RE: Pietenpol-List: Spars, trying again
Posted: Tue Nov 04, 2008 12:11 pm
Original Posted By: AMsafetyC(at)aol.com
John et al,In addition to the different ways aluminum and wood respond to temperature changes,the behavior under stress is going to be critical. There are well established calculations for loaded members made of very dissimilarmaterials. Steel-reinforced concrete is one example. The different materialsare usually intended to carry different loads.However, in a wing spar, virtually all the significant loads are from flying athigh G-loading. In a typical spar of web and caps, the caps carry primarily tension and compressionwhile the web carries a shear load that keeps the tensioned cap from goingone way while the compressed cap goes the other. As the spar flexes under apositive load, the cap on the bottom takes on more tension and the cap on thetop takes more compression while the web joins the two under shear loading. These stresses increase predictably because the modulus of elasticity is similarbetween the spar caps and web.In an all wood spar or an all aluminum spar, the caps and web will flex similarlyand the onset of stress with increased load will be shared more predictably,resulting in stresses much more like a stress calculation would indicate. In your composite aluminum/wood wing, you may have to overbuild substantially tosafely carry the loads.When the assembled aluminum/wood wing is flying and has to carry an increased load,the aluminum and wood take on the increased stress at different rates. Becausealuminum and wood have vastly different moduli of elasticity, the aluminumweb will be stressed much more than the wooden spar caps as the wing flexesunder load. While the assembly of web and caps could theoretically carry theload, in practice the aluminum web will carry almost all of it, possibly exceedingthe tension or compression limit of the material.This is not an area where "toying around" is a very good idea. The Piet is loadedwith things to experiment on, but your wing spars are a little too importantfor experimentation.Mike Hardaway -------------- Original message ----------------------
John et al,In addition to the different ways aluminum and wood respond to temperature changes,the behavior under stress is going to be critical. There are well established calculations for loaded members made of very dissimilarmaterials. Steel-reinforced concrete is one example. The different materialsare usually intended to carry different loads.However, in a wing spar, virtually all the significant loads are from flying athigh G-loading. In a typical spar of web and caps, the caps carry primarily tension and compressionwhile the web carries a shear load that keeps the tensioned cap from goingone way while the compressed cap goes the other. As the spar flexes under apositive load, the cap on the bottom takes on more tension and the cap on thetop takes more compression while the web joins the two under shear loading. These stresses increase predictably because the modulus of elasticity is similarbetween the spar caps and web.In an all wood spar or an all aluminum spar, the caps and web will flex similarlyand the onset of stress with increased load will be shared more predictably,resulting in stresses much more like a stress calculation would indicate. In your composite aluminum/wood wing, you may have to overbuild substantially tosafely carry the loads.When the assembled aluminum/wood wing is flying and has to carry an increased load,the aluminum and wood take on the increased stress at different rates. Becausealuminum and wood have vastly different moduli of elasticity, the aluminumweb will be stressed much more than the wooden spar caps as the wing flexesunder load. While the assembly of web and caps could theoretically carry theload, in practice the aluminum web will carry almost all of it, possibly exceedingthe tension or compression limit of the material.This is not an area where "toying around" is a very good idea. The Piet is loadedwith things to experiment on, but your wing spars are a little too importantfor experimentation.Mike Hardaway -------------- Original message ----------------------