The Concept of Morphing in UAVs

UAVs or Unmanned Aerial Vehicles are the aircrafts that don’t have a human crew on-board to control and fly. They can be fully automatic, semi-automatic or controlled remotely by a human pilot. This helps the aircrafts to carry high risk missions like disaster-stricken area (e.g., Landslide, Flood, and Earthquake) and also for surveys & mapping of cities and conservation of wildlife with slight modifications [1]. Their sizes can vary from radio controlled small hobby-plane to even large commercial airlines. One such UAV variant from Team RAKSHAK is as follows:

 

Figure 1: Smokey, Team RAKSHAK [1]

 

Wing morphology can play a crucial role in various flight states of UAV like cruising, maneuvering or hovering. For example a commercial aircraft can spend around 90% of its flight time in cruising mode. In such cases high lift-drag ratio will be required for optimal cruising performance. The general flight state cycles for most of UAVs comprise loitering, cruising, fast ascents and fast descents. The figure below shows the difference of flight states for a commercial UAV as opposed to a Surveillance drone/ UAV.

 

Figure 2: Flight Comparison [2]

 

Morphing originates from the field of Biomimetics. Biomimetics involves studying the nature’s perfect examples like the eagles, kites, bats, etc. who have a proven wing shape adaptability (Morphing). To most extent in comparison to winged creatures’ bats, a mammal family ave evolved the ability of powered flight from last 50 million years. The modern day bats are efficient flyers and researching on the bat flight has revealed many interesting results. By using the techniques like particle image velocimetry, the wake vortices generated can be captured thus enabling the aerodynamic force estimation across a morphed wing. Following figure shows one such instance during the experimentation.

Figure 3: Particle image velocimetry [3] 

 

In terms of aerodynamics, Morphing can be defined as the ability of aircraft wings to change the wing profile seamlessly to suit various flight states and mission profiles, by providing optimal performance. Thus the flow characteristics over the airfoil will dramatically change the Lift and drag values over the morphed shape [4].

The current research in the domain of wing morphing in UAVs can be broadly classified in following 4 sectors [5]:

1. High power density actuators- Involves study and development of various closed loop controlled motors mostly servos for Airfoil twists.

2. Wing Structural Mechanics- Involves study and development of the Structural linkages constituting the wings like multi-bar mechanisms to Cellular solids with micro-scale lattice structures.

3. Flexible skin development- Involves study and development of Shape memory polymers which have the ability to change the wing wrapped skin tightness (mechanical term: stiffness) based on electric signal actuation.

4. Collective Morphing control- Involves development of System and control of mechatronic circuitry for wing morphing.

 

Figure 4: A morphed wing via effective chord length change [6].

 

Classification of various Wing morphing techniques:

 

Figure 5: Classification of various Wing morphing techniques [7].

 

1.    Wing Span Length Resizing:

In this morphing technique, the span of wings are constructed from a telescopic actuators. The telescopic cross-sections get gradually reduced till the wing tip, such that each individual wing span can slide and fit in its adjacent wing span

 

Figure 6: Wing Span Length resizing illustration [8].

 

 

2.    Effective Chord Length Change:

Effective Chord length allows morphing via increase in the wing area. They are commonly found in Fixed wing UAVs. The changes are performed by extension of leading edge flap and trailing edge flap. The actuation is mostly performed by a combination of DC motor with a lead screw. In the compressed wing state just like the following figure, there are little gaps between the rib connectors. However in fully extended state via linear actuation (DC motor + lead screw), these gaps become significant, effectively changing the wing area and wing length.

 

Figure 7: Wing Span change before and after actuation [9].

 

3.    Morphing via Sweep Angle control:

This technique of sweep angle variation is used by birds during fast descent flight stage. This can be adopted in Fixed Wing UAVs easily by pulling the wings backward towards the root during the fast descent. This effectively leads to a streamlined thinner airfoil profile.

 

4.    Morphing via Camber variation:

To change the lift/drag forces across the wing span, the angle of attack of a wing can be varied. The same concept is incorporated here by varying the camber of airfoil. The following steps illustrate such morphing:

 

Figure 8: Angle of attack variation via Camber change [10]. 

 

5.    Lateral Wing bending using a truss filler:

To achieve a higher lift-to-drag ratio as compared to earlier morphing techniques, NASA incorporated the lateral wing bending structure. It is analogous to bird-wing folding during the flights. To achieve more flexible bending, a truss structure can be incorporated as filler, reducing the wing density. The actuation of bending can be done by heating the either of top-bottom faces of the wing, causing thermal expansion-contraction [11].

Figure 9: Lateral Wing bending using a truss filler actuation [11].

 

6.    Morphing via wing twist:

A UAV wing can be looked upon as a ribbed structure with large number of airfoils mounted over a supporting Eccentuator Arm (shown in the figure). The twisting of these individual airfoils in the wing can account for a curved, twisted surface, provided the wrapping membrane continuity is maintained. This helps in changing the angle of attack of the wings thus leading to control on Lift/Drag acquired.

 

Figure 10: Wing twist illustration [12].

 

 

References:

1. https://rakshakiitb.org/

2. Bolkcom C. Bone E. Unmanned aerial vehicles: Background and issues for congress. Technical report, Congressional Research Service, 2003.

3. Bat flight: aerodynamics, kinematics and flight morphology Anders Hedenström and L. Christoffer Johansson, The Journal of Experimental Biology (2015) 218, 653-663 doi:10.1242/jeb.031203

4. Bowers A. The wright brothers and the future of bio-inspired flight, 2007.

5. Sanders B. Reich G. Introduction to morphing aircraft research. J Aircraft, 44:1059, 2007.

6. Larson R. R. Afti/f-111 maw flight control system and redundancy management description. Technical report, NASA, 1987.

7.  Tan K.T. Yeo W.K. Sofla A.Y.N, Meguid S.A. Shape morphing of aircraft wing: Status and challenges. Materials and Design, 31:1284_1292, 2010.

8.  Supekar A. H. Design, analysis and development of a morphable wing structure for unmanned aerial vehicle performance augmentation. Master's thesis, The University of Texas at Arlington, 2007.

9.  Pelley B. M. Havens E. Reed Jr. J. L., Hemmelgarn C. D. Adaptive wing structures. In: Smart structures and materials 2005: Industrial and commercial applications of smart structures technologies. Proc SPIE, 5762:132-42, 2005.

10.  Breitbacha E. Monner H. P., Hanselkaa H. Development and design of flexible fowler flaps for an adaptive wing. In: Smart structures and materials 1998: industrial and commercial applications of smart structures technologies. Proc SPIE, 3326:60-70, 1998.

11.  Wadley H. N. G. Elzey D. M., So_a A. Y. N. A shape memory-based multifunctional structural actuator panel. Inter J Solids Struct, 42:1943-55, 2005.

12.  Martin C. A. Kudva J. N. West M. N. Bartley-Cho J. D., Wang D. P. Development of high-rate, adaptive trailing edge control surface for the smart wing phase 2 wind tunnel model. J Intel Mater Syst Struct 2004, 15:279-91.