# How is the bandwidth of an actuator selected for a primary flight control surface?

How the bandwidth of an actuator is selected for a primary flight control surface? Is there any standard? Based on what factors this decision is made?

• – ymb1
Dec 14 '19 at 13:24
• This question is about actuator bandwidth. They are dissimilar.
– JZYL
Dec 14 '19 at 15:33

## 2 Answers

Actuator bandwidth requirements typically flow down from guidance documents like MIL-STD-1797 (Flying Qualities of Piloted Aircraft). Essentially, actuator bandwidth is selected to ensure the flight control system has a sufficient stability margin. For example, see p. 235 of the above referenced document where the bandwidth of the open-loop response from pilot command to control effectors has to fulfill a minimum gain and phase margin.

In particular, this paragraph is relevant:

A measure of the handling qualities of an aircraft is its stability margin when operated in a closed-loop compensatory tracking task. We refer to the maximum frequency at which such closed-loop tracking can take place without threatening stability as bandwidth ($$\omega_{\mathrm{BW}}$$). It follows that aircraft capable of operating at a large enough value of bandwidth will have superior performance when regulating against disturbances. A bandwidth criterion is especially useful for highly augmented aircraft in which the response characteristics are non-classical in form (i.e., have large mismatch in equivalent system fits).

In any case, there are other reasons for actuator selection of which bandwidth is a major one. For example, here's the rationale for actuator selection for the A380 flight control system. The selected electrohydrostatic actuators have a higher actuator bandwidth versus the conventional hydraulic actuators fulfilling the stability margin requirements with the potential for improved disturbance rejection.

Higher actuator bandwidth enables the potential for even better handling quality, e.g., gust load alleviation, however that would need to be assessed in relation to actuator lifetime reliability and overall power requirements.

I understand the "bandwidth" of an actuator to be a measure of how quickly it can respond to a change in input. It is not a measure of the time it takes the actuator to go from one extreme to the other, but rather a measure of how quickly it can change from extending to contracting and back again. If that is what you are asking about then no, there is no standard as such that I am aware of for the actuator but standards do exist in some fields (such as the military) for the complete pilot to control surface system of which the actuator is a component. They design constraints involved are quite easily understood if you replace the actuator for cables and pulleys. The control surface must be able to change direction quickly enough to be able to give the pilot the feedback they require to control the aircraft in difficult conditions, but how much that is will vary based the control surface and the circumstances involved. A C172 in a cross-wind has different requirements from an A320 on final which are different again from an F16 in a knife fight.

• Dont you think it is the SAS system that determines the bandwidth of the actuator as it contains more high frequency signals? Dec 17 '19 at 13:40