SAFETY INT

RAUOL J. HOFFMAN

ly increase their weights.

theoretical design similar to the

structural units, especially airplane structure, is the transmission of a load of the suspension points with a minimum weight and a minimum number of parts.

The

principle of designing

Sec. 3 shows another combination of members, the unbraced frame design. Load will deflect an unbraced frame a great amount; therefore it is

one shown in Sec. 7. It should be mentioned that punched holes must be rounded, flanged or rolled in order to prevent the

The magnitude of the deflection should be small; it is always present when members are stressed, but should not introduce new secondary stresses that were not originally taken

not suitable for airplane designs.

into consideration when dimensioning the structure. The simplest transmission of

a force is through a member in tension; the weight is directly proportional to the length of the member. A unit in tension is the lightest structural design possible. Reversing the di-

rection of the force and keeping it in line with the axis of the member, will place the member under compression. The weight of the member will now

increase faster than its length. Therefore, in connecting structural parts, the part under compression should be made shorter than the one in tension. (Sec. 1).

Placing

the

line

Comparing the weights of the

three structures, the weight of the angular truss will be the

lowest, if a certain loading condition is assumed; one example is

indicated

in Sec. 4.

This

leads to a deduction that, by resolving the force into new forces, or by sending the load through two new members, continually added, towards the supports will make a very light structure possible. Sec. 5 indicates the simplicity of graphic estimation of stresses in a structure statically determinate if the number of members is equal or smaller than

twice the number of joints less three. Taking for example the number of joints to be 5, then the number of members should be seven or less. In case the

number of members is higher,

of force

the above method cannot be apsquare to the axis of the mem- plied. For space frame strucber and beyond the supports re- tures that have members in three sults in a cantilever beam. Sec. directions the number of mem2 shows a loaded cantilever I- bers to be equal or smaller than

beam; the load creates bending

three times the number of joints

stresses that place tension in the upper flange and compression in the lower flange; the shear forces are transmitted by

(less six), in order to be statically determinate. Usually the main forces are resolved into forced laying in one plane and the forces from

the web of the beam. Using the same force and support conditions, and replacing the I-beam by two members,

each plane added to get the resulting force.

The previously mentioned unbraced frame may be reinforced by attaching a very thin sheet member and compression in the of material. Loading this frame, lower member; the shear apcorrugations will appear at cerpears only as a component of tain intervals and at a certain the members at the supports. angle, which is close to 45 deThe forces in this simple struc- grees, as shown in Sec. 6. These tural combination are easily 45 degree shear lines are very computed if the load is applied obvious on the walls of a buildat the joint, the point of inter- ing that is under stress. Addsection of the center lines (neu- ing loads to the reinforced frame tral axes) of the members. The will over-stress it if stiffeners stress estimation becomes more are not added at a distance about complicated if the load is moved equal to the height of the frame. past the joint, as shown in Sec. Frames placed under load act3, for new bending stresses are ing up and down, their unstressadded that will increase the size ed web portion removed by as shown in sketch, the load will put tension in the upper

of the members and consequent-

punching round holes, give a

starting of cracks; this operation is not required for webs that are over-dimensioned. The 45 degree slope of the corrugations on the web mem-

I3 aircraft are built without plans and possess quality and flying

February 25, 1957

"phe meeting was called to or-

the grain in a plywood at the angle. Sec. 8 shows application of the frame and plywood combination in designing a wooden spar and a wooden rib. Viewing all mentioned above from analytical standpoint the trend of efficient structures is towards angular combinations; these combinations include the thin webbed frames that create their own angular folds. This relative placement of members - the fold in the web is considered a mem-

ber-continually

radiating from

each consecutive point (joint) may be designed under one term; triangulation. At least three members should meet at each joint forming a tripod with another one transmitting the load. In panel layouts at least two members must meet at a joint and one or two transmitting the load. This triangulation is carried

out for landing gears shown in

Sees. 10 and 11. Section 12 shows a landing gear of 1908,

which we may call "over-triangulated". This landing gear had a tendency of preventing a

take-off when tall grass covered the airport or the grass was

dew-covered.

The early Junk-

ers machine were over-triangulated, but lately the zig-zag de-

signed spars were substituted

by spars with flanges, and the number of joints and members

greatly

reduced,

show in Sec. 13.

as

sketches

The triangulation of a glider (Sec. 14) and a float bottom (Sec. 15) is a simple matter. The fuselage design in Sec. 16 uses cross-wires for bracing

potential hazard. Mrs. Wittman suggested that an teur built aircraft built plans, have the plans go

all changes from the original drawings noted on the plans.

lower chord. For comparison the same section shows the design for very light loading used by the Wright Brothers. Loads

A Treasurer's report was given. Receipts for January, 1957 totaled $2730.53, disbursements totaled $1974.28, leaving a net gain of $756.25. The bank balance as of January 31, 1957 was $3753.39. It was announced that correspondence received from members indicates many are having difficulties in getting their homebuilt aircraft certificated by C.A.A. Maintenance Agents in

indicated in Sec. 23 and Sec. 24

of present Civil Air Regulations on amateur built aircraft have

kinds of trusses have their individual advantages for strength and economy. Sec. 21 shows triangulation of a large metal rib, and Sec. 22 triangulation of a wooden rib

with all loads carried by the

should always be applied at the joint directly (or indirectly) as

except when an added load, usually a bending load, is consider-

ed when dimensioning the member. Triangulations of tail surfaces and tail skid are shown in Sees. 25, 26, 27, and 28. Lately, however, the member clobe

to the cloth covering is placed

parallel with the line of airflow in order to reduce turbulencies. Usually, in experimental planes, the engineer encounters the problem of removing a diagonal

for access to the inside of the

structure with no reduction of

the strength of the design. These

openings are doors, handholes, unbraced space for tanks and accessories. Sees. 27, 30, 31 and 32 show solutions of the problem by carrying the load around corners within the frame or outside of

the field because broad scope

allowed these

agents

varied

opinions and caused confusion on the procedures used for such certification. It was felt the

regulations

should

be

more

angulate their first design, which is commendable, for it adds

strength to the structure and

therefore safety. Nevertheless, each concentrated load should

The large amount of recent

aircraft accidents in this country due to mid-air collisions and

etc., has caused considerable attention and action in the C.A.A.

and C.A.B. and large scale investigations are under way. A great deal of attention is being

given to air safety. It is believ-

mit to fly.

In accordance with

Civil Air Regulations adopted in 1938, all civil aircraft

flown in the United States must

take tension and compression

ard welded steel tube fuselage designs are shown in Sec. 20;

triangulations of wing, I-strut,

shown in Sec. 19. These diagonal "web" members alternately

one inch.

This model will not

stresses and they may be streamlined if so desired. The two

manufacture of the plane or the mock-up case one is wanted. •

It was also suggested that the aircraft's operation's limitations

should also accompany the ship

when it changes owners. Under the present regulations, the

builder

need only

present a

three view drawing or three pictures showing the configuration of the aircraft when applying for his certificate plus a set of log books to be used for the aircraft and engine. A big discussion formed around the Wittman "Tailwind" and Nesmith

"Cougar".

Mr.

and Mrs. Wittman were present

and

strongly

expressed their

as a service to its readers.

brief account of the homebuilt

only help simplify the design, but also will aid speeding up

is always under tension. Stand-

This would give the buyer an

idea of the aircraft's structure.

activities in England under the Popular Flying Association.

N-strut are shown in Sees. 17 and 18. The Howe and Warren type truss designs are used in laying out the welded steel spars

could be eliminated by using

crossed tubings; the tubes are welded to the frame, and one

the aircraft upon resale, having

viewpoints. It was decided the E.A.A. will take a neutral stand

ed this action may have prompted C. A. A. attorneys to init; or placing the triangulation vestigate the legality of allowing into an adjacent panel. The amateur built aircraft to fly years - ago popular "picture without an airworthiness cerframe" (bent tubing) design tificate. Under the present proshould not be used (Sec. 31). visions of the C.A.R., amateur The airplane designer and the built aircraft are issued a per-

model engineer usually overtri-

S. J. amafrom with

specific to guide the agent rather than leaving it up to his discretion, likes or dislikes. George Hardie, Jr. gave a

have a valid airworthiness certificate. Poberezny said he inbe at a joint braced three ways; tended to go to Washington, these loads are carried by pul- D. C. in the near future to meet leys, bearings, walking-beams, with C.A.A. officials in an atseats, safety-belt connections, tempt to get a clarification of fittings connecting wings and the regulations governing amalanding-gear. To aid the spe- teur built aircraft. Word was cial conception of the structure received at headquarters that a model may be built using Vs the C.A.A. may eliminate the inch balsa sticks. The reduction selling of an amateur built airof the general dimensions should craft from the builder to another be to a scale of one foot equals individual. This would mean

that require turnbuckles, which

Some people feel this is a

der by President Poberezny at 8:25 P.M. at Curtiss Wright Airport, Milwaukee, Wisconsin. The minutes of the previous

meeting were approved as read.

ber is tne reason lor placing

characteristics known and familiar only to the builder.

the builder would have to retain

his aircraft until it was dismantled or destroyed. It is believed

this was brought about by the fact that many amateur built

and endorse neithek- product,

but will present material in the "Experimenter" on both aircraft

A letter from Bill Robinson of Chapter 11 (Los Angeles) was read. Bill's letter listed the names of the winners of the events at the Jan. 20th combined Fly-Ins of Chapters one and Eleven held at the Flabob Airport, Riverside, California. The E.A.A. Aircraft Design

Contest was discussed. Many are of the opinion that the proposed judging date of the 1958 Fly-In is too soon to allow a good

number

of

contestants

ample time to design, build and fly their entries. It was suggested all entries be of original design and not a modified homebuilt, presently flying. Those designers who have already built an airplane and desire to modify their original design would be welcome

to do so.

This is to discourage those people who have bought plans for a Playboy or Tailwind from

altering the original design. It was also suggested a dead line date should be set up for the finished rules for the contest.

The meeting was adjourned at 10:40 P.M. After the meeting

movies entitled

"Aeronautical

Oddities" and "Aircraft Structures" were shown. Refresh-

ments were then served by Lois Nolinske and Audrey Poberezny.

Respectfully submitted, Bob Nolinske Secretary-Treasurer