As for a Bipe. The top wing is above the fuselage and will behave somewhat like a high wing design (a BIG "somewhat" as some Bipes behave quite well and don't have a problem with outside loops).

The point I'm trying to make is that the wing is above the center of mass of the plane. To illustrate, try and balance a broom upside down on the palm of your hand (left or right, which ever is easier ). The center of mass is WAY above your hand and as a result tried to get below your hand so that it can hang like a pendulum. This is why you need to keep moving your hand in the direction the broom starts to fall towards. This is to try and get the center of mass directly above your hand. If it is not, it start to fall over.

The same thing might be happening with the Bipe, if the top wing is quite high above the fuselage. If you look at a Pitts Special Bipe you will see that the wings are close together and the the plane does a wonderful job in aerobatic manouvers, but I'm sure it's less stable in an outside loop than a fully aerobatics "pattern ship".

There is also a change that the difference in incidence of the two wings is the problem. The bottom wing might be stalling before the top wing due to lower incidence and the effects of the top wing.

Having said all of this, is the plane very heavy? Also, is the top wing very forward compared to the bottom?

I find it surprising that it does not do an outside loop, as most planes can be MADE to do one if the pilot fights with it a bit.

-Q.