#stifferwing

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.”