A Review of Current Research in Subscale Flight Testing and Analysis of Its Main Practical Challenges
Abstract
:1. Introduction
2. Analysis
2.1. Definitions
- The test object is flown unconstrained in the open atmosphere, which excludes flight inside wind-tunnel facilities.
- The test object does not have any crew on-board, independently of its control method.
- The test object represents a significantly larger, more complex system or technology and is therefore far from a final product, which excludes conventional flight testing of unmanned aerial vehicles (UAVs) but does not exclude technology demonstrators.
- geometric similarity, which implies equivalence in shape and proportions;
- kinematic similarity, which implies equivalence in motion;
- and dynamic similarity, which implies equivalence in motion and forces.
2.2. Common Scaling Methods
2.2.1. Aerodynamic Scaling
2.2.2. Dynamic Scaling
2.2.3. Aeroelastic Scaling
2.2.4. Demonstrative Scaling
2.3. Recent SFT Projects
- Project or platform has produced at least one publication in English in a scientific journal or conference.
- Project or platform has shown signs of activity (publications or related research activities) during the last decade (2010–2020).
- Project or platform utilisation fits the definition of SFT given in Section 2.1.
- Keyword- and keystring-based search using established search engines for scientific publications (SCOPUS, Google Scholar) with an extended timespan (1990–2020).
- Filtering and removal of duplicates, resubmissions, drafts, and publications outside the field of interest.
- First filtering of valid SFT projects based on information from title, abstract, main features and conclusions.
- Tracing of citations in the already selected publications.
- Expert consultation for additional references.
- Second filtering of valid SFT projects according to the selection criteria detailed above.
- Grouping of publications based on the project they relate to.
- Analysis of each project’s aim, methods and platforms.
- Elaboration of a final list of SFT platforms according to its utilisation.
2.4. Most Common Issues Associated with SFT
- Scaling issues:
- −
- Not possible to attain sufficiently high Reynolds and Mach numbers to ensure aerodynamic similarity.
- −
- Flow distortion due to the need for model actuation, instrumentation, propulsion, and manufacturing constraints.
- −
- Not possible to attain dynamic similarity with Froude and Mach numbers simultaneously.
- −
- Similarity in mass ratio and inertia is severely constrained by operational, economic and practical limitations.
- −
- Dissimilar mass and inertia variations during flight due to different fuel fractions and fuel system.
- −
- Quick angular motion at small scales impose hard requirements on actuation and data acquisition systems.
- −
- Not possible to match aeroelastic similarity criteria with dynamic similarity at practical model scales and speeds.
- −
- Difficult to attain similarity in stiffness, mass distribution and inertial characteristics in a functional aeroelastic model.
- Flight testing issues:
- −
- Severe airspace and operational constraints for remotely piloted aircraft.
- −
- Optimum flight test organisation and procedures differ from those typical of full scale testing.
- −
- Lack of appropriate instrumentation and data acquisition systems for small vehicles.
- Data analytics issues:
- −
- Measurements usually disturbed by turbulence due to ground proximity and operational constraints.
- −
- Lack of an appropriate specialised framework for data conditioning and visualisation.
3. Tackling SFT Issues
3.1. Approaches to Scaling Issues
3.1.1. Management of Flow Differences
3.1.2. Management of Dynamic Similarity, Mass, and Response Time
3.1.3. Aeroelastic Considerations
3.1.4. The Demonstrative Scaling Approach
3.2. Approaches to Flight-Testing Issues
3.2.1. Optimising SFT at Levels 1 and 2
- Automation of test manoeuvres: Custom-made software that is able to precisely command any kind of pre-programmed excitation manoeuvres without the need for a closed-loop flight controller or an on-line ground station, hence avoiding eventual redundancy and certification requirements.
- Optimised manoeuvres for flight mechanical characteristics: Reduce exposure time by exciting different axes and controls simultaneously using multisine signals [94].
- Optimised manoeuvres for performance evaluation: Executing certain dynamic manoeuvres that allow for the exploration of a wide area of the polar in a short time. Figure 6 is an example of the lift characteristics of the GFF demonstrator identified from a single flight using some of the manoeuvres proposed in [93].
3.2.2. Specific Data Acquisition Solutions
3.3. Approaches to Data Analytics Issues
4. The Role of SFT in Aircraft Development and Opportunities for Future Research
- Implications of partial similarity and scaling inaccuracies on the measurability, fidelity and extrapolability of flight characteristics: Beyond basic aerodynamic considerations such as Reynolds number or compressibility deviations, the effects of not fulfilling other similarity parameters is still a controversial topic, especially if the purpose of the SFT experiment is to estimate the flight or handling characteristics of a full-scale vehicle. While this topic has been widely discussed in the wind-tunnel literature, little open information is available for free-flight models. Recent publications [8,135,174] show ongoing efforts to identify and quantify these effects using different approaches.
- Benefits of early subscale experimentation in the maturation of new technology using a demonstrative scaling approach: While the growing interest in using demonstrative subscale platforms to increase the technology readiness level (TRL) of new technologies may indicate that the method has a positive effect in the development process, no scientific studies have tried to interpret or quantify these benefits in comparison to other development strategies.
- Suitability of SFT for the evaluation of handling qualities with a human pilot in the loop: The usefulness of SFT for experimenting with automatic flight control laws is, at this point, indisputable. However, its suitability for obtaining human-pilot ratings of handling qualities is unclear. Earlier experiences from NASA [2] suggest that SFT may not be appropriate for this purpose while Mandal et al. [70] suggest wide variations in pilot behaviour. Specific studies taking into account modern control and information augmentation systems would be desirable.
- Specific flight-testing methods, measurement and analysis techniques for efficient subscale experiments: The testing environment, procedures and even the measurement solutions often seem to play an important role in both the capabilities and the results of SFT. While this is a wide area ranging from unmanned aircraft operations to manoeuvre design and data acquisition techniques, its understanding is key to enabling efficient and useful SFT experiments.
5. Conclusions
Author Contributions
Funding
Conflicts of Interest
Abbreviations
AOA | Angle of attack |
BVLOS | Beyond visual line-of-sight |
CFD | Computational fluid dynamics |
COTS | Commercial off-the-shelf |
EVLOS | Extended visual line-of-sight |
GFF | Generic Future Fighter |
RC | Radio control |
RPA | Remotely piloted aircraft |
SFT | Subscale flight testing |
TRL | Technology readiness level |
UAV | Unmanned aerial vehicle |
VLOS | Visual line-of-sight |
Symbols | |
a | Linear acceleration [m s] |
Angle of attack [ or rad] | |
Generalised aircraft attitude relative to airstream [ or rad] | |
Angle of sideslip [ or rad] | |
Lift coefficient [-] | |
c | Speed of sound (in the pertinent fluid) [m s] |
Control surface deflection angle [ or rad] | |
E | Modulus of elasticity [Pa] |
Bending stiffness [N m] | |
F | Force [N] |
Froude number [-] | |
g | Acceleration due to gravity [m s] |
Torsional stiffness [N m] | |
I | Mass moment of inertia [kg m] |
l | Characteristic linear dimension [m] |
M | Mach number (context dependent) [-] |
M | Moment (context dependent) [N m] |
m | Mass [kg] |
Dynamic (absolute) viscosity [Pa s] | |
Kinematic viscosity [m s] | |
Generalised angular rate [rad s] | |
Frequency of oscillation [rad s] | |
Generalised angular acceleration [rad s] | |
Reynolds number [-] | |
Mass density (of the pertinent fluid) [kg m] | |
t | Time [s] |
Time constant or reduced-time factor [-] | |
V | Linear velocity [m s] |
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Index | Description | Formulation |
---|---|---|
(a) | Reynolds number | |
(b) | Mach number | |
(c) | Control surface angular deflection | |
(d) | Relative density or mass ratio | |
(e) | Relative mass moment of inertia | |
(f) | Aeroelastic-bending parameter | |
(g) | Aeroelastic-torsion parameter | |
(h) | Aircraft attitude relative to the airstream | |
(i) | Reduced linear acceleration | |
(j) | Reduced angular velocity | |
(k) | Reduced angular acceleration | |
(l) | Reduced oscillatory frequency (Strouhal number) | |
(m) | Froude number | |
(n) | Reduced-time parameter |
Method | Focus | Relevant Similarity Parameters |
---|---|---|
Aerodynamic scaling | Similarity of the flow field, disregarding similarity of the aircraft self-motion | (a)(b)(c)(h)(j)(l)(n) |
Dynamic scaling | Similarity of the rigid aircraft motion as well as the aerodynamic loads that cause it | (a)(b)(c)(d)(e)(h)(i)(j)(k)(l)(m)(n) |
Aeroelastic scaling | Builds on dynamic scaling and includes similarity for vehicle deformations | All (a) to (n) |
Demonstrative scaling | Scaled demonstration of a particular technology, system, or capability; partially or fully disregarding the vehicle’s similarity conditions | Variable |
Organisation | Platform Name | Full-Scale Reference | Scale Factor | Scaling Method | Years Active | References |
---|---|---|---|---|---|---|
ONERA, NLR, CIRA, Airbus (EU) | Scaled Flight Demonstrator (SFD) | Airbus 320-200 | 0.12 | Dynamic | Under dev. | [8] |
Military University of Technology (Poland) | Tu-154M model | Tu-154M | 0.10 | Aeroelastic (partial) | Under dev. | [9,10] |
Warsaw University of Technology (Poland) | Numerical Design Results Demonstrator (NORD) | n/a | n/a | Demonstrative | 2020–n/a | [11] |
Airbus (UK) | AlbatrossONE | Twin-engine transport (A321) | 0.07 | Demonstrative | 2019–present | [12] |
FLEXOP consortium (EU) | FLEXOP demonstrator | n/a | n/a | Demonstrative | 2019–present | [13,14,15,16,17,18] |
Univ. of Illinois at Urbana–Champaign (USA) | Cub Crafters CC11-100 Sport Cub S2 | Cub Crafters CC11-100 Sport Cub S2 | 0.26 | Demonstrative | 2019–present | [19] |
Massachusetts Institute of Technology (USA) | KESTREL demonstrator | KESTREL Hybrid eSTOL | 0.30 | Demonstrative | 2019–n/a | [20] |
Univ. of Stuttgart (Germany) | e-Geius-Mod | e-Genius | 0.33 | Dynamic | 2018–present | [21,22] |
ETH Zurich (Switzerland) | Scout B1-100 | Adaptive landing gear concept | n/a | Demonstrative | 2018–n/a | [23] |
Georgia Institute of Technology (USA) | Quad-tiltrotor aircraft | n/a | n/a | Demonstrative | 2018–n/a | [24] |
San Jose State Research Fdn., U.S. Army Aviation Development Dir. (USA) | C-182 UAV | Cessna 182 | 0.12 | Demonstrative | 2018–n/a | [25] |
Univ. of Illinois at Urbana–Champaign (USA) | Cirrus SR22T | Cirrus SR22T | 0.21 | Dynamic, demonstrative | 2018–n/a | [26] |
Aeronautics Institute of Technology (Brazil) | ITA-BWB | ITA-BWB | 0.07 | Demonstrative | 2017–present | [27] |
Univ. of Manchester, BAE Systems (UK) | MAGMA | Boeing 1303 UCAV | n/a | Demonstrative | 2017–present | [28,29,30] |
United States Air Force Academy (USA) | Sub-scale ICE aircraft | ICE/SACCON UAV | 0.10-0.14 | Demonstrative | 2017–present | [31,32] |
NASA (USA) | E1 | Extra 330 SC | 0.40 | Demonstrative | 2017–n/a | [33,34,35] |
NASA (USA) | Super Guppy Foamie | MiG-27 | n/a | Demonstrative | 2017–n/a | [33,34] |
NASA (USA) | Wodstock | n/a | n/a | Demonstrative | 2017–2018 | [33,34,35] |
Univ. of Illinois at Urbana–Champaign (USA) | GA-USTAR aircraft | Cessna 182 | 0.22 | Dynamic | 2017–n/a | [36,37,38] |
Univ. of Kansas (USA) | XQ-139micro | XQ-139A | n/a | Demonstrative | 2017–n/a | [39] |
Stanford University (USA) | 1/10-Taylorcraft | Taylorcraft | 0.10 | Demonstrative | 2015–n/a | [40,41,42] |
Stanford University (USA) | 1/5-Super Cub | Piper Pa-18 Super Cub | 0.20 | Demonstrative | 2015–n/a | [40,41,42] |
Technical Univ. of Munich (Germany) | HYPE Edge 540 | Zivko Edge 540 | n/a | Demonstrative | 2015–n/a | [43,44] |
Univ. of Illinois at Urbana–Champaign (USA) | UIUC Subscale Sukhoi | Sukhoi 29S | 0.35 | Demonstrative | 2015–n/a | [45] |
DLR, Technical University of Munich, Airbus (Germany) | SAGITTA demonstrator | SAGITTA UAV | 0.25 | Demonstrative | 2014–present | [46,47,48,49] |
Berlin University of Technology (Germany) | Flying V | Flying V | n/a | Demonstrative | 2014–n/a | [50] |
Inst. of Aviation, Air Force Inst. of Technology, Warsaw University of Technology (Poland) | MOSUPS | MOSUPS, inverted box-wing | n/a | Demonstrative | 2014–n/a | [51,52,53] |
Istanbul Technical University (Turkey) | TURAC | TURAC VTOL UAV | 0.33-0.50 | Demonstrative | 2014–n/a | [54] |
Univ. of Illinois at Urbana–Champaign (USA) | Great Planes Avistar Elite | n/a | n/a | Demonstrative | 2014–n/a | [55,56,57] |
Virginia Tech (USA) | Telemaster | n/a | n/a | Demonstrative | 2014–n/a | [58,59] |
NASA (USA) | GL-10 Greased Lightning | GL-10 tilt-wing UAV | 0.15-0.50 | Demonstrative | 2013–present | [60,61,62] |
NASA, Area-I (USA) | Prototype-Technology-Evaluation Research Aircraft (PTERA) | Twin-engine transport (various) | 0.11-0.16 | Dynamic, demonstrative | 2013–present | [63] |
Univ. of Illinois at Urbana–Champaign (USA) | UIUC Aero Testbed | Extra 260 | 0.35 | Demonstrative | 2013–n/a | [64,65] |
Virginia Tech (USA) | Sig Rascal 110 | n/a | n/a | Aeroelastic (partial), demonstrative | 2013–n/a | [66,67] |
West Virginia University (USA) | Phastball | Twin engine transport | n/a | Demonstrative | 2013–n/a | [68,69,70] |
Japan Aerospace Exploration Agency (Japan) | S3CM | JAXA’s low sonic boom concept | 0.16 | Demonstrative | 2013–2014 | [71,72] |
Rotorcraft Operations Ltd. (UK) | 1/10th scale centre-line tiltrotor | Centre-line tiltrotor concept | 0.10 | Demonstrative | 2011–n/a | [73] |
University of Kansas (USA) | Yak-54 UAV | Yak-54 | 0.40 | Demonstrative | 2011–n/a | [74,75] |
University of Michigan (USA) | X-HALE | n/a | n/a | Demonstrative | 2011–n/a | [76] |
AFRL, Lockheed Martin, NASA (USA) | X-56A Multi-Utility Technology Testbed (MUTT) | AFRL SensorCraft UAV | n/a | Aeroelastic (partial), demonstrative | 2013–present | [77,78,79,80,81,82,83,84] |
National Institute of Aerospace (USA) | Puffin demonstrator | NASA Puffin Electric Tailsitter | 0.33 | Demonstrative | 2010–n/a | [85] |
Japan Aerospace Exploration Agency (Japan) | NWM, LBM | JAXA’s low sonic boom concept | n/a | Demonstrative | 2010–2013 | [71] |
NASA, Boeing (USA) | X-48C | NASA N+2 | 0.09 | Dynamic | 2010–2013 | [86] |
University of Colorado (USA) | Hyperion 1.0, 2.0, 2.1 | Hyperion BWB concept | n/a | Demonstrative | 2010–2013 | [87,88,89] |
Linköping University (Sweden) | Generic Future Fighter (GFF) demonstrator | Generic Future Fighter (GFF) | 0.14 | Demonstrative | 2009–present | [90,91,92,93,94] |
Linköping University (Sweden) | Rafale | Dassault Rafale M | 0.13 | Demonstrative | 2009–present | [95] |
Beihang University (China) | BB-1, BB-2, BB-3, BB-4 | BUAA-BWB | 0.03 | Demonstrative | 2008–2012 | [96] |
Linköping University (Sweden) | Raven | Raven | 0.14 | Dynamic, demonstrative | 2007-2018 | [7,95,97] |
AFRL, Boeing, Virginia Tech (USA) | SensorCraft RPV | Joined Wing SensorCraft (JWSC) | 0.11 | Aeroelastic (partial) | 2007–n/a | [98,99,100,101,102,103] |
Cranfield University (UK) | ECLIPSE | FLAVIIR Demonstration Vehicle (DEMON) | 0.87 | Demonstrative | 2007–n/a | [104,105,106] |
University of Stuttgart (Germany) | VELA 2 | Very Efficient Large Aircraft (VELA) | 0.03 | Demonstrative | 2007–n/a | [107] |
NASA, Boeing (USA) | X-48B | NASA-Boeing BWB | 0.09 | Dynamic | 2007–2010 | [86,108,109,110] |
NASA, University of Minnesota (USA) | FASER (Ultrastick 120) | n/a | n/a | Demonstrative | 2006–n/a | [111,112] |
NASA (USA) | GTM-S2 | Lockheed L-1011 TriStar | n/a | Demonstrative | 2005–n/a | [113,114] |
NASA (USA) | GTM-T2 | Twin-engine transport (Boeing 757) | 0.06 | Dynamic | 2005–n/a | [113,115,116,117,118,119,120,121,122] |
NACRE consortium (EU) | Innovative Evaluation Platform (IEP) | Twin-engine transport | n/a | Dynamic, demonstrative | 2005–2010 | [123,124,125] |
Univ. of Applied Sciences Hamburg (Germany) | AC20.30 | AC20.30 BWB concept | 0.03 | Demonstrative | 2004–2013 | [126] |
Picture | ||||
---|---|---|---|---|
Index | (a) | (b) | (c) | (d) |
Aircraft type | Generic Future Fighter (GFF) | Light business jet | Light-Sport Aircraft (LSA) | Human-Powered Aircraft (HPA) |
Take-off mass | 15,400 kg | 4000 kg | 290 kg | 100 kg |
Wingspan | 11 m | 14 m | 5 m | 25 m |
Cruise speed | 300 m s | 160 m s | 50 m | 8 m s |
Cruise altitude | 9000 m | 11000 m | 3000 m | 5 m |
SFT scale factor | 0.14 | 0.14 | 0.33 | 0.24 |
SFT Take-off mass | 19.2 (42.3) * kg | 11.0 kg | 10.8 kg | 1.4 kg |
SFT Wingspan | 1.5 m | 1.9 m | 1.7 m | 6.0 m |
Level | Vehicle Mass | Operation | Procedures, Safety | Examples |
---|---|---|---|---|
4 | >150 kg | BVLOS, segregated airspace, full redundancy | Professional, near full-scale | X-48B/C [86], X-56A [84] |
3 | <150 kg | BVLOS/EVLOS, segregated airspace, advanced redundancy | Professional, high-level | SAGITTA [48], IEP [123] |
2 | <60 kg | VLOS, over airfield, limited redundancy | Professional, mid- to high-level | GFF [86], FLEXOP [14], MAGMA [28], AlbatrossONE [12] |
1 | <25 kg | VLOS, over airfield, limited/no redundancy | Relaxed, similar to leisure aeromodelling | Raven [7], Taylorcraft [40], ITA-BWB [27] |
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Sobron, A.; Lundström, D.; Krus, P. A Review of Current Research in Subscale Flight Testing and Analysis of Its Main Practical Challenges. Aerospace 2021, 8, 74. https://doi.org/10.3390/aerospace8030074
Sobron A, Lundström D, Krus P. A Review of Current Research in Subscale Flight Testing and Analysis of Its Main Practical Challenges. Aerospace. 2021; 8(3):74. https://doi.org/10.3390/aerospace8030074
Chicago/Turabian StyleSobron, Alejandro, David Lundström, and Petter Krus. 2021. "A Review of Current Research in Subscale Flight Testing and Analysis of Its Main Practical Challenges" Aerospace 8, no. 3: 74. https://doi.org/10.3390/aerospace8030074
APA StyleSobron, A., Lundström, D., & Krus, P. (2021). A Review of Current Research in Subscale Flight Testing and Analysis of Its Main Practical Challenges. Aerospace, 8(3), 74. https://doi.org/10.3390/aerospace8030074