26 July 2008
Look Ma, No Tail Rotor II !
(If you missed the first exciting installment of "Look Ma, no tail rotor", you can catch up with the rest of us here.)
I can never remember whether it's Newton's first, second, or twenty-third law...
"For every action there is an equal and opposite reaction." For conventional twin-engine airplane drivers, it's what makes takeoffs interesting because if you lose your "critical engine", Mr. Newton's law might make the last seconds of your life exciting.
Newton also keeps most helicopter pilots on their toes by constantly keeping them aware of that little turny-thingy on the back of the machine, the "anti-torque rotor".
The turning main rotor tries to twist the fuselage of the helicopter in the opposite direction. The tail rotor is used to stop or control the amount of "equal and opposing" force.
Tail rotors are dangerous.
Back when I was still flying Bell LongRangers, one of my main worries on scene flights was having someone walk into the tail rotor. The bottom of the tail rotor arc reached down to a level less than 4 feet above the ground, so even a child was at risk. I always tried to park the aircraft in such a way that the tail rotor was opposite the direction ground EMS personnel would approach the aircraft... Chaotic scenes with everyone running around willy-nilly were nerve-wracking.
You have to be more than 6 feet tall to be concerned about encountering the tail rotor on the BK117 I now fly, (assuming the aircraft is sitting on level ground). So I can now be a little more relaxed at accident scenes.
The other consideration about tail rotors is their vulnerability. They are quite fragile, and they are at the rear extreme of the helicopter, helping them to be a better lever and do the job they have to do. That position also makes it more likely the pilot will screw up and strike them against something in a hovering turn or extreme nose-high attitude. As we have seen before, a conventional helicopter suddenly losing its tail rotor is an interesting study in physics.
So for a number of reasons, efficiently doing away with the tail rotor is a good thing.
Sitting in my office the other day I heard what sounded like a truck going down the road with the tread of a tire coming off. When the sound didn't fade away into the distance I ran to see what was causing the noise and saw one of the above pictured aircraft land and taxi to the fuel pumps. It's a Kaman Aircraft company "K-Max". The two main rotors cancel out torque. Eliminating the tail rotor also increases the amount of power supplied to the main rotors... and since this machine is designed for lifting, having that extra power to lift is a great thing. And it's an efficient lifter... it can pick up its own weight. There is a penalty in the design... the pilot was flying his machine cross-country to do another job, and when I asked his cruise speed he replied, "Ninety knots".
Other machines use two main rotors to counteract torque...
here's an interesting shot of one you know. Here's another that has just started its U.S. military career.
Finally, there is one more example of a two main rotors to cancel torque shown here. This example is a Russian Kamov, with its coaxial rotor system.
Now, a mental exercise for ya...
Think of how hovering turns are accomplished in all these machines!