It really depends on aircraft design. Almost any aerobatic plane on the market will fly knife edge flight with the right CG and control inputs. CG tends to be the most important. A more rearward CG will tend to make knife edge flight easier but will not correct for the plane pulling towards the gear. I have a Extreme Flight Extra 300 EXP that will fly the prettiest knife edge I have ever seen. My GP Reactor flys a great knife edge as well but requires elevator inputs to fly straight. Some airaft will simply not fly a perfect knife edge with out mixing or inputs. Set your CG back a bit then experiment with different elevator throws and exponential to figure a combo that requires the least amount of input to hold the airplane in staright knife edge flight.