This started as an e-mail to answer a question from a new member of Flight Adventures. Perhaps it might be of interest to others. I submit it below:
Just checked out the Canadian Labrador. It is a CH46 tandem rotor turbine helicopter which is still being used by the US Marines. They call it the Sea Knight.
A brief spot of history. Shortly after our Korean adventure Bell Helicopter started working on the Huey utility helicopter. At roughly the same time Boeing-Vertol started working on a heavy lift helicopter using the tandem rotor concept. The result from Boeing was two models. One, slightly smaller, and adapted for carrier use with folding rotors was the Sea Knight. It is still in use by the US marines and still plays a significant role. This is the model Canada refers to as the Labrador. Another model, larger and heavier was developed as the CH47 Chinook and used by the US Army. Both entered service in the early 60's. Both are in heavy use today. Both served in Vietnam, Desert Storm, and Afghanistan. Neither are antiquated machines. Both models have been updated with new engines, airframe enhancements and improved avionics and glass cockpits.
Boeing is still producing new models of the CH47 Chinook, as well as refurbishing older models to new capabilities. For the foreseeable future tandem rotor helicopters will have a significant role.
Let me explain why. All of the power produced in tandem rotors translates into lift. In conventional helicopters, the tail rotor is used to counter-act the torque of the rotating wings. Although small, maybe 10 to 20 percent of power available is used to control torque. This energy contributes nothing to lift or speed. This waste of energy translates into decreased useful load. That is why Boeing can brag of their lift to weight ratio.
This efficient use of available power does not come without problems. Stability is severely compromised. All tandem rotors do not like to fly straight. The rear rotor is constantly trying to swap ends with the front rotor. Not a happy thing to happen. This was overcome by gyro stabilization called "Stability Augmentation System." This system, called SAS is critical to operation of a twin rotor. Gyros feed input into the control system to maintain stability. Part of my training was to try to fly the beast with SAS off. Only at slow speeds and small loads would this be possible. That is why even the earliest Chinooks had dual SAS systems. If one SAS was acting flaky it was possible to switch to the other. If both failed in a critical flight regime recovery would be doubtful. Complete failure of SAS never happened to me, or anyone else that I know of.
Another curious fact of these tandem rotor beasts is that they are inherently faster than single rotor craft. I can already hear the cries. No way these tubs were faster than a Cobra! Allow me to explain why. Every single rotor craft is limited in speed because of a physical property called "retreating blade stall." At some point, every single rotor craft is flying through the air fast enough so that the retreating blade can not keep in advance of the relative wind to produce lift on the retreating blade side. Hence a stall and roll. Tandem rotors are counter-rotating, hence as the retreating blades on one rotor lose efficiency, the other rotor is increasing efficiency on the other side to balance the effect. Granted a torque effect on the fuselage structure is induced, but popping a few rivets was a small price to pay to outrun the Cobra!
To clear up one other common misconception each rotor was not turned by one of the two engines. The rotors were connected by a drive shaft along the spine, and an efficient transmission funneled each of the engines output to a single source. Should one engine fail, both rotors were still supplied with the remaining power available. These aircraft are still heavily used by the military of numerous countries. The success of Desert Storm depended greatly on flanking the Iraqi's by placing troops, equipment, fuel and ammunition hundreds of miles into the Iraqi desert.
I am sure that I told you more than you asked, but I could not resist the impulse to pontificate.
Barry Echols
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