Building Light and Strong
Design Chapter 13 opened with the familiar quote, "Simplicate and add lightness." The point was also made that the design of a model aircraft is strongly influenced by how it will be used. Most designs will fly better and be more competitive if their weight is kept to a minimum, as long as the structural integrity is not compromised. This is especially true for free flight models flown indoors or outdoors.
A heavier model will have to fly faster for the wing to generate added lift to compensate for the added weight. When these models strike an object or the ground the resulting damage is greater.
In the past our airborne radio equipment weight was expressed in pounds.
When an RC model encountered an unplanned impact and stopped suddenly the heavy radio equipment kept going until it also hit the object or the ground. So we built heavy structures to restrain the equipment.
Today with our much lighter radio equipment and lighter structural materials we can build the model lighter and less likely to self-destruct on impact.
The title of this discussion sounds contradictory. How does one take weight out of a model and at the same time make it stronger? In the real, full scale world it's being done every day by designers of cars, boats and airplanes. Finding ways to do it with our model aircraft is a very useful learning experience.
The design requirements for an indoor rubber stick model are somewhat different than those for an outdoor gas powered RC model. But the models have some requirements in common:
The weight must be distributed properly with respect to the design center of gravity (CG), the point where the entire weight of the model is assumed to be located.
Sufficient structure should be planned for in highly stressed areas.
Appropriate materials should be selected for each component of the design.
We will first look at useful record keeping, two typical designs and that all- important CG.
In chapters two and three we have seen the great diversity in model aviation. Models range from super light, microfilm covered, indoor models to large, turbojet powered, radio controlled, fire breathing, giant model aircraft. They have two things in common. Their power source lifts them up and gravity brings them down.
E.1.1 CREATE A BASELINE
If you build from kits having an established design, build a first model stock, and follow the plan exactly. As you build, record the weight of each component before and after covering, and after applying finish and details. Keep a log of flight performance for this build. See 13.2 for details.
Then build a second model form the same plan, but incorporate weight reduction changes.
Record the component weights to determine the weight savings or additions.
Again, check and record the flight performance.
If you design your own models such record keeping from build to build is equally important.
E.1.2 STRUCTURAL CONSIDERATIONS
The Moth versus the Dreamer.
These two models reflect the evolution of good design over more than a half-century. The reader is asked to download the plans for the above designs from the Internet so that we can compare their characteristics.
The Flying Aces Moth first appeared in the Flying Aces Magazine in 1937. It remains popular to this day. See: www.ualberta.ca/~khorne, and click VOL. 5 No. 1 December 2003 and click Flying Aces Moth 1937 design.
Note the following:
⋅ Long nose to balance the rubber motor that is extending far to the rear
⋅ Effective use of fuselage diagonal pieces to strengthen the landing gear attach points
⋅ Wing tips and rudder outlines are heavier than required
⋅ Probable heavy hardwood commercial wheels located well forward to protect the propeller. They also help to balance the heavy tail
The Dreamer was designed in 1995. It is intended for competition in the Flying Aces Club Embryo endurance competition. The rules require that the fuselage volume must enclose a space 1.25 x 1.50 x 3.00 inches or larger. A monoplane's wing area is not to exceed 50 square inches, and the model is to be covered with Japanese tissue or equivalent. Wheels must be three-fourths of an inch or larger in diameter.
See: www.parmodels.com and click the dreamer an embryo endurance model and click download 91k.
Note the following:
⋅ The fuselage transitions to a triangular cross section aft of the wing, eliminating some structure (part of a fourth longeron and some cross pieces), saving weight aft of the CG
⋅ Effective use of diagonal pieces to strengthen landing gear attach points and all of the forward fuselage
⋅ Light laminated wheels and wheel enclosure
⋅ Light weight windows
⋅ Gussets used at transition from rigid structure to rectangular open structure, and at landing gear attach points.
⋅ Light weight stabilizer and rudder, gussets used to add strength
⋅ Sliced wing ribs used to reduce weight
E.1.3 CENTER OF GRAVITY
The center of gravity (CG) is the point where the entire weight of the model is assumed to be located. A model will balance at that point. The recommended CG location is sometimes shown on the plan. It is usually located to the rear (aft) of the wing's leading edge about one-third of the distance to the wing's trailing edge. If the model is found to balance behind the recommended CG it is said to be tail heavy. If the actual CG is too far aft of the Design CG it effects stability during flight. This can be compensated for to some degree with controlled flight (RC), but it causes trimming problems with free flight. Most modelers add weight to the nose of the model when they find the CG too far aft. The better alternative is to build the tail lighter.
For radio controlled and electric models, one can position the batteries or RC components forward to adjust the CG. In the case of rubber-powered models, the rubber motor is distributed all along the fuselage. One can shorten the motor by moving the rear peg (attach point) forward to compensate for the tail heaviness.
Do not overlook lateral balance. If one wing is heavier than the other the usual fix is to add weight to the light wing tip. The better approach is to weigh the structural components for each wing half before assembly. Redistribute a few of the heavy items such as wing ribs to the light side to achieve balance.
E.2 COMPONENT WEIGHT REDUCTION
More detailed discussion relating to individual components follows.
The fuselage is subjected to many stresses by other components.
When a wing tip strikes an obstacle during flight the distance (the moment) from the tip to the attach point on the fuselage contributes to a high stress at the attach point. The same applies to the landing gear.
The damage caused by impact stresses such as these can be reduced by using flexible or breakaway attachment methods. The most common are the attachment of components with rubber bands. Wings may be attached with nylon bolts that will break first on impact. Tiny magnets have also been used to hold wings in place on small models. An example is Radio Shack item 64-1895. Also see wondermagnet.com for a wide range of such magnets.
If a wing or landing gear is to be glued in place, provide a solid hard point on the fuselage of balsa or thin ply sheet. Be sure to cut away covering material that would prevent a wood-to-wood glue joint.
When an abrupt change in stiffness occurs in adjacent parts of a given structure, any applied stress will give rise to a failure at the point of change.
This happens when we fill panels in a framework with balsa adjacent to open panels.
Simple triangular gussets can be added to the inside corners of the open frame next to the sheeted portion to spread the stress out to relieve the problem. Sharp inside corners create such stress risers. Note the gusset locations on the Dreamer plan at E.1.2 above.
The fuselage is subjected to stresses when handling. Examples include holding it during the cranking of a gas engine, winding a rubber motor, or hand launching for flight. Tow hook attach points on gliders also undergo stress. As a result of the above, lightening a fuselage must be done with care.
Longerons should be made from hard balsa or basswood, cross pieces should be medium density material. Diagonal sticks can be added to fuselage side frames to create triangles that afford rigidity to high stress areas. Use low-density sheet balsa for formers and any between-stringer fill, especially aft of the CG.
Consider half-shell construction to minimize structure and former material. See: members.aol.com/KMyersEFO/shaw3.pdf Use plastic foam for contoured filler material such as a tail cone or a wing-to-fuselage fillet.
Wing structure designs vary considerably regarding spar location and configuration. One common arrangement consists of a spar on both top and bottom of a rib with a vertical shear web between them creating an I-beam shape. Or, with the web glued alongside of them creating a channel shape. These shear webs are usually inserted between ribs. The ribs can be reduced in thickness on outer wing panels and still be effective. On a small rubber-powered model the web can be made of tissue paper.
Another alternative structure for small models features a continuous full depth spar. The front part of each rib is cut to fit between the wing leading edge and the spar and butt glued it to the spar. Then butt glue the rear part of the rib to the spar. For small models the spar and ribs can be reduced to one thirty second of an inch in thickness. If a single wing spar is used, place it on the top of the rib to provide wing stiffness and reduce covering sag between ribs.
Wing ribs can be made from airfoil shaped sheet balsa, sheet balsa outlined with cap strips of balsa or carbon fiber (an I-beam cross section), plastic foam or simply strips of balsa along the top and bottom surfaces.
Strip ribs can be sliced or formed. Sliced ribs are cut from sheet balsa with the grain running from leading to trailing edge. If the rib airfoil has a very thick cross section the grain will run out near the front resulting in a weak structure. Sliced ribs can be cut using a simple airfoil shaped template as a guide. Move it across a balsa sheet one sixteenth of an inch at a time slicing off a new rib with each move. Or, you can build a high-tech jig. See: this article
If you use a Simplex airfoil (logarithmic spiral airfoil) you can slice all ribs as long as the widest chord of the wing. For shorter ribs you then cut away any surplus length from the trailing edge without significant change to the airfoil shape.
To plot a Simplex airfoil see: www.smallflyingarts.com/Downloads/Downloads.htm and click Airfoil Plotter.
You can make stronger strip ribs with the grain following the rib contour by forming a balsa sheet between forming blocks. See E.4.2 for details.
Wing strut attachment
When wing struts are used between the wing and fuselage or between wings on a biplane their attachment is dependent upon several factors. Is the wing to be removable for transport? Is a firm attachment needed to ensure a rigid assembly? Does the wing have a proper hard point to attach to?
An interesting solution involves the use of fishing line leader (monofilament) inserted between wing strut ends and the wing to provide some flexibility at the attach point. A small piece of line is glued into each end of the strut and is inserted into a preglued paper tube in the wing. It may be glued into the wing tube or left unglued to pull out on impact or disassembly.
In E.1.3 above we have seen the impact that a heavy tail has on the CG. The structure of the horizontal stabilizer (which includes the elevator) and the vertical stabilizer (which includes the fin and rudder) must be designed as light as possible but strong and resistant to warping.
Weight reduction options include:
⋅ Selecting light wood that is stiff as possible
⋅ Laminated wood structural outlines
⋅ Position internal structure diagonally to create stiff triangles
⋅ Minimize applications of finish such as multiple coats of dope
Weight increases that may be needed to compensate:
⋅ Minimal use of carbon fiber tow (threads) on internal spars to stiffen and prevent bending
⋅ Small gussets in corners to reduce covering wrinkles due to covering shrinkage pulling light, thin structure out of position
Most small model designs specify a structure composed of a flat single layer. Typical horizontal stabilizer construction consists of a spar extending from tip to tip. The fore and aft strips on this single layer are interrupted by the spar. The late Earl Stahl added a second fore and aft strip on top of the first on many of his designs. The second strip was kept full thickness at the spar location, but was sanded to a featheredge at the leading and trailing edges of the stabilizer. His approach changed a flat airfoil to a lifting airfoil that may not be best for some designs. However, if the same approach were used on both top and bottom of the stabilizer, one would have a symmetrical non-lifting airfoil that would be stronger and less likely to warp. Such added pieces should be made of the lightest wood available.
The following discussion will be limited to propellers for rubber-powered models. There is a wide selection of commercial propellers available for electric or gas powered models.
Propeller weight is thought to be of minor importance since we often need to add weight to the nose anyway. But if the tail has been properly lightened and we keep the propeller weight down, we can add structural strength to the nose needed for handling and to minimize impact damage.
Commercial plastic propellers are very heavy compared to balsa or other built-up types. They can be lightened by carefully scraping the blade surface to reduce thickness to a minimum. Take care to retain proper balance during the process.
Propeller sizing and carving information is found at:
For propeller fundamental math see:
Using a laminated blank to ease carving:
A lightweight propeller using formed sheet blades:
Scroll down to Double Whammy-Pennyplane and click PDF
Source for a commercial lightweight propeller:
E.2.5 WING WIRES AND OTHER BRACING
Aviation pioneers learned early that strategically placed wires provided rigidity to structures. Wing structures could be much lighter if they were well braced with wires.
Modelers who fly very light indoor models use the same techniques to strengthen flying surfaces and fuselages. Wire materials range from .001 to .007 inch diameter tungsten to Kevlar or polyester thread. Thread can also be used in place of balsa diagonal strips in some fuselage structures for large rubber models, for example.
To reduce wing warping on large rubber models some modelers glue thin strips of tissue in a diagonal direction between the wing leading and trailing edges. Apply from the front of one rib, across the next rib to the rear of the rib after that. Make an X with a second strip on top of the first. The above strips are applied before covering. Tissue grain must run parallel to the length of the strip to be effective. Note that some tissue has no grain structure. See E.3.6 for more details.
E.3 - MATERIALS
Balsa wood has been a favorite choice of many model builders for over seventy-five years. Beginners find it easy to cut and shape and above all it is light. Metals and plastics normally have relatively common properties throughout but balsa, like other woods, can have varying density, grain type and direction and imperfections.
When selecting balsa for a given application note that it is subject to density variations within a single sheet. This can be a problem when one wants to strip sticks of uniform strength and weight. By holding the sheet over a lighted electric bulb you can see the variations. The denser areas appear darker. Also look for indications of objectionable grain variations such as the grain suddenly veering off to one side on a thin stick.
Balsa is sold in densities ranging from four to sixteen pounds per cubic foot or higher. The lightest range, from four to six pounds is rare and commands a slightly higher price when it is available. It is referred to as contest balsa. The average range for modeling use is six to 16 pounds.
Balsa is also classified by the grain type that results from the way it was cut from the log. Each type has characteristics that make it best for a particular application. The Sig Manufacturing Company is one of the major suppliers of balsa to modelers. Their catalog has useful information regarding balsa characteristics. The following Web sites contain a reprint of some of that information:
When Balsa trees are bent by heavy winds the wood can develop small cracks in the grain called checks. If one strips sticks from a sheet containing such checks, the sticks will break easily at the check location. If you break up a sample stick every six inches or so you can see if the stick breaks clean indicating the above defect.
Before stripping a balsa sheet it's a good idea to apply a different colored magic marker along the end grain of each sheet. After sticks have been stripped from the sheet you will know which sheet a stick came from. That is more useful of course if you have recorded the weight, stiffness and color of the original sheet.
E.3.1.2 GRAIN TYPES
Balsa sheets are classified by their grain characteristics that result from their relationship with the growth rings in the original log. See 11.6 for more information.
Images showing the appearance of the grain types can be seen at the Web sites listed above at E.3.1.1
Some suppliers print the density range in pounds per cubic foot directly on each sheet of balsa. Especially for contest grade sheets.
But what if you are purchasing balsa at a shop and just happen to have a pocket scale with you, and want some eight-pound balsa one-sixteenth inch thick, but need to know what that sheet should weigh? You will find charts that specify the weight for a given sheet, stick size, or block on the Web sites above at E.3.1.1 A very nice chart for sheets giving the weights in both grams and ounces will be found at: www.geistware.com/rcmodeling/balsa_weight_chart.htm A nice inexpensive pocket scale for the above purpose can be bought from Staples. See: www.staples.com
Balsa wood varies in the ability to resist bending. This is referred to as stiffness or springiness. This characteristic is important when choosing sticks to be used as fuselage longerons. If longerons vary significantly in stiffness, one may end up with a banana-shaped fuselage.
The following methods of measuring stiffness provide approximate results that are usually sufficient for modeling purposes.
⋅ Hold one or more sticks of balsa on the edge of a table with most of the stick extending out horizontally unsupported. Place a weight on the secured ends and press the unsupported ends down and note the spring back. One can feel the resistance. If two or more sticks are tested side by side and the resistance is the same, you have a good set of longerons, providing they also weigh about the same.
It is said that two sticks of wood of the same density can vary over one hundred percent in stiffness.
[Ross, Don. Rubber Powered Model Airplanes, Hummelstown, PA: Markowski International Publ., 1998, 12]
⋅ A plan for a home made Deflection Meter for sticks can be seen at:
⋅ Support a balsa sheet on two horizontal dowels placed near the ends of the sheet. The dowels are secured to a vertical surface. Place a known weight on the balsa (such as a dry cell) halfway between the dowels and measure the deflection of the balsa sheet.
One can compare the amount of the deflection to other sheets tested. However, Don Ross provides useful tables of recommended applications for various deflections at various balsa densities. [Ross, Don. Flying Models, Rubber, CO2, Electric and Micro Radio Control, Hummelstown, PA: Markowski International Publ., 1998, 32]
⋅ To see how a violin maker does it with more sophisticated equipment:
⋅ Another method used by some for measuring the weight and stiffness information on a stick involves the use of a sensitive scale with a horizontal pan for holding the part to be weighed.
Support the stick vertically on the pan and record the weight. While continuing to position the stick vertically press down on the stick until it bends. Take a "weight" reading and record. You will find that continuing to press further down at this point the weight will remain relatively constant. To match sticks, choose those with the same weight (density) and stiffness "weight" reading.
Construction plastic foam is light in weight and has many applications in model building. It can be carved, sawn, or cut with a hot wire. It sands easily and when pores are filled with a material such as Spackle it presents a smooth surface. Care must be taken with adhesives and finishes since some will dissolve the foam.
Entire model airplanes made with foam can be fragile. However, it serves well as the shaped filler that is covered with sheet wood to make lightweight wings. On small models filler blocks of foam can be used to create contours to save weight. A tail cone at the rear of a fuselage is such an example. For more details on plastic foam see:
Explanation of fanfold foam
Descriptions of various foam materials
Chart of weight for various foam types and source for thin foam sheets
and click Hot Wire Equipment and Foams
homemade foam cutter
another homemade foam cutter
Thin fiberglass cloth can be applied as reinforcement to high stressed areas such as the location where wing halves join.
It is most commonly found in the molding of curved components such as RC glider fuselages and wings. See the following:
The vacuum bagging process
CST Sales - Fiberglass supplies
Lone Star Models - Fiberglass supplies
A tough, relatively light, paper-like material called Tyvek has been found useful in model building. It's the stuff used to wrap buildings during construction as vapor barrier. A lighter weight version is found as envelopes that must withstand rough handling. They can be bought at the U. S. Post Office. It's almost impossible to tear it by hand. Tyvek has been laminated with balsa for catapult-launched glider fuselages. It can be glued to fuselage formers over landing gear wires to hold them in place and many similar applications.
E.3.5 CARBON FIBER
Carbon fiber based materials are available in many forms such as rods, tubes, cloth and strips.
See the following for applications of its use:
www.cstsales.com/How_to_Articles and click Build a Wing with Carbon Rods and Tubes or click Build a Carbon Fiber Spar
Links to various carbon fiber applications
Wide ranging discussion of glass, carbon and aramid woven materials
Wide range of supplies
Wide range of carbon fiber supplies
homefly.com and click rod and tube supplies
E.3.6 COVERING MATERIALS
In the past the lightest covering material used for indoor models was microfilm. The modeler poured the liquid onto the surface of water and retrieved the hardened film with a wire coat hanger. It is very fragile stuff. Then there was condenser paper. Originally used to separate metallic layers inside electronic capacitors. It also is still available from some sources. Again, somewhat fragile.
Modelers often use mylar covering for small models. Thin mylar is available from mail order sources such as www.modelresearchlabs.com
An inexpensive mylar used by modelers is Reynolds Plastic Wrap. For details see: this article
Japanese tissue paper was the standard for small rubber powered models. Today, under trade names such as Esaki, it is still available from some sources. Tissue paper is found in a wide range of weights, graininess and wet strength. Some tissue has a grain that runs in one direction. Most less expensive tissues have none. If a grain is present it will be noticeable when the paper is torn. The best stuff is found through specialist mail order locations such as:
Peck Polymers and
FAI Model Supply
For the everyday fun flyer Hallmark stores carry a medium weight tissue in a variety of colors.
Some modelers that fly outdoors and require a durable covering have turned to covering with thin mylar followed by a second layer of tissue. The logic is that the mylar is more puncture resistant and the tissue is more warp resistant. Also the mylar limits the bleed through of dope applied to the tissue, keeping finish weight down. For details see: www.mindspring.com/~thayer5/ffpages/tips/mylartissue.html
There is a wide variety of other covering materials. Some have adhesive backing and are applied with a hot iron, some don't. Some are available only in white, others in various colors. They span the weight spectrum. The following Web site provides more detail:
Dope, acrylics and enamels contribute significantly to aircraft weight. Especially those containing color.
Modelers have found ways to reduce the color weight penalty.
Dyes used to color cloth can be applied to covering materials. Inks available from art stores work as well.
Colored chalk can be rubbed on tissue and fixed with a spray of Krylon clear from a spray can bought from a hardware store. See: www.easybuiltmodels.com/chalk.htm
These all keep the weight down.
When just the right color can't be found some modelers buy model railroad paint, let the solids settle, pour off the carrier liquid and add the color to clear dope. For a source of paint intended for model railroaders see: ehobbies.com - Floquil
When two or more sheets of wood are glued to one another the result is a much stiffer sheet than a single sheet of the same overall thickness. This is especially true if their grain runs ninety degrees to each other. One needs only to try to bend a small piece of common plywood to see this.
Modelers laminate strips of balsa or basswood on wing tips and tail outlines. The presoaked strips are coated with glue and two or more are wrapped around a form that matches the outline desired, carefully pinning or taping as one bends. They are left to dry and are trimmed to fit the rest of the structure. For a description of the process see:
These thin outlines replace previous designs that use much wider sheet balsa pieces.
When balsa sheets are soaked or steamed for a sufficient time they can be bound to a form and molded to the form's contour(s). This method can be used to create cambered sheet wings, or rolled tube fuselages.
For the above wings the wood's grain runs parallel to the wing leading edge. If the wood is positioned so the grain runs fore and aft the form will create a sheet with the airfoil's upper contour.
If a form block is made with the desired contour of an airfoil, one can soak the balsa sheet and bind it to the form with an ace bandage until dry. A second option is to soak two sheets, apply glue between them and bind them to the form until dry. (don't forget to protect the form with waxed paper). In either case you can use a balsa stripper to strip wing ribs from the molded sheet(s).
If the form block is created using a band saw you have a concave and a convex mold. You can place the balsa sheet on the concave mold block and use a steam iron to form the balsa to the form required. See: smarttoolsinc.com/Model_Kits/TigerFly/TigerFlyConstruction.html. With any of the above options you end up with light and strong strip ribs.
The molding process can also be used to mold tissue paper. The paper mache process has been around for years. A molding form is carved and layers of tissue are glued to one another over the form. See: www.mindspring.com/~thayer5/ffpages/ffgallery/buxton/ma2.html
Use more tissue layers or heavier paper for larger models.
Some scale model competition event rules award points for adherence to scale detail as well as flight performance. Lightweight materials such as foam, paper and even dried grasses have many applications.
Rolled tissue tubes can be made by rolling the tissue around a wire or dowel, applying glue or dope as you build up the desired laminations. Small tubes made this way serve as sockets for wing mounting sticks on light indoor models.
Other applications for this technique include the use of larger tissue tubes made this way. They can be painted black and wrapped with silver thread to simulate engine cylinders. A similar approach can be seen here.
Some modelers collect grass and weed stems and dry them. They can be used to simulate gun barrels or slipped over wire landing gears to increase its diameter making them closer to scale. They add little weight.
Look for lightweight materials that can be used to simulate aircraft components such as the Duct Foil windshield frame featured here.
Lightweight finishes can be used to color and simulate weathering. Dry brushing can be used to simulate depth or a rounded surface giving a paper image of a power plant a three dimensional appearance. Penciled lines can be applied to simulate panel outlines.