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Week 10 Testing

After having completed the first test week 9, troubleshooting the model, and figuring out how to use the water channel, the team is prepared for testing during week 10.  During week 9, the team was not able to run dye through both the wing and fuselage ports simultaneously due to time constraints.  Next week, we will run both channels at the same time, and place a water proof GoPro in the water channel.  We hope to have an even better flow visualization session next week.

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First test of the wing port!  Unlike the fuselage ports, it actually worked the first time.  Again, the canopy is in the non-optimal position for inverse pressure gradient matching.  The streamline from the dye port appears to be forced outward from the fuselage by the high pressure zone from the canopy, and then forced inward from the low pressure zone behind the wing.  There is also a region of separation in the same place behind the wing.  

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Success!! After some initial difficulties with getting the dye to actually exit the model, we finally got it to work.  The team found that air was getting trapped in the dye channels, and that the pressure required to get past this air bubble forced the dye out of the interface instead of the ports (notice the giant glob of clay at the rear of the aircraft; this was an attempt to fix this issue).  The best solution was to “prime” the model with dye while it was out of the water.  Once the channels were full of dye, the model was placed into the water and the dye flowed seamless out of the ports.  In this photo, the canopy is in the NON-optimal position for inverse pressure gradient matching.  It appears that there is a large zone of separation behind the trailing edge at the root of the wing.  

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The final model with functional dye ports!  Credit goes to Paul Gilles for getting an amazing finish on the ABS, especially at the wing root.  In my previous post, I mentioned how the wing ports were working flawlessly, but the fuselage ports were clogged.  To resolve this issue, the team drilled into the fuselage, removed the blockage (which turned out to be some ABS scraps), and covered over the drill hole with Depron.   After some preliminary tests of running water through the channels, this solution appears to have been successful. 

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Here is an example of the drilled dye exit ports in the wing.  We chose to drill them instead of have them be 3D printed because the printer would not be able to achieve the desire resolution in this orientation.  Similar holes will be drilled in the fuselage.

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The model is nearing competition!  It has been sanded and covered with primer.  It appears that one of the die channels is functioning, however the other is blocked.  We are currently discussing the optimal solution to this problem.  One current option is to place the die wand in the front of the fuselage, near the ports.  This isn’t entirely optimal because it might disturber the flow upstream of the model. Another option is to use pressurized air or water to force the blockage out.  The last option is to cut into the model and attempt to remove the blockage ourselves.  This option risks damage to the model as well as to the other functioning channel.  Our test is Friday, so we will have to figure out a solution soon. 

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Seemingly successful prints!  Next steps are to sand a smooth finish, drill exit ports for the dye, and paint everything white so that the dye is more visible. Although it is note visible in this photograph,the dye will enter through the fuselage, just in front of the vertical stabilizer.  A series of channels will then route it through the wing, and to the front of the fuselage.  The main concern while printing was that these channels would be blocked (which would be really bad because they are completely internal, and therefore difficult to clear or repair).  However, upon initial inspection, the channels appear to have printed well.  Perhaps the team will devise a  method to test them before testing in the water channel.

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Our current Solid Model.  It will be printed in 3 separate pieces, so that each piece can be printed in the optimal orientation.  Although they are not visible in this screenshot, there are two channels for dye running through the fuselage.  Dye will enter at the rear of the aircraft (so as not to disturb the flow over the fuselage), and some will exit through ports around the fuselage in front of the wing.  The rest will be routed through a series of channels and exit ports on the leading edge of the wing.  There will be pictures of a (hopefully successfully) 3D printed model soon! 

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Flow Viz

A feature from other flow viz projects that I found interesting was the use of thermal imagining.  It seems to be a more precise technique for observing laminar and turbulent boundary layer than using smoke alone.  I would be interested in using a combination of both techniques to obtain a more accurate picture of the flow field.  

I am extremely excited for our mini-project.  My team will be assisting in the development of a new racer aircraft called the SR-1.  The SR-1 is utilizing the concept of “inverse pressure gradient matching” to reduce drag.  Our team will 3D print a model with a movable canopy in order to find the optimal placement in relation to the wing.  This model will be placed in the water channel and, with the help of dye, the flow around the canopy will be visualized.  Perhaps the most challenging feature of the model is a series of ports on the surface of the model that will dispense dye.  I am concerned about the ability of the 3D printer to cleanly print these channels, seeing as we will have no way to remove blockage since these channels are buried deep within the fuselage.  The solid model is almost complete (pictures to follow), and we hope to 3D print it this weekend.

This course has reinforced that, above all, the practicality of a test must be of the utmost importance.  I have noticed that often it is not the complexity of a concept being tested that causes bad results, but rather the execution of the test itself.  It is with this in mind that we go into the last project, trying to maintain the practicality of our experiment. 

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Flow-Viz

We have successfully concluded our flow viz and have started working on the presentation.  For our flow-viz project, the team designed vortex generators, attached them to the red NACA 4412 wing.  Vortex generators are essentially small fins placed on top of a wing, offset around 15 degrees about the vertical axis.  This angular offset creates a vortex that energizes the boundary layer.  This energized boundary layer then stays attached to the wing at higher angles of attack.  Effectively, vortex generators delay the stall.  

For this project, them team designed vortex generators based off experiments done by RC flight test group, Flitetest (https://www.flitetest.com/articles/vortex-generator-design-tips-and-experimentation).  These vortex generators were then 3D printed and mounted to the red NACA 4412 wing.  A fog machine was used to show the vortex created by the generator.

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4412 Conclusion

, As we near the end of the 4412 extravaganza, several thoughts come to mind:

It has been difficult keeping track and being organized with our data.  This is entirely that fault of the group, and I’m sure wouldn’t be an issue if we were to repeat the class, but data organization has been poor, as well as group preparedness for testing.  We did some (very light) write-ups of our first experiments, but nonetheless it will be difficult to recall the exact details during our report.

I have found flow visualization most interesting. Although it hasn’t gone as smoothly as I’d like, we have seen some interesting results (such as vortices off our winglets).  I am excited for the flow-viz challenge although, like most things, I wish we had more time (or used the time we had better!) to prepare for it.

Our team is very good at handling unexpected errors.  Fortunately, this hasn’t happened too often.  However when it has, (mostly MATLAB giving odd results), we have worked quickly and efficiently to discover the source of the error.  More often than not, it was a bug in the code, and we were quickly able to fix it.

This class has been a good continuation of 304.  The experiments were more challenging and frankly, more interesting.  It seemed a little closer to how I imagine actual wind tunnel testing in industry is.  Again, there were several things I wish we had done better as a team in order to get more out of the class, but it has been a valuable learning experience nonetheless.  

The 3 biggest skills I will take into the open-ended part of the course are: preparedness, organization, and more in-person collaboration.

Being unprepared has plagued our group from the beginning.  Whether it was things like not having code up and running before lab (not always entirely our fault), or assigning individual tasks at the wind tunnel right before testing, or not having briefed everyone on exactly what we were doing, there was room for improvement.  Similarly, we can definitely be more organized.  Files were uploaded to Google drive, and put into semi-relevant folders.  Meeting times were haphazardly planned, not everyone could make them.  One solution to this disorganization is the 3rd skill to improve upon: in person collaboration.  I have learned time and time again that work is best done when everyone is present.  No one will ever “go home and work on it.”  It just doesn’t happen.  My goal for the remainder of the quarter is to set strict meeting times and hold all group members, including myself, more accountable for getting work done during meetings.    

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In addition to nice winglets, we also got some nice flow-vis.  It is clear that there is a vortex at the winglet tip.  However whether or not it is smaller than it would be if there were no winglet is the real question.  According to the preliminary results ( which still ahve some error that needs to be reduced), our winglet did in fact increase the L/D ratio.  How much it did so will be determined shortly.

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Winglets turned out nicely.  We attempted to make the tip chord as short as possible to mitigate the vortex it would produce.  Our largest fear while printing was that the trailing edge would not turn out nicely.  However thanks to the Ultimaker 2+ and some good old high quality ABS, our TE turned out almost perfectly.  The dowels also fit really nicely inside so mounting went smoothly. 

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This is all of the data from the rake overlaid onto one graph.  The x-axis is the dynamic pressure, in Pascals.  The y-axis is the probe number on the rake.  For our procedure, we started the rake outside of the region of influence of the wing  (about 33 cm from the tunnel wall), and moved it across the wake of the wing in increments of 10 cm using the traverse.  The trends we observed are quite interesting.  It appears that there is a large amount of error from probe to probe, but that error seems to be very consistent.  This means that we can filter it out relatively easily.  The few curves that deviate exceptionally far from the others are due to the wing lowering the flow velocity (and therefore dynamic pressure) behind it.  Once the error is filtered out, we will be able to calculate the lift and drag produce by the wing from this drop in velocity.

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This is the Cp at an AoA of 20 degrees, after the airfoil has stalled and flow has separated.   The max value of the Cp is less than half of the max value before the stall.  This indicates a large reduction in lift, and explains why stalls are so dangerous to aircraft.  Although error bars have yet to be added, the team expects them to be larger in general after the airfoil has stalled. This is because the wing is now shedding vortices and the conditions at the pressure ports are much more unsteady than before the stall.  Therefore, there will be larger amounts of variability in the data, and larger error bars. 

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This is the Coefficient of Pressure distribution of the black wing at an AoA of 16 degress.  This is the AoA just before flow separation (stall) occurs.  It is also the AoA where the theoretical Cl_max will be reached.  It is clear that the largest value of the Cp (~6.1) occurs very near the leading edge of the wing section.  This is what causes a large pitching moment on the airfoil.  This pitching moment is proportional to the Cp (and therefroe to Cl), and explains why pilot’s must add large amounts of up elevator right before the stall.

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Here is the Coefficient of Pressure distribution over the black wing.  Error bars are coming soon.  The AoA is -4 degrees, very close to the 4412′s 0-lift AoA.  This can be seen graphically because the magnitude of the Coefficient of Pressure on the upper surface is close to the magnitude of the Coefficient of Pressure on the lower surface.  That data point for port 8 has been removed, since the port was clogged.  It appears that an additional pressure port is giving incorrect reading on the lower surface, although further analysis will have to be conducted to determine if this is actually the case.