Experimental Investigation of an Insect-based Flapping Wing Hovering Micro Air Vehicle Pranay Seshadri
∗
, Moble Benedict
†
, Inderjit Chopra
‡
Alfred Gessow Rotorcraft Center, Department of Aerospace Engineering, University of Maryland, College Park, MD 20740
ABSTRACT A dierential-four-bar based apping mechanism was designed and built to emulate insect wing kinematics with an active pitching ability for a hovering apping-wing micro air vehicle (MAV). The apping mechanism was designed to have symmetric maximum upstroke and downstroke motions. Numerous wings with the same planform area were built and tested using this mechanism. The wings were designed to be sti in bending and compliant in torsion. The operating Reynolds numbers for these wings were 12,000 38,000. Lift and power measurements were obtained for each of these wings at frequencies ranging from 4 Hz to 10 Hz, for 65, 75 and 90 degrees pitch angles (at midstroke). The maximum lift obtained with one wing was 60 grams at a frequency of 10 Hz and at a pitch angle of 90 degrees. Vacuum chamber tests were conducted on wings so as to isolate the aerodynamic and inertial contributions to power. Typically inertial power contributed 40 60% of the total power cosumption. A motion caputre system (VICON) was utilized to track the motion of the wing, thus providing an insight into passive wing deformations. Flow visualization studies were carried out on these wings, which clearly showed the presence a leading edge vortex even at these relatively high Reynolds numbers (38,000). Finally, two such optimzied wings were implemented on a apping MAV weighing 56 grams and tethered hover was achieved. plications such as tra c monitoring, biochemical sensing, border surveillance, re and rescue operations and
INTRODUCTION
wildlife surveys. Many of these scenarios are extremely challenging and would require the vehicle to operate out-
The need for small autonomous aerial vehicles to
doors in gusty environments and also within con ned
perform a myriad of operations has fostered tremendous
spaces such as buildings, caves, etc. In order to be able
research and development activities in the emerging eld
to sucessfully execute these missions the MAVs should
of micro air vehicles (MAVs). A MAV can be broadly
be capable of e cient hover and also be extremely ma-
classi ed as a ying vehicle having a maximum wing
neuverable. Many xed wing MAVs have been successfully ight
span of 15 cm and weighing from 100 200 grams [1]. While MAVs can be extremely useful assets to the mil-
tested.
itary especially for operations in highly populated ur-
close to over 30 minutes. However, they lack the ability
They are extremely e cient with endurance
ban environments, it can also be used for civilian ap-
to hover, and therefore cannot operate in highly constrained environments. Rotary-wing MAVs, on the other hand, have the capability to hover, but their e ciency is
Graduate Research Assistant,
[email protected]
signi cantly lower compared to their xed-wing counter-
Graduate Research Assistant,
[email protected]
parts [1]; also they have limited maneuverability, mak-
Alfred Gessow Professor and Director,
[email protected] Presented at the American Helicopter Society Specialists' Confer-
ing them less eective in gusty environments. This ba-
ence on Aeromechanics, Jan 20-22, San Francisco, 2010.
sically means that scaling down full-scale concepts such
right
c 2010
by P. Seshadri et al.
Copy-
Published by the American
Helicopter Society International, Inc. with permission. All rights
as xed-wings and helicopters may not be the right ap-
reserved.
proach for operating in a completely dierent aerody-
1
from these previous studies and create a simpli ed lightweight mechanism that can emulate insect kinematics and also ap at high frequencies. With this thought in mind; a simpli ed mechanism, implementing only the translational and rotational modes in the wing apping kinematics is designed and built in the present work. Most of the previous studies were performed using rigid wings and at low frequencies [3] [5] because their focus was on gaining a fundamental understanding of the aerodynamics of the insect-based apping ight and therefore it was essential to decouple the structural Figure 1:
Insect wing kinematics
deformation of the wings from the study. However, in an actual insect wing, or even in a apping MAV, the wings have to be made light and they operate at high frequencies, which result in signi cant elastic deforma-
namic regime. Therefore, it is important to investigate
tions. This makes it more of an aeroelastic problem and
alternate solutions such as apping wings, for operation
therefore more complex to analyze. In the present study,
in low Reynolds numbers. However, the greatest chal-
the importance of tailoring the exibility and mass dis-
lenge in utilizing the apping wing concept for MAVs lies
tribution of the apping wing to improve its aerody-
in emulating their wing kinematics and understanding
namic performance will be examined through various
their associated complex aerodynamics.
wing structural designs.
The wing kinematics involves a combination of dif-
There are not enough studies in the literature to
ferent active modes as shown in Figure 1. In the course
prove or disprove the fact that natural apping wing
of a complete stroke, the wing translates from the ab-
animals are more e cient than conventional man-made
domen towards the thorax, in what is termed the down-
MAVs. The power measurements made on actual insects
stroke. Even during the translational phase, the angle
are not direct and therefore, the e ciencies measured for
of attack of the wing continuously changes due to a con-
various insects are not consistent. One of the primary
stantly varying pitch angle. Upon reaching the end of
goals of the present study is to perform direct power
its apping stroke the wing rotates so as to have a posi-
measurements based on motor torque to evaluate the ef-
tive pitch angle for the subsequent stroke backwards to the abdomen.
ciency of insect-based apping concept from the point
This rotational phase is called supina-
of view of power loading (thrust per unit power) and
tion. The translational phase that ensues is referred to
compare it with conventional micro-scale rotors.
as the upstroke. Finally, upon reaching the end of the
Vac-
uum tests were also performed to breakdown the total
upstroke the wing once again rotates through prona-
power into its aerodynamic and inertial components.
tion back into the downstroke [2]. Insects can also tilt
Signi cant improvements in thrust and power load-
the reference plane of apping; termed the stroke plane,
ings were obtained by systematically testing large num-
thereby varying the net direction of the aerodynamic
ber of wing designs at dierent wing kinematics. There
forces.
is not much quantitative information available in the lit-
Numerous mechanisms have been built to emulate
erature on the dynamic deformations of a exible ap-
insect kinematics [3] [5]. However, most of these are
ping wing.
mechanically complex and are designed for operation at
A motion capture system (VICON) was
utilized to track the motion of the wing, thus provid-
extremely low frequencies because these are scaled-up
ing quantitative information on the deformations of the
benchtop models that matched actual insect Reynolds
wing at dierent instants of the apping cycle. A ow
numbers. Moreover, the goal of these studies was to ex-
visualization study was also performed, which clearly
actly replicate insect wing kinematics in order to under-
captured the leading edge vortex and some of the sec-
stand the associated complex aerodynamic phenomena.
ondary ow structures.
For instance, Caltech's Robo y mechanism [2] which
Finally, in order to prove the
ightworthiness of this concept, a hover capable ap-
closely mimics the wing motion of a fruit y was scaled
ping wing MAV was designed and built.
up 100 times and therefore, aps at 0.167 Hz compared to 220 Hz for the y [6]. Cambridge University's robotic insect, designed based on a hovering hawkmoth operates at 0.3 Hz [7]. Even though these mechanisms can emulate the insect kinematics very accurately, they are too cumbersome to be used on a ying vehicle. Therefore, for a ying MAV application, the key is to learn
2
Figure 2:
Insect wing kinematics [7]
Figure 5:
anism
Schematic of the actual apping mechFLAPPING MECHANISM
Operating Principle As described in the previous section, the present apping mechanism incorporates apping and wing rotations by using a dierential four-bar mechanism.
If
examined carefully, the insect wing kinematics ( apping and wing rotation) can be interpreted as both leading and trailing edges undergoing a periodic motion with a phase dierence (with the trailing edge always trail-
Schematic explaining the operating principle of the mechanism Figure 3:
ing the leading edge). This periodic motion is shown in Figure 2. The present mechanism incorporates this and uses two four-bar mechanisms to ap the leading edge and trailing edge of the wing at the same frequency and amplitude but with a phase dierence with respect to each other. Therefore, if the trailing edge is apped with a phase delay with respect to the leading edge, it always trails the leading edge producing a periodic change of angle of attack of the wing as shown in Figures 3 and 4. Even though in the actual apping mechanism, the wing is apped with a four-bar mechanism, for explanation sake, it can be assumed that the wing is apped in a harmonic fashion in a two-dimensional space as shown in Figure 3. Then the motion of the leading edge can be written as
Asin(ωt) and the motion of the trailing edge Asin(ωt − φ). The pitch angle of the
can be written as
wing (θ ), at a particular instant is given by:
Variation of pitch angle through apping cycle Figure 4:
θ = tan
−1
Asin(ωt) − Asin(ωt − φ) h
(1)
Therefore, the pitch angle depends on two parame-
3
Figure 6:
Schematic of four bar mechanism
ters, the phase dierence (φ) and the vertical separation (h) between the leading edge and trailing edge.
The
variation of wing pitch angle obtained for a complete apping cycle obtained using this concept is shown in Figure 4. However, as shown in Figure 5, in the actual ap-
Pitch angle obtained using four bar mechanism compared the one using simple harmonic motion Figure 7:
ping mechanism, the leading and trailing edges move in a three dimensional space and their motion is acheived using a four-bar mechanism instead of a simple harmonic motion.
This will cause signi cant dierences in the
YT = L2 sin(θ2 − φ).
kinematics of the wing compared to the simple harmonic
and
case depending upon the lengths of the four-bar mecha-
the apping mechanism was designed to produce a total
nism
(L1 , L2 , L3
and
L4
stroke angle of
in Figure 6), the phase lag (φ)
108o
Using the above relation,
with symmetric pitch variation in
and the vertical separation (h) between the two four-bar
both upstroke and downstroke. The linkage lengths used
mechanisms. prises of four distinct linkages that are connected to-
for the nal mechanims were were [L1 = 3.068, L2 = 0.5, L3 = 3.006, L4 = 0.62] inches and h was 1.5 inches. The phase angles (φ) were varied to produce dierent
gether using pin joints, typically to convert the rotary
pitch angle variations.
motion of a motor into a apping motion.
15 and 30 degress to produce
As shown in Figure 6, a four-bar mechanism com-
ping angle of the linkage azimuthal variation in
θ4 = tan where
−1
Y X
θ2
L4 (θ4 )
The ap-
− cos
X = L1 − L2 cos(θ2 )
as a function of the
and
65o
degrees
7 compares the pitch angle variation of the wing during a
L23 − L24 − X 2 − Y 2 √ 2L4 X 2 + Y 2
and
90o , 75o
mid-stroke pitch angles (Figure 4), respectively. Figure
is given by:
−1
The phase angles used were 0,
apping cycle produced by the four-bar mechanism with
that of a simple harmonic apping.
(2)
It can be clearly
seen from the gure that the four-bar based mechanism causes some asymmetry between the upstroke and down-
Y = −L2 sin(θ2 ).
sroke pitch angles. A closer look at the graph also shows
As explained before, in the actual apping mechanism,
that the four bar motion permits greater time of the
there are two four-bar mechanisms (top and bottom
stroke to be spent at lower pitch angles as compared to
four-bar) vertically separated by distance, h (Figure 5).
the simple harmonic case and this may improve the lift
The bottom four-bar mechanism lags the top four-bar
production of the apper.
with a phase dierence of
φ.
Moreover, four-bar mecha-
As shown in Figure 5, the
nism is the most simpli ed way of implementing a ap-
angle subtended by the line connecting the points A and
ping motion. The implementation of the four bar con-
B (tips of leading and trailing linkages) with the hori-
cept for a apping wing system is shown in Figure 8.
zontal plane is the pitch angle (θ ) of the wing at any instant of the ap. This is given by:
Benchtop Flapper
Θ = sin−1 q
h 2
2
2
The apping mechanism,
which has been built
based on the above principle, is shown in Figures 9 and
(3)
10. In the present mechanism, the phase dierence be-
(X − XT ) + (Y − YT ) + (h)
tween the top and bottom four-bar mechanisms can be where the variables with subscript T denote the
changed by changing the phase of one of the ywheels
XT = L1 −L2 cos(θ2 −φ)
relative to the other until the desired angle of attack
trailing edge and are de ned as:
4
Figure 9:
Insect-based apping wing mechanism attachment on the pin as shown in Figure 10. The minimum geometric angle of attack for the wing that can be obtained by this mechanism is limited by the maximum swivel angle allowed by the rod-end bearing in the trailing linkage. The mechanism is apped using B-20 brushless Hacker motor, which has an internal planetary gear box. The overall gear reduction in two stages is about 16:1. A high gear ratio is of extreme importance because it allows the motor to operate at the optimum RPM which is much higher than the operating frequency of the apper. High speed videos of insects apping at frequencies of the order of 100-200 Hz have shown that during the entire apping cycle, the leading edge of the wing re-
Figure 8:
mains sti in bending while there is signi cant torsional
Flapping mechanism design
deformation (passive pitching) [8]. The design of all the wings fabricated followed this governing principle. In order to simulate the sti leading edge of an insect-based
variation of the wing is obtained. Therefore, using this
wing, a unidirectional carbon rod was placed along the
mechanism, the wings can be tested for dierent geo-
leading edge of the wing.
metric angle of attack variations. To keep the weights of
Thin strips of unidirectional carbon ber sheets
the moving parts minimum, these are fabricated out of
were used in the chordwise direction hinged o the car-
Delrin plastic. To attach the wing, the ends of the ap-
bon rod (Figure 11). The bending deformation of these
ping linkages are connected by a pin. Since the distance
chordwise spars manifest as the torsional deformation
and the angle between the linkage ends are changing, a
or passive pitching of the wing (Figure 12). Therefore,
double hinge mechanism is installed between the leading
the passive pitching of the wing mainly depends on the
linkage and the pin on one end as shown in Figures 8, 9
bending stiness and the orientation of these chorwise
and 10 to allow the rotation of the joint in two planes.
spars and to some extent on the torsional stiness of
The other end of the pin is free to stoke inside a hole
the leading edge spar. The wing frame is then glued to
on a rod-end bearing installed on the trailing linkage to
a thin layer of Microlite which acts as the skin for the
allow for the rotation and change in length between the
wing.
ends of the linkages (Figure 10). The wing is xed to an
5
Side view schematic of wing de ection at midstroke pitch angle of 90o Figure 12:
Figure 11 shows some of the wings that were built using this method. Each wing had the same planform Figure 10:
shape, chord and span dimensions. The main dierence
Double hinge and rod-end bearing
in the construction of the three wings was the orientation of the two ancillary spars. In Wing-1, the two spars are oriented perpendicular to the leading edge, while in Wing-3 they are simply rotated an angle of
30o .
Wing-2
has one of the ancillary spars hinged at the root, and with a higher angle of
50o .
All three wings had the
same leading edge carbon spar diameter and the same thickness of the chordwise spars. Three other wings (Wing-4, Wing-5 and Wing-6) were made with varying bending and torsional stinesses, maintaining the same spar arrangement as Wing1. Wing-4, had the same bending stiness as Wing-1, however, a lower torsional stiness. Wing-5 and Wing6 used a thicker leading edge spar, which consequently increased their bending stiness. Torsional stiness of Wing-6 was also increased by increasing the number of carbon layers for the chordwise spar. Characteristics of all six wings are shown in Table 1.
EXPERIMENTAL SETUP Lift values were obtained by mounting the apping wing apparatus onto a load cell. For the purposes of this Figure 11:
experiment, the focus was not on the instantaneous val-
Wings designs
ues but rather on the average values. As a result the lift values are re ective of only the aerodynamic forces and
Wings description Name Spar Dia. Chord Thick. Mass
not the inertial. This is because the averaging eliminates
Table 1:
Wing-1
1.5 mm
0.20 mm
the inertial forces. The output mechanical power of the motor is mea-
2.07 gm
Wing-2
1.5 mm
0.20 mm
2.15 gm
Wing-3
1.5 mm
0.20 mm
1.93 gm
Wing-4
1.5 mm
0.10 mm
1.82 gm
Wing-5
2 mm
0.20 mm
2.42 gm
Wing-6
2 mm
0.30 mm
2.51 gm
sured through a special arrangement of the motor and torque cell (Figure 13) so that it only measures the torque provided by the motor to ap the wing and not any of the inertial body moments produced by the wing. The motor was mounted on four coaxial bearings so that all degrees of freedom except the rotation about the mo-
6
Experimental setup
Figure 13:
Variation of lift with apping frequency for Wing-1 Figure 14:
tor axis, is constrained. As a result, when the motor is connected to the torque sensor, the torque sensor only registers the motor torque.
The power consumed was
obtained from the torque and frequency measurements. A hall sensor was used to obtain the apping frequency.
LIFT RESULTS Three dierent sets of wings were designed and tested to investigate the eect of spar orientation, spanwise bending stiness and chordwise torsional stiness on wing performance.
Eect of Spar Orientation Lift measurements obtained for each of the three wings (Figure 11) at dierent pitch angles are plotted in Figures 14 16. It should be noted that for all these
Variation of lift with apping frequency for Wing-2
results, the only parameter that is varied is the orien-
Figure 15:
tation of the two ancillary unidirectional carbon ber spars. Among these results, there are only small dierences in the lift measured. For instance, the maximum lift recorded for these tests was 50 grams by Wing-2 and Wing-3 at a pitch angle of 90 degrees at 10 Hz. Note that the pitch angle of
90o
means the wing surface is
normal to the motion in both the up and down strokes. Under the same conditions, Wing-1 produced a maximum of 45 grams of lift. From the graphs it is evident that at low frequencies the lift values for all pitch angles are quite similar, however at higher frequencies, the inates.
90o
pitch case dom-
The occurrence of maximum lift at
90o
pitch
maybe attributed to the bending of the chordwise spars at high frequencies.
This bending manifests as a pas-
sive wing pitching and therefore reduces the geometric angle of attack of the wing. The small dierences in lift
variation of lift with apping frequency for Wing-3 Figure 16:
7
Variation of lift with apping frequency for Wing-5 (with 2mm leading edge carbon spar)
Variation of lift with apping frequency for Wing-1 (1.5 mm) and Wing-5 (2 mm) for a mid-stroke pitch of 90o
Figure 17:
Figure 18:
as observed in Figures 14 16 can be attributed to the dierences in deformation because of the varied orien-
it is clear that an additional six grams of lift is produced
tations of the chordwise spars which can cause signi -
at 10 Hz by increasing the bending stiness.
cant geometric coupling between bending and torsion.
Eect of Torsional Flexibility
Therefore, the actual geometric angle of attack of the wing is a combination of the prescribed wing kinemat-
The chordwise spars of the wings in the spar orien-
ics (pitch angle) and the torsional deformation (passive
90o ,
the angle
tation section used two layers of unidirectional carbon
of attack maybe markedly dierent. In the subsequent
ber (chordwise-spar thickness = 0.2 mm). Two layers
sections, the wing deformation will be discussed and an-
were used primarily because they allowed the chordwise
gle of attack ascertained with the help of the measured
spars to bend resulting in signi cant torsional deforma-
deformations using the Vicon motion capture system.
tion or passive wing pitching. In order to evaluate the
pitching).
Thus, while the pitch is at
extent of exibility, a wing was fabricated using only
Eect of Wing Bending Stiness
a single layer unidirectional carbon ber for the spars. This new wing is called Wing-4 and has a chordwise
The three wings tested in the prior section used a
spar thickness of 0.1 mm. It should be noted that both
1.5 mm diameter leading edge carbon rod. During ap-
Wing-1 and Wing-4 have the same bending stiness,
ping, limited bending of this leading edge rod was ob-
because they both employ the 1.5 mm diameter leading
served. In order to explore the eect of bending stiness
edge spar.
Wing-5 was fabricated, as mentioned in the wing design
Figure 19 plots the lift variation for Wing-4.
section. Wing-5 maintained the same torsional stiness
with the other wings, lift at low frequencies is identical
as Wing-1, however its bending stiness was higher due
to other cases.
to the 2 mm diameter leading edge carbon spar. Wing-
lift at high apping frequencies, however compared to
1 only employed a 1.5 mm diameter carbon spar. The
the other wings it produces a maximum lift of only 35
underlying assumption here is that the bending sti-
grams. A comparison of the
ness of the wing is mainly governed by the stiness of
and Wing-4 is shown in Figure 20.
the leading edge spar and the torsional stiness by the
quencies the results are identical, at higher frequencies,
bending stiness of the chordwise spars. Figure 17 plots
the 0.1 mm wing underperforms.
the variation of lift with apping frequency for Wing-5.
dierence of 11 grams between the two cases. This may
90o
be because of the fact that Wing-4 is too compliant in
A comparison between Wing-5 and Wing-1 for the pitch case is shown in Figure 18.
90o ,
90o
pitch again yields maximum
90o
pitch case for Wing-1 While at low fre-
At 10 Hz there is a
torsion and therefore, the geometric angle of attack at-
Figure 17 reveals that for Wing-5 maximum lift occurs at a pitch angle of
The
As
tained by this wing during apping may be extremely
similar to the case with
low producing signi cantly lower lift compared to Wing-
wings in the spar orientation section. From Figure 18,
1.
8
Variation of lift with apping frequency for Wing-4
Variation of lift with apping frequency for Wing-1 and Wing-4 for a mid-stroke pitch of
Figure 19:
Figure 20:
90o
The maximum lift was achieved using a 2 mm diameter leading edge spar and 0.2 mm thick chordwise
spars perpendicular to the leading edge.
spars (Wing-5).
Also, in the previous section it was
Wing-3 had the spars at a subtended angle which would
shown that reducing the chordwise-spar thickness below
in turn cause geometric bending-torsion couplings. Six
0.2 mm degrades the performance. The next step was to
re ective markers were placed on Wing-1; one on each
investigate the eect of increasing torsional exibility by
spar towards the leading edge, and one on the trailing
increasing the thickness of the chordwise spars. There-
edge (Figure 23). The idea was to track the motion of
fore, a wing with a 2 mm diameter carbon spar and a
these individual markers with respect to time and thus
0.3 mm chordwise spar thickness wing was fabricated.
obtain a quantitative measure of wing de ection distri-
This wing is called Wing-6. Figures 21 and 22 show the
bution at high frequencies. The two points on each spar
performance of Wing-6.
could be connected via a line, and thus the geometric
Wing-6 generates about 60 grams of lift at a fre-
Wing-2 and
angle of attack could be estimated.
quency of 10 Hz. This result represents the highest lift
Utilizing VICON for Wing-1 for the dierent pitch
generated on the present apping wing system. It is im-
angles at a frequency of 8 Hz, the geometric angle of at-
portant to compare the lift produced for this wing with
tack variation measured during a apping cycle is shown
Wing-5.
Figure 22 shows that once again, at low fre-
in Figures 24 26. The angle measured using VICON
quencies there is hardly any dierence in wing lift for
is the net sectional pitch angle of the wing, which is
a change in torsional exibility. However at higher fre-
a combination of the prescribed pitch variation by the
quencies the three layered wing (0.3 mm) produces 10
mechanism and the wing elastic torsion. For purposes
grams more lift than the two layered case (0.2 mm).
of nomenclature, the spars are named A, B and C; with
Both wings are fabricated using the 2 mm inch car-
spar A being the most inboard or the root spar of the
bon rod for the leading edge spar. These results clearly
wing, spar B the middle spar and spar C being the out-
shows that torsional exibility is an important param-
board spar or the tip spar (Figure 11). Also presented
eter in the design of apping wings as it signi cantly
is the prescribed wing pitch angle variation obtained us-
aects the passive wing pitching and thereby the lift
ing the dierential four-bar analysis, that shows the pre-
produced.
dicted pitch angle variation of the wing. From the results it is evident that the wing tor-
VICON TESTING
sion at spar C (wing tip) was the highest and therefore the lowest geometric angle followed by spar B and then
In order to quantify the extent of torsional defor-
spar A. This trend is expected because the aerodynamic
mation and the corresponding angle of attack, a VICON
and the inertial forces experienced by the wing increases
motion tracking system was utilized on the apping wing
from the root to tip. For instance, in the
system with Wing-1. For this task Wing-1 was chosen
(Figure 24), the angle of attack at spar A averages be-
because of its construction. The wing had the ancillary
tween
9
65o
to
80o
90o
whereas that of spar C is
pitch case
55o
to
65o .
Variation of lift with apping frequency for Wing-6
Variation of lift with apping frequency for Wing-5 and Wing-6 for a mid-stroke pitch of
Figure 21:
Figure 22:
90o
Figure 27 plots the angle of attack variation for spar B
Comparison of experimental and analyzed lift in grams Pitch Experimental (gm) Analysis (gm) Table 2:
at the three mid-stroke pitch angles. For a pitch angle
60o to 75o , o o o while at a pitch of 65 the angle lies between 40 to 55 . of
90o ,
the spar has an angle of attack of
Analytical Model
65
19.46
16.87
75
24.46
18.82
90
27.35
19.25
A blade element theory based model with uniform in ow was formulated in order to explain analytically
90o pitch angle case produces the highest lift o o followed by 75 and then 65 . It should be noted that, as
in this paper are far higher, the general trend of the
shown by the VICON results, even though the prescibed
experimental results.
why the
mid-stroke pitch angles are
90o , 75o
and
65o ,
CL − α
curve can be used to explain the trend in the Table 2 compares the lift values
the wing
obtained from the above analysis with the experimen-
is actually operating at much lower pitch angles due to
tal results at 8 Hz for wing Alpha at the various pitch
elastic twist. Therefore, a linear interpolation was per-
angles.
formed to obtain the spanwise distribution of geomet-
From Table 2, the lift is under predicted mainly
ric angle at the three spanwise locations measured us-
because the lift due rotational circulation and other un-
ing VICON at dierent instants of ap (VICON data
steady eects were not factored into the model. How-
gathered from Figures 24 26).
ever, the analysis also predicts that the
From the geomet-
75
o
90o
pitch case
65o .
ric angle of attack, the aerodynamic angle of attack at
produces maximum lift followed by
each spanwise location was obtained using a uniform
though the wings are prescribed to translate at high an-
in ow model based on the total average lift and ap-
gles of attack, due to the bending of chordwise spars
ping sector area. A translational lift coe cient given by
and in ow eects, the average angle of attack of the
CL = 0.225 + 1.58sin(2.13α − 7.20)
wing may be below the stall angle.
was then used to
and
Even
obtain the sectional lift which was then averaged along
In the previous section the lift results for Wing-5
the span to obtain the lift of the wing at each azimuthal
and Wing-6 were compared. Both wings were tested us-
location.
ing the VICON system, and compared for the
A plot of this curve is shown in Figure 30.
The instantaneous lift values were then averaged over
90o
pitch
angle as shown in Figure 28.
a apping cycle to obtain the steady lift over an entire
The plot shows the geometric angle variation for the
stroke.
two cases for spar C at a pitch angle of
90o .
Lift values
This value of the lift coe cent was obtained from
of both these wings were compared in Figure 22. Fig-
studies conducted on a dynamically scaled fruit y,
ure 28 shows that Wing-6 is able to maintain a higher
Drosophila melanogaster, for a Reynolds number of 192
geometric angle as compared to Wing-5.
[9]. While the Reynolds number of the results presented
the mid-stroke angle of attack for the 0.3 mm case is
10
Thus while
Wing-1 geometric angle of attack variation at mid-stroke pitch angle of 75o Figure 25:
Figure 23:
Vicon experimental setup with Wing-1
Wing-1 geometric angle of attack variation at mid-stroke pitch angle of 65o Figure 26:
Wing-1 geometric angle of attack variation at mid-stroke pitch angle of 90o Figure 24:
11
70o ,
for the 0.2 mm case it is
58o .
This result can be
substantiated by the fact that for greater torsional rigidity, the amount of torsional de ection seen on the wing will decrease and it is this eect that yields an extra 10 grams of lift (Figure 22). This once again establishes the mid-stroke angle of attack for maximum lift is around
70o -75o .
FLOW VISUALIZATION The complex ow regime at low Reynolds numbers and corresponding unsteady aerodynamic mechanisms, account for the production of lift at such high angles of attack. A ow visualization study was performed on Wing-1, to obtain qualitative data as the wing translates and rotates. For this experiment the ow was seeded using mineral oil fog and the ow eld was captured at different phases of the apping cycle using a stroboscope synchronized with apping and a high resolution digital camera. Figure 29 shows the ow over wing alpha
Wing-1 geometric angle of attack variation for spar B at dierent mid-stroke pitch angles
through various instances during translation and rota-
Figure 27:
tion. Previous research has shown that a form of dynamic stall ensues during the translation phase of apping and is observed in the form of a strong leading edge vortex (LEV) [10]. The theory is that circulation of the LEV enhances the bound vortex of the wing and as a result can produce aerodynamic forces greater than those predicted by quasi-steady ow [11]. Even in the present experiment a strong leading edge vortex is observed on the wing as shown in the gure. Traditionally, at high angles of attack on conventional airfoils, the LEV rapidly moves downstream and then sheds into the wake. As a result the increase in lift from dynamic stall is seen only brie y, before lift plummets. However in apping wing ight there is a dominant span-wise ow that convects the vorticity towards the wing tip. Here it coalesces with the tip vortex and prevents the LEV from breaking away from the surface [12]. While the LEV is a dominant factor in lift augmentation, studies have shown that it is responsible only for 65 percent of the increased lift [11]. There are two other aerodynamic mechanisms that also play a signi cant role, namely rotational circulation and wake capture.
During the rotational phase
of the wing, additional lift is produced through rotational circulation which is analogous to the Magnus effect. This circulation is developed so as to maintain the rear stagnation point at the trailing edge, thus enforcing
Comparison of angle of attack for change in torsional exibility
the Kutta condition [13]. During the translational and
Figure 28:
rotational phases, the wing moves through the wake generated from the previous stroke. It has been illustrated that this phenomenon increases the eective uid velocity over each successive stroke, thus increasing the lift that is generated solely through translation. This con-
12
Figure 29:
Flow visualization images
13
tinues until a steady state circulation is reached.
Fig-
ures 29 (a, c) shows the wing apping through the wake generated from the previous strokes. Various other secondary ow structures such as eddies and swirls can also be seen. While prior studies have predominantly focused on visualizing these unsteady phenomenon at Reynolds numbers of the order 150-200, this study illustrates their presence at Reynolds numbers of 38,000 which is more typical of a MAV.
POWER RESULTS The power measured using the torque and frequency measurements include the aerodynamic power of the wing (induced power, pro le power and unsteady loses), the inertial power of the wing, parasitic power
Variation of power with apping frequency for Wing-1
and the inertial power associated with the rest of the
Figure 30:
apping mechanism (other than wings) and the balance associated losses.
The power associated with the rest
of the mechanism and balance associated losses are obtained by measuring the power consumed by the apping mechanism in the absence of the wing. This is referred to as the tare power. The tare power is then subtracted from the total power measurements to obtain the aerodynamic and inertial power consumed by the wing (wing power). It is this power that is plotted for the dierent wings tested in Figs. (30 35). Figures 3032 plots the variation of wing power with apping frequency for Wing-1, Wing-2 and Wing3 respectively.
For a maximum frequency of 10 Hz,
Wing-1 consumes the least power amongst all the wings. Maximum power is consumed by Wing-3 at for a midstroke pitch angle of
90o
at 10 Hz.
It is evident from
the graphs that for all wings, the power requirements for the
90o
pitch case are the highest for most frequen-
cies, followed by
75o
and then by
65o
Variation of power with apping frequency for Wing-2
. A relationship
Figure 31:
for translational drag for the dynamically scaled fruit y model as a function of angle of attack is given by
CD = 1.92 − 1.55cos(2.0α − 9.82)
[9]. The above rela-
tion reveals a steady increase in the drag with angle of attack. Even with the passive wing pitching,
90o
pitch
angle had the maximum angle of attack throughout the entire apping stroke compared to the other pitch angles. This produces higher average drag which increases the pro le power for the
90o
pitch angle case.
The eect of a change in bending stiness on wing power is shown in Figure 33. An increase in the leading edge carbon spar diameter by 0.5 mm, leads to a sixty percent increase in power consumption. Lift results for this case show Wing-5 producing 6 more grams of lift than Wing-1, which is less than a 10 percent increase (refer to Figure 18). Figure 34 and 35 show the eect of increasing the chordwise-spar thickness on wing power. An increase in
Variation of power with apping frequency for Wing-3 Figure 32:
14
Variation of power for Wing-1(1.5 mm) and Wing-5 (2 mm) at a mid-stroke pitch angle of 90o
Variation of power for Wing-1 and Wing-4 at a mid-stroke pitch angle of 90o
Figure 33:
Figure 34:
powerloading. the chordwise-spar thickness from 0.1 mm to 0.2 mm results in a 0.5 Watt increase in power at 10 Hz.
In Figure 40, the wing with the best power loading
At
obtained from the present study is compared to a con-
the same frequency, a transition from a 0.2 mm to 0.3
ventional rotor of similar scale at the same disk loading.
mm chordwise spar results in a 2.5 Watt increment. In-
It can be clearly seen that the power loading for the
creasing the chordwise-spar thickness increases torsional
apping wing is signi cantly lower than that of a con-
stiness along the chord. As a result the angle of attack
ventional rotor.
while apping stays at higher values yielding greater
apping wing, in addition to the aerodynamic power,
drag. This drag manifests itself in the form of a higher
a large part of the power is used for accelerating and
power consumption. The dierences in power consumed
decelerating the wing during the apping cycle.
in many of these cases is also because of the dierences in
is termed as inertial power and this can be a signifcant
the mass of the wings which changes the inertial power
fraction of the total power depending on the mass of the
required to ap the wings. This aspect is further inves-
wing [14]. However, for a conventional rotor, the iner-
tigated in the section on vacuum chamber testing.
tial power is zero because the rotor rotates at a constant
This is because of the fact that for a
This
speed. To investigate this further a series of tests were
POWER LOADING
conducted in both vacuum and air to obtain the fraction of inertial power in the total wing power measurements for a typical wing.
The overall hover performance of an MAV system can be de ned by the power loading, which is the thrust produced per unit power.
VACUUM CHAMBER TESTS
Thus, the higher the power
loading the more e cient the hovering system. Figures 3638 show the power loading for Wing-2, Wing-4 and
To quantify inertial wing power consumption, vac-
90o pitch case producing largest lift
uum chamber tests were conducted. The vacuum cham-
force, its power loading is the lowest because of the large
ber setup with the apper is shown in Figure 41. The
power consumption. A key point that is highlighted is
maximum gauge pressure achieved was 27 in of mercury,
Wing-5. Despite the
90o
pitch case that results
which corresponds to a 90 percent vacuum [15]. Given
in the lowest power loading for all three wings because of
the dimensions of the vacuum chamber, it was not possi-
the high drag produced at these higher angles of attack.
ble to test the mechanism with the present wings. As a
the relative ine ciency of the
While in Figures 36 and 38, the
65o
pitch case produces
result, a scaled down version of the Wing-1, called Wing-
75o
7 was built. It is important to note that the mass distri-
the highest power loading, in Figure 37 it is the
bution of both the wings were identical. Thus, the ratio
mid-stroke pitch variation for Wing-4.
of inertial power to total power would remain the same
Figure 39 shows the power loading values for all three wings at a mid-stroke pitch angle of
65o .
for both Wing-1 and its scaled down version, Wing-7.
For gen-
Power was measured for the wing in air and in vac-
erating the same amount of lift, Wing-5 has the greatest
15
Variation of power for Wing-5 and Wing-6 at a mid-stroke pitch angle of 90o s Figure 35:
Figure 36:
Wing-2
Figure 37:
Variation of power loading with lift for
Figure 38:
Variation of power loading with lift for
Wing-4
Variation of power loading with lift for
Wing-5
16
Comparison of power loading for Wing2,4 and 5 at a midstroke-pitch angle of 65o Figure 39:
Figure 41:
Comparison of power loading values for a conventional rotary based MAV and apping wing system utilizing Wing-5
Vacuum chamber setup
Figure 40:
Variation of power with apping frequency for Wing-7 Figure 42:
17
Power composition for three dierent mass cases at 10 Hz Figure 45:
Variation of power with apping frequency for Wing-7 with 0.20 gram added mass Figure 43:
Powerloading for Wing-7 in the presence and absence of inertial power Figure 46:
Variation of power with apping frequency for Wing-7 with 0.50 gram added mass Figure 44:
0.50 gram mass. Figure 45 compares the ratio of inertial to aerodynamic power for the original Wing-7 with
uum. The inertial power corresponds to the power con-
the Wing-7 with the added mass. In all three graphs,
sumed in vacuum, and the aerodynamic power is simply
as expected, the aerodynamic power remains almost the
the inertial power subtracted from the power consumed
same, while the inertial power increases proportional to
in air. Figure 42 shows the average power results where
the mass added. This substantiates the vacuum cham-
the inertial power accounts for 60 percent of the total
ber tests.
power consumed. In order to substantiate the above re-
The eect of the inertial and aerodynamic contri-
sult, a simple experiment was conducted where a mass
butions to power loading was explored by utilizing the
was glued to the leading edge spar of Wing-7. Since the
power results for the small wing (Wing-7). The lift of the
leading edge spar is sti, and no de ection occurs, it is
wing was measured for the same frequencies as tested in
assumed that the addition of the mass does not aect
the chamber. Figure 46 plots the powerloading as func-
the aerodynamics of the wing in any way. However the
tion of lift using both the total power and the aerody-
inertial power would increase with an increase in mass.
namic power.
Figures 43 and 44 present the results with the ad-
As shown in Figure 46, the powerloading values ob-
dition of two separate masses on the wing. In the rst
tained based on the aerodynamic power is two and a half
case a 0.20 gram mass was added and in the second a
times greater than obtained using the total power. This
18
Figure 47:
Close up of apping wing MAV
Figure 48:
Tethered apping wing MAV with
Figure 49:
Tethered apping wing MAV in hover
guide rods
result clearly highlights the relative ine ciencies caused due to the large inertial power associated with apping wings. Therefore, while translating to a ying MAV, it is extremely important to keep the mass of the wing as low as possible.
FLAPPING WING MICRO AIR VEHICLE All the above tests were aimed at developing an optimized apping system to be used on a hover capable MAV. Therefore, the next step was to design and build a apping wing micro air vehicle utilizing the present apping mechanism and one of the optimum wing designs.
Translating from a heavy bench-top apper to
a light-weight ight-capable apping MAV was a major challenge. The vehicle had to be designed such that it is
required electronics. The nal goal of the project is to
strong enough to handle the large dynamic loads due to
develop an autonomous optimized apping wing MAV
apping and also light enough to be able to y. The de-
that is capable of hovering for about 30 minutes.
sign went through numerous interations until the MAV could lift o in a tethered condition. Figure 47 shows
CONCLUSIONS
the nal apping wing MAV. In order to reduce weight the main body was made using a rapid prototyping technique. The wings along
This study implemented a dual four bar mechanism
with a corresponding four-bar based apping mechanism
in order to emulate insect-wing kinematics by incorpo-
described before were attached to the body. The mech-
rating the translational and rotational degrees of free-
anism was driven by an outrunner motor weighing 25
dom. The mechanism featured an active pitching capa-
grams. A gear ratio of 7:1 was used so that the motor
bility that was crucial in determining the optimized wing
operated at the peak e ciency frequency. The mass of
kinematic parameters.
the entire vehicle with all the components was 56 grams.
dicted the wing kinematics was formulated and yielded
The MAV was then tethered between two vertical steel
satisfactory results. Various wing designs were system-
guide-rods as shown in Figure 48.
Upon apping at
atically tested to obtain the lift and power measure-
the required frequency, tethered hover was achieved (see
ments. The main factors that impacted wing design are
Figure 49).
This demonstrates the ightworthiness of
orientation of chordwise spars, bending stiness of the
this apping concept. Control methodologies are being
leading edge spar and torsional exibility of the wing.
developed for the attitude control of the MAV. Since the
The wing deformations at dierent spanwise loactions
wings can produce a total lift of almost 120 grams, there
were obtained using a Vicon motion capture system.
is still a 64 gram weight allowance for batteries and the
A hover capable apping wing MAV was designed and
19
An analytical model that pre-
built to demonstrate the ightworthiness of this concept.
Joseph Mait (ARL) and Mr. Mark Bundy (ARL-VTD)
Speci c conclusions derived from this work are:
as Technical Monitors. The authors are greatful to Greg
1. Lift measurements were carried out at
90o
65o , 75o
Gremillion, Joseph Conroy and Dr.Sean Humbert at
and
the Autonomous Vehicle Laboratory at the University
mid-stroke pitch angles, for the various wings.
of Maryland for their assitance with the Vicon testing.
For the majority of wings, it was observed that at
The ow visualization was carried out with the help of
low frequencies (4 6 Hz) the lift generated was identical.
Joseph Ramsey.
However for all wings tested at higher
90o
frequencies (8 10 Hz), the
pitch angle case
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21
