1950’s REVIVAL

AUGUST RASPET

Credited by many as the first major instigator of revival of interest in HPF. A paper by Raspet was published by the Mississippi Academy of Sciences in 1952. In the 1950's, tests on boundary-layer (see Glossary) control were being done for proposed use on aeroplanes of many types. In at least one case an engined-aircraft was modified by having a sleeve added over a short part of the wing span. This sleeve was very accurately made to a mirror finish. Then small holes were drilled in the surface through which suction was applied in flight. The objective was that the boundary-layer would be constantly sucked away thereby preventing transition to a turbulent boundary-layer. It was thought by some that this principle could have relevance to HPF, it being considered that HPF was not quite possible with ordinary wings but that it might just be made possible with the reduced wing drag resulting from a totally laminar (see Glossary) boundary layer, controlled by suction. Amongst these were August Raspet of Mississippi and Professor T.R.F.Nonweiler of the U.K. (Glasgow, Belfast & Cranfield).

B.S.SHENSTONE, T.R.F. NONWEILER

The renaissance of interest in human powered flight which arose in the UK in the 1950s, culminating in the figure-eight prize offer which marked the end of the decade, was further stimulated and given direction by the writings and other activity of these two. Beverley Shenstone was Chief Engineer of British European Airways, and Terence Nonweiler was on the staff of Queen's University Belfast. The March 1956 issue of the Canadian Aeronautical Journal carried Shenstone's "The problem of the very light weight highly-efficient aeroplane". In October 1958 Nonweiler boldly chose the title "The Man-Powered Aircraft" for his piece published by the Journal of the Royal Aeronautical Society.

The MAN-POWERED AIRCRAFT COMMITTEE,(MAPAC)

Shenstone, Nonweiler and five other eminent enthusiasts met in Cranfield in January 1957 and formed this ad hoc committee with the purposes of reviewing relevant literature, assessing the prospects of HPF and promoting its realisation. It was the opinion of its members and of most other aeronauts that flight powered by muscle power alone was only questionably possible. Haessler and Villinger's Mufli, 1935, had always been towed into the air and other claims were mythical or unsubstantiated. Shenstone was to be an active member of the MAPAC and its successor for over twenty years. Prof G.M.Lilley, an early member of MAPAC, remains active, and became an aero-engine himself in 1977 when he flew Gossamer Condor on one of its last flights.

EMIEL HARTMAN

In England in 1958 an ornithopter was built by a glider-repairer who followed the sketches of Emiel Hartman, a sculptor. It used a mechanical linkage to provide the necessary twisting of the wings during the flapping cycle. Only towed flights were made, but the builder told the author in 1961 that by flapping the wings, forward progress had been made on the ground. In common with the earlier and more successful Lippisch machine the downstroke was assisted with springs, which became tautened on the upstroke. It would appear that its one good feature was that the natural frequency of oscillation of the wings, with the springs used, was the same as the usual human rowing cadence. This was also equal to the frequency calculated to be correct for aerodynamic propulsion. Various researchers had observed a range of insects and birds. Extrapolating from these observations, they hypothethised a flying creature of the weight of one person plus machine, and found that, conveniently, the frequency comfortable to rowers is what one would expect to find used on a successful flapping-wing HPA.

DANIEL PERKINS

Daniel Perkins was a civil servant, an engineer working at the Royal Aircraft Establishment at Cardington. This was at the time Britain's biggest experimental airship facility. Perkins decided to build an inflatable wing HPA. His first test was a propeller driven trolley and since the rider of this heavy crude trolley could accelerate to 14 mph, Perkins concluded that all was well with the drive and propeller system. Transmission was a rope belt. However, having built the plane, they found that this could only be pedalled to 14 mph, (6.26 m/s) the same speed as the test-trolley. Perkins took the wing off. Same result, 14 mph maximum, even though the rider was now in a streamline shaped pod, and the total weight was much less than the test-trolley. The configuration was a pod and tail-boom fuselage (the first such built), and a parasol wing (see Glossary). Various tests and modifications including a virtual rebuild did not improve this ground-speed. Perkins became convinced that the efficiency of his propeller was adversely affected by proximity to the ground, and that this explained the apparent 14mph barrier. However he persisted (see Reluctant Phoenix).

ROYAL AERONAUTICAL SOCIETY MAN POWERED AIRCRAFT GROUP, (RAeS MPAG)

In October 1959 this group of the society was formed, having been proposed by, and essentially consisting of (ex-)members of the MAPAC. Meetings were then held at the Society's offices, as they are to this day. The name was changed to "Human Powered Aircraft Group" in 1988 in recognition of the many successful flights by women pilots.

S.S.WILSON

This Oxford University lecturer described his HPA transmission test-rig to the RAeS MPAG on 17th March 1961. Wilson appreciated that HPA drives have a heavy peak torque and low speed compared with most machines, and that a 1% improvement in drive efficiency is as effective in reducing the power required from the pilot as a saving of 2 pounds (0.9 Kg) of weight. The power output from pedals is cyclic, with a burst of power as each leg pushes down, but it is preferable for the propeller to run at a constant speed and torque. The transmission system can reduce these variations in power by an effective change in gear-ratio during the cycle, for instance with non-circular chain-sprockets or belt-wheels. During his series of tests, the drive being tested was mounted in the rig and a further power transmission of known high efficiency linked the shaft back to the pedals. This mechanism was then strained when static so that the load in the drive was as it would be when in use. The rig was driven & the power to drive it measured. A flexible shaft showed the highest efficiency found, 99%. In use, this might cushion out cyclic-power-variations by absorbing part of the energy supplied as each foot goes down and resupplying this to the propeller during the remainder of the cycle. (Strictly, of course, this is stored energy). Wilson quoted 98.5% for chain drive, but declared chains to be untwistable! Chains impose less load on the supporting structure than belts, which unless toothed are less efficient. A double-crank system (as Seehase) with a 1 1/2 inch crank length was made to work using rubber bushes, and showed an efficiency of 96.5 %. Wilson recommended the use of self-aligning-bearings throughout an HPA drive system because of the flexibility of the supporting structure, and their lesser friction. He quoted a coefficient of friction of 0.001 for self-aligning bearings and 0.0015 for single-row bearings. Needle rollers were not recommended. Tyres need 5 hours running in to reduce rolling resistance which decreases with temperature.

REDUCTION OF CYCLIC VARIATIONS

Four methods of reducing cyclic variations are :-

1. Elliptical sprocket as mentioned by Wilson and used on Musculair.
2. Energy stored in drive path as suggested by Wilson's flexible shaft.
3. Energy stored outside drive path, such as Bradshaw spring, as proposed for Jupiter.
4. Weighted propeller tips to provide a flywheel effect.

Builders have never used this last principle presumably because they felt that the extra weight was not worthwhile.

TRANSMISSION SYSTEMS IN FLIGHT

It should be remembered that actual conditions can never be duplicated by ground-rig tests. Such tests as those of S.S. Wilson are of value as are those of the manufacturers or a group's own ground-rig-tests; but experience has shown that flexure of the airframe in flight has affected the performance of the transmission.

HENRY KREMER

In November 1959, Henry Kremer, an industrialist, offered a prize of £5,000 for the first man-powered flight. £5,000. At the time, that sum would have bought a moderate size house. The money was donated to the RAeS and the rules, composed by the MPAG were published in the February 1960 Journal of the Society (JRAeS). ( These clear, succinct and unambiguous rules were written, agreed and published in two months in the days when emails were unheard of.). The winner would would have to demonstrate that sustainable flight after an unassisted take-off was being achieved, and that left and right turns and moderate climbs could be flown, both with or into any wind. To fly a figure of eight course, with a minimum of ten feet height at start and finish, necessitates all those manoeuvres. This was the original Kremer Prize course. Entry was restricted to the British Commonwealth. Henry Kremer has continued to sponsor competitions ever since (three are current in 2007). Henry Kremer was a quiet spoken man of few, well chosen, words. His views on the rules for each competition, as well as his generosity, have influenced the progress of human powered flight as much as anybody. Even with regard to the Daedalus flight, which one might have thought had nothing to do with Henry Kremer, (the course having been established a few thousand years earlier !), Professor Mark Drela, co-designer, said that the flight would not have been possible but for the experience that members of their team had gathered while flying Kremer courses. On February 19th 1967, the purse was doubled and the competition made international. Also in 1967, the "Slalom" competitions were started, effectively for a figure "S" around three pylons. There were first, second and third prizes of £2,500, £1,500 and £1000 for the Slalom competition, but it had no entrants and was finally withdrawn. In 1973 the Kremer figure "8" prize was increased to £50,000, more than covering the rate of inflation. Accounts of the Kremer World Speed Competition, and the three current Kremer competitions follow later. Henry Kremer was himself the recipient of awards when in 1986 he was made an Honorary Companion of the Royal Aeronautical Society and was awarded the prestigious Gold Medal of the Federation Aeronautique International at their annual conference in Sydney, Australia on 10th October 1988. Kremer led technological developments in industry for over 50 years. These developments include the first chipboard and the first usable remote-control bomb-defuser. 

The late Mr Henry Kremer. RAeS collection

EFFECT OF FIRST KREMER PRIZE

The announcement of the first Kremer Prize in 1960 spurred many projects to be started. The success of these projects varied widely. No review of human powered flight which covers this period is complete without a description of at least one of the large number of eager but inexperienced pioneers.

ALAN STEWART was building human-powered-ornithopters before 1959, and was still making attempts in 1979. In fact these were all unsuccessful Stewart-powered-ornithopters, although one glided once. He was perhaps the most persistent and notorious of the many young men of this period who had more optimism than aeronautical knowledge. The passage that follows shows the technical level of many of the enthusiasts of this period, and illustrates the organisational problems faced by anyone operating in the real world such as Stewart's village, Greenhill.It illustrates one of the necessary attributes. a strength of audacious pioneering despite the mockery of the neighbours. Perhaps the main lesson to be learned here is that on any project which is seen as bizarre, as HPF still is, it is necessary to develop a technique for coping with the curious.

To quote Alan Stewart :-

It wasn't long before someone leaned over the garden wall.

"What's that you're making then ? "

I tried to avoid giving a straightforward answer in the hope he would go away, but others appeared asking the same question.

"An aeroplane", I admitted at last, hoping that would get rid of them.

"Get away ! You are having us on. You never are ! Come on, tell us what you are making"

"An aeroplane", I repeated, preparing to meet their criticism with thick skin. At first they were puzzled.

 People didn't make aeroplanes in their backyards.

"It's a practical joke !", grinned one at last, believing he had the answer. The others looked questioningly at me. I nodded. If they cared to think it was a joke, I wasn't stopping them. But as time went on, the people of Greenhill got to hear of the Kremer Prize and began to take me seriously. £5,000 was no joke. ( Stewart 1980 )

A different answer could have been something along the lines of Dr Paul MacCready's reply 18 years later to similar questioning at a time when he was as yet unsuccessful in muscle powered flight. (See Gossamer Condor.)

However, it wasn't Alan Stewart who lost out during a later series of questions, this time from the press.

Stewart again :-

The film sequence was carefully arranged....I was told to pedal as though really going somewhere. At a given signal I was to stop pedalling. This was a cue for the interviewer to duck under the wing and have a little chat with me. I pedalled until the wings began to fan up and down, the cameraman signalled me to stop and the interviewer came forwards, ducking smilingly under the wing to ask his first prepared question. "Tell me, Mr Stewart, don't you think this is all rather dangerous ?" But although I had stopped pedalling, the wings hadn't quite finished. They came steadily down to conclude the final beat. The main spar hit the interviewer on the head with such force that he fell to his knees. "It is a bit", I had to confess." (Stewart 1980)

PRESS ACCOUNTS cc1960

It was the less well-considered designs which were most written about and pictured in the daily press of this period, and the reporters found no shortage of material. Their vocabulary and accuracy however was more limited. Every overweight and under-wingspanned machine was pedalled "furiously", and every run was "an attempt on the Kremer Prize". In fact there was no official entry during the 1960s.

ROUGH ESTIMATE OF POWER REQUIRED

Sir Isaac Newton, Alan Stewart and those journalists in common did not understand the principle of the wing. Newton thought a wing worked by pushing air downwards. Since a propeller works by pushing air backwards, this would seem a reasonable assumption. A propeller creates a jet of backward moving air, but a wing creates a vortex around itself. A vortex is swirling fluid, as seen when a bath empties. At each wing-tip the vortex persists, and the wing leaves in its wake a pair of vortices. Thus the total vortex system is horseshoe shaped. At certain altitudes these trailing vortices give rise to observable vapour trails. These vortices are necessary for lift and will be present no matter how "perfectly" the wing is made. The creation of these vortices absorbs energy, and for efficient flight will be as small as possible. The beneficial effect of the vortex, the lift, acts all along the span so for the same vortex strength, the long wing produces more lift for the same energy. Hence, to generate the same lift a longer wing will need to generate a smaller vortex than a short wing, and will require less power from the pilot or engine. That is why success comes to HPAs with long wings. This energy loss manifests itself on the plane as a drag-force known as "induced drag". Added to the energy absorbed by this vortex generation will be the power which would be necessary to push the plane forward even if no lift were being generated. This is reduced by making it a smooth or "streamlined" shape, as for any vehicle. In practice it usually turns out that these two components of required power are of the same order of magnitude. Some optimisation procedures aim to make them equal The magnitude of this required power can be estimated at various degrees of sophistication as the project proceeds, but a rough estimate good enough to check the viability of a proposed design can be obtained as follows, from knowledge of three values and two empirical constants :-

total weight W lbs

wing span b ft

wing area S sq ft

empirical constant 700

empirical constant 1

V, the speed at which you might expect to fly is found from :-

V² = 700 x W / S

P, the total power required is found from :-

P = 1 x W x S x V / b²

This equation gives the value of P in the units of ft lb/sec.

550 ft lb/sec = 1 HP ( One horsepower ) = 746 watts

1 ft lb/sec = 1.356 watts

This rough estimate method assumes a lift coefficient of 1.2, ground effect factor of 0.80, pilot weight of 165 lbs, ratio of induced drag to total drag of 0.43, propulsive efficiency of 0.85. These are typical values for HPA. The ground effect factor mentioned will only apply near the ground; more power will be needed to fly higher. The propulsive efficiency figure mentioned here is intended to cover both the mechanical transmission and the propeller. This estimate is good enough to show that most of the "planes" shown in the press of the 1960s on the ground were almost certain to stay there.

EXAMPLE

Stewart-1976 Span 30 feet, Weight 250 lbs, Area 180 sq ft

V² = 700 x 250 / 180 = 972

V = 31 ft/sec = 21 mph

P = 250 x 180 x 31 / ( 30 x 30 ) = 1550 ft lb / sec (2102 Watts)

POWER AVAILABLE

The power that a person can produce depends on how long they have to produce it for, but however brief the flight, some time will be spent under exertion during the take-off run, say a total of half a minute. A person of average fitness can produce no more than 385 ft lb/sec (522 Watts) for this time, and 1550 is clearly more than 385.

Making the same calculation for Mufli and Daedalus, and comparing with more sophisticated estimates : -

*Precise values from (Sherwin 1971) and (Langford 1989).

The rough estimation method is only suggested for use in estimating the likely success of a proposed design at the "back-of-envelope" stage, or on first sight if only weight, span and area are known. For the classical method see (Glauert 1948). John McIntyre writes "Yes, a rough estimate of induced drag is useful to start with. To get it accurate later on, refinements will include considering the flexural and torsional deflection of the wing. The load distribution on the wing in straight flight is as you would expect from classical theory, except that the flexural deflection of the wing causes inward rotation of the local lift vectors. This reduces the lift of the wing and the aircraft must fly slightly faster than predicted - for an accurate solution, some iteration is necessary."

CALCULATOR

A calculator for the above calculations has been added. Clicking here will open it up in a new window

PUBLIC ATTITUDES

1959 were typified by those of Alan Stewart's neighbours portrayed above. The spaceships of fiction foretold real spaceships with reasonable accuracy. But in 1959 most people did not know what a human powered aircraft would look like. Those who responded actively to Henry Kremer's challenge did not know what one would look like either, but that did not worry us as it did others. The ability to conceptualize the not-yet-existent, an essential talent for creators, does not appear to be universal. After the prize announcement, "man powered flight" to the majority of people meant "you have to do a figure eight".

RAeS GRANTS for HPA DEVELOPMENTS

In the UK In June 1960 a grant-fund was set up to which Henry Kremer among others contributed. This fund still exists in 2007. An applicant for a grant must show primarily that the proposal is likely to lead to the development of human-powered-flight. In theory, grants are open to anyone who intends to research some aspect of the subject rather than build or modify an aeroplane, but such a grant has never been made. The applicant is asked to present a comprehensive assessment of the project, covering many relevant aspects to show that it will not falter for want of any essential ingredient. Also one is expected to have made the initial design decisions and show for instance the flight-envelope. The preparation of the grant application itself is thus useful in ensuring that one has not ignored anything of importance. (The PROGRAM section below, may hopefully serve a similar purpose.) The grant is not intended to cover the entire cost, and one must anticipate needing to defray expenses occurring while awaiting final approval. These grants have been instrumental in making several projects possible in the UK.

FIRST RAeS LECTURES on HPF

The Royal Aeronautical Society held several lectures on the subject of HPF around 1960. Here and there in the Lecture Theatre one would be able to see little knots of people feverishly taking notes; these were the rival groups. The lectures were often published a few months later (see Bibliography), but we wanted the information straight away. The RAeS HPAG continues to provide this service; typically now there is one event a year with a varying number of speakers. Many are reports of experience with HPF. Any prizes won during the year are presented.