Grandad builds rollercoaster in his backyard for his grandkids!

If there was a 'Grandpa of the Year' award, it would have to go to American man Paul Gregg who built a rollercoaster park in his backyard for his 
The retired Boeing aerospace engineer has posted a series of videos to Youtube showing his granchildren enjoying the coasters.
The 'Out'n'Back Negative-G' starts from the top of a picnic table placed on his porch and speeds over a series of bumps and dips, before climbing a large 
slope and retracing the course backwards.
Another ride dubbed 'Smaug the Terrible' after the character from The Hobbit barrels its way through the backyard, weaving between trees and looping under itself.
The gifted grandfather has so far constructed three coasters, applying the same processes he used when he worked on aeroplanes and military vehicles
at Boeing. The materials a bit of a step down from the cutting edge alloy he's used to - mainly lengths of timber and PVC pipe.
In a post to Reddit, Mr Gregg described how he carefully designed each coaster with CAD schematics, calculated velocity, kinetic energy and G-forces for 
each twist and turn using spreadsheets and the accelerometer in his phone. 
He carried out extensive safety tests with sandbags strapped to an infant car seat, before finally letting his grandson ride.
Mr Gregg has plans for more coasters and has even published his own book, Backyard Coasters, in order to pass his knowledge along to the next
generation of budding home-coaster enthusiasts.
His efforts already have some dubbing him 'Granddad of the Century' - a claim which would be hard to deny.