The Cessna Citation CJ3 (model 525B) has that device circled below, what is it?

enter image description here
Source: airplane-pictures.net

I initially saw it as a mid-chord fixed spoiler-like device. As @programagor commented, it's parallel not perpendicular to the airflow.

  • $\begingroup$ Is that not just a vortex generator? It seems to be parallel to the airflow, not perpendicular. $\endgroup$ – programagor Apr 26 at 3:25
  • $\begingroup$ @programagor: You're right about the orientation! Thanks for the hint. It turns out it's not a VG. I submitted an answer. $\endgroup$ – ymb1 Apr 26 at 4:03

I found a hint in a worldofaviation.com article; it's an "aileron fence":

... a small 'aileron fence' rises from the upper surface, just inboard of the ailerons.

Now knowing what to look for and the angles to consider, it's now much easier to spot (and to not spot—knowing that shallower head-on angles would not show it).

enter image description here
Source: wikimedia.org

Wikipedia's Aileron article says:

Aileron fences Some aileron designs, particularly when fitted on swept wings, include fences like wing fences flush with their inboard plane, in order to suppress some of the spanwise component of the airflow running on the top of the wing, which tends to disrupt the laminar flow above the aileron, when deflected downwards.

But, it's citation needed

As seen below, when downward deflected, the aileron actually takes the fence down with it, so Wikipedia gets it wrong.

enter image description here
Source: wikimedia.org; this high-res photo shows it attached to the aileron

There's little literature on the device; I was able to find only one source where it's properly explained and makes sense:

In the presence of a sideslip from the wingtip direction of a given aileron, the fence creates a local high-pressure area on the upper surface of the aileron causing it to deflect downward and lifting the opposite aileron. This causes a stabilizing rolling moment away from the sideslip. Fences can also be installed on the lower surface at the outboard edge of the aileron and function in much the same way to create a low-pressure area to “suck” the aileron down. Although aesthetically preferable, these are usually less successful. Some examples of aileron fences used for static lateral stability can be found on a wide range of aircraft: Cessna 206 (single engine piston), Cessna 208 (single engine turboprop), and Cessna Citation CJ3 (twin engine turbojet, Figure A-7).

— Williams, Russell Loy. Reduced static lateral stability in airplanes. 2005. (PDF)

Another thing Wikipedia gets wrong as shown by that thesis, is that it's present on three Cessna models with straight wings.

  • $\begingroup$ Fwiw, from Cessna, taller aileron fences provide static lateral stability below approach speeds. $\endgroup$ – mins May 21 at 17:23

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