Load Tests On A Scale Spar Structure Part 2 of 2 Parts

By Frederic K. Howard Photos and drawings by the

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author

lthough it might seem that the deflection at any point on the half-size spar when subjected to some particu-

lar load would amount to one-half the deflection to be

expected on the actual spar at the same relative point

when subjected to a corresponding load, this is not the case. It can be shown that the deflection on the actual spar will vary as the square of the scale factor. (A mathematical derivation of the relation between deflections in box spars of the same design but different sizes will be published next month). In other words, in the case of one-half size spar, its deflection under load is Vs 2, or one-fourth that to be expected in the actual beam when subjected to a corresponding load. It becomes necessary

then to multiply measured deflections in the half-size test spar by a factor of four to obtain estimates of the corresponding deflections expected in the full size design. A general observation on cantilever flexure might be made here. It has often been noted that a marked increase in wing flexure occurs in the larger cantilever de-

Uniformly Distributed Load of 75 Ibs. with 38 Ibs. Additional at Free End. Total Tip Bending of 1.625 in.

This information of course relates to the design test-

S-COMBINCD OfSICft LOAD FACTOR AHD SAFETY FACTOR a imtC*T[S HtAiUtca CCFIKTIOH

ed — only in a general way can conclusions be drawn from the data applicable to other cantilever spar designs. Similar curves could be expected for any other tapered beam loaded in the same pattern but the actual deflections would in each case be unique. It will be noted from the graph that the test beam reached a deflection at the tip of .813 in. upon being loaded to 119 lbs., with the load at any point approximately proportional to the spar depth at that point. Although this corresponds to a design load factor of 7 on the full size beam, the inherent increase in flexure as the design is scaled up to full size will cause the actual beam to bend a maximum of 4 X .813 in., or 3.25 in. when loaded to the same factor. The flexure of 3.25

UNDER

TAPfgfp CANTILEVER VARIOUS LOADS

signs - an increase which appears to De out of proportion to the increased dimensions of the aircraft. This can be explained as a tendency for flexure in cantilever wings to increase geometrically with arithmetic increases in size. Larger cantilever designs are of course not simply scaled up versions of smaller designs so it would be incorrect to refer to the characteristic as other than a tendency. The first series of load tests on the one-half size structure can be described briefly as follows: The spar was inverted, pinned in horizontal position by its root fittings to a rigid vertical support and loaded with weights (building materials and books substituting for the usual sandbags) to design load factors ranging from two through eight. In each of these cases the load was distributed throughout the span to approximate, as described earlier, a distribution varying as the spar depth varies. The flexure of the beam at four different places along the span was measured in each instance. The data obtained is summarized in the accompanying graph.* 10

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The actual deflections are noted in the accompanying table.

TOTAL LOADS AND MEASURED DEFLECTIONS ON ONE-HALF SIZE SPAR Total Load (Lbs.)

34 51 68 85 102 119 136

DEFLECTIONS

At 38% At 57.5% At 76.5% At 100% of Spar of Spar of Spar of Spar Span Span Span Span 5/32 3/64 3/32 D/32 or .047 or .094 or .156 or .031 7/64 11/64 17/64 1/16 or .172 or .266 or .062 or .109 3/16 17/64 3/8 7/64 or .187 or .266 or .375 or .109 3/8 33/64 1/4 9/64 or .516 or .250 or .375 or .141 31/64 21/32 21/64 13/64 or .203 or .328 or .484 or .656 19/32 13/16 27/64 17/64 or .266 or .422 or .594 or .813 17/32 47/64 63/64 11/32 or .734 or .984 or .344 or .531 JULY 1959

expressed per in. of spar span is .0325 in. The one-half size spar, however, at the same factor will have a maximum deflection, expressed per in. of its span, of .813/50 or .01625 — exactly one-half that of the actual spar. What must be determined then is that the spar design has sufficient flexibility to undergo a bending of .0325 in., per inch of span without permanent distortion or other failure. For although the first tests on the structure showed that the design had the strength to support loads to the desired factors, these tests did not prove that the design had the flexibility to carry these loads when scaled up to actual size. If the actual spar is too rigid to "give" the amount that is required (an average of .0325 in. per in. of span) it cannot, of course, support the design load.

A bending throughout the 50 in. of span on the test

spar of .0325 in. per in. of span will produce a tip deflection of 1.625 in. The only loading of the test structure which will result in bending throughout the span

total amount of 238 lbs. However, when this load had reached 210 lbs. it was noted that the structure to which the spar was attached was itself beginning to distort. As a result, the spar deflection could not be measured with accuracy. By these tests it was confirmed that the particular design could sustain the bending required to undergo design load factors through 7 when scaled to full size without exceeding its "elastic limit".

Some of the conclusions reached on other points may

be of interest. Although maximum deflections of under

IVz in. are indicated up through load factors of four, these deflections are on a spar span of only 100 in. This design has, at the lower factors, greater flexure than was anticipated. Although a decrease of about 20 percent in bending could be obtained by substituting birch plywood for the mahogany plywood used in the test spar, the

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Total load of 136 Ibs. distributed proportional to spar depth. (Scales shown were used to measure flexure at selected points.)

of the beam exactly to scale to the bending anticipated in the full size spar would be a load distributed along the span as was done in the first load tests but reaching a total amount sufficient to produce 1.625 in. tip deflection in the scale structure. It can be shown mathematically that this total load would have to be 238 lbs. — 129 times the weight of the spar itself. There are, however, ways other than loading the test structure in excess of the design requirements to produce an average bending of .0325 in. per in. of span. One of these would be to load the test beam uniformly along its span to some convenient amount and add a concentrated load at or near the tip sufficient to produce a deflection of 1.625 in.

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second, and more realistic method,would be to load the spar to an amount corresponding to a design factor of seven and then add whatever additional load required at or near the tip to increase the maximum deflection from .813 in. to 1.625 in.

Finally, the test spar could be

loaded — in this case overloaded — following the same weight distribution as in the first tests, to an amount sufficient to produce the desired deflection. All three of these load tests were carried out on the

one-half size structure with the spar pinned inverted and

horizontal as before. It was found in the first test that a uniformly distributed load of 75 lbs. with a concentrated load of 38 lbs. additional near the free end produced a total bending measured at the tip of 1.625 in. In the second test a load of 119 lbs. distributed as in the first series of tests with additional load of 21 lbs. near the free end also caused the required average bending of .0325 in.

per in. of span. Finally, the test spar was loaded to a

SPORT AVIATION

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Total Load of 210 Ibs. producing average deflection of about -0325 in. per inch of span. (At this load, about .375 in. of indicated deflection is from distorition of spar support.

strength requirements do not warrant this substitution. It seems sensible to take advantage of the 9 percent weight savings by use of the mahogany plywood. However, because the normal flexure is larger than was first estimated, the interplane struts are being retained so that the respective upper and lower panel will not move about independently. For those who have some reservations about the strength of light-weight glued, plywood cantilever structures, these tests may be informative. Although this box beam carried loads up to 129 times its own weight — some of which were supported for extended periods — this is not out of the ordinary for wood box beams. One of the advantages of the monospar cantilever biplane — the general design to which this beam belongs — is the

relatively smaller loads that each wing spar must support. For example, the single spar in the French cantilever monoplane, the Emeraude CP-30, must support almost 20 lbs. per in. of unsupported spar span at a design load factor of around 6. The cantilever spar design discussed here, by contrast, must carry less than 9 lbs. per in. of unsupported span at the same factor. For the amateur designer-builder interested in original and perhaps unconventional designs, the scale test structure should be a particularly useful idea. The design of many aircraft components other than cantilever beams might profitably be explored at the scale size. This is of course especially true if the design is unconventional or the builder-designer somewhat inexperienced. A fact to remember is that it's a lot cheaper — and quicker in the long run — to find your design errors before production gets under way. A

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