Trouble Keeping it Stiff
On Monday our group practiced installing the wing on the sting in the wind tunnel. We were able to run up the tunnel with smoke and foundt that 300 rpm is fast enough to see what we wanted to see. We blacked out the test section and fiddled with lighting. We were supposed to take pictures with the lab camera, but the battery was dead. We couldn't pitch the wing up because the pitch controller was not hooked up, so for the remainder of the lab period we went up the DBF lab and foam cut a wing and covered it with microlite, a plastic wrap, to create a smoother surface.
On Wednesday our group was able to plan, design, and laser cut the VG's. At first, Reynolds Number and boundary layer calculations said that the VGs should only be on the order of 10^-5 m, but after considering the practicality of attaching them to the wing and accurately embedding them to the correct height, we decided this was too small. After delving further into the matter, we found that real planes have VG's on the order of about 1% of the chord. This gave us a more realistic VG height of about 2 mm. The VGs were designed as small spikes that we will tap into the wing until only the rectangular section of appropriate height and length are exposed. An accompanying equation for spacing recommended placing the VGs about every half an inch across our 20 inch span, but this seemed like overkill. We settled on an inch spacing because our VGs are already bigger than they should be so the effect should still be magnified. Our VGs stick well above the laminar BL at the quarter chord, so they are causing more drag than is optimal, but that is OK because we are just trying to visualize some flow features.
We decided to switch to mounting the wing vertically in the test section on the swivel instead of on the sting because there were some difficulties with the sting that may have taken a while to sort out. In the picture above we were messing with the lighting because we want to get shots from above that have the wing and smoke lit, but not the floor of the test section. In order to acheive this we think it is best to light from the bottom angled up at the wing.
As you can see in the second picture, when we pitched the wing up, the subsequent increase in lift and drag caused the wing to bend quite substantially. In order to mitigate this during the next time we test, we will shove a carbon rod into the exposed wing tip so that it can span the entire test section. This should increase the wing's stiffness because it will introduce another fixed boundary condition.
The final picture shows a picture of the lab camera's view of the test section when wedged in the skylight and the test section is lighted appropriately. I'm very excited to see what type of flow visualization we will get
