. The advantages (if it is built properly) are low weight and high strength. Built-up structures are mainly used on large wings or fins, where weight Rib Spar Skin Built-up fin framework A simple built-up fin using hardwood strips and a skin of light cardboard or balsa Figure 6-16 shows the key parts of a built-up structure. The spars carry bending loads, the ribs give support and contour shape to the skins, and the skin gives the wing an outer cover. When the skin is used to give strength to the structure, the wing is said to have a semi-monocoque construction. Skins that can be used effectively this way include balsa sheet, thin plywood sheet, and fiberglass sheet. This construction is also suitable for fins with sharp leading or trailing edges. The skins on these have to be very thin and stiff; light cardboard, thin fiberglass sheet, or Continued on page 6 • Holds Aerotech’s Reload Propellant • Sizes: 24mm To 98mm Diameter • Power Range: E Through N • Cases For Any Project • Rouse-Tech Quality • Affordable! ISSUE 328 DECEMBER 18, 2012 www.ApogeeRockets.com • Reusable Rocket Motors Save Money Your Source For Everything Rocketry High-Power Reload Casings Page 5 Continued from page 5 Designing And Constructing Fins Fig 6-17 Thin cardboard skin thin plywood 0.39 mm (1/64”) or 0.79 mm (1/32”) work well. Figures 6-13 and 6-14 illustrate how they could be constructed. Foam Core Fins: To increase the strength of a Wedge-shaped fins are easily built-up wing, withmade using a built-up method out significantly increasing its mass, you can insert Fig 6-14 expanded polystyrene into the hollow areas. This will add strength and stiffen the Folded cardboard skins. The added fins without spars strength is usually remarkable, so that many structural members inside the skin can be eliminated, and this decreases the mass of the wing. Balsa spar Skin from balsa or cardstock Hardwood caps Fig 6-15 Cardboard end-caps Constructing a foam core structure is similar to building a built-up structure. Create a skeleton framework around the fin and cut a piece of foam to fit any hollow area. Shape the foam using a sanding block, so it exactly matches the Plastic foam core Foam core fins are good canidates for high power rockets perimeter defined by the skeletal framework. Bond the skin directly to the foam and all structural members. Epoxy is usually a safe adhesive to use on foam, but polyester resins and instant glues may melt it. Again, the type of skin should be appropriate for the expected loads and the shape of the fin. Flat and stiff skins such as light cardboard, thin balsa, thin plastic, or thin plywood are suitable only to fins with flat surfaces. For curved surfaces, fiberglass cloth soaked in epoxy is generally better. Several layers of fiberglass can be added to give extra strength to the fin. This technique is especially applicable to larger and higher-powered rockets. Plywood Fins: Plywood is not usually used as a fin material on small rockets, because it is difficult to cut and shape, and its density is too high for the strength it gives. Continued on page 7 The Most Versatile Alignment Jig Ever Manufactured • Get Perfectly Aligned Fins Every Time • Holds the Tube In a Horizontal Orientation to Prevent Glue Drips • Self Adjusts to ANY Size Tube From 13mm (BT-5) to 66mm (BT-80) • Securely Holds The Fin While The Glue Dries • Kid-Friendly! Helps Them Make Stronger Fins, Resulting in Straighter Flights • Can Accomodate Fins Up To 1/2” Thick • Allows Any Number of Fins on the Tube www.ApogeeRockets.com Page 6 ISSUE 328 DECEMBER 18, 2012 www.ApogeeRockets.com Guillotine Fin Alignment Jig Continued from page 6 Designing And Constructing Fins About The Author: For small fins that need accurately shaped airfoils, however, plywood does have some nice characteristics. Use only 0.39 mm (1/64”) or 0.79 mm (1/32”) thick wood. For fins thicker than this on small rockets, balsa is highly superior. Plywood is also acceptable for use as a skin on a builtup or foam core fin. When used this way, only thickness of 0.39 mm (1/64”) or 0.79 mm (1/32”) are acceptable. Thicker than this, it adds too much weight without any significant increase in strength. Many rocketeers think they need thick, solid plywood fins on larger model rockets and on higher-powered rockets. This is not true. You can make strong and stiff fins with many of the methods described above. The density and weight of plywood outweighs any benefits it might have. Just take your time making the fins and do the job right. Plywood doesn’t have the right strength-to-weight characteristics to make a really good fin. Even more important, plywood is so heavy it reduces the safety margin in your rocket designs. Tim Van Milligan (a.k.a. “Mr. Rocket”) is a real rocket scientist who likes helping out other rocketeers. Before he started writing articles and books about rocketry, he worked on the Delta II rocket that launched satellites into orbit. He has a B.S. in Aeronautical Engineering from EmbryRiddle Aeronautical University in Daytona Beach, Florida, and has worked toward a M.S. in Space Technology from the Florida Institute of Technology in Melbourne, Florida. Currently, he is the owner of Apogee Components (http:// www.ApogeeRockets.com) and the curator of the rocketry education web site: http://www.ApogeeRockets.com/education/. He is also the author of the books: “Model Rocket Design and Construction,” “69 Simple Science Fair Projects with Model Rockets: Aeronautics” and publisher of a FREE e-zine newsletter about model rockets. You can subscribe to the e-zine at the Apogee Components web site. Need A Parachute? We’ve Got The Right One For You! Rocket Jewelry • The Perfect Launch-Range Accessory • Subtle, Tasteful, Fashionable & Distinctive • Makes A Great Gift for Family and Friends • Display Your Passion for Rocketry www.ApogeeRockets.com We’re Paying Cash For Great Articles for This Newsletter Are you a writer looking for some serious pocket change? We’re paying up to $350 for good how-to articles for this newsletter. If you’re interested, see our submission guidelines on the Apogee web site. www.ApogeeRockets.com/Newsletter/Newsletter_Guidelines ISSUE 328 DECEMBER 18, 2012 Page 7