Structural design of a ULM PrandtlPlane wing system

Chapter1
Introduction
In the last years,the increasing cost of fuel and the need for a reduction in air traffic
pollution has led civil aviation to move its attention toward the construction of more
efficient aircraft. In order to achieve this goal, the aeronautic industries have usually
tried to improve aerodynamics and to build lighter structures but, recently several
solutions have been proposed in order to lead a radical change in the standard aircraft
configuration. Among them, the attention of the aeronautic community has been
focused on the Blended Wing Body and the multiplane configurations and currently,
NASA and Boeing are developing the first one.
This thesis aims to examine one of the multiplane configuration, which is called
PrandtlPlane . In particular the present work faces the problem of finding the minimum
weight solution for the main wing structures of a n ultralight PrandtlPlane . In the
last years, Pisa University Aerospace Engineering department has started this project
following the idea of Best wing System, an alternative aircraft configuration elaborated
by L.Prandtl in 1924.
1.1 Best Wing System
The Best Wing System idea was presented by L.Prandtl in the TN 182 NACA
report, in which Prandtl showed that once wingspan and total lift are given, as the
number of wings used increases, the induced drag decreases. Then, for instance a
biplane is more efficient than a monoplane, triplane is more efficient than biplane and so
on. In this respect the best result available is a box-shaped wing, shown in Figure 1.1,
then called Best Wing System. The main characteristics of this configuration, as shown
in [1], are an equal lift on upper and lower wing and a butterfly shaped (lateral) forces
with nil resultant on the vertical winglets (Figure 1.2). Even though such conditions
are necessary to minimize the induced drag, they aren’t suitable for practical use, since
1
1. Introduction 1.1. Best Wing System
Figure 1.1: Best Wing System
they could not verify the stability requirements related to the aircraft aeromechanics.
Moreover, the induced drag reduction, depends on the lift acting on the two wings
Figure 1.2: BWS lift distribution
and on the mutual induction between them, becoming more important as the distance
between the upper and lower wing decreases. The equation below represent the global
drag which affects a biplane wing system
D
i
=D
11
+D
22
+D
12
+D
21
(1.1.1)
where the first two terms represent the self induction, while the third and fourth terms
represent the mutual induction. In particular the last terms strongly depends on the
h
b
ratio
1
, in fact the drag decreases asymptotically when it increases. Thus, when there is
a sufficient gap between upper and lower wings, the lift is not affected by the induction
of each other, as shown Figure 1.3. These topics are fully illustrated and demonstrated
in [1]
1
Where h is the gap between upper and lower wing an b is the wing span
2
1. Introduction 1.2. The PrandtlPlane
Figure 1.3: Effect of h=b ratio on induced drag
1.2 The PrandtlPlane
The airplanes builded in the first years of aviation were biplanes since the aircraft
were made using wood and thus a monoplane solution didn’t allow to achieve an
adequate stiffness. Later on, tanks to the rising of light materials (like aluminium alloy)
the development of aircraft has been moved toward single wing solutions, that actually
are widely diffused in all the aviation branches. The Best Wing System solution, can
be transformed by moving the two wings in longitudinal way.This configuration, shown
in Figure 1.4, is called PrandtlPlane . In this case the wing and the horizontal tail
are integrated in a wing system able to produce the same total lift of a traditional
wing-tail solution and to satisfy the stability and controllability requirements as well.
Figure 1.4: Prandtl Plane Configuration
This kind of configuration is mainly suitable for commercial aircraft, in which
3
1. Introduction 1.3. Ultralight Aircraft
fuel consumption has a great influence on operating costs; for them, an induced drag
reduction leads to a higher probability of reduction of operating costs in terms of fuel
consumption. Moreover, this solution presents several advantages also in terms of
controllability. Indeed, the control surfaces are arranged on both wings (front and
rear), so that is possible to move them with opposite rotation allowing the pilot to
generate a longitudinal pure moment without affecting the total lift and, therefore,
increasing the accuracy of the command during the maneuvers.
Concerning structures, the over-constrained nature of the wing-box leads to a
greater stiffness of the overall system with advantages from the static and aeroelastic
standpoints(see [1]).
1.3 Ultralight Aircraft
The induced drag reduction is not one of the main goals of the "light aviation",
since fuel consumption has small influence on operating costs, if compared to a long
range aircraft. In fact,for this kind of airplane the parameters that affect the operating
costs are especially the landing fees and maintenance. However, there are other aspects,
especially related to safety, for which the use of PrandtlPlane in this category should
be considered.
Concerning the stall phenomenon, this kind of planes presents some advantages; in
fact, the downwash caused by front wing leads it to stall as first, so that he resulting
nose down moment keeps the aircraft out from such a situation.
Finally,unlike the typical ultralight airplane, the shape of this kind of aircraft
allows the designer to place the engine behind the cabin rather than inside aircraft
nose, allowing greater visibility for pilots and providing more distance between cabin
and engine compartment. Furthermore the rear engine solution provides an easier
installation of any cabin safety devices to protect the cabin against frontal impact.
To summarize, the characteristics that lead to prefer a PrandtlPlane configuration
rather than a traditional one can be summarized as follows:
 reduction of induced drag with a consequent fuel consumption;
 accuracy of longitudinal control system;
 smooth stall behavior;
 distance between pilots and dangerous components (fuel tank and engine)
4
1. Introduction 1.4. The purpose of the present thesis
1.4 The purpose of the present thesis
The purpose of this job is to design the wing structure of a very light PrandtlPlane
aircraft. This thesis starts from several previous works, in particular these are shown
in [2] and [3]. The first one defines the prototype, which is the object of this study and
shows the results of a finite element analysis performed on the initial configuration.
These results will be used to compare the behavior of the prototype with the one of
the new structure designed. The second, instead, provides a tool useful to build the
design code.
The prototype used as started model is ULM PrandtlPlane builded using as carbon
and several kind of wood. Mixed material instead has been used mainly because the
full carbon structure is not suitable for this purpose, concerning the requirements. In
fact the most part of very light aeroplanes are entirely built using wood, so that in the
years this solution has been classified as the most secure and reliable, in this concern a
full carbon structure could not found the agreement of authorities, because of the poor
backgrounds available.
The design is done through an analytical process, which aims to define the thickness
of the flanges in each point of the structures, and to verify wether the webs are suitable
to bear the loads. The first task is absolved by a code developed inMatlab
R  language
while the verification is done using the F.E.M. model exposed in [2]. This model is
built using Patran
R  as preprocessor and postprocessor and Nastran
R  as solver.
The thickness of flanges is defined in order to verify the stress constraint and, even
though this constraint takes only the strength of material into account(no buckling
or other instability effect are considered in this paper), the ultimate stress imposed
accounts itself the compressive strength of material. Moreover, the main structure of
this kind of aircraft is composed of beams with box shaped cross-section, hence the
ratio between the width of the plate that compose the spars and them thickness is
quite low. In this respect, especially concerning flanges, the buckling effect should be
negligible. Nevertheless in order to verify this extended buckling analysis would be
requested. Furthermore the only condition of limit load factor will be investigated in
order to avoid non linear effect, which could lead to a not consistent results.
Finally, a brief exposition of a multiobjective optimization process will be given.
This extended optimization could be implemented in several optimization tools, and
can involved several sets of geometrical and technological parameters in order to find
the configuration which minimize both the induced and parasite drags and thus fuel
consumption.
5
Part I
ULM
6
Chapter2
The prototype
This chapter aims at depicting the prototype of a PrandtlPlane ultralight aircraft
(ULM), currently under construction at SG Flyevolution in Udine. This work aims to
optimize the wing structures, with particular attention on spars. The features of such
prototype will be used as starting point to develop a numeric algorithm that aims to
define the minimum weight solution. In the next sections the main properties of the
airplane, regarding geometry, materials, structures and regulations, will be illustrated.
These topics are fully illustrated in [2].
2.1 Regulations
The present thesis is based on the requirements given by the Italian RAI-V.EL
regulation, which provides a conservative estimation about load factors compared to
European regulation CS-VLA, as sown in [2]. Unlike the "certified airplanes", the flight
envelope of very light aircraft is not obtained by superposition of gust and maneuver
diagrams, but only on maneuver and only at the zero altitude. Table 2.1 gives the
main specifications of the regulation used. The European regulation CS-VLA,requires
expensive certification methods, in terms of both times and certification costs. For
this reason, every country has developed his own regulation; in particular, the Italian
aviation, refers to RAI V.E.L, the United Kingdom refers to BCAR S and, finally,
Germany refers to BFU. As shown in Table 2.2, all these regulations are similar each
other, hence an aircraft builded in Italy has a good chances to fly also in the rest of
the Europe. This thesis makes reference to the Italian regulation.
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2. The prototype 2.2. Geometry
RAI-V.EL
Wto[Kg] 600
nz
max
4
nz
min
2
nz
gust
4 for
W
S
< 350
N
m
2
nz
gust
through graphical interpolation for
W
S
> 350
N
m
2
Table 2.1: RAI-V.EL: Load factors
CS-VLA BCAR S RAI-V.EL BFU
(Europe) (United Kingdom) (Italy) (Germany)
MTOW[Kg] 750 450 600 450
V
maxland
[Kts] 45 35
1
40:5
2
35:1
Climb speed[m=s] > 2 > 2 1:5 1
Landing speed[m=s] 1:3V
st
1:3V
s0
1:3V
st
 Table 2.2: Regulations used in Europe
2.2 Geometry
The ULM external shape, is the result of a previous thesis ([4]),in which the
aerodynamic configuration was optimized. The resulting configuration is shown in
Figure 2.1. Figure 2.2 gives the main parameters of the fuselage, which is made of a
carbon fiber shell and is equipped with two appendices, called Karman, that are used
to connect the wings. Figure 2.3 gives the shape of wing system.
Table 2.3 gives the parameters used to define the aircraft
3
Corresponding to karman ribs
4
Corresponding to karman ribs
8
2. The prototype 2.2. Geometry
Figure 2.1: Final model
Figure 2.2: Fuselage external dimensions
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2. The prototype 2.2. Geometry
(a) Top view
(b) Front view
Figure 2.3: Wing system
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2. The prototype 2.2. Geometry
Parameter Size
General
W
eng
[HP ] 100
V
cruise
[m=s] 69
V
land
[m=s] 19
Cl
cruise
0:13
Cl
m
ax 1:35
Wing span [m] 8
S
ref
[m
2
] 14.37
MTOW [Kg] 500
h
cr
[m] 1000
V
cr
[m=s] 69
Front wing
dihedral[deg] 7
 25
[deg] 24
Cr[m]
3
0:98
Ct[m] 0:612
Airfoil GOE 398
Rear wing
dihedral[deg] 0
 25
[deg] 11
Cr[m]
4
1:16
Ct[m] 0:57
Airfoil GOE 398
Table 2.3: Wings geometric parameters
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2. The prototype 2.3. Wing Structures
2.3 Wing Structures
The primary structure of each wing is a single spar, with a box shaped cross section.
Several materials are used, the spar flanges of both front and rear wings, are made of a
laminate composed by several layers of spruce and carbon fibers; the spar of winglet
is made of using a carbon fiber reinforced plastic laminate in order to simplify the
manufacturing process. A structure called karman,which is a part of the fuselage, joins
each wing to the fuselage. The spar is connected to the karman through a pair of
transversal pin, as shown in Figure 2.4. Moreover, in order to sustain the hinges of
control surfaces, there is an auxiliary floating spar on both wings. These spars are
not linked to the fuselage and thus they don’t bear loads. Furthermore unlike the
main spars the auxiliary ones have a "C-shaped" cross section. Table 2.4 gives the
(a) Spar connection
(b) Karman connection
Figure 2.4: Wing-karman connection
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2. The prototype 2.3. Wing Structures
characteristics of the main spars at wing root and wing tip; this set of parameters will
be used, to initialize the optimization process.
Wing spar Height[m] Width[m]
Front @ root 0:1217 0:09
Front @ tip 0:0602 0:097
Rear @ root 0:1217 0:09
Rear @ tip 0:0602 0:097
Table 2.4: Main spars dimension set
Wing ribs are bonded on the spars and the volume of winglets is filled with a
polystyrene foam. Ribs provide the airfoil shape and transfer loads from skin to spars
while Bulkheads are stiffer than ribs.
There are 9 ribs on each wing; the bulkhead are positioned close to the karman, and
at wing tip to connect the winglet. Both of them are manufactured stacking twelve
plies of birch, each one 1mm thick. Both ribs and bulkheads are shown in Figure 2.5.
Moreover, in order to reinforce the zone of load transfer from ribs to spar, several pads
Figure 2.5: Ribs layout
composed by a laminate of birch are bonded inside spar, next to ribs location.
Finally, skin panels, made by stacking several plies of okumè, are connected to ribs.
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2. The prototype 2.4. Materials
2.4 Materials
Several kind of material are used to build the prototype; in particular, carbon
laminates and wood. Carbon laminates are used to build the entire fuselage shell,
some layers of the upper flanges of both spars and the entire spar of winglet. Wood
is used in ribs, webs of main spars, skin and also in a great number of layer in the
flanges. In particular there are three types of wood: European Spruce, birch and okumè.
European Spruce is used to laminate the flanges of the main spars, birch is used in
ribs, reinforcing pads and also is the webs of the spars. Finally, okumè is used to build
the skin. An overview of material properties is available in appendix A.
In order to maximize strength and stiffness of the flanges against the bending
moment, the fibers are directed parallel to the beam axis. The spars of front an rear
wings are identical. Table 2.5 gives the longeron layout, where W means Wood, C
means Carbon and the number before them indicates the quantity of layer staked. The
carbon layers have a thickness of 1:2mm and are arranged on the flange outer surface,
while the thickness of the spruce layers is 8mm for each ply. Concerning the thickness
Section location[m] Upper flange[mm] Lower flange[mm]
0) 0:45 26:4(3L; 2C) 24(3L)
0:45) 0:95 34:4(4L; 2C) 24(3L)
0:95) 1:2 26:4(3L; 2C) 24(3L)
1:2) 1:6 26:4(3L; 2C) 16(2L)
1:6) 2:32 18:4(2L; 2C) 16(2L)
2:32) 3:72 9:2(1L; 1C) 8(1L)
Table 2.5: Flange thickness
distribution, as shown in the previous tables, it is assumed the minimum value at tip
and the maximum one at root because the regulation requires that the static test of
the structure is more confident without the winglet and, thus, both front and rear
wings are tested as cantilever beams and designed accordingly. Figure 2.6 gives the
spar layout
Finally, concerning the material of laminates, both carbon and wood was used for
the following reasons: first, a full carbon structure could be lighter than a mixed one,
due to the higher strength characteristic but when the thickness has to be large due to
buckling, a mixed solution using carbon and spruce could be most efficient, so even
though both strength and stiffness are greater than the wood ones, the weight resultant
could not be definitely lower. Finally, the main reason because the full carbon structure
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