The last talk of
2006 was given on the 8th November by aerodynamicist Afandi Darlington,
a graduate of Imperial College, onetime employee of BAe, member of the
British Gliding Team and president of the Imperial College Gliding
Club. He also led Richard Noble's Farnborugh F1 design team and went
with the project to the USA when it was taken on and built by the Gulf
Aircraft Partnership as the Kestrel. His colleague, Peter Masson, was
due to cover the competition aspects but, as he was indisposed, Afandi
covered that as well as performance.
objective of glider design is to improve performance so that the
aircraft flies faster and further, and climbs more quickly in thermals
or waves. The key is low drag achieved by developing a clean airframe
with laminar flow regions. A successful modern glider will demonstrate
a lift:drag ratio (L/D) of up to 70; i.e. the glider will travel 70
miles for a mile of height. At low speed induced drag is dominant and
this is reduced by winglets.
Gliders And Optimising Performance
At high speed profile drag dominates, mainly from the wing, and this is
reduced by achieving laminar flow and by careful design of profiles and
the junctions between the wings and tail and the fuselage.
Water ballast is used to increase the wing loading which results in
increased speed for the same L/D and yields an increased L/D because
the operating Reynolds number is increased. The boundary layer can be
laminar - thin and well ordered - or turbulent - thicker giving more
drag. Glider wing sections are designed to achieve large regions of
laminar flow with low drag and reasonable stalling behaviour.
Afandi illustrated glider performance improvements by historical
examples. The 'modern era' started in the 1930s with the German
'Wiehe', an 18 m span wooden design, which achieved a L/D of 29 at 41
kts and a minimum sink rate of 1.2 kts.
The 'fs 24' Phoenix of 1957, a 16 m glass fibre design incorporating
laminar flow technology, had a L/D of 40 at 43 kts with a sink rate of
1.1 kts. The equivalent numbers for the 'Nimbus' of 1971, a 20.3 m
glass fibre design, were 49, 49 and 1.0.
By 2000 the carbon fibre/glass fibre/Kevlar 31 m 'ETA' demonstrated 70,
59 and 0.9, the current 'state of the art.' By 2020 an L/D of 80 should
be achievable by laminar flow control. It was laminar flow which gave
the big jump in L/D post WW II.
This is all put into perspective by Sir George Cayley's 1853 9 m span
glider which had a L/D of 5 at 25 kts and flew 200 yds. In 1985 Werner
Pfenniger designed a 32.4 m glider with a predicted L/D of 100 using a
windmill to suck away the boundary layer. The honeycomb carbon fibre
skin was laser drilled with holes of sizes which matched suction to
local pressure and was successfully tested in a wind tunnel.
The UK built 21 m 'Sygma' had a variable chord wing to be
extended in thermals. Seal difficulties caused failure but the concept
was later tried successfully in Germany. Also investigated have been
the 15 m 'SB 13' flying wing and a variable span 19 to 30 m design with
extension by hand crank.
Current wing design tools include computational fluid dynamics and
infra-red boundary layer visualisation in the wind tunnel; laminar flow
is cool, turbulent is warm. Materials include Kevlar and carbon fibres
aligned to tailor strength, using traditional wet lay-up methods or
pre-impregnated cloth; 'pre-preg'. The latter reduces weight by 5% with
increased strength but costs rise by 300%. Computer aided design and
manufacture (CAD/CAM) techniques give accurate, smooth moulds allowing
airframes to be hand finished to a mirror-like surface, aiding laminar
Modern gliders have
safety cockpits, carefully shaped with high sills and made with a
mixture of composites (70% Kevlar, 30% carbon or glass fibre) for peak
energy absorbtion. They are equipped with electronic instruments
including total energy displays and GPS. Some gliders have high
powered, compact electric motors, the latest of which can take a glider
from launch to 10,000 ft. The future holds the promise of stronger
fibres, active boundary layer control, adaptive geometry, better
instruments, fuel cell powered electric motors and weather information
via datalink. Advances will probably spill across from military UAV
(unmanned aerial vehicle) research.
Turning to the sport of gliding, Afandi noted that the current distance
record, which must be set in daylight, is 3009 km, the speed record is
247 km/hr over 500 km, and the altitude record is 49,009 ft, although
Steve Fosset, in a flight yet to be homologated, has exceeded this
wearing a pressure suit. Thermal, hill or ridge and wave lift are
utilised but new records depend on the latter; wave lift is known to
extend up to 100,000 ft in New Zealand.
Competitions consist of nine races over nine days. Competitors launch
when they choose, fly through the start line, then fly a triangular
course back to the finish line. The United Kingdom is the top gliding
Afandi closed with a
video of the Gulf Kestrel first flight and then answered questions from
the floor. He encouraged the audience to go gliding, perhaps by a
course at Lasham. (The Editor can thoroughly recommend this having done
such a course in September.) The vote of thanks was given by Ralph
Hooper, a once keen glider restorer and pilot who also worked with
Afandi on the Farnborough F1.