Another builder took it one step further with two Allisons resting side - by - side and using the twin - prop set - up .
One idea had a single Allison with a duo of counter-rotating propellers with only one gearbox .
Another builder took Allison one step further with two Allisons resting side - by - side and using the twin - prop set - up .
One idea had a single Allison with a duo of counter-rotating propellers with only one gearbox .