1. Wind Turbine Final Report
WindTER – Wind Turbine Energy Resources
Kristina Monakhova – Program Manager
Elizabeth Yasuna – Executive Director
Dominick Farina – Business Development
Kyle Zalud – Technical Lead
EAS 140 D2-E, Zack Bauer, Nikita Ranjit Goraksha

2. Project Objectives
Purpose: Design efficient wind turbines for small and large scale applications
Goals:
Build and improve a wind turbine
Strive for continuous improvement
Create a scientific foundation for future improvements/innovations
Focus on simplcity and reliability
WindTER is a wind turbine research company headquartered in Buffalo, NY. We design, build, and manufacture wind turbines for small scale and large scale energy harvesting operations.
Currently, we are in the research and planning phase of our business and are seeking funding to perfect our turbine design and move on to manufacture and sell our high efficiency wind turbines.
The goals of this project were to research existing wind turbine designs, understand factors affecting turbine performance, design a preliminary wind turbine, and experiment and test out different configurations to improve wind turbine performance. Through our experimentation and research, we hope to gain technical expertise and a firm scientific understanding that could facilitate future innovations. We aim to create a simple yet reliable wind turbine that will stand the test of time and provide a dependable energy source for many years to come. We want to design an efficient turbine with a high yet stable power output. Above all, we strive for continuous improvement.
Source:

3. Background Research - Design Factors for Wind Turbines
•Number of blades
•Angle of blades
•Shape of blades
•Blade Twist
•Blade Length
•Blade materials
•Generator
•Gear ratios
•Oil/Lubricant used
•Height of tower
•Rotational Speed
Number of blades - The number of blades the wind turbine has. Having more blades increases weight and means each blade must be thinner and narrower, making it harder to build them strong enough. Generally, wind turbines use no more than 3 blades since aerodynamic efficiency is marginally improved as you increase the number of blades, but blade stiffness is compromised.
Angle of blades - The blade angle changes the angle of attack the turbine, affecting rotational speeds. When the blades face directly into the wind, the turbine will stop. Generally, blades are angled at 12° at the base to maximize speed. Angles greater than 14° cause stalling.
Shape of blades - The shape of the blades affects the rotational speed of the turbine and therefore the power output. Generally, the blades become narrower towards the tip to maintain a constant slowing effect across the swept area. The turbine shape should maximize lift while minimizing drag.
Blade Twist - To maintain the optimum angle of attack along the blade, the blades must be twisted down their lengths. Generally, blades are tapered at tip and slightly twisted, creating a greater angle-of-attack near their root where rotational velocity is slowest. Velocity is higher at the tip of the blade, so the angle-of-attack there is smaller. Wide tips add drag.
Blade Length - The blade length affects the swept area of the turbine, which affects the power. Longer blades increase power since power equals 1/2 the air density times the swept area times the wind speed cubed. However, longer blades increase weight which decreases a turbine's ability to increasing winds.
Blade materials - The blade material determine the blade weight and stiffness. Lighter blades have less inertia and can accelerate rapidly if the winds pick up, providing more power. Heavier blades have more inertia which buffers changes in wind speed to create a more stable power output over time. The blade material should be stiff enough to ensure minimal blade deflection, which reduces aerodynamic efficiency.
Generator - The type of generator used can affect the power output. Some generators rotate at a constant speed while others turn at whatever speed generates electricity most efficiently.
Gear ratios - Different sized gears connect the generator to the wind turbine. More power is produced the faster the generator spins, so different sized gears are used to maximize the generator rotational speed. To maximize power, a smaller gear should be on the generator and a much larger gear should be on the turbine, driving the smaller gear.
Oil/Lubricant used - Using lubricant at the joints could minimize friction and maximize rotational speeds.
Height of tower - Wind speeds increase with height, so the height of the wind turbine will affect the wind speeds present and therefore the potential power generated by the turbine, however taller towers are expensive, heavier, and harder to install.
Rotational Speed - Some generators rotate a selected constant speed to match the electric grid while others can adjust to the rotational speed of the turbines. A rotational speed of seven to ten times the wind speed is selected for certain turbines.
Physical Tradeoffs
Lift vs. Drag: A larger blade will generate more lift as well as increase the power available at the cost of increasing drag
Rotational Speed Transfer vs. Resistive Torque: A larger gear ratio will increase the rotational speed transfer but also increase the resistive torque, slowing the rotation of the blades
Blade length – Longer blades will increase the swept area and the potential energy harvested from the wind, however will increase the system weight, which will make it more difficult to get the blades to start spinning.
Rotational Inertia – Heavier blades will have greater inertia which will produce a more steady power output but take more time to spin up. Lighter blades have less inertia and are able to start spinning faster yet provide an inconsistent power output when wind speeds vary greatly.

4. Initial Build - Design Blades Gears: largest and smallest3
Balsa wood material
Flat
Roughly 30° tilt
Attached to single wooden dowel with duct tape
Gears: largest and smallest
Base: provided, no support structure
For our initial design, we used balsa wood material for the 3 flat blades. Each blade was tilted roughly 30 degrees and attatched to a dowel rod using duct tape. The blades were at the end of the dowels and far apart from the center of the turbine. We used the provided base without a reinforcing support structure and had a large gear ratio with the small gear attatched to the generator and the large gear driven by the spinning turbine blades.

5. Initial Build - Performance
max Voltage: 3.78V
max Current: 7mA
max Power: .026W
Bulb used: LED (lit)
For our initial build, the peak voltage was 3.78V, the peak current was 7mA, and the max Power output was .026 W. We used an LED bulb. The turbine was able to light the LED.
Unloaded, the max voltage was 3.72V, the max current was 55.6mA, and the power was .2W. With the incandescent bulb, the max voltage was .2V, the max current was 46mA, and the max power was .009W.

6. Overview of Design Rationale
Design Factor
Possible Influences on Performance
Configurations for Experimentation
Real World
Testable in Model
Research
Physical Law
Exp. 1 – Blade shape
Exp. 2 - # of blades
Exp. 3 – Blade Angles
Exp. 4 – Type of Blades
Number of blades
yes
More = greater weight, solidity  less speed, more torque
fewer  more speed, less inertia
Solidity = # of blades * area of blade / total swept area
Baseline (3)
2, 3, 4
Angle of Blades
Affects angle of attack – certain tilt to capture more wind
Lift to Drag Ratio=
(blade area)(net pressure)/(.5xDrag coefficient × mass density×area×velocity2),
Baseline (30)
Baseline level (30)
0, 15, 30, 45
Baseline (15)
Shape of blades
Narrower at ends, airfoil shape to maximize lift and minimize drag
(blade area × net pressure)/(1/2 ×Drag coefficient × mass density×area×velocity^2
Rectangular, air foil
Baseline level (air foil)
Baseline (air foil
Baseline (air foil)
Blade twist
Twisted down length to maintain angle of attack
Baseline level (none)
Blade length
Longer blade increases swept area, but increase weight
(blade area)(net pressure)/(.5xDrag coefficient × mass density×area×velocity2), Power in wind : P=.5ρ(Πr2)v3
Baseline level
Baseline level (some variation)
Blade material
Lighter = accelerate rapidly, heavier = more stable
Rotational Inertia, I=.5mr2 , I = 1/12 ML2 +M(L/2)2
Basswood
Balsa wood
Balsa wood, posterboard, corrugated plastic, basswood
Gear ratio
Larger gear ratio = more speed, less torque, more resistive torque
Ressitive Torque = force × Radius, rotational speed transfer: rlωl=rsωs
Baseline (largest)
Baseline
(largest)
Generator no
Tower Height
From our background research on wind turbine design, we sited the following real-world turbine design factors: number of blades, angle of blades, shape of blades, blade twist, blade length, blade material, gear ratio, generator, and tower height. Out of those design factors, all except for generator and tower height could be tested in our turbine model. For our turbine testing, we focused on the blade shape, the number of blades, the blade angles, and the type of blades.
We decided to test blade shape because we believed the shape of the blade was a key factor in the power output because the blade needs to maximize lift while minimizing drag. We tested a rectangular shaped blade and an airfoil shaped blade to examine this design criteria. Next, we tested the number of blades because our research showed that the number of blades significantly affects the turbine power output. We tested 2, 3, and 4 blades to determine the optimal blade configuration. For our 3rd experiment, we tested the blade angles to maximize the angle of attack, testing 0, 15, 30, and 45 degree configurations. Lastly, we tested different blade materials with varying masses in order to change the inertia of the blades. We tested balsa wood, poster board, corrugated plastic, and basswood to find the optimal blade material.
Based upon our research, we marked blade twist, blade length, and gear ratio as low priority tests due to budget and time constraints. We believed these factors would not have as large of an effect on the power generated as the other factors. From the beginning, we selected a large gear ratio to maximize the speed of the gear connected to the generator. We assumed that the resistive torque from the generator would be negligible compared to the power benefit of having a large gear ratio as opposed to a medium or small gear ratio. Due to limited funds, we decided not to test different blade lengths since testing different lengths would involve the destruction of the blades. The blade twist was our last priority due to the complexity of the endeavor. We believed testing and optimizing the other design factors would be more important than twisting the blades for stage 1 testing.

7. Experiments – Blade Shape
Configurations: Rectangular, Air foil
Experiment 1 - Blade Shape
Configurations: Bulb: LED Motor: B1 Fan Distance: 8ft
3 blades, 30 degrees, balsawood, large gear ratio
Shape
Max Voltage (V)
Max Current (mA)
Power (W)
RPM
Cut-in Time (s)
Rectangular
3.03 17 80
3.5
Airfoil
3.3 20
0.066
100
Rectangular
Airfoil
Constants: LED bulb, B1 motor, largest gear ratio, 30 degrees, 3 blades, balsawood material
We tested two different blade shapes – rectangular and an airfoil shape. For each experiment, the blades were positioned at 30 degrees and were placed the same distance away from the hub. Each blade shape had a similar cut-in time, but the airfoil shape had a higher voltage, current, and power than the rectangular shape. It is seems that the airfoil shape was more aerodynamic than the rectangular shape and was able to better maximize lift while minimizing drag.
According to our research, the tips of the blades should be thinner than the root of the blades to give the blade a constant slowing effect on the wind over the blade. This slowing effect helps to make sure that none of the air leaves the turbine too slowly or quickly, causing turbulence or wasted energy. The airfoil design accounts for this, however rectangular blades have wide tips and could cause turbulence. It is evident that an airfoil design is needed for an optimal turbine.
Conclusions: Airfoil – maximize lift, minimize drag

8. Experiment - Number of blades
Configurations:
Experiment 1 - Number of Blades
Configurations: Bulb: LED Motor: B1 Fan Distance: 8ft
30 degrees, large gear ratio, balsawood blades, rectangular shape
Number of Blades
Max Voltage (V)
Max Current (mA)
Power (W)
RPM
Cut-in Time (s) 2
 3.3
 30.7  
 110 3
 2.8
 25.7  
 102
3.5 4
 2.1
 15.0
 0.0315
 98 A a A a a A
Constants: LED bulb, B1 motor, largest gear ratio, 30 degrees, airfoil shape, balsawood material
We tested different configurations of using 2, 3, and 4 blades on the turbine. For each experiment, blades were places symmetrically around the hub and positioned at 30 degrees. Using 2 blades produced a faster cut-in time than 3 or 4 blades and also produced a higher voltage, current, and power. It seemed that 2 blades worked better than 3 or 4 blades in our initial testing.
We tested our 2 blade configuration on the official testing stand and had a significantly lower power output than we had calculated in our initial experiment. This testing discrepency might be attributed to the fact that our testing occurred in a narrow passageway, creating an artificial windtunnel which channeled the wind to the turbine, providing a more consistent windspeed. The official testing occurred in a more open area, enabling the wind from the fan to dispurse, causing less wind to hit the wind turbine blades. With a weaker, more variable wind, 2 blades simply did not provide the necessary weight or surface area to generate a stable power output, whereas with a stronger, more constant wind, 2 blades performed very well since it was able to spin up quickly.
Our initial experiment showed that 2 blades produced the highest power due to the high speeds of the blades caused by a lower rotor solidity. Rotor solidity is the ratio of the total rotor platform to the total swept area.
Solidity = # of blades * area of blade / total swept area
Using fewer blades results in a lower solidity, which corresponds to high speeds and low torques. Using more blades results in a higher solidity value, corresponding to a lower speed and a higher torque.
Following the results of our official test and research on turbine blade factors, we decided to switch to 3 blades to increase the rotor solidity and increase the surface area to capture more energy from the wind. Although 3 blades did not perform as well as 2 blades in our initial tests, 3 blades would perform better in conditions with more variable wind since 3 blades could better capture the wind and would also provide greater solidity.
2 blades
3 blades
4 blades
Conclusions: 2 blades
3 blades

9. Experiment - Angles of Blades
Configurations: 0 °, 15 °, 30 °, 45 °
Experiment 2 - Blade Angle
Configurations: Bulb: LED Motor: B1 Fan Distance: 8ft
2 blades, balsawood blades, large gear ratio, rectangular shape
Angle (degrees)
Max Voltage (V)
Max Current (mA)
Power (W)
RPM
Cut-in Time (s) 15
3.3
34.1 81 3 30
2.95 19 85 45
2.8
2.3 49
3.5 0°
15°
Top View
Constants: Light balsa wood blades, LED bulb, B1 motor, largest gear ratio, airfoil shape
For this experiment, we changed the angles of the blades to determine the optimal angle of attack. We tested 0, 15, 30, and 45 degrees. We found that 15 degrees consistently produced the highest RPM, voltage, current, and power.
It appears that 15 degrees is the optimal angle of attack and works well to maximize lift while minimizing drag. At 0 degrees, the blades did not spin at all since the angle of attack is too small to provide any lift. At angles greater than 15, the angle of attack was too great; although it provided lift, it created too much drag, causing the blades to spin slower. 15 degrees provided the optimal lift/drag ratio for the maximum power output.
Side View
Conclusion: 15° is optimal

10. Experiments – Blade Material
Configurations:
Experiment 3 - Blade Material
Configurations: Bulb: LED Motor: B1 Fan Distance: 8ft
3 blades, 15 degrees, large gear ratio, airfoil shape
Material
Max Voltage (V)
Max Current (mA)
Power (W)
RPM
Cut-in Time (s)
Balsawood
2.8 18
0.0504 81
3.5
Posterboard 2
2.2
0.0044 92 3
Corrugated Plastic
2.3
1.6 90
Basswood
3.03 17 80
Balsawood
Basswood
Constants: LED bulb, B1 motor, largest gear ratio, 15 degrees
For this experiment, we changed the type of materials used for the blades, maintaining the angle of attack constant between experiments. We tested balsawood, posterboard, corrogated plastic, and basswood. For each experiment, the blades had an airfoil shape. We found that the cut-in time was smaller for posterboard and corrogated plastic since the blades were lighter and could spin up quicker than the slightly heavier balsawood and basswood. Despite this, balsawood and basswood had higher power output than posterboard or corrogated plastic. This might be due to the fact that the heavier balsawood and basswood had greater inertia than the posterboard or corrogated plastic and would continue to spin in the inconsistent wind input from the fan. The posterbaord and corrogated plastic had no problem spinning up, yet their spin speeds fluctuated and were highly inconsistent, resulting in lower power outputs.
Furthermore, the posterboard blades deformed greatly, bending with the wind rather than capturing the wind’s energy. The balsawood, basswood, and corrogated plastic had higher power outputs than posterboard because they were more rigid and did not deform in the wind.
From this experiment, we concluded that basswood performed the best. The basswood blades were slighly more rigid than the balsawood blades, so they did not deform very much in the wind. Also, the basswood blades were heavier than the balsawood, so they had more inertia to keep a constant spin speed in the variable winds.
Posterboard
Corrogated Plastic
Conclusions: basswood – more inertia

11. Final Improved Design Blades Gears: largest and smallest3
Bass wood material
Flat
Roughly 15° tilt
Attached to single wooden dowel with wood glue and duct tape
Gears: largest and smallest
Base: duct tape and poster board support structure
For our final design, we used the basswood material for the 3 flat blades. Each blade was tilted at 15 degrees and attached to a dowel rod using wood glue and duct tape. The wood glue kept the blades from twisting during our test runs and changing the angle of attack, which would negatively affect the power output. The blades were positioned as close to the hub as possible to limit air losses in the center of the turbine. We used the provided base with a reinforcing support structure made of poster board and duct tape to prevent the turbine from falling over or moving backwards when the fan was switched on. Washer weights were placed inside the turbine base to help weigh down the turbine and prevent tipping. The turbine had a large gear ratio with the small gear attached to the generator and the large gear driven by the spinning turbine blades. Duct tape was used on the back of the turbine to keep the large gear from slipping off.

12. Final Improved Design – Rationale and Innovations*
Blades – Basswood *
Heavier
Longer
15° tilt
Base*
stability
Blades: Making the blades longer and using a denser material added weight and increased the inertia of the blades, causing them to continue to turn, thus continue to produce power, for a short time after the wind stopped. The airfoil shape provided a maximized power output compared to a rectangular shape. 3 blades instead of 2 increased the solidity of turbine and power harnessed by the turbine. The optimal angle for these blades was 15 degrees, as seen through the blade angle experiments.
Blade Material- Instead of Balsa wood, we used basswood. The bass wood is much stiffer and heavier and also longer than the balsa wood sheets we were given in the kit. The blades worked well as they didn’t flex and covered a wider surface area than the other blades.
Base – The unbalanced blades were heavy and caused the entire system to wobble back and worth, lowering the overall efficiency and increasing the risk of tipping over. To counter this obstacle we made a base for the turbine and weighed it down with washers. This acted somewhat as an anchor to stop the wobbling effect. Increasing the surface area and weight of the base eliminated the possibility of tipping, allowing the turbine to function properly and safely.

13. Results - Final Testing
2.05Ws
3V, .02A
160rpm
Calculated Values:
Power in wind: 2.7 W
Turbine Efficiency:
Relative to power available in wind : 2.2%
Relative to power available at blades: 3.75%
Rotational Speed of high speed shaft: 1011rpm
Final Testing Conditions: LED light bulb, high fan speed, wind speed: 2m/s, B1 motor
Results: Estimated blade rpm: 200 rotations in 75s, 160rpm, Peak current: .02A, Peak Voltage: 3V, Energy Generated in 75 seconds: 2.05 Ws
Calculations
Power generated (W) = IV
Power generated = .02 A * 3V
Power generated = .06 W
Wind speed: 2m/s , Air density: kg/m3
Radius of Swept Area: .426 m
Power in Wind
Power available in the Wind = ½ρAV3
Where ρ = density of air, A is the area swept by the blades, V is the velocity of the wind
Power available in the Wind = ½ ( kg/m3) ((π)(0.426m)2)(2m/s) 3
Power available in the Wind = 2.7 W
Efficiency of Turbine relative to power available in wind
Efficiency of Turbine = (Power generated / Power in Wind) * 100%
Efficiency of Turbine = (0.06 W / 2.7 W) *100%
Efficiency of Turbine = 2.2%
Efficiency based on power available at blades
Fp = ½ [ 1 – (v2/v1)2](1+ v2/v1)
Let v2/v1 = R
Fp = ½ [ 1 – (R)2](1+ R)
= ½ (1+R –R2 – R3)
Maximize Fp by deriving Fp and setting equal to zero
Fp ‘ = ½(0 + 1 – 2R – 3R2)
0 = 1 – 2R – 3R2
3R2+ 2R – 1 = 0
(3R – 1)(R+1) = 0
R = -1, 1/3
Reject negative number, R=1/3
Plug back into equation:
Fp = ½ [ 1 – (1/3)2](1+ 1/3) = 0.592
Fraction of power available at blades = 0.59 or 59% (Bentz Limit)
Power available at blades = Power available in wind * Fraction of power available at blades
Power available at blades = 2.7 W * .59 = 1.6 W
Efficiency relative to power available at blades = power generated / power available at blades *100
Efficiency relative to power available at blades = .06 W / 1.6 W = 3.75 %
Trace speed transfer - estimated rotational speed for high speed shaft
200 rotations for 75 seconds  220rotations/ 75 sec = x / 60s
x = 160 rpm
r1w1=r2w2 where r1= radius of large gear, r2= radius of small gear, w1= rpm of large gear, w2=rpm of small gear
(0.053m)(160rpm) = (0.007m)(w2)
w2 = 1011 rpm

14. Interpretations of results
Successful:
Very consistent voltage
Fairly consistent current/power
Kept on spinning after 60s
Unsuccessful
Low current and power
High cut-in time
Why?
Blades too long – larger than fan diameter
Blades too heavy
No twist to blades
Unbalanced
Tip-Speed Ratio: 3.5
Successful/Unsuccessful
Our turbine was successful in that we were able to maintain a consistent voltage and a fairly consistent current and power output. Also, the turbine kept on spinning after the 60 second mark when the fan was shut off due to the inertia of the blades. Our turbine was unsuccessful in that it had a relatively low current, which resulted in a low power. In addition, the turbine had a high cut-in time due to the heavy, long blades which require more force to move.
Why?
We attribute our successes to our optimal angle of attack (15 degrees) and our simple, yet effective air foil blade design. Our turbine was able to keep spinning after the fan was turbed off due to it’s long, heavy blades with increased mass and inertia which kept the blades spinning and producing power after the wind had stopped. In addition, we carefully sanded down the surface of our turbine blades to eliminate surface flaws. This smooth surface was aerodynamic and served to facilitate the blades’ movements.
As our testing progressed, we noticed that our turbine was unbalanced in that one of the blades was heavier than the other blades and when the turbine was stationary, the heavier blade tended to migrate towards to bottom. This unbalance was due to inconsistencies in the blade densities and imperfections in cutting out the airfoil shapes of the blades. The unbalanced blades increased the cut-in time since the heavier blade had a greater tendency to remain at the bottom and negatively impacted the performance of the turbine, resulting in a slight rattling during operations.
Our turbine was unable to produce a high power and was fairly inefficient largely due to the length of the blades. The turbine’s long blades were larger than the diameter of the fan, so the wind hitting the tips of the blades was fairly weak and choppy. Although the power available in the wind depends upon the swept area and therefore the length of the blades, longer blades do not always necessarily produce more power. Longer blades increase the weight, which makes it harder for the blades to begin spinning. If we had used shorter blades, our turbine might have been able to spin up faster and spin faster in general, producing a greater power output. In retroscpect, for this scenario, it seems that blades the same length as the fan’s radius would be ideal, since the maximum airflow occurs directly in front of the fan and anything outside the fan’s diameter receives less airflow and therefore has a smaller wind speed. Since power depends on the area and the cube of the velocity, the wind speed is a greater determinant of the power than the radius of the blades, so it would be beneficial to sacrifice blade radius in order to obtain a relatively constantly high wind speed along the entire length of the blades rather than just the middle of the blades.
We did not factor the tip-speed ratio (TSR) into our turbine design. The tip-speed ratio is the ratio of the speed of the rotating blade tip to the speed of the incoming wind. There is an optimal TSR due to the fact that the angle of attack is dependant on the wind speed. The power coefficient of a turbine varies with the TSR with the optimal TPS hovering around 8 (see diagram above). Our calculated TSR is 3.5 (see calculations below), which produces a much lower power coefficient than a TPS of 8. To increase our TSR, we would need to either increase the radius of the turbine’s blades or increase the rotational speed of the turbine. Since our radius is already maximized, we would need to somehow increase the rotational speed of the turbine to increase the TSP. Using lighter or shorter blades could potentially increase the TSR, although shorter blades could also decrease the TSP due to a smaller radius. A balance between blade length and rotational speed must be found to create an optimal TSR.
Calculations for TSR for our turbine design:
TSR = (ΩR/V) where Ω = rotational speed in radians/sec, R=rotor radius, V= wind velocity
TSR = (16.7 radians/s) (.426m) / (2m/s) *note 160rpm = 16.7 radians/sec
TSR = 3.5
Source:

15. Future Research Blade twist from root to tip
Curved Hub to guide wind to blades
Different blade lengths for variable wind speeds
Different blade widths
curved
Given additional funding, we plan to look into different blade lengths and blade twist for future research. According to our experimentation and analysis, using different blade lengths would greatly facilitate the optimization of our turbine. Furthermore, we would incorporate a curved hub into our design. This simple addition could help guide wind from the center of the turbine to the blades, minimizing lost wind energy from the center of the turbine. In addition, we would look into twisting the blades to maintain the optimal angle of attack along the entire length of the blades. Blade twist could greatly improve the efficiency of the turbine. Furthermore, we would experiment with different blade widths to see how changing the solidity of the turbine affects the power. Throughout the experimentation process, we plan to create a model of the turbine in Autodesk Inventor and document design changes electronically as they occur in real life. With this model, we could run computer simulations to aid in the optimization of our wind turbine. Having the turbine modeled in this CAD software, we would be able to use a 3D-printer to fabricate our turbine blades, enabling high precision in the blade twist.
Too long
Optimal
Too short

16. Questions?