If you’re testing a UAV of some sort, dealing with scaling issues isn’t usually a big concern.
A full-sized model, or even production asset, of a small vehicle can easily fit in the test section at a typical low-speed tunnel. Slightly larger aircraft can fit with some simple modifications.
But what if you do need to test a scale model? Doesn’t the scaling mess everything up and make your data useless?
Here’s where our use of coefficients and Reynolds numbers comes to the rescue.
In a wind tunnel, the balance is reading the exact forces and moments that are being experienced by the model. These will be significantly lower than for a full-sized airplane, just because the model is smaller.
But if you take those forces and moments and non-dimensionalize them—that is, remove the units using the dynamic pressure and reference area—then you get coefficients of those forces and moments for that specific aircraft shape, regardless of its size.
You can use these coefficients in your analysis of the full-sized vehicle just as if you had tested a full-sized aircraft. You can even use them to evaluate a larger or smaller version of your configuration, if you wanted.
This does require your model to be a relatively precise scaled-down version of your aircraft geometry. However, there is an entire industry specifically for designing and manufacturing wind tunnel models that have this kind of precision. If you contract with them to make your scale model, you’re guaranteed to be in good hands.
What about Reynolds number though? Remember, this is a way we quantify how much “energy” is in a flow, by comparing its inertial forces to its viscous (friction) forces.
Reynolds number (Re) is proportional to the length of whatever surface a flow is moving across, and the speed of that flow. If your full-sized aircraft has a 3-ft wing chord while your model has a 1-ft chord, and you test both at the same speed, the Reynolds number will be different for each case. And this changes how the airflow behaves.
So what do you do? You speed up.
If you match the Reynolds number by increasing your wind tunnel airspeed, the flow moving around your model will behave exactly the same as it will for the full-sized aircraft. And your resulting data will be even more representative.
This is most important for low-Reynolds number and stall behavior. As Re increases, your stall angle of attack and maximum lift coefficient also increase, and often your drag decreases too. The reverse is true for decreasing Re.
So if you want the most accurate stall and drag data, you’ll want to match Reynolds number as best you can.
For proper flow similarity, you should also match Mach number (your velocity relative to the speed of sound). But this only really matters once you start to reach around Mach 0.3, which is around 230 miles per hour.
If you’re testing at a low-speed tunnel, you’ll probably never come close to reaching that speed. So check your Mach number, but don’t worry about it.