Stephan..this plane will fly very easily if...

1. The wing loading is low enough
2. The power / thrust is strong enough
3. The model has the correct balance
4. The model has the correct amount of control surface travel
5. The model is assembled geometrically "true" and the controls have no slop or defects.

I have a few biplanes in the size range in your picture. They hand launch very easily with just a slight push away from me. They all weigh about 14 ounces or less.
Any combination of factors that I listed can add up to failure, especially if the .049 engine isn't producing about as much thrust as what the model weighs.