What is really convenient is that, quite often, choosing a tunnel facility that provides the flow conditions and instrumentation you need will determine what balance you use, without any extra effort on your part.
Wind tunnels built to operate in high-subsonic to supersonic regimes (i.e. speeds just below, at, and above the speed of sound) will almost exclusively use internal balances.
Models at these tunnels are built small to reduce the physical loads they need to withstand from the air moving so fast:
- A tunnel operating at Mach 0.7 (70% of the speed of sound, about 537 miles per hour) will generate a dynamic pressure of around 5.1 pounds per square inch on everything it touches.
- A tunnel operating at Mach 2 (twice the speed of sound) will create a staggering 41.8 pounds of force per square inch.
- For reference, baseline atmospheric pressure is around 14.7 pounds per square inch. Driving down the road at 60 mph (97 kph) is a measly 0.064 pounds per square inch.
Despite these absurd loads, the actual deltas in the data between things like control deflection angles may be pretty small. Internal balances have the finer measurement resolution needed to help discern between inherent variation and actual data changes.
Using an internal balance plus sting mount also provides some additional flexibility—literally. Many stings are made with one or more joints, or knuckles, that allow the model to be rotated in more than just two axes and achieve more varied test conditions.
Meanwhile, external balances are mostly found in low subsonic-speed tunnels (under 200 mph). These tunnels are often meant to be pretty plug-and-play, with a customer able to come in, install their model within a day, and get up and running.
Installing an internal balance within a model and attaching it to the sting can be a multiple-day affair in some cases. And while you still need to be careful with an external balance, they’re a lot more resilient to random taps and jerks than an average internal balance.
These balances can also take much larger, heftier models. I’ve seen (and helped mount) ones that go up to 700 lbs! This makes it easier to get the flow conditions you want, and adds space for additional instrumentation like pressure ports.
And if we’re being honest, usually tunnels with external balances are much cheaper for testing. Rates at one well-known low-speed tunnel are around $36,000 for 40 hours of occupancy time; meanwhile, that same 40 hours at a transonic tunnel will cost you at least an order of magnitude more money.
The choice really boils down to this:
- If your test program requires testing at higher airspeeds, a larger test section, or any other “unique” needs, you will likely end up using an internal balance just based on the tunnels available to you.
- But if you only need low speeds, don’t need to control ambient air pressure or temperature, and don’t have a tiny model, just go to a low-speed tunnel with an external balance. You’ll still get fantastic data and an excellent experience. And I’m sure you can find other ways to spend that $260k you’ll save!