A CLOSER LOOKAT SOME ALTERNATIVES

TO SITKA SPRUCE INAIRCRAFT CONSTRUCTION

By Eugene L. Parker(EAA 49871)

624 Valley View DriveMedford, Oregon 97501

OlTKA SPRUCE (Picea sitchensis) has become so popu-lar and common in wooden components of airplane con-struction that builders tend to believe it is the only accept-able material. Certainly, species native to North America,and more suitable, are either of limited supply or non-exis-tent. However, there are a number of acceptable speciesavailable, and at considerably lower cost. Sitka spruce israted low in durability, as are most of these other trees,and all should be treated with preservatives. PerhapsSitka spruce is so popular because it is apparently the onlywood that is singled out for "aircraft" inspecting and grad-ing. Agreed; it is strong for its weight, soft and workable,and for the forseeable future, in good supply.

Its natural habitat extends from Kodiak Island andCook Inlet (southeastern Alaska) south along the Pacificcoast to northern California. Logging has depleted air-craft-grade spruce in Washington, Oregon and California,even though in past years stands in Oregon and Washing-ton contained some of the most splendid trees of record.Now only occasional trees bear witness of the past. Re-forestation and management is programmed to supplyingthe paper-pulp industry or 50 to 75 year-old sawtimberwhich would contain too many knots for aircraft use.Hence, our reliable supply for aircraft use must come fromBritish Columbia and Alaska. There are seven or eightspruce species native to the United States and Canada.Wood anatomy studies of conifers (cone-bearing trees)show little if any differences in quality and strengthamong species of a given genus, such as spruces and firs.Where problems arise is in availability of straight, clearand flawless timber in large enough pieces for our use.The common spruce of the Cascade and Rocky Mountains,Engelmann spruce (Picea engelmannii), serves well whenone can find logs of proper quality. Some research peoplemaintain that under a microscope it is impossible to sepa-rate by species, wood specimens from our native spruces,as well as among the firs. Other American spruces are:white spruce (P. glauca), black spruce (P. mariana), bluespruce (P. pungens), red spruce (P. rubens), and Brewerweeping spruce (P. breweriana).

A REVIEW OF ALTERNATIVESDOUGLAS FIR (Pseudotsuga menziezii), (formerly P.

taxifolia) is actually not a fir, that is, not a "true" fir. Firsare of the genus Abies, while Douglas fir is of the genusPseudotsuga or "false-hemlock." The wood fibers differfrom the "true" firs by containing resin cells (or ducts)giving it a certain durability factor. This wood is strongerbut heavier than either the spruces or firs, but can varyin hardness, density, and especially in weight, to the ex-tent that aircraft material must be carefully selected.

(Photo by E. L. Parker)RGURE I: NOBLE FIR (Abies procera) the largest of Americanfirs, has a very high form factor yielding excellent sawtimber;Cascade Mountains of southern Oregon near Mt. McGlaughlin.

The best Douglas fir comes from northerly Pacific Slopestands, even though it is native to all States of the conti-nental divide and westward, as well as northern Mexicoand southwestern Canada. Fine-grained old-growth wood,with thin summer-wood (the rings or "grain") and thickerspring-wood (the lighter-colored area between the rings)

SPORT AVIATION 57

(Photo by O. V. Matthews)FIGURE II: PACIFIC SILVER FIR (Abies amabilis) displays itsexcellent form in this northwest Oregon forest in ClatsopCounty.58 SEPTEMBER 1984

is softer, best to work, and usually lighter in weight. Sinceold-growth logs are usually peeled into plywood veneer,good material is increasingly difficult to find. Still, this isundoubtedly the most common "substitute" for Sitkaspruce. A big percentage of old-growth Douglas fir is har-vested from U. S. Forest Service and Bureau of LandManagement reserves. These supplies are expected to lastanother 50 to 75 years; well into the 21st century. Onedrawback in Douglas fir is the occasional appearance ofhidden pitch pockets which may show up only duringmanufacture.

PORT ORFORD (white) cedar (Chamaecyparis law-soniana), is also known by horticulturists as "Lawson'scypress." Actually, there are no "true" cedars native tothis hemisphere (of the genus Cedrus). Many Americanjunipers, cypresses or arborvitaes are called "cedars" be-cause of certain botanical similarities. Port Orford cedaris one of three native species of Chamaecyparis (or "groundcypress") in North America, and if it were not such a raretree, would be my choice for airplane construction. Itsvery limited habitat is extreme southwestern Oregon andadjacent northwestern California, confined mainly to anarea from protected coastal lowlands inland 30 or 40 milesin canyons and ravines where moisture is abundant. Oncemore plentiful, it was popular for wooden boats, battery-plate separators, venitian blinds and arrow shafts. Thiswood is of a light straw-color, straight and fine-grained,strong for its weight (exceeding Sitka spruce) and ex-tremely durable. Its former popularity for venitian blindsbears out another feature; its resistance to warping. Somebuilders have been led to believe the wood is too brittleand that its natural preservative "oils" resist proper glu-ing. Such is absolutely, not the case. Anyone who canobtain this wood has the very best. In fact, one wouldreally not need to varnish the wood except to perhapsenhance keeping it clean and dry. Very old pieces of PortOrford cedar, when sawed or sanded, quickly emit thepleasant and uniquely aromatic odor of the species. Inrecent years, Japan has purchased nearly all harvestedPort Orford cedar, and it brings the highest price of allwestern confiers, including redwood (Sequoia). Its limitedavailability is further threatened by a root-rotting fungusnow attacking some of the remaining stands.

ALASKA YELLOW CEDAR (Chamaecyparis noot-katensis), a close relative, is more abundant and foundmainly in the Pacific Coast region from southeasternAlaska southeast to western British Columbia, westernWashington and Cascade Mountains in Oregon (higherelevations). This "cedar" is also strong and decay-resis-tant, but a little darker in color and heavier than PortOrford cedar, and a favorite of woodcarvers.

ATLANTIC WHITE CEDAR (Chamaecyparisthyoides) is the third native "ground cypress," but a smalltree, confined chiefly to the coastal plains from southernMaine to northern Florida, west to Mississippi. Regretta-bly, I am not familiar with the wood, but suspect thatbecause of its rarity and small size, would not be a suitablesource for our use.

WESTERN HEMLOCK (Tsuga heterophylla) isanother Pacific Slope native, from southern Alaska (KenaiPeninsula) south to northwestern California. This wood isvery strong, workable, and has been popular for flooring,furniture, ladder stock, dimensional lumber, paneling andoccasional aircraft construction; as durable as spruce, butnormally has a dense grain of thicker "rings" and thuslyheavier.

Another less abundant western hemlock of good qual-ity is mountain hemlock (Tsuga mertensiana) of hightimberline regions from the Sierra Mountains north tosouthern Alaska, and east to southeastern British Colum-bia, western Montana, northern Idaho, and northeasternOregon.

Our other two hemlocks are eastern hemlock (Tsugacanadesis), of the eastern United States and adjacentsouthern Canada, south to the mountains of Georgia andnorthern Alabama; and Carolina hemlock (Tsugacaroliniana) from the Appalachian Mountains of Virginiasouth to northern Georgia.

THE PINES (Pinus)Of the many American pines, few offer qualities for

our use. Most of adequate strength are either overly resin-ous (pitchy), heavy, or both. Of our three commercial whitepines; western white pine (Pinus monticola), sugar pine(P. lambertiana), and eastern white pine (P. strobus), onlythe latter has a strength rating possibly adequate to air-craft use.

RED PINE (Pinus resinosa) of the north easternUnited States and adjacent Canada rates with Douglas firin both weight and strength. The builder should be awarethat among the more resinous pines, though naturaldecay-resistance properties may be present, much lumbertends to warp and shrink excessively. The softer species,such as sugar pine, western white pine, and ponderosapine are excellent for non-structural uses, such as triangu-lar corner blocks, etc.

THE FIRS (Abies)Nine species are native to the United States and

Canada. These are: balsam fir (Abies balsamea), Eraserfir (A, frasier), subalpine fir (A. lasiocarpa), Pacific silverfir (A. amabilis), grand fir (A. grandis), white fir (A. con-color), noble fir (A. procera), California red fir (A. mang-nifica), bristlecone fir (A. bracteata), a rather small, inac-cessible tree, not harvested (protected) in the CaliforniaSanta Lucia Mountains.

The firs of suitable size and quantity for our use arenoble fir, red fir, white fir, and to a lesser extent, grandfir and Pacific silver fir.

Fir wood is strong, lightweight, light in color, straight-grained, often fine-grained, and is easily worked. Theabsence of resin in the wood, as in spruce, lowers itsdurability rating, but certainly this can be corrected withpreservatives, proper varnishing and proper storage of theairplane. Fir is relatively abundant and most often usedfor dimensional lumber, boxes, paneling, plywood core-stock, and was once popular for butter containers for itsnon-resinous factor. Though there are a number of colorsassociated with the names of some firs (red, white andsilver) these are for either bark or foilage. Fir woods arevisually indistinguishable except for grain density, basicweight and timber quality.

NOBLE FIR (Abies procera) is our premium "true fir".In fact, legend has it that noble fir was successfully substi-

(Photo by O. V. Matthews)FIGURE III: WHITE FIR (Abies concolor) the most widely distri-buted important lumber fir in North America is harvested exten-sively; filmed near Klamath Falls, Oregon in the Cascade Moun-tains.

WoodSpecies

Port WordCedarSitkaSpruceWesternHemlockNobleFirDouglasFirMahoganyYellowBirch

GRADING CODE: 1 - RELATIVELY HIGH 2 • INTERMEDIATE 3 - RELATIVELY LOW

Behavior And Working TraitsLightnessIn WeightWhen Dry

2

2

2

1

22

3

ResistanceTo

Shrinking

1

2

2

2

21

3

ResistanceTo

Warping

1

1

2

2

21

2

WorkingEase

1

2

2

2

31

3

ResistanceTo

Decay

1

3

3

3

21

3

Strength And Load FeaturesBendingStrength

2

2

2

2

12

1

Rigidity

1

1

1

1

12

1

ColumnarStrength

2

2

2

2

12

2

Hardness

2

3

2

3

22

1

Toughness

2

2

2

2

22

1

NailHolding

2

2

2

22

1TABLE I Adapted from AN 01-1A-7 A.P 26754 Repair Handbook 1944 SPORT AVIATION 59

TABLE II

WoodSpecies

SpruceSpp.WesternHemlockPortOrtordCedarDouglasFirCaliforniaRed FirNobleFirPacificSilver FirWhiteFirAlaskaYellow CedarWesternLarchRedPineEasternWhite Pine

Weight PerCubic Fool

At12% Moisture

Content

31

29

29

34

27

26

27

26

31

36

34

25

Static BendingStress AndProportional

LimitLimit Lb. PerSquare-Inch

6,700

6,800

7,700

8,100

7,200

6,600

6.200

6,500

7,100

7,900

8,400

6,000

Modulus OfRupture

10,100

10,100

11,300

11,700

11,200

10,100

9,400

9,300

11,100

11,900

12,500

8,800

Modulus OfElasticity

1,OOOLb.PerSquare Inch

1,510

1,490

1,730

1,920

1,590

1,580

1,530

1,380

1,420

1,710

1,800

1,280

Work ToMaximum Load

In.-Lb.PerSquare Inch

8.8

7.5

9.1

8.6

9.5

8.8

9.3

6.7

10.4

8.0

10.0

6.7

CompressionParallel To Grain

Stress AtProportional

LimitLb.Per

• Square Inch

4,500

5,340

5,890

6,450

__

4,960

4,660

3,590

5,210

5,950

5,330

3,680

MaximumCrushingStrengthLb.Per

Square Inch

5,650

6,210

6,470

7,420

5,290

5,550

5,550

5,350

6,310

7,490

7,340

4,840

CompressionPerpendicular

To Grain •Stress At

ProportionalLimit

Lb. PerSquare Inch

650

680

760

910

850

640

490

600

580

1,080

830

550

ShearParallel To

Grain -MaximumShearingStrengthLb.Per

Square Inch

1,120

1,170

1,080

1,140

1,050

980

1,050

930

1,130

1,360

1,160

860

TensionPerpendicular

To Grain -MaximumTensileStrengthLb.Per

Square Inch

360

310

400

300

350

220

___

260

360

310

490

300TABLE II Adapted from AN 01-1A-7A.P. 26754 Repair Handbook 1944

Based on extensive tests at the U.S. Forest Products Laboratory

(Photo by E. L. Parker)FIGURE IV: BREWER SPRUCE (Picea breweriana) of limitedoccurrence at high elevations in a small area of southwesternOregon and adjacent northwestern California, yields wood ofhigh quality; filmed near Oregon Caves National Monument.60 SEPTEMBER 1984

tuted for Sitka spruce in British Mosquito Bombers duringWorld War II. The wood is plentiful, of good quality, FPLrated and comparatively inexpensive. Its habitat is athigher elevations from the Cascade Mountains of centralWashington through western Oregon to the Siskiyou andKlamath Mountains of southern Oregon, and to the Trin-ity Mountains in northwestern California (HumboldtCounty). In these southern limits it is more commonlycalled "Shasta fir" or "Shasta red fir" (but not to be con-fused with the shastensis variety of California red fir ofthe Sierra Nevada Mountains and the Mount Shasta reg-ion).

CALIFORNIA RED FIR (Abies magnifica) thoughprobably not as plentiful as its close relative, noble fir, isof good quality and is also FPL rated. This fir occurs fromKern County (high elevations of the Sierra Nevada Moun-tains) northward, and in the highest coastal mountains ofnorthern California, in the Mount Lassen and MountShasta region (Cascade Mountains), and thinly into theCascade and eastern-most Siskiyou Mountains of southernOregon, where some suspect intergrading with noble fir.The recognized variety Shasta red fir (A. mangnifica var.shastensis) is not separated except by its cones; is indistin-guishable in all other respects, and generally not noticedby foresters or lumbermen.

WHITE FIR (Abies concolor) is the most widely distri-buted commercially important (lumber) fir in the UnitedStates. Often cultivated as an ornamental, it occurs in thecentral and southern Rocky Mountains (typical-varietyconcolor) to the Pacific slope in Oregon and California(var. lowiana) and into northern Mexico, and as withnoble and red firs, is of large size and plentiful. The SierraNevada and Cascade Mountains (to central Oregon) pro-duce most white fir lumber.

At this point I should emphasize that commercial ordimensional lumber from all the true firs, at least inOregon and California, is usually marketed as "white fir";chiefly because of the very light color of the wood, as wellas the established marketing practice. Therefore, if one

(Photo by W. R. Parker)FIGURE V: ENGELMANN SPRUCE (Ptcea engelmannll) the com-mon spruce of the Rocky and Cascade Mountains can yieldexcellent spruce lumber. This mature tree was filmed In theCascade Mountains in Southern Oregon. Large tree in back-ground is a Douglas fir.

wishes to select a given species such as noble fir, he mustusually follow that species from its timber delivery source,through the sawmill and planing mill, grading and finaldelivery. Millworkers are only interested in separatingtrue fir lumber from other conifers (Douglas fir, pines,cedars, etc.). In southern Oregon, where noble fir is mostabundant and usually called "Shasta fir", sawmills alsoprocess white fir and grand fir, and all three are marketedas "white fir".

GRAND FIR (Abies grandis) is also called "lowlandwhite fir", and found generally at lower elevations. Itgrows from the coastal regions of northern California, andOregon and Washington northward into southern BritishColumbia, inland to western Montana, northern Idaho,northeastern Washington and northeastern Oregon.Where this fir meets with pacific-slope white fir (var.lowiana) many intergrades (hybrids) are found. No effortis made to separate the two, and all lumber is simply"white fir". Rather a fast-growing tree at lower elevations,the better trees for our use would probably come from thehigher elevations of the Inland Empire region.

PACIFIC SILVER FIR (Abies amabilis), named for itssmooth, light-colored bark and silvery foliage, is not abun-dant; usually growing in mixed stands and probably sel-dom marketed by its name. A handsome tree of good sizeand quality, its habitat extends from extreme southeast-ern Alaska and coastal British Columbia south to themiddle Cascade Mountains of Oregon, with tiny outpostoccurrences on south in the Siskiyou and Marble Moun-tains of extreme northwest California.

Our other American firs, subalpine fir (Abieslasiocarpa), a western timberline tree, balsam fir (A. bal-samea), and Fraser fir (A. fraseri) are comparatively smalland would seldom yield lumber suitable for airplanes.Balsam fir, widely distributed in Canada, the Lake Statesand northern Appalachian Mountains is commonly har-vested for pulp. Fraser fir of the southern AppalachianMountains is closely related to balsam fir and once formedgood timber stands. These three boreal forest trees arenow important as Christmas trees, but have sufferd dev-astating attacks from aphids and budworms in the 20thcentury.

WESTERN LARCH (Larix occidentalis), the second-most widely distributed larch or "tamarack" in America,is apparently the only one that is FPL rated. Study of thetables shows that while very strong, probably more-sothan any other American conifer, it is comparativelyheavy. This larch attains excellent size, much more-sothan does the more widely distributed "tamarack" (L.laracina) of the northeastern United States, Lake States,Canada, and Alaska. Interestingly, western larch is a richsource of Galactan, a water soluble gum concentrated inthe lower part of the tree. This oleoresin can produce"Venice" terpentine and related products.

Study of the tables clearly illustrates that except forits selection for "aircraft" grading, Sitka spruce has veryclose rivals for our use. If only from the standpoint ofeconomics, our other usable trees should eventually berecognized and more fully utilized. On the basis of durabil-ity, strength and weight, and in the order of my personalpreferences, I would not hesitate to use any of the follow-ing in the construction of my own project:

1. Port Orford cedar 6. Noble fir or red fir2. Alaska yellow cedar 7.3. Sitka spruce4. Western hemlock5. Douglas fir

Engelmann spruce8. White fir9. Red pine

Some DefinitionsDENSE GRAIN Generally with annual rings

having equal thickness of spring-wood and summerwood, andusually harder and heavier thannormal.

INLAND EMPIRE That region generally includingnorthern Idaho, western Mon-tana, eastern Washington andnortheastern Oregon; a slight ex-tension of the Rocky Mountains

* - into the Columbia Basin.OLD-GROWTH Trees of old age having annular

rings close together as a result ofretarded or past-prime develop-ment; resulting in "fine grain".

PACIFIC SLOPE That region generally extendingwestward from the Sierra Neva-da and Cascade Mountains ofCalifornia, the Cascade Moun-tains of Oregon and Washington,and from the Rocky Mountains ofBritish Columbia, to the PacificOcean.

REFERENCESREPAIR OF WOOD AIRCRAFT STRUCTURES AN-01-7A.P. 26754 — 1944.

SILVICS OF THE FOREST TREES OF THE UNITEDSTATES Fowels, H. A. — 1965Agriculture Handbook No. 271

CHECK LIST OF NATIVE AND NATURALIZED TREESOF THE UNITED STATES Little, Elbert L. — 1953Agriculture Handbook No. 41THE GEOGRAPHIC OVERLAP OF NOBLE FIR ANDRED FIR Parker, Eugene L. — 1963Forest Science Vol. 9 No. 2

SPORT AVIATION 61