What is the smallest aircraft with stick-fixed controls?

From : Don Stackhouse

That could have a number of different answers depending on the point of view of the person asking the question. However, radio-controlled models qualify in most cases. The servos typically used for control actuation appear to be essentially rigid from the point of view of the control surface (unless the model builder made some serious errors in design and construction practices), so most R/C models operate in what amounts to "stick-fixed" mode.

The exception is some of the tiniest R/C models that use magnetic actuators instead of servos. In these, a varying current in a coil acts against a small magnet to create a force that tries to push the control surface away from neutral. Hinge stiffness, a centering spring of some sort, and/or air loads try to push the surface back to neutral. The balance between these opposing forces of magnetism vs. everything else is what closes the feedback loop. This type of control acts in a "stick-free" mode in most examples.

Of course the ultimate example of "stick fixed" controls would be the smallest of free-flight models. These are typically trimmed on the ground before flight by careful and deliberate warping of the flying surfaces, so in effect they have control surfaces that are essentially rigid in flight. I'm not sure what the smallest successful free-flight models on record are, but they can get quite small. For example, I personally have built free-flight gliders with wing spans of less than 5/8" (the smallest was a balsa stand-off scale Schweitzer 1-34, with an L/Dmax of approx. 2.5:1), and flyable kites that were scale models of Monarch butterflies (including the little knobs on the ends of the antennae)with wingspans as small as 5/16". BTW, these kites were of composite construction using Kevlar fibers in a lacquer matrix, molded with precise dihedral and cambered and reflexed airfoils, and the same techniques would work for making a small model airplane. I've attached a picture of some of these gliders and kites. I don't have the 5/8" span glider anymore, I gave it to a friend. The one in the photo is 7/8" span.

These are the smallest I've made, but I'm sure there is somebody somewhere who has gone even smaller than these.

Then of course there's the question of insects and whether their "controls" via muscles, bio-hydraulics and structural flexing qualify as "stick-fixed" or not, but this also depends a great deal on the exact definition of "aircraft" in your question, and whether a living creature can qualify as one.

Don Stackhouse
DJ Aerotech