On the other hand, the relative wind flows under
the outside of the wing because the pressure on the lower surface is greater
than the one outside of the wing. The air tries to turn around the wing-tips
from the lower surface to the top skin. The way to explain why a higher
aspect ratio is better than a shorter one, would be to say that the greater
the aspect ratio, the smaller the air quantity escaping outside from the
wing-tips is. The air turning around the wing-tip is not useful to produce
lift; this is often called ‘a marginal loss’.
As these two airflows, the one from the top skin and the one from the lower
surface, meet on the trailing edge at a certain angle, they create whirlwinds
turning clockwise (seen from the back) behind the left wing and counter
clockwise behind the right wing. All the vortices from the same side have
a tendency to gather in order to form a great vortex that escapes from each
wing-tip. This is called tip vortex or easer vortex.
Many pilots have seen this vortex or, more precisely, the central part of
them that condensation makes perceptible. The air humidity condensates because
of the pressure loss in the heart of the vortex. You must not confuse these
visible vortices at take-off with those created by the exhaust gas of engines
at altitude.
If we now watch the direction of rotation of the vortex we perceive that
there is a draught towards the top at the outside of the wing span and a
draught downwards behind the trailing edge. You must not confuse this downwards
draught with the normal deflexion that occurs. In this last case the deflexion
downwards always goes with an upwards deflexion in front of the wing so
that the final direction of draught is not modified. But in the case of
tip vortex the deflexion occurs upwards outside of the wing and not in front
of it, so that the draught leaving the wing is finally directed downwards.
By consequence, the lift which is perpendicular to airflow is slowly inclined
towards the back and contributes to drag. Result of the vortex, this part
of drag is called induced drag.
How long have we been trying to decrease or eliminate the wing
tip vortex ?
Since before Clément ADER, the aeronautical world has always had
its fair share of letters of nobility, titles which married performance
against an aesthetic value. Every year, something new is announced to
the public; it is always a world stopping event - a new motorisation technique,
faster speeds, new shapes of fin or fuselage, extraordinary wing profiles
or more wings of all shapes and sizes with multiple functions etc….
Many innovations, it can be said, only stand on compromise - the manufacture
of a plane being the way it is.
Presently equipped with sophisticated technology, all these planes fly
around the word with a common problem: airport traffic congestion and
the inherent danger in the approach. This problem increases regularly
by 5 % each year. Due to this fact we are, politically, trying to find
places to build new airports which do not exist. Into this race against
the clock, colossal budgets are given to aerodynamic specialists all over
the world:
- Project AWIATOR (Aircraft Wing With Advanced Technology) to which l’IRPHE,
EADS Airbus and CERFACS are associated,.
- Aerodynamics research centres like ONERA (France), CIRA (Italy), DERA
(U.K.), D.L.R. (Germany), F.F.A. (Sweden), I.N.T.A. (Spain), N.L.R. (Netherlands),
- All the European scientific flight schools,
- European program C. WAKE,
- And also the projects inscribed at the American program AGATE,
How to reduce the induced drag and how to eliminate the vortex
?
As research will tell you, it is a Frenchman, Georges BABAUDY, who was
the first inventor to propose a solution to the problem of induced drag,
adapting a conic device on each wing-tip. His patent, published on the
22nd of December 1909, was aimed at achieving better equilibrium but as
it did not take into account all types of drag this project never succeeded.
And, furthermore at this time, compressible speeds were not obtainable.
Since this time a multitude of patents have been issued in the world.
Only two or three are available at present time.
The most well known and most reliable invention remains that of American
Richard T. WITCOMB (1985) with his « winglets », a kind of
vertical heightening and prolongation of each wing, which one can see
in place on heavy aircrafts and on some business jets.
Presently, the gains are limited, but there is no other possible way,
as we have not the panacea.
A cruising Airbus A 340, for instance, generates 33 % of induced drag
in its total drag. The same plane equipped with « winglets »
has a registered gain of 1,2 % on this induced drag (net gain), and 3
to 4 % on its total drag (brute gain). The vortex still remains.
Does the future of aerial transport hang on the mastering of
these tremendous perturbations?
All pilots know the strength of these dangerous vortices. Even at large
distances, they are advised to keep away and to move aside. Anecdotes
are plentiful on this subject. As soon as one follows the trajectory of
another aircraft, you are in its wake and escape becomes a miracle. These
problems happen very often in intense aerial traffic areas, holding patterns
before approach, descent before landing (ILS approach) or departure (take
off and initial climb).
The last crash at New York Airport, as we all remember and which was front
page news at the time: an Airbus A 320 pilot took off too early behind
a Boeing 747 and was not able to control his aircraft. The vortex force
added to the force of recovery of the aircraft clean broke its vertical
stabilizer.
There is no alternative. Either we eliminate this by adapting adequate
devices on wing-tips, or we have to build a new plane which does not generate
vortex, i.e. an aircraft equipped with another type of wing such as a
lozenge or rhomboidal, joining the main wing to the back ground-plan wing,
joined and fixed over the head of the fuselage. Economically, such a fabrication
is at presently also a dream.
Maybe we shall have the chance to witness these planes flying in 15 or
20 years?
Put another way, 2006 will be the year of A380’s first flight. Here
are some figures :
- wing span : 79,80 meters
- length : 73 meters
- height : 24,10 meters
- thrust of the engine : 333 KN
- maximum take-off gross weight : 583 tons
- maximum lending weight : 427 tons
- empty weight : 249 tons
- fuel tankage : 325.000 liters
- approach speed : 150 Kts CAS (278 kms/h)
- cruising speed : 0,85 M (1016 Kms/h à 0°)
- usual cruising level : FL 330 (10.058 mètres)
- range : 10.410 Kms
It is said that this airway superweight will use, for take-off or landing,
the same runways that are used by the Airbus A340 or Boeing’s 747!
(Information given by Airbus France in Toulouse):
Which airport will be able to receive such a plane generating so much
wing-tip vortex (moreover it is imperative and vital to wait for its total
dilution before any new take-off)?
How shall we manage the problem of boarding and departing with this unusual
amount of passengers using the same airport infrastructure?
Will we be able to face the 5% aerial traffic increase each year?
To conclude, we have not yet won: vortex, vortex, alea jacta est…..
C. Hugues.
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