Hello, and welcome to this video lecture
on novel aircraft configurations. My name is Carmine Varriale,
and I am Assistant Professor of Flight Mechanics at the section of Flight Performance and Propulsion,
in the Faculty of Aerospace Engineering of the TU Delft. In this video, we are going to look at
the unique aerodynamic properties of the box-wing, and we are going to try to understand
why it is interesting to consider a box-wing aircraft as a potential solution towards
a more sustainable aviation in the future. First of all, what is a box-wing
and why is it called like that? A box-wing is a closed wing system
consisting of two main wings connected to each other by side panels,
or side wings, at the tip. It is called like this because,
if you look at it from the front or from the back, the profile of the wing looks like a box,
or more in general like a closed curve or polygon. Depending on the application
and category of the aircraft, there are many possible designs
for the geometry of a box-wing. And the same is true about the possibilities
to integrate it with a fuselage and vertical tail, in order to make
a proper aircraft configuration. The examples you see in the pictures
have already flown for light and ultralight applications, but there are many more
that are currently being studied in research. For flight in transonic conditions,
which is the current standard for transport jets, all wings of the box-wing
have to be given a certain sweep angle, and, as a consequence,
a significant horizontal distance, which is called stagger. This particular aircraft configuration, with such a staggered and swept box-wing,
a wide-body fuselage and twin vertical tails, has been recently referred to as the “Prandtlplane”. This is from the name of one of the fathers
of modern aerodynamics, Ludwig Prandtl, who was the first one to discover
the important properties of the box-wing, and in 1924 he called it
the “best wing system”. Why did he call the box-wing like that? Because no matter
the particular shape or design of the box-wing, the box-wing achieves the minimum induced
drag for a given wing-span and weight. You may already know
that induced drag is related to the creation of lift, and, as such, it is associated
with the creation of vortices behind the wing. The intensity of these vortices
can be reduced by increasing the wingspan, as in the case of a truss-braced wing, or by introducing winglets,
as in the case of many existing airplanes, already in the real world. In the case of the Prandtlplane, you can imagine that the rear wing
acts as a huge winglet for the front wing, and also vice versa. In this way, you can convice yourself
that the intensity of the vortices behind the wing is minimum, and so is induced drag. Now, how can we put
this property to good use? Consider the two most numerous
aircraft configurations currently in service: the Airbus A320 family,
and the Boeing 737 family. They are both characterized
by a wingspan of approximately 36m and can carry about 150 passengers
in a 2-class cabin layout. If you were to design an airplane
based on the box-wing concept, but with the same wingspan, you would, first of all, retain the possibility
to use the same ground infrastructure already available
in all airports of the world: I’m talking about passenger gates,
hangars, or maintenance facilities, for example. Note that this would not be true
for the truss-braced wing, which needs a larger wingspan
to achieve improvements in aerodynamic efficiency and is quite a relevant aspect in a world where population
and travel demand keep growing, while space for new facilities
becomes less and less available. Secondly, thanks to
the improved aerodynamic efficiency, the box-wing would make it possible to either
transport the same weight of the A320 or B737, with less drag,
and hence consuming less fuel. or carry more weight
with the same drag of the A320 or B737, hence being able to transport
more passengers per flight and overall needing
less flights from point to point. From some preliminary studies
comparing the flight performance of a reference aircraft resembling
the Airbus A320 equipped with new generation engines, and a Prandtlplane with 36m wingspan, it can be seen that these scenarios
are actually not so unrealistic. For various short-and-medium mission
ranges, from 2000km to about 6000km, and for two different flight strategies,
it can be seen that the A320 consumes about 17.5g of fuel per passenger per km
with 150 passengers, while the Prandtlplane consumes
a minimum of 14.5 and a maximum of 16.5g of fuel per passenger per km
with 308 passengers. In other words,
the Prandtlplane uses less fuel per passenger, to move more passengers
on a short-medium range route, and hence can be defined as
a more efficient means of transportation. Another aspect to be considered stems
from the fact that the geometry of the staggered box-wing makes it possible to install
multiple, redundant control surfaces all over the two main wings. By coordinating the deflection
of these control surfaces, it is then possible to perform
any maneuver in infinite different ways, and also to choose an optimal way
to achieve some desired performance. For example, control surfaces
can be coordinated to achieve Direct Lift Control, which makes it possible to change the aircraft lift,
without any change in angle of attack. This could be especially useful
to make the aircraft feel more precise
and agile to pilot commands, and to make it react faster
to external disturbances. Direct Lift Control
would reduce the vibrational load felt by passengers
and structures in a turbulent airfield, resulting in more comfort on board,
and an overall safer flight. Also, it could allow precise control
of the flight path angle during descent and landing phases, allowing for steeper continuous descent routes
and lowering the noise footprint of landings close to urban areas. So, what have we learned in this video? The box-wing is a closed-wing system
which achieves minimum induced drag, regardless of its particular shape or design By integrating the box-wing geometry
with a wide-body fuselage, the Prandtlplane could transport
approximately double the number of passengers of the A320 or B737 while consuming less fuel per passenger
and using the same ground infrastructure Finally: redundant control surfaces
on both wings of a staggered box-wing can be coordinated
to achieve Direct Lift Control, which results in
more comfort and safety in flight, and in a lower noise footprint
during descent and landing. Of course, there are still many challenges
to be faced for the box-wing and the Prandtlplane, but research is ongoing
to explore this novel configuration and its operation as a potential solution
towards more sustainable aviation.
