Determining Toy Helicopter Orientation From Image
For yaw it's sufficent to have three distinct data points fixed to the helicopter in a plane parallel to the rotor rotation: different colored leds for example. You could put them on three thin radial arms fixed to the landing gear. Maybe colored reflective materials would work, too, but you'd have to compensate for environmental light changes.  

Then getting the yaw angle is simple.  Suppose the lights are red, green, and blue 120 degrees apart and ccw when viewed from the top.  



Then in the image you will see these lights in various x-coordinate orders. As the helicopter rotates ccw fromt the position shown in the diagram:

G R B   // green and blue in front; red in back
R G B   // green in front; red and blue in back
R B G   // red and gree in front; blue in back
B R G   // etc...
B G R
G B R


(Of course at the borders of these regions two lights will merge to a single point.  These can be handled as special cases.)

So with 3 lights you have split all posible yaws into 6 x 60 degree segments.  

You can further get down to 30 degree resolution by just comparing the left and right "gap" widths. If pointing directly away from the camera is azimuth theta zero degrees and the red light is on the nose (as in the picture), then the algoritm is:

Xr = -sin(theta)
Xg = -sin(theta + 60)
Xb = -sin(theta + 120)
if (Xg &lt; Xr &amp;&amp; Xr &lt;= Xb) {
  // Green and blue are in front of red
  dxLeft = Xr - Xg
  dxRight = Xb - Xr
  yaw = (dxLeft &lt; dxRight) ? 15+-15 : -15+-15;
}
else if (Xr &lt; Xg &amp;&amp; Xg &lt; Xb) {
  // Green is in front of red and blue
  dxLeft = Xg - Xr
  dxRight = Xb - Xg
  yaw = (dxLeft &lt; dxRight) ? 30+-15 : 60+-15;
}
else ... four more cases


The X+-15 above mean X degrees plus or minus 15: a 30 degree segment.  To get the exact offset, you can construct a preprocessed table that converts the ratio 

R = dxLeft &lt; dxRight ? xLeft / xRight : xRight / xLeft


into an an exact azimuth offset between -15 and 15.  This is pretty simple trigonometry to work out, and the nice thing is that it works independent of how far away the helicopter is, so long as it never turns upside down!

Another side benefit: the image distance between the outer two lights gives you inverse distance to the camera.