Conventional Tail Rotor Solution

Helicopter Anti-Torque Concepts

Intro

Today once again we going to look closer to conventional helicopter build with focus on anti-torque solutions. Recently we talked about engines, controllers, airfoils types, and main rotor solutions. Same as before, we going to dive a bit deeper then we need. But the reason behind this is that I want to introduce some of those features in further updates. O.K. Let’s dive in and see what we can learn from this topic and what to adopt from it to next update 04 in “One Man Helicopter” project for Unreal Engine 4.

Conventional Tail Rotor Solution
Preview in a clay – Early concept.

As Newton’s 3rd Law of motion states, in every action, there is equal and opposite reaction. Torque is a force that is the reaction of the engine turning the rotor. When the engine turns the rotor, there is an equal-and-opposite reaction that tries to turn the body of the helicopter in the opposite direction. Anti-Torque solution is to prevent helicopter against this phenomenon. However, before the concept comes into effect, engineers have to keep in mind other ground rules during work on anti-torque solution design for single-rotor helicopters.

Torque

Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration. Torque is a vector quantity. The direction of the torque vector depends on the direction of the force on the axis. Anyone who has ever opened a door has an intuitive understanding of torque.

The magnitude of torque of a rigid body depends on three quantities: the force applied, the lever arm vector connecting the origin to the point of force application, and the angle between the force and lever arm vectors.

T = F * r * sin(theta)

  • T – is the torque vector
  • F = linear force
  • r = distance measured from the axis of rotation to where the linear force is applied
  • theta = the angle between F and r. Thetais needed to take into account the direction from which the linear force is being applied. The force will not always be pushed from straight on like a door. It can come from many different angles.

In equation, sin(theta) has no units, r has units of meters (m), and F has units of Newtons (N). Combining these together, we see that a unit of torque is a Newton-meter expressed in (Nm).

Anti-Torque

Etymology anti- +‎ torque

Anti-Torque in simple words is the opposite force to torque and it’s value we can deduce from the above formula. A typical helicopter has three flight control inputs—the cyclic stick, the collective lever, and the anti-torque pedals to control the anti-torque force. Also engineers have to take into account further arguments.

Ground rules

  • Hazard to ground personnel.
  • Vulnerability to ground-contact damage.
  • Vulnerability to small-arms fire.
  • Reduced susceptibility to high-speed forward flight flapping instabilities.
  • Improved reliability, maintainability, noise, erosion, and foreign object damage characteristics.

An underlying objective is that the selected tail rotor substitute should be of a nature that could be adapted to existing helicopters by a retrofit modification program at reasonable cost.

Categories

Many concepts had similar basic characteristics which permitted, grouping them into the following categories:

  • Conventional tail rotors
  • Ducted fans
  • Nozzles
  • Immersed aerodynamic surfaces
  • Horizontal-axis rotary-wing airfoils
  • Future concepts

Conventional tail rotor

In general, it is mounted at the aft end of the fuselage structure and exerts thrust 90 degrees to the centerline of the fuselage, it is mounted on one side of the fuselage completely exposed, and it is shaft driven from the main-rotor gearbox.
Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, which also provide directional control by allowing the pilot to rotate the helicopter around its vertical axis. Some of the same problems which designers encountered with main rotors occur with tail rotors. Often the solution is similar or identical to the solution used on a main rotor.

Ducted fans

This anti-torque device is usually submerged within the vertical tail or the tail cone and is shaft driven off the main rotor to permit operation during engine-out, autorotation conditions. Essentially the same drivetrain used for conventional tail rotors is used. Collective pitch of the fan blades is provided to modulate thrust. Ducted fans have between eight and eighteen blades arranged with irregular spacing so that the noise is distributed over different frequencies. The housing is integral with the aircraft skin and allows a high rotational speed; therefore, a ducted fan can have a smaller size than a conventional tail rotor.

All fans improve the forward flight performance relative to an exposed rotor due to the reduction in vehicle drag when the fin provides the anti- torque force in forward flight.

Nozzles

The nozzle can be single and rotatable, or two opposed nozzles may be used. This solution eliminates the use of the tail rotor on a helicopter. Modulation of thrust can be accomplished by adjusting the throat or by varying the upstream plenum pressure. Plenum pressure can be supplied by a compressor driven by the main rotor. Another source of power identified showed main engine high-pressure exhaust gases routed directly to the nozzle. However, this concept is not attractive due to low thermodynamic efficiency and lack of operating capability during engine-out conditions.

A unique nozzle arrangement found in the search suggests an array of aspirators ejecting high-pressure air which induces a secondary flow of outside air through concentric nozzles.

Immersed aerodynamic surfaces

Aerodynamic surfaces have often been considered for providing anti-torque forces by acting as airfoils immersed in the wake of an airflow generator such as the helicopter main rotor or pusher propeller. They fall in two general categories: those primary systems which can be designed to produce the entire anti-torque moment; and those auxiliary systems which generate forces to supplement other anti-torque devices.
A basic concept for an anti-torque device which is best applied as a supplementary system is a fixed airfoil in the main rotor downwash, oriented to generate thrust in the direction to produce main rotor anti-torque. By making this surface movable, a modulating anti-torque and lateral control system is accomplished.

Horizontal-axis rotary-wing airfoils

A distinct general type of anti-torque system employs airfoils rotating about a horizontal axis parallel to the spanwise direction in paddle-wheel fashion, and often referred to as a “cyclo-gyro”. The axis of rotation is generally in the fore-and-aft direction, and cyclic pitch controls the direction and magnitude of the anti-torque force. This type is generally penalized by complex design and high drag.

Future concepts

When studied in detail, the concepts presented in this category may show an apparent conflict with time-honored conservation laws, as well as the action-reaction principle. This, however, should not be particularly disturbing since, for example, nuclear physics has shown that absolute conservation of mass and absolute conservation of energy are not longer inviolate laws. Likewise, the action-reaction assumption expressed in Newton’s third law, although generally valid for two infinitesimal, infinitely rigid particles, admittedly does not hold for magnetic interactions between moving charges. In the manner that purely “mechanistic” theories are not directly applicable in the realm of subatomic micro-phenomena of the Quantum Theory nor in the large-scale macro-phenomena of the Theory of Relativity, it is not inconceivable that important exceptions to the traditional laws of physics may exist in the vast middle ground of engineering between subatomic and intergalactic dimensions, with practical applications of unmeasured possibilities.

A number of advanced concepts were examined that are outside the present state-of-the-art. They are included for completeness in covering the full spectrum of anti-torque concepts.

  • Electromagnetic rotation
  • Acoustic radiation pressure
  • Three-dimensional vortex
  • Compound precession of multiple gyroscopes

Summary

Above knowledge is just scratching the surface of iceberg, but I think we have more informations then we need to design our solution for single seat helicopter. Based on what we’ve learned I am going to provide with the first approach a conventional tail rotor solution. Traditionally mounted at the end of the fuselage structure completely exposed, with two blades and with shaft driven from the main-rotor gearbox. With one exception to traditional build, the tail rotor airfoils angle of attack will be transferred with a linear electromechanical servo actuator mounted on the back of the tail rotor gearbox. More information about this solution could be found in Helicopter controls.

Now I can begin to pre-shape the main frame for all those components: engine with electric, cooling and lubricant system, semi-rigid main rotor with two blades, a controls with servo motors and tail rotor solution. In the future I would love to experiment with ducted fans and nozzles as anti-torque solutions.

Next in queue

Here is a list of remaining parts in line for modeling process before moving to Unreal Engine 4. Presented list is in a non-chronological order.

  • pilot seat,
  • dashboard,
  • navigation & flying instruments,
  • fuel tanks,
  • main frame,
  • landing gears,
  • battery,
  • electronic.

The good think is that those remaining parts are simpler to understand, and mostly with out moveable parts. This will speed up a little the whole process. However, there are several challenges ahead.

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