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

This entry was posted in Instrumentation, Wind Tunnel and tagged , , . Bookmark the permalink.