THE

CONCENTRATION

AND

By John S. Harris (EAA 119974) Brigham Young University 3146 JKHB Prove, UT 84601

OF LOADS IN AIRCRAFT DESIGN

(Illustrations by Brant Gifford)

I

N THE PROCESS of aircraft design, once the shape is determined for proper aerodynamics, the design of structures can begin. This portion of design - the calculation of loads and forces and the design of members to carry those loads and forces - is much like the process of designing a railroad bridge or designing the structure of a building, except that the scale and materials are different. In aircraft design, a crucial part of that process and one that occupies a very large proportion of the designer's time and effort is designing for load and force concentration and load and force distribution. Often amateur builders do not realize the implications of concentration and distribution of loads in design and may make casual modifications that interfere with the efficiency of the design or, worse yet, transfer the load to another location that is weaker and thus reduce the strength of the entire structure. While engineers designing all sorts of structures have to deal with the problems of load distribution and load concentration, for aircraft designers the problem is especially challenging because in aircraft there is always the need to keep weight as low as possible and because aircraft present unique combinations of distributed loads and concentrated loads that must both be figured into the total structure. In an aircraft, the concentrated loads are from the engine, the passengers and the landing gear. The distributed loads are mainly air loads on the wings and tail surfaces. A thousand pound light aircraft may have a wing loading of as little as seven pounds per square foot. Thus the surface that is in contact with the air can be thin doped fabric or very light aluminum skin. But that air load on the wing surface must

leaves. The flat surface of each leaf is supported by ribs, which absorb the load and transfer it to the leaf stem. The loads from leaf stems are transferred to small twigs then to branches and, finally, to the trunk, the load carrying members get progressively larger and more rigid as they approach the trunk, and the trunk itself is largest and most rigid at the ground. Thus, the tree, like an airplane has the thinnest and most flexible structures near the distributed loads and the heaviest and most rigid structures close to the point loads.

be somehow transferred to the whole structure so that

that fabric or aluminum skin actually lifts hundreds of pounds of engine and passengers. Further, that transfer of load must be made efficiently, with as little weight as possible. Similarly, the concentrated load of the engine,

weighing perhaps two hundred pounds, must be transferred to the wing fabric or skin, and that load must be distributed evenly so that the very light weight structure

is not torn to pieces when those loads are multiplied by the G forces during a hard landing or during maneuvers. In the same fashion, the landing gear is built so that it not only carries the entire weight of the aircraft, but so that it can, through its attachment structure, transfer the landing shock to every other part of the aircraft. If the landing gear is mounted on the wing, the wing must be

A tree is nature's model of load distribution. Air loads on the leaves in wind are distributed downward and inward through progressively larger branches in the same manner as the airloads on aircraft skin are concentrated to larger structural mem-

bers.

designed to not only handle area loads in flight but also

If you should decide to convert your Cessna 150 to a

the concentrated loads of the landing gear during landing. The load transferring structure is much like a tree. For

taildragger, you would face some very challenging problems with load distribution. Cutting off the nose gear is

a tree in the wind, the force of the wind is mainly on the

easy enough, and even deciding where to relocate the

34 MAY 1984

main gear is not awfully difficult, but finding something to attach that relocated main gear to • that is the problem. In the conversion that Ralph Bolen does, that load distribution problem is elegantly worked out. On a 150, the gear legs are made of spring steel and the gear legs are separate. The legs are about % inch

thick. At the top, they are about four inches wide. The shank that enters the fuselage is about nine inches long.

Thus, all of the upward and rearward forces on the gear during a hard landing are transferred to that r>/M by four by nine shank. When you consider the loads and the leverages involved, the transfer of those loads to the light aluminum fuselage is a formidable design problem. On the Bolen conversion, the gear leg shank passes through slots in two heavy aluminum plates about '/•> inch thick and about six inches high and eight inches long. The leg is bolted securely to heavy aluminum brackets on the inner plate. The outer plate serves as a fulcrum, with the gear leg as an unmoving lever. Thus the gear loads are transferred to those two thick plates. The plates in turn are bolted to a box structure. This box structure is made of '/K inch aluminum and runs the width of the fuselage. The gear legs are thus solidly mounted in the box. The box itself is then riveted to the fuselage bulkheads in front and behind. To further distribute the loads to the skin, a '/IB inch sheet is placed under the belly of the plane and outside the original skin. This sheet is then riveted to the box and to the bulkheads through the skin. Since the bulkhead in front of the box serves as the attachment point for the wing struts and the one behind supports the passenger seats, the whole structure is thus made very strong and capable of withstanding the stresses of rough field landings. It certainly cannot be easily pulled loose

truss fashion. But even in the building up of the ribs, the principles ofload distribution apply. The plywood gussets on each joint of the ribs are there to distribute the loads of the joint to the other members and also to provide a larger glue joint surface - which amounts to the same thing. The wooden box spar on which the ribs are mounted has spar caps - the principal load bearing pieces on the top and bottom of the spar - that taper from about 4 inches in width and 2' 2 inches in thickness at the wing root to

only :1/4 inches in width and rV» inches in thickness near the tip. Again, the purpose of this tapering is load distribution and concentration.

from the aircraft, since it is literally the aircraft itself.

On a wooden airplane the airloads on the plywood skin are

transferred to ribs and eventually to a tapered spar.

For composite aircraft such as the VariEze, load distribution and concentration is dealt with primarily by varying the number of plies of glass in the layup at a given point in the structure. The material allows for a very precise tuning of the strength of a given structure, but the caculations of the stresses in such design are beyond the capabilities of most amateur designers.

An example load distribution in a composite aircraft occurs in the wing-to-winglet joint in the VariEze. The

winglet is joined to the wingtip and extends about three

On an aluminum airplane the point loads of the gear are transferred to the skin through a series of successively larger and

lighter plates.

In a wood airplane the same principles ofload distribution and load concentration apply, except that wood as a

building material imposes a few more limitations of the material itself. In the Falco, for instance, the outer skin on the wings is mostly 2mm plywood. Air loads on it are

transferred to ribs built up of % x V* spruce in the usual

feet vertically. Where they join, neither the wing nor the winglet has any substantial internal structure. Each has a Styrofoam core and four plies of fiberglass skin laid up directly over the foam. They are joined by eight plies of

glass on the inside of the joint and eight more on the outside. But all the plies are not the same length. On each surface there are eight plies at the joint - the point of greatest load - but some plies extend only eight inches from the joint. Others extend as much as fifteen inches from the joint. The result is a gradual transfer of the joint load to the skin of the wing and winglet.

SPORT AVIATION 35

A fiberglass fishing rod will bend in a parabolic curve

if it is properly designed: that is, if the load distributions from handle to tip have been properly made. If you slip a section of rigid tubing over the middle section of that rod and try casting with it, you will not only destroy the action of the rod, you will likely destroy the rod, because the flexing loads will be concentrated at the ends of the tubing, and the rod will probably fail at those points. Laying up

extra plies of glass on a composite aircraft may have the

same effect on the airplane as the tubing has on the fishing rod. Instead of making the structure stronger, it may actually weaken it by shifting loads to weaker areas. The same sort of thing may happen with wood or metal aircraft. A well-intentioned beefing up of the outer portion of a wing spar on a wood or metal aircraft will make that

portion of the structure less flexible and have the result of concentrating the stresses inward toward the wing root.

The result will probably be a weakening of the wing. Designing for load concentration and load distribution is both science and art. Most of it is done by computer now, though it is said that Anthony Fokker did it by eyeball. If he did, it was an exquisitely calibrated eyeball. But done by computer or by eyeball it is an extremely important factor in both the strength and efficiency of an aircraft design.

Search for Flight

The bending loads on the joint between the wing and the winglet on the VariEze are distributed to the skin through multiple plies of fiberglass with the most plies at the joint and fewer plies outward.

With composite construction, special attention must be paid to the flexibility of the structure. Even metal structures flex. The wing tips of a B-52 are said to be capable of flexing as much as seventeen feet. But in small aircraft, those built of composites probably flex more than the others, and the design must take such flexing more into

account.

I've envied red-tailed hawks in flight And eagles with their bowed and tilting wings — Whose primary feathers fingered banks Into the thermal's rise. I've seen them feel the breeze To row, and pull, and drag And sweep through depths of wind And curves of air. I've been hauled by silver jets From place to place With herds of others — In a massive streamlined box Of computerized trajectory — More like an elevator's lift Than like the flight of birds. The trainer that I learned to steer Gave me some control But lacked identity — A mere machine, built in a distant city, By faceless men To a formula — and a market.

A fishing pole bends in a parabolic curve, but if some part of it Is reinforced as by putting a rigid tube around it, the bending loads will be concentrated at the ends of the tube and the pole will probably break at those points. 36 MAY 1984

But this plane I built, I know. I know and trust the strength Of spars as I trust my bones. I felt and shaped the gusseted longerons. The glued and bradded ribs Are part of me. I feel the tension of the cables; I know the friction of each pulley And the crimp of every sleeve and thimble — They're like my joints and sinews. I will my wings into a climbing turn And feel the g's against my structure — At last I know This flight is mine. - John Sterling Harris