Re: Runway excursion - canard bent


David J. Gall
 

The LS-1 canard was designed to support a higher gross weight and higher flight speeds than its predecessor. Assuming the same design structural load factors and margins, it would, of necessity, have to be designed stronger -- capital letters notwithstanding.


But I disagree that the declining number of spar cap layers toward the tip is unsuitable design for the canard, whether GU or LS-1. The carbon spar in the LS-1 is also tapered, not only in diameter but also in thickness.


A little analysis: Both designs are (apparently) adequate for the design load conditions, which happen to be (for simplified analysis such as follows) two distinct loading cases: ground loading and flight loading. We must consider the bending and shear loads for each of these cases.


The root bending moment imposed by tip ground loads at 1.0g (stationary) is about 8/3 the root bending moment caused by distributed flight loads at 1.0g. (from Roark's Formulas for Stress and Strain.)


Landing gear design calls for a 2.0g load factor, so any flight load factor over 16/3g (5.33g) would require a spar stronger in root bending than the ground loads require. So a six-g spar has adequate root bending strength for tip gear design loads.


The bending moment of the distributed lift load in flight accumulates quadratically from tip to root, which would call for a spar having a quadratically-tapering spar cap thickness toward the tips. However, the tip loading of the landing gear imposes a bending moment that accumulates linearly from tip to root, so a linearly-tapering spar is the right solution for landing gear loads, and is over-built for flight loads. Thus, the linearly-tapering spar is absolutely the right solution for this application.


The other consideration is whether the wing's shear strength has been carried to the tip adequately to carry the landing gear shear loads, since a normal wing need not be concerned with such loads. The shear load is carried by any portion of the shear web or wing skin or other structural part that has a vertical extent.


BID has a shear strength of about 290 lbs per inch per ply, so we need about eight lineal inches of vertical extent, or a two-ply shear web four inches deep, or a four-ply web two inches deep, etc. The shear strength of the wing skins may also be employed to help carry these loads. Add it up: In analyzing the existing canards we can easily find ample material to carry the shear loads and also (from experience) ample evidence that the deflections are minimal and properly accounted for by the designers. The shear strength requirement is so minimal as to be almost incidental to the task of designing an adequate spar and a wing skin that can handle the torsional flight loads and hangar-rash loads.


In summary: If the canard broke while off-roading through the wilderness, it likely encountered loads more than two times greater than it was designed for (safety factor of two), or else the previous repair was inadequate. Since other un-repaired canards have met similar fates while off-roading, I presume the repair was adequate.


Just my humble opinion,


David J. Gall

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