Yikes - 80% charge gone in 20 minutes!!
What iOS do you have installed - I think 9.0 or maybe 9.2.1 is their latest. Do you have a lot of apps running in the background that might drain your battery while flying?
Despite any apps running, losing 80% in 20 minutes is just not normal at all and there is no way the controller charging for the iPod could replenish when loosing a charge that fast.
Another question here for you - have you ever, or do you regularly deep cycle charge the battery? One of the reasons you might want to do this is when you notice battery discharge is occurring quickly (like in your case).
Regardless, you really should carry out at least one complete charge cycle per month.................
1 Charge battery to 100%
2. Let it run down completely (run a video or something that will drain it quickly).
3. Re-charge to 100% and test it out during your normal usage and see if that helps (I have ran a complete cycle charge 2x in row to achieve best battery health and to improve usage time for a single charge (usually 1x per month is enough).
Really does sound like you need to do a complete cycle charge - discharge and charge (your battery is showing 80% and gone in 20 minutes really screams complete cycle charge required (that is unless you have iOS 8 installed - there was problems with battery charging and false % values of full charge with that OS).
Best of luck.