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Builders guide to safe aircraft materials
Wood beams in aircraft
Rev. 4 — page content was last changed 6 January 2010
The illustration depicts a board supported at each end on its narrow face and bending under a load (i.e. acting as an elastic beam). The application of the load causes bending stresses. The fibres in the upper edge of the beam are resisting compression while those on the lower edge are resisting separation i.e. the lower part of the beam is experiencing tensile stress (the strain will tend to extension in beam length) while the upper part is experiencing compressive stress (the strain will tend to contraction in length). There is also shear stress around a neutral axis — where length doesn't change — but the axis moves lower as the load increases. (Sorry but the eight red arrows indicating direction of stresses are difficult to see in the image.) Wood is very much stronger in tension than in compression, so the top of the beam will fail first if the load is increased beyond the beam's ultimate strength.
The distribution of the tensile, compressive and shear stresses have application in engineering beam theory for the design of wing spars and other structural members, leading to the use of more complex, but lighter, beams in place of solid beams. The theory applies to metal and composite beams, and is also applied in sandwich construction where high-strength materials are used for the outer layers with a lower-density material as the core. The core material separates the load-carrying layers but still binds them together so that the load forces are properly distributed.
The mechanics: let's say the cross-section dimensions of the beam shown are 100 mm in depth by 30 mm thick, the 'clear span' is 1000 mm, the depth/span ratio is 100/1000 = 0.1 and the load is a constant value. If the thickness of the beam is doubled (to 60 mm) the deflection distance under the load will be halved, whereas if instead the depth of the beam is doubled (to 200 mm), the deflection will be reduced to just one-eighth of that shown. Increasing the depth to 200 mm will double the depth/span ratio to 0.2.
Similarly if the thickness is doubled, the beam will carry double the load. But if the depth is doubled it will carry four times the load — with the same span between the supports. (Of course there is a limit to the amount the depth can be increased before other factors become significant.)
Conversely if the supports are moved closer together so that the distance spanned is halved (the depth/span ratio is doubled) then stiffness is increased eight-fold and the beam will carry double the load.
So, the breaking strength of a beam is:
• directly proportional to the width
• proportional to the depth squared
• inversely proportional to the span.
The stiffness of a beam is:
• directly proportional to the width
• proportional to the depth cubed
• inversely proportional to the span cubed.
Thus whether the depth or width of a solid beam is doubled, the mass is the same but the strength/stiffness/weight ratios for a beam are very much better with increased depth rather than increased thickness.
(Lighter wing structures can be achieved by using external struts, which carry some of the load from the spars to the fuselage and reduce the depth/span ratio in accordance with the location of the strut connection on the spar.)
If a solid spar must normally cope with compression stresses along the upper edge, tension stresses along the lower edge and shear stresses in between, then the solid wood in the shear area need not be so massive. That wood could be reduced in thickness (by routing) to save weight. But a better solution is to retain the solid wood as upper and lower flanges, and replace the rest with a plywood shear web of sufficient thickness to supply the necessary depth and handle the shear stress. In the diagram at left the solid wood of the I-beam is now just the flanges trenched so that the plywood web can be glued in position. However, this is not representative of the usual aircraft I-beam construction; see beam types below.
Another alternative is the box beam where plywood is laminated to both sides of the flanges forming a box structure. The diagram shows part of the cross-section of a World War 2 Mosquito wing spar, with the lower flange now consisting of three pieces laminated together with the plywood webs between them — note the symmetry of laminations in the lower (tension) flange. All aircraft wing spars are solid, I-beam or box-beam variants and their flanges tend to be called spar caps.
4.20. TYPES OF BEAMS. The types of beams shown [below] have been used frequently as wing spars, control surface spars, floor beams and wing ribs. The terms "beam" and "spar" are often used interchangeably and both are used in this chapter.
The wood-plywood beams (box-, I-, double I-, and C-) are generally more efficient load-carrying members than the plain wood types (plain rectangular and routed). A discussion of the relative merits of these various beam types is presented in succeeding paragraphs.
The box beam is often preferred because of its flush faces which allow easy attachment of ribs (fig. 4.24 below). The interior of box beams must be finished, drained, and ventilated. Inspection of interiors, is usually difficult. The shear load in a box beam is carried by two plywood webs. By checking shear web allowables by the method given in section 2.73, it will be seen that for the same panel size a plywood shear panel half the thickness of another will carry less than half the shear load which can be carried by the thicker panel.
The preceding statement points to an outstanding advantage of the I-beam since its shear strength is furnished by a single shear web rather than the two webs required of a box or double I-beams. Also, the I-beam produces a more efficient connection between the web and flange material than the box beam in cases where the web becomes buckled before the ultimate load is reached. This is because the clamping action on the webs tends to reduce the possibility of the tension component of the buckled web cleaving it away from the flange.
The double I-beam is usually a box beam with external flanges added along that portion where the bending moments are greatest. This type allows a given flange area to be concentrated farther from the neutral axis than other types.
The C-beam affords one flush face for the flush type of rib attachment but it is unstable under shear loading. C-beams are generally used only as auxiliary wing spars or control surface spars.
Plain rectangular beams are generally used where the saving in weight of the wood-plywood types is not great enough to justify the accompanying increase in manufacturing trouble and cost. This is usually the case for small wing beams, control-surface beams, and beams that would require a great deal of blocking.
Routed beams are somewhat lighter than the plain rectangular type for the same strength but not so light as wood-plywood types. Usually this small weight saving does not justify the increase in fabrication effort and cost.
In determining the relative efficiency of any beam type, reduction in allowable modulus of rupture due to form factors must be considered.
4.21. LAMINATING OF BEAMS AND BEAM FLANGES. Beam flanges and plain rectangular and routed beams can be either solid or laminated. A detailed discussion of methods of laminating beams and beam flanges is in section 2.4 of ANC Bulletin 19, Wood Aircraft Inspection and Fabrication (ref. 2 - 24).
Since the tension strength of a wood member is more adversely affected by any type of defect than is any other strength property, it is recommended that all tension flanges be laminated in order to minimize the effect of small defects and to avoid the possibility of objectionable defects remaining hidden within a solid member of large cross section.
4.22. SHEAR WEBS. Although square-laid plywood has been used extensively as shear webs in the past, the present trend is to use diagonal plywood (fig. 4-12) because it is the more efficient shear carrying material (see. 4.14).
(Note: para. 4.14 states – BEHAVIOUR UNDER SHEAR LOADS. Diagonal plywood [face grain at 45° angle to the edge of the panel] is approximately five times stiffer in shear than square laid plywood and somewhat stronger. When shear strength or stiffness is the controlling design consideration, diagonal plywood should be used.)
It is desirable to lay all diagonal plywood of an odd number of plies so that the face grain is at right angles to the direction of possible shear buckles. In this way the shear web will carry appreciably higher buckling and ultimate loads because plywood is much stiffer in bending in the direction of the face grain and offers greater resistance to buckling if laid with the face grain across the buckles (fig. 4-13). This effect is greatest for 3-ply material
Figure 4-14 illustrates various methods of splicing shear webs. If the splices are not made prior to the assembly of the web to the beam, blocking must be inserted at the splice locations to provide adequate backing for the pressure required to obtain a satisfactory glue joint.
4.23. BEAM STIFFENERS. Shear webs should be reinforced by stiffeners at frequent intervals as the shear strength of the web depends partly upon stiffener spacing (fig. 4 -15). In addition to their function of stiffening the shear webs, the ability of beam stiffeners to act as flange spreaders is very important, and care must be exercised to provide a snug fit between the ends of the stiffeners and the beam flanges. External stiffeners for box beams are inefficient because of their inability to act as flange spreaders.
Stiffeners are usually placed at every rib location so that the web will be stiffened sufficiently to resist rib-assembly pressures.
4.24. BLOCKING. Any blocking, introduced for the purpose of carrying fitting loads (fig. 4-16), should be tapered as much as possible to avoid stress concentrations. It is desirable to include a few cross-banded laminations in all blocking in order to reduce the possibility of checking.
(Extract from ANC–18 ends)
The next module in this group is 'Selecting aircraft timber'
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Builders guide to aircraft materials – wood, plywood and adhesives modules
| Guide contents | Properties of wood | Properties of plywoods |
| Wood joints and adhesives | [Wood beams in aircraft] | Selecting aircraft timber |
| Basic strength and elastic properties of wood |
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