Windfree 100" glider with a 480. What was wrong?

[Frank v B]

A club member gave Paul Scuse a 100" glider with a 480 in it.

Here are the symptoms.  You guess what was wrong.

All 4 attempts were in very gusty, swirly winds.

flight 1- A quick check of the CG put it on the spar.  Upon launch at half throttle, it went skyward.  Full down trim and full down elevator and it would not fly straight.  Up and down and managed to bring it back near the landing area.

Flight 2- doubled the elevator throws by moving the pushrods on the servo and elevator horns.
Same thing except full down elevator and down trim would make it flyable but just barely.  Whenever I powered up, it would go skyward.

Flight 3- a club member thought it was a rearward CG problem and added some nose weight.  Still the same problem.

Flight 4- very gusty winds and still the same problem.  The gusts were so bad that I felt the wings would fold.  Gently settled it down anywhere.  Thank you Paul for doing my walk of shame.

Then it hit me when I was halfway home.

Frank

[Frank v B]

Decided to take it back to basics so we could check the motor thrust line as well since this plane was designed in the '70's before electric power went mainstream.

Approach
step 1- a power-off glide test- chuck it into the wind to eliminate thrust line issues.  A good toss should have it fly 50'-100'.  It flew fine. No nose-dive (indicates nose-heavy) or scalloped flight path (indicates tail-heavy)

step 2- a 50% power-on flight- it went fine.

There was one change made before these flights. When we put a 4' straight-edge (a spirit level) on the bottom of the wing and noticed two things:
1) about a 3 degree positive angle of attack on the wing (leading edge 3 degrees higher than the trailing edge).
2) neutral angle of attack on the lifting stab.

Theory: since a plane flies on the wing, it would place the stab at a -3 degree angle of attack (leading edge lower than trailing edge)....or a permanent 3 degrees "up" elevator.

Adjustment: we screwed the elevator clevis inward to give it a permanent 3 degree down elevator.

Result:
1) the power-off test flight went about 75' with very little stick adjustment.
2) the power-on flight was very controllable in very difficult windy conditions.

Here are the photos of the issue.
67- you can see the wing relative to the stab.
68- the wing is at a very noticeable "up" angle (leading edge higher than trailing edge).
69- this is the elevator set at neutral flight.  Notice how far it is down from the previous neutral where the centre section goes through the fuse.
70- Paul holding the 4' straight edge on the bottom of the wing.  I then measured the stab's leading edge and trailing until they were equal. During flight both the wing and elevator are a 0/0 to each other (in line).  The fuse is now crooked.

Phew- now if we can only make the wind go away.

Frank