Your UAV’s pitch angle was low enough, and airspeed high enough, during the flight. But a sudden updraft has stalled your aircraft’s wings and now it’s nose-diving. With the right autopilot logic and enough altitude, it might recover.
There’s no way to know the exact conditions up at altitude, and you can be surprised by small pockets of turbulence. This example was an extreme case, and there’s only so much you can do to prevent unexpected cases like it.
But for less severe conditions, you can mitigate the risk. Here are three recommendations to consider; they’re not the only options by any means, but they can help kickstart your risk-reduction efforts.
- Encourage awareness of the difference between the reported “pitch angle” and the aerodynamic “angle of attack”, especially with operators.
- It’s unreasonable to expect all UAV operators to have a substantial understanding of aerodynamics. Part 107 certification may be the only aerodynamics education an operator gets: study materials talk about stall, but largely in the context of high-load maneuvers, and there may not be a clear definition provided for angle of attack.
- Pitch angle is a vital indicator, but it can only tell us so much—we need to remember what it’s not showing us, and be mindful of how that could impact the aircraft’s flight.
- Where possible, consider installing a properly-calibrated angle of attack sensor on the UAV. It’s a relatively straightforward risk reducer, especially if you can plan for the hardware in the initial design stages.
- As a bonus, whoever ends up doing your flight data analysis will love having this extra measurement!
- Try to design your aircraft to fly with reasonable stall margin. I would aim to have your angle of attack in cruise be at least three degrees lower than your stall angle of attack.
- Preferably, you’d have cruise AoA three degrees lower than when your lift curve starts to significantly round over. The airflow gets more and more separated and unreliable as you angle up, so you can’t be certain of exactly when you’ll lose your lift.
- You often get a bit more endurance flying at higher angles, making it very tempting to fly as slow as you possibly can. Adding just a few knots to your cruise speed gets you that extra stall margin, and might help prevent the airplane from getting pushed to where you don’t want it.
You can’t remove every possible risk from flight (well, you can…by not flying). But you can at least try to reduce them.
Best case, flying a little faster at a lower angle of attack gives your aircraft more leeway to restabilize after a strong gust. Worst case, at least your angle of attack measurement can help give you a better picture of what went wrong, and the lessons you can learn for next time.