There is one primary piece of equipment that we use to collect all the “physical”—that is, force and moment—data for a given wind tunnel test. It’s called the balance.
This is a delicate, complex apparatus designed to measure the forces and moments exerted upon it and output them as voltages.
Its other main purpose is to isolate the wind tunnel model entirely from the world around it, so that it only captures the physics of the model without any interference from other sources.
There are two types of balances: external and internal. Each type has clear advantages and disadvantages.
Neither of the two is objectively better or worse than the other; like most things in engineering, it’s all about how well it fits your use case. See if a situation comes to mind where an external balance might be better suited than an internal balance, or vice versa.
External balances…
- Are big collections of fixtures, mounted outside of the test section either underneath or next to it. Imagine something the size of a car engine.
- Have a central column, called a strut, that extends into the test section for the model to mount to. This is designed so that the strut never touches the walls of the tunnel to maintain that physical isolation.
- Some balances are even on their own foundation, separate from the rest of the building, to ensure zero interference with the balance readings.
- Have large load limits in the hundreds to thousands of pounds, but variable data resolution. This means they can handle the loads generated by large models very well; meanwhile, if you’re testing something much smaller, it may be difficult to discern any changes in your data from the natural variations.
While internal balances…
- Are much smaller, about the size of your hand and forearm, and are mounted inside a model.
- Are commonly mounted using a long pole called a sting—the model will be bolted to the balance, and the balance then connects to the sting. This is what lets you collect isolated data.
- Have internal strain gauges for force measurement and are therefore far more delicate to handle. They usually have smaller load limits, but make up for it with higher data resolution.
What if the choice between an internal or external balance isn’t very clear-cut? Your model won’t be generating enormous loads, but it’s still of a notable size. How do you choose what to design for and use?