Joining the green aviation club 01 June 2021

Flying under their own power has fascinated many people since the dawn of humanity. Next year, a human powered aircraft race across the English Channel will test both pilots and planes. What fastening technologies will keep them together?

Since humans evolved enough to think about birds, they’ve wondered if they could fly like them. Greek mythology tells the story of Daedalus and Icarus who had wings of feathers and wax, but the challenge with the pilot providing the power is not the problem of getting too close to the sun (as Icarus did) but that humans can produce only about a third of the horsepower necessary by pedalling. This is one hundredth of the power in the smallest aircraft engine. Due to this, Human Powered Aircraft (HPAs) have to be extremely light, but strong enough to survive some turbulence and maybe a few gentle crashes. Joining methods have to be equally light and strong.

HPAs fly slowly and just above the ground (or water), so crashes or structural failures are not serious (no one has ever been seriously hurt in an HPA crash). Therefore, no licenses are needed and anyone can build one. The risk is less than riding a bicycle.

Structures are made largely of composites materials (very often carbon fibre reinforced plastic), foam and a thin covering, so they are very light and strong. The adhesives and fasteners have to be equally light and strong. In the early 1980s, I flew an inflatable HPA called Phoenix, which was made mostly of Mylar glued together with Bostik 1755 adhesive. All fasteners were designed and made by the aircraft designer/constructor Fred To. I crashed it in a London park with neither me nor the plane being damaged.

Later, flying in London’s Docklands (they were not built up then), we achieved a cruise speed of 4mph and accidentally gained the world record for the slowest aeroplane (balloons and helicopters are aircraft, but not aeroplanes). The HPAs that will fly across the Channel next summer will cruise faster, but not dangerously so. Fred To conceived the race and has put together the team that will run it.

The race will commemorate the 60th anniversary of the first recorded HPA flight, which was Southampton University’s Man Power Aircraft (SUMPAC) piloted by Derek Piggot at Lasham airfield in 1960. He covered a distance of 64m (210ft), climbing to a height of 1.8m (6ft). The race was to have been held this year, but it’s been delayed a year because of the Covid-19 lockdown. It’s not just about aircraft design, but also encouraging fitness and interest in the STEM subjects (science, technology, engineering and mathematics). Large scale aviation is under pressure to be more green. You won’t find any aircraft greener than HPAs.

Up to four aircraft can enter the race and designers and builders worldwide are putting forward their aircraft. Take off will be from The Warren, about four miles west along the coast from Folkestone and landing will be on the beach at Cap Gris Nez in France. Each aircraft will be accompanied by a boat in case it has to ditch along the route. The boats will carry rescue swimmers to ensure the pilot gets out safely.

The route is the same taken by the first and only crossing of the Channel by an HPA, which was Paul MacCready’s Gossamer Albatros – flown by cyclist Bryan Allen in 1979. Allen completed the journey in 2 hours 49 minutes in his 31kg HPA. He is on the Great Race organising team.

One of the main aims of the race is to encourage the designing and building of HPAs, particularly by young people, to encourage their interest in technical subjects and keeping fit. Very short flights can be made in many places and because they fly low and slow, HPAs do not involve high risk. A longer-term aim is to encourage people to take up HPA flying as a sport. In 2012, the Icarus Cup was launched, which sees several aircraft and teams meeting on a UK airfield each year to compete.

 

Building an HPA

Designing and putting together a HPA is now within the grasp of many groups because of advances in aerodynamics, materials, structures and fastening systems. Years ago, the involvement of an aeronautical engineering department was recommended because the science was not that well understood. One of the most successful HPAs was the NASA/MIT Daedalus, which was flown from Crete to Santorini in the late 1980s by a Greek pilot, re-enacting the mythical flight of Daedalus, of just over 70 miles.

US HPA designer/pilot and team leader Alec Proudfoot recently told The Telegraph – a UK newspaper: “Crossing the English Channel in a human powered aircraft has been done, once before – barely. It was one of the most amazing athletic achievements of our time. To think that several international teams are going to attempt the same feat, on the same day, in a race to see who is the fastest, seems almost a bit bonkers. It will be a huge technical and logistical challenge, but most of all a supreme test of athletic and piloting skill.”

Lined up to take part so far are Wiltshire-based Team Aerocycle and a team from Bordeaux University, with others expected to join over the coming weeks. The fastest to complete the crossing will receive a GB£50,000 prize, with GB£10,000 going to the second fastest and a prize of GB£5,000 for the fastest female pilot.

“Flying a human powered aircraft takes between 300W and 400W of power,” says HPA designer Fred To, the race instigator. “To keep this up over the several hours crossing will require levels of stamina close to what is needed in the Tour de France.”

One of the most successful current HPAs is DaSH (Dead Simple HPA), designed, built and flown by Alec Proudfoot. “The bulk of the joining of things on the airplane is done by glues and epoxies, which are carefully apportioned and weighed out, with glues used in very thin layers (or where appropriate, in small spots rather than full layers) with many tests done before final parts are built, to minimise the total amount of glue used to keep the weight down.”

These include:

  • Cyanoacrylate  (CA) – small spots of CA are used to hold some pieces in place for epoxies to dry, for example at the interface between the rear of ribs and the trailing edge.  Foam safe CA is also used for repairs on things such as the rib caps and rib doublers and to repair breaks in the XPS and EPS foams used (usually with balsa doublers to help strengthen the break). All CA glues used on the plane are foam safe, as they can get confused with foam melting types.

 

  • Laminating epoxies – the tubes used for the fuselage and wing spars use pre-preg carbon fibre with laminating epoxy, baked in an oven around aluminium mandrels, to make the tubes. The trailing edge and various pieces used in the joystick and handlebar assembly, and the bottom bracket assembly, use laminating epoxy with either Kevlar or carbon fibre cloth. In some places, carbon fibre cloth is used with Nomex honeycomb to make lightweight honeycomb panels (used in the joystick and handlebar mount, and bottom bracket mount, as well as fuselage fairing mounting, for example).  All of the above parts are vacuum bagged and built with the minimum epoxy to make a strong part (multiple test parts are built and tested to destruction to determine the minimum epoxy needed to maintain good strength). The main laminating epoxy used is MGS epoxy – 285 resin with the MGS epoxy 285 and fast hardener.

 

  • Structural epoxy (adhesive epoxy) –  DaSH uses Hysol-20HP and  Hysol-120HP to join aluminium pieces to each other and to composite parts. An example is gluing the aluminium brackets that join the wing sections into the ends of the rear spar and front support tubes. These epoxies are sometimes used for non-aluminium parts due to the convenience of their squirt gun style applicator (for example joining the ribs to the main spars).

 

  • Uhu Por – a contact cement used to glue the Depron foam leading edge material to the thin plywood rib caps that top the XPS foam rib.

 

  • Pliobond  – a contact cement that can be heat activated is used to attach the 0.0005 inch Mylar skin for the wings and tail surfaces to the 0.4mm plywood rib caps.

 

  • Polyurethane glue – DaSH uses both Elmer’s E9416 Glue All Max and Gorilla’s polyurethane glue to make lightweight but very strong connections between wood and XPS foam in the ribs, for both the rib caps and the strengthening balsa doublers that go around the holes for the main, and rear spars, and the trailing edge attachment point. The technique for doing this is to wet the XPS or EPS foam used for the ribs, then apply a very thin layer of glue to the 0.4mm plywood rib cap, or to 1mm or 1.5mm balsa doublers, thinning the layer out with a squeegee until the glue is almost gone, leaving just a very thin sheen on the part. Then the piece is clamped (vacuum bagged around the rib blank for the rib caps, or clamped with each rib between a board for the doublers) while the part dries over about 20 minutes.  The moisture put onto the foam causes the polyurethane glue to foam and make a very lightweight but strong bond.

 

  • Aliphatic wood glue – DaSH uses Titebond III in several places, to join wood. One example is the lightweight pieces used to form the shape of the ribs and formers used to shape the exterior of the fuselage frame. The ribs are made by cutting 6mm wide strips of 1.5mm thick balsa and gluing four strips together around aerofoil shaped forms made with finishing nails – staggering any gaps between layers (and forming a 6mm by 6mm cross sectional piece, four layers deep), by using thinned out Titebond III glue.  When this dries, laminating epoxy is used with very thin carbon fibre strips to create an outer carbon fibre layer on these laminated balsa shapes, which adds quite a bit of stiffness for not too much extra weight.

 

One example of how the DaSH team worked out the ‘right’ way to make a part is the rib caps on the wing and tail ribs. The team tried virtually all types of adhesive (epoxy, aliphatic wood glue, polyurethane glue, various contact cements) to see which worked best and make the lightest, strongest parts (50mm wide XPS or EPS foam blanks are hot wire cut out, then 0.4mm, three layer birch plywood is glued onto the blanks and vacuum bagged.  When dry, 6mm wide ribs are bandsaw cut from the rib blanks, then balsa doublers are added to the 6mm wide ribs). After running many tests, all types of glues were found to work to some extent, but the polyurethane glue made the strongest and lightest bond.

 

Time is tight

HPA designs are mainly about keeping the weight down. Most current HPAs are about half the weight of the pilot. This gives the pilot/aircraft combination endurance – it can stay up quite a while. To win the race, speed will be important, so it will be a speed/endurance combination that will win the day. The flight of just over 20 miles might be competed in just over two hours. A windless dawn will be chosen for the race, but a light tail wind might arise; the route is about eastward and the prevailing wind westerly.

Teams have just over a year to get their aircraft built and ready to take off from near Folkestone, England. The sciences of materials, joining techniques, aerodynamics and fitness will be pushed to the limit in aircraft where efficiency means everything. A well-known phrase in aircraft design is “simplicate and add lightness”. In planning and building an HPA, it could not be more appropriate.

Claire Aldridge Deputy Editor t: +44 (0) 1727 743 889

Having joined the magazine in 2012, Claire developed her knowledge of the industry through the numerous company visits, exhibitions and conferences she attended both in the UK and abroad.

Claire prides herself on keeping readers well informed and up to date with the latest industry news.