Well, I've had another three flights with the Spitfire since last week.
Sunday's flight didn't go so great because the battery came adrift and sent the Spit into tail-heavy mode! I managed to get it down in one piece, but the wind was too strong for a second test, so home I went to put in a brace to stop the battery from sliding so far back.
(Before anyone asks, yes, I had velcro on the battery, but it obviously wasn't secure enough.)
Tonight's first flight went utterly pear-shaped from the start. From launch, the Spitfire went straight up as though I was trying to loop it. It bottomed out about 18ins from the ground, and after that it was all over the sky. We trimmed it right down and I managed to get it back down on the ground. I checked the battery, and it hadn't shifted, but even with the motor throttled back it was wanting to climb. I'm sure the wind had a part to play in that, but it certainly wasn't helping.
After I added another 30g of nose weight, I tried again, and this flight was much better. The wind shifted her about a bit, but she was nowhere near as skittish as before. We still had the down-trim wound in, but this time it was much more manageable.
The consensus is that there's a warp in the tailplane, making the model want to climb the whole time. A look over the tailplane seems to confirm this.
I might have to try getting some lollypop sticks to try bracing the tail better, unless anyone can suggest something else?