TECH­NI­CAL­I­TIES

How is it best to build a bird? Go be­neath the skin to find out.

Flying - - CONTENTS - By Peter Gar­ri­son

In our March is­sue, a short ar­ti­cle about the Vul­canair V1.0 - an Ital­ian four-seater strongly re­sem­bling a Cessna 172 - men­tioned that ist uses "a steel-frame and alu­minum struc­ture, which was the stan­dard for decades". I beg to dif­fer. It's true that the Vul­canair has a steel frame un­der skin - I sus­pect only it is not quite true that this has ever been the stan­dard style of con­struc­tion, let alone for decades. It is a rel­a­tively unsual hy­brid of two styles. Two other com­pa­nies that have used this sys­tem, Mooney and Meyers, come to mind; I’m sure there have been oth­ers.

Own­ers of air­planes built this way of­ten claim that the steel cage in which they sit makes them safer, but the man­u­fac­turer’s rea­son for us­ing it might have been some­thing else en­tirely. Thin-walled steel tub­ing, welded into a web of tri­an­gles, does make a rigid and light struc­ture. It is also con­ve­nient. Hard points for at­tach­ing wings and land­ing gear are eas­ily pro­vided. Big holes for win­dows and doors can be read­ily framed. Per­haps more im­por­tant, a hack­saw, a file, a weld­ing rig and a fix­ture to hold things in place are all the equip­ment a fledgling ama­teur builder or an un­der­cap­i­tal­ized startup needs to get into air­plane build­ing. For all th­ese rea­sons, steel-tube frames,

usu­ally skinned with wood and fab­ric, were preva­lent well into the 1930s. They re­mained pop­u­lar with home

builders un­til, in the 1960s and ’70s, first alu­minum and then com­pos­ite struc­tures sup­planted them.

A steel cage sounds safe, but where crash­wor­thi­ness is con­cerned, the light­weight truss-work sur­round­ing the cabin of an air­plane is a far cry from, say, the roll cage of a race car. It is ap­prox­i­mately sim­i­lar in strength to an all-alu­minum or

all-com­pos­ite struc­ture, as it log­i­cally must be since air­planes are de­signed not to be su­per­flu­ously strong but rather as weak as they can be while still ful­fill­ing fed­eral cer­ti­fi­ca­tion re­quire­ments. Strength is weight, and weight is bad. As we have learned from well-pub­li­cized ef­forts to make cars safer, the crash­wor­thi­ness of a pas­sen­ger com­part­ment owes more to as­tutely de­signed en­ergy-ab­sorb­ing “crumple zones” ex­tend­ing out­ward from the cabin than it does to a su­per­strong shell or cage im­me­di­ately en­cas­ing it.

The type of struc­ture that has ac­tu­ally been “the stan­dard for decades” among air­plane man­u­fac­tur­ers is alu­minum semi-mono­coque. This was the struc­tural medium of every Cessna, Beech, Piper, Boe­ing and what-have-you built in the half-cen­tury fol­low­ing World War II.

The term mono­coque, which means “sin­gle shell,” prop­erly de­notes struc­tures that have no in­ter­nal fram­ing what­so­ever. A tip tank, for in­stance, might be a pure mono­coque: Its outer shell car­ries all the weight of the fuel, with­out any in­ter­nal spine or frame other than what­ever lo­cal re­in­force­ment is re­quired for at­tach­ment to the wing. A fuse­lage could be a pure mono­coque, but large cutouts for doors and win­dows, and hard points for at­tach­ing wings and en­gines, re­quire lo­cal re­in­force­ments that be­gin to look some­thing like a par­tial frame. Con­sider, for in­stance, the fuse­lage of a Cessna Sky­hawk. Open the doors, and there ap­pears to be al­most noth­ing be­tween the tail cone and the en­gine. All the loads pass through the floor, the ceil­ing and the wind­shield posts. Strong built-up sec­tions sur­round and re­in­force the door and win­dow open­ings. From the point of view of crash­wor­thi­ness, there is no rea­son th­ese bent and stamped alu­minum an­gles, chan­nels and box beams should be in­fe­rior to a welded-steel frame.

The dis­tinc­tion be­tween skin and frame is not al­ways easy to make. If you peer back into the tail cone of any metal air­plane, you see a series of sheet­metal rings, typ­i­cally a cou­ple of feet apart, that give the cone its shape. A num­ber of stiff­en­ers, ex­truded or else bent up out of sheet metal, run length­wise. Al­though this in­ter­nal struc­ture gives the im­pres­sion of be­ing a “frame” — and in­deed the ring­like ele­ments hap­pen to be called “frames” — it is ac­tu­ally a col­lec­tion of stiff­en­ers that are loosely con­nected to one an­other, if at all. It has no in­tegrity of its own. Take away the outer skin, and the in­ter­nal ele­ments sag shape­lessly or fall to the ground.

Stiff­en­ers are needed be­cause, in sheer vol­ume, the largest struc­tural mem­ber in an air­plane is its skin. For the air­plane to be light, the skin must be thin, but the thin­ner it is, the less able it is to carry loads, es­pe­cially com­pres­sive ones, with­out de­form­ing. Typ­i­cally, the alu­minum skins of air­planes in the un­der-4,000-pound weight class are be­tween a 30th and a 50th of an inch thick. Thin­ner skins im­ply a more ef­fi­cient struc­ture, but ones that re­quire more closely spaced stiff­en­ing mem­bers and are ac­cord­ingly more costly and la­bor-in­ten­sive to man­u­fac­ture.

Thin skins be­tray them­selves by a wrinkly or wavy ap­pear­ance un­der load, and by more nu­mer­ous lines of riv­ets where in­ter­nal stiff­en­ers are at­tached. Thicker skins, though heav­ier, re­quire less in­ter­nal stiff­en­ing and so can be smoother. If you com­pare a Chero­kee’s wing to a Bo­nanza’s you read­ily see the dif­fer­ence; the skins of the Piper are smoother and the rows of riv­ets are far­ther apart. The dif­fer­ence is due in part to Piper’s de­sire for sim­plic­ity and low parts count, but also to the hope that by elim­i­nat­ing rivet lines ahead of the main spar the Chero­kee (or the ear­lier Co­manche) might get a lit­tle speed ben­e­fit from its lam­i­nar-pro­file wing.

Sur­face smooth­ness is a muchtouted ben­e­fit of com­pos­ite con­struc­tion. Pre­sum­ably, it con­trib­utes some­thing to per­for­mance. There is am­ple ex­per­i­men­tal ev­i­dence of the drag penalty as­so­ci­ated with sur­face rough­ness or wavi­ness. Any self-re­spect­ing sailplane uses ex­treme lam­i­nar pro­files, well buffed, and its pi­lot will no­tice changes in its be­hav­ior when rain­drops dis­turb the flow on its wings. But most of the sur­face area of a sailplane is wing. How large the ben­e­fit of smooth­ness is on air­planes with com­par­a­tively high wing load­ings — air­planes, that is, whose wings ac­count for a rel­a­tively smaller frac­tion of their to­tal drag — is more doubt­ful. The dif­fer­ence be­tween a dirty sur­face and a clean one is a frac­tion of a frac­tion. Nev­er­the­less, it is in­dis­putable that if you care­fully wash and pol­ish the bug ceme­tery that is your lead­ing edge you will no­tice that your air­plane flies a lit­tle faster, just as, af­ter you clean the wind­shield of your car its en­gine runs more smoothly.

Al­though alu­minum mono­coque con­struc­tion is still the stan­dard to­day, it is grad­u­ally giv­ing way to glass- and car­bon-fiber com­pos­ites, which have, in ad­di­tion to the virtue of sur­face smooth­ness, those of su­pe­rior strength-to-weight ra­tio and of ready forma­bil­ity to any de­sired shape. A more sub­tle, but not neg­li­gi­ble, as­set of com­pos­ites is the uni­tary qual­ity of their con­struc­tion. Un­like alu­minum struc­tures, which are stitched to­gether out of many small pan­els us­ing thou­sands of riv­ets, com­pos­ite struc­tures tend to be made in sin­gle large parts — half of a fuse­lage, or the top or bot­tom of a wing. Th­ese ma­jor com­po­nents are then bonded on long, lightly loaded seams, elim­i­nat­ing the stress con­cen­tra­tions that can lead to even­tual fa­tigue and crack­ing in a riv­eted sheet-metal struc­ture.

This phys­i­cal con­ti­nu­ity of struc­ture is a prop­erty that com­pos­ites share with frames of welded steel tub­ing. Their joints too con­sist of con­tin­u­ous and ho­mo­ge­neous ma­te­rial, with­out a dis­tinct boundary be­tween a part and its neigh­bor. Those who built air­planes of welded steel in days of yore must have been in dis­be­lief when new­com­ers — who turned out to be prophets of the fu­ture — pro­posed dis­card­ing an in­ter­nal frame en­tirely. How would they build the air­plane? Out of a patch­work of metal sheets. And how would they con­nect them to­gether? Why, by punch­ing holes along the edges, stick­ing nails in them and squash­ing them flat!

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