On 19 November, the RAeS's General Aviation Group held its conference  'Light Aircraft design: Methods and Tools 2012'  looking at the design of GA aircraft. Report from the event.   Once again the General Aviation Group heard a suite of papers reflecting experience and views relating to the design of light aircraft at its one-day conference on 19 November, supported by the sporting organisations the Light Aircraft Association (LAA), the British Microlight Aircraft Association (BMAA) and the British Gliding Association (BGA), and the software company Strand7. The Group’s principal objective is to encourage innovation and entrepreneurship in design. Its recent series of conference programmes has been specifically aimed at providing useful and practical information to those interested in the design process and keen to take part in it. Thanks to the Society’s move towards publishing more material on its refurbished website, the Group has posted more of its past proceedings on its web pages; anyone interested is encouraged to refer to them. We plan to recover and publish more earlier material in due course. As the range of topics covered expands, we hope that these will form a body of practical and up to date references which will complement the existing literature.  

The Swift aircraft

[caption id="attachment_7895" align="alignnone" width="390"] The Swift aircraft (Swift Aircraft)[/caption] This year’s design case study was the Swift two-seat composite aeroplane, being developed by the Swift Technologies Group under the direction of its founder and owner David Stanbridge. David, who is an aeronautical engineering graduate, keen pilot and enthusiastic ambassador for British aviation, set up the Swift Technology Group in 2008. Early on the Group acquired Europa Aircraft. Now it is in the late stages of developing the Swift VLA (Very Light Aircraft), a new two-seat trainer and the first new certified aircraft in the UK for a generation. It is to be part of a range of composite, aerobatic-capable training aircraft suitable for the general aviation market, flying school training and the military. Existing aerobatic training aircraft are all ageing; even the RAF’s Grob trainers first flew nearly 30 years ago. David spoke in detail about the specification and his determination to make his aircraft practical in terms of cockpit roominess, payload flexibility and good cg range to cope with different loading combinations. Also he stressed the importance he places on the design having an attractive appearance and his choice of Spitfire-like elliptical wing plan to emphasise its British origin. He outlined the principal factors covered in the design and the need for considerable emphasis on iteratively refining the basic design to accommodate the many necessary compromises. To speed up this process, Swift uses an Excel programme it has developed to co-ordinate its work with 3D CAD, CFD, 3D graphics and FEA. CFD (Computational Fluid Dynamics) is used to model flows over the exact aircraft geometry, establish forces and moments, examine drag trends with different design features and include propeller slipstream effects. 3D graphic simulation is used for rapid and accurate visualisation. For example, it is used to resolve ergonomic issues such as visibility from the cockpit and for refining cockpit space for pilots of differing sizes. How conspicuous the aircraft is in different colours against differing backgrounds has been studied in consideration of the safety aspect of avoiding in-flight collisions. David spoke of the advantages of composite materials learnt from experience with the Europa aircraft. Two main options for lifting surfaces are (1) foam filled and (2) composite sandwich, each with advantages in different parts of the airframe. He noted that FEA (Finite Element Analysis) is really the only way to optimise double curvature composite shells. David closed by saying that he plans to make design spread sheets freely available soon for download from the Swift Aircraft website. An e-mail help desk is envisioned. Of course, the design process continues all the way from concept, through applied aerodynamic and structural theory and systems integration to the details of construction and testing. There are frequently problems in the details.  

View from the LAA 

[caption id="attachment_7896" align="alignnone" width="403"] Rutan Long-EZs at the 2012 Sywell LAA Rally. (Light Aircraft Association).[/caption] The LAA has some 8,000 members and it oversees 2,600 aircraft of over 300 different types currently flying, not to mention a further one thousand aircraft in the course of home construction. Typically five or six new designs are taken on each year for the assessment and clearance; today the majority are designs which originate overseas. From his experience of 20 years as Chief Engineer of the LAA, Francis Donaldson has seen more than his share of mistakes in design or in the proving stages. Common problem areas include structure, structural testing, flight handling, configuration, and weight and balance. In his talk on mistakes to avoid, Francis concentrated on structures and structural testing. Francis outlined the various steps in a traditional wing stress analysis, including spanwise load distribution, tail load increment, inertia relief, wing torque, drag and anti-drag loads, effect of struts, and the stressing of the wing root. He advises the designer to avoid solutions that will take a lot of effort to analyse and emphasises the importance of ‘sanity checks’. There are mistakes to avoid in the structural testing stages too. For instance, sandbags on a wing load test should not be overlapped to become self-supporting like bricks in a wall if a representative loading distribution is to be ensured. The behaviour of a loaded structure should be anticipated and carefully observed at the approach to proof and ultimate conditions. Safety precautions are advisable to contain extreme deflections.  

Modelling the design

[caption id="attachment_7897" align="alignnone" width="383"] CAD modelling of a light aircraft (Strand7)[/caption] FEA, which has been used in the aerospace industry since the 1960s, is increasingly available in lower cost CAD programs and can be performed more flexibly as part of the design iteration. However, it remains important to have a clear understanding of the limitations of FEA and of the software being used. Gerard Carè gave a brisk but comprehensive demonstration of the process of creating and analysing a wing box structure. Gerard is Managing Director of Australian company Strand7 and has been principal developer of its FEA for over 20 years. His wing box example was made up of two spars, top and bottom stressed skins, and a number of ribs and skin-stiffening stringers. Setting up and 'cleaning' the model geometry from the CAD file is followed by defining loading zones and boundary constraints. Meshes can be applied automatically to the shell elements. Linear static stress analysis is the most basic structural analysis, determining displacements, stresses and strains. Nonlinear static and dynamic analyses are also available as are Euler buckling analysis and inertia relief analysis. Composite laminates can be modelled and analysed, based on the classical laminate theory. Various graphical means are used to illustrate the resulting deflections and element stresses. The displacement distribution is normally reviewed first to ensure the results make sense. For thin shell elements the shapes taken on from local buckling are shown.  

The Sherwood Ranger

[caption id="attachment_7898" align="alignnone" width="403"] The Sherwood Ranger biplane from TLAC. (TLAC)[/caption] The Sherwood Ranger is a two-seat open cockpit biplane. Originally designed by the late Russ Light, it is now being made by The Light Aeroplane Company (TLAC) at Little Snoring airfield in Norfolk. Managing Director, Paul Hendy-Smith, described how he acquired the design and manufacturing rights in 2007 and set about creating the current business. Paul’s own career was in manufacturing, both in the UK and in Germany. His talk focused on the business basics and the stark realities of venturing into this marketplace. Before purchase, the project had fallen dormant. Practically everything had to be worked over, from significant work on the design, creating manufacturing drawings, sorting stock, designing and making new tooling, to purchasing replacement equipment. Marketing and business plans were drawn up and the aircraft is now available as a microlight or Group A aircraft. In the UK it is available in kit or Fast-Build format; overseas it is also available as a finished aircraft. One of the more difficult areas is in the acquisition of materials, both aluminium in imperial sizes from a shrinking number of manufacturers, and wood, with fewer suppliers, a world shortage of spruce, severe price increases and extended lead times. Paul cites Vans Aircraft as an example of a company that has achieved ‘critical mass’ in business terms, with 18,000 customers and 7,500 aircraft flying. Looking at manufacturing light aircraft today, in the UK we are down to half a dozen companies compared with the 30 odd of 60 years ago, the market is shrinking and costs are rising. Nevertheless, he has ambitious plans for TLAC in its chosen market niche.  

The insurance angle 

[caption id="attachment_7899" align="alignnone" width="312"] The RAeS/Boeing Schools-Build-a-Plane Rans S-6 Coyote at Farnborough Air Show in 2012. (SBAP).[/caption] The GA Group continues to lobby the CAA on the question of a UK experimental category, which it originally proposed in its specialist paper of 2006. Clearly risk is a predominant factor in these discussions so, with these in mind, Norman Pocock, Managing Director of H R Jennings and Co, aircraft insurance brokers, was invited to give a view on behalf of the insurance business. Norman has spent his career in insurance, specialising in the aviation business. He is a Companion of the Royal Aero Club and has been involved with the Boeing/RAeS Build-a-Plane scheme. He spoke about the customer- broker relationship and the importance of the insurer's credibility. In the UK the business is closely regulated by the Financial Services Authority (FSA) to ensure adequate financial stability. Although insurance cover may be sought overseas, the level of regulation may not be as stringent. Discussion extended over topics such as aerobatic and display flying, airfield liability cover and finding cover for microlights and the ‘new’ SSDR (Single Seat De-Regulated) category. It appears that the light aircraft community would benefit from an improved dialogue with appropriate people in the insurance business.  

Crashworthiness of GA aircraft

[caption id="attachment_7901" align="alignnone" width="375"] Aircraft crashes can generate much more vertical accelerations than horizontal ones. (Cranfield University)[/caption] After retiring from 30 years as a family doctor, Dr Tony Segal studied at the RAF Institute of Aviation Medicine (IAM) and the accident investigation course at Cranfield. Then, over some 25 years, he carried out full size experiments into glider cockpit crashworthiness at the RAF IAM and RAF Bicester. He is himself an enthusiastic flyer with some 600 hours in gliders, as well as time on light aircraft and a number of parachute jumps. He has made it his mission to improve safety by promoting design changes to increase the crashworthiness of gliders and light aircraft. He opened with a review of the levels of acceleration that the human body can withstand before damage to organs or the spine become severe. These levels are highly dependent on the duration of the acceleration and for short impact periods can be surprisingly high. Conversely, there are resonant effects which can be particularly damaging. Important data has been plotted by the NASA researcher Martin Eiband. It is worth noting that human tolerance to short duration acceleration without severe injury can be substantially higher than those required by CS22 for restraining structure. An aircraft crash is more likely to generate damaging vertical accelerations than horizontal ones, distinguishing this type of accident from those of motor vehicles. So how to design the cockpit? Tony gave a number of pointers to good practice, including the design of a strong cockpit cage and an energy absorbing nose. The seat harness needs to be adequate both for in-flight loads and accident impact conditions. Glider undercarriage design with a single main wheel close under the cockpit needs careful attention. Accident data for gliders show the prevalence of spinal injuries. Tony has given considerable thought to the best means of protecting against these and gives a number of recommendations including seat shape to control posture, seat structural support and the use of energy absorbing foam on the seat pan to prevent rebound accelerations (see his presentation and accompanying paper on the Group's web pages).  

Summary

[caption id="attachment_7902" align="alignnone" width="349"] Vans RV-8A (TimothyMN/Wikipedia).[/caption] This year there seemed to be a significant development in our audience with delegates from diverse backgrounds, more university staff and, most importantly more young people, both men and women. Hopefully this is something to build on.The reader is encouraged to refer to the Group’s web pages to see the presentation materials from this and other recent conferences. As always the Group is keen to hear views from members on topics of interest, especially design issues which have not yet been examined. This article previously appeared in Aerospace Professional.  

Royal Aeronautical Society
15 March 2013