What is a minimal step angle deflection for flight control surfaces?

If the flight control surfaces (ailerons, elevators, rudder) are controlled digitally by servo motors, yet disregarding the way how the input to the servo motors get generated (manually or autopilot), what is the suggested minimal single step deflection of the control surface to be of practical value for a large UAV or piloted airplane?

By "practical" I mean not a ridiculously small angle that slows down the servo with unnecessary many steps, yet sensitive enough to allow for a smallest practically useful deflection or ensure the precision of a small input.

If the minimal step of deflection is not the same between them, then please suggest your answer for each control. Please give answer in degrees within 360 circle.

• Welcome to aviation.SE! Apr 18, 2018 at 15:56
• I would suggest find the angle of deflection of the control surface, (such as +/- 15 degrees), and make the servo range match that. So if the servo can move +/- 42.5 degrees from center for example (more than that and you have to worry about reverse movement with simple control horn), than make it move as smooth as you can over that range to make the control surface feel as analog-like as possible for the piloted airplane at least. When I fly, I don't want any noticeable steps in my control movement, I want the aileron and stabilator following my hand action as smoothly as possible, & rudder. Apr 18, 2018 at 19:00
• @CrossRoads that's indeed one of the results I want to accomplish - smooth control adjustments, also efficient and fast controls in terms of electronics and programming. I still hope that somebody has data from experience of the smallest angle deflection - that makes the development so much easier. Apr 19, 2018 at 14:48
• servo has no steps, stepper has Apr 20, 2018 at 19:14
• one way to implement a good precision servo is to make it and control it as a stepper motor. Stepper is one of the types used for a servo implementation. Apr 28, 2018 at 14:55

There are several phenomena that may affect the asked resolution. The easier one is the static (steady case), and the dynamic case which requires several more analyses.

Statically, the resolution of any surface will determine the moments that that particular surface can generate. Consider the elevator, the resolution will determine which aoa and which speeds the aircraft will be able to fly (with a given weight and a given thrust configuration).

Dynamically, the stability of the plane will derive the required resolution. Consider the elevator, and suppose your resolution is 1 degrees. The inputs will generate short period and long period (phugoid) modes simultaneously and your controller will become a bang-bang controller, because you can only command increments (decrements) of 1.

Hope these give some clues to understand the physics. For more details, control theory will be useful.

• thank you - you well covered the relationships between the controls resolution, surface area, speed and stability. But can you give a simple applied example of the minimal resolution, for example take a composite glider with high l/d ratio - what would be a good practical control surface deflection resolution of ailerons, elevator? Any known numbers from practical experience? Or any example of other 3-axes aircraft with a good L/D ratio (not a supersonic jet)? Apr 23, 2018 at 19:04
• I would simply check some rotary servos that are in use with aircraft, e.g. part of autopilot systems or UAV’s. Some of them have ranges in 0.1 to 0.25 degrees. However, the actuated surface resolution depends on the gearing mechanism, as you’d expect. Jul 20, 2018 at 17:01

The correct answer depends on flight speed. If your UAV only flies at one speed: Take the deflection maxima and sprinkle your steps across that range, just as CrossRoads suggested. If you find out during flight test that part of the deflection range is never used, you may consider reducing that range in oder to gain finer resolution over the remaining range.

If, however, the craft covers a wide range of speeds, make sure that you have a finer resolution towards the center of the range. For big deflection angles, it does not matter how close two steps are - you will need them only at low speed and for as long as the condition which you desire to end lasts. At high speed, control effectivity is much larger and even fine variations in deflection angle will result in disproportionally large flight path deviations. Those are unavoidable in a stepped control surface, but should be held as low as practical.

In order to still have a reasonable control reaction, details depend on your servo and the aircraft, so I feel unable to give a concrete number. But make sure that you can react quickly enough in order to dampen out all eigendmodes.

• Thank you - these all cover much of the flight specs theory. Good for all who search for this topic here. I personally am familiar with them - I still hope to get a specific practical example. Preferably for a modern glider (for ex. a standard class competition glider) or whatever particular example you know of please share - max deflection and minimal step angles as implemented for different stages of flying (if possible include links to the usual specs of the known aircraft). May be somebody here is from among actual designers and builders who knows actual numbers for a particular design? Jul 22, 2018 at 13:58