Bio


Lentink's group studies biological flight as an inspiration for engineering design. We focus on key biological questions which we probe with new engineering methods to find inspiration for innovative flying robots. Our comparative biological flight research ranges from maple seeds and insects to birds such as swifts, lovebirds, and hummingbirds. For in-depth biomechanics research we focus on bird flight. Our fluid mechanic research of dynamically morphing wings ranges from studying vortex dynamics to fluid-structure interaction. We apply our findings through robot designs centered on flying in complex cluttered environments under realistic atmospheric conditions.

Academic Appointments


Honors & Awards


  • CAREER Award, National Science Foundation (2016)
  • Recognized as one of 40 scientists under 40 by the World Economic Forum, Annual Meeting of the New Champions, Dalian, China. (2013)
  • 100kE Dutch Academic Year Prize, 100kE (2010)
  • Biophysics thesis award, Dutch Society for Biophysics and Biomedical Technology (2009)
  • Zoology Award, Royal Dutch Zoological Society (2009)
  • Bolk Prize, Netherlands Society for Anatomy (2008)
  • Ritsema van Eck Award, Delft University of Technology (2006)
  • Most Exotic Micro Air Vehicle (MAV) Award, First American-European MAV contest (2005)
  • Elsevier Young Scientist Award, Society for Experimental Biology (2005)
  • AIAA best Fluid Dynamics conference paper, AIAA (2003)
  • Dobbinga Award, Delft University of Technology (2003)

Boards, Advisory Committees, Professional Organizations


  • Alumnus, Young Academy of the Royal Netherlands Academy of Arts and Sciences (2011 - 2016)

Professional Education


  • PhD, cum laude, Wageningen University, The Netherlands, Experimental Zoology (2008)
  • MS, BS, Delft University of Technology, The Netherlands, Aerospace Engineering (2003)

2016-17 Courses


Stanford Advisees


All Publications


  • Power reduction and the radial limit of stall delay in revolving wings of different aspect ratio JOURNAL OF THE ROYAL SOCIETY INTERFACE Kruyt, J. W., Van Heijst, G. F., Altshuler, D. L., Lentink, D. 2015; 12 (105)

    Abstract

    Airplanes and helicopters use high aspect ratio wings to reduce the power required to fly, but must operate at low angle of attack to prevent flow separation and stall. Animals capable of slow sustained flight, such as hummingbirds, have low aspect ratio wings and flap their wings at high angle of attack without stalling. Instead, they generate an attached vortex along the leading edge of the wing that elevates lift. Previous studies have demonstrated that this vortex and high lift can be reproduced by revolving the animal wing at the same angle of attack. How do flapping and revolving animal wings delay stall and reduce power? It has been hypothesized that stall delay derives from having a short radial distance between the shoulder joint and wing tip, measured in chord lengths. This non-dimensional measure of wing length represents the relative magnitude of inertial forces versus rotational accelerations operating in the boundary layer of revolving and flapping wings. Here we show for a suite of aspect ratios, which represent both animal and aircraft wings, that the attachment of the leading edge vortex on a revolving wing is determined by wing aspect ratio, defined with respect to the centre of revolution. At high angle of attack, the vortex remains attached when the local radius is shorter than four chord lengths and separates outboard on higher aspect ratio wings. This radial stall limit explains why revolving high aspect ratio wings (of helicopters) require less power compared with low aspect ratio wings (of hummingbirds) at low angle of attack and vice versa at high angle of attack.

    View details for DOI 10.1098/rsif.2015.0051

    View details for Web of Science ID 000351230700018

    View details for PubMedID 25788539

  • In vivo recording of aerodynamic force with an aerodynamic force platform: from drones to birds JOURNAL OF THE ROYAL SOCIETY INTERFACE Lentink, D., Haselsteiner, A. F., Ingersoll, R. 2015; 12 (104)

    Abstract

    Flapping wings enable flying animals and biomimetic robots to generate elevated aerodynamic forces. Measurements that demonstrate this capability are based on experiments with tethered robots and animals, and indirect force calculations based on measured kinematics or airflow during free flight. Remarkably, there exists no method to measure these forces directly during free flight. Such in vivo recordings in freely behaving animals are essential to better understand the precise aerodynamic function of their flapping wings, in particular during the downstroke versus upstroke. Here, we demonstrate a new aerodynamic force platform (AFP) for non-intrusive aerodynamic force measurement in freely flying animals and robots. The platform encloses the animal or object that generates fluid force with a physical control surface, which mechanically integrates the net aerodynamic force that is transferred to the earth. Using a straightforward analytical solution of the Navier-Stokes equation, we verified that the method is accurate. We subsequently validated the method with a quadcopter that is suspended in the AFP and generates unsteady thrust profiles. These independent measurements confirm that the AFP is indeed accurate. We demonstrate the effectiveness of the AFP by studying aerodynamic weight support of a freely flying bird in vivo. These measurements confirm earlier findings based on kinematics and flow measurements, which suggest that the avian downstroke, not the upstroke, is primarily responsible for body weight support during take-off and landing.

    View details for DOI 10.1098/rsif.2014.1283

    View details for Web of Science ID 000351227000028

    View details for PubMedID 25589565

  • Hummingbird wing efficacy depends on aspect ratio and compares with helicopter rotors. Journal of the Royal Society, Interface / the Royal Society Kruyt, J. W., Quicazán-Rubio, E. M., Van Heijst, G. F., Altshuler, D. L., Lentink, D. 2014; 11 (99)

    Abstract

    Hummingbirds are the only birds that can sustain hovering. This unique flight behaviour comes, however, at high energetic cost. Based on helicopter and aeroplane design theory, we expect that hummingbird wing aspect ratio (AR), which ranges from about 3.0 to 4.5, determines aerodynamic efficacy. Previous quasi-steady experiments with a wing spinner set-up provide no support for this prediction. To test this more carefully, we compare the quasi-steady hover performance of 26 wings, from 12 hummingbird taxa. We spun the wings at angular velocities and angles of attack that are representative for every species and measured lift and torque more precisely. The power (aerodynamic torque × angular velocity) required to lift weight depends on aerodynamic efficacy, which is measured by the power factor. Our comparative analysis shows that AR has a modest influence on lift and drag forces, as reported earlier, but interspecific differences in power factor are large. During the downstroke, the power required to hover decreases for larger AR wings at the angles of attack at which hummingbirds flap their wings (p < 0.05). Quantitative flow visualization demonstrates that variation in hover power among hummingbird wings is driven by similar stable leading edge vortices that delay stall during the down- and upstroke. A side-by-side aerodynamic performance comparison of hummingbird wings and an advanced micro helicopter rotor shows that they are remarkably similar.

    View details for DOI 10.1098/rsif.2014.0585

    View details for PubMedID 25079868

  • Flying like a fly. Nature Lentink, D. 2013

    View details for DOI 10.1038/nature12258

  • Leading-edge vortices elevate lift of autorotating plant seeds. Science Lentink, D., van Dickson, W., B., van Leeuwen, J., L., Dickinson, M., H. 2009; 324: 1438 – 1440
  • Rotational accelerations stabilize leading edge vortices on revolving fly wings. J. Exp. Biol. Lentink, D., Dickinson, M., H. 2009; 212: 2705 – 2719
  • How swifts control their glide performance with morphing wings. Nature Lentink, D., Müller, U., K., Stamhuis, E., J., de Kat, R., van Gestel, W., Veldhuis, L., L.M. 2007; 446: 1082 – 1085
  • Fruit fly scale robots can hover longer with flapping wings than with spinning wings. Journal of the Royal Society, Interface Hawkes, E. W., Lentink, D. 2016; 13 (123)

    Abstract

    Hovering flies generate exceptionally high lift, because their wings generate a stable leading edge vortex. Micro flying robots with a similar wing design can generate similar high lift by either flapping or spinning their wings. While it requires less power to spin a wing, the overall efficiency depends also on the actuator system driving the wing. Here, we present the first holistic analysis to calculate how long a fly-inspired micro robot can hover with flapping versus spinning wings across scales. We integrate aerodynamic data with data-driven scaling laws for actuator, electronics and mechanism performance from fruit fly to hummingbird scales. Our analysis finds that spinning wings driven by rotary actuators are superior for robots with wingspans similar to hummingbirds, yet flapping wings driven by oscillatory actuators are superior at fruit fly scale. This crossover is driven by the reduction in performance of rotary compared with oscillatory actuators at smaller scale. Our calculations emphasize that a systems-level analysis is essential for trading-off flapping versus spinning wings for micro flying robots.

    View details for PubMedID 27707903

  • Flapping wing aerodynamics: from insects to vertebrates JOURNAL OF EXPERIMENTAL BIOLOGY Chin, D. D., Lentink, D. 2016; 219 (7): 920-932

    View details for DOI 10.1242/jeb.042317

    View details for Web of Science ID 000373212600011

  • Flapping wing aerodynamics: from insects to vertebrates. journal of experimental biology Chin, D. D., Lentink, D. 2016; 219: 920-932

    Abstract

    More than a million insects and approximately 11,000 vertebrates utilize flapping wings to fly. However, flapping flight has only been studied in a few of these species, so many challenges remain in understanding this form of locomotion. Five key aerodynamic mechanisms have been identified for insect flight. Among these is the leading edge vortex, which is a convergent solution to avoid stall for insects, bats and birds. The roles of the other mechanisms - added mass, clap and fling, rotational circulation and wing-wake interactions - have not yet been thoroughly studied in the context of vertebrate flight. Further challenges to understanding bat and bird flight are posed by the complex, dynamic wing morphologies of these species and the more turbulent airflow generated by their wings compared with that observed during insect flight. Nevertheless, three dimensionless numbers that combine key flow, morphological and kinematic parameters - the Reynolds number, Rossby number and advance ratio - govern flapping wing aerodynamics for both insects and vertebrates. These numbers can thus be used to organize an integrative framework for studying and comparing animal flapping flight. Here, we provide a roadmap for developing such a framework, highlighting the aerodynamic mechanisms that remain to be quantified and compared across species. Ultimately, incorporating complex flight maneuvers, environmental effects and developmental stages into this framework will also be essential to advancing our understanding of the biomechanics, movement ecology and evolution of animal flight.

    View details for DOI 10.1242/jeb.042317

    View details for PubMedID 27030773

  • The biophysics of bird flight: functional relationships integrate aerodynamics, morphology, kinematics, muscles, and sensors CANADIAN JOURNAL OF ZOOLOGY Altshuler, D. L., Bahlman, J. W., Dakin, R., Gaede, A. H., Goller, B., Lentink, D., Segre, P. S., Skandalis, D. A. 2015; 93 (12): 961-975
  • The role of passive avian head stabilization in flapping flight. Journal of the Royal Society, Interface / the Royal Society Pete, A. E., Kress, D., Dimitrov, M. A., Lentink, D. 2015; 12 (110)

    Abstract

    Birds improve vision by stabilizing head position relative to their surroundings, while their body is forced up and down during flapping flight. Stabilization is facilitated by compensatory motion of the sophisticated avian head-neck system. While relative head motion has been studied in stationary and walking birds, little is known about how birds accomplish head stabilization during flapping flight. To unravel this, we approximate the avian neck with a linear mass-spring-damper system for vertical displacements, analogous to proven head stabilization models for walking humans. We corroborate the model's dimensionless natural frequency and damping ratios from high-speed video recordings of whooper swans (Cygnus cygnus) flying over a lake. The data show that flap-induced body oscillations can be passively attenuated through the neck. We find that the passive model robustly attenuates large body oscillations, even in response to head mass and gust perturbations. Our proof of principle shows that bird-inspired drones with flapping wings could record better images with a swan-inspired passive camera suspension.

    View details for DOI 10.1098/rsif.2015.0508

    View details for PubMedID 26311316

  • Folding in and out: passive morphing in flapping wings. Bioinspiration & biomimetics Stowers, A. K., Lentink, D. 2015; 10 (2): 025001-?

    Abstract

    We present a new mechanism for passive wing morphing of flapping wings inspired by bat and bird wing morphology. The mechanism consists of an unactuated hand wing connected to the arm wing with a wrist joint. Flapping motion generates centrifugal accelerations in the hand wing, forcing it to unfold passively. Using a robotic model in hover, we made kinematic measurements of unfolding kinematics as functions of the non-dimensional wingspan fold ratio (2-2.5) and flapping frequency (5-17 Hz) using stereo high-speed cameras. We find that the wings unfold passively within one to two flaps and remain unfolded with only small amplitude oscillations. To better understand the passive dynamics, we constructed a computer model of the unfolding process based on rigid body dynamics, contact models, and aerodynamic correlations. This model predicts the measured passive unfolding within about one flap and shows that unfolding is driven by centrifugal acceleration induced by flapping. The simulations also predict that relative unfolding time only weakly depends on flapping frequency and can be reduced to less than half a wingbeat by increasing flapping amplitude. Subsequent dimensional analysis shows that the time required to unfold passively is of the same order of magnitude as the flapping period. This suggests that centrifugal acceleration can drive passive unfolding within approximately one wingbeat in small and large wings. Finally, we show experimentally that passive unfolding wings can withstand impact with a branch, by first folding and then unfolding passively. This mechanism enables flapping robots to squeeze through clutter without sophisticated control. Passive unfolding also provides a new avenue in morphing wing design that makes future flapping morphing wings possibly more energy efficient and light-weight. Simultaneously these results point to possible inertia driven, and therefore metabolically efficient, control strategies in bats and birds to morph or recover within a beat.

    View details for DOI 10.1088/1748-3190/10/2/025001

    View details for PubMedID 25807583

  • How Lovebirds Maneuver Rapidly Using Super-Fast Head Saccades and Image Feature Stabilization. PloS one Kress, D., van Bokhorst, E., Lentink, D. 2015; 10 (6)

    Abstract

    Diurnal flying animals such as birds depend primarily on vision to coordinate their flight path during goal-directed flight tasks. To extract the spatial structure of the surrounding environment, birds are thought to use retinal image motion (optical flow) that is primarily induced by motion of their head. It is unclear what gaze behaviors birds perform to support visuomotor control during rapid maneuvering flight in which they continuously switch between flight modes. To analyze this, we measured the gaze behavior of rapidly turning lovebirds in a goal-directed task: take-off and fly away from a perch, turn on a dime, and fly back and land on the same perch. High-speed flight recordings revealed that rapidly turning lovebirds perform a remarkable stereotypical gaze behavior with peak saccadic head turns up to 2700 degrees per second, as fast as insects, enabled by fast neck muscles. In between saccades, gaze orientation is held constant. By comparing saccade and wingbeat phase, we find that these super-fast saccades are coordinated with the downstroke when the lateral visual field is occluded by the wings. Lovebirds thus maximize visual perception by overlying behaviors that impair vision, which helps coordinate maneuvers. Before the turn, lovebirds keep a high contrast edge in their visual midline. Similarly, before landing, the lovebirds stabilize the center of the perch in their visual midline. The perch on which the birds land swings, like a branch in the wind, and we find that retinal size of the perch is the most parsimonious visual cue to initiate landing. Our observations show that rapidly maneuvering birds use precisely timed stereotypic gaze behaviors consisting of rapid head turns and frontal feature stabilization, which facilitates optical flow based flight control. Similar gaze behaviors have been reported for visually navigating humans. This finding can inspire more effective vision-based autopilots for drones.

    View details for DOI 10.1371/journal.pone.0129287

    View details for PubMedID 26107413

  • Gliding Swifts Attain Laminar Flow over Rough Wings PLOS ONE Lentink, D., de Kat, R. 2014; 9 (6)
  • Bioinspired flight control. Bioinspiration & biomimetics Lentink, D. 2014; 9 (2): 020301-?

    View details for DOI 10.1088/1748-3182/9/2/020301

    View details for PubMedID 24854957

  • Gliding Swifts Attain Laminar Flow over Rough Wings. PloS one Lentink, D., de Kat, R. 2014; 9 (6)

    Abstract

    Swifts are among the most aerodynamically refined gliding birds. However, the overlapping vanes and protruding shafts of their primary feathers make swift wings remarkably rough for their size. Wing roughness height is 1-2% of chord length on the upper surface--10,000 times rougher than sailplane wings. Sailplanes depend on extreme wing smoothness to increase the area of laminar flow on the wing surface and minimize drag for extended glides. To understand why the swift does not rely on smooth wings, we used a stethoscope to map laminar flow over preserved wings in a low-turbulence wind tunnel. By combining laminar area, lift, and drag measurements, we show that average area of laminar flow on swift wings is 69% (n = 3; std 13%) of their total area during glides that maximize flight distance and duration--similar to high-performance sailplanes. Our aerodynamic analysis indicates that swifts attain laminar flow over their rough wings because their wing size is comparable to the distance the air travels (after a roughness-induced perturbation) before it transitions from laminar to turbulent. To interpret the function of swift wing roughness, we simulated its effect on smooth model wings using physical models. This manipulation shows that laminar flow is reduced and drag increased at high speeds. At the speeds at which swifts cruise, however, swift-like roughness prolongs laminar flow and reduces drag. This feature gives small birds with rudimentary wings an edge during the evolution of glide performance.

    View details for DOI 10.1371/journal.pone.0099901

    View details for PubMedID 24964089

  • Small aspect ratio differences impact hover efficacy among 12 hummingbird species Kruyt, J. W., Quicazan-Rubio, E. M., Van Heijst, G. J., Altshuler, D. L., Lentink, D. OXFORD UNIV PRESS INC. 2013: E118-E118
  • Flight Artists: An outreach project that enables the general public to film natural flight using the worlds most advanced high-speed camera Lentink, D., Fiaz, A. W. OXFORD UNIV PRESS INC. 2013: E124-E124
  • Vortex interactions with flapping wings and fins can be unpredictable. Biol. Lett. Lentink, D., van Heijst, G., J.F., Muijres, F., T., van Leeuwen, J., L. 2010

    View details for DOI 10.1098/rsbl.2009.0806.

  • Nature inspired flight – beyond the leap. Bioinspir. Biomim. Lentink, D., Biewener, A., A. 2010; 5
  • Structural analysis of a dragonfly wing. J. Exp. Mech. Jongerius, S., R., Lentink, D. 2010; 50: 1323-1334
  • Biofluiddynamic scaling of flapping, spinning and translating fins and wings. J. Exp. Biol. Lentink, D., Dickinson, M., H. 2009; 212: 2691 – 2704
  • The scalable design of flapping micro air vehicles inspired by insect flight. In: Flying insects and robots. Lentink, D., Jongerius, S., R., Bradshaw, N., L. edited by Floreano, D., Zufferey, J. -C., Srinivasan, M., V. Springer. 2009
  • Automated visual tracking for studying the ontogeny of zebrafish swimming. J. Exp. Biology. Fontaine, E., Lentink, D., Kranenbarg, S., Müller, U., K., van Leeuwen, J., L., Barr, A., H. 2008; 211: 1305 – 1316
  • Vortex-wake interactions of a flapping foil that models animal swimming and flight. J. Exp. Biology. Lentink, D., Muijres, F., T., Donker-Duyvis, F., J., van Leeuwen, J., L. 2008; 211: 267 – 273
  • Wake visualization of a heaving and pitching foil in a soap film. Exp. Fluids Muijres, F., T., Lentink, D. 2007; 43: 665 – 673
  • Turning on a Dime. Science Müller, U., K., Lentink, D. 2004; 306: 1899 – 1900