Billowing jacket

[trophyrider]

Could only put this here I feel but .....
Out today and for the first time in a couple of years I took someone on the back seat. My son decided he would like to come out with me for my usual sunday morning run, he is 6 foot and about 12 stone, I have a Givi touring screen with madstads fitted. Centre hole and all the way forward, barn door style. Anyways got to about 70 ish and my jacket started to fill with air and billow out like the Michelin man, now I need no help on the Michelin man thing as I am a big lad but the Belstaff jacket felt like it was going to fill up and come off. Never happened when I was solo and at post motorway speeds in fact never happened at any speeds before.
Any ideas? and no I did not have wind nor was my son trying to get warm inside my jacket as it was pissing down at the time.

[Piglumps]

I have found that I get more air in my jacket when the current Lady Piglumps is riding pillion, it could be either of two things.
 First the addition of a passenger changes the airflow (no sh*t Sherlock) and you now have a changed airflow either running up your back or sleeves. I doubt it's sleeves as this would happen all the time.
 Secondly the envelope could be at a considerably lower pressure causing your jacket to billow.

Advice: Adjust Madstad or sell child!