:) My teacher gave the task to figure out the common relation between stability and control. I know definitions and only thing I could find on the internet was "Control is directly opposed to stability" but I am not sure if I get it. Thank you for your answers :)

  • $\begingroup$ Simply put... If something is very stable, it is difficult to get it to change attitude. If something is unstable it is easy to change attitude (not necessarily in a controllable manner). Civilian aircraft tend to be very stable (easier, safer operation and more comfortable). Fighter jets tend to be (very) unstable for manoeuvrability. Some fighters are so unstable (F-16) that they need artificial stability (electronics) or no human would be able to fly it. $\endgroup$
    – Chris V
    May 25 '17 at 12:20
  • $\begingroup$ "Control is directly opposed to stability" -- yet stability is not always directly opposed to control. Example-- if a hang glider "tumbles" due to getting too slow at the top of a loop, it becomes very difficult to control. Consider also an aircraft that will enter an unrecoverable flat spin at the drop of a hat-- very difficult to control. $\endgroup$ Apr 28 at 19:54

You are right.

Stability is the tendency of a system to return to its old state after a disturbance, be it a gust or a control input. It needs more control effort to change the state of a more stable airplane, such that stability can be measured by measuring control forces instead.

A famous example which is often mentioned in literature is the BE.2, a British two-seater which was designed as a training aircraft in 1912. As a trainer, it had ample stability, so its control responses were sluggish. That proved to be its undoing when war broke out in 1914. The BE.2 was easily outmaneuvered by dedicated fighters, so it had to be withdrawn from frontline service.

Quote from Wikipedia:

While it proved fundamentally unsuited to air-to-air combat it had a relatively low accident rate, and its high degree of inherent stability actually proved helpful in its artillery observation and aerial photography duties (although it also made it more difficult to maneuver quickly)


BE.2 (picture source)

  • 1
    $\begingroup$ I was blown away by the size of that aircraft, until I realized the person in front was a small child --! $\endgroup$ Apr 28 at 20:32

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