Quoting SilverComet (Reply 1):
*All that being said, the closest defined speed to what you are looking for is the minimum unstick speed, Vmu* |

Actually the speed

**Jaylink** is trying to determine

__is__ defined, as

**Vmr** (minimum rotation speed).

As well as the five things

**SilverComet** listed there are some others you would need to take account of:

Aircraft mass (CG position alone is not much use).

Stabiliser trim angle.

Approximate Cl v AOA for the stabiliser.

Stabiliser area.

Zero lift pitching moment of the wing.

Stabiliser angle is a key parameter, because if this is mis-set the aircraft may not rotate at all. It will certainly affect Vmr.

So there are lots of veriables involved, and much guess work.

As for the basics of control surfaces, there's some basic stuff here:

http://www.grc.nasa.gov/WWW/K-12/airplane/rotations.html
Basically, control surfaces get more effective as airspeed increases, in fact their effectiveness increases with speed squared, but reduces with air density. This explains why it can be next to nothing at 20 knots, yet very significant at 100 knots.

Quoting SilverComet (Reply 1):
*Downforce on elevators will then be (ideally) equal to: (1/2) x density x area x speed squared x K* |

**(1/2) x density x speed squared** is the air flow's dynamic pressure, multiplying by the area gives a force. K would vary according to elevator deflection. For simplicity it could be assumed to be a linear relationship.

Quoting Jaylink (Thread starter):
*Presumably this will be before the aircraft would begin to rotate with only a partial extension.* |

Correct. At half deflection you would need roughly twice as much dynamic pressure to get the same pitching moment, so about 41.4% more speed, i.e. square root 2 times the Vmr for full elevator deflection.

The glass isn't half empty, or half full, it's twice as big as it needs to be.