while working with Geoff McBroom
This is an extract from his latest Book
People laugh at this now, but part of my job description while employed by Geoff McBroom was ‘Hang Glider Test Pilot’. It is important to emphasise that designing and building a really successful state of the art hang glider in the 1970’s was a far from easy task. It was all too obvious during flight testing, sometimes the moment your feet left the ground, when a promising looking new design had failed to live up to expectations. Others were found to be wanting after more extensive trials and some even after they went into production.
I started work at McBroom Sailwings in 1974 and in the following five years that the company traded we never suffered the setback of a recall. Other (sometimes larger) manufacturers were not so fortunate. Geoff is a really top notch engineer with a degree in aerodynamics and McBroom ‘kites’ were always designed and built with safety as a first consideration, consistent with the knowledge available at the time. However, not long before Geoff took me on, he was obliged to offer a ‘kingpost modification’ as it was proven that bracing the kite’s top surface with rigging wires had a very beneficial effect in helping to prevent divergent ‘luffing dives’. Catching up exercises like this were the price to be paid for being one of the first in the UK to produce hang gliders.
From then on, integrity of design and manufacture were synonymous with McBroom machines, which included the Argus standard Rogallo, the very successful Cobra (over 300 sold) the Lynx and the Super Lynx. To deliver such an improved range of kites required not only design flair but a cautious approach to issues that might affect safety. Hang glider development could hold some nasty surprises and progress was sometimes hard won.
True, building a prototype was not that costly in terms of materials or time, particularly if you had the necessary facilities as all the manufacturers did. In fact it was quite possible to build and test fly a newly configured machine in the space of a week. This partly explains the very impressive improvement in hang glider performance during this period. However, when someone with the undisputed qualifications, ability and experience of Geoff McBroom suffered his fair share of disappointments at this endeavour, ‘learning by ones mistakes’ could become an expensive and demoralising experience.
This new art/science of ‘hang glider technology’ tended to be unpredictable when applied to original designs. For instance, before initially flying a prototype, inverting the wing in the workshop was usually the only way to simulate the shape (camber and twist) it might adopt in flight. However gravity often produced different results to actual flight loads. It really was a case of suck it and see. This could be frustrating to a formally trained aerodynamicist as inevitably an element of luck was involved.
Another example of the way in which this contrariness could manifest itself was in the development of different sizes of the same basic type of glider. It is important to match the weight of the pilot to the size of the glider, as wing loading has a marked effect on the control authority available and the best sink rate achievable. Unfortunately simply scaling up or down the dimensions of a good prototype seldom resulted in a satisfactory larger or smaller version of the design. Something often got lost in the resizing process and frequently modifications were required or in the worst case the variant had to be abandoned altogether. We at McBroom Sailwings faired well in this respect but there were some other manufacturers who suffered badly from this syndrome.
Of course the rewards for creating a new a ‘winner’ tempted many to try – myself included. Just to be able to turn up at the hill with something different, that you had made yourself and that would ‘out fly’ other kites, guaranteed a tremendous sense of achievement. This was quite possible for any enthusiast with sufficient motivation, skills and resources and in fact there were notable amateur homebuilders that produced some excellent machines.
For the manufacturer, considerable profit could be made by translating a good prototype into successful series production. The home market was strong for top performing gliders as many pilots were keen to ‘upgrade’ on a regular basis in order to remain ‘top of the stack’. There also existed a very lucrative export market in some countries without indigenous manufacturers.
These powerful incentives further explain the phenomenal rate of improvement in hang glider performance and safety during this period. Although, as Geoff McBroom stated more than once, if this vast progress had been gained in the traditional aircraft industry with all it’s traceability paperwork, quality approval requirements, and rigorous testing it would have taken five times as long or not happened at all.
In contrast, being a new aviation sport, hang glider airworthiness issues were dealt with using simpler, more practical methods. A hang glider manufacturer’s association was formed which laid down minimum standards for hardware to be used and guidance on constructional methods to be employed. A series of flight tests had to be passed before a particular model could enter production. Based on common sense and not overly onerous to comply with, these requirements allowed small to medium sized manufacturers to be relatively free to experiment and innovate, driving development forward at a rapid pace.
The crux of the problem they had to solve was to produce a machine that more closely matched the superior performance of a conventional glider (sailplane) but which would still satisfy the definition of a hang glider – an unpowered aircraft that could take off and land using the pilot’s legs and feet as the undercarriage. Of course there were other considerations; weight, portability, ease of rigging/de-rigging, durability, ease of control, safety and cost. So, this dictated that, while other configurations were tried, the advantages bestowed by the ‘flying wing’ concept of the original delta shaped ‘Rogallo’ was retained together with much of it’s triangulated aluminium tube and steel cable structure and sailcloth wing.
As regards performance, an important aspect that needed to be improved was the ability to turn efficiently. Some of the earlier ‘kites’ had respectable sink rates when flown straight, but ‘plummeted’ when banked in a turn. This meant that they could not climb well when circling in a thermal. Greater wing spans and broader wing tips solved this problem as clearly demonstrated by Dick Boone’s Phoenix 6B. Nigel Milnes proved this conclusively in 1977 by being the first in the UK to fly 22 miles straight line distance from the Malvern Hills, mostly by circling his 6B in thermal lift.
Some features from the conventional glider such as increased wingspans, narrower wings (chords), smoother more accurately defined aerofoils, ‘double surface’ wings and the removal drag producing appendages were adopted when the necessary ‘breakthroughs’ in design were made and proven to work.
Being involved directly with this effort, I sometimes wondered how Mr Joe Public perceived the evolution of hang gliders over these short years. Comedian Jasper Carrot likened hang gliding to ‘jumping off Beachy Head attached to a frame tent’. Despite this amusing although uninformed description, the radical difference in shape between a typical 1975 ‘kite’ and the flying wing planform of a 1980’s machine must have surely made some impression on even the most casual outside observer.
Was this obvious transformation achieved by evolution or revolution? Sometimes the new ideas and developments came so thick and fast it felt more like a revolution at the time. Looking back now, the progress made can perhaps be more accurately described as a rapid evolution, characterised by a series of incremental steps forward. These were made by talented individuals answering particular design problems or taking an existing idea and developing it to a further significant level. The role call of honour included:Roy Haggard (USA), Bob & Chris Wills (USA), Dick Boone (USA), Bill Moyes (Australia), Miles Handley (UK) and Gerard Thevanot (France).
Interestingly, Roy Haggards’ contributions featured strongly at both the beginning and end of this 5 year period of development. His early ‘Dragonfly’ design, which had truncated wing tips, clearly showed that the ‘standard Rogallo’ was capable of being improved. He later developed the wonderful UP Comet which became available in its earliest form in 1980. This combined the best of previous designs together with ‘floating cross tubes’ (actually the keel ‘floated’) and was really the culmination of much of the effort to improve hang gliders during this time. The Comet was the ‘benchmark’ against which subsequent 1980’s machines were judged. However without the other vital advances made by the others named in the intervening years, the Comet would not have been possible. As will be appreciated, the above list is far from inclusive and apologies now for omissions, this a purely ‘personal’ take on what is an understandably complex subject.
© Copyright Andy Billingham 2009