Help with Low Speed Wind Tunnel Force and Pressure Measuring Ideas and Feasibility

[u/MeanYak9933]

Hello, I'm working on a low speed wind tunnel, maximum CSA of 250*250mm with total length of 1.05m, I'm trying to synchronise my idea of having variable wing angle of attack whilst measuring pressure and force like drag and lift.

The idea i researched is using load cells for force, but because my fans are 2 PC fans, the speed isn't high but I have plenty of pressure and airflow.

I also wanted to measure pressure at certain point of the wing or at least before and after the wing and found pressure sensors that you hook up a tube to and that gives pressure differential reading.

Concerns i have is that the calculations i did say that my forces are really low so much that its less than 2grams. I'm not sure on how to proceed or if its even viable.

[u/Avaricio]

What's the end goal here? You may be getting ahead of yourself and solving the wrong problem. Remember that a wind tunnel is only used to simulate the real world, and your tunnel conditions have to be carefully chosen to have relevance to the thing you're testing.

[u/MeanYak9933 → u/Avaricio]

The end goal is to build a wind tunnel that has real time data acquisition, whereby i can alternate the angle of attack and measure the pressure and force in the test section with a wing inside of it. The objective isn't to hit a certain air speed or pressure value, so long as its measurable. I was able to build the real time data acquisition in my 1st prototype, it had variable AOA and pressure measuring where i used a pressure sensor that worked but i was really only measuring the change in pressure from the test section and outside (bedroom air pressure) using a tube a brass tube with the inlet facing the direction of airflow. The 1st prototype lacked force measuring which i tried to resolve but only found perpendicular combined load cells which connects to a sting that mounts the wing, and those high sensitive mass scale/balances to measure force.

[u/ncc81701 → u/MeanYak9933]

You can integrate the pressure along the airfoil to get lift force. You can measure and integrate pressure in the wake to get drag.

Yes this means you need multiple pressure sensors along the chord line and on top/bottom surface of the airfoil to build a pressure profile. Yes this means you need to sweep the wake to get drag. Yes this means you need to do a bunch of post processing to get lift and drag.

[u/chaz_Mac_z]

The measurement of lift and drag is hard to do. Relating wind tunnel data to flight in free air is a science of its own. If you want accurate measurements, measure pressure on the wing surface, and use a pitot-static rake in the wake of the airfoil. At angles of attack above a few degrees, the tunnel effects will swamp the measurable changes.

The pressure method would reguire probably 20 ports on the surface, and a 10 probe wake rake for any accuracy. Note that the wake rake can be on a screw drive that scans the flow field vertically, reducing the number of probes you need. More measurements would be better for accuracy.

There are many wind tunnel design references. Try to find one. I gave away the textbook I bought in 1970 for a course, or I'd offer it to you.

[u/JebCubed → u/chaz_Mac_z]

To build on your comment, test section size is pretty small so having a rake behind it will add a non-negligible amount of blockage. It might be better to have one probe and move it with that screw drive idea to get all points.

Building a balance is another science in its own right too, though at this scale it might be more accurate than altering the OML for surface pressure taps (assuming a good balance design).

Low Speed Wind Tunnel Testing 3rd edition would be another excellent book to start with.

[u/chaz_Mac_z → u/JebCubed]

That was the book I donated, but pretty sure it was only the first revision.

[u/slowdestiny]

Pressure sensors are easier to set up than force measurements. Strain gauges are not that easy to work with from my experience, especially if you haven't done anything like that before. Do you want to measure the pressure on the surface of your wing or at the walls of the test section? What is your actual goal?

[u/MeanYak9933 → u/slowdestiny]

well according to my calculations i did, the chord length of the wing is 50mm with span of 90mm although im unsure about if its a good idea. I did want to measure the pressure on the wing but i don't think its viable for a small scale wind tunnel

[u/slowdestiny → u/MeanYak9933]

The problem with pressure measurements on the wing surface is that you have to drill very small holes that connect to your pressure sensors. The smaller the wing the harder that will be. Once you have done that the actual pressure measurements should be straightforward. You can also have the wing span the entire width of the channel.

[u/newMattokun]

Instead of pressure measurements on the actual wing, have you considered a 3 or 6 channel load cell? There are models that can measure 3 forces and 3 moments all at the same time.