Man-Powered Aircraft

European Airways and has done much aviation writ- ing. We should like to ... to be that, since it has not yet been done, it never can be done. The assumptions ... designed to try to win a prize offered in 1933 for the first man-powered flight in a ...
1MB taille 2 téléchargements 364 vues
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

DURATION

SECONDS

Fig. 1

EMPTY WEIGHT yMO.£ b-172 —

LBS

/

/ x/

-" C)

10

^-._-

20

c ,--'

30

SPAN

FEBRUARY 1966

^r

40 FEE!

Fig. 2 16

X

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

O

6b 86

5o

e0

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

^ '

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. ®