Relative Merits of Conventional, Tandem and Coaxial Rotor Configurations

The first successful helicopters were nearly all of the side-by-side or coaxial rotor configurations. This ensured no torque reaction on the airframe by equally balancing the torque supplied to both rotors. While an obvious principle, supporting, powering, and controlling two separate rotor systems led to complications in building the machine and often made for slow progress. The simpler idea of using a single rotor with a separate tail-rotor for anti-torque was known in the early days of helicopter experimentation, but this configuration was not widely used. Igor Sikorsky was to adopt the single main rotor configuration in the 1930's, and was ultimately the first to put the type into successful production during the early 1940's. Today, the single rotor helicopter is the most common design, comprising over 90% of all helicopters currently flying. To describe the relative merits of the single-rotor versus twin-rotor configurations, one must consider aerodynamic (performance) issues, as well as mechanical complexity, weight, handling qualities, vibration, noise and maintenance issues.

• For the single-rotor helicopter, the purpose of the tail rotor is to counteract main-rotor torque reaction, and also to provide directional (yaw) control and stability. On tandems and coaxials, the rotors turn in opposite directions to cancel torque. Yaw control on coaxials is obtained using differential collective pitch, which adjusts the relative torque balance between the rotors. Tandem machines use differential lateral cyclic to control yaw. It is generally argued that for single rotor helicopters, the power supplied to the tail rotor is 'lost' power because it does no useful lifting or propulsive work. This is compared to tandems or coaxials where both rotors provide lift and propulsion. However, both tandems and coaxials suffer from strong aerodynamic interference effects, where the two rotors have a detrimental effect on each other. This is particularly the case for the lower rotor on coaxials and the rear rotor on Tandems. Tail rotors typically consume up to 5-10% of the total power, whereas twin-rotor interference effects can account for at least 10\% or more of the total power.
• Tandems also suffer larger rotor/airframe interference effects, including a download penalty compared to single rotor machines, because the fuselage of a tandem is directly located below the region of the rotor flow where the downwash is highest. In this case, extra power is obviously required to achieve the same net vertical lifting force because of increased vertical drag. The coaxial helicopter is a relatively compact rotating-wing design, with an overall fuselage length that can be lower than a single rotor machine because a tail rotor does not have to be located on a long boom structure. However, the coaxial type also suffers from a download penalty because of the higher rotor slipstream velocities generated by the two lifting rotors that are located directly above the fuselage.
• One major disadvantage with twin-rotor machines (tandems and coaxials), is the high parasitic drag of the rotor hubs and controls in forward flight. Generally, the parasitic drag (quantified in terms of equivalent flat-plate area) of tandems and coaxials is higher than a single-rotor helicopter when compared at the same gross weight. This means that single-rotor helicopters generally have the edge when it comes to climb and forward flight performance.
• One obvious advantage of the single main rotor helicopter configuration is mechanical simplicity and lower empty weight for the same payload. On the single rotor machine, the effects of the rotor controls are essentially independent of each other. On tandems and coaxials, the interference between rotors introduces significant 'coupling effects,' and is unavoidable in all three control axes - roll, yaw, and pitch. These effects can be exacerbated by fuselage interference effects. Therefore, not only do tandems and coaxials have a duplication of flight control systems, but appropriate 'mixing' of control inputs is required to give the machine acceptable flying qualities. This leads to greater design, production, and maintenance costs.
• The maintenance costs of all aircraft are roughly proportional to the number of moving parts. Twin-rotor helicopter have more complicated transmissions compared to single rotor machines, mainly because of the need to cross-shaft the rotors. This leads to greater design and production costs. For similar reasons, the maintenance costs of a twin-main rotor system will also be higher than a single main rotor system.
• It is often claimed that single rotor helicopters have a much lower allowable center-of-gravity (c.g.) travel than tandem machines, which because of the relatively widely spaced rotors, allow for larger c.g. flexibility. This is not necessarily true. While early single rotor machines had a limited c.g. range and had to be carefully loaded, the use of a flapping 'hinge-offset' on the rotor gives the machine a much wider c.g. range,especially for the larger machines. The c.g. range on tandem rotor helicopters is generally limited because of a variety of other issues, including stability and control. It is fair to say that for the same gross weight, modern tandem and single rotor helicopters have very comparable c.g. ranges. Interestingly, the useable c.g. range of the modern helicopter is usually much larger than that of a fixed-wing aircraft.
• Rotor noise has its origin from the rotor blades (so-called thickness noise) and from the blade tip vortices. While the vortices themselves generate little noise, large amounts of noise are produced when the blades interact with these vortices - so-called blade vortex interaction or BVI noise. On single rotor helicopters, this mainly occurs under conditions of descending and maneuvering flight. However, for coaxials and tandems this condition is more prevalent under all flight conditions as the wake system from one rotor is ingested into the other. Thickness noise is related to the volume of air displaced by the rotating blades, which can in turn be related to rotor solidity. For the same gross weight, single and twin-rotor systems tend to have similar overall thickness noise, but may have significantly different noise directivities. Both coaxial and tandem rotor machines produce strong BVI noise, which tends to dominate the noise signature of the machine in the frequency region that is sensitive to the human ear, and so makes them subjectively 'noisy.'

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