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The DLR A320 Advanced Technology Research Aircraft,
ATRA, glides gently across the fields on its final approach. A
glance out of the cabin window reveals that slats and flaps on
the leading and trailing edges of the wings are being deployed.
This event is accompanied by a gentle whirring from the electric
actuation motors, while the wings appear to have double the
width they had during the cruising portion of the flight. The
undercarriage locks down into the landing position. Still travel-
ling at 200 kilometres per hour, ATRA crosses the fence around
its home base in Braunschweig – finally, it touches down on the
runway.
An occurence observed by passengers on a daily basis
conceals an engineering masterpiece. The wings on large,
modern passenger aircraft are designed for high speeds, around
850 kilometres per hour, while cruising at altitudes of about
10,000 metres. During approach and landing, they reveal a
different capability – the flight crew keeps the aircraft airborne
over the last few metres of its flight at less than 250 kilometres
per hour. This is accomplished by utilising an aerodynamic trick –
the magic phrase employed by aircraft engineers is ‘high-lift
system’. With it, they refer to the various mechanical flaps on
the wings that are deployed during take-off and, above all,
when landing an aircraft, together with their complex control
system.
The decisive factor here is an increase in the curvature of
the wing and its surface area, which delivers more lift. The third
act in the performance increase for low-speed flight is devoted
to the aircraft pitch attitude – to increase lift, the pilot pulls up
the nose, reaching up to 10 times the angle it adopts at cruising
altitude. During this process, the flap and slat system ensures
that high angles of attack can be reached without stalling of the
wing.
The gaps that open between the slats on the leading edge
of the wing, the main wing and the trailing edge flaps play
another key role in the performance of a high-lift system. All of
these elements must be precisely aligned with each other to
achieve the optimum high-lift configuration. The simulation of
this complex situation at the limits of the flight envelope in
terms of maximum lift remains an enormous challenge.
Researching at maximum lift and minimum speed
By Ralf Rudnik and Falk Dambowsky
Studying the low-speed flight regime
An aircraft delivers holidaymakers and business travellers to their destination swiftly and directly. Just prior to the
arrival at destination, pilots and aircraft are faced with a set of very specific requirements. The aircraft must remain
stable in the air during the low-speed approach. DLR is collaborating with Airbus to study this low-speed flight regime
for civil aircraft in the HINVA (High-lift IN-flight VAlidation) project, to explore the potential benefits in terms of quieter
approaches, shorter landing distances and higher passenger capacities. To accomplish this, scientists employ dedicated
flight-testing, wind tunnel experiments and computer simulations.
Port wing of the ATRA research aircraft with
pressure measuring strips visible as reflective
silver strips
HINVA – Simulation and
flight-testing go hand in
hand
Since numerical methods and modern high-performance
computers allowed the analysis of realistic high-lift confi-
gurations about 15 years ago, research has concentrated
on gaining a detailed understanding of the complex aero-
dynamic phenomena involved, and on ways of improving
the numerical processes. Primarily, this took place within
European projects. Right from the start, DLR played a
prominent role – not least because of the significance
of high-lift systems for the Airbus site in Germany. Until
recently, the experimental data for comparisons between
measured and computed data originated from wind
tunnel testing. Assessing all of this research work, it
became apparent that comparisons with wind tunnel test
results alone would not be sufficient to deliver the level
of precision needed to determine aerodynamic high-lift
performance or to further develop numerical processes.
Restrictions on size and the influence of wind tunnel
walls and model mountings are factors that prevent the
achievement of these goals. As a consequence, extended
validation was required, allowing verification of predic-
tions against flight test data – reason enough to start the
HINVA project in 2010.
aerodynamics | DLR ma Gazıne 136 · 137 | 39