Flying - - CONTENTS - By Rob Mark

Air­foils and how a wing cre­ates lift

An air­foil, or a wing, as pi­lots more com­monly re­fer to it, when viewed from the wingtip look­ing back at the fuse­lage, nor­mally of­fers a curved shape, with the front, or lead­ing edge, be­ing thicker than the rear, or trail­ing edge. Air­foils are, of course, also used in the con­struc­tion of pro­pel­lers and he­li­copter ro­tor blades.

Air­foils are cre­ated in a va­ri­ety of shapes, many of which orig­i­nated with the Na­tional Ad­vi­sory Com­mit­tee for Aero­nau­tics (NACA) in the late 1920s and through the 1930s. Some NACA air­foils come with ra­zor-thin lead­ing edges. Some are flat on the bot­tom, while oth­ers are nearly con­cave at the trail­ing edge, with oth­ers be­ing sym­met­ri­cal above and be­neath the wing. Each of­fers very dif­fer­ent flight char­ac­ter­is­tics and pe­cu­liar­i­ties from which en­gi­neers choose de­pend­ing upon the de­sired per­for­mance char­ac­ter­is­tics of the air­craft they’re de­sign­ing.

Some air­foils cre­ate great climb per­for­mance, while oth­ers pro­duce faster cruise speeds. No mat­ter the shape, all air­foils are cre­ated for the same pur­pose: to cre­ate lift as air moves over both the up­per and lower sur­faces. Physics tells us that the faster the air moves across the sur­face of an air­foil, the more lift a wing cre­ates. An un­der­stand­ing of how an air­foil ac­com­plishes its work does de­mand a grasp of some fun­da­men­tal terms, in ad­di­tion to lead­ing and trail­ing edge.

En­gi­neers cre­ate an imag­i­nary ref­er­ence point in the de­sign process, called the chord line, that runs from the

lead­ing edge straight through to the trail­ing edge. The air­foil’s over­all cur­va­ture is known as the wing’s cam­ber. The an­gle at which a wing is at­tached to the fuse­lage that of­fers a slightly up­ward tilt is re­ferred to as the an­gle of in­ci­dence.

How­ever, un­der­stand­ing how an air­foil ac­tu­ally cre­ates lift is much like study­ing weather; sim­ply know­ing the ter­mi­nol­ogy and the shapes is only a starting point. So im­por­tant is a deeper com­pre­hen­sion of what makes an air­foil work that Wolfgang Langewi­esche, a famed test pilot for Cessna Air­craft and Chance Vought, ex­plained the “why” in the first few pages of his 1944 aero­dy­nam­ics clas­sic, Stick and

Rud­der. “At this very mo­ment, thou­sands of men try­ing to learn to fly are wast­ing tens of thou­sands of air hours sim­ply be­cause they don’t re­ally un­der­stand how an air­plane flies; be­cause they don’t see one fact that ex­plains just about every sin­gle thing they are do­ing; be­cause they lack the one key that with one click un­locks most of the se­crets of the art of fly­ing.” The key el­e­ment to which Langewi­esche was re­fer­ring was an­gle of at­tack. An in­ad­e­quate un­der­stand­ing of an­gle of at­tack’s role in the per­for­mance of a wing is one rea­son that to­day, loss of con­trol is the largest cause of fatal accidents in air­craft of all shapes and sizes.

To un­der­stand how an­gle of at­tack fits into the role of an air­foil de­mands defin­ing a few more im­por­tant terms. One is rel­a­tive wind, the name at­tached to the air mov­ing op­po­site the di­rec­tion the air­craft is trav­el­ing. The an­gle mea­sured be­tween the wing’s chord line and the rel­a­tive wind then is the true def­i­ni­tion of an­gle of at­tack. As the an­gle of at­tack in­creases, so does the lift gen­er­ated by the air­foil. How­ever, the an­gle of at­tack can only be in­creased to a point. The crit­i­cal an­gle of at­tack de­fines the re­gion in which the wing be­gins to stop pro­duc­ing lift be­cause air no longer flows smoothly above and be­neath the wing. This point at which a wing stops pro­duc­ing lift is called a stall. Wings can stall in any flight at­ti­tude, from nearly level flight to de­scents, climbs and in turns.

Avoid­ing stalls de­mands a thor­ough un­der­stand­ing of both Bernoulli’s the­o­rem and Sir Isaac New­ton’s three laws of mo­tion, although Langewi­esche saw Bernoulli’s the­o­rem as noth­ing more than “an elab­o­ra­tion and a more de­tailed de­scrip­tion of just how New­ton’s law ful­fills it­self” in the pro­duc­tion of lift.

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