Design of a Very Light Jet and a Dynamically Scaled
Demonstrator
Christopher Jouannet*, David Lundström†, Kristian Amadori‡ and Patrick Berry§
Linköping Universitet, Linköping,58432, Sweden
A dynamically scaled model of a Business-Jet has been realized by master students at
Linköping University. The goal of the project was to understand the difficulties of dynamical
scaling and try to extract usable data from flight testing. This project is also a good way to
gives students a chance to experience a complete aircraft project from conceptual design to
flight testing.
Nomenclature
AR:
dfs:
dM:
k:
IMU:
lfs:
lM:
Wfs:
WM:
Aspect Ratio
full scale air density
model air density
scale factor
Inertial Measuring Unit
dimension on the full scale aircraft
dimension on the model aircraft
full scale aircraft weight
model aircraft weight
I.
Introduction
IRCRAFT design is a complex multidisciplinary task and teaching can easily tend to be too theoretical, not
providing the students with the tools they need to successfully participate in industrial projects. The approach
chosen at Linkoping University is intended to create the right balance between theory and practice, and to place
the student in the centre of the problem, in order to achieve an overall perspective of the aircraft design process. This
article presents the aircraft design project course given in the last year of the aeronautical masters program, in which
flying hardware is designed and build, in response to a design challenge.
Based on a fictive configuration which has been designed at Linköping University, a dynamically scaled model
has been designed and built. This approach was inspired by the work performed by NASA within the AirStar
program1. The goal was to acquire experience and understanding on building and flying dynamically scaled models.
The starting point was a Business-Jet/Medivac-Ambulance aircraft designed by master students at Linköping
University.
A
A. Research coupling
One axis of the current research in aircraft design at Linköping University is focused on fast concept evaluation
in early conceptual design stages. This covers multidisciplinary optimization using tools of different level of
complexity and low cost subscale testing. In some cases a flight test will give far more answers than several
computations. In order to achieve this goal a methodology is developed to allow fast creation of subscale flying
concepts. The methodology is still under development and one important part of it is the scaling methodologies and
the imposed requirements on manufacturing. The master student projects performed at Linköping University are
*
Assistant Professor, Department of Mechanical Engineering, [email protected], AIAA Member.
PhD, Department of Mechanical Engineering, [email protected], AIAA Student Member.
‡
PhD Student, Department of Mechanical Engineering, [email protected], AIAA Student Member.
§
Lector, Departement of Mechanical Engineering, [email protected]
†
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American Institute of Aeronautics and Astronautics
used to assess those requirements. Several recent examples of sub-scale fly testing can be found, such as the AirStar1
program or even more recently the X-532 Blended Wing Body project.
B. Educational challenges
Over the years there has been a dramatic reduction in the number of ongoing aircraft projects.
Today’s aircraft design engineers are lucky if they will be involved in one or two complete projects during their
entire careers. This is in sharp contrast to the golden age, when an engineer was likely to be part of several projects
during his career, see Table 1.
Table 1. Design career length versus military aircraft design projects by decade (adapted from Scott3)
This situation creates an issue regarding the education of aircraft design engineers. When they start their
professional life they will be integrated into an ongoing project and they may be involved in that process for a long
time before anything new appears. The teaching approach as proposed by Linköping University is to allow future
aircraft design engineers to participate in a complete aircraft project, from requirements to flight testing as a
preparation for their very first steps in industry.
The other main challenge in aircraft design education is changing demands from the industry regarding the type
of knowledge the yet to be engineers should be educated for. Almost all the educational systems, in aeronautical
engineering, are focused to develop the analytical skills of students and not to develop the synthesis capability or the
innovative perspective need for design. Recent changes in educational perspective such as the CDIO initiative4,
initiated by the Aerospace Institute at MIT and three Swedish universities, Linköping University being one among
them, try to apply a more synthetic view on engineering education, by introducing small practical works into the
regular courses. This approach is adopted in a larger scale for the Aircraft Design education at Linköping University,
and was adopted before the creation of the CDIO initiative. In the same spirit Young5 argued in favour to design
projects in engineers’ education.
Nowadays it is more important to work together in teams and the new engineer needs to be able to perform as an
individual in that team. Being able to present results and ideas in a selling manner is also becoming increasingly
important. Another important thing is to be able to convert his/her own ideas into something practical and useful.
This is something which Universities seldom care very much about, but which is no doubt important, i.e. to bridge
the cliff between a previous mostly theoretical life into a more practical one in industry. One of the most important
issues is to be able to gain a holistic viewpoint from the very start in working life, i.e. to possess a kind of
“helicopter view” with regard to the product or project you are involved in. Possessing this holistic view point
makes life easier for everyone and saves time as well as money in the long run for industry. One way of preparing
for that insight is to carry out projects like the Aircraft Design Project in Linköping.
II.
Raven Full-Scale Model
The Raven aircraft, as the project was named this year, was designed with its main role as a business jet and
secondary role as an ambulance/medivac aircraft. The aircraft was designed according to FAR23/EASA23 rules,
around two Williams FJ-33 engines, with short take-off and landing performance for rural operation in mind.
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American Institute of Aeronautics and Astronautics
The students on this project worked in two groups. One group was aircraft design related while the other group
concentrated on interior layout and ergonomic design. The ergonomic design group focused on the air
ambulance/medivac role of the aircraft but also did a lot of work on the business jet interior as well. A full scale
mock up of the centre piece of the aircraft was built to be able to decide on which cross section to use but also to
help out in the ergonomic design of the interior.The ergonomic design group made several study visits to different
air ambulance operators to follow up how they operate and to check on current and future requirements with regard
to ground handling, working conditions inside the cabin and equipment requirements.
Based on these mock up cross sectional studies and a preliminary layout of the cabin layout, the design of the
aircraft could proceed. The aircraft design group finally settled on a low winged configuration with rear placed
engines. The wing was designed slightly foward swept to make it possible to locate a wide side door on the front
fuselage without interfering with the wing while loading. Another obvious benefit with such an arrangement is
improved stall behaviour. Care was taken as not to sweep the wing too much forward with regard to potential
structural stiffness problems and directly related increase in weight
As always in aircraft design there´s a delicate balancing in between the differrent roles of the aircraft. The
obvious is not to let one role overdesign the other. Such a situation was obvious when the students started to figure
out where to position the door to load patients through. We had previously planned for a wide side door for both
passengers and patients to be used, hence the forward swept wing, but as the problem was looked through more
deeply, another solution surfaced. Why not use a back door solution, which could double as a cargo hold door, in the
business jet version? Since this was obviously a more demanding task and a real selling point if it could be realised,
work concentrated on this solution.
The students finally settled on a back door solution which borrows much of its design from previous rear
engined aircraft designs. One such an example is the MD80 aircraft, where one of the emergency exits is a door
through the rear pressure bulkhead. Instead of using it as an emergency exit, the student made use of the rear
pressure bulkhead door to load patients and cargo/luggage through it. To be able to do this the rear fuselage had to
be structurally strengthened to carry the loads through the fuselage from the fin and stabiliser.
The Raven aircraft is designed as a dual usage aircraft, small Business-Jet and Medivac-Ambulance application
compliant to the FAR23/EASA23 regulation. A general layout is presented in Figure 1. The aircraft was designed
with the following specification:
-
Two roles: Business-Jet or Medivac-Ambulance
− Quick configuration change (30 min max.)
− Two pilots
− Sized around two Williams FJ33 engines
-
In Medivac-Ambulance role:
− 575 kg payload (max 700kg)
− Range 1300 nm
− Two patients, one doctor and one nurse
− Enable one stretcher to remain inside while the other is embarked/disembarked
-
Business-Jet role:
− 4 to 6 passengers
− Offer space and high class interior
− Able to use runways 800m long (ISA+20)
The main geometrical characteristics of Raven are the following:
-
Wing span
Overall length
Wing Area
AR
Mcruise
Cross section
14,4 m
12 m
21,8 m2
10
0,55 at 40 000 ft
1.6 m
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Figure 1. General view of Raven.
Some artistic illustrations of RAVEN are presented in Figure 2 and Figure 3.
Figure 2. Business Jet layout and interior details.
Figure 3. Medivac-Ambulance layout and interior details.
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III.
Subscale Flight Testing
Subscale flight testing is a mean of allowing the design team to evaluate the free flight characteristics prior to
building a full-scale prototype and to investigate extreme, high risk portions of the flight envelope without risking
the prototype air vehicle. A further possibility, and one that is advocated in this work, is to use subscale flight
vehicles as a mean of evaluating and demonstrating high risk platforms and technologies without the prohibitive
expense of a full-scale vehicle. There are several recent examples of this, the NASA funded McDonnell Douglas X366,7 and Rockwell HiMAT8, Saab UCAV9, NASA X-43A-LS10 and proposed Gulfstream Quiet Supersonic Jet11. In
all cases the configurations are highly unconventional and thus there is a desire to demonstrate the configuration's
feasibility without the cost or risk of a manned, full-scale vehicle.
The testing of subscale free flight models is not a new concept. Particularly for high risk testing such as high
angle of attack and to study departure modes, the restrictions imposed by a rigid connection as in the wind tunnel
has been prohibitive. Spin models for updraft wind tunnels have been a standard practice since the 1940s and
remotely controlled drop models from helicopters have often been used to complement spin tunnel testing (a typical
example being the Saab Viggen test program12. Among the more unique examples of subscale testing, in order to
test the novel aerodynamics of the double delta Saab Draken aircraft a subscale manned aircraft “LillDraken", with a
similar plan-form, was tested prior to full-scale development. In addition, numerous drop models and models fixed
to the nose of rockets for high speed tests were adopted. Free-flight models have also been built for conventional
wind tunnels, such as the NASA Langley Free Flight Facility13. Also for fighter configurations, drop models have
been widely used; recent examples being the X-3114 and F/A-18E/F15. Subscale drop models of space vehicles such
as the Lockheed Martin X-38 and Japanese HOPE-X16 have also been employed. Recently the usage of subscale
flight testing has been extend to civil aircraft such as the NASA AirStar research program, where scale model are
used to explore a larger flight envelope for a civil transport aircraft. This in order to evaluate the risks that can be
encountered during take-off, landing or due to heavy gust. For the Blended Wing Body concept, the X-53 project
from Boeing and NASA is currently using a scaled model to demonstrate the concept and obtain more data without
going to full scale.
IV.
Dynamic Scaling
In order to convert the “paper” study into a project that can be build and flown, the students were required to
produce a dynamically scaled model based on the designed full scale configuration. The purpose of the dynamic
scaling was to design the demonstrator aircraft so that it got the same dynamic properties as the full-scale aircraft.
This does not mean that the demonstrator should be able manoeuvre exactly like the full scale aircraft. It means that
the demonstrator should have the same response according to scale. The concept of dynamic scaling has been of
most importance throughout the entire process.
A. Scaling methods
Key similarity conditions that must be met if full similarity is to be achieved are (these are discussed in detail by
Wolowicz et al.17):
−
−
−
−
−
−
Geometric similarity
Aerodynamics
Reynolds number (fluid inertial-to-viscous forces ratio)
Mach number (fluid inertial-to-pressure forces ratio)
Inertial scaling
Froude scaling
Note that the scaling problem becomes even more difficult when aeroelastic effects need to be considered; they
are however neglected for the purposes of this discussion. For complete similarity the seven parameters as listed in
Table 2 must be met (if the effect of structural flexibility is neglected).
Significant questions exist as to what degree, and whether, all of these parameters need to be closely matched to
ensure similar characteristics between the subscale and full-scale vehicles. They are more than likely dependent on
the vehicle itself and characteristics which are being sought. For example, departures such as spin require correct
inertial scaling whereas takeoff and landing performance tests are more dependent on aerodynamics and thrust
matching.
The effect of these parameters on the vehicle flight characteristics will now be briefly discussed, as applicable to
the work performed here.
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American Institute of Aeronautics and Astronautics
Table 2. Key similarity parameters.
1. Geometric Similarity
Geometric scaling can result in problems with regard to structural design and internal volume. Modifications to
the outer mould line may be necessary to a subscale vehicle. Typical examples include different aerofoil sections to
account for the Reynolds number and modifications to the engine layout. These modifications may have a
significant impact on the right characteristics, and therefore need to be accounted for in evaluating flight test results.
To evaluate the handling characteristics of an unconventional configuration however, these differences may be
acceptable.
2. Aerodynamic Similarity
The principal issue here is the Reynolds number; compressibility is considered to be insignificant for high angle
of attack, departure and landing regimes which are the primary focus. The Reynolds number may typically be in the
order of 500,000 or thereabouts (based on wing MAC) for the subscale models that are the subject of this work
(length around 2m, weight under 20kg). This would be around one to two orders of magnitude less than the fullscale.
3. Inertia Scaling
While the relative density and inertial factors would in all likelihood be impossible to meet on a subscale vehicle,
this is perhaps not so critical. More importantly from a point of view of departure analysis (especially spin) is the
relative values of Ixx, Iyy, Izz and Ixz. In the case of inertia coupling, departure can occur in yaw or pitch. In case of
the former it is the ratio of Ixx to Iyy that is relevant and for the latter Ixx to Izz. These ratios should therefore be kept
consistent between the subscale and full-scale air vehicles to ensure similar inertial characteristics (but note the
damping effect of aerodynamics on such motion, and hence the Reynolds number similarity that is typically
required).
Similarly, spin characteristics are critically dependent on both the inertia ratios and aerodynamic characteristics.
The inertia-yaw coupling is determined by the ratio of Ixx to Iyy and the spin attitude by the ratio Ixx to Izz. We are
thus left with two critical inertia ratios: Ixx/Iyy and Ixx/Izz which should be set as a minimum.
Experience gained from the present work is that these inertia ratios may be met readily if a large wing span is
available, such that small masses may be added at the wing tips. For smaller wing spans it becomes more difficult to
adjust Ixx because of the excessive mass required. Longitudinally there generally is sufficient volume to allow the
distribution of heavy items (such as battery packs) such that both Iyy and the CG position can be set as required.
4. Froude Scaling
As discussed in Wolowicz et al.17 the Froude number is relevant to manoeuvring vehicles. At low Mach numbers
Froude scaling can be readily achieved. When compressibility effects are not negligible it is required to satisfy both
Froude scaling and Mach scaling. Generally this is not possible, if not in particular pressurized wind tunnels using a
specific refrigerant as medium or in case of a specific scale factor. However this scale factor may not be of practical
use.
V.
Scaled Model
A. Design requirements
The design requirements were defined from the scaling method chosen and from the size of the jet engines to be
used. One constraint was the fact that the model should be transportable in a station wagon. That imposed the wing
to be at least partially detachable.
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The model aircraft was derived directly from the full-scale model. As the full-scale configuration was modeled
in CATIA V5, the external geometry was scaled to the wanted size and used to create the structural layout. CATIA
was then used to place all components and ensure that the right center of gravity position and inertia matrix were
obtained. The model includes a retractable landing gear that is pneumatically actuated.
B. Scaling method
In this project Froude scaling is used, originating from the similarity parameter Froude number:
V2
NFr =
Ag
V = speed
A = characteristic length
g = gravitational acceleration
(1)
The essence of the method is that it compensates for inertial and gravitational effects, thus assuming that two
objects flying at different speed, altitude, etc. have the same Froude number. Hence conversion factors can be
derived for a wide spectrum of quantities: forces, moments, Reynolds number, angular rates, etc. For instance, if a
characteristic length ( A ) is to be scaled (subscript M denotes model, subscript fs denotes full-scale):
AM
=k
A fs
→ A M = k ⋅ A fs
(2)
The scale factor k was determined to n = 0,138. This is from the wingspan ratio specified in the project outline.
When the scale factor had been determined every other quantity could be scaled.
The scaling factor was decided from practical considerations and weight restrictions. The weight was calculated
from the following equation:
WM = k 3 ⋅ W fs ⋅
dM
d fs
(3)
It can be seen from equation (3) above that the model weight is determined from full scale aircraft weight and
altitude, or conversely, a given model weight can represent different combinations of aircraft weight and altitude.
The Swedish Civil Aviation Administration does not allow radio controlled models over 25 kg. In order to keep the
landing speed down without need for complex high lift devices on the scaled model, it was decided to keep the
weight relatively low. After considerations, a scale of 13% was chosen. The density factor was set to one assuming
that only sea level flight would be simulated. However other altitudes could be simulated by adding weight to the
model.
From scaling and similitude requirements, a subscale model must respond faster than a full scale model by a
factor k . Mass moments of inertia of the model are related to the inertias of the full-scale aircraft by a factor k5.
The following table summarizes the main characteristics of the full scale and the model aircrafts.
Full scale
Model
Length
12 m
1,66 m
Weight
3890 kg
10,42 kg
Wing span
14,4 m
2m
Altitude
0m
0m
Table 3. Summary of the main characteristics of the full-scale and subscale aircrafts.
Different altitude scaling could be achieved by adding weigh. However, in order to keep the models weight as
low as possible, to minimize stall speed, the scaling was performed for takeoff and landing condition, assuming an
altitude of zero meters.
C. Inertia determination
The inertia for RAVEN in full scale was determined by two distinct methods, the first one is based on equivalent
body, see Figure 4, the second one from the CATIA model, in both case the calculation are based on the conceptual
design that has been performed. The inertia matrix was then multiply by k5, the obtain matrix was used as input for
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the design of the internal structure of the scale model. The main goal when designing the scale model in CATIA V5
was then to obtain the right inertia. The inertia of the realized aircraft will then be measured with a cradle. In the
current status the inertia is not verified and no action will be taken on that prior to completion of a first flight.
Figure 4. Inertia determination.
D. Flight test equipment
The rapid development of low cost and miniaturised electronics has been a key enabling technology to the
development of subscale vehicles. The objective of this part of the work has been to construct an instrumentation
package consisting of both the ground and airborne systems. The basic system layout used in this work is given in
Figure 5.
Key drivers in the layout of the instrumentation system were minimum cost and simplicity, such that a minimum
amount of time would be required to integrate the hardware and build the software.
In order to be able to log data during flight tests the aircraft is equipped with:
−
−
−
−
−
−
CPU board with data logging capability
IMU
Potentiometer at all control surfaces
Alpha and beta vanes
Dynamic and static pressure sensors
Telemetry with stall speed warning for the pilot
The heart in the data acquisition system is a
pc104 computer, “Athena”, from Diamond
Systems18. This computer comes complete with
all the analog and digital inputs that are needed to
connect the additional sensors for flight testing.
To measure the accelerations and angular
positions an AHRS (IMU) from Xsens19 is used.
For position estimates a 16 channel, 4 Hz GPS
module from Ublox is employed20. To further
enhance the precision of the position estimate
during flight, a Kalman filter is used to fuse the
data of the GPS and the IMU together.
A simple camera will also be mounted
onboard for live streaming during flight in order
to give a visual feedback to the ground. The
control of the airplane is realized by usage of a
RC radio, the standard receiver has been replaced
by a redundant system to minimize transmission
losses. The usage of a RC radio link for the
control was to simplify the system in a first stage. Figure 5. Main layout of the data acquisition system.
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If the first fly will prove to be successful the system could be completed with a ground control, see Figure 6.
Figure 6. Ground control station possible layout.
E. Manufacturing
The subscale model aircraft is realized in composite materials with the internal structural elements of the
fuselage made of plywood. The composite was realized as a sandwich of two glass-fiber layers and one HerexTM
sheet, cured in vacuum bags. The pictures in Figure 7 show stages from the manufacturing. The moulds were milled
from RenShapeTM 5460 blocks directly from the outer mould line defined in CATIA V5. The wing is mounted with
four screws to the fuselage. Both the fuselage and the wing were realized as two separated halves then glued
together.
Figure 7. Manufacturing of the model. Left: one of the fuselage moulds. Right: the molded fuselage. Bottom:
one of the wing moulds.
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F. Engine installation and Fuel System
5. Engine installation
The engines chosen for this project were two FunSonic FS70. The engines are rear mounted and are installed
according to Figure 8, which illustrates the set up for both start and operation. The gas tank and the display are not
installed on board and are only used for start operation. The display is also used for monitoring the engine prior and
after each flight.
Figure 8. Engine set up for start and installation.
6. Fuel System
Since two engines are used, the fuel system was divided into two separated identical systems. Both systems are
fully independent.
Two common problems with fuel system for RC jet engines are the following:
-
-
Air bubble problem: Any air bubble
reaching the engine may cause engine
flame-out. Therefore air needs to be
remove from the system, especially after
the pump.
Cleanness problem: The fuel need to be
perfectly clean and free of particles. If
particles reach the engine, it may block the
injection and that will result in engine
losses.
Different type of tanks can be used; in this project
soft tanks made from plasma bags are used. This
reduces the risk for air in the fuel system. The main
layout of the system is displayed in Figure 9.
ENGINE
MANUAL ON/OFF VALVE
PUMP
FILTER
BUBBLE TRAP
FUEL TANK(S)
Figure 9. The fuel system.
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REFUELLING
VI.
Status and Future Developments
The aircraft has been completed under late fall 2007 and system tests have been performed. The model will start
flight testing during late spring. Prior to flight testing a series of car top tests will be performed to check all systems
and ensure that there are no stability issues. A large part of the work will be performed by three master students, one
focusing on car top testing, one implementing the flight test equipment and the third one working on the extraction
of aerodynamic data from recorded flight data. Figure 10 shows the aircraft as it appears today.
Figure 10. Raven: a dynamically scaled Business Jet.
Acknowledgments
The authors would like to thank the students for their devotion and work during this project. The authors would
also acknowledge that this project would not have been possible without the financial support of Linklab21.
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