Actuator Types

Actuators are critical components for modern aircraft. An actuator is a device that is used to rotate an element from one discrete position to another. They are used on everything from control surfaces, landing gear, doors, and all the way to thrust vectoring nozzles. Actuators should not be confused with propeller motors - actuators are auxiliary devices and are not used for propulsion purposes. In terms of VTOL aircraft, actuators are critical in helping the flight computer to establish full control authority of the aircraft during hover and transitions (except for quadcopters which don’t need any actuators). Actuators come in various configurations but ultimately depend on the power source. Today there are three power sources available for driving actuators: pneumatic, hydraulic, and electric.

1-?????? Pneumatic: The actuators draw their power from a source of compressed gas. A pneumatic system has several advantages: it is safe because it runs off of air, which is easily refillable on the spot, relatively inexpensive, and has excellent response time (speed). This type of system does require a heavy air-pump to run non-stop in order to maintain a constant pressure in a large buffer tank. Pneumatic systems are used on a whole lot of terrestrial applications (nails guns, jackhammers, etc...) but due to their poor power density are rarely found on aircraft. Interestingly enough, steam locomotives (the first mechanized mode of transportation) were actually pneumatic systems as they used steam to directly drive the wheels.

2-?????? Hydraulic: For the past century, hydraulic actuators were the most popular choice for aircraft. Hydraulic systems rely on a pressurized liquid to drive the actuator as opposed to a pressurized gas. A liquid, unlike gas, is incompressible and as a result, a hydraulic actuator can deliver up to 25 times more force than a pneumatic actuator of similar size. These types of actuators can also handle long pipe routes but are susceptible to leaks and depending on the hydraulic oil employed may even be toxic. These systems require a hydraulic pump often times consisting of interleaving gears to deliver a steady, high-pressure source of hydraulic fluid.

3-?????? Electric: As batteries become smaller and electric motors ever more powerful, electric actuators are replacing hydraulic actuators in several applications. When considering eVTOL vehicles which already have a large battery bank available, it makes sense to use electric actuators throughout the aircraft. Smaller, fueled VTOL vehicles also greatly benefit from electric actuators as they simply do not have the mass or the volume availability to accommodate a heavy hydraulic system. Electric actuators often employ a high-speed brushless motor coupled with a reduction gearbox to generate a lot of torque in a small package.

When it comes to actuators on VTOL aircraft, it is safe to say that electric power is the future. Electric actuators are safer, easier to use, and do not require the complex systems that go along with pneumatic or hydraulic motors. Although VTOL rockets are not necessarily part of modern aviation, the greatest example to summarize this point is the SpaceX Starship Booster. The booster used to rely on a complex hydraulic system to gimbal the individual engines which caused quite a lot of issues during the first test-flight. Once SpaceX jumped to electric actuation, the booster became significantly more reliable, and the starship reached orbit - a clear sign that electrical actuation is the way of the future.