Check out the double-rotor helicopter that could be the US Army's next Black Hawk
Why Do Helicopters Have A Tail Rotor? » Science ABC
Tandem-rotor aircraft - Wikipedia
MH-47G Chinook Special Operations Helicopter, United States of America
Solved Question 13 Helicopters are designed with two sets of | Chegg.com
Dual rotors could make the Defiant one of the world's fastest helicopters
How does a helicopter work? - Explain that Stuff
Why do some Russian helicopters have two sets of rotor blades but such design could not be found in the West? - Quora
These dual rotors are perfectly timed so that none of them crash with each other : r/Damnthatsinteresting
How do helicopters with two rotors operate? - Quora
Helicopters & Vertical Flight – Introduction to Aerospace Flight Vehicles
upload.wikimedia.org/wikipedia/commons/5/51/Ukrain...
Sikorsky tests radical dual rotor helicopter that can reach 276 mph | Daily Mail Online
What military helicopter has two propellers? - Quora
Helicopter - Wikipedia
Helicopter Rotor Systems Configuration | SKYbrary Aviation Safety
aerodynamics - Number of blades in a helicopter rotor - Physics Stack Exchange
The Osprey: Half-airplane, half-helicopter, totally badass
Why don't helicopters have two sets of blades that rotate in opposite directions (one clockwise and one counterclockwise)? - Quora
33 Things You Probably Don't Know About the CH-47 Chinook
Twin rotor helicopters hi-res stock photography and images - Alamy
Twin rotor helicopters hi-res stock photography and images - Alamy
Why should a helicopter necessarily have two propellers? - Quora
What Are the 6 Different Types of Helicopters? | Chopper Spotter
