DEVELOPMENT OF A COUNTERPART VERTICAL 

FIN OF PAPREG FOR THE AT-6 AIRPLANE 

December 1945 

This Report is One of a Series 

Issued in Cooperation with the 

ARMY-NAVY-CIVIL COMMITTEE 

on 

AIRCRAFT DESIGN CRlTERlA 

Under the Supervision of the 

AERONAUTICAL BOARD 

No. 1594 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison, Wisconsin 

In Cooperation with the University of Wisconsin 
DEVELOPMENT OF A COUNTERPART VERTICAL FIN 

1 

OF PAPREG FOR THE AT-6 AIRPLANE 

By E. C. JUNGMANN, Engineer 

Summary 

To explore the possibilities of making primary aircraft structures of papreg, 

a vertical fin for the AT-6A airplane was designed and fabricated from 

papreg, and was tested under static loads representing gust and steady dive 

condition. The metal fin for the same airplane is of conventional two-spar 

construction with ribs. In the development of the papreg fin, trials of 

complete fins and of several rear spar designs led to the adoption of a 

semimonocoque design as most suitable because of the properties of the 

materials used. The papreg fin, equipped with an I-beam rear spar of 

papreg, supported 200 percent of the design load in static gust-load tests. 

Stress data obtained indicate that, while the papreg fin did not exceed the 

aluminum fin in weight, its weight can be reduced. 

Introduction 

Development by the Forest Products Laboratory of papreg,

2 a laminated paper 

plastic of high tensile strength, during World War II led to the decision 

to investigate the utility of this material in a primary structural assembly 

in cooperation with the Army-Navy-Civil Committee on Aircraft Design Criteria. 

The vertical fin of the AT-6A basic training aircraf't was selected as a suitable structure, as in it papreg could be employed for spars, stiffeners, and 

skin, both alone and in combination with veneer and plywood in sandwich-type 

materials. The metal AT-6A fin, which is of conventional two-spar construction with ribs, was duplicated in papreg, but exploratory tests (appendix) 

indicated that, for the papreg and sandwich materials to be used, a semimonocoque design would be most suitable. 

1

This report is one of a series of progress reports prepared by the Forest 

Products Laboratory relating to the use of wood in aircraft. Results here reported are preliminary and may be revised as additional data become available. 

2

"Strength and related properties of Forest Products Laboratory laminated 

paper plastic (papreg) at normal temperature," FPL Rept.No.1319, rev. 1945. 

Report No. 1594 -1-
In this report are presented the results obtained from tests of a fin with 

papreg-faced, sandwich-construction skin and solid papreg spars, stiffeners, 

and formers. The types of loading used, gust and steady dive-condition 

static loads, were believed to represent the most critical loads to which 

the structure would be subjected in service. The objective of the tests was 

twofold: (1) to determine the suitability for aircraft structures of the 

materials used, and (2) to ascertain the most suitable design for a spar 

and complete fin built of these materials. 

Method of Test 

Gust-condition loads were applied to the fins so as to impose critical 

bending stresses, Steady dive-condition loads were applied so as to impose 

torque, which is primarily critical for the skin. 

For steady dive-condition tests, the fin was mounted as for the gust-load 

tests (fig. 1). The simulated air load was applied to the fin surface in 

accordance with a specified pattern (table 1) and the rudder load was 

applied in the opposite direction, upward, through a pulley system which 

permitted use of the trays to carry the loads. During loading, jackscrews 

were used as in the gust-load test's. 

In the gust-load tests, the fin was mounted horizontally on two wood stanchions (fig. 1). The rear spar root fitting was fastened to one stanchion 

and the front spar fitting to the other. Simulated air load was then applied 

by means of sand bags and other weights distributed on the surface of the fin 

in a specified pattern and hung by means of trays from the rudder-hinge fittings, The fin surfaces and rudder-hinge fittings were loaded simultaneously 

(table 2). Loads on fin surfaces and fittings were in the sane direction. 

While the weights were put in place as shown in the diagram accompanying 

table 2, the fin was supported by jackscrews, which were removed to apply 

the load. 

The preliminary tests of spars were made with tho same apparatus as was used 

to test the fins. Each rear spar was fitted to a simulated internal structure of the fin consisting of a front spar and three internal ribs formed 

from 16-gage galvanized iron (fig. 2). The high rigidity of this frame permitted concentration of stress in the rear spar. A flat wood platform the 

shape of the fin in plan view was placed loosely on this metal frame to 

support the simulated air load. Jackscrews were used for support as in the 

fin tests. Four deflection readings were taken along the length of each 

spar tested as the load was applied. 

Deflections produced in the fins and spars by application of the loads were 

read by means of a cathetometer (fig. 1,B) equipped with a vernier measuring 

device. Two deflection readings were taken along the spar and two on the 

leading edge of the fin. Strains produced by the stresses applied were read 

by means of type SR-4 electric strain gages. Where the direction of strain 

was known, simple gages were attached to the skin and spar; where the direction was not apparent, rosette-type gages were attached. The strains were 

read on an automatic strain recorder. 

Report No. 1594 - 2-
Preliminary Investigation and Design 

Preliminary experiments with a fin made by duplicating in papreg and papregplywood sandwich materials the various parts of an aluminum AT-6A fin showed 

that these materials were not adapted to the conventional design of the 

aluminum structure (appendix) because the stresses could not be properly 

distributed. These results led to the decision to adopt a semimonocoque 

construction, for which special attention was given to the design of a rear 

spar that would support the bending loads without assistance from the shell 

structure . 

A series of spars was therefore investigated to determine the efficiency 

of various types of cross sections, including C-, I-, and box-sections. 

Materials from which these spars were made included papreg, staypak,3 and 

such conbinations as papreg and staypak, papreg and wood, and a papregbasswood veneer sandwich (figs. 3-9). Results of steady dive-condition and 

gust-condition loading tests on these spars (appendix) indicated that a 

papreg rear spar of I-section (fig. 6) would be entirely satisfactory through 

200 percent of the design ultimate load. 

In the semimonocoque papreg design, much of the internal structure was 

eliminated, A front stub spar 10 inches long replaced the front spar to 

transfer the drag load to the fin. Angles and formers replaced the ribs 

and acted only as skin stiffeners. The formers and stiffeners thus acted 

as auxiliaries that enabled the skin to transfer air loads directly to the 

rear spar, Since stresses beyong the proportional limit are likely to 

cause creep in the plastic, the test fins were designed to stresses below 

the proportional limit. This design limitation also made it possible to 

ignore the amount of additional shear which could be taken by a buckled 

spar web or section of skin. By ignoring tho shear loads taken by the 

diagonal tension field, a conservative structure was produced. The margin 

of safety calculated for the spar web at its buckling stress in shear was 

7.18. Reduction of this extreme margin of safety was not attempted, because the primary goal was to design a plastic fin that would equal or 

exceed the aluminum fin in performance without exceeding it in weight. 

Consequently, the fin design did not represent the best attainable strengthweight ratio. 

A preliminary fin of this design (fig 10) was given a single test in 

gust-condition loading. Results essentially substantiated the theoretical 

assumption that the semimonocoque design was more suitable for the 

materials used than for a conventional two-spar design, although several 

modifications in the materials used were indicated (appendix). These 

modifications were incorporated In the construction of the final papreg 

fin. 

3"Heat-stabilized compressed wood (staypak)," by R. M. Seborg, M. A. 

Millett, and A. J. Stamm. FPL Rept. No. 1580, rev. 1944. 

Report No. 1594 -3-
Description of Materia1s and Fabrication of Parts 

The semimonocoque design selected for the AT-6A fin called for the following parts: a leading edge, side skin panels, a rear spar, a front stub 

spar, skin stiffeners, and formers. Materials used in these parts and 

their method of' fabrication were as follows: 

Loading Edge 

The leading edge part consisted of inner and outer papreg facings on a plywood core and constituted the skin of the fin to a chord point 10-1/2 

inches back from the leading edge. The papreg facings were 0.028 inch 

thick, with the fiber direction of adjacent sheets of the phenolic-resin 

impregnated paper parallel to each other and to the leading edge. They were partially cured flat in a press at 200 pounds of pressure per square 

inch and 284° F. for 8 minutes, then drawn without cooling and preformed 

to approximate final shape on a wood mold (fig. 11) by means of a piece of 

cotton duck fastened to a mood bar at either side for convenience in pulling it down over the papreg during the forming operation. The papreg was 

partially cooled on the mold, then removed. It was held in clamps to retain its approximate curved shape pending the final assembly operation. 

The core consisted of yellow-poplar plywood. It was made of two-ply 

sheets 1/80 inch thick which were 90° cross banded and glued together with 

a dry film phenolic-resin glue. The grain direction of the veneer was 45° 

with respect to that of the leading edge. The two-ply sheets were trimmed 

while flat so that they were 2 inches shorter than tho papreg facings. 

This was done so that only the facings would be scarfed to the side panels 

when assembling the fin. Each two-ply sheet was preformed on the mold after 

being wetted down the center on both sides to soften it, then stored at 

80° F. and 65 percent relative humidity until the final assembly of the 

leading edge, being held in shape during storage by means of clamps. 

Assembly of the leading edge was done by placing the inner preformed facing 

on the mold, laying two sheets of resin-impregnated paper on it, next placing the three preformed two-ply sheets alternately with two sheets of 

impregnated paper, and finally laying 4the outer preformed facing in place 

over two sheets of impregnated paper. Over the whole was laid a sheet of 

moistureproof cellophane, about which 2-inch-wide webbing was tightly 

4 

It was found by trial that the phenolic-resin impregnated sheets of paper 

provided a better bond between the components of the sandwich than either 

liquid phenolic glue or dry-film glue. This was probably due to the fact 

that the moisture content was not so critical for the paper, that flow in 

molding was not so severe as in the liquid glues, and that the paper had 

higher strength during bonding than did the dry-film glue and consequently 

stayed in place better. Because the laminations pulled down better than 

with other bonding agents, the impregnated paper appeared to have better 

slip qualities. 

Report No. 1594 -4-
wrapped in order to pull the material firmly against the mold. No caul was 

used, because the outer papreg face was stiff enough to resist indentation 

by the webbing. 

This assembly was bag molded for 30 minutes in an autoclave at 75 pounds of 

steam pressure per square inch and a temperature of 320° F., using a 

neoprene bag, and was withdrawn while hot from the autoclave. 

Side Panels 

The side panels were of papreg 0,060 inch thick made up of sheets of resinimpregnated paper laid so that the fiber direction was at right angles in 

adjacent sheets. The panels were molded in a flat press at 200 pounds of 

pressure per square inch and 320° F, for 15 minutes. The grain direction 

of the panels was 45° with respect to the spar center line. 

Rear Spar 

An I-section papreg spar of a design that had previously withstood gustload tests up to 200 percent of the design load (fig. 8) was used. This 

spar consisted of a cross-laminated papreg web 0.104 inch thick and flanges 

and papreg angles 0.120 inch Chick. The outstanding legs of the angles 

were tapered in width from 1-1/2 inches at the root to 3/4 inch at the tip. 

The angles were glued to the web with a room-temperature-setting, urearesin glue. 

Skin Stiffeners 

The skin stiffeners, consisting of 0.120-inch cross-laminated 3/4- by 3/4­

inch papreg angles, were die molded at 200 pounds of pressure per square 

inch (fig. 12). 

Formers 

The skin formers for the top and bottom ends of the fin consisted of facings 

of 0.038-inch parallel-laminated papreg on a 3/8-inch core of parallellaminated, 1/8-inch, basswood veneer. The formers were cut with the grain 

of the core lengthwise from panels that were press molded in a single 

operation at 100 pounds per square inch and 320° F. for 20 minutes. 

Front Stub Spar 

The front stub spar (fig. 13) was made of the same papreg-basswood veneer 

combination as were the formers. This material was chosen because it could 

be easily worked, would provide sufficient glue area for the transfer of 

the drag load to the fin, and was of the correct stiffness for the air loads 

applied. 

Report NO. 1594 -5-
Assembly of Fin 

With one exception, all assembly gluing was done with a law-temperature-setting, urea-resin glue. The single exception was the gluing of the lending 

edge to the side panels, this scarf joint being glued under about 15 pounds 

of air pressure per square inch on a steam-heated strip heater, using a hightemperature-setting, phenolic-resin glue. 

The leading-edge section was scarfed so that it overlapped the side panels 

on the outer surfaces of the fin. Since the trailing edges of the leadingedge section had been fabricated so that they consisted only of papreg, the 

total thickness of each was only 0.060 inch, the same thickness as the side 

to which it was scarfed. The plywood core of the leading-edge section 

had been sanded during fabrication of the section so that its thickness 

receded gradually to the point where the papreg facings were joined. The 

effects upon the smoothness of the skin surface produced by the greater 

thickness of the sandwich construction in the leading edge were thereby 

minimized. 

After the leading edge had been glued to the side panels, the panels were 

glued to the rear spar, pressure being applied with woodworking clamps. 

The stub spar, stiffeners, and formers were then glued in place also with 

clamps; in the case of the stiffeners, a metal bar was used to distribute 

the clamp pressure. 

In volume production of such fins, of course, jigs could be designed to 

facilitate the assembly gluing operation. Gluing could be speeded by the 

use of thermosetting glue and strip heaters. 

This papreg fin, which possessed a smooth surface unimpaired by rivets or 

other external fastenings (fig. 14), weighed 13.2 pounds. The actual 

weight of the original aluminum fin, complete with electric fixtures and 

all fittings, was 13.75 pounds. Subtracting the estimated weight of these 

fittings, for comparison purposes, the aluminum fin was calculated to weigh 

13.25 pounds. For practical purposes, the two fins were therefore of the 

same weight. 

Discussion of Test Results 

Static load tests applied according to tables 1 and 2 were successfully 

carried by the papreg fin. 

In steady dive-condition loading, a deflection of 0.82 inch was read with the 

cathetometer at both the spar tip (station C on diagram accompanying table 3) 

and the leading edge (station D, table 3) at 100 percent of the design ultimate load, which was the maximum applied in this test. This loading applied 

torque to the fin, since rudder loads are opposed to air loads. Under gustcondition loading, at 100 percent of the design ultimate load the spar tip 

(station C on diagram accompanying table 4) was deflected 1.02 inches and 

Report No. 1594 -6-
the leading edge (station D, table 4) 0.72 inch. At 200 percent of the 

design ultimate load in gust-condition loading, the spar tip was deflected 

1.97 inches and the leading edge 1.47 inches (table 4). These results indicated that the structure was rigid enough to maintain its shape and was not 

critically displaced from position through any practical range of loading. 

Tables 3 and 4 present data on set, observed immediately after test. The 

preliminary tests on the spar had indicated that, even at 200 percent of the 

design load, the spar was essentially straight, thus assuring that no binding 

would take place in the rudder hinges (appendix). 

Strain data obtained from gages installed at various stations on the upper 

surface of the fin (figs. 15 and 16) indicated that the most highly stressed 

area in the skin under either loading condition was in the upper corner near 

the spar (table 5). In gust-condition loading, stresses approached the 

proportional limit for the material. The spar flanges between the bottom 

rudder fitting and the fin showed a high concentration of stress at 200 percent of the design ultimate load in gust-condition loading. The direction 

of highest stress in the spar web indicated that an incomplete diagonal tension field was developed in the spar between the stiffeners. The tension 

stresses were considerably under the proportional limit. 

The evidence of set produced by the deflection readings indicates that 

strain gages were not placed in some critically stressed area of the fin. 

The fact that strain gages Nos. 22 and 4 produced evidence of unusually 

high stress at two points indicates that the proportional limit may have 

been exceeded in the vicinity of either of these gages. Further tests would, 

however, be necessary to locate these points of set exactly. The set data 

given in table 3 are well within the margin of error permitted in reading 

deflections. Since the proportional limit for the material was used as the 

basis for 100 percent of the design load, the set at 200 percent (table 4) 

is not regarded as critical. 

Conclusions 

The results of the steady dive-condition and gust-load tests indicated that, 

with proper design to develop the properties of the material, papreg is a 

promising material for use in fabricating primary aircraft structures. 

While the tests made did not include all conditions to which such a structure 

would be subjected to establish definitely its practicality, they did represent the most critical loadings. The papreg fin under load was not deflected 

sufficiently to interfere with the proper operation of the rudder, and it 

proved to be rigid enough to maintain its aerodynamic shape throughout all 

loadings applied, so that flight characteristics would not be impaired. 

Aerodynamic smoothness of the skin was attained. The skin was, moreover, 

rigid enough to permit adoption of semimonocoque design, with its advantage 

of a virtually unobstructed interior. Such design, while not important in 

the fin, offers definite advantages for wings, affording greater space for 

such parts as retractable landing gear, gasoline tanks, and armament, 

with which an elaborate internal rib structure is likely to interfere. 

Report No. 1594 -7-
The parts of the papreg fin are simple to make. Only one shape of molding 

die is necessary. The die-molded pieces can be turned out in a single 

operation of a hot-plate hydraulic press. All necessary fabrication and 

assembly of parts can be done with ordinary woodworking tools. Neither highpressure hot presses nor autoclaves of high-pressure capacity are needed. 

While no attempt was made to produce a fin of less weight than the one tested, 

the fact that the fin tested carried 200 percent of the design ultimate load 

in gust loading without failure indicates that a fin of adequate strength can 

be made with reduction in weight considerably below that of the aluminum 

AT-6A fin. 

APPENDIX 

The design, fabrication, and test of the papreg AT-6A fin were preceded by 

preliminary investigations intended to establish the most promising of 

various designs and combinations of wood, papreg, and sandwich materials 

for use in the construction of tho final fin. 

These investigations were begun by planning and fabricating, with wood and 

papreg, equivalent parts of the aluminum fin used in the AT-6A aircraft. 

In this preliminary fin, all structural. parts were made of papreg, while 

the skin consisted of a sandwich of papreg facings and a yellow-poplar core. 

The fin was assembled with a thermosetting glue cured by moans of electric 

strip heaters (fig. 17). Skin stiffeners were eliminated because the 

sandwich material was calculated to be 5.6 times as stiff in bending as 

was the aluminum in the leading edge and 4.6 times as stiff as the aluminum 

in the side panels. 

This preliminary fin carried the design ultimate load in steady dive-condition loading, but failed in gust-condition loading at 60 percent of the 

design ultimate load because the rear spar lacked the necessary lateral 

stability. The outstanding leg of the spar (fig. 18) buckled on the compression side, When the leg was stiffened (fig, 19) a tension failure at 

the root fitting resulted. Stiffening of the flanges of the spar and increase of the tension area by addition of compreg plates at the root (fig. 

20) strengthened the spar somewhat, but two successive gust-condition loadings to 120 percent of the design ultimate load caused a lateral failure 

in the spar above the intermediate rudder fitting (fig. 21). 

From those results it became clear that the most important factor in 

achieving a suitable fin with papreg was a modified design that would 

permit proper distribution of stresses. For this reason, the two-spar-and 

rib design of the original aluminum fin was abandoned and a semimonocoque 

structure adopted. Special attention was given to the design of a rear 

spar that would support all bonding loads regardless of the shell structureReport No. 1594 -8-
Spar Design Tests 

The tests of spar designs were undertaken first. A series of spars with 

five types of cross sections, including C, I, and box, was made of various 

materials, Including papreg, solid stay-pak, and combinations of papreg, 

staypak, papreg and wood, and a papreg-basswood sandwich material. Equipped 

with new fittings, these spars were made as follows: 

(1) A spar of papreg-bssswood sandwich construction (fig. 3) was made of 

parallel-laminated papreg faces 0.038 inch thick pressed and bonded to a 

three-ply, parallel-laminated, 1/8-inch basswood veneer core in a single 

compression and assembly operation. The spar was reinforced with extra 

papreg plates 0.050 inch thick at all points of attachment except at the 

main attachment point at the root of the spar, which was reinforced with 

four tapering lower-hinge fastenings of compreg made from yellow birch 

veneer. The aluminum lower-hinge fitting was replaced by a steel block to 

adapt the fitting to the construction. 

(2) A solid staypak spar (fig. 4) approximately 1/2 inch thick was made 

from 17 plies of 1/16-inch parallel-laminated yellow birch. Lightening 

holes were cut to reduce the weight. A set of flat steel plates was 

designed to hold this spar at the root for the fuselage attachment, such 

plates being considered more efficient than the standard aluminum attachment. 

(3) A box-beam spar (fig. 5) was made with staypak flanges and papreg webs. 

The staypak was made from 1/16-inch parallel-laminated yellow birch veneer 

and cut into tapered sticks 3/4 inch square in cross section at the root end 

and 1/2 inch square at the tip, with widened zones at points of attachment. 

The 0.052-inch cross-laminated papreg webs were glued to the flanges with 

a room-temperature-setting, urea-resin glue. 

(4) A spar of I-section was made with a cross-laminated papreg web 0.104 

inch thick and papreg flanges 0.120 inch thick (fig. 6). The papreg angletype flanges were tapered from 1-1/2 inches on the outstanding leg at the 

root to 3/4 inch at the tip. These angles were glued in pairs to the web 

with a room-temperature-setting, urea-rosin glue to constitute the flanges. 

(5) An I-section spar was made of papreg and Sitka spruce with a 0.104-inch, 

cross-laminated, papreg web and spruce flanges (fig. 7). The flanges were 

tapered from 1 inch square in cross section at the root to 1/2 inch square 

at the tip. The flanges and several spruce stiffeners located at various 

points along the spar were glued to the web with a room-temperature-setting, 

urea-resin glue. 

Spar Test Results 

The spars were tested in gust-condition loading with the apparatus shown 

in figures 1 and 2. 

Report No. 1594 -9-
beam 

The papreg-basswood spar failed first at 140 percent of the design ultimate 

load in the compreg because of a lack of sufficient tension area. After 

correction of this failure, the spar buckled above tho juncture with the 

lower rib at 160 percent of the design ultimate load. 

The solid staypak spar failed in the staypak on the tension side at the 

bottom lightening hole in shear at 120 percent of the design ultimate load. 

The box-beam spar with papreg webs and staypak flanges developed a tension 

field in the web, which sheared from the staypak at 180 percent of the design 

ultimate load. 

The I-beam spar with papreg web and flanges was entirely satisfactory through 

200 percent of the design ultimate load. (This spar was essentially the 

same as the one used in the tests of the final fin described in the main 

body of this report.) 

The I-beam spar with papreg web and spruce flanges failed at 120 percent of 

the design ultimate load in bearing at the root fitting. 

The tests of these spars indicated that the I-beam papreg spar and the 

papreg-staypak box spar, each of which showed a maximum bowing of 0.020 inch 

at 200 percent of the design ultimate load in gust condition, might with 

modifications be suitable for incorporation in fins of semimonocoque design 

for further tests. The web of the I-beam spar was strengthened (fig. 8) 

by angle stiffeners glued to it with a low-temperature-setting, urea-resin 

glue and the flanges at the root with additional angle plates also glued in 

place with the same glue. The box spar, when stiffened in the webs by papreg 

angles glued in place as for the Ispar/(fig. 9), also carried 200 percent 

of the design ultimate load. 

Development of Semimonocoque Fin 

Theoretical considerations upon which the semimonocoque design of the fin was 

based are discussed in the main body of this report. To test these considerations, a trial fin (fig. 10) was fabricated. This fin corresponded to the 

final fin described in the main body of this report in all details of construction except the side panels. These were of sandwich construction instead 

of the solid papreg used in the final papreg fin. Tho faces consisted of 

0.020-inch, parallel-laminated papreg. The core consisted of three-ply, 1/60­

inch, yellow-poplar plywood glued with a high-temperature-setting, phenolicresin film glue and cross banded at 90°, with the grain at 45° to the fiber 

direction of the papreg facings. The grain of the facings was parallel to 

the leading edge. These panels were press molded in one operation at 200 

pounds of pressure per square inch and 320° F. for 15 minutes. After being 

trimmed to shape they were scarfed to a slope of 1 in 12 for bonding to the 

leading edge. The strengthened I-beam papreg rear spar (fig. 8) was incorporated in this fin. All parts of the fin were bonded together with a lowtemperature-setting, urea-resin glue after being sanded thoroughly with No. 0 

sandpaper. 

Report No. 1594 -10-
A single test in gust-condition loading was made. At 160 percent of the 

design ultimate load, tho plywood core in the side panels delaminated; inspection revealed 80 to 90 percent glue failure in the glue lines of the 

plywood, Delamination is believed to have been due to poor gluing. The 

increase in stress imposed upon the papreg by failure of the plywood, on 

tho other hand, caused failure that was entirely in the plastic rather than 

in the glue between the laminations. 

Although the fin did not sustain the high loading sought, this test indicated 

that the semimonocoque design developed the inherent strength properties of 

the material satisfactorily. It was therefore decided to fabricate a final 

test fin in which papreg side panels would replace the sandwich construction 

used in the preliminary fin. The test results with the fin so modified are 

presented in the main body of this report. 

Tests of Papreg-staypak Box Spar Fin 

Tests of the various types of spars had indicated that, in addition to the 

I-beam papreg spar, the box-beam spar fabricated with papreg webs and staypak 

flanges (fig. 5), which failed in test at 180 percent of its design ultimate 

load, also held promise of meeting load requirements in a semimonocoque 

design. Accordingly, this spar was modified with the addition of papreg 

angles to stiffen its webs (fig. 9) and incorporated in a fin similar in 

other respects, except that bag-molded papreg hat-section skin stiffeners 

replaced tho angle stiffeners, to the final fin discussed in the main body 

of this report. 

Tho box-spar fin carried steady dive-condition loading successfully through 

100 percent of tho design ultimate load. The spar tip was deflected 0.72 

inch and the leading edge 0.88 inch (table 6). This structure showed more 

eivdence of torque than did the fin incorporating tho papreg I-beam spar. 

At 100 percent of the design ultimate load in gust-condition loading (table 

7) the spar tip was deflected 0.72 inch and the leading edge 0.70 inch. 

The fin failed at 180 percent of' the design ultimate load (figs. 22 and 23). 

At 160 percent of the design load, the spar tip was deflected 1.51 inches 

and the leading edge 1.10 inches. Gust-condition loading did not deflect 

the box-spar fin at 100 percent of the design ultimate load so far as it did 

the papreg fin, the spar in the papreg fin having been deflected 1.02 inches. 

The leading-edge deflection of the papreg fin was practically the same as 

that of the box-spar fin. 

Strain gages were installed on the box-spar fin in the same locations as 

on the papreg fin. Stress calculated from strain-gage data indicated that 

higher stresses ware developed in the spar root (table 8) than in the papreg 

spar. The skin at the upper corner near the spar was not stressed nearly 

SO much in either loading condition as that in the papreg fin, Relatively 

speaking, the box spar did not work so effectively as did the I-section spar. 

At 100 percent of the design ultimate load in gust-condition loading, the 

spar was again the weak part of the structure. In the papreg fin, the 

most highly stressed areas had approached the same stress values, 

Report No. 1594 -11-
indicating equal strength. This condition in the papreg fin persisted 

throughout the tests. The box-spar fin failed at the top corner of the 

skin, even though the stresses did not indicate that ultimate strength 

had been reached. 

Report No. 1594 -12-
Z M 64002 F 
Z M 64003 F 
Z M 64004 F 

Z M 64006 F 
Figure 1.--Test setups for static load tests of vertical fins. A, loads applied for gustcondition test. B, loads applied for steady dive-condition test; cathetometer used to 

measure deflections is shown at right. 

Z M 63993 F 
Figure 2.--Rear spar test setup for gust-condition loading. The spar shown is of papreg-basswood 

sandwich construction. The air-load platform, not yet in place, can be seen between the jacks. 

Z M 64841 F 
Figure 3.--Papreg-basswood sandwich-construction rear spar is shown 

with its test fittings. 

Figure 4.--Staypak rear spar. Note the root fitting for fuselage 

attachment. 

Figure 5.--Papreg-staypak box-type rear spar with fittings. 

Figure 6.--Papreg I-section rear spar with fittings. 

Z M 63997 F 
Figure 7.--Papreg-Sitka spruce I-section rear spar with fittings. 

Figure 8.--The papreg I-section rear spar used in the final design 

of the fin. 

Figure 9.--The papreg-staypak box-type rear spar which had been redesinged 

to include papreg angle web stiffeners. The position of the spar flanges 

and web stiffeners is outlined for clarity in this picture. 

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Figure 10.--Vertical fin assembled for preliminary trial of semimonocoque 

design. 

Figure 11.--Laminated wood mold for molding leading-edge section, The 

inner radius of the leading-edge section at the tip, shown in the foreground, is 3/32 inch. At the bottom it enlarges to 1/2 inch. 

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Figure 12.--Skin stiffeners glued in place to skin interior. The bottom 

formers appear in the foreground and the top formers in the background 

interior. 

Figure 13.--Front stub spar in position in the Pin. The drag-load 

fitting is in place. Bolts through the fitting go through the former 

and through a molded papreg angle glued to the root of the stub spar. 

Figure 14.--Side elevation of papreg fin. Note the smooth surface 

unimpaired by external fastenings, such as rivets. 

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Figure 18.--View looking forward at base of rear spar of first vertical 

fin tested. Shear failure occurred between flange and web at proof 

load in gust condition. 

Figure 19.--View looking forward at rear spar of first vertical fin 

tested. Fracture developed at 80 percent of the design ultimate 

load. The spar had been stiffened by clamping 1 inch by 1 inch 

oak strips to it from fitting to bottom rib. 

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Figure 20.--Vertical fin with compreg spar-root stiffeners. 

Figure 21.--Lateral failure above the intermediate rudder fitting of 

the rear spar in the first fin tested. 

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Figure 22.--Fin in which was incorporated the papreg-staypak box spar. 

Locations of the strain gages on the skin surface are shown. Note 

the hat-section stiffeners on the skin interior. 

Figure 23.--Position of strain gages on spar web. The spar web is shown 

sheared away from the spar f'langes. The web stiffeners are illustrated 

in the broken section. 

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