Construction Problem with Hovercraft

[SciFi1]

For my science fair project, I've decided to build a miniature working model of a balloon-powered hovercraft. I am instructed to use two balloons of the same size; I'm planning to use 9-inch-wide balloons. The hovercraft is being judged on its ability to navigate on a linear path, its ability to not touch the floor/fly in the air, and its ability to glide in the air for a long period of time. Hence, I have followed the instructions from many sources including the Science Buddies website, and built several models based on my own ideas.
I placed one balloon facing outside the center of the bottom of a paper plate which is flipped upside-down to make the hovercraft float in the air, and attached the other balloon onto a large straw. The paper plate itself is the size of a dinner plate. The rim of the paper plate acts like the skirt of a genuine hovercraft. Then, I attached the straw holding the balloon through the diameter of the paper plate to act as a force that would thrust the hovercraft forward. So far, the rate of the amount of air that is released when I let go of the balloons on my hovercraft is controlled. My hovercraft also moves according to how I planned it to move, but it does not do so very efficiently. The entire hovercraft does not float in the air; it seems as though it is tilting on each side of the paper plate's rim rather than actually flying up into the air. The hovercraft moves forward at a slow pace, does not move very far (hardly half a metre), and stops inching forward when there is still a decent supply of air left to thrust it forward.
Perhaps it may be that the paper plate may be too large for the balloons to move with the air, hence I will implement this design scheme to a smaller paper plate - maybe to the size of a quarter plate. However, if there are any other faults in my design scheme, or if there are any ways in which I can improve it, please inform me. I would greatly appreciate the help.

[paulsdecarli]

As a regular science Fair Judge, I like to see experiments to test hypotheses. Now you just hypothesized that a smaller plate would work better. Why? Think about the variables in your hovercraft...weight of system...area of plate...size of balloons...etc.
can you vary one thing and hold the others constant? I hope this post gives you some ieas about a suitable project.