Lesson 1: Types of Helicopters

I always thought about how I would talk to a bunch of school kids about helicopters, and figured I would start by asking the class to tell me how many different kinds of helicopters there were. The Rotor and Wing helicopter identification poster shows over 100 different models. The UH-60 Black Hawk line is currently up to variant R; the R-22 line includes about 5 different variants. I’ve lost count with what Bell and McDonnell Douglas (formerly Hughes, now Boeing) are offering. Within the ‘basic’ helicopter design there are at least three different main rotor systems and three different anti-torque (tail rotor) systems. So how do you break this down for a rational discussion?

Let’s start with a brief discussion of the controls. In the cockpit are the cyclic and collective sticks, and the pedals. The cyclic stick controls aircraft pitch and roll (fore-aft and sideward translation). The collective stick controls the vertical lifting force. In some models (particularly turbine aircraft) this also provides direct control of the engine throttle. The pedals provide yaw control (rotation about a vertical axis), anti-torque to the main rotor (in conventional helicopters), and turn coordination. In some aircraft, anti-torque is automatically coupled to application of collective.

Now consider that there are 4 basic rotor configurations: conventional, tandem, coaxial, and synchropter. While these different configurations may fly somewhat similarly, and are all capable of vertical flight, their control systems are so different that each warrants its own discussion. After a brief discussion of each type, a table will show how control is accomplished ‘behind’ the cockpit. Autogyros/ gyrocopters will not be discussed here.

Conventional Helicopters

Conventional helicopters are what most of us think of when we hear the word. The configuration consists of a large main rotor rotating in a nominally horizontal plane and a smaller tail rotor rotating in a nominally vertical plane parallel to the aircraft axis to provide anti-torque. I say ‘nominally’ because many helicopters have the main rotor canted forward (after all, the vehicle is manufactured to primarily perform forward flight) a bit, and sometimes to the side to accommodate subtleties of rotor dynamics.

For instance, the Sikorsky Sky Crane has the main rotor tilted forward and to the side to allow the cyclic control stick to be better centered during a hover (this machine was designed for lifting, not transport). The US Army’s Black Hawk has the tail rotor canted at a slight angle to provide some lift as well as anti-torque, vastly improving the CG capability of this utility aircraft.

There are three principle designs for main rotors: rigid, semi-rigid (teetering), and fully articulated. The differences between these will be the topic of a later discussion.

Similarly, there are several designs for tail rotors: conventional (which may be teetering, fully articulated, or rigid), NOTAR (as found on the McDonnell Douglas aircraft), and fan-in-fin (as found on the AS-365 Dauphin, for instance). Benefits for the last two are a degree of safety for ground personnel and from obstacles (trees) in confined areas.

Aircraft designs may incorporate various combinations of main and tail rotor types. For instance, the Enstrom line uses a fully articulated main rotor and a teetering tail; Robinson and Bell use teetering designs for both main and tail rotors. Some rotors turn clockwise, others counter clockwise. Similarly, tail rotors may turn one way or the other, or may be on either side of the aircraft.

Regardless, the fundamental criteria for a conventional design is a relatively large lifting rotor and a smaller device for reacting the torque created by that rotor. To a large extent, the nature of the basic design defines how the pilot will fly the aircraft.

Controls:

 Pitch Provided by changing the lift of the main rotor blades on one side of the rotor disc to allow forward or rearward translation of the aircraft Roll Provided by changing the lift of the main rotor blades on one side of the rotor disc to allow left or right translation of the aircraft Yaw Provided by changing the thrust of the tail rotor to allow left or right rotation of the aircraft, turn coordination, or react torque when collective is applied Collective Provided by changing the lift of all the main rotor blades to permit vertical motion of the aircraft or to accommodate power changes for changes in aircraft speed or rate of climb.

Tandem Helicopters

Tandem helicopters are defined as having two large main rotors, one in the front and one in the rear. Examples include the Boeing CH-46 and 47. Because lift of the aircraft is provided by both rotors, which are widely separated, these aircraft are capable of lifting heavy loads across a wide CG range. Because both rotors are well away from the ground, personnel safety is enhanced. The design mandates synchronization and high torque shafting between the rotors. The rotor system is typically of the fully-articulated design.

Controls:

 Pitch Provided by using differential collective pitch between the forward & aft rotors Roll Provided by changing the lift of the main rotor blades on one side of both rotor discs to allow left or right translation of the aircraft Yaw Provided by changing lift of the main rotor blades on one side of one rotor disc and on the other side of the second rotor disc to allow clockwise or counter-clockwise rotation of the aircraft (one left, one right) Collective Provided by changing the lift of all the main rotor blades on both rotors to permit vertical motion of the aircraft or to accommodate power changes for changes in aircraft speed or rate of climb.

Coaxial Helicopters

This design consists of two main rotor systems mounted on a common mast, but rotating in opposite directions. Examples of this design are the Sikorsky ABC and the MIL-29. Rotor systems are either fully-articulated or rigid.

Controls:

 Pitch Provided by changing the lift of the main rotor blades on one side of both rotor discs to allow forward or rearward translation of the aircraft Roll Provided by changing the lift of the main rotor blades on one side of both rotor discs to allow left or right translation of the aircraft Yaw Provided by increasing lift on one rotor system while decreasing it on the other to allow clockwise or counterclockwise rotation of the aircraft Collective Provided by changing the lift of all the main rotor blades on both rotors to permit vertical motion of the aircraft or to accommodate power changes for changes in aircraft speed or rate of climb.

Synchropters

This is an extremely unusual design consisting of two rotor heads at a slight angle to each other, driven by a common transmission. The rotor heads usually consist of teetering (semi-rigid) systems. The blades intermesh but magically avoid collision. Aircraft of this type have been produced by Kaman, the latest of being the K-Max, used in the helicopter logging industry.

Controls:

 Pitch Provided by changing the lift of the main rotor blades on one side of both rotor discs to allow forward or rearward translation of the aircraft Roll Provided by changing the lift of the main rotor blades on one side of both rotor discs to allow left or right translation of the aircraft Yaw Provided by changing lift of the main rotor blades on one side of one rotor disc and on the other side of the second rotor disc to allow clockwise or counterclockwise rotation of the aircraft (one forward, one rearward; see Tandem helicopters) Collective Provided by changing the lift of all the main rotor blades on both rotors to permit vertical motion of the aircraft or to accommodate power changes for changes in aircraft speed or rate of climb.

Next time, we will discuss rotor and control design. With this background, we can better understand control operation and move into maneuvers. All further discussion will be around the conventional helicopter. This is because these basics should apply across all helicopter types, but X-Plane only models conventional helicopters.

Helicopter Tutorials