I was noticing a problem with it descending power off. It was heading away, and quite high at the time it started down.
The plane doesn't have the incidence the GL is supposed to have.
I oughta fix that!
I was more than pleased when it didn't munch into the ground!
I have a CF rod pulling the elevator in the up direction. When the boom bends down, that adds "up".. hmmm...
More "up" just adds to the boom down load.

Hmmm... descending and heading away.... If it was quite a ways out I'll bet the speed was building up a bit? Especially if the wing to tail decalage was a trifle scant? Sounds like you may have had the same thing happen as what I had. In my case the high drag center from it being a T tail ensured a nice terminal velocity and very vertical dive.

Hi ...your "design" looks like the Shadow 200 UAV.

Be careful to properly calculate your tail volume. should be around 0.8 S.tail/S.wing*distance between N point Mac of wing and N point Mac of stab / wing Mac...

you should position your main undercarriage at a point which is 15 degrees behind your C of G . and your nose gear should be positioned at a point that it will carry 12% of your plane's weight.

the offset of your main wheels position will determine your "force" amount to lift your nose of the ground. The far the wheels are from your C G the more force it will need.

Your plane looks like it can easily fly on a 10Hp engine with ~5hp for cruise. what is your wing loading....??? At your application usually each horse power can lift 5~7 kg with a Rate of climb of ~800 ft/min. What airfoil did you use what CL-max and at what CL did you position your wing?Also how did you calculate your tail boom and your wing spar?

It looks like you are using a 3W-150cc....A prop of 28/16 will fly this bird very nicely.