My guess is that you have some up thrust on your motor. Try placing some washers on the screws between the motor and the firewall for the screws on "top" of the motor, causing the motor to have some down thrust.
This goes to the issue of trimming philosophy. There are different ideas on how to setup a plane. This is how I do it. I start with CG @ 25% and on low rates.
1) Trim the plane to fly straight and level at full throttle
2) Move the CG aft until I get the amount of instability I want, usually around 30%. If you want a lot of stability keep CG at 25%.
3) Do the dive test.
Dive test: fly very high, cut the throttle and put the plane into a straight down dive. The goal is to be at to dive 150-200 feet straight down. If the plane quickly pulls out of the dive right side up, you need less down thrust. If the plane. If the plane pulls out bell side up, you need more down thrust.
Why it works, when you trim for straight and level at full throttle, you trim out any errors in thrust or incidents. When you dive without power, you remove the thrust and yet maintain good speed. This will allow you to see what was trimmed out.
How quickly the plane pulls out will give you an idea how much thrust angle change is needed. If your plane pulls out in less than 50 feet, you need a large change 4 degrees plus. If it pulls out in 75-100 feet, you’ll need 2-3 degrees. If the it pulls out is 100-150, 1 degree or less.
When you get it right the plane will fly well at any speed.
No, If I were building from plans or a kit, I would start at the FT recommended CG. I have calculated the recommended CG on 3-4 FT planes, it has been 25%. I have found 25% to be a great starting place regardless of the plane. It gives stable flight performance which is what you want for trimming and/or training.
However, when I was at flight fest 19, I shared the table with 3 noobs who all built the tiny trainer from a kit. They built their planes independently from each other. I did not watch how they built the planes as I was flying. When it came time, I was given the honor to maiden their planes. All three were balanced at the recommended mark. All 3 were tail heavy. They were all flying squirrels. I immediately landed and moved the CG forward 1/4 inch, then they flew great. I'm not sure if the kits were off a bit or if they did something incorrectly in the build.
I seldom build a stock FT plane, mine are always highly modified. I've never build an FT kit.
Even with the CofG in a suitable position is it possible to make a plane fly without climbing or diving at every throttle setting?
Surely without some form of on board gyro stability control the answer has to be no.
To be 'human flyable' a plane has to have a degree of natural stability. Which means that when in a steady state if it speeds up it will climb causing it to slow down and vice versa. To alter the speed of a steady state you have to re trim.
Yes, you can use engine down thrust to reduce the climb effect of the planes natural stability but it is not a perfect compensation, particularly during the period whilst the speed is changing form one steady state to another.
All full size aircraft have to be re trimmed to achieve a steady state at the desired speed and throttle setting. Either the pilot has to do it or it is done by the auto pilot but it still happens.
It may be possible to build a craft that will fly the same at any throttle setting but it will be a big, neutral stable, large motored, highly aerobatic, symmetrical wing, zero incidence, flying stabilizer, and generally a double handful to fly. Highly useful for combat however.