I took the Tin Goose to the testbench to get an idea of the exact power/thrust. First test is a test with the complete original setup. 3 x fk180 motor, 3 x 4.3 inch prop and the 7.2v Nimh battery.
As you can see in the movie I only got 9500rpm with the Nimh battery charged up to 8.12volt. With the 4.3 inch prop this will result in 40grams of thrust per motor. A total thrust of 120gram on a plane that has a 450gram of total weigth. The conclusion is simple. This amount of thrust is not enough to fly this plane.
A RC Trainer should have 1/3 (trust/weigth) ratio.So for this plane it should be at least 150gram of thrust. The original setup is bad. Don't follow the manual because it just don't work that way. You can try to mount a 7cell or 8cell Nimh battery or a 6cell Nimh battery with more capacity. But then your battery pack won't fit in to the battery slot an you will get a more weight. These little brushed motors are just not effecient enough to get good results (thrust) with a nimh battery.