I bought a Parkflyers Decathlon about 7 years ago and for various reasons never tried to fly it until this month. A few weeks ago when I tried, the plane would cut power at high throttle and start to beep so never got off the ground. I figured that the original NiMH battery it came with (1000mah, 9.6V) could no longer hold a charge since it was probably 8 years old or more. So I bought a LiPO battery on ebay using the spec that Parkflyers recommends for the LiPO upgrade for this plane (1800mah, 7.4V 2cell). Today with the fully charged LiPO battery I tried 6 or 7 takeoffs all unsuccessful both from the ground and tossing by hand. At full throttle it just didn't have enough thrust and no lift whatsoever. Never got off the ground and would just nose dive about 10 feet away by hand.

When holding the plane and giving full throttle I feel it lightly pulling forward but nowhere near enough I imagine to produce lift. The voltage on the LiPO is lower than the NiMH but that is what Parkflyers sells as a compatible battery.

Any thoughts on what to do?