The Whirligig in Whirligig Sentry stands for Wireless High-Reliability Independent Government-Issue Gunship. It's a fancy name for mini Vertibird sentry.
If you're sick and tired of using the same old defenses to protect your settlements, try calling in this bad boy to do your dirty work.
The Whirligig Sentry come with four fire modes (Minigun, Gatling Laser, Gauss Cannon, Prototype Disruptor), upgraded models that come with improved damage, and amazing mobility. It will fly around and attack anyone that gets close to your settlers.
These sentries aren't invincible though. They can be damaged and destroyed by your enemies. Same goes for their helipad, their base of operations.