Lift and Drag Measurement: Post 2

Fellow Basement Scientists,

The last installment brought about the development of the lift and draft measurement mechanism. However, we still needed to build the device hold it.

Before designing our anchor contraption, we must understand all of what it is meant to do. It must let the anchor screw rotate but not move, and anchor the springs while allowing their length to be adjusted. The first is easy, just drill a hole and stick the screw through it. The second is more difficult. My idea was to have instead of a hold, a slot. The spring would then be attached to a long screw which could more back and forth through the slot and tightened in place. Ok, lets build it.

As usual, I have available wood and decided to use it since it is easily shaped. I also decided it would be best to make two identical sides with the levers in the middle, for stability's sake. Again, it is important to know for which dimensions accuracy is important and which are not. Here is what I determined.

-Length between two sides: Doesn't matter, just smaller than the length of my screws
-Position of hole for anchor screw: Not important, just so that there is enough room on the board for the spring screw slots.
-Position and length of spring screw slots: I positioned these off of the hole for the anchor screw. They must be as perpendicular to the corresponding lever as possible and at the position of the spring. Length is not too critical, just long enough for the extent of spring stretch.
-Quality of box construction: This box should be strong and stable for consistent data.

A little preview so you can understand what I am talking about.

Alright, I took a 1x5 and cut two pieces about about 8 inches long and stacked them on top each other to make sure they had equal dimensions. Given the size of the lever mechanism, this one board would not work, but needed another placed lengthwise perpendicular to make room for the other slot for the spring screw. I cut two more similar pieces and placed them in what would be their final position. Holding the lever mechanism up to these pieces, I estimated the locations of the hole for the anchor screw and spring screw slots. Clamping the two sides together, I drilled a hole for the anchor screw. Making the slots now requires a little more precision.

I put the anchor screw through the lever and into the board to simulate its final position. With it in place, I then marked on the board where the spring screw slots must be. This may be breaking some rules, but to make the slots I used the drill press and bit as a router. As basement scientists, you can only use what you have, and for me it is the only thing that would do the trick. To make the slot straight, I clamped a board to the drill press table to act as a guide rail.

Right now, we got all our slots and holes and the four pieces making up the two sides. We need to connect the two sides. I took my 1x5 and ripped a 12in section in two using a table saw. These two pieces will serve to connect the halves, one across the very bottom and another shorter section a little above it. Drill your pilot holes, get your 2" wood screws and we have our measurement contraption!

Congratulations, We have our measurement mechanism. Hooray for basement science! We wont know if it really works until we start using it. I imagine there will be some debugging at the end of all the construction.

Next, we must determine the spring coefficient of our springs.

Best Regards,
Ben Washington

Lift and Drag Measurment Gauge

On to the Next!

In order to conduct science you need measurements. For our Wind Tunnel tests investigating fluid mechanics and aeronautics we need to measure lift and drag. There are a lot of ways to measure lift and drag, but with all basement science we need to ask, “What’s available?” As usual, my what’s available is wood. This time I have a box of old balsa hobby wood. This wood was my grandfathers and was used to make little glider airplanes.

In the box are long thin sticks I thought would be useful. My idea was to hold the test specimen in the wind tunnel with these balsa sticks, then, the force on the stick could be measured. I knew I could not buy digital force meters for cost reasons, so I needed something else to respond to the lift and drag…Springs! Cheap, available, and easily characterized, springs would do the trick.With few springs in the house, I bought a bag of assorted small springs from Home Depot.

The next challenge was to design a mechanism which turns the forces on the stick to an extension of the springs, and the length of extension could then be measured. This goes back to High School physics class where we all learned the equation for the force of a linear spring, F=-k*x : Force equals the negative of the spring coefficient times the length extended. (Later we will determine the spring coefficient of our springs.) We have two forces here, lift and drag, which happen to be perpendicular. Therefore, if we keep the springs perpendicular and in the direction of lift and drag, we will then be measuring these forces. If the springs are not perpendicular, things will start to get convoluted.

If we just attached the springs to the stick there would be no stability, so I chose to go with levers. Usually, the best mechanism is the one that performs the task in the simplest way and it’s hard to get more simple than the lever. With two connected levers one could measure lift and the other drag. Using wood glue I stacked a few sticks to make two thicker ones, drilled some holes, and here is what I got:

At the yellow arrow, the test specimen would be attached and held in the wind tunnel (haven't yet figured out the attachment yet). The levers would be anchored at the blue arrow, attached and free to rotate at the green arrow, and springs attached at the red arrows. The idea is that the length of the springs would be adjusted to bring the levers back the this square position once the wind tunnel is turned on. The length of the springs would then relate to the force of the lift and drag. To actually get the lift and drag we must use the equation of torque, T=L*f : torque equals length of arm times force. The drag force and spring force will create equal torques. With known arm lengths drag and lift forces may be calculated with this relation! Simple mechanics is very handy.

I have talked some about knowing which parts of construction need to be accurate. This lever needs to be very accurate. You need to know as accurately as possible the distances between the rotation points and specimen and spring attachments. This is a great reason to get a digital micrometer, and they are fairly cheap. As for bolts, the 3 inch long screw anchor is from an old Xbox and the rest are the smallest bolts I could scrounge up.

Looking back, it always seems like an easier task than it actually was. That's probably because you have to think of all the ideas for the first time. In manufacturing you can be making the same part a million times, so you get pretty fast at it. In our basement science, we are designing and making individual parts, which can be difficult if you try and do it right.

Our next task will be to make a box of some sort to provide an anchor for the anchor screw and springs.

Best Regards,
Ben Washington