At first glance it seems like picking out your guiding performance requirements would be pretty easy. Counterintuitively, despite having fewer options here, it can be a bit harder to choose what should be a “guiding” requirement.
Performance requirements are anything around the UAV’s capabilities:
- Maximum endurance or range to hit—that is, how long you can fly and the furthest you can go
- Design cruise altitude, or even maximum achievable altitude, the “service ceiling”
- Loiter or cruise airspeed, often fast enough to counter higher winds at cruise altitude
There can be some pretty unique requirements that cross over categories, like an aircraft needing to carry a pod to a location, drop it, and then return to base. It’s a performance requirement (your airplane needs to have the altitude/airspeed range to do a thing) but also an intrinsic requirement since you need to handle the change in weight and its distribution after dropping the pod.
You might also have requirements for certain roll rates or g-loadings if you’re designing a more agile, faster aircraft, much like Kratos’ XQ-58 Valkyrie.
No matter how spicy they get, one of these requirements will be the characteristic of most concern for the customer or end user.
This is something you’ll need to drill down into though, because much of the time if you ask “what really matters here?” you’ll get the reply that all of it matters, of course!
Your performance requirement is the most critical guiding requirement to pin down because it heavily influences your design. To fulfill your guiding intrinsic requirements and not have something absurdly expensive/long lead/tough to fabricate, you often cannot easily meet all of the given performance requirements. You need to make tradeoffs in wing planform, airfoil, and more. Physics is not generous that way.
So as an engineer, it’s your job to basically interrogate whoever it is that’s setting the requirements. What do they really, genuinely want? At the end of the day, do they need the endurance number they specify? Are they flying in areas with high winds aloft so they have to hit a certain cruise airspeed?
Getting them to name the one metric they desperately care about hitting will make your job not easier, but at least clearer. You know what you can sacrifice to achieve what actually matters.
As a reminder, none of this discussion about guiding requirements means the other requirements don’t matter. Of course they do!
Picking out the one or two intrinsic and performance requirements that you care the absolute most about, however, helps you shrink your aero-mechanical design space to a manageable size. It helps you make the hard choices in your trade studies, prioritize what you truly need, and defend those decisions if they’re challenged.
It gives your team the true reason this aircraft needs to exist at all. And honestly, that can be pretty darn motivating.