Thanks to all for the suggestions,

The light bulb went off in my head to try decreasing the exit diameter by creating an adjustable temporary adaptor to
reduce the diameter at various interval and measure the thrust levels. We'll see!

PS. Thanks Ian, I already thought about selling to someone else who flys off hard surfaces and it may end up
going that direction. Unfornunately, I promise the buyer I would sell it to him and I thought for sure I could
get this thing to fly off grass for his use.