My Low D Whistle takes a lot of air in the upper octave to play at pitch. Is this a common thing or is it just my whistle? Note I play GHBs and if I say it takes a lot of air it probably does.
Lows often tend to go phlat on the high notes. See if there is something else you can do to compensate, like changing the direction of the breathing, or changing the shape of the inside of your mouth by bringing the tongue up, etc, besides blowing harder. Or just get used to the flat notes. 
I was really just wondering if it was me or the Whistle I guess it is me I have trying been trying to get used to blowing harder.