Tapered Wings Are For Birds....And Very Large Airplanes

I do not profess to know why theCreator elected to install tapered

wings on birds. I am sure at leastthat there is a tie-in with the me-chanics of "flapping wing" flight. Iam also sure that tapered wingsshould NOT be used on little air-planes - those of a size buildableby amateurs (whose wings do notflap much).

Why then are tapered wings sofrequently used by amateur and pro-fessional builders alike? Ignorancecan be charged in some cases, butin the case of at least one majormanufacturer it is known that thedecision was based upon purelyaesthetic reasons, even at a recogniz-ed cost penalty. It costs a manufac-turer at least a little more to buildtapered wings than straight ones, andit costs the homebuilder countless ad-ditional hours of needless labor.

How did it happen then that taper-ed wings became the style? It cannow only be surmised that in an effortto eliminate external wing bracingearly aeronautical engineers were in-spired by cantilever bridges, whichprobably could not even support theirown weight were they not tapered!The weight advantage of greaterbeam depth at point of maximumbending is theoretically true in a can-tilever airplane wing; also, taperingbrings the lateral center of pressure

EDITOR'S NOTE: The accompany-ing graph shows the relationship be-tween lift and angle of attack foran NACA 0015 airfoil at two differ-ent Reynolds numbers. The lower Rey-nolds number corresponds to a 1 ft.chord at 35 mph. The higher curvewould then be about a 4 ft. chordat this same speed of 35 mph. Thisthen is a taper ratio of about 4 to 1.

Note that at the higher Reynoldsnumber (larger wing chord) the liftis 20% higher. However, the impor-tant feature is the angle of attackat which the stall occurs. Note thatthe stall for the low Reynolds numberoccurs at an angle of 12° while theangle of stall for the larger wingchord is 15°. This shows that the tipwill stall much sooner than the rootof a tapered wing - the cause of theviolent rolling tendencies at stallpointed to by Mr. Thorp in his arti-cle.

The graph is taken from data foundin NACA Technical Report No. 586(1937).

By John W. Thorp, EAA 1212

of the wing closer to the side of thefuselage, thus reducing the wingbending that the structure must sus-tain.

In the application of tapered wingsin the design of small airplanes, how-ever, two important factors are over-looked. First, because some weightof material is required to give awing its aerodynamic shape the struc-ture thus employed is capable oftaking some wing bending. Theadditional material required to makethe wing strong enough to sustainall the bending is a small percentageof the total wing weight - thereforweight saving by greater structuralefficiency through tapering is small,even if the wing area is the same,tapered or rectangular. Secondly,aerodynamic scale effect makes itpossible to use a smaller rectangularwing for a given stalling speed, thusthe tapered wing loses its smalltheoretical weight advantage.

Reynolds number is a device usedby technicians to relate scale effectsin both aerodynamic and hydrody-namic flow. Reynolds number is pro-portional to velocity and proportionalto length of body in the flow direc-tion, and inversely proportional tothe kinematic viscosity of the fluid.Applied to a wing, Reynolds numberis proportional to the speed of flightand proportional to the chord of the

Of

wing. You can forget about kine-matic viscosity if comparisons aremade under conditions of standardair.

The maximum lift coefficient of awing section is a function of its to-

tal Reynolds number. At large Rey-

nolds numbers (big wing, high mini-mum speeds, or both) the effect ofscale on maximum lift may be small,but at low Reynolds numbers (smallairplanes and low landing speeds)the scale effect is very large.

When a wing is tapered, the Rey-nolds number at the tip and rootof the wing are in the same orderas its taper ratio. A highly taperedsmall airplane wing, then, has a muchlower Reynolds number and maxi-mum section lift coefficient at thetip than at the root. A moderatelytapered wing has somewhat lowertip section Reynolds number andmaximum lift coefficient than theroot, while a rectangular wing has aconstant Reynolds number and essen-tially the same maximum section liftcoefficient all along the span.

Wing stall starts where some sec-tion along the span first reaches itsmaximum section lift coefficient. Atapered wing because of Reynoldsnumber tends to stall at the tip first,while a rectangular wing tends tostall at the center of the span becauseof Reynolds number and span-wiseload distribution.

The loss of lateral control associat-ed with tip stall is obvious, and manysmall airplanes with tapered wingshave exhibited violently bad rollingtendencies at stall. I won't singleout any of these, but almost all smallairplanes with notoriously bad stall-spin accident histories have had ta-pered wings.

The tip-stall problem of small ta-pered wings can be solved by oneor more of the following techniques:

1. Wing twist (decreased inci-dence at the wing tips).

2. Increased section camber atthe tips.

3. Blunt radius on leading edgeof tip airfoil.

4. Wing slots (both fixed andautomatic opening).

Wings may be twisted so that thetip section is at a low enough angleof attack as to not stall when theroot section gets to its maximumangle of attack. This means that thewhole wing lifts less and more areawill be required for a given stallingspeed than if a rectangular wing isused.

Increased section camber and bluntleading edge radius increases drag.

Fixed slots increase drag, and auto-matic slots increase weight.

Continued on page 30SPORT AVIATION 7

"Queen Bee" . . . from Page 6the tail tips, the canopy, the backlighted instrument panel, tail cone,and several interior appointments. Asimple flash light serves both as acanopy dome light and an instru-ment safety light. It is also remov-able for night walk-around inspec-tions.

The wing flaps are continuous fromaileron to aileron and constitute six-teen percent of the airplane's totalwing area. Short field landings havebeen demonstrated with these veryeffective electrically powered flaps.The wing flap control switch is in amost convenient location . . . on topof the stick.

Where most aircraft employ twocontrol wheels or sticks and onethrottle, the "Queen Bee" utilizes onecenter control stick and two throttlesfor its fully dual control arrange-ment. This feature has really madea business man's dream of this air-plane with plenty of leg room andspace for four or five passengers tobe comfortable on long range flights.

People who have flown the "QueenBee" claim that it is truly a pilot'sairplane when it comes to instrumentarrangement, power plant controls,circuit breakers, wheel brakes andtrim tab control placement. Every-thing seems to be right where it be-longs, which makes the pilot veryhappy.

Homebuilders . . . from Page 2new understanding of your organi-zation and its financial status, andof course the March issue of SPORTAVIATION added to your knowledge.

I would like to add some personalcomments to this column regardingmy work with this movement. First,I would like to say thanks for themany kind and personal letters thatmany of you have taken the time towrite. These have done much to keepme inspired as it is quite a task toanswer the many thousands of let-ters, perform Association administra-tive duties, receive personal visitsby members and prospective mem-bers, and many other items too num-erous to mention, and at the sametime completely fulfill my employ-ment obligations as an Aircraft Main-tenance Officer for an ANG jet fight-er squadron, maintain my militarypilot flight proficiency in both jet andreciprocating aircraft, and of course Icannot let slip by my obligations as afather to my two wonderful chil-dren, and husband to Audrey, oneof aviation's most understandingwives who has been most patientand invaluable to our movement.30 APRIL 1960

There are others here at Head-quarters who also have shown ex-treme patience and kindness, suchas Bob and Lois Nolinske, GeorgeHardie, and Val Brugger. I believemany of you will attest that it is noeasy task to get human beings to allpull together, but I am proud tosay that we here have thus farachieved this goal. For without it,EAA would not be what it is today.

In visiting many of the membersand chapters I find that one of thegreatest problems confronting thegroup is that of human relations, un-derstanding each other, and tryingto develop team work. Once the lead-ers, using a little diplomacy, makethe break through, great advancescan be obtained. True, by nature weall do not fit into the same pattern.Some are more tolerant than othersand the barbs of criticism sting withvarying degrees. When it comes tothis I often times wish that my skinwere a little thicker, but as the oldsaying goes, "one will feel and re-member a jab in the ribs a lot longerthan a pat on the back."

A few closing reminders for thoseof you who have not as yet sub-mitted your EAA Membership Sur-vey ballot, please do so. It is veryimportant that we have this informa-tion.

For you chapter members — haveyour local officers been reading themonthly chapter bulletins to your.croup? This bulletin has been pub-lished monthly for the past twoyears. In checking our records wehave found that some chapters havefailed to furnish National Headquar-ters with a copy of their minutes oftheir monthly meetings. This was tohave been accomplished by March1, 1960 (Dec. 1959 chapter bulletin).Check with your chapter officers tosee if this has been accomplished tokeep your chapter on our active list.

Tapered Wings . . . from Page 7On large airplanes the Reynolds

number effect is small and the weightsaving is large, hence the additionalcost of construction is justified. Onsmall airplanes designed for a givenstalling speed and stalling accept-ability the airplane with the rectang-ular wing will have less wing area,therefor less wing drag, and will gofaster on a given power. It will al-so be much simpler for the amateurto build.

So-o-o-o . . . "Handsome is as hand-some does". In time we may beable to convince ourselves that rec-tangular wings do look better thantapered wings. Even some birdshave almost rectangular wings! £

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FACTS ON AILERONS . . .From page 16to be rather stiffly made to take all the control andair loads at its pivot point and distribute it without de-flection. In short, tip ailerons and their controls canwork out to be heavier and more complex than plainailerons, and the problem grows more acute with in-creased span.

A tip aileron is of course an extension of the wing,and normally would have the wing's airfoil if a noticeableand objectionable discontinuity is to be avoided wherewing ends and aileron begins. Given any commonly usedwing airfoil, the center of pressure of a tip aileron willhave that airfoil's normal amount of center of pressuretravel with varying angle of attack. This quirk leadsdirectly to problems of control linkage and leverage.NASA found that tip ailerons usually resulted in ap-preciably heavier control stick loads—up to three timesas great for plain ailerons giving the same amount ofrolling action. If the pivot point is located reasonablyfar back on the tip aileron, there is of course a greateraerodynamic balancing effect, which would be usefulin reducing control loads if it were not for one thing . . .it appears to aggravate tip aileron flutter problems. Youcan stop flutter by moving the pivot point forward, butin so doing you reduce aerodynamic balance and increasethe control forces. The use of aerodynamically efficientlong, narrow tip ailerons leads to a reduction in struc-tural stiffness and an increased proneness to flutter. Toget away from this by going into short, wide tip aileronsleads to increased control loads because an airfoil ofgreater chord and consequently greater center of pres-sure travel is obtained.

In the end, the lightest, simplest, best-performingmethod of minimizing adverse yaw effect proved to bethe Frise type aileron, and retention of aileron controlwell into the stall is now obtained by wing twist, whichmakes the root stall first and the tip stall last. Tipailerons mess up the wing tip vortices and add drag, theyspoil climb, they are structurally heavy, and pose somuch of a flutter and control force problem that theyseem a profitless path for designers to follow. This isnot to say that they won't work or can't be made to work,but that since it is possible to gain the desired advantagesmore easily and reliably by using Frise ailerons andtwisted wings, the fellow who wants to get into the airwith minimum effort and maximum safety would dowell to stick to common aileron forms. After all, commonfeatures of today's airplanes are used not because of thedesigner's lack of imagination but because they are theresult of many decades of work to find the most practicalways of making airplanes! Q

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BUSHBY'S "LONG MIDGET" . . .From page 6 'higher pitch prop would give an improvement. A con-trollable pitch prop would of course be the best.

The ship turned out to be heavier than the 525Ibs. empty weight of the original built by Dave Long -Bob's weighs 572 Ibs. empty. Additional weight isdue to the canopy, cowl and thicker spar stock usedin the wing. Wing loading works out to about 10 Ibs.per sq. ft. The ruggedness of the ship is evidencedby the 7% G's registered in flight test.

Bob is completely sold on the superiority of metalconstruction over "tube and rag". He says the actualhours required for construction are no more for metalthan the more common fabric job, if everythingis counted. He figures that it's possible for the averagebuilder to build a "Midget" in about two years ofspare time work if he's not held up by a lack of capi-tal, an overly-demanding full-time job, etc. Durabilityof metal over fabric, with economy of upkeep as an add-ed feature, makes the former extremely attractive.He says he was comparatively inexperienced at the startbut close study of CAM 18 and the assistance of availableinformation helped a lot. In particular he found the"Tin Bender" articles by EAAer Jim Graham (seethe EXPERIMENTER for July, September, October andNovember, 1957) to be very helpful.

Bob works in the research department of SinclairOil Co., and is a licensed A & E mechanic and a com-mercial pilot with flight instructor's rating as well.He does A & E work part time, and his "spare" timehas been devoted to the "Midget" project. He hasoffered to assist other "Midget" builders in EAA -his home address is 14612 S. Edbrooke Ave., Dolton,111. A

Error in "Tapered Wings"An unfortunate typographical error appeared in

John Thorp's article "Tapered Wings are for Birds -and Very Large Airplanes" in the April issue of SPORTAVIATION, which changed the meaning of one sen-tence. On page 7 at the bottom of the center columnthe sentence should read "The maximum lift coef-ficient of a wing section is a function of its local Rey-nolds number".

Author John Thorp is preparing other factual andeducational articles which will appear in future issuesof SPORT AVIATION. Next to appear will be "WhichAirfoil Section?" in the June issue. "How Much As-pect Ratio?" and "Performance at a Glance" will betwo in the series to follow. We indeed appreciate theopportunity to present this valuable information author-ed by one of aviation's outstanding aircraft designers.

ONEMAN

HELICOPTER• True copter, no belts, chains or power plant shortcuts.• You build from our drawings.• Parts available.• Send $2.00 for photo, 3-view drawing,

specifications and design information.• Send $2.00 for our book "Basic Helicopter Aerodynamics",

uses above helicopter as example problemfor performance calculations.

H E L I C O P T E R R E S E A R C H C O .BOX 121

LA MIRADA, CALIFORNIA

28 MAY 1960