Originally Posted by small_rcer
If you place the contact point too far aft of the CG, you may find it hard to rotate the aircraft on take off. If it is too far back you need a large elevator deflection which causes drag and can slow you down at the critical take off point. Then once the plane starts to rotate you might over rotate and stall.
You do not want to be in a position where you have to pull a lot of up and then have to push the stick forward once it starts to rotate. You could get into a PIO or Pilot Induced Oscillation. Very unpleasant position to be in at take off.
So on that size plane I "guestimated" about 3/4 inch aft of the CG in a 7 to 10 deg nose up position. The reason I suggested that the LG be mounted further forward and slope aft and down is to get the weight of the mounting point structure and the LG itself as far forward as possible to make it easier to balance the plane.
When you have the plane sitting on the ground with the CG where it should be, push down by hand on the tail to rotate the nose of the plane up to 10 deg nose high. The downforce you use to push the tail plane down is what the elevators will have to generate to rotate the plane. Too much and the elevators may not have enough force to do it or require a large deflection.
Cheers for the info... I am going to do this next on my task list. I have measured the distance between contact point and vertical between CG at 7-10 degrees, a i think its currently sitting 2.5 inches behind. so needs to be moved 1.5 inches.
I've worked out i need to make tail 215g total. A friend of mine is up to the challenge. I am going to use DOW Blue foam instead of EPS foam as it gives a smoother finish. the the resin will go on smoother and not fill so many voids causing weight.