16.622
Final Presentation
Eleanor Lin, Eric Liu
May 8th, 2008
1
Project Summary
• Flapping flight induces large angles of attack
• Flow likely to separate without flow control which reduces
performance
• Nature has solved flapping flight, a problem which still puzzles
engineers:
– Bats may use fine hairs on their wings to decide how to change geometry,
pitch, flight speed, etc for optimal performance
Figure by M. Drela
2
Project Summary
• We simulate the function of bat hairs using Preston
tubes mounted to the surface of a simulated bat wing
– Bat wing simulated by a thin, symmetric, rectangular wing
• Preston tubes measure average total pressure across a
boundary layer
– Indicates separation and will drive a servo that controls pitch
via a feedback controller to reduce the effective angle of
attack
Figure by M. Drela
3
Literature Review
• Biologists long aware of bats’ wing hairs, but the exact function
of the hairs is not known.
• Zook tested the idea that the hairs provide feedback on the state
of the boundary layer over the wing.
– Put haired and hairless bats through a sharp, 90° turn
• Compared to bats with wing hairs, bats with bare wings
exhibted significantly more vertical deviation in flight.
Zook’s haired vs. hairless bat flight results.
Photograph of bat wing hairs.
4
Hypothesis
Figure by M. Drela
One pair of Preston
tubes mounted on
the top and bottom
surfaces of a HT13
airfoil at Re = 140K
can be used for the
purpose of active flow control on a wing in a freestream at
13.4m/s, heaving vertically with amplitude 0.1524m and frequency
3 Hz; the Preston tubes’ output can then be fed to a PD controller
with feedback that will drive the wing pitch to reduce the angle of
attack in the presence of flapping.
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Objective and Success Criteria
• The objectives of this project are to fabricate the apparatus
which will heave vertically in the Wright Brothers Wind Tunnel.
Using our feedback controller fed by readings of average p0
across the boundary layer, we will assess its ability to reduce α by
measuring the servo-commanded angle and the wing’s vertical
velocity.
• We will consider the experiment to be successful if the controller
can reduce α compared to the uncontrolled case.
• Assessment: we were able to reduce α via active control
methods. However, the experiment was performed under
conditions that did not quite meet our specifications.
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Experimental Setup
7
Experimental Design
• Parameter Selection:
k  wc
2V
– Try to match a bat’s typical reduced frequency:
which lies in the range [0.1…0.4]: chose k = 0.11  f = 3Hz
h
– tan  max   2k 0 implies  max  12 ensures a large separation
c
region
• Controller—Lead Compensator (PD):
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Methods of Measurement
• Pressure:
– Pressure transducer mounted at the tip of the beam
– Measures differential pressure across the top and bottom surfaces
• Servo Position (  t  ):
– Potentiometer connected with tygon-tubing to the shaft in the wing
– Measure variation in voltage across the pot as the shaft rotates from servo
commands

• Vertical Velocity ( h t ):
– Accelerometer mounted at the tip of the beam
– Measures vertical acceleration which can be integrated to obtain vertical
velocity
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Test Matrix
Kp
K d 100
150
200
250
300
350
400
100
120
140
160
X: Indicates that the test was not performed.
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Static Results
• Disconnected the flapping mechanism
• Proof of concept:
– Held the beam with the wing in the freestream at
30mph
– Simulated changes in α by twisting the beam
– Observed the controller matching the motion of my
hands
• Trial and error to determine Kp and Kd
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Static Results - Video
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Data Reduction and Error Estimation
•
•
•
•
•
•
•
 t  
 controlledt    t   tan  h    t    uncontrolled t 
V 
 

We seek to compute
Raw θ and h t  are both recorded in LabView.
h t  integrated to get h t  using Forward Euler
αuncontrolled computed from h t  since V∞ is known
αcontrolled computed, smaller than and αuncontrolled

To quantify, computed 
for each gain pair
Error bars computed based on twice the standard
deviation 
in, giving a 95% confidence level
1




controlled
uncontrolled
controlled
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Flapping Results:
Kp
K d 100
150
200
250
 controlled
 uncontrolled
300
350
59%
400
100
59%
120
54%
56%
52%
55%
57%
60%
61%
140
53%
56%
56%
64%
64%
68%
69%
160
68%
68%
76%
79%
78%
82%
86%
X: Indicates that the test was not performed.
57%
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Controller Gains ( K p and K d ) vs.
αcontrolled
αuncontrolled
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Error Mitigation and
Analysis
• Random error exist in both accelerometer (noise)
reading and potentiometer (dead band, direction)
N
2
• Standard Error:


x

x
x

k
k 1
x 

N N  1
N
• By averaging our results over many samples, we
significantly reduce the error in the mean
• When integrating accelerometer data, constant drift was
removed by subtracting a window averaging of the raw
integrated data
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Error in Implementation
• Flapping:
– Motor cannot provide enough torque
– Result is that flapping occurs at 2Hz instead of 3Hz
– Lower frequency reduces αuncontrolled but the controller was
still operative
• Wing Construction:
– Mistakes in construction resulted in a slight negative camber
– Results in a slight bias in pressure
• Transducer:
– Connected to Pitot tubes by small (diam.) Tygon tubes
– May introduce large lags in the pressure reading
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Comparison to Theory
• Body of theory for our work is small
• Expected:


1  h t  
 t    t   tan
V 
 

–
(angle of attack) was reduced by active
control
– Expect controller attempt to minimize variations in α
• Unexpected:
– Differential pressure was not notably affected by control
– Explanation:
• αcontrolled not sufficiently reduced, max value ≈ 5° in the best case
• Accounting for 3D effects, XFOIL predicts that the Preston tubes sit
at the beginning of a separation bubble.
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Evaluation of Hypothesis
• Static test indicates that the concept is sound
• Actual experiment did not operate at the specified
parameters:
– factual = 2Hz ≠ 3Hz
– Reactual ≈ 140K is out of the [30K…100K] range
– k ≈ 0.07 (too small) and θnax ≈ 8° (large enough)
• Nonetheless: successfully reduced the local angle of
attack with active control
– Evidence does not refute the hypothesis
– Further testing is needed to verify whether control is
maintainable when operating at the prescribed experimental
parameters
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Conclusion
• Using only total pressure as an input, we reduced the
operating α of a flapping wing
• However we were unable to eliminate separation
• Idea that bats use their hair bending moments to
evaluate the flow is supported by our results
– Inability to eliminate separation (i.e. achieve an even lower α)
indicates shortcomings in the controller design
– Our operating parameters do not quite mirror bat flight
conditions, but we think there is little reason to believe that
the theory is unsupportable.
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