Bxcar32

Dave,

To explain the aerodynamics associated with a tandem rotor helicopter I’ll discuss a few general helicopter aerodymanic phenomina.
Helicopter controls are associated in the pitch, roll and yaw axis and are called pitch, roll, and yaw accordingly. I have noticed throughout the R/C world where most R/C helicopter pilots consistently call these inputs elevator, aileron, and rudder inputs. It is important that when we talk about helicopter controls we use the associated helicopter term. This will eliminate confusion among those who fly both fixed and rotary wing R/C aircraft. This is a general discussion and certain specifics are omitted in the interest of basic aerodymanics.

In a single rotor helicopter, pitch inputs tilt the rotor disc forward and aft. This has the effect of changing the attitude of the helicopter into a nose low or high attitude respectively along the pitch axis. Example: given a set power setting in cruise flight, applying forward cyclic will increase airspeed and decrease altitude. Conversely, applying aft cyclic with decrease airspeed and increase altitude.

Applying lateral cyclic tilts the rotor disc left or right. This has the effect of changing the attitude of the helicopter into a left or right bank along the roll axis. The degree of which is proportional to how much you move the cyclic. In a hover, this input slides the helicopter to the left or right according to the input selected. In flight, this input is used, in conjunction with pedal inputs, to turn the helicopter.

Yaw inputs (or pedal inputs if you prefer), rotate the helicopter along the yaw axis (referenced as the vertical shaft). In flight, pedal inputs are primarily used to control aircraft trim and assist with turns.

Collective pitch increases or decreases the pitch of all blades in a rotor system regardless of quantity of blades and location of each blade in rotation. Due to Torque Effect, application of collective pitch requires adjustment of yaw inputs to keep the alignment of the helicopter on your intended track.

Torque Effect is the helicopters natural desire to offset the torque applied to the rotating rotor disc by spinning the non rotating porting of the drive system in the opposite direction. Most helicopters have a vertical “wing” at the location of the tail rotor to provide lateral “lift” in forward flight to offset the amount of yaw inputs needed to maintain helicopter control. Since single rotor helicopters are designed with either clockwise or counter clockwise rotation, yaw control will be designed accordingly. For the pilot, you just need to know which direction of rotation your helicopter has in order to apply correct yaw inputs with power application.

For a Tandem Rotor helicopter these inputs have the desired effect through different application principals.

To control the pitch of a tandem rotor helicopter, you must understand Differential Collective Pitch or DCP for short. DCP is differential pitch applied to the forward and aft blades according to the position of the cyclic. As you apply forward cyclic, the pitch in the forward head is decreased while simultaneous pitch in the aft head is increased. This is accomplished in a balanced manor where the helicopter will rotate along its pitch axis (located at half the distance between each head). Applying aft cyclic accomplishes the opposite effect; pitch in the aft head is decreased while simultaneous pitch in the forward head is increased. This is contrary to a single rotor helicopter since forward cyclic tilts the whole rotor disc. On a side note, it’s rather humorous how the effects of control application seem to be centered around the cockpit. Either that was by design or helicopter pilots think everything revolves around them. Hmmmm.

Roll control is accomplished by tilting both the forward and aft rotor head in the direction of lateral cyclic input. Left or right input will cause the helicopter to enter a left or right bank.

Yaw control is accomplished by tilting the forward and aft heads in opposite directions when yaw inputs are applied. When left yaw (pedal) is applied the forward head will tilt the same as a left roll and the aft head will tilt as if you had applied right lateral cyclic. Since the forward and aft head turn in opposite directions (coaxial rotor systems included), they cancel out torque effect. All power supplied from the engines go towards lift. Thus, tandem rotor helicopters apply 100% power to lift where as single rotor helicopters apply anywhere between 85% to 92%, (as example, specifics are per each helicopter design) engine power to lift, since single rotor helicopters must use some of their engine power to maintain directional control.

The helicopter has the ability to turn about the axis of either the forward or aft head. The trick in understanding how to do this is to understand how the control inputs work on the rotor system. To turn about the forward or aft head, you should understand how yaw inputs work on the rotor system and also how roll inputs work.

In flight, the Chinook has an Advanced Flight Control System (AFCS) that couples turns for us at airspeeds over 40kts. It also has the ability to adjust our airspeed with a few clicks of the “coolie hat” on the cyclic. This adjusts our DCP and power applications accordingly and in conjunction with each other.

I will leave you to research Gyroscopic Procession, Translating Tendency, Transverse Flow Effect and Retreating Blade Stall, Loss of Tail Rotor Effectiveness (LTE), etc at your own pace as they tend to be a little confusing to discuss in a forum as this.