(PART ONE)

Wing Span... The Vital Factor By Bob Whittier, EAA 1235 Box 543, S. Duxbury, Mass. Thirty years later the same aercdynamic principles have been put to use by EAA member Jim Bede in his BD-2. With only 210 hp it is able to carry enough fuel to fly

around the world non-stop. Low span loading is the key to this capability.

UST TO REACH a sensible cruising altitude of 1,000 ft., the early 40 hp Taylor Cubs had to use full throttle all the way up when carrying two men on a hot summer day. But the same basic airframe, cleaned up, beefed up, and fitted with a 125 hp engine, went out one day and quite handily set a world altitude record of 30,203 ft. for light aircraft in its class. Powered by the 1,600 hp Rolls-Royce Merlin engine, the Supermarine Spitfire Mark VII had extension wing tips which increased the span to 40 ft. 2 in., where the normal span was 36 ft. 10 in. And where Spitfires with the same engine and normal span could climb to 37,750 ft., the Mark VII could ascend to 41,000 ft. for photo reconnaisance work. Another version of the Spitfire, the Mark XII, was equipped with the 2,000 hp Rolls-Royce Griffon engine and had stub wing tips which reduced span to 32 ft. 8 in. It was a fast machine, and could climb at a rate of 4,750 fpm. Yet the Mark XIV with the same Griffon engine but having the normal 36 ft. 10 in. span could ascend at

J

5,500 fpm! Every airport frequenter has noticed that the fa-

miliar Seabee amphibian has a mediocre rate of climb. As manufactured by Republic, it was fitted with an engine having a special extended shaft to carry the propeller behind the wing trailing edge. Because of this special engine, it has not been feasible to repower the ship with later and more powerful engines to improve the climb. So a popular modification worked out in recent years features the addition of extension sections to the wing tips. Climb is improved.

Powered w4th the usual 100 hp Kinner engine, the faithful old Fleet biplane was just an ordinary climber. But in recent years crop dusters and sport aviation enthusiasts have attached more powerful engines to this ship's

nose and some of the results have been spectacular — 5,000 to 5,500 fpm with 190 to 220 hp engines! When you look through aviation history books for pic-

tures of airplanes that set distance and altitude records, you notice they all had one thing in common — unusually

long wing span. Coming down through the years to our day, the famous U-2 spy plane, able to fly great distances at exceptionally high altitudes, has as its most noticeable feature a very long wing. So does Jim Bede's BD-2 roundthe-world non-stop airplane.

So what does all this have to do with homebuilt airplanes? Plenty! Every "eyeball engineer" knows all about wing loading and power loading — but it's a rare one indeed who has even heard of span loading, much less understand how to make use of it in design work. The term "span loading" is a familiar one to professional designers and they are apt to think of it first before going on to wing loading and power loading. An airplane's span loading is simply its weight divided by its wing span. A 700 Ib. homebuilt with 25 ft. span has a span loading of 20.8 Ibs. per ft. of span. This probably means no more to you that the formulae a nuclear physicist has scrawled on his blackboard. But span loading is very important. There happens to be a rea-

son why successful lightplanes of the 1930s had wing spans of from 35 to 39 ft. There happens to be a reason why successful midget biplanes of today have 65 hp as a very minimum, and more often have 85 to 125 hp. This reason involves the mathematical laws governing induced drag.

"Induced drag" is one of those weasel-like engineering terms you can get quite accustomed to reading and hearing, but find it hard to visualize in practice. But to understand span loading you've got to understand induced drag! You've probably read or heard that two-thirds of a wing's lift comes from the upper surface. Interpreted literally this statement is misleading, for it implies that air sucks up so hard on the top surface that even if all lift on the lower surface were spoiled, you'd still have twothirds of your lift. We can't go off into airfoil theory (Continued on next page) SPORT AVIATION

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WING SPAN . . . (Continued from preceding page)

here, but we've got to straighten you out on this notion. A less misleading way of explaining how an airfoil works is to say that if it is shaped so that air pressure is reduced over its top surface, the lower surface can push up to better advantage. But fundamentally, and disregarding the complexities of airfoil theory, it is absolutely true that when all is said and done, a wing creates lift, forcing a mass of air downward. After all, a propeller creates thrust by hurling a mass of air backward, and a propeller is merely a rotating wing. The fact that a wing taken as a whole produces lift by forcing air downward can be visualized in a smoketype wind tunnel. Anyone can readily demonstrate it to his own satisfaction in any small airplane by attaching a length of string to the wing's trailing edge with scotch tape. In flight the string can be seen to maintain an angle with the wing chord noticeably greater than the wing's actual angle of attack with the relative wind. This downward deflection of the air is equivalent to the airplane climbing out of its own down-wash. Level flight is merely a condition of climbing in which the plane's rate of climb has been reduced to a point where its upward velocity is exactly equal to the downward velocity its wing has imparted to the air. In other words, in level flight an airplane is continually climbing away from the air that it has forced downward. Power is expended to maintain this climbing process. For the airplane to gain altitude it must climb through the air at an increased rate, so more power must be applied. "Induced drag" is the power penalty involved in forcing air downward. It is the drag which results from the

creation of lift by a w.'ng. It is that portion of the engine's power expended in keeping the plane aloft and making it climb, cs opposed to that which is expended to pull the plane forward.

An airplane's climbing ability depends on how much power it h?s left over after the engine has been set to keep it in level flight. If it takes most of a very small engine's power to maintain altitude, there is little left with which to make it climb. One of the performance phenomena mentioned at the beginning of this article involved the cmazing increase in a Fleet biplane's climbing ability brought about by doubling the engine power. The extra climbing ability comes from the generous surplus

(Stouffer's Phvoto Service)

Here is a 1935 Aeronca C-3. With only 36 hp it nevertheless had useful performance while carrying two persons. Wing span 36 ft., chord 50 in., aspect ratio 9 to 1. This long wing thus had low induced drag and that gave acceptable climb in spite of low power. When power is fixed, you increase climb by increasing wing span. 28

JANUARY 1968

( S t o u f f c r ' s Photo Service)

This 1929 Fleet Biplane originally had a 100 hp engine. When repowered with a 220 hp engine its rate of climb increased spectacularly. When wing span is fixed, you increase rate of climb by adding power.

of power left over after using some of it to maintain level flight. Adding power obviously improves rate of climb very well. But when power is limited, such as when we are trying to fly with some interesting small engine which happens to be available, then what? Well, we could try to reduce the induced drag, to make more of our limited power available for climbing. Increasing the wing span reduces induced drag. This statement is based on the aerodynamic calculations of Dr. Prandtl of the University of Gottingen, one of the early students of the mathematical laws of flight. At any given air speed and density, his calculations demonstrate that induced drag is determined by the ratio of weight to span. Said another way, if weight remains constant and span is reduced, induced drag increases. An increase of induced drag robs power and reduces the amount available from a small engine for climbing. Said yet another way to try to help you grasp this ephemeral thing, increasing the span (while weight remains the same) spreads the plane's weight out over more feet of wing span. With each foot of span therefore lifting

And here's wing span at work in modern form. Designers of the Lockheed U-2 spy plane had to get both maximum range and maximum altitude to do the job. Not needing extreme high speed, they employed a medium powered jet engine for moderate fuel consumption. And to get extremely high ceiling cut of mcderate power they turned to generous wing span. That in turn produced a bonus in the form of long range. Span and power always work together.

less of the plane's weight, less power is needed — or more of what power is available is left over for climbing. Climbing ability and ceiling are of course closely related. If you want good climbing ability for taking off and clearing obstacles, you also get good ceiling, or height to which the plane can ascend. One way to assure that a new design will take off and climb out well is to design it to

have a reasonably high ceiling, say 15,000 ft. Here is a

table which has been worked out to design to that particular ceiling:

Span Loading in Lbs. Per Ft.

35 31 27 25 23 22 21 20.5 20

Power Loading Req. in Lbs. Per HP

15 17.5 20 22.5 25

27.5 30 32.5 35

If generous wing span helps takeoff and climb, it stands to reason that it will also help ceiling. And so it does. This Bristol Type 138A was built in England in the mid 1930s for high altitude experimental work and for a time held world altitude records. Note the generous span. That is the way to get as high as possible on given horsepower.

plane or biplane configuration. To secure a desired wing

area, the two wings of a biplane work out to be consider-

Taking the two top figures, if wing span is short it

will require more power to climb to 15,000 ft. Taking the

two bottom figures, if wing span is generous a small engine will do the job. Note in this table that power loading increases by 2.5 times, but at the same time span loading decreases at a lower rate. This again means that when horsepower is limited, span must be appreciably increased if acceptable climb and ceiling are to be obtained. Wing loading, or the load per square foot of surface, affects only the take-off and landing speeds. In practical design, wing loading is chosen to give the desired landing speed. When the wing loading has been chosen the wing span and chord can be arranged so as to give the most favorable span loading for the area involved. The only

limit is that span cannot be made unduly great, for this will result in small chord, which in turn will lead to long,

shallow spars and consequent strength difficulties. Incidentally, assuming that wing loading has been decided upon, one is confronted with the choice of mono-

ably shorter than the single wing of a monoplane. And

when span is reduced, ceiling and climb are also appreciably reduced! A biplane of 20 ft. span weighing 1,000 Ibs. has a span loading of 50 Ibs./ft. and that isn't even on the foregoing table! It won't climb to 15,000 ft., which means that its rate of climb is also poor. Now you see why successful

small biplanes have high horsepower. The only way to compensate for the induced drag caused by such high span loading is to bring the power loading way down. Power loading and wing loading are useful figures

for comparing one existing airplane with another, especially where commercial airplanes are involved. But in laying down a new design the primary consideration is the relationship between span and power. Increase power and it's possible to reduce span; reduce power and it is

essential to increase span. You can mix them as you wish, to get what you want. But to enter into design work with no appreciation of the direct connection between span and power is to head down the road to disappointment or even disaster.

In recent years we have been lucky to have access

to a good assortment of used engines in the 65 to 180 hp

range. The ability of these engines to overcome induced drag is very good, and there has been a trend to smaller

and smaller wing spans as witness the Tailwinds, Cougars, Midget Mustangs, Miniplanes and Thorps.

However, many of us dream of low-cost ultra-light air-

craft powered by small air-cooled automotive, utility and other engines. A lot of designs have been doodled up to

take such power and some have been built. Many never got off the ground and as for those that did, it soon be-

• S p c c d ' s Photo)

We don't know if the Russians ever claimed to have invented it, but long wing span is an essential factor in any aircraft designed to fly as far as possible on whatever quantity of fuel its power can lift into the air. High aspect ratio reduces induced drag when wing is flying at the higher than normal angle of attack at the ship's best speed

for greatest range. Ship here flew over the North Pole from Russia to California in 1937, setting a distance record. It's an ANT 25.

came apparent that when it came to such practical matters as getting out of grassroots airstrips, clearing power lines, coping with downdrafts, climbing to cruising altitudes on hot days and getting over ranges of hills, they

just didn't have it.

So if we in EAA are going to accomplish anything worthwhile in ultra-light aircraft — we've got to under-

stand that wing span is the key to satisfactory all-around

performance.

(To Be Continued) SPORT AVIATION

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Known as the "Long Tom Project", this military Beechcraft was fitted with detachable wing tip extensions containing additional fuel for long distance delivery flights. The outer auxiliary wing panels were hinged to the wing tips and "flew" themselves Superficially you'd say the increased wing area is what added to the lifting capacity. But by increasing wing span, they also increased cruising range. Basically, it's span loading at work.

WING SPAN...THE VITAL FACTOR By Bob Whittier, EAA 1235 Box 543, S. Duxbury, Mass. (PART TWO)

N A PRECEDING article it was pointed out that wing Ifactors span and engine power working together are the prime governing an airplane's ability to take off and climb.

Practical experience with a wide variety of aircraft has demonstrated that when for any reason wing span is fixed, such as on an existing airplane, rate of climb may be increased by adding horsepower. And when horsepower is fixed, such as when a particular engine is to be used, rate of climb can be increased by increasing the wing span. Not only has the point been proven by practical experience, it has also been explained mathematically by some of the best minds in the field of aerodynamic theory. Shortly after World War I, the Germans developed a keen interest in gliding because the peace treaty forbade them to do much in the field of powered flight. Gliders being about the only thing in the air in the early 1920s, they naturally attracted the attention of air-minded men in German universities. Before long such men as Drs. Prandtl, Bctz and Munk of the University of Gottingen were taking a scientific look at glider design. Mathematicians and hydrodynamicists for the most part, they took to aerodynamics readily and in a short time had developed principles and formulae by means of which glider designers could scientifically design their craft to obtain desired performance characteristics.

At about the same time, aviation enthusiasts in France and England were busy with ultra-light airplanes powered chiefly with motorcycle engines. They adopted the German glider designers' methods in their search for acceptable performance from very limited power and in the years 1922, 1923 and 1924 produced a very interesting assortment of machines. The news made its way across the Atlantic and caught the attention of aeronautical engineer Ivan Driggs, designer of the Driggs Dart lightplane. (See the 1932 edition of Flying and Gliding Manual, available from EAA). In the December, 1924 and January, 1925 issues of a long-forgotten aviation magazine called "The Slipstream Monthly", he wrote some articles on the 18

FEBRUARY 1968

subject of span loading which became classics. They were reprinted by NACA (now NASA) as Technical Memorandums 311 and 326, and in that form were widely read by designers and students of light aircraft. Such well-remembered airplanes as the Aeronca, Curtiss Wright Junior, American Eaglet, Spartan C-2 and others with broad wings

showed the influence of Prandtl and Driggs. TM 311 is divided into two parts. The first is a general survey and summarization of the light airplane field as it was in those days. It goes over the history of the European ultra-light experiments in the early 1920s and the specifications of many ships of that time are tabulated. Some of the observations it makes are of interest even today! 1. Experienced pilots often remarked that they hesitated to fly low (meaning around 1,000 ft.) in heavy, high-powered airplanes due to the amount of space needed to maneuver, their steep glide following engine failure and the dangers attendant to forced landings, and consequently cruised at altitudes of 5,000 ft. and more. 2. At such altitudes travel by air can be very monotonous.

3. The ultra-light is safe to fly at lower altitudes because it needs little room in which to maneuver; it glides well and can make forced landings in small clearings with minimum damage due to light weight and low landing speed. 4. The ultra-light pilot has more of a feeling of moving along swiftly when flying at 1,000 ft. than at 5,000 ft. in a faster plane, and since he can see the sights below so much better, he has a more interesting time of it. 5. Provided that adequate wing span is used, a pilot can fly an 18 to 25 hp single-seater in a useful manner, while a two-seater can be flown from 35 to 40 hp. 6. The safety of ultra-light aircraft in forced landings is a very valuable recommendation for their use in sport aviation. Part 1 ends with a summary of the differences between gliders, powered gliders, ultra-light airplanes and

regular airplanes, and is so specific and pertinent that we will reprint it here:

"A great deal of general discussion has been offered on the subject but as yet no attempt has been made to define the term 'light airplane.' That is a question that is receiving a great deal of attention, both abroad and in this country today. Is a light airplane an engined glider? Is it an underpowered airplane? In the light of what has

been accomplished it is neither. "Of course, as pointed out previously, the original idea was the outgrowth of glider and soaring machine development. In fact, one of the British single-seaters, the

'Wren' could very truly be called a powered glider, as in

1922 the same airframe without an engine had been used in soaring competition.

"However, the problems of gliding and of flying from place to place are widely separated. The glider receives its sustentation from a wind which has a strong upward component. Such a machine is designed so that its sinking speed will be at a minimum and • equal to or less than the rising of the wind in which it is flying. This necessitates a very high ratio of lift to drag at a very low speed. The aim of soaring is to stay off the ground as long as possible.

"Powered flight, on the other hand, has for its purpose the accomplishment of useful work, namely, the transportation of a pay load through the maximum distance in the shortest possible time and at the least cost. This is a problem of range of flight, rather than of duration as is the case of the glider. Winds cannot be depended upon for assistance as it may be necessary at times to fly against them. An airplane will be most efficient in meeting the demands of commercial work when it is least affected by wind. "This means that the cruising speed should be high in order that the percentage reduction in ground speed

experienced in average air conditions may be low. The practical airplane should have a margin over its most efficient cruising speed at least equal to the average velocity of the winds liable to be encountered. "The light airplane, therefore, must have a very high

ratio of lift to drag. The powered glider will have a phenomenal duration but will not be a practical airplane. "Light airplanes are not underpowered in the true

sense of that term. The number of pounds carried per horsepower is much greater than designers have previously deemed advisable in the construction of military

Not all European designs built around the Volkswagen engine have short wings. The Fournier Avion-Planeur makes use of a long wing to achieve good take-off and climb ability. Span is 37 ft. 6 in., loaded weight 770 IDS., and

ceiling 16,000 ft.

No, this is not a "His and Hers" version of the P-51 built

for Paul and Audrey! It is a North American F-82 Twin Mustang. Developed from the basic P-51, its job was to serve as a long range fighter, long range reconnaissance aircraft, night fighter and interceptor. Aerodynamically the arrangement accomplished two things — it increased power and increased wing span. This resulted in increased climb, ceiling and range and better fitted it for these duties. Span and power always work together!

types. This high power loading is the reason for being of the light airplane. For commercial work the greatest

possible load must be carried by the minimum feasible power. Everything else being equal, that airplane which has the highest power loading will be the cheapest both in first cost and in operation. An airplane is underpowered only when it is unable to properly function in the service for which it was intended. "If that service is used to transport a pilot and baggage 200 miles at a speed of 75 mph and passing over a mountain range 12,000 ft. high on the way, that airplane which fails in the accomplishment of this task is underpowered whether it carries 15 or 30 Ibs./hp. "The advocates of the light airplane believe that there are two ways to increase performance, namely, by increasing either the engine power or by decreasing the amount

of power required for flight — and that the latter method is by far the most logical and scientific.

"An increase of power necessitates an increased fuel load, and therefore greater total weight. Consequently,

the cost of the total airplane as to original outlay and maintenance increases. Everyone has heard the statement, 'Give me enough power and I can make a barn door fly.' The light airplane is diametrically opposed to a powered barn door. It may be defined as a scientific attempt to obtain the greatest possible useful work from the least power. Incidentally, this results in an airplane extremely cheap in all respects." Part 2 of TM 311 is an interesting and useful discussion of the formulae developed by Dr. Prandtl of the University of Gottingen for calculating induced drag, parasitic drag and power required, with emphasis on desirable characteristics for ultra-light aircraft. Even today it could serve as a useful text for the amateur designer. It is no longer easy for private persons to borrow copies of old NACA reports from today's space-oriented NASA, but public libraries in some large cities and many technical universities maintain files of NACA publications and one can inquire there. Some are able to do copying work at about lOc a page or $5.00 for all of both TM 311 and 326. Of course, the same information found its way into texts on aeronautical engineering and may be found by looking into them. So to get on with the discussion of wing span, here are some design rules which appear at the end of TM 311: (Continued on next page) SPORT AVIATION

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WING SPAN . . . (Continued from preceding page)

Rule I: Make the ratio of span to weight as small as possible compatible with structural requirements. Rule II: Build as light as possible. Rule III: Reduce parasite drag to the absolute limit, even at the sacrifice of weight. Rule IV: Use large diameter, narrow-blade propellers. It is very interesting to think over these rules. Rule I is simply the span loading matter expressed another way. Rule II explains why eyeball engineering alone is hardly adequate to design a really able ultra-light airplane. Rule III explains why the designers of Jodel, Druine and Taylor monoplanes went to the trouble and weight of building cantilever wings. Rule IV is a guide in studying the rotational speed circumstances of engines under your consideration. TM 326 is a continuation of the discussions in TM 311 and deals specifically with how to go about selecting the most suitable wing span for any given power and the kind of performance desired. Because this data is so valuable to the aims and objectives of EAA, the entire text of this publication including graphs will be reprinted next month. Although TM 326 dates back to 1925, the air we fly in today is the same. So the mathematics in TM 326 are as sound today as when originally published. But some mention must be made of developments which have taken place since 1925 and which may confuse or mislead

This Italian AM 12 aircraft, powered by a Praga engine of only 75 hp, set an altitude record in 1964 of 28,661 ft. Ample wing span is obvious — and is the reason why the ship could attain such a great altitude on its modest power.

the present-day amateur who is familiar with things that Ivan Driggs had never seen. TM 326, of course, deals with the climbing properties of light aircraft fitted with fixed pitch wooden propellers. It is perfectly true that metal and variable pitch propellers will give a worthwhile improvement in rate of climb. To cite one case for which figures are available, the 95 hp Super Cub fitted with a wooden propeller climbs at 624 fpm and when fitted with a fixed pitch metal propeller climbs at 710 fpm. With the wooden propeller, absolute ceiling is 16,000 ft., and with the metal prop it rises to 17,750 ft. But of course it isn't easy to find a metal propeller tailored to fit the Volkswagen or Cushman engines! So anyone designing a modern ultra-light is more or less limited to wooden props and the data in TM 326 is still applicable. It is also true that the airfoil used in the wing influences climbing ability. A book can be written about this subject so we can't explore it deeply here. It is necessary to do some slide rule work, comparing various possible airfoils to find one that looks good from the climb standpoint. What might be chosen is an airfoil that has a rapid accumulation of lift at lower speeds and at the angles of attack associated with taking off and climbing out. It is also well to look into the power required to pull various airfoils along at your design's climbing and cruising speeds. Many a hot-eyed amateur designer has fallen for a dramatic looking airfoil developed for high speed cantilever wings — and later wished he had used some older section more suited to a slow and light ship.

The beginnings of wisdom. In the early 1920s little was known about the importance of wing span to climbing

ability with low power. Under the impression that wing

area controlled climb, designs such as the Mix Arrow biplane appeared. As knowledge of the importance of wing span began to distil! out of European experiments with

ultra-light aircraft, designers everywhere began to get better results by using longer wings, such as in the Mummert monoplane. 20

FEBRUARY 1968

And it is also true that wing flaps, Hoerrner wing tips and wing tip plates can improve climbing ability. But that does not mean they will transform each and every poor climber into a skyrocket. As to flaps, since any ultralight ship is going to be a slow one, they are hardly needed for landing purposes so it is a moot question whether their complexity and added weight will add anything worthwhile to take-off and climb. Hoerrner wing tips may indeed add 3 or 5 percent or even more to the climb and ceiling of an already fast airplane. You can sense and put to use the difference between 2,000 and 2,050 fpm rate of climb, but as between 300 and 315 fpm in an ultra-light It can be hard to detect that degree of improvement in practical flying. Air velocities and pressures associated with larger airplanes can make such wing tips pay off, but on an ultra-light the pressures and velocities are so much lower that the result may hardly be worth the effort involved.

No doubt wing tip plates can improve the performance of some small airplanes, but don't make the mistake

of believing that through their use alone you can make any low-powered ship climb snappily. You may have seen them do a lot for some airplane used for crop dusting — but don't forget that this plane already had a favorable power-to-span ratio before the tip plates were added!

Generous wing span is still the main key to good builtin climbing ability. In this discussion of climbing ability, we are talking about every-day climbing from a standing start. It is possible to hold an ultra-light down after the wheels leave the runway and build up enough momentum to perform a dramatic zoom. But this is not real climbing ability. Some day you are going to want to take off from a runway that may be too short to allow zooming speed to be picked up!

There are in existence some small aircraft designed

for the Volkswagen engine. Their wings are from 21 to 23

ft. in span, they weigh about 600 Ibs. loaded and this works out to an average span loading of 27 Ibs./ft. Assuming the 1200 cc. Volkswagen engines used in them to produce 30 hp, they have a power loading of 20 Ibs./hp. As you will note, a span loading of 27 Ibs./ft. goes with such a power loading in the table near the beginning of this article. That shows the basic accuracy of the table. It also suggests what kind of climbing ability such planes have. Some published performance figures claim rates of climb ranging from 600 to 900 fpm. These figures are either giddily over-optimistic, the misleading results of the difficulty of making accurate performance measurements without fancy equipment, or the results of flying in rather cool air or with a decidedly small man in the cockpit. If you have been to Rockford and if you are observant, you will have noticed that most of the flying in airplanes of this class is done early in the morning or in the cool of the evening — which is a tip-off! This is not meant as a criticism of such airplanes. It is said to give you a realistic view of climbing ability. Remember that summer weather in northern Europe is generally cooler than in North America, and that temperature directly affects climbing ability. When it gets to be 80 or 90 deg., the climb of an ultra-light airplane

falls off very noticeably. In cool European weather these machines do quite all right in getting off the long runways of the former military airfields which are commonly used by private flyers over there. By the time such a plane reaches the end of a 5,000 ft. runway it is up quite high

enough to clear any trees or houses.

Their cantilever wing construction does make them aerodynamically clean, and their wing spans of 21 to 23 ft. doubtless facilitate the construction and transportation of one-piece wings in the workshops available to the average amateur. But their existence should not be taken as proof that 22 and 23 ft. wings are necessarily ideal for low power. Consider such departures from that range of wing span as the Fournier Avion-Planeur, the single-seat Cvjetcovic, Flaglor's "Scooter", and others. They have wings spanning from 27 ft. up. It would be exceptionally interesting to see what a converted VW might do if mounted experimentally in a Heath LNB Parasol of 31 ft. span, in a Longster of 30 ft. span, or in an Aeronca C-2 of 36 ft. span. You know the saying, "You pays your money and takes your choice." It is like that in deciding upon suitable wing span. Perhaps you live in a flat, cool country and would prefer to sacrifice some rate of climb in favor of more speed. You could get by with less span than the fellow who lives in hot or hilly country and who feels safer with the best possible rate of climb that can be coaxed out of the engine he's using. Very great span is to be avoided because eventually a point is reached where the profile and skin friction drag of a long wing takes too much away from cruising speed. An exceptionally long wing also poses structural problems and can end up being too heavy. In general, unsatisfactory climbing ability will result from putting Volkswagen and similar engines into ships of less than 20 ft. span, satisfactory big-field and cool weather climb can be had with spans of around 22 to 24 ft., and for real short-field ability, hot weather flying and carrying husky pilots, spans in the high 20s and low to middle 30s are well worth considering. Mainly, when working with low horsepower — span's the thing!

(2)

Huge liolli.i Bomber Being Built Clarence Atkinson of Charlotte, N.C. has contracted with Paramount Pictures to construct a full scale replica of a German Gotha biplane bomber of World War I. The ship, which spans more than 80 ft., will be used in the forthcoming film "Darling Lillie." We understand that Atkinson is working against a pretty tight deadline to complete the ship, and no information is available yet as to what engines will be used. Try to get that one out of your basement! Also to be constructed is a 30 ft. long radio-controlled model of a Zeppelin for the film. In England, several specially built Currie "Wots" have been completed for the film, and these have been modified to resemble as closely as possible S.E.5A types. One of these special ships, G-AVOT, is shown in the ac-

companying picture. A modern opposed engine is fitted, necessitating a widening of the nose section. The ship is somewhat smaller than the S.E.5A. ® SPORT AVIATION

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