Technically they don't have an overall charge......individual bits do, you just have to be awfully clever to use them.
The Drakh are actually flying backwards as part of a fiendish plan to lure the enemy into a false sense of security.
or
There was an unfortunate typo in the contract they signed - this being a fictional universe, it immediately came true.
Somewhat akin to a Warhammer-playing friend of mine reporting that he'd just managed to kill off a Chaos Lord by charging him with a Brettonian Duck.