The Bossi-Bonomi "Pedialante" was an Italian attempt at man-powered flight.

Man-Powered Aircraft By B. S. Shenstone Chief Engineer of British European Airways (Drawings by Robert McRoberts, EAA 2271)

(PART 1 OF 2 PARTS)

(EDITOR'S NOTE: This article originally appeared in SHELL AVIATION NEWS, which publication kindly gave us permission to reprint it. We present it for two reasons: It is an outstanding example of how a skilled writer can present an ab-

stract technical subject in an interesting, understandable way; and it is informative and thoughtprovoking to all who are interested in the study of aeronautical design. Mr. Shenstone is chief

engineer of British

European Airways and has done much aviation writing. We should like to point out that the article was written a few years ago; since that time several experiments have been made in Europe and the United States with man-powered aircraft). MANY irresponsible people have played with the ScultOidea of man-powered flight that it is becoming diffito take the matter seriously, even when it is discussed by technically qualified people. The main objection seems to be that, since it has not yet been done, it never can be done. The assumptions here are two-fold. On one hand is the thought that if man could fly, he would have done so long ago. After all, we are so very clever in these brilliant technical times that if a little problem like this sort of flying were soluble we would have solved it. It is, of course, obvious that this attitude is nonsense. It would be better to say that, if something hasn't been done, it is high time to do it. On the other hand, there is no money in it, so why bother? Some people make money out of

anything, but I assume there are far easier ways of making money than out of man-powered flight, and I hope we steer clear of it and concentrate simply on achieving the heretofore impossible. This brings me to the other assumption. Bees and birds fly, and bees fly more easily than birds, and big birds find it most difficult to fly. There is no known case of a creature weighing more than 32 lbs.

that can fly. The bigger that creatures grow, the less their specific strength. To be factual, the basal metabolism

increases at less than the first power of the weight; in fact, as the 0.734 power. This is most discouraging and it is easy to stop here and say it is obvious that man cannot

fly.

In addition to all this, man-powered flight has always had a bad name, even in mythical times. It will be recalled that Icarus, on his flight from Crete to Sicily, had a structural failure and fell into the sea. This is the whole story to most people, but what must not be forgotten is that the chap who organized Icarus' flight was Daedalus, his father. They set off together and Daedalus, who had sense enough to fly at a low altitude, succeeded in reaching Sicily. He was entirely correct in keeping to a low altitude, as I shall indicate later. Whether or not it was the sun which melted the wax on Icarus' wings or whether he fell as a result of other troubles will never be known, because nobody made any accurate observations at the time, and this has been clearly exposed by the well known painting by Brueghel in which everybody is going about his own business and paying no attention to Icarus and, in fact, all that you can see of him is two legs sticking above the water. It has been stated hundreds of times that men have always wanted to fly like birds because of the freedom of action which the birds uniquely enjoy. Most people now tend to say that we are far better than the birds with our airplanes and gliders and, in fact, in certain conditions of gliding we are practically on a level with the birds. But full freedom of action the glider has not, and never can have, because it depends on an external source of power. Man-powered flight is often thought to be far from reality, but in fact it has already been nearly done. And, when something has practically been done, it usually takes only a moderate addition of ingenuity and knowledge to do it properly. It is not generally known that in 1936 in Germany, a single-seat man-powered aircraft, the Haessler-Villinger, made at least four flights over 200 yds. in length at an altitude between 3 and 15 ft., and that these flights were officially observed! Each of the flights lasted about 20

seconds. Indeed, there was one flight which achieved a

distance of 440 yds. These flights were made on an aircraft (Continued on next page) SPORT AVIATION

15

MAN-POWERED AIRCRAFT . . .

(Continued from page 15)

designed to try to win a prize offered in 1933 for the first man-powered flight in a closed circuit without intermediate landing around pylons 500 meters apart. The prize has never been won, but at least it was close to being won. In Italy at about the same time, the Bossi-Bonomi "Pedialante" was built with great care and skill, but not successfully flown. These are the facts and we can now approach the problem from several points of view. We can discuss the 32 lb. bird and argue about the man's strength. We can find out why the German aircraft of 1936 could not fly more than 4CO yds. We can deal with the problem from scratch and merely use the 1936 flights as encouragement. Probably it is better to use all methods of approach so that we can see whether there is now, over 30 years later, a good case to be made for the possibility of

such flying.

To argue from the birds does not lead us anywhere. We are not attaching wings to man and pushing him off a branch. The birds that work hard at flying are either fast or inefficient aerodynamically for very practical reasons. Their spans may be limited for lateral obstacle clearance, or they may not need to fly very often or very far. They have heavy wing-folding gear which leads to inefficiency. The aerodynamically efficient birds, such as the albatross, do not waste much effort on flying, quite apart from their use of thermals, obstacle updrafts or Katzmayr effect. (See any textbook on aerodynamic theory for a detailed explanation). Man can wrap himself up in an aerodynamic shell, which may be better than a bird and much simpler. Even so, he will have to work harder than the albatross, but why not? It is admittedly a fight against nature and the elements, but that nature can be beaten is now commonplace. The German Haessler-Villinger aircraft failed mainly because the power transmission was not efficient, it being 20

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FEBRUARY 1966

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be expected in a man-to-propeller drive is not known. Using arms and legs, a man has an instantaneous

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transmitted from bicycle-type pedals through a belt to a propeller shaft on which was a small propeller well below the optimum size (under 5 ft.). It also failed because knowledge of aerodynamics at that time was not sufficiently advanced. Another reason for failure was that optimum power was not available. In other words, the optimum number of crews is not one, but is considerably more than one. I think it is common experience that a two-place aircraft does not weigh twice as much as a single place, nor does it have twice the drag. However, a two-place man-powered aircraft has rather more than twice the power available because, as a single-place, it is probably impracticable to use more than leg power of a man for locomotion, as he will need his hands to guide and control the aircraft. In the case of a crew of two, the second man can exert all of his energy, arms and legs for driving the aircraft. An estimate of the power required to drive the Haessler-Villinger is 0.82 hp which, as can be seen from Fig. 1, can only be exerted by a man for a very short period, which is certainly less than a minute. It would be necessary to redesign this aircraft so that it used only about 0.6 hp which would, of course, be a major improvement. It will be noted fas Fig. 1 clearly shows) that a man cannot produce his full power through his legs alone and that arm-power is significant. A paper by Nonweiler in the Journal of the Royal Aeronautical Society for October, 1958 shows how the problems can be approached by a particular design solution and the assumptions involved are clearly stated, so Nonweiler's approach is the approach from scratch and is, of course, the approach that would have to be used by any practical designer or investigator. He starts out by showing quite clearly what the limitations of the poweravailable are. Chief among the unknowns is the loss in drive. When these figures are brake horsepower, there is some mechanism between man and brake in which there must be some loss. Whether such loss is of the order to

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The Haessler-Villinger machine was flown in Germany in 1936.

7

power of something of the order of 1.5 hp, but for a minute's duration he can only exert about 0.8 hp. However, for longer periods up to about one hour he can exert

0.5 hp. We can. therefore, use these powers with reason(Continued on page 18)

MAN-POWERED AIRCRAFT . . . (Continued from page 16)

able assurance but with full realization that if calculations are wrong or the usual things occur, such as the actual aircraft having more drag and weight than the design, the only way to get over it is to find a stronger man, and there isn't much scope in that. As a result of these figures, we can draw the usual power-required and poweravailable curves for flights of any reasonable duration. The aircraft then must be designed to fit one of these curves or it will be a failure. Power-required curves for three actual lightweight aircraft . . . the "Windspiel", a very light, efficient and expensive glider . . . the Haessler-Villinger man-powered aircraft . . . and the Bossi-Bonomi "Pedialante" . . . show that none could be flown for more than a few seconds by man-power. We can also cross-plot the thrust powers available against time. By this is meant the actual power output of man or men reduced by the inefficiency of propeller and drive, i.e., 80 percent propeller efficiency and 90 percent drive efficiency, giving an over-all efficiency of 72 percent. This figure is conservative, but it is best at this stage to be careful. The result is disappointment! Even the two-place aircraft does not fly very well. On the other hand, with double the weight, and more than double the power available, the power-required increase is only about 30 percent. It is the right direction and it would not take much more ingenuity to bring success. Just slightly better assumptions would show that reasonably continuous flight is possible. It is essential to do a "cut and try" design study varying the assumptions and then seeing whether the result fits the desired curve. Such a design study is full of important unknowns, such as structural weight, wing-profile drag, fuselage drag, propeller efficiency, efficiency of mechanical drives, and weight of the driving mechanism. Information on actual weights of single-seaters in the past indicates that an empty weight for a single-seater can be

METAL WORKING . . . (Continued from page 17)

Bend allowances are admittedly a little problem. If these can't be figured from the plans or instructions, you can make up some test samples of the different thickness materials. I always do this anyway, for something usually doesn't come out just right on the average brake. The problem is that the run-of-the-mill sheet metal brake does not have radius shoes. To use a brake without various radius blocks, you must bend up one or more layers of scrap metal to use as a shoe. Just experiment until you obtain the proper radius. Never allow the brake to mark the finished part or to make a sharper bend than the plans indicate. Forming straight bends with a mallet requires that the metal be clamped firmly between two blocks. A radius should be shaped on the inside block to prevent cracking. It is very important to use an adequate radius because 2024-T3 can be quite brittle. Use a hard rubber mallet to prevent denting the metal. Work the bend down slowly to minimize bowing. Since some bowing is unavoidable, it should be straightened out. 4.

Straightening: It is virtually impossible to form

a bend with a mallet without getting some bowing. This 18

FEBRUARY 1966

as small as about 80 lbs. (see Fig. 2). It would need a very clever designer to achieve this and it will be noted that Nonweiler's assumed empty weight for a two-place aircraft is 170 lbs., which is certainly not optimistic. The design difficulty on structural weight is essentially one of experience. To my mind, it is quite clear that such a machine should be made of wood and fabric. The trouble is that there is hardly anyone left with experience with very light wooden structures. It is easy enough to design the spar but that is not where the weight goes. The weight disappears into the ribs and frames, secondary structures and fittings, and very careful calculations would have to be made, including reproduction of complete test sections to insure not only that the strength was of the right order, but that the weight was acceptable. The wing section problem was until recently extremely difficult, but there is now some information available at the requisite Reynolds Numbers which are of the order 0.7 x 106. It is clear that what one wants to have is a wing section which has as much laminar flow as possible at this Reynolds Number and have the bottom of the laminar flow bucket at a lift coefficient of the order of 0.9. No normal airplane wing section will give this characteristic but fortunately, at least two sections give some promise of it ... NACA 65A(10)12 and the German section FX05H-126. The wing section thickness which is practical on such an aircraft may be difficult to determine. Informal discussion with the Air Registration Board indicates that an ultimate load factor of not more than 2.5 will be acceptable with very little gust loading. This means that the wing would be very flexible and that the detail design would have to be directed toward permitting it to be as flexible as possible, thereby saving weight and still avoiding flutter or control reversal at operating speeds. ® (END OF PART 1)

can be taken out of flanges (which will later be riveted) with a crimping tool which slips on the jaws of vise-grip pliers. Crimp between the rivet holes. 5. Making Holes: A Whitney punch is a must for transferring all edge holes. A hand drill is used for all other holes in sheet stock. To transfer holes from templates, use a nibless Whitney punch for edge holes and transfer punch all other holes first with a nibbed Whitney punch and hammer, deepen with a center punch and then drill. Virtually every hole in a fitting which will receive a bolt must be drilled undersize and then reamed.

6. Bending Skins: To bend all leading edge radii, simply mark the center line of the bend on the outside of the skin, fold over by hand and clamp the two trailing edges in the proper position with a board and C clamps. Lay another board near the bend and work the bend down by pressing on the board until the C clamps can be slipped on. Screw down the clamps and make proper adjustments to keep the bend in the proper position. Inside flanges in fuseJage frames can easily be bent down to almost 90 deg. without cracking. This gives a much stiffer frame than the 45 deg. bend. The corners don't need to be bent down as far as the straight portions. ®

Man-Powered Aircraft By B. S. Shenstone Chief Engineer of British European Airways (Drawings by Robert McRoberts, EAA 2 2 7 1 )

(PART 2 OF 2 PARTS)

ERHAPS THE most difficult probP lem is that of the propeller and the drive for it. It is probable that the most efficient sort of drive is an adaptation of bicycle practice, i.e., pedals, sprockets and chains. It is complicated by two factors: (1) the propeller tends to be distant from the crew position, and (2), the propeller shaft will be found to be at right angles to the initial drive shaft. This involves the use of bevel gears which tend to be heavy, or a twistable chain

yet to be developed. There is also the detail design for minimum weight for shafting absorbing less than 2 hp with rotational speeds of about 120 rpm on the initial drive shaft and up to 500 rpm at the propeller shaft. The difficulty here will be designing the shaft small enough and light enough. As for the propeller, which would probably have to be 6 ft. or more in diameter, normal propeller construction would be too heavy. Possibly a propeller made entirely of balsa wood would be sufficient, but no thoughtful work has been done on this matter yet, so it is not out of the question that a properly designed propeller might look very different from the old wooden propeller. Ground clearance of the propeller is important and you will see one way of achieving this in Nonweiler's de-

sign which involves making a very high fuselage and fin combined (Fig. 3). One solution I put forward myself a couple of years ago (Fig. 4) involved a propeller half way along the fuselage, which is a tricky mechanical problem although good aerodynamically. It is essential to get the slipstream free of the wing and that is certainly not easy to do. It is also desirable to have a light compact drive. The Haessler-Villinger achieved the compact drive by a simple twisted belt, but the propeller was not well located (Fig. 5). The Bossi-Bonomi had normal tractor propellers in the wing leading edge and a long chain and shafting. Dr. D. R. Wilkie of University College in London has suggested a very compact two-place mechanism which involves both crewmen working the same crank. There are other possible solutions suggested by Dr. Alexander Lippisch who spent many years in thinking about this problem. He considered the tail-first machine as a possibility but rather prefers the tailless type of machine with sweepback. There is much to be said for both layouts. However, Lippisch has made an attempt to reduce the length and weight of the driving mechanism and that is something which requires very careful consideration, for if the driving mechanism is a critical part of the

design, it may be necessary to compromise to get the best over-all result. The use of a tailless aircraft such as Lippisch suggests is difficult, except for somebody like Lippisch who has a great deal of experience on the detailed difficulties of such a design, or like the late Professor Geoffrey Hill who did so much tailless development work. Figs. 6 and 7 show how easy it is in such a layout to make a compact propeller drive. In this respect, the tail-first aircraft is more attractive because it is an easy job from the aerodynamic and control point of view, and it fits in well mechanically. Whether or not it has inherently too much drag and interference is a matter which would need

some study. This point brings up a fundamental problem in the design of the manpowered aircraft. It is quite clear that there are many different ways of approaching the problem. There is the way just described, which is an aircraft of normal configuration and using a single propeller. This approach seems too prosaic for many people and the flapping wing has often been suggested as the natural method of propelling man-powered aircraft. I am not attracted by arguments about Nature, because practically all successful mechanical inventions have been against Nature, and rightly so. We have found it possible to override Nature's basic limitation, which is that a revolving shaft is naturally impossible, although it would be a tremendous advantage were it possible. But we can use the shaft for our machinery and we do so. The use of the flapping wing seems to be suggested mainly because, if man wants to fly like a bird, he wants to fly like a bird and not like something else. The flapping wing is an extremely complex mechanism which suffers severely from structural scale effect and its aerodynamics are not fully understood. The use of a flapping wing is the hard way towards the solution of man-powered flight. Flapping should not be the solution of man-powered flight. Flapping should

not be the first method to be tried in an attempt to achieve man-powered

FIG. 16

MARCH 1966

3

flight, because then you are attempting to solve two distinct problems at one time. First, you are trying to make

FIG. 4

the flapping work, which has not yet been done on this sort of scale and, secondly, you are trying to accomplish man-powered flight which has also not been accomplished. It might be difficult to know, if success were achieved, where the fault was, that is, the man with his power, or the flapping wing. Let the flapping wing be first tried and found to be good when driven by an engine. When that problem has been solved and the aerodynamics known and the performance measured, then is the time for it to be applied to flight by man-power. However, there is considerable backing for the flapping wing and Walter Filter of Germany has built one and in the United Kingdom, Emiel Hartman has also constructed one. Another approach has been the suggestion that the wing be designed to use Katzmayr effect. It is quite possible that at this scale, the Katzmayr effect is usable, although for wings of larger chord the ratio between speed and order of the natural wind variations is wrong. However, this again is something which is not sufficiently known to be the basis for a proper design, even if an attempt were made to help the Katzmayr effect by the use of a vertically oscillating wing.

falling down a few yards later. This is a point which anybody building a man-powered aircraft must take into account. If a man-powered aircraft is designed in the simplest fashion, it

will not be able to take off under its own power and it will therefore have to be gotten into the air by some outside agency and then allowed to fly (Continued on next page)

However, my main point is that the

best way to attack the problem of man-power is to do it in the simplest possible way, using available knowledge to the greatest possible extent and thereby experiment to the minimum degree. This attitude, I feel, has the best chance for initial success, although it may not produce the most efficient man-powered aircraft. However, it does seem that there is enough information to indicate that, with present knowledge, man-powered flight is

possible. In the British technical press there was a discussion on what constitutes merely being thrown into the air and

FIG. 5 The controls of the Haessler-Villinger are above the pilot's knees. Stirrup pedals are used and, although the pilot reclines, his body is in the same attitude as a bicycle rider. SPORT AVIATION

17

(AEROPLANE Photo)

FIG. 6 The Westland-Hill "Pterodactyl Mk. I" rode on a tripod landing gear, had the rudders at the end of the wing struts and had movable wing tips. The

RAF roundels denote that the military had an interest in the project.

Man-Powered Aircraft . . . (Continued from preceding page)

on its own. Arguments will then arise as to whether the initial locomotion was sufficient to allow the aircraft to glide to its landing place without the man-power or to what extent the manpower extended its flight, if at all. With this in view, it should be noted that both the Man-Powered Aircraft Committee and Nonweiler suggest that an effort should be made to design the man-powered aircraft so that it can take off under its own power. This involves the use of wheel traction during the initial phases of the take-off because the propeller is very inefficient under these conditions. It would mean a gradual transfer of power from the driving wheels to the propeller near the lift-off point. The danger here is that a crew might become so exhausted during the takeoff run that, although they are able to take off, they are unable to keep

it flying. Twenty-five years ago, the Germans suggested that the best method was to allow the crew to store power for not more than 30 min. immediately prior to take-off and to carry the stor-

age mechanism in the aircraft. This might typically be arranged as follows: A very long bungee might be pegged into the ground well ahead of the aircraft and wound up on a drum within the aircraft before takeoff, the aircraft being held in position during the winding operation. When

fully wound, the tail could be released, the aircraft could jump into

the air and would take the bungee

away with it, and it could then be

wound up and withdrawn into the air18

MARCH 1964

craft. This adds quite a lot of weight to the aircraft, and in the case of the Haessler-Villinger added 33 lbs. to the weight. Nonweiler's suggestion of a wrapped bungee is also of interest. Another solution, for the initial development at least, is simply to tow the aircraft by motor power so that it takes off at a speed no greater than the cruising speed and at an altitude of, say, 3 ft. This would give very little opportunity for a long glide under no power and would minimize any momentum effect which would be present if the speed were higher than that required for cruising. This is practical as long as figures of speed and height can be reasonably determined. At present, there is no likelihood of the man-powered aircraft rising more than a few feet above the ground because the power required within the ground cushion is so much less than the power required at about 15 or 20 ft. In fact, at about 10 ft. altitude, the ground effect doubles the effective aspect-ratio of the wing which has a large effect on the power required, because the wing incidence is quite high. Hence, the possibility of man-powered flight in the initial stages would be over flat fields, such as an airfield or over a moderately sized stretch of water. No doubt the duration achieved over water would be greater than that over land, because of the natural incentive to avoid a ducking. The question now arises . . . "Who is really interested in this sort of problem?" The answer about 10 years

ago was "practically nobody", but now

there is a slowly increasing interest on the part of technical people quite apart from the perennial interest of the frantic fringe. But the real interest in this problem is increasing because it is gradually being realized that the problem is one which is very close to solution and the technical background is almost all available. What is being done to channel the development of this interest is still on a very small scale. In 1957 the Man-Powered Aircraft Committee (MAPAC) was formed at a meeting held at the College of Aeronautics at Cranfield, England. Its chairman from the beginning has been H. B. Irving, and its members come from universities, the aviation industry, research workers, and official representatives from the Royal Aeronautical Society and the British Gliding Association. The efforts of this committee have been directed toward

reviewing work, published and unpublished, on the subject, and encourag-

ing the discussion of ways and means of promoting the realization of man-

(AEROPLANE Photo)

FIG. 7 Looking quite ferocious, the WestlandHill "Pterodactyl Mk. IV" had a threeplace cabin and was powered with an inverted 120 hp deHavilland "Gipsy."

It had a pivoted-cradle or bicycle-type

landing gear with outriggers and wingtip rudders. In its final form, the "Pterodactyl" had a tractor-mounted Rolls-Royce "Goshawk" engine and

was intended as a two-place fighter aircraft, having a ball turret mounted in the rear of the fuselage.

powered flight. It has also spent some time in drafting an appreciation of the position at present which would be used as background for obtaining sufficient financial interest for manpowered aircraft to be built and flown. However, let it be quite clear, success means a great deal of work by professional people and this work will have to be paid for and, so far, there is no money for this purpose. The outlook for flight by man-power is certainly promising and, compared with the development of even a light airplane, the development of a manpowered airplane will probably be quite inexpensive. The main difficulty lies in the small tolerances allowed. If the aircraft is not correct from a structural point of view, from the weight point of view, from the aerodynamic point of view and the mechanical point of view, it will not fly. It is worth reiterating that it is unlike any other form of aircraft in this respect, there being no reserve of power to call upon, and

therefore no way of correcting mistakes except by redesigning and rebuilding. A failure on the first serious attempt to construct and fly such an aircraft would obviously leave widespread doubts in the minds of people as to its ultimate practicability, and therefore if any form of success is to be achieved, the problem must be approached in a sound technical manner and taken as seriously as would be the design of any powered aircraft. ®