The Pennsylvania State University Schreyer Honors College

THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE

DEPARTMENT OF AEROSPACE ENGINEERING

Integrating Wake Rake (Development and Profile Drag Analysis)

ANTHONY GONZALEZ SPRING 2020

A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Aerospace Engineering with honors in Aerospace Engineering

Reviewed and approved* by the following:

Mark D. Maughmer Professor of Aerospace Engineering Thesis Supervisor

Robert Melton Professor of Aerospace Engineering Honors Adviser

* Electronic approvals are on file.

ABSTRACT

Profile drag drastically affects an airplane’s performance. Therefore, determining a reliable way of measuring profile drag over an airfoil is crucial. This research focuses on the design and fabrication of an integrating wake rake to be used for in-flight measurements of airfoil profile drag, specifically looking at how 3D printing can simplify the fabrication process.

It was found that the wake rake provides accurate measurement data similar to that obtained via analytic solutions and from wake surveys performed in wind tunnels and consequently that integrating rakes appear suitable for university flight testing. Furthermore, 3D printing proved to be an effective way of producing the wake rake, albeit future experiments should focus on developing a more streamlined version to reduce drag.

NOMENCLATURE

c = AIRFOIL CHORD LENGTH

푐푑 = SECTIONAL DRAG COEFFICIENT

D = DRAG

퐷푝푟표푓푖푙푒 = PROFILE DRAG

퐹 = INTEGRATING FACTOR

푛 = NUMBER OF PITOT TUBES

NCV = NEW CONTROL VOLUME

푃0 = FREE-STEAM PRESSURE

푃푡 = TOTAL PRESSURE

푃푡푟 = RESERVOIR PRESSURE

푞0 = FREE-STREAM DYNAMIC PRESSURE

r = TUBE RADIUS

S = WING AREA

u = AIR VELOCITY

푢̅ = MEAN AIR VELOCITY

푢′ = HYPOTHETICAL VELOCITY

푈0 = FREE-STREAM AIR VELOCITY

y = CROSS FLOW AXIS iii

휇 = AIR VISCOCITY

휌 = DENSITY

휁푟 = RAKE HEIGHT

휁푤 = WAKE HEIGHT iv

TABLE OF CONTENTS

LIST OF FIGURES ...... iii

LIST OF TABLES ...... iv

ACKNOWLEDGEMENTS ...... v

Chapter 1- Introduction ...... 1

Background on Profile Drag ...... 1 What is Drag ...... 2 Profile and Induced Drag ...... 3 Importance of Profile Drag ...... 5

Chapter 2 - Measurement of Profile Drag...... 6

Inconsistencies with Wind Tunnel Testing and Computational Analysis ...... 6 Objective ...... 6

Chapter 3 – In-Flight Determination of Profile Drag ...... 8

Past ways of Measuring Total and Static Pressure ...... 8 The Work of Froude (1874) ...... 9 The Work of Betz (1925) ...... 10 The Work of Jones (1936) ...... 12 Evaluation of Betz and Jones Method ...... 13 Equipment and Experiment Flaws ...... 14 A “New” Experimental Method...... 14

Chapter 4 - Integrated Wake Rake Design ...... 19

Objectives ...... 19 Necessary Design Parameters ...... 19 Wake Construction...... 20 Method of Construction ...... 23 Alternative Design ...... 29

Chapter 5 Testing and Results ...... 30

Wind Tunnel Testing and Set Up...... 30

Chapter 6 Results and Conclusion ...... 32

The Wake Rake ...... 32 Recommendations for Improvement...... 32 v

Plaisance Data ...... 33

REFERENCES ...... 36

ACADEMIC VITA ...... 38

LIST OF FIGURES

Figure 1 Drag Curve for a Lifting Body in Steady Flight ퟏퟏ ...... 1

Figure 2 Aircraft Force Analysis ...... 2

Figure 3 Skin Friction Drag Over the Upper Surface of an Airfoil ퟖ ...... 3

Figure 4 Airflow from Bottom to Top Surface ퟒ ...... 4

Figure 5 Isometric View of Integrating Total-Pressure Rake ퟏퟓ ...... 8

Figure 6 Betz's New Control Volume ퟏퟑ ...... 10

Figure 7 Jones' New Control Volume ퟏퟑ ...... 12

Figure 8 Laminar Flow Velocity Profile ퟓ ...... 15

Figure 9 Schematic of Pipe Flow ...... 16

Figure 10 Isometric View 1 of Wake Rake ...... 21

Figure 11 Isometric View 2 of Wake Rake Model ...... 22

Figure 12 Isometric View 3 of Wake Rake Model ...... 22

Figure 13 Wake Rake (Frame) Dimensions ...... 23

Figure 14 Wake Rake (Solid Pad Extrusion) ...... 24

Figure 15 Wake Rake (Hole Creation) ...... 25

Figure 16 Wake Rake (Hole Pattern Creation) ...... 26

Figure 17 Wake Rake (Side Wall Pocket) ...... 27

Figure 18 Wake Rake (Pad Definition) ...... 28

Figure 19 Wake Rake (Added Support) ...... 28

Figure 20 EPPLER 521 Wake Rake Design ...... 29

Figure 21 Penn State Wind Tunnel ퟏퟐ ...... 30

Figure 22 Total Pressure Deficit at a Reynolds number of 720,000 and Aplha=-6 deg ퟏퟑ .... 33

Figure 23 Total Pressure Deficit at a Reynolds number of 720,000 and Alpha=0 deg ퟏퟑ ..... 34

Figure 24 Total Pressure Deficit at a Reynolds number of 1,500,000 and Alpha=-6 deg ퟏퟑ . 34 1

Chapter 1- Introduction

Background on Profile Drag

For most aircraft reducing drag is a major concern. The drag acting on an airplanes wing consists of both induced and profile drag. Referencing figure 1 below, as the flight velocity increases so do the effects profile drag have on the aircraft.

Figure 1 Drag Curve for a Lifting Body in Steady Flight ퟏퟏ

Therefore, reducing profile drag is very important for sailplanes and small general aviation aircraft, as lowering profile drag results in an increase in the aircraft’s performance.

What is Drag

In aerodynamics drag refers to the force that retards the motion of the aircraft, which can be seen in figure 2 below.

Figure 2 Aircraft Force Analysis

If someone sticks their hand outside a moving vehicle they feel the force of air pushing back on their hand. The force one’s hand exerts on the air as it pushes the air molecules out of the way is the exact same force the air exerts back. This force exerted by the air on one’s hand can be thought of as the “drag force.” As one can predict, reducing drag is a top priority since drag retards the aircraft’s forward motion. If drag increases, more thrust is needed to overcome this drag, which results in an increase in fuel consumption. 3

Profile and Induced Drag

Profile drag is the combination of skin friction and pressure drag acting over an aircraft.

Referencing figure 3 below, as an airfoil moves through the air it carries some of the air particles near its surface along with it.

Figure 3 Skin Friction Drag Over the Upper Surface of an Airfoil ퟖ

The air particles closest to the surface, once moving with the free-stream velocity (푉∞), now move with the airfoil in the opposite direction. Changing the direction of fluid particles requires a momentum transfer from the aircraft. This momentum transfer is known as skin friction drag, one of the components of profile drag.

Pressure drag is also a component of profile drag and is formed by separated flow, total pressure losses in the viscous boundary later, and unbalanced pressure forces created on the surface. Separated flow, in particular, is caused by an adverse pressure gradient. Separation over an airfoil can be visualized in figure 4 below.

Figure 4 Separated Flow Diagram ퟗ

In potential flow air that flows over a surface causes an equal pressure distribution.

Therefore, uneven airflow results in an imbalance in pressure acting on the object, which leads to an increase in pressure drag. In steady level flight (assuming no gusts, etc.) there is more pressure on the bottom surface than the top surface of the wing. Air flow travels the path of least resistance, and so air on the bottom surface flows around the wingtips to the upper surface. This phenomenon, associated with induced drag, can be seen in figure 5 below.

Figure 4 Airflow from Bottom to Top Surface ퟒ 5

Importance of Profile Drag

As mentioned previously, profile drag becomes a larger contributor toward the total drag as the flight speed increases due to the drag being proportional to the square of the velocity, as seen in the profile drag equation

1 퐷 = 휌푢2푆퐶 (1) 푝푟표푓푖푙푒 2 퐷

Therefore, reducing the profile drag is of major concern for aircraft flying at higher velocities. Reducing profile drag has many benefits, some of which include a reduction in fuel consumption and increase in aircraft performance.

As seen in equation 1, the profile drag is also proportional to the profile drag coefficient. This term is highly dependent on the points of boundary layer transition and separation along the airfoil. Therefore, if an aircraft wishes to reduce its profile drag, “the laminar flow should extend as far as possible on the airfoil surface, and the boundary layer should become fully turbulent before the start of pressure recovery over the aft part of the airfoil”

(Castillo and Zubia). Thus, the ability to accurately estimate and measure where these phenomena occur is vital for reducing drag. Turbulators, such as zig-zag tape or bump tape, can artificially produce transition points to reduce drag caused by separation bubbles. That said, turbulators and other transition devices are only effective if placed in proper locations over the 6 airfoil. These locations can be determined either by experimental method or computational analysis.

Chapter 2 - Measurement of Profile Drag

Inconsistencies with Wind Tunnel Testing and Computational Analysis

Most generally, the determination of airfoil profile drag is obtained from computational analysis or wind-tunnel testing. Computational analysis is valuable, as it can give results that are very difficult to measure in a wind tunnel. The results, however, should be taken with careful consideration since many computational methods are based on a number of simplifying assumptions (Plaisance 3). Similarly, wind-tunnel results can also be imperfect, as certain test environments can lead to imperfect data. For instance, the “level of turbulence in a wind tunnel can affect the location of the transition on the airfoil and thus the values of force and moment coefficients (Schlichting 572 – 574). An alternative method for obtaining airfoil profile drag is via an integrating wake rake. Integrating wake rakes can deliver realistic in-flight data, however, it is worth noting they too have many approximations and assumptions. The benefit with the wake rake is that it can provide useful data and is relatively simple and inexpensive to build.

Objective

The overall objective of this experiment is to design an integrating wake rake capable of making accurate profile drag measurements. This wake rake, ideally, should be inexpensive and 7 easy to construct through the aid of 3D printing technology. Chapter 3 will focus on methods and calculations used in measuring profile drag. Chapter 4 will focus on the wake rake design, specifically focusing on how 3D printing can help expedite the manufacturing process and reduce production costs. Chapter 5 will focus on the implementation and testing of the wake rake and then Chapter 6 will discuss the results and give recommendations for future efforts.

Chapter 3 – In-Flight Determination of Profile Drag

Past ways of Measuring Total and Static Pressure

Prior to wake rakes there were two popular ways of measuring in-flight wake pressures.

In one method, measurements of total and static pressure are made simultaneously by a rake of pressure tubes. The pressure tubes are routed to an averaging manometer, which takes each pressure reading and averages them together (Silverstein and Katzoff 295). This method of measurement is considered valuable because it provides instant readings of all pressures in the wake, however, the design requires a large number of pressure connections and a multiple manometer. The large number of components make the design complex, bulky, susceptible to breaking, and expensive. In the other method a pitot tube sweeps across the wake, taking measurements as it traverses through point by point (Silverstein and Katzoff 295). This method provides valuable results but again is susceptible to inaccurate readings due to changes in flight conditions. This paper will discuss the implementation of an integrating rake, which takes the best features of both designs and combines them into one. The integrating wake rake, as shown in figure 5, will be discussed in more detail in Chapter 4.

Figure 5 Isometric View of Integrating Total-Pressure Rake ퟏퟓ 9

The Work of Froude (1874)

William Froude was the first to develop principles for determining the amount of drag from a pitot tube (Plaisance 10). Through his work, Froude demonstrated that the skin friction loss of a ship can be accounted for by the momentum it delivers to the water around it. Using

Froude’s logic, one can derive an integral expression using the x-momentum integral. In deriving this expression, one must use a control volume with boundaries taken far enough so the static pressure equals that of the undisturbed free steam pressure. Doing so results in the momentum deficit expression

푤푎푘푒 ( ) (2) 퐷 = 휌 ∫0 푢 푈0 − 푢 푑푦

One can then use Bernoulli’s equation to obtain the profile drag co-efficient as a function of pressure measurements

2 푤푎푘푒 푃푡−푃0 푃푡−푃0 푐푑 = ∫ √ (1 − √ )푑푦 (3) 푐 0 푞0 푞0

There are still complications given the fact that one cannot make measurements at an infinite distance away from an airfoil. For practical reasons measurements are more likely to be made closer to the trailing edge of the airfoil, where the static pressure is not fully recovered. 10

The Work of Betz (1925)

In 1925 Betz attempted to correct the complications with Froude’s analysis. Betz started with a similar approach mentioned earlier, however, condensed the control volume so the boundaries would be close to the body of the airfoil, as seen in figure 6 below.

Figure 6 Betz's New Control Volume ퟏퟑ

Using the new, restricted control volume, and the assumption that the total pressure ahead of the airfoil was equal to the total pressure of the undisturbed free-stream, Betz derived the momentum deficit expression

푁퐶푉 푁퐶푉 1 퐷 = ∫ (푃 − 푃 )푑푦 + 휌 ∫ (푢 2 − 푢 2)푑푦 (4) 푡0 푡2 2 1 2 0 0

It is worth noting the second integral needs to be expressed in terms of free-stream pressures. To solve this issue Betz introduced a hypothetical flow and a source within the control volume. In doing so, Betz obtained the drag equation

푁퐶푉 푁퐶푉 1 퐷 = ∫ (푃 − 푃 )푑푦 + 휌 ∫ (푢 ′ − 푢 )(푢 ′ + 푢 − 2푈 )푑푦 (5) 푡0 푡2 2 2 2 2 2 0 0 0

where 푢′ represents the velocity in the introduced, hypothetical section. Betz, using the fact that dynamic and static pressure summed together equal total pressure, then derived the drag coefficient equation

푁퐶푉 푁퐶푉 1 푃푡0 − 푃푡2 1 푃푡0 − 푃2 푃푡2 − 푃2 푃푡0 − 푃2 푃푡2 − 푃2 푐푑 = ∫ ( ) 푑푦 + ∫ (√ − √ )(√ + √ − 2)푑푦 (6) 푐 푞0 푐 푞0 푞0 푞0 푞0 0 0

At last, Betz was able to derive a drag-coefficient equation which took into account measurements made close to the body of the airfoil, making it practical for experimental testing in the wind tunnel.

The Work of Jones (1936)

Melville Jones developed a slightly different yet easier function to use in 1936 (Plaisance

14). In his calculation, Jones altered the control volume to include two sections downstream of the airfoil, which can be seen in figure 4 below.

Figure 7 Jones' New Control Volume ퟏퟑ

The further section is far enough from the airfoil so that its pressure equals that of the free-stream pressure ahead of the airfoil. The second section is close to the trailing edge of the airfoil, where the wake rake makes measurements. Using a similar approach to Betz, Jones determined the drag equation for the section furthest from the trailing edge of the airfoil

푁퐶푉

퐷 = 2 ∫ 푢1(푈0 − 푢1)푑푦 (7) 0

Jones related the velocity values of the first section to those of the second using the continuity equation and Bernoulli’s incompressible equation. This allowed Jones to obtain the expression for the coefficient of profile drag in terms of total and static pressure measurements in the wake

푁퐶푉 2 푃푡2 − 푃2 푃푡2 − 푃0 푐푑 = ∫ √ (1 − √ )푑푦 (8) 푐 푞0 푞0 0

Comparing Jones’ expression to Betz’s, one can see Jones’ is simpler. Therefore, as long as the two compare in accuracy one would prefer to use Jones’ equation.

Evaluation of Betz and Jones Method

The methods of Betz and Jones have been subject to multiple theoretical discussions, wind-tunnel experiments, and free-flight experiments over the years. Though widely used, their methods were not received critique free. Taylor, for instance, determined that the no pressure loss assumption is inaccurate, and was able to provide an example where Jones’ method resulted in a 10 percent error (Taylor). That said, the general consensus is that these two methods work well in conventional circumstances.

Analysis and experiments by Jones, Silverstein, Goet, Shrenk, Serby and Cooper proved that Betz and Jones methods were accurate for profile drag analysis, so long as measurements were performed on two-dimensional wing sections (Plaisance 16). Furthermore, an experiment 14 by Medina in the Penn State wind tunnel proved there is no difference in accuracy between the

Betz and Jones methods (Plaisance 16).

Equipment and Experiment Flaws

In order to utilize the Betz and Jones methods, one must take measurements of static and total pressures across the wake and in the undisturbed free-stream. One way of doing this is by stacking multiple pressure tubes, one on top of another, in the wake to record all the pressures, in all locations, at the same time. Though this theoretically could provide all the proper data, this requires a lot of difficult to acquire equipment like a multiple manometer. As a result, this method can be very expensive, susceptible to failure, (due to the large number of moving components), and heavier. Another approach to measuring pressures in the wake uses one static and one total pressure probe, which traverses through the wake of the airfoil to collect data. This approach was first used by Shrenk in 1930. Although this method is a lot simpler, the procedure again is susceptible to failure. If flight conditions change while the probes are traversing, this could lead to inaccurate data. That said, for testing in a wind tunnel this would not be an issue.

A “New” Experimental Method

In 1940 Silverstein and Katzoff developed a new method for measuring profile drag over an airfoil which incorporates parts of both experimental methods described previously (Castillo and Zubia). Similar to the first experiment, one can stack multiple pressure tubes vertically behind the wake, however, instead of having each pressure tube make a measurement, these 15 tubes are connected to a common chamber where the pressure drag calculation is obtained. Using this method, one no longer needs a multiple manometer and does not have to worry about changes in flight conditions.

For the integrating wake rake to function properly, the fluid flow entering each tube needs to be laminar in nature. One way of ensuring this is by making the hole diameters relatively small (less than 1.5 mm inner diameter). If the flow in each tube is laminar, then the decreases in pressure are proportional to the mean decreases in velocity in each tube, and the flow does not change shape along the axial direction. An example of a laminar velocity profile can be seen in below figure 8, in which the velocity profile is parabolic with zero velocity at the walls of the pipe.

Figure 8 Laminar Flow Velocity Profile ퟓ

In the case of an integrating wake rake where multiple pressure tubes are used, each pressure can be obtained using the same principle. 16

Figure 9 Schematic of Pipe Flow

As stated earlier, the pressure drop is proportional to the mean velocity drops, which can be expressed as

8휇푙푢̅ 푃 − 푃 = 푃 − 푃 = (9) 2 1 푡2 푡1 푟2

Then, applying the previous equation to all n tubes gives

푟2 ∑ 푢̅ = ∑ (푃 − 푃 ) = 0 (10) 푛 8휇푙 푛 푡 푡푟

Then, the total pressure in the reservoir can be calculated using

1 푃 = ∑ 푃 (11) 푡푟 푛 푛 푡

Silverstein and Katzoff then assumed the drag on the airfoil is proportional to the average total head loss across the wake, and used this to determine the drag coefficient equation 17

퐹 푤푎푘푒 푃푡0−푃푡 휁푟 푃푡0−푃푡푎푣 푐푑 = ∫ 푑푦 = 퐹 (12) 푐 0 푃푡0−푃0 푐 푃푡0−푃0

In this equation, 푃푡0 − 푃푡 represents the free stream dynamic pressure, c represents the chord of the airfoil, and 휁푟 represents the height of the rake. Therefore, there is only one unknown variable, F, also known as the correction factor.

Silverstein gathered pressure distribution data for a variety of different airfoil in the late

1930s, and in 1940 Silverstein and Katzoff set out to solve for the correction factor. Using the pressure distribution data, the two developed the equation

2 푦휋 푃푡0 − 푃푡 = (푃푡0 − 푃0)휂푐표푠 ( ) (13) 휁푟

in which 휁푟 represents the height of the rake and 휂 represents the maximum total pressure loss. Combining the two previous equations gives rise to the coefficient of drag equation

퐹휂휁 푐 = 푤 (14) 푑 2푐

Combining equation 13 and equation 8 gives rise to the drag coefficient equation

푤푎푘푒 2 푃 − 푃 푦휋 푦휋 0 2 2 푐푑 = ∫ √1 − − 휂푐표푠 ( ) ∗ (1 − √1 − 휂푐표푠 ( )) 푑푦 푐 푃푡0 − 푃0 휁푤 휁푤 0 (15)

One can then compare these two equations to solve for the correction factor, resulting in

푤푎푘푒 4 푃 − 푃0 푦휋 푦휋 퐹 = ∫ √1 − − 휂푐표푠2 ( ) (1 − √1 − 휂푐표푠2 ( )) 푑푦 (16) 휂휁푤 푃푡0 − 푃0 휁푤 휁푤 0

After finding a solution for the correction factor, F, Silverstein and Katzoff developed three more correction factors to account for different testing environments. At high Mach numbers compressibility effects become more significant and effect pressure results. In addition to this, if the pressure tubes are large in size this can create an entrance loss. Finally, in a study by Young and Mass in 1936 it was determined that a pitot tube measures total pressure not at the geometric center of the tube but more towards the side of higher pressure (Plaisance 23).

Everything taken into consideration, Silverstein and Katzoff determined the errors caused by flow conditions are minimal (less than 1 percent) for testing performed under 300 mph using a properly designed wake rake (Castillo and Zubia). Reducing the speed limits the compressibility effects on the experiment, and reducing the tube diameters limits the entrance pressure loss. 19

Chapter 4 - Integrated Wake Rake Design

Objectives

The primary objective of designing a wake rake is to effectively measure the profile drag behind an airplanes wing. In addition, a design that is easy to reproduce, easy to construct, and cost efficient is highly desired. Though initial design inspiration came from previous work done by Christophe Plaisance and Richard Castillo, modifications were made to better meet the aforementioned requirements.

Necessary Design Parameters

As mentioned earlier the inner diameter of the pressure tubes must be relatively small to prevent the entering flow from transitioning. Additionally, in Silverstein and Katzoff’s design they chose to place 20 tubes evenly spaced in the wake behind the airfoil. To ensure all pressure drops are accounted for the two decided to add 20 more evenly spaced tubes to encompass twice the width of the wake (Silversetein and Katzoff). Therefore, in their design the bottom and top pressure tubes were placed at the ends of twice the wakes width.

Wake Construction

Past models, like the one produced by Richard Castillo, welded metal plating together to form the wakes chamber. While this is durable, recreating this is time consuming and there is no room for error, especially if one does not have familiarity with welding or the thermal properties of metals. Therefore, to make manufacturing easier 3D printing methods were explored.

Both Richard Castillo and Christophe Plaisance, in designing their wake rakes, chose a rake height of approximately 6 inches, which is large enough to measure the wake behind standard sailplane wings. That said, the rake designed in this study was designed to be 8 inches in height, two inches bigger to accommodate the testing of small general aviation aircraft. 3D visuals of the wake rake can be seen in figure 10, 11 and 12 below.

Figure 10 Isometric View 1 of Wake Rake Model 22

Figure 11 Isometric View 2 of Wake Rake Model

Figure 12 Isometric View 3 of Wake Rake Model

Furthermore, it was determined that a total of 30 tubes was sufficient to measure the wake deficit, similar to the number used in Castillo’s experiment. To simplify the manufacturing process brass tubing was chosen since it’s easier to machine. To keep the flow laminar, brass tubing with an OD of 1.5 mm and 0.2 mm wall thickness was chosen for this rake design. The tube spacing was determined so the tubes are equally spaced across the rake. Though our rake was designed to be 8 inches in height, simple adjustments to the extrusion can be made to increase or decrease the rakes size so it is usable with other aircraft.

Method of Construction

To begin designing the rake one first needs to create the frame. The frames dimensions are outlined below in figure 13.

Figure 13 Wake Rake (Frame) Dimensions 24

After the frame is created one then pad extrudes to the appropriate height of the wake.

For this rake it was decided an extrusion of 8 inches was sufficient, however, this dimension can be modified to meet the requirements of a different rake. After extruding the frame, the wake rake should look as follows:

Figure 14 Wake Rake (Solid Pad Extrusion)

Then one must create a shell to hollow out the inside of the wake rake. Hollowing the inside allows space for the pitot tubes to be inserted and connected properly. For this project a 25 shell with default inner thickness of 0.09 inches and outer thickness of 0.0 inches created a wall thick enough for testing.

After developing the shell one must then work on creating slots for the 1.5 mm OD brass tubing. To do this, the first hole was created and placed near the bottom of the leading edge, as exemplified in figure 15 below. It is important to make sure the hole lies at the center of the leading edge.

Figure 15 Wake Rake (Hole Creation)

After the preliminary hole was created, a rectangular pattern was used to create the remaining 29 holes, running them up the leading edge of the rake at even intervals of spacing.

The final product can be seen in figure 16 below. It is worth noting that the pattern runs 7.5 26 inches up the leading edge. Given that the rake is 8 inches in total height, this gives a little room on the top and bottom for sealing purposes.

Figure 16 Wake Rake (Hole Pattern Creation)

After this a side panel was created to give access to the internal tubing. To do this, a pocket was created within one of the side-walls of the rake. Removing a side panels allows for easy access to the rake itself and the instruments within. After everything is in place, one can 3D print a rectangular slab to cover up the void created by the Pocket. A diagram of the pocket with dimensions can be referenced below in figure 17. 27

Figure 17 Wake Rake (Side Wall Pocket)

After creating the pocket and shell there is now a need to cover up the top of the wake rake. To do this, one can copy the sketch used in figure one and place it on the top surface of the rake. Then, this sketch can be extruded 0.09 inches to create a top surface for the wake rake. A top surface of 0.09 inches created a sturdy top for the rake without using too much printing material or taking up too much space. This feature can be seen below in figure 18.

Figure 18 Wake Rake (Pad Definition)

Given the length of the pitot tubes, it became necessary to secure the pitot tubes during flight and keep them normal to the wake at all times. To assure this, an insert was added slightly behind the leading edge, as suggested by Prof. Richard Auhl, and each tube runs through the leading edge and the support. The two, when combined, help prevent the pitot tubes from shifting around.

Figure 19 Wake Rake (Added Support) 29

After 3D printing the body the pitot tubes were inserted into their respective slots and epoxied into place. Then static tubes were placed at the top and bottom of the rake. These tubes were created by epoxying the ends of pitot tubes and then drilling straight through the top of each tube with a small drill bit. Additionally, to make the wake rake more aerodynamic some of the sharp edges were filleted to make the wake rake more streamlined.

Alternative Design

Understandably one might wish to make the wake rake design more aerodynamic in nature. While the rake designed in this project was composed mostly of geometric shapes, one can also import airfoil coordinates and create a wake rake centered around this shape as well.

Referencing the figure below, the coordinates of an EPPLER 521 were imported and used to create the wake rakes body. That said, other airfoils such as the EPPLER 472 or EPPLER 476 can also be used to fit this function.

Figure 20 EPPLER 521 Wake Rake Design 30

Chapter 5 Testing and Results

Wind Tunnel Testing and Set Up

In installing the wake rake it is important the device is placed an ample distance, 0.15c-

0.30c, behind the trailing edge of the airfoil so the entire wake is captured. Adjustments can be made for different airfoils used in testing. The wake rake developed in this project was designed for use in the Pennsylvania State University wind tunnel, a closed-circuit, single-return tunnel. A schematic of the tunnel can be seen below in figure 21.

Figure 21 Penn State Wind Tunnel ퟏퟐ

The test section of the wind tunnel is 6.1 feet long, 5 feet wide, and 3.25 feet tall. The wind tunnel at Penn State is capable of generating velocities up to 220 feet per second. If necessary, slight modifications can be made to allow testing in a different environment. 31

All measurements are made using pitot tubes connected to pressure transducers, which take pressure readings, convert them to voltage measurements, and send these measurements to a recorder (or computer).

Chapter 6 Results and Conclusion

The Wake Rake

Overall, 3D printing made building the wake rake a much easier process. That said, certain parts of the rake lacked in structure. The tube reinforcements, for instance, could have been bulkier. Due to its thin frame, 3D printing this reinforcement was particularly difficult, and ultimately the reinforcement had to be removed because of sloppy printing. Additionally, outside of epoxying the openings with plastic inserts there was no great way of making the chamber of the rake accessible without damaging the integrity of the rake entirely. Finally, since the rake was developed from non-streamlined geometric shapes (triangles and rectangles), the rake produced unnecessary drag.

Recommendations for Improvement

Moving forward, there are ways one can improve the overall quality of the wake rake designed. For one, the wake rake was not very aerodynamic. In this research project the idea of using airfoil coordinates as the frame for the wake was explored, but never fully developed. If one were to re-do the experiment it may be a good idea to recreate the wake rake using airfoil coordinates. 33

Plaisance Data

Given time constraints due to the COVID-19 outbreak, wind tunnel testing was never accomplished with the wake rake developed in this thesis. That said, data from Christophe

Plaisance’s thesis is included to give the reader a better understanding of the rakes data output if one decides to run the experiment in the future. The data was obtained using a S824 airfoil tested at The Pennsylvania State University wind tunnel. All readings were made placing the wake .51 chord length behind the trailing edge of the airfoil, slightly further back to assure the entire wake was accounted for.

Figure 22 Total Pressure Deficit at a Reynolds number of 720,000 and Aplha=-6 deg ퟏퟑ 34

Figure 23 Total Pressure Deficit at a Reynolds number of 720,000 and Alpha=0 deg ퟏퟑ

Figure 24 Total Pressure Deficit at a Reynolds number of 1,500,000 and Alpha=-6 deg ퟏퟑ 35

After running the experiment, Plaisance found the values obtained from his wake rake closely match those obtained via analytic solutions.

REFERENCES

1 Anonymous. “Pressure Drag.” How Things Fly, howthingsfly.si.edu/aerodynamics/pressure- drag.

2 Byadmin, Published. “What Is Drag? All Explained Here!- Part 2.” Knots Aviation, 5 Feb. 2018, knotsaviation.com/what-is-drag-all-explained-here-part-2/.

3 Castillo, Richard, and Adolfo Zubia. “Design and Test of an Integrating Wake Rake for In- Flight Measurements of Profile Drag.” The University of Texas at Austin, ASE/EM Department, 2005.

4 “Drag.” Drag, FAATest.com, www.faatest.com/books/flt/chapter17/drag.htm.

5 “Entrance Length.” Wikipedia, Wikimedia Foundation, 11 Nov. 2019, en.wikipedia.org/wiki/Entrance_length.

6 Flight, Aerodynamics of. “Forces Acting on the Aircraft – Drag.” Flight Literacy, 19 Dec. 2019, www.flightliteracy.com/forces-acting-on-the-aircraft-drag/.

7 Flight, Aerodynamics of. “Forces Acting on the Aircraft – Drag.” Flight Literacy, 19 Dec. 2019, www.flightliteracy.com/forces-acting-on-the-aircraft-drag/.

8 “Flight Training Fixed Wing Aircraft.” Aerodynamics of Flight Drag, www.pilotfriend.com/training/flight_training/aero/drag.htm.

9 Gendrich's, Chuck. “Dynamic Stall: A Catastrophic Loss of Lift.” Dynamic Stall Research, www.egr.msu.edu/tmual/CHuck/dyn_stall.html.

10 “How Fast Do Commercial Airliners Fly?” FlightDeckFriend.com, www.flightdeckfriend.com/how-fast-do-commercial-aeroplanes-fly.

11 “Parasitic Drag.” Wikipedia, Wikimedia Foundation, 13 Mar. 2020, en.wikipedia.org/wiki/Parasitic_drag.

12 Premi, Amandeep. “QUALIFICATION OF AIRFOIL TRANSITION MEASUREMENTS TAKEN IN THE PENN STATE LOW-SPEED, LOW-TURBULENCE WIND TUNNEL.” Semantic Scholar, 1 Jan. 1970, www.semanticscholar.org/paper/QUALIFICATION-OF-AIRFOIL-TRANSITION- MEASUREMENTS-IN-Premi/48984f3d36df4f065da255cb8f47bbb6e7a99053/figure/0. 37

13 Plaisance, Christophe. “The Development of an Integrating Wake Rake for in-Flight Measurements of Profile Drag.” The Pennsylvania State University, Department of Aerospace Engineering, 1997, pp. i-105.

14 Schlichting, Hermann. “Boundary Layer Theory.” McGraw-Hill, New York, 1979

15 Silverstein, A., and S. Katzoff. “A Simplified Method for Determining Wing Profile Drag in Flight.” Journal of the Aeronautical Sciences, vol. 7, no. 7, 1940, pp. 295–301., doi:10.2514/8.1127.

16 Taylor, G. I. “The Determination of Drag by the Pitot Traverse Method.” British Aeronautical Research Council (A.R.C.), R & M no. 1808, 1937

ACADEMIC VITA

Anthony González

Education

The Pennsylvania State University University Park, PA Schreyer Honors College Graduation: May 2020 Bachelor of Science in Aerospace Engineering Spanish Minor

Summer Study Abroad Program (Ronda, Spain) 5/17 - 6/17 • Developed Spanish communication and leadership skills through a 6-week accelerated Spanish immersion program Student Pilot • Accumulated hours towards private pilot’s certificate in accordance with FAA and ICAO requirements

Work Experience

Pratt & Whitney Manufacturing Engineer Intern (East Hartford, CT) 5/19 - 8/19 • Increased efficiency by developing automated thermal detection for F100 first stage and F135 third stage cooling holes • Optimized part flow for military cell by using 3P (Production, Preparation, Process) to develop new floor plan layout • Led cross functional team of 5 interns in developing social networking app for Pratt & Whitney employees Triumph Group Design Engineer Intern (Arlington, TX) 5/18 - 8/18 • Reduced build time by adding nutplate retainers in lieu of individual nutplates • Reduced aircraft weight by removing fillet seals in areas not exposed to the environment • Reduced manufacturing cost on the inboard flap by using Bombardier spray templates • Modeled aircraft parts and assemblies using CAD software such as CATIA V5 and Enovia Penn State ITS Lab Consultant 8/18 - Present • Assisted students, faculty, and staff with technology concerns and hardware operations in Penn State computer labs Penn State Learning Spanish Tutor 8/17 - Present • Taught students Spanish concepts to improve their writing and speaking skills • Organized review sessions to cover and reinforce key exam topics

Research

Penn State Sailplane Lab Keel and Drivetrain Lead 8/17 - Present • Fabricated the keel of a human-powered airplane using pre-impregnated carbon tubes and wet-layup composite joints • Developed build-schedule and tracked team performance • Analyzed stresses in keel joints to guarantee product reliability under different loads Profile Drag Analysis Undergraduate Researcher 7/19 - Present • Developed a wake rake to analyze profile drag over an airfoil • Explored the effects turbulators (Zig-Zag Tape, Bump Tape, Pneumatic Turbulators) have on profile drag

Activities

Penn State Spanish Club President 8/17 - Present • Organized club activities (volunteer, study abroad info sessions) to foster group camaraderie and cultural awareness • Coordinated Spanish conversation hours to promote language skills Penn State Alternative Breaks Site Leader (Columbia, SC) 8/19 - Present • Facilitated volunteer trip to educate peers about the juvenile justice system and risk factors that fuel the prison pipeline • Mentored incarcerated youth working towards life outside correctional facilities Schreyer Orientation Leader 5/18 - 8/18 • Led 22 orientation mentors in acclimating first-year Schreyer scholars to Penn State • Organized 8 team building activities to create an inclusive environment for mentors and incoming students Penn State Eye to Eye Chapter Leader 3/19 - Present • Mentored teenagers with ADHD to develop metacognition skills, build self-esteem, and identify strengths/challenges Penn State Lion Ambassadors 1/19 - Present • Conducted tours and student events to preserve Penn State traditions and showcase campus history • Led campus tours for prospective and accepted students

Software and Language Skills

Software - Proficiency in C++, MATLAB, CATIA V5, CAPP 2, Solumina, Nx, Enovia, Autodesk Fusion 360, and SolidWorks Language Skills - Proficiency in Spanish