In the Wikipedia article on the [F-82 Twin Mustang](http://en.wikipedia.org/wiki/North_American_F-82_Twin_Mustang) it says:

> 
> After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area. The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved"
> 
> 
> 

Why did this direction of propeller turn cause this problem?

![enter image description here](https://i.stack.imgur.com/NdyLa.jpg)

Image source: <http://www.au.af.mil/au/afhra/wwwroot/photo_galleries/aaf_wwii_vol_vi/Captions/078_Experimental.htm> 
(This image or file is a work of a U.S. Air Force Airman or employee, taken or made as part of that person's official duties. As a work of the U.S. federal government, the image or file is in the public domain.)