Tandem Rotor Helicopters

By Glenn Bloom of www.helicopterpage.com

Tandem Rotor Helicopters

Tandem rotor helicopters operate a little differently than do the single rotor variety. In a tandem rotor helicopter, you have no tail rotor, so there is no translating tendency to deal with, but you still have pedals for directional control at a hover. Your cyclic control, which is used as it always has been in single rotor helicopters, has not changed either. The only thing different in terminology for tandem rotor aircraft is the term "Thrust control", which is used to describe the collective pitch control lever. It is used in the same way as any other collective, but the tandem guys use the term thrust control. (Who am I to argue???)

The cartoon you see is a computer generated version of the one I first drew while in Saudi Arabia during Desert Storm. I made a shelving unit out of scrap lumber and placed it in our tent, next to my bed ("The Mushroom Tent"...keep us in the dark...you know the rest. Our commander was not pleased with our choice of names for the tent. We lived next to the "Warrants of Arabia" tent which was a much more popular name with the command.) I took a magic marker and drew a cartoon of a Chinook (Like this one)on the side of the shelving unit. Using an Arabic to English dictionary that one of the guys found in an Iraqi bunker, I copied the phrase "East or West, Home is Best" under the picture in Arabic. I had people stopping in from other camps to see the picture I had drawn on my shelving unit. Once the word got out, I had a few visitors a week come to see it. Anyway, I hope you enjoy it. (BTW...If you give a helicopter pilot enough room on a web page, he will put at least one of his war stories on there!)

Rotor Position for Level Hovering

Tandem rotor helicopters operate in forward flight by using "Differential Collective Pitch" or "DCP". DCP is basically just increasing more pitch in one rotor system then the other to make the aircrafts attitude change. By increasing the pitch in the aft system more than the forward system, the aircraft will tilt nose low, and accelerate forward. To climb without changing airspeed, more pitch is placed in both systems simultaneously. It is really a matter of aircraft attitude more than anything else. If the aircraft is in a nose high attitude, it will climb and bleed off airspeed. If it is too nose low, it will dive and increase airspeed. The amount of pitch put in each rotor system will dictate airspeed and altitude. The pilot will fly the aircraft just like any other, but the rotor systems will act in a way peculiar to tandem helicopter flight. The picture here is of a tandem helicopter in level hovering flight. Notice both rotor systems are depicted as level. The actual blades will "Cone" a bit. What this means is they will bend upwards to the tip. The more the weight on the aircraft, the more the blades will cone. For our descriptive purposes, the pictures will not depict coning. The cartoon above has exaggerated the blade coning a bit for humorous effect (Ok, you can laugh now).

Here's a chinook doing slope operations.

Rotor Position for Maneuvering

You will notice in the first of these two pictures both of the rotor systems are tilted to one side. This is what allows the aircraft to fly sideways. This is accomplished by using lateral cyclic, and a corresponding pedal input to maintain directional control. What the cyclic and pedals do is actually put inputs to the rotor systems to make them both tilt the same direction, as if there were two cyclics both putting in duplicate inputs to the two rotor systems. In the second picture, you see the front rotor system level, and the aft system tilted, allowing the helicopter to pivot around the forward mast. This can be a handy maneuver when there is little room to move the nose, but plenty of room to swing the tail of the aircraft. This is accomplished by using large pedal inputs and small cyclic inputs to make the aircraft pivot around its nose. I am sure you wonder how one learns to do these types of maneuvers, and how the thinking process is involved while trying to make these types of maneuvers. I can tell you that there is none. Once you learn to fly, and get acquainted with your aircraft, you do not think about maneuvers. You just do them. Just like you do when you drive your car. In your car, do you think to yourself; "I am going to make a right turn, so I need to start slowing down, signal, turn the wheel, accelerate through the turn, and recover the steering wheel?" I never do. I just start to do it by feel as I judge the distance from the curb, and monitor my speed. My mind is not on every move, it is concentrated on certain aspects of the maneuver. Flying a helicopter is the exact same thing. You concern yourself with altitude, clearance from obstacles, and power settings. The maneuvers just happen mainly by feeling your way through it while you judge your distances, make sure your engine, or engines are working properly, and that you are not going beyond the aircrafts limitations. You are so busy doing other things, you do not actually think about the actual control inputs to induce the maneuver.

More Rotor Position for Maneuvering

In the first of these two pictures, you see both rotor systems are tilted in opposite directions. This is accomplished by pedal only inputs. By depressing one pedal over the other, cyclic inputs are put in both systems in opposite directions to pivot about the center of the aircraft. Both rotor systems receive equal cyclic inputs, and the helicopter just spins nicely at its center without the pilot having to move his cyclic control at all. In the second picture, a pivot around the tail is depicted. This is accomplished by heavy cyclic inputs by the pilot, and little or no pedal inputs. This will make the tail stay in one place, and the nose of the aircraft moves laterally until it spins about the aft mast. (Also known as the aft vertical shaft, due to its height). Now, I could try to sit here and tell you what happens to the rotor systems when left cyclic is put in, and then what happens to them when pedal inputs are induced. It would take me a few hours of saying, "Wait, Wait, no, it does this, no, it actually does this, wait, ok, yes, it does that too." I will spare you all of this and just say it is a complicated system that requires a lot of linkages, a lot of control tubes, and a pilot who trusts his maintenance crew to make sure it all was put together properly (Even if you trust them with all of your heart, do a good preflight anyway!).

Since we are on the subject, I will tell you a quick preflight story:

In the Army, as with civil helicopter operations, it is the pilot in commands responsibility to do a meticulous preflight inspection of the aircraft before each flight. Some Army pilots will expect their crew chief to do the inspection for them. This is not a good practice, as the crew chief is not the one who is responsible if something goes wrong. It is always the pilot in command who is ultimately responsible for the safety of his or her aircraft. There was this one pilot who always had his crew chief do the preflight for him. He was a Major (Commissioned Officer) and he always ordered his crew chief (A Sergeant First Class) to perform the preflight, even though it was not his responsibility, the crew chief did as he was ordered. One day the crew chief decided that he had enough of this so he walked out while the Major was starting the aircraft, and in his hand was a Jesus Nut (Which is the one main part that holds the rotor system on the aircraft). The crew chief started flipping the Jesus Nut in the air, and when the Major saw this, he went into emergency shut down procedures, cutting the engine, and securing the aircraft. In a panic, he jumped out and yelled at the crew chief. "How come you were going to let me start this aircraft without a Jesus Nut?" The crew chief replied, " I was not going to do any such thing." The Major who was quite perplexed said, "Isn't that Jesus Nut off of this aircraft Sergeant?" The crew chief replied, "Sir, If you had done a proper preflight, you would have known that it was not." Need less to say, the Major was quite humiliated by all of this, and he was sure to perform his own preflight inspections from then on.