This is an attachment so you can use your iPhone 4s and the Clinometer app http://plaincode.com/products/clinometer/ as a RC heli rotor blade pitch gauge. Print out the parts and bolt together with M3 x 10mm and M3 x 18mm socket head cap screws. Note: this is designed to work with a naked iPhone. No cases, screen guard. It's a nice snug fit to keep down on errors. There is an internal channel in the top clamp part for rubber bands, but it doesn't really need it.The "pg Phone Mount Main v2" file has a small rectangular piece. Print that part out and cut that piece off and bolt to the front of the Mount main part.