An educational unit is presented for building and evaluating simple wing structures, in order to learn about materials choice and lightweight construction. This unit is appropriate for a high school materials science class or lower-division college courses in structural engineering, materials science, or aeronautical engineering

Bunnell, L. R.

Piippo, S. W.

UNT Digital Library

j°PNL-SA-- 21578DE93 007882..+_,...%" ,.,I,,,,,.oA DEMONSTRATIONOF SIMPLEAIRFOILS- F_O 18 19,93STRUCTURALDESIGNAND MATERIALSCHOICES,_.._.,+,_ _++ ;_ ++_+"__e- ---.-=_o L R. Bunnellp_ o +,._ _'_m -- _'_" > _al_ ,. I= I= G _ _ 0"i=s. 0 _ _," ..., tj _: _=.. o_3 R _ E =Rra--" = _ o tj..• ___--- o_.= o. January 199= _ _ .,- m 1_: o_@m,..., u.._ E _ _ ,-m u _ 0t_ _ ,', _ '.' _ _ - i_.',+_.,_ ,_,;Lc lhc-__--++,,+==-o.= i_atio_alEducator'sWorkshop: Update '92tj -) _I_-I{:• -_= + + ° _ _ _ e November11-13, 1992_'_ I_ +--" = m m_. 0z._ +._.o_ o+ Oak Ridge,Tennessee_-S _,_B2 _o__.IO_ ," U +."_ _ _ 0 _- _•=-_ _,:- _ ___ _ _.=__l;;r_ tj _ _.tj E S _Work supportedbythe U.S. Department of Energyunder ContractDE-ACO6-76RLO1830PacificNorthwestLaboratoryRichland,Washington 99352+2',";++.,.,,,++,U +iIi.||. D|_"T_g_B_-t_ONOF THIS DOCUMENT IS UNLIMI-FED"_ ,, ,,A DEMONSTRATION OF SIMPLE AIRFOILS: STRUCTURAL DESIGNAND MATERIALS CHOICESL. Roy BunnellPacific Northwest Laboratory, (a) Richland, WashingtonSteven W. Piippo, TeacherRichland School District, Richland, WashingtonKEY WORDS: airfoils, composites&strength/weight ratio, structural testingPREREQUISITE KNOWLEDGE: This unit is appropriate for a high schoolmaterials science class or lower-divisioncollege courses in structural engineer-ing, materials science, or aeronautical engineering. Prepare for the,project byexplaining the following: the way that an airfoil shape generates lift (Bernoullieffect), the importance of airfoil shape regarding lift/drag ratio, and the struc-tural requirements of a wing.OBJECTIVE: Students will build and evaluate simple wing structures, andin so doing will learn about materials choices and lightweight constructionmethods.EQUIPMENT AND SUPPLIES: Listed below are the materials required toconduct the project. The suggested quantities (see Table 1) should be suffi-cient material to construct three each of the six airfoil variations listed in Table 2,TABLE 1. Materials for Building 18 Wings (including 50% excess)Quantity,Each Description (a)36 Balsa sheet 1/16"x 3"x 36"18 Balsa leading edge 3/8" x 5/16" x 36"16 Balsa trailing edge 1/8"x 3/4" x 36"12 Balsa 1/8" x 1/4" x 36"9 Spruce 1/8"x 1.4"x 36"(a) Wood is typically measured in English units.!(a) Pacific Northwest Laboratory is operated by Battelle Memorial Institute forthe U.S. Department of Ener_ly under Contract DE-AC06-76RLO 1830.TABLE 2. Wing Structural VariationsVariation Description Wt, g(a)BW Ribs 6 cm apart, balsa spars 38.9BN Ribs 3 cm apart, balsa spars 55.4BWS Ribs 6 cm apart, balsa spars, shear webs 53.0SW Ribs 6 cm apart, spruce spars 52.1SN Ribs 3 cm apart, spruce spars 56.9SWS Ribs 6 cm apart, spruce spars, shear webs 55.4(a) Weights are those of the set built for this presentation, includedhere for reference. These were the weights used for thestiffness/weight and strength/weight ratios calculated below.including 50% allowance for wastage and mistakes. Ali required supplies canbe purchased at any good hobby shop for under $200 (1991 prices).• a piece of fibrous ceiling tile about 60 x 120 cm, wi," a flat finish.Use back side if no flat tiles are available. (Before applying adhesive,the parts of the airfoil are pinned to the ceiling tile to ensure properalignment; reference lines are used to control component orientationand spacing.)• cyanoacrylate adhesive for model building; common brands are Zapor Hot Stuff• bicarbonate of soda• T-head pins, medium size• transparent polyester covering material, Monokote (or equivalent)brand• special iron designed for use in model construction• special high-temperature heat gun designed for use in modelconstruction• steel templates, approximately 3 mrn thick (cut to Clark Y airfoilshape, with notches for spars).X-Acto or equivalent hobby knives, with straight pointed blades,4'PROCEDURE: The various parts of a wing are illustrated and labeled fororientation in Figure 1. Figure 2 illustrates the actual airfoil size, which may bescaled larger or smaller using a photocopier. [Note that the two notches areplaced to fit spars sized 0.64 cm by 0.32 cm (1/4 in. by 1/8 in.).] The six con-struction versions, ali using the Clark Y airfoil, are listed in Table 2. The Clark Ywas chosen for construction ease, since it is nearly flat-bottomed. These sixversions are identical in cross section, but differ in the amount and placement 0Shear Web - Vertical Grain BalsaSection B-B' (without covering)nc,.r.YSection'A-A'.......................... Trailing EdgeLeading Edge RibRib Spacing _ A'"l ,r..........Shear WebSparTop View of Wing Structure39208111.1FIGURE 1. Construction Details of a Model Airplane Wingt..--'''''---I_L ___.._O OI'339210034.1FIGURE 2. Clark Y Airfoil, Actual Size Used in This Projectof internal reinforcements and in the use of either balsa or spruce as spar mate-rial. Assign each team of two or three st_udents the task of constructing one ofthe six airfoils.Construction: 1o make the ribs, the students will need to cut rectanglesslightly larger than the templates from 1.6_.mm(1/16-in.) sheets of balsa. Sincethe balsa rectangles will be clamped between the templates, oversized holesshould be drilled in the balsa rectangles to match those in the templates. Next,carve and sand the ribs to the shape of the templates (avoid sanding the tem-plates themselves). To make the notches for the spars, glue a 10-cm strip of150-grit sandpaper to the 1/4-in. face of some scrap spruce spar wood. Sandthe spar notches in the ribs, using care to avoid enlarging the notches in thetemplates.Cover the building board (ceiling tile) with wax paper to prevent bondingthe wing to the board. Pin the bottom spar in place on the building board.Pour bicarbonate of soda into a long narrow tray and dip each rib into the sodabefore placing it onto the bottom spar. The tiny amount of soda that clings tothe rib accelerates the reaction rate of the cyanoacrylate and strengthens thebond. Hold each rib perpendicular to the building board, taking care that eachclosely follows its reference lines. Apply the cyanoacrylate; the bond will becomplete in 2 to 3 seconds. (Use the cyanoacrylate in a well-ventilated areaand avoid breathing the fumes.) Set the top spar securely into the notches,and apply cyanoacrylate to the joints. Pin leading and trailing edges in placeusing a 3-mm (1/8_-in)shim under the leading edge, since the airfoil is not quiteflat; then bond with cyanoacrylate.If shear webs are to be attached t,_ the spars, cut the webs so the grain isperpendicular to the building board, th_,n bond to the front and back surfacesof the top spar with cyanoacrylate. The wing may now be removed from thebuilding board and the shear webs bonded to the bottom spars. Glue adoubler of scrap spar material to the outer ribs at either end; this will preventwarpage as the covering material shrinks. Carefully sand the structure as nec-essary, and use a vacuum cleaner with brush to remove balsa dust. At thispoint, prepare the covering material by cutting it approximately 3 cm larger on!i ali sides than the wing. Follow manufacturer's instructions for applying the cov-i; ering, first tacking it in place using the iron, then using the heat gun to shrink• the film.Testing" Record the weight of each airfoil before testing for strength andstiffness. Then, using a clamp that fits the shape of the airfoil's top profile,fasten the airfoil to the edge of the workbench, allowing approx. 35 cm of thej airfoil to hang free, like a cantilever. Since deflection under load is a cube func-tion of length, the free length should be carefully controlled. Attach a plastic orcloth bag or small plastic pail to the top spar using strong string and duct tape.See Figure 3 for details. Using a meter'_tick fixed to a base, measure the dis-tance to the nearest millimeter from the lower wing surface to the floor.To measure the relative stiffness of the airfoil, apply weights (used wheelweights from a tire store will do) until a deflection of 1 cm is attained; weigh thecontainer and weights, and record this weight. Continue to load the airfoil untilfailure occurs; record the failure weight and the approximate deflection at fail-ure. Be careful to avoid destroying the wing when failure occurs by suspendingthe weight container within 5 cm of the floor. When the airfoil fails, the distancethe weight must drop to the floor is limited. After failure, examine the structureto locate the failure site, taking this information into consideration along withobservations made during testing.Discussion: In discussing cause for failure, students should speculate onwhat could have been done differently during construction. For instance,because ali of our test structures failed in compression at the interface betweenthe wing and the clamping fixture, a buttress at this point would be appropriate.If time permits, have students build and test a second airfoil (same cross sec-tion as the first) using their proposed remedies for increasing strength and stiff-ness and for minimizing weight. Alternatively, students may wish to simplyduplicate their first airfoil, being more careful with wood selection and construc-tion. Allow interested students to construct original designs, as long as theirairfoils remain within the initial parameters of dimension, shape and material.An example of an original construction would be an airfoil cut from polystyreneinsulation foam, using electrically heated wire and templates on each end. Thisairfoil could be covered with balsa sheeting bonded with epoxy. In spite of theweight penalty for such an airfoil, the improvements in strength and stiffness areimpressive.Plot the class's test results, using a simple bar graph like Figures 4 and 5.On the x-axis plot structure type; on ,_hey-axis, plot either the weight to cause1-cm deflection divided by the wing weight or failure weight divided by the wingweight. Observe and discuss scatter and trends. (Instructor should saveresults to use for comparison with subsequent classes.)!i,IBased on tests conducted to prepare this presentation, Figure 4 showsthe weight required to cause 1-cm deflection divided by the airfoil weight, whichis proportional to the stiffness/weight of the various wing variations. On thestrength of this single data set, the stiffness of wings built with either spar mate-rial appears to be substantially improved when shear webs are used. This islogical, because the combination of shear web and spar forms a box beam.The lightest structure, BW, shows stiffness/weight superior to airfoils built withspruce spars, except where shear webs were used. Finally, the airfoil withspruce spars had the highest stiffness/weight ratio, but only when shear webswere provided. Figure 5 shows the weight required to cause structural failuredivided by the wing weight, for each of t_e six versions. The structures madewith balsa spars and shear webs performed impressively for its weight, betterthan either spruce-spar wings not using shear webs. Although the shear-webbed wing made with spruce spars had the highest strength/weight, the dif-ference was less than 10% more than the shear-webbed wing made with balsa39210034.3FIGURE 4. Stiffness/Weight Ratio of Six Structural Version of Same AirfoiliFIGURE 5. Failure Weight/Structure Weight Ratio for Six StructuralVersions of Same Airfoilspars. Of course, in an actual airplane wing design, the strength and stiffnesswould be critical factors in themselves and would only be divided by weight forpurposes of comparison, as in this project.INSTRUCTOR'S NOTES: The model airplane hobby offers students anexcellent introduction to the basic techniques of producing the light, strongstructures that make flight possible. Based on the principals of aircraftmodeling, this project requires students to build an airfoil of constant crosssection and to compare several variations of structural design and materials.Simple tests of the resulting airfoils enable comparisons of performance as afunction of structural weight. Students should discuss whether the moreelaborate construction methods are justified on the basis of added weight.Using modern materials and methods, airfoil construction and testing can beaccomplished within the time and economic constraints of the classroom with-out requiring advanced craftsmanship of the stl_dents.Note that the materials and construction techniques used to constructmodel airplanes may be unfamiliar to the average student. However, one ortwo students in many classes may have had experience in this particular hobby._rThese students will be a valuable resource to the class, especially during thefirst round of airfoil construction; make use of such students.SAFETY: Avoid breathing the fumes of reacting cyanoacrylate. In addi-tion, take care not to bond fingers together with the cyanoacrylate, whichadheres quickly and tenaciously to skin. If this should happen, use thedebonding chemical available at hobby shops, or wait ten minutes beforeslowly rolling the bonded surfaces apart. Do not pull fingers directly apart oruse sharp blades to cut skin surfaces apart. Take extra care to avoid gettingglue into eyes.t

A demonstration of simple airfoils: Structural design and materials choices

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A demonstration of simple airfoils: Structural design and materials choices

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