#aero307

Monday was pretty uneventful. After going through our mounting plan and seeing what happened when we actually spun the football, it was decided that we needed something way more solid to mount the football. This was to try to reduce the amount of oscillation created by the rotated football. 

We did this by attaching the motor, esc, battery, and receiver to this large wood block that had mounting holes that matched up with the super solid linear drive. There was a minor-ish detriment with doing this though. This would mean that the block would be a blunt face that would affect the flow visualization after the flow went over the football. Despite this, it was decided that this was the best approach going forward due to time constraints.

Moving on to Wednesday and actual testing day. It was basically the last day we could get some actual data to present on the next Monday. Will and I quickly mounted the block onto the linear drive and tested the spinning football again. The amount of oscillation was very reduced, it was still there but it wasn’t so bad that it was shaking our entire mounting system. Due to the line laser being burnt out earlier in the day, we had to rely on making the test section super bright with 2 floodlights to adequately illuminate the test. 

Will then setup the go pro and we finally got ready to get some actual visualization. I turned on the windtunnel and then communicated with will to adjust the smoke to where it contacts (roughly)  the center of the football. It actually looked good! The flow was attaching to the football and you could clearly see what is going on. We then did the scary thing and began spinning the football. At a quick glance, the boundary layer got smaller and there was definitely some different behaviors going on.

We then moved the gopro to a different view and repeated the test. Overall I would say it went much better! Hopefully he presentation goes well next Monday.

Week 5, the halfway point! Only one wind tunnel test this week, the rest was spent in the conference room starting to put together a presentation, and painstakingly stepping through a video frame-by-frame trying to calculate the frequency of vortex shedding on our wing after the flow detaches (preliminary estimates place this number at 40 Hz).

After the Great Wing Bending of April 2018, we decided to fix the wing flexibility problem by performing the ever-elegant solution of shoving a carbon rod in the wing. We figured that placing the carbon rod at about the quarter chord of the wing would allow it to counteract most of the bending the wing experienced. We drilled a hole out of the wing, then tried to stuff the carbon rod in further. All told, we probably managed to get the carbon rod in to a depth of about 4 inches. That should be good enough, right?

That’s still not ideal, and even though the reinforced wing did actually bend less than the original wing, Dr. Doig still likened it to a banana. Such a strong curve definitely affected the angle that we were able to observe stall (it’s safe to say that most airfoils, let alone the 4412, stall well before 22°).

The reason that the wing, reinforced or not, stalled at such a late angle of attack is due to changes in the local angle of attack of the wing. At first I thought the bending would cause such changes, but thinking it over further, I realized that this wouldn’t change the local angle of attack that much. I realized that if anything, the wing must be twisting in a sort of weathervane effect, causing it to have a much lower angle of attack than what is indicated on the winch. This effect must have been stronger on the unreinforced wing, considering we never actually observed stall on that wing.

This twist comes from two likely places: one being that the wing we made didn’t fit in the turntable perfectly, and two that the wing was made of EPP foam, which is a lot less stiff than typical wing materials.

The final thing that we did was attempt to calculate the Strouhal number for the wing once the flow had separated. This necessitated finding the frequency of vortex shedding off the wings. We didn’t actually know how fast the slow-mo was on a Galaxy S8, so some trickery was required: by filming a timer in slow motion, we were able to find out how many seconds passed with each frame, which allowed an fps calculation. (The Galaxy S8 takes slow mo video at 240 frames per second). Then came the meticulous flipping frame by frame through the video to find the exact moment the vortex gets shed off the wing. We found that a similar image showed up every six or so frames, meaning that the vortices were being shed at a pace of about 40 Hz. What does this mean? A little more interpretation is needed on that question.

Today’s fact of the week: did you know a pangram, or a holoalphabetic sentence, is a sentence that uses every letter in the English alphabet? They’re often used to display typefaces, as you can see we did on our slides below:

My favorite pangram is “sphinx of black quartz, judge my vow”, which I think we can all agree is objectively the far cooler way to display an alphabet. There are far more examples, but I think perhaps the most applicable to aerospace engineers is: “ Pack my box with five dozen liquor jugs.”

Monday began the final quarter of my third year at Cal Poly, and it started with AERO 307. After meeting in the Wind Tunnel Lab and going over some of the specifics of the course, we moved to 322 to prepare for the first lab. After sitting with the group and tossing around ideas, we decided to analyze the difference between the flow over a normal wing and the flow over the wing if the flow was tripped, using some sort of rough surface on the upper edge. We then decided to analyze the differences in stall angle. We would take pressure measurements at many angles of attack and use a CL vs. alpha plot to show the stall angle of both the tripped and non-tripped wing.

 After inspection of the wing, we decided that the ideal place to trip the flow would be between ports 8 and 9 on the wing. Ideally, we wanted the flow to be tripped just prior to the max thickness of the airfoil (upper end of the leading edge). We figured out that it should be between ports 8 and 9, which measured to be the quarter chord of the wing. We also decided to used 200-grit sandpaper to trip the flow, citing that it was rough enough to just trip the flow, but not to completely disrupt it. We agreed to test at every other angle, with each angle closer to the expected stall angles.

 On Wednesday, we went to the lab for the experiment. We didn’t think that 200-grit sandpaper would be rough enough, so we used 100 instead. During the lab, I operated the wind tunnel, LabView, and changed the angle of the wing for different runs. After we took our pressure measurements on the wing without sandpaper, we attached a .5-inch wide strip of sandpaper long enough to cover the locations of the ports. We taped it outside of the port locations. When we turned on the tunnel, we heard a lot of flapping and realized it was the sandpaper. After searching the lab, we found tape used to protect flooring from high traffic areas. It is very similar to sandpaper and had adhesive the whole way down. After attaching that, we had no problems with the rest of the experiment.

 Personally, I helped a lot with the initial ideas of the lab. I helped with discovering our experiment, and finding the necessary data needed to perform it. I also helped a lot with the test plan itself. During the lab, I operated the wind tunnel, LabView, and changed the angle of the wing for different runs. I was used our panel code, creating in an earlier class, to compare our data with what was generated in class (See Below).

 The stall angle for the tripped wing was very odd. Initially, we only expected to go up to 18 degrees for both runs. However, while plotting the data, we noticed that the tripped wing had not yet stalled when we reached 18 degrees. We went up to 20 degrees, and noticed that it stalled around 19, so our extra data was needed. Since this experiment was fairly standard, there were no huge insights.

 This week, we will continue our analysis on part 1 of the lab. We still have to do our CL and CD plots for both runs. We will also begin to write our lab report. Most of it can be done before the next part of the experiment. On Wednesday, we will perform the second half of the experiment—the wake study.

 This week saw the launch of the Lockheed Martin/NASA supersonic aircraft. The main part of this x-plane is to reduce the noise effects of going supersonic. Although this doesn’t apply to 307, it definitely applies to 303. This technology could then be applied to commercial aircraft and would likely change the dynamic of commercial aviation as we know it.

Figure 1. The wing with the new trip-tape on it

Figure 2. Using Panel Code, I created this graph of the Cp vs. Wing Location all all angles of attack without the trip strip.

Figure 3. Cp vs. Wing Location with no Strip from the Data

I was placed in Group 4, which means that I won’t be in the wind tunnel lab until next week, so my work this week was pre-lab analysis.

As I had some familiarity with the XFLR-5 program, I was in charge of that analysis for my group. As a group, we decided that some of the things we would be looking for during the experiment was the zero-lift angle of attack, the stall angle of attack, and the reattachment angle. I figured that using XLFR-5 to generate a Cl/alpha plot for the NACA 4412 would be a good place to start to determine what angles of attack we want to test during the experiment.

Above is what was generated when I ran an analysis on the NACA 4412. I simulated Reynolds numbers 100k to 300k in increments of 25k, through a range of angles of attack from -10 to 25 degrees, in increments of .25 degrees. The specific Reynolds number range was chosen because our group wanted to stay within the laminar flow regime. The angle of attack range was determined using the limits specified in the lab manual. As is strongly evident in the drag polar and the lift curve, XFLR generates some erratic data for the 4412 at low Reynolds number and high angle of attack, so comparing experimental data to the XFLR data in those conditions will probably not generate any useful insight.

However, given the kind of information we are looking for, looking at only the lift curve is necessary. The lift curves for most of the Reynolds numbers simulated reach 0 lift between angles of attack -5 to -3 degrees, so we focused a few of our test matrix points around these angles. The lift curves peak, indicating stall, anywhere from 10 to 15 degrees angle of attack, depending on the Reynolds number, so we focused some more points in the test matrix in that range of angles.

Before we can begin obtaining aerodynamic data in a college laboratory setting, I had to learn what tumblr was. My group consisting of John, Luis, Bennett and myself began this adventure by deciding what aspects of infinite wing aerodynamics we wanted to dive into. After careful deliberation, we decided that we wanted to find the angle of attack (AoA) that would produce the highest coefficient of lift as well as the AoA that would produce the highest Cl/Cd ratio.

Our group then dove into ideas of how we would go about finding these AoA’s. The consensus we reached was starting at 0 degrees and increasing the AoA until the coefficient of lift begins to decrease. At this point we would refine out interval between AoA conditions. An issue we expect to encounter is having to run the data through MATLAB each run of the tunnel. Bennett has volunteered to head the development of the MATLAB code we will be using.

To supplement experimental data and provide a prediction for data, John and I will be running our test conditions through Xfoil to obtain a rough idea of what to expect during testing. By this coming Wednesday, we are planning to have our test plan done which will require Bennett to finish the MATLAB code and for John and I to provide expected results from Xfoil. Luis has already finished everything that can be done so far regarding the test plan document. 

Well, here we are. Last post of the quarter. So the first piece we printed came out ok. The layers split a little, but I think with some putty it’ll be usable. The second piece wasn’t so successful. The printer is having a problem with the heating element, so it stops about 20 minutes in every time. The other printers aren’t working so well either, so we won’t be able to have the fairing manufactured, but we ‘ll make sure to include documentation for doing that later. Here’s some pictures of the prints.

The most memorable moment in 307 for me was making the wing tips for the winglet challenge. I had never used a 3D printer before this class, so it was excited to take a design, prototype it using the printer, then get to test it in the tunnel. I really enjoyed this because it felt like for the first time in a lab, every group wasn’t doing the exact same experiment.

The most challenging aspect of the quarter was correcting for error coming from the equipment, determining what could be causing the error, and then finding a way to still get something meaningful out of that data. It was a pain, but I think it’s not the last time I’ll be doing it.

Our test piece printed pretty well, and it looks like it should fit the traverse just fine. A couple of problems we ran into were that it was a bit tight at the back of the traverse plate, and we also didn’t accommodate a couple of bolts running along the front of the traverse plate. I’m glad we decided to print the test piece instead of wasting time and filament on a full scale one.

Next, we went to work updating the model to deal with the problems. The next thing to consider was mounting. We used the engineering drawings from the traverse documentation to find mounting points, but on further thought decided not to print the fairing with the mounting holes. Since our model was going to be printed in multiple pieces we were concerned that the holes wouldn’t be in the right spots when we were done. Instead, we decided to make some circular cuts where we could seat the bolts and washers without interfering with airflow. Once the pieces were assembled, we could drill the required holes.

After some further discussion, we decided that 3D printing would be the best option for manufacturing our design. This would allow for the design to be easily be changed or remade later and also ensures that the geometry is known and can therefore be modeled in CFD.

With that decision made, we went to work on obtaining detailed dimensions of the traverse and drawing a top view of it.

Here’s what we came up with. Rather than trying to fit the geometry of the traverse exactly, we decided that the best way to design the fairing would be to generate an airfoil geometry that can accommodate the worst case top profile of the traverse, then extrude this profile through the solid fairing.