Jupiter Continued


Flight Lieutenant John Potter MA RAF had contacted the Royal Aeronautical Society asking about HPF and learnt of the plight of Jupiter. A plane without a group. Well, actually the plane was in almost as bad a state as the group. He arranged to meet Sir Stuart and myself at Woodford, Essex to see the hardware. Number one, chronologically, in the actions for which he deserves special credit is having the imagination to see a heap of rubble and think `I am going to transform that into an aeroplane'.  As I saw it then, my options were to allow this man to turn it into a static exhibit at best, or to let it carry on rotting; so I chose the former.

MY FEELINGS (with hindsight ) about the HANDOVER

Jupiter was to become the only HPA which performed better after being handed over to new leadership. Is this a point for me to be proud of ? Yes, insofar as John could generally follow drawings originally prepared only for myself.   Yes, insofar as it meant that as mentioned above, Jupiter became in some ways the first non-experimental HPA. No, since if the mistakes I made had been different ones, the original group could have seen the project through. I had built the fuselage big enough for my 6ft 2inch (1.88 m) self, but realised that others have more flying skill. While being sole leader of the project, my intention had been that when it came to flying, that at the moment of take-off the pilot would become the boss. Thus you are flying your aeroplane. It is one system, maybe not the best, but the decision on my part to adopt this policy had partially prepared me for relinquishing control.


On arrival at Halton John and I dumped all the components in the canoe-club shed. The task of convincing others that here there was a worthwhile project began. Sir Stuart Mallinson wrote to Air Commodore Weighill, the commanding officer at Halton. `When Flight Lieutenant John Potter came here, he took away what was left over from a fire of an aeroplane built by Hodgess Roper.  ...  worked in two of my garages building what he hoped ... Unfortunately, after 5 1/2 years he had an accident which prevented him for over a year doing anything, and after some 6 1/2 to 7 years a fire broke out and destroyed part of the machine. It is quite impossible for him in his condition to carry on or replace, and eventually he apparently arranged for it to be taken over to your Base. I think there is no question that Hodgess Roper had a good idea and there was a reasonable prospect that he might have been successful. ..  I hope it may be possible for you to encourage the finishing of this machine.    ` Stuart Mallinson'


In much the same way as at Woodford,Essex, the plane was moved step by step into better premises. At one time it occupied the only room at Halton College which had access without going around twisting corridors. This was necessary because of the length of the wings. Other staff, Mrs Potter, and the more senior apprentices became interested, joined the Halton group and helped with the repair and later with the flying operations.


John's policy was that if anyone turned up, then he would find them something to do. I took a friend with me to Halton on one occasion and they were promptly given the task of making sanding blocks. In this way, if a person persisted they could then proceed to other tasks, if they didn't, then at least they had done something. As the group built up, he was able to delegate entire components, and 3/4 of the work of restoration was done in 20 weeks. (Potter 1973)


As far as I was concerned, from now on, Jupiter was nothing to do with me. But this situation did not last very long. John was working to the detail-working-drawings that I had prepared at Woodford, Essex for my own use, and they were such that he was able to do this. Well, there was an exception to this. On one drawing, I had written .028" x 1/4"  which taken literally indicates a strip  0.7mm x 6 mm and John quite rightly didn`t believe that this was what I intended. So he phoned me. I replied that this meant a tube 1/4" diameter and .028 thickness; a stocked size. Being asked to resolve this ambiguity was all it took to re-awaken my interest in the project. I visited Halton from time to time. I made a few new drawings or new copies of old ones.


The original tailplane and elevator needed replacing, and I took the opportunity of a redesign here, in consultation with John. The new surface was all-moving, had a higher aspect-ratio and was positioned slightly further aft. This component was built at Woodford, Essex in 1971. The original control linkage was retained. Unfortunately this led to flying problems, a small movement at the control bar having too much effect. I had made outrigger wheels and fitted these just inboard of the ailerons. John decided to dispense with these and they were not used in flight. I feel that this was probably the right decision. The Puffin I outriggers had been removed after early flights. However on Toucan, outriggers at 40% of the semi-span had proved satisfactory. The Halton group finalised the detail design of the aileron control linkage, and made and fitted these parts Much of the trailing edge had warped because of damp storage. This would increase induced drag because the lift distribution would be affected. Looking at it another way, in flight each warp would create its own little additional trailing vortex. So, a lot of the trailing edge was cut away and replaced. The outer wing sections were found to be in good condition and were left with the original blue Melinex covering which Susan and I had put on at Woodford, Essex. The other three sections needed various repairs and were then covered in John's choice of aluminised Melinex.


When the plane was nearly assembled, I noticed that it was very easy to move the top of the fin from side to side because the rear-fuselage is easily twistable. This didn't seem right, but apparently didn't matter, being less critical than wing torsional stiffness. Ironically, it was partly because of the observed low torsional stiffness of the tail-boom that I had rejected it.


Halton is a grass airfield. When the aeroplane was in one piece, John tried some runs there, but was finding that the tail was staying on the grass and with only a small wheel, that end was being alarmingly shaken. Hence he moved the aeroplane twenty miles (32 Km) to RAF Benson. This meant that during all the flight trials, people from Halton had first to make this journey. During the first runs on the Benson concrete runways the tail still would not lift. Then John found that all that was needed was more speed to make the tail lift. This is due to the constraints of this type of layout with the concentric wheel and pedals, affecting the centre-of-gravity position relative to the mainwheel (see Puffin). On Jupiter, more weight is on the tailwheel than one wants for take-off.

 I arrived at Halton on February 10th 1972 with a list of a few jobs which I considered needed doing before the plane was airworthy, to be informed that Jupiter had flown the previous day, twice. Three more flights were made on the 13th. On the last of these the wheel buckled on landing. I had seen none of these flights, and it was not until after the 25th February that I was able to write :-


"Dear Geoffrey                    28th February 1972

"A great shame you couldn't be there on Friday (25th), actually we only just managed to get a flight as by the time we had put the wheel back on and a new chain and got it outside it was nearing darkness. I believe I told you the wheel was damaged on the fifth landing, and the people at Halton had only just repaired it and took it to the aerodrome with me on Friday, When we had put the wheel back, a task which was accomplished not without my local knowledge of the particular machinery, it was discovered that the wheel-drive chain had also been damaged during the landing, but luckily we had a spare length and the special tools at Benson. So this was then trimmed to the exact number of links and fitted.  (The old chain which had made five flights was cut up and a short length given to several of those who had helped with the project.) We then opened the hangar doors, having got clearance to use the runway, and seeing that the weather was calm enough and trundled the machine outside, the wingtips just clearing the sides of the doors of the hangar which could take a Concorde. Taxi to end of runway, and you wish you had taken a taxi because you get more puffed out running alongside than the pilot who can cycle the machine at ten miles an hour with very little effort. Surprisingly, since the aeroplane only has two wheels like a bicycle this can be done without the assistance of anyone at the wingtip to hold it level because aileron becomes effective at quite an early speed, but assistance is required to help turn sharp corners on the ground since the wheels don't swivel to steer. So, having got to the end of the runway you have to point the plane in the right direction. There were only three of us there - John Potter, Ernie Moore and myself on Friday, which is the minimum, one inside and two ground assistants for handling and to put the cockpit hood on which is quite a knack as it has to mate up in several places at once. Having fulfilled these tasks, I was going to act as timekeeper of the flight, and Ernie as photographer. Two youths leaning against the enclosing fence of the aerodrome thought fit at this stage to shout to us "You'll never fly". On our first attempt that day we actually didn't, this was because I hadn't grasped the fact mentioned above re wing tip holding. I as wing tip holder had held on far too long as I ran alongside whereas I should have let go when the aircraft reached walking pace. All my previous  wing  tip  holding  had been with gliders. Ernie claims that it took off and he could see daylight under the wheel while I was still holding the tip. He may well be right, it turns out that the pilot finds it not clear to tell whether he is still on or just off the ground, and on this occasion the feel was further complicated by me holding on. I was looking forward and forty feet from the wheel all the time, and it was beginning to get dark. We waited for John P to get his breath back before making another attempt. Meanwhile I was thinking that if what I had just witnessed was flying then it was a bit low. This time he reached flying speed much quicker and when he left the ground there was no doubt at all that he was going up. What impressed me most was the rate of climb, particularly as my calculations had shown and I had read in several places that this would be a most difficult manoeuvre. The aircraft then levelled out to a height well above our heads and giving us the illusion of hovering. Then John must have given an extra burst of energy because the machine once more started to climb and then went off into the distance. Then I realised that I hadn't pressed the stopwatch. It certainly was a sight worth seeing. The day's events had very much taken the form of going upto a bicycle, fixing the wheel and the chain and then just wheeling it out of the shed and pedalling it into the sky.  I have up to now felt insulted when the machine has been described as a "flying bicycle" rather than as an aeroplane, but once airborne the size is apparently lost amongst the vastness of the sky, and what has happened is that he's pedalled and its flown. It is a Flying Bicycle. He landed and we ran to the aircraft, and he commented on some minor adjustments we had made to the instruments. I felt that watching the flight was a worthwhile recompense for having travelled to Halton, travelled from there to Benson, worked on the wheel and the chain and done quite a lot of running along the side of the runway. The true cost, of course, is all those, but multiplied by you-name-it and then put a nought on the end, or two noughts. And whether I feel it was worth this true cost I can't say. Thinking about it since, sometimes I feel I can't say yes and sometimes I feel that I definitely can't say no.'

 Geoffrey, to whom this letter was sent, came to Benson in June 1972 and observed the record-distance flight. As it happened he had the aggravation of a minor car-accident on the way. I asked him ( 1990 ) whether he felt it was worth it. `Yes, definitely', he replied.

(In 2002, Geoffrey himself became airborne under his own power pedalling the hovercraft, Steam Boat Willy. I remember that one too. An ex-marathon-runner, at the age of 62 he pedalled excessively on his first take-off and the craft shot away uncontrollably.  But that`s another story. )

Two more flights were made on the 27th February, and three more on the 1st March.  By the end of 19th March, twenty five flights had been  made, the longest about a quarter of a mile.

Jupiter flying at RAF Benson circa 1975


John Potter had made several flights before having the opportunity to observe one from the outside. It was Ernie Moore who gave him this opportunity, by making a few flights. Based on his observations, John was able to establish the best speed and attitude-in-pitch for Jupiter. Following this, flights were of longer duration as they were less exhausting.


It was found that aileron became effective at 5 knots, rudder at 8, but the tail did not lift until 13 knots. ( 1 knot = 1.15 mph = 1.69 ft/sec = 0.52 m/sec ) After this point, John recommends holding the acceleration steady. He later analysed the optimum pedalling effort during the ground-roll in order to be the least exhausted at the point of take-off (Potter 1975). However this analysis omits that the relevant quantity is anaerobic energy. Such an analysis will be vital for take-off from water.


It took some time to locate the source of an occasional loud banging noise that was occurring during flight. I discovered that on the ground this could be cured by holding my thumb against the moving chain as John pedalled, and we deduced that the propeller drive chain was sometimes jumping a tooth. Halton group members fitted a spring-loaded tensioner.


The Community Relations Officer at RAF Benson arranged a press-conference for a Sunday, always a good move, namely March 19th. This was to include a demonstration flight. As mentioned in the foregoing letter, I had been impressed by the flight-pattern of holding one height for a while, and then doing a further climb. I discussed this with John, who for most flights had been inside. We agreed to do this during the demonstration flight since with the capabilities of the aircraft as it then was, this was one of the most impressive series of manouevres that could be done. The first flight that day was disappointingly brief, as was the second. By the time the aeroplane was repositioned for another attempt, the wind had sprung up.  Take-off into wind is considerably easier in Jupiter, thus conserving the pilot's energy. John was able to start the series of manoeuvres we had planned. As it happened, just as John started the second climb, the plane met a gust with the result that the climb was much more impressive than we had expected. The effect of the gust was to increase the effective airspeed, and the plane climbed like a kite, the mass of the aircraft being the "string". John levelled out, and for a while held a straight course. Then a second gust hit the plane, this time from the side. The effect of the gust and the wind-shear put the aircraft out of control Suddenly I noticed that Jupiter looked different, but it had a familiar appearance. Somewhere I had seen that view before. It took me a few milliseconds to remember that it was on my drawing-board that I had seen that view. It was a front elevation - spot on. Jupiter was heading straight for my bit of grass, it was out of control and losing height. Luckily I had thought out the appropriate response for just such an eventuality. I dropped to the ground. The rationale is that the wing will pass over you. It did. Seconds later I was on my feet and observing the heavy landing, wing-tip first. I was in line with the wing at this moment and observed it to be deflected into a curve I would not have thought possible, but nothing broke on the Balsa and spruce wing. A second later the fuselage came down with a thud and its height was reduced to a dimension that was surely not enough to house a living John Potter.   It was a worrying few seconds until Tony Gilchrist and I reached the cockpit and tore away at the fairing, to find John all in one piece.  `Actually, that wasn't supposed to happen', said John for the benefit of the pressmen who were arriving. It had been Tony who had installed the aileron control linkage, and since it was failure of lateral control which had occurred, he promptly checked to see if it was the linkage that had been at fault. It was not so. Photographs show the controls at full deflection during the descent.  The weather conditions had just been too much for it.

Jupiter flying at the press conference at RAF Benson in 1972. RAeS collection

DAMAGE On inspection later it was found that damage was only to the wheel, the cockpit canopy and a dozen spruce to spruce glue joints on the top and bottom of the portion of the fuselage under the wing.

`IT'S DOWN, LET'S KICK IT' Journalists at Benson 1972, who would not have deigned to touch it when it stood majestically in one piece, swarmed forward to have a prod after Jupiter was seen to crash-land. There was already damage which turned out to take six weeks to repair. We didn't want any more. I was the only civilian in the group who was present at the time, and the only person who made any attempt to defend the aeroplane from this aggression.  The Royal Air Force had been invaluable in enabling John Potter to restore and fly the machine, (Seven years later the RAF helped Gossamer Albatross), but my belief in the benefits of national armed forces in general had been declining for several years and took another downward turn when observing their inaction while our aeroplane was being attacked. This demonstration received greater media exposure than that of any other aircraft prototype except the Concorde, but as is usual, some of the descriptions were imaginative. One photograph showing Tony Gilchrist and myself running is captioned "..as running sightseers keep pace with it ..". Flt Lt A J Gilchrist was effectively the chief maintenance engineer, and as I remember, I had some sort of connection.

NAME During this much publicised flight the aircraft still did not have a name. The previous evening I had thought up two; both retained the tradition of the first vowel being "U". "Unicorn" because the pylon is like the single horn of this mythical beast, and because it was now flying from Benson, the home of the Queen's flight, and the Unicorn is one of her symbols. "Jupiter", after the planet, astronomy being one of my hobbies at the time. I had once managed to see this planet in the daytime without binoculars (we couldn't afford binoculars); it can be done if you know just where to look. This had reminded me of the discovery of the planet Uranus in 1781. Herschel first observed this through a telescope, then realised that it can be seen without such aid. Similarly, flight was first made only with the aid of engines (or winches), and now we were doing it without. I wasn't calling it "Uranus" and John didn't like "Unicorn" so that settled it.

REPAIRS On most trips to Benson, the Halton group would need to do some minor repair in addition to any flying, though not as extensive as after the landing of the 19th March. As a result the aircraft was gradually getting heavier.

MIST The inside of the cockpit used to mist up, and John double glazed a small area for forward vision, and cut some slits. It was necessary for the pilot to do some physical warming-up exercises before a flight. This would mean that the screen would become misted during taxiing. To obviate this John developed the technique of a "cold pilot" and a "hot pilot". The cold pilot who had not warmed up and was not perspiring would taxi the plane to the end of the runway.

INSTRUMENT To indicate attitude in pitch, John Potter added a mercury-switch controlling a red and a green light, powered by a hearing-aid battery. On test it was found that the best attitude was when the green light was blinking. The pilot could tell which light was shining without turning the head. This has been recognised by people outside the Jupiter project as a significant innovation. It certainly made a lot of difference to the actual performance of the aeroplane.

RECORD DISTANCE By May 1972, John had learnt exactly what speed to fly at for minmum power and how to take-off with minimum energy, and he had done a lot of cycle training. He had told me he was going to give up smoking. `But you don't smoke', I replied `Oh, I smoke about one a week', he said. It became clear to both of us that Jupiter was not capable of the figure-eight course. Arguably the power-requirements were too high for the climbs and distance alone. Having to do turns made it out of the question. What was worth attempting was a new distance record. The length of the Benson runway had been covered several times, and on the evening of June 29th 1972, John Potter was observed to fly 1171 yards (1070 m). The configuration in which it established the distance record was the 1963 design from my drawing-board with only the addition of the chain-tensioner and the instrumentation.

FIRST "COMMERCIAL" HPA We established another `First' by carrying payload on an HPA, namely printed envelopes stamped `Worlds First Man-Powered AirMail, Jupiter, 1972'. These philatelic covers were sold to collectors, and  raised funds for the RAF Museum.


HPA`s have appeared at Air-Shows, for which they sometimes get fees. The current RAeS "Sporting Aircraft" Competition implies that the winner will go into production and sell copies of the winning craft to athletes for use in the sport. Some of the Japanese groups are partially funded by the prizes that they win. Most prizewinners have found that the cash won does not cover the costs. Now you`ve got this free on the web, I don`t even get paid for this book, but you are welcome.

CRANWELL In 1974 John transported Jupiter to Cranwell, where it hangared alongside the Dumbo/Mercury which he also acquired, but no great success was achieved with either of these planes at this airfield. In 1978 the Jupiter was retired to the Shuttleworth Collection and remained there until 1982 along with Toucan and SUMPAC.

THAMESMEAD FESTIVAL of HUMAN POWER 1984 This was a weekend meeting of HPVs of land, sea and air. The Jupiter was on static display and the only aircraft, since no current HPA was in Britain at this time. The purple Melinex on the outer wing-panels and ailerons was still drum-tight 19 years after Susan and I had glued it on (maybe only ten years after John had retensioned it). The propeller still spun. Festival-goers enjoyed sitting in it betwen HPV races.


Jupiter is now located at the Filching Manor Motor Museum, Polegate, near Eastbourne, Sussex, England. It may be viewed by appointment.

What I did right:- Design. Used the conventional answer whenever there was one; and if there was not, then thoroughly bench-tested the new idea before incorporation into the airframe.  Organisational. After it became impossible to continue at Woodford,Essex, I handed over as soon as I could.

What I did wrong:- Design. The structural design of the wing to fuselage junction, though satisfactory, could have been neater and lighter. This is one of the ways in which the Stork aircraft was better than Jupiter. Organisational. There could have been contact with other HPA groups. During most of the time that Jupiter was at Woodford, Essex, we were located not far from SUMPAC (London) & Toucan (Herts). As it was, with no such contact, the sense of isolation was contributory to loss of morale. Links with other groups might also have helped solve the other organisational problems. Made plenty of other mistakes as mentioned above.

What John Potter did right:- Took the opportunity of taking over in the first place.  Organised the best use of all the people and facilities that he had available, or could make available, to restore and fly the plane. This included, at the flying stage, intensive use of the "facilities" of the aeroplane itself and his own legs.

What John Potter did wrong:- During re-building, no components were weighed. There are a few details of the Jupiter which are badly constructed. Most noticeable is the wrinkled covering of the rear-fuselage. This could have been avoided by more consultation with me. In general the standard of construction of the Woodford-built parts is higher. With regards to flying, as he realised himself and wrote (to me) in 1973:- `I hope we shall be able to conduct our (further) trials in a rather more professional manner than we had time for last year (1972). It was all rather a rush, and I am surprised we achieved our results in a very short space of time '

CHIGWELL  My next design never flew. It was based on what I misguidedly thought to be the best points of Jupiter and the best points of Puffin II. My February 1973 drawing shows a span of 80 ft (24.4 m), area 380 sq ft (35.3 m2), much of it constant chord for ease of construction. Ailerons moving up only and tip spoilers. Upright pilot. Twisted chain transmission to 10 ft (3 m) propeller mounted behind tip of fin. No rudder. It was designed to take off from grass.

All moving tailplane, also at tip of fin. Primary structure as Puffin II/LiverPuffin. Secondary structure Balsa and EPS. With its low wing position, tapering rear-fuse and "T" tail it looked like a jet transport, and had enormous fuselage wetted area. Optimisation was based on turning flight.  Constructional test-pieces made.   Propeller built by Mike Knight of Essex Gliding Club. Ergonometer made.

METAL CHAINS CAN TWIST - OFFICIAL Lack of sufficient appropriate support ended the Chigwell project, but not before I had written to the Application Services Manager of Renold Ltd with regard to use of their chain 51308/01 (aluminium, fits bike sprockets). He replied  `... we confirm that this chain will twist through 90 degrees over 54 inches (1.37 m) and although adequate for the loading figures given [cruise 75 lb(34 Kg), climb 144 lb (65 Kg), ultimate 352 lb (160 Kg)], we are concerned with the effect of the twist and reverse twist under load conditions and suggest that the system be tested before any flights take place. Also the chain should be visually checked at regular intervals to confirm that the security of the chain parts is still intact. [It can be arranged that there is no reverse twist, and if there is any, that it is on the slack side.] `Effective lubrication is most important. The lubricant must penetrate between the the pin and bush and pin and roller, and to achieve this we suggest dipping into a bath of good quality light machine oil prior to assembly, and re-lubricating regularly.'

1975 Frank Vann, John Potter and the author met in May 1975 to consider a new project, but collectively there was not enough enthusiasm at the time for this to proceed