The Secrets of Wood Designs

The Secrets of

46 JUNE 2005 SCOTT SPANGLER Wood Designs Building with wood means you must leverage its strength and accommodate its weaknesses Neal Willford

art one of this article looked at the structural char- acteristics of wood for aircraft use. This month we continue the discussion by looking at the practi- cal application of wood to aircraft design details. ThereP is a new spreadsheet available to download from the EAA Sport Aviation page on the EAA website at www.eaa.org that will be helpful for those who are interested in designing wood aircraft. Despite the growth of composites and popularity of alumi- num, wood is an excellent choice for new airplane designs. It is helpful to study the structural details of wooden airplanes that have stood the test of time to see how their designers solved various structural problems. EAA has published the plans to a couple of wood aircraft, including the Bowers’ Fly Baby (in 1963) and the Turner T-40 (in 1965). Copies of these articles can be obtained from the EAA library. EAA also sells reprints of the 1932 Flying and Glid- er Manual, which contains the plans for the Pietenpol Aircam- per. In this article, we will only focus on a few of the major structural details. SCOTT SPANGLER

EAA Sport Aviation 47 Rib Design planes with dive speeds below 150 rib design where the diagonal piec- Regardless of its material, the job of miles per hour. Airplanes with high- es were in compression under the the wing skin is to transmit the air er dive speeds required a reduced rib worst flight loads (as shown in Fig- loads to the wing ribs. The chord- spacing that can be calculated using ure 1). This is because the diagonal wise air load distribution tends to be Equation 1. pieces were still pushing into the triangular, especially at high angles Though the requirement was for joints when the glue joints started of attack, as shown in Figure 1. This fabric-covered wings, these spacing failing. Applying the chord load dis- distribution can be estimated for the guidelines can also be used as a tribution as shown in Figure 1 to different loading conditions The face grain [should] be the rib would try to bend the using the spreadsheet from cantilever portions of the rib Reference 1. oriented vertically for solid ahead of the forward spar and The ribs in turn trans- aft of the rear spar upward. mit the resulting loads to plywood ribs and horizontally The diagonal pieces in these the wing spar(s). Certifica- portions of the rib act like the tion requirements back in when lightening holes are used. struts on a low-wing airplane the 1930s required that ribs and are in compression. be designed for an additional safe- starting point for plywood-skinned The diagonal pieces between the ty factor of 1.25 for fabric-covered wings. spars are also in compression only wings and 1.2 for plywood- or met- Comprehensive testing in 1930 if the load is applied to the bot- al-skinned wings. They also defined found that the truss rib was the tom of the rib. Most of the air loads a maximum rib spacing of 15 inch- most efficient design (Reference 2). typically act on the upper skin, but es for fabric-covered wings in air- Specifically, the best was a truss when rib stitching passes around

Figure 1. Suggested rib truss arrangement based on NACA Figure 2. Box spar showing the neutral axis location. testing. Joint gussets not shown for clarity.

Table 1. Ultimate strength (lbs./ sq. in.) for aircraft grade wood.

Sitka Spruce Douglas Fir Tensile Stress (parallel to grain) 9400 11500 Tensile Stress (perpendicular to grain) 165 145 Compressive Stress (parallel to grain) 5000 7000 Compressive Stress (perpendicular to grain) 840 1300 Shear Stress (parallel to grain) 850 920 Modulus of Elasticity 1300000 1700000

48 JUNE 2005 Equations: height was reduced accordingly. The AT-10 had a fairly high wing Dive Speed(mph) 1. Max Rib Spacing (inches)=23.6- loading, which likely contributed 17.5 to the need for the apron strip. Bending Moment xc 2. Bending Stress= Several successful wooden home- Moment of Inertia built designs have used a three-piece Torsion Moment 3. Torsion Stress= rib design without such a strip, so 2 x Area x Skin Thickness depending on the wing loading, your design may not need one. Plywood ribs have also been a the complete rib, then it is correct the stress level. This apron strip was popular choice among designers. to apply the load to the lower sur- made from thicker plywood than Although they are the least efficient, face. If the rib stitching (or what- the wing skin, so the gain in spar construction simplicity is often a ever attach method is used) does not go completely around the rib, then 75 percent of the ultimate air load is applied to the upper rib and 50 percent to the lower. These two percentages add up to the additional 1.25 safety factor mentioned ear- lier, so be careful not to apply this twice. Looking at several popular homebuilt designs showed that the rib trusses were made from ¼-inch by ¼-inch or ¼-inch by ½-inch spruce, with most being the latter. These dimensions are a good starting point, but should be checked for sufficient strength for the actual loads in a new design. Plywood gussets 1/16-inch thick are typically used on the rib joints to increase the joint strength and stability. Wings that use single-piece ribs require filler strips between adjacent ribs to account for the rib cap thickness. These strips are not counted as spar height when sizing the spars. As we will see shortly, the spar’s height has a significant impact on its strength. Using ¼-inch rib caps would lower the spar’s height by ½ inch total, so it is not surprising that some designers use a three-piece rib that allows the spar height to extend to the skin. Development of the Beechcraft AT-10 indicated that this can cause skin cracking unless an additional plywood apron strip is used between the spar and skin (Reference 3). This strip was wider than the spar cap and also had tabs that extended a short distance onto each rib, help- ing reduce the abrupt change in

EAA Sport Aviation 49 bigger consideration. Two success- skin usually have a single main spar on the spar locations. Historical- ful wood homebuilts, the Fly Baby that resists the shear and bending ly, the front spar for fabric-covered and VP-1, both have plywood ribs. moment due to the air loads. This wings is located 12 to 17 percent Reference 5 recommended the face spar is often located near the maxi- chord from of the leading edge, and grain be oriented vertically for solid mum thickness point on the airfoil the rear spar 65 to 70 percent chord plywood ribs and horizontally when or about 25 to 30 percent back from aft of the leading edge. lightening holes are used. Look- the leading edge. The rear spar on A spar’s stress at different points ing at the plans for both of these these wings primarily serves as an along the span can be calculat- designs shows that their designers attach point for the ailerons and ed using Equation 2. The bending followed this recommendation. flaps. moment is for the particular wing Fabric-covered wings do not have station being evaluated. The “c” Spar Design skins capable of resisting the tor- term is the distance between the The wing spar is one of the most crit- sional loads, so this requires that the neutral axis and the outer edge of ical pieces of an airplane’s structure rear spar share in the load carrying the spar cap. The neutral axis is the and needs to be carefully designed burden. Reference 1’s spreadsheet center of gravity of the spar’s cross- to ensure that it won’t fail under will estimate the amount of shear sectional area and can be seen in normal operation. Small general avi- and bending moment each spar will Figure 2. Under positive “g” maneu- ation airplanes with a stressed wing experience, which largely depends vers, the upper part of the spar is in compression and the lower in tension. The dividing line between tension and compression is at the neutral axis. At this point there is no load along the axis of the spar. The inertia term in the equation is the moment of inertia of the spar cross section. It is approximately equal to the sum of the cap area times the square of the distance from the neutral axis to the center of gravity of the cap area. Since the distance is a squared term, it indicates we want to get the spar cap material as far away from the neutral axis as possible. The resulting stress must be less then the wood’s ultimate strength or the spar will break. Table 1 Figure 3. Ratio of ultimate compressive stress to tensile stress shows that wood is much weaker for wood spars. in compression. However, the ultimate allowable compressive stress also depends on the spar geometry as shown in Figure 3 (from Refer- ence 4). The key parameters are the ratios of web to overall thickness and compression cap to total spar height. For a solid rectangular spar, the web thickness ratio is one, and the chart indicates that the ultimate allowable compressive stress is equal to the tensile stress. The other extreme is a box beam, where the web thickness ratio is zero. In this case the Figure 4. Forty-five degree plywood face grain orientation for maximum bottom curve of Figure 3 is used. strength and stiffness (when up-bending and nose-down pitching moment For example, the upper ratio of are critical). cap thickness to spar height is 0.25

50 JUNE 2005 for the Figure 2 box spar. A Large Compression Load on is below the wing (like on The bottom curve of Fig- a high-wing design), posi- ure 3 indicates that the ulti- the Spar Due to the Strut Can tive-g maneuvers result in mate allowable compressive an additional compression stress would be 65 percent Cause additional Problems. load. The opposite occurs of the ultimate tensile stress when the wing experiences of the spar’s material. The curves in spar inboard of the strut attach a negative-g loading. between these two are for routered point will experience an addition- In either case, the additive load spars, which some designers use al compression or tension load. is equal to the horizontal compo- to help reduce the weight. This is Whether this stress is tension or nent of the load in the strut and okay as long as the minimum web compression depends on the flight is shared equally by the upper and thickness isn’t less than ½ inch. condition and the strut location rel- lower caps. The end result is that The spreadsheet for this article will ative to the wing. When the strut inboard of the strut attach point, automatically calculate the allowable compressive stress based on the ANC-18 method. Box spars are more efficient because they concentrate the cap area at the outer edge. Plywood is used to tie the two caps togeth- er and resist the shear load. The face grain orientation of the plywood shear web plays a key role in the spar’s stiffness and ultimate strength. Orienting the grain at 45 degrees allows the spar to withstand much higher shear stresses. Ply- wood’s shear strength at 45 degrees is greater when the face sheets are in compression (as shown in Figure 1928 Stearman C-3B, 5). This is particularly true for birch restored by Poly-Fiber plywood with poplar core. It is also true for mahogany/poplar plywood up to ¼ inch thick. Plywood with 45-degree face grain is more expensive than sheets with the grain parallel to the length. A spar web’s shear strength is also Even Poly-Fiber, the ruggedest, easiest-to-use much lower if it is oriented with the face grain horizontal (or verti- fabric coating system, won’t last forever! cal) and consequently may require a However, being pretty nearly foolproof and thicker web or two webs. No matter what orientation is used, the mini- consistently yielding spectacular results have mum recommended plywood spar a lot to do with why Poly-Fiber is still the web thickness is 1/8 inch. Testing in Reference 5 indicated top-selling system out there. that the ultimate shear of a plywood spar web also depends on the spar stiffener spacing. This was particu- www.polyfiber.com larly true when the ratio of the stiffener spacing to the spar web height e-mail: [email protected] exceeded 1.5, where the ultimate 800-362-3490 shear stress value started decreas- ing rapidly due to buckling failure Call for a free info pack! of the web. If the wing is strut braced, the

EAA Sport Aviation 51 the upper compression spar cap Plywood is an that measure ¾ inch by ¾ inch will get bigger and the lower ten- or 1 inch by 1 inch. Some smaller sion cap smaller. outstanding material homebuilts use even smaller ones. A large compression load on In the early days of aviation, the the spar due to the strut can cause for wing skins. fuselage longerons were diagonally additional problems, though. braced with wires. This tended to This load, combined with the tion is that bounded by the leading be a maintenance headache and spar geometry, the bending moment edge skin and main spar. This kind also proved dangerous in a crash, outboard of the strut, and the air of wing often has an angled rib back where splintering longerons could loads inboard can actually result in to the rear spar that, along with the skewer the occupants. the bending moment inboard of the root rib, has the plywood skin con- The de Havilland Co. is common- strut attach position increasing to a tinuing back to the rear spar. ly credited with developing the fuse- point where it becomes the critical The equations get more compli- lage design that replaced the brac- design condition. This is called sec- cated when the wing is a “two cell” ing wires with plywood skins. This ondary bending and is more com- design where the skin goes back to resulted in a safer, maintenance-free mon on designs where the strut the rear spar. This article’s spread- design that has been universally attachment is far out on the wing. sheet will estimate the skin stress adopted by wooden aircraft design- This month’s spreadsheet can be for this kind of design. Like the spar ers ever since. These skins are typi- used to check this special condi- web, face grain orientation affects a cally 3/32-inch to 1/8-inch thick tion. wing’s torsional stiffness and shear on homebuilt designs. It is impor- When a wing spar is under a com- stress. Figure 5 (from Reference 3) tant to check the members for col- pression load due to the strut, it can shows a wing skin’s ultimate shear umn buckling strength and design act like a column and will buckle if stress as a function of rib spacing accordingly. it is not laterally stable. The leading and face grain orientation. Figure It is important to remember that edge and fabric covering provides 4 shows the desired wing skin face new designs should be structurally some stability to the spars, but not grain orientation for 45-degree ply- tested to ensure their strength. Stress as much as a fully skinned wing. wood when a nose-down pitching calculations are important in sizing moment is the critical torsion case. and optimizing an airplane’s struc- Wing Skins ture, but there is nothing like actual- Plywood is an outstanding material Fuselages ly testing to know that it is actually for wing skins. The torsional shear Boxy fuselages are really just big strong enough. stress in a wing with a “D” cell lead- spars and can be analyzed in a simi- ing edge skin can be calculated using lar fashion. Several popular designs References: Equation 3. The area in this equa- have longerons and cross members “Estimating Air Loads,” Willford, Neal, EAA Sport Aviation, June 2003. NACA TR-345, “The Design of Airplane Wing Ribs,” Trayer, George, 1931. Available at http:// naca.larc.nasa.gov. “Wood vs. Metal Aircraft Con- struction in Aircraft,” Rawdon, Herb, SAE Transactions, December 1945. Design of Wood Aircraft Struc- tures, ANC-18, 1944. NACA TR-344, “The Design of Plywood Webs for Airplane Wing Beams,” Trayer, George, 1931. Available at http://naca.larc.nasa. gov.

more at www.eaa.org Figure 5. Maximum torsional shear stress for mahogany plywood skins. 52 JUNE 2005