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?

Did some research and it seems motor speed is directly proportional to voltage applied. So I have to assume that since the LiPO battery voltage is much lower is why there is not enough RPM generated. Parkflyers said the LiPO was an upgrade for this plane - interestingly they also have an upgrade 480 motor with stronger magnets as well. So maybe the upgrade requires BOTH the LiPO and the motor. Guess I have no choice but to get both.

Would going to an 11.1V LiPO be possible with the stock 480 motor? Or is that just going to fry the motor....or the ESC which is rated at 30Amp

Thanks for the tips. yes prop and motor are correct. It does roll forward on the ground with throttle up.

I ordered the himag 480 motor upgrade which they say generates 40% more power and another LiPO 7.4 V battery 1500mAH 35C. Not sure if the battery I had was fresh but it did register about 8.5V fully charged. So should know in about a week if the upgrades worked.

Assuming the problem wasn't the prop being on backwards (a common issue) you could have probably fixed the problem with a bigger/higher pitch prop. Yes, you've lost some RPM but can make the power back up with more prop.

Its not possible to fit the prop backward cause there is an indent that fits the hex collat on the prop adapter only on one side. Therein, also is another problem in finding another prop that fits this adapter

It should be fairly simple to find a replacement adapter that fits your motor shaft. It sounds like a moot point now that you've ordered a new motor, but for future reference there are standard collet type adapters for every size motor available.

It sounds like you are just starting in the hobby, so if that's the case do you have someone who can help you with flying skills? A Decathalon in a nice flying plane, but it's not the most beginner friendly. Someone to at least do the initial trimming and talk you through your first few flights would give you a much better chance at success.