Pitch trim issues - this is one of those very basic things about R/C RBG's. The elevator trim for boost needs to be less than the elevator trim for glide. Early on, I modified my trasmitter so I could use a toggle swtich to change from using channel 6's lever for the boost trim, to using the elevator trim for glide trim (The transmitter had separate potentiometers for trim, making that possible). With computer transmitters, it's easier to do, no rewiring (well, the computer Tx's I've used, anyway)
So, I have one toggle switch that I use as a Boost or Glide switch. For boost, the elevator trim is set for less elevator than used for glide, also that switch goes to low rate for elevator and "turn" (aileron or rudder depending on the model type). When flipped to glide mode, the elevator trim is higher, and the rates are higher.
Also, perhaps the model needs more up trim than you gave it. That might be simple enough to fix in the computer options in the transmitter (increased servo throw, especially for up), or might require a mechanical fix too such as a longer servo output arm hole location for the elevator pushrod, to get more pushrod travel. Or at the other end, use a shorter distance between the elevator contorl horn hole and the pushrod. I'd go for the longer servo arm, myself, if I wanted more mechanical throw (a bit less torque, but not introduce as much sensitivity for slop/hysterysis in the system as a shorter control horn).
BTW - ALWAYS be on the lookout for hysterysis. Bascially, move elevator up, then release to center. Move elevator down, then release to center. If it does not return to dead center but is biased a bit in the direction it came from, that's a problem. Causes for that are usually either too much friction in the pushrod system, or mechanical "slop" (looseness). Sometimes both. Also long ago, soetimes the servos, but servos these days are better in that regard. If it does not re-center, you need to fix it or it will always be flaky to fly since effectively you have wandering trim. I suspect your original model had that problem. Certainly that is a basic flaw of the T-25/Strato-Blaster to begin with, the original elevator pushrod layout snaking around for too much friction.
Flaperons: Thing about flaperons on the T-25, is you are not going to have real flaperons.
Real flaperons would mean a full-span trailing edge control surface, from root to tip.
On the T-25, only the outer 2/3 of the wing can act like a flaperon, the inner 1/3 is fixed.
My concern is that the model may tend to tip-stall pretty badly. The center 1/3 won't be stalling as soon because it will have less of an angle of attack than the outer "flaperon" portion of the wing once the flaperons are down. But when it comes to a wing stalling you always want the center to stall first, never the tips.
See this post on RC Groups:
https://www.rcgroups.com/forums/showthread.php?891870-Tip-Stall-and-Flaperons
The thin airfoil is good for low boost drag, not so good for low speed. So when you try to slow that wing down, if PART of the wing is going to stall first, you want the center of the wing to stall first, tips to stall last. Which is why on a lot of planes that use flaps that are not full-span, the flaps are along the middle of the span, like with the classic P-51, Piper Cub, Cessna, Lear Jet. Or even for aircraft that have "full span" flaps, often the flaps are segmented and the outer flaps do not have as much angle as the inner flaps, so that aerodynically the wing has a twist to it with less angle at the tip than near the center.
But OK, at this point it would be very difficult to try to make the ailerons full span, to cut and rebuild into the wing to make the inner part of the TE hinged and move as one with the outer ailerons. But you can try a few things. One, not use much flap angle. I mean for example if you had the ailerons droop 10 degrees as flaps, I could almost guarantee that model would tip-stall any time you tried any significant roll command. Another trick would be t set up for differential ailerons, so that for aileron control the inboard trailing edge would go UP (Say left aileron when rolling left) perhaps twice as much as the outboard (Say right aileron when rolling left) trailing edge would move down.
Another trick, I recommend for models like this one anyway. Add turbulator strips to the tips.
https://en.wikipedia.org/wiki/Turbulator
For our purposes, it trips the boundary layer early enough for a separation bubble to re-attach and therefore can fly at a higher angle of attack before it stalls
For models like these, the turbulator strip can be vinyl tape about 2 to 3 layers thick, about 1/8 to 3/16" wide. Locate the strip about 25% back from the leading edge. For this model, the last 6" of span. See the photo below.
There have been various models thru the years where I used turbulator strips. Some, to prevent tip-stalling. Others, models that had tip-stall problems and this either cured them or made the tip-stalls a lot less severe.
Having said all that.....I think it'd be best to not do flaperons at all. Stick to the ailerons. Plan out and fly the landing pattern so that you can level it out and keep it wings level for landing. And plan out the landing area so you can undershoot and most importantly overshoot by a long distance. Not try to "spot land", this ain't the kind of model for that.
I mean, you want it to be more forgiving but I really think if you did flaperons on it, it may be LESS forgiving and bite you even worse (tip-stalling)
BTW - in the first photo, that T-25 Centurian has several modifications to it. If has some dihedral. It was modified to use a BT-55 engine mount, to allow using 32mm reloads like F13 and G12. That required doing the aileron pushrods differently. On the outside, then a "Z" bend to go thru slots cut into the blow-molded pod. And actually, that is similar to the original pushrod arrangement that Larry Renger's prototype Estes Strato-Blaster used. It was later changed, against Larry's wishes.
And for those of you who wonder what the "Strato-Blaster" was, well, the T-25 Centurian is the old Stato-Blaster, albeit prefabricated (the Estes R/C R/G kits of the early 1990's all required the foam core wings to be built like model planes of the era, vac-bagged or weights or clamping set-ups to glue the wing skins on and all that).
Link to a kit I sold on eBay years ago, several photos:
https://georgesrockets.com/GRP/ebay/2011/StratoBlaster.html
