Despite how often I espouse the benefits, I’ll admit—sometimes your UAV development project really doesn’t need a wind tunnel test.
Note that I didn’t say shouldn’t do. In many cases, you can certainly get value out of a test.
Wind tunnel testing is expensive, though. Tunnel occupancy time alone costs at least $50,000 for a single week at a low-speed university tunnel, and that’s considered cheap.
The return on that investment over the life of the vehicle can be fantastic. But in the moment, it’s funding you can’t spend on additional prototypes, or flight testing, or even just another set of wing molds. That also doesn’t include the necessary cost of a purpose-built model, or sacrificing a production airframe to the irreversible modifications that make it a usable test asset.
So let’s look at a few criteria that can indicate you’ll be fine without a wind tunnel test. I won’t provide any decisive recommendations here. But if your project fits one or more of these criteria, your decision calculus is less cut and dry than you might think.
Cheap, quickly repairable assets
If an airframe is relatively inexpensive, and/or is easy to repair after a hard landing, you’re probably fine skipping a typical wind tunnel test.
I’ve worked with a number of designs like this. Maybe the tails all use the same components and have simple connections to the empennage. Any spare can replace any tail, and on impact they just snap off instead of ripping the fuselage apart. Or the design has intentional “sacrificial” components, like replaceable feet, so in a hard landing that part fails but preserves more delicate components.
And if your aircraft is relatively cheap to manufacture, that means you can build more of them and have more assets available for when one inevitably goes into the ground.
Flight test data is the holy grail of the aircraft development process. It’s the real world function of your vehicle, and a single flight exercises every system on board. If you have flight assets and extra parts to spare it can be far more valuable to go through the cycle of flying, “landing”, fixing, and flying again. Spending that money and effort on a wind tunnel test might only provide insight into the aerodynamics—helpful, but just one facet of the entire design.
KISS configurations
We’ve gotten good at modeling and testing simple lifting surfaces over the past 100 years. So if your UAV design is pretty conventional—think a rectangular or slightly-tapered wing, a benign NACA 4-series airfoil, cruciform horizontal and vertical tails—there are likely better things to spend your budget on than a wind tunnel test.
Lower-order tools, like AVL, work great for predicting the aerodynamics of these conventional configurations. And handbook methods will fill in the gaps, like estimating parasitic drag and fuselage contributions. You can even use these to validate your CFD instead of needing wind tunnel data.
There also may already be wind tunnel data out there for a configuration similar to your aircraft. It probably won’t match the fuselage, but even finding data for just the same wing is a huge benefit. Make sure you pay attention to the scale and speed of your vehicle versus the test data—your aircraft is probably much smaller and slower than whatever design that wind tunnel testing was meant for.
No clear questions to answer
If you have a feeling that you “should” wind tunnel test your aircraft, but you’re not quite certain what data you actually want to collect, then you can probably find a better use for your project’s funds.
This is different from having an idea of what you are investigating, but still needing to lay out the exact runs, or maybe better phrase and refine your test objectives. Instead, you feel the importance of the project calls for a test entry, but you’re still pretty vague on the plan for the data once you get it.
And even if you do have an idea of what you want to get out of the test, you need to make sure it aligns with what a wind tunnel can actually give you. Outside of extremely specialized facilities, a wind tunnel is NOT the place for structures testing, payload troubleshooting, or anything similar.
You should not assume you can use a tunnel to test the strength of some part of your vehicle. In fact most tunnels, especially ones that are larger or operate at high speeds, require very thorough loads calculations to ensure your wind tunnel model will withstand all the forces it will experience. Every individual part is analyzed to be certain the pins and bolts that hold it on will not fail under planned wind speeds.
Any object that goes down the tunnel—even just strips of metal tape—has the potential to be damaged, cause damage, or both. And many wind tunnel facilities are so old and so unique that if they experience substantial damage or mechanical failure, they will be shut down forever. There’s just no one to make the replacement parts anymore. So please, be kind to our precious tunnels.
There are many different types of data you can get from wind tunnel testing; I’ve talked about balance data, pressures, and flow visualization before. But before you go get yourself penciled in on a facility’s schedule, make sure you know what data you actually need, and what you want this investment to do for your project.