Prince William and King Charles don't always see eye to eye apparently ... with a new book claiming they've gotten into some major arguments -- over a helicopter.
In an excerpt from "Catherine, The Princess of Wales," obtained by The Daily Mail, biographer Robert Jobson says Charles called out his eldest son over using his helicopter with his wife Kate Middleton and their three kids.
In fact, Charles was so concerned about all the flying, Jobson says, he demanded William sign a formal document acknowledging the risks and taking responsibility for his actions -- not exactly a warm, father-son bonding moment.
Apparently, the King of England was just taking after the late Queen Elizabeth II ... who Jobson adds regularly voiced her displeasure with Will flying Kate Middleton and their kids 115 miles from Kensington Palace to their manor Anmer Hall.
The book claims KC first brought up his frustration with PW after he'd been diagnosed with cancer in February ... with Jobson implying he's especially concerned about what could happen to William given his own mortality.
BTW ... William and Charles apparently aren't strangers to father-son conflict ... with Jobson claiming Middleton often acts as a stabilizing force in their relationship.
Of course, Middleton's going through the wringer these days ... finally stepping back out into the world after months away due to her own cancer diagnosis -- so, it's hard to say how much she's been up to playing Royal Peacemaker.
The new book chronicling her life comes out August 6 -- and, we got a feeling this helicopter story is just the tip of the royal iceberg.