## Abstract

A new vortex model of a helicopter rotor with an infinite number of blades is proposed, based on Shaidakov\'s linear disk theory for calculating inductive speeds at any point in space in the helicopter area. It is proposed to consider the helicopter rotor and the behind vortex column as a system of discrete vortex cylinders. This allows building a matrix of the influence of the vortex system under consideration on any set of points, for example, the calculated points on the rotor itself, on the tail rotor, etc. The model allows calculating inductive velocities at any point near the helicopter using matrix multiplication operation. It is shown that the classical results for the momentum theory remain constant even in the discrete simulation of the helicopter rotor vortex system. The structure of the air flow behind the rotor and the simulation results obtained by the proposed method is compared with the structure of the tip vortices and the results of the blade vortex theory. In addition, the experimental data were compared with the simulation results to verify the correctness of the model under real operating conditions by the helicopter trimming.

### Keywords

- induced velocity
- intensity of circulation
- discrete vortex cylinder
- influence matrix
- Shaidakov’s linear disk theory

## 1. Introduction

The character of the load distribution on the disk rotor vortex theory affects induced velocity. In turn, the inductive flow is the most important factor affecting the value of the inductive losses, as well as forces and moments acting on the helicopter’s rotor. Therefore, the efforts of many authors are aimed for creating theories and methods for the simplest way to calculate the induced-velocity field, without which it is impossible to calculate the determination of the aerodynamic characteristics of the rotor.

In conditions of low velocities, induced-velocity field is particularly irregular. This leads to significant changes of aerodynamic forces acting on the blade. The blades begin to oscillate with higher amplitudes, causing significant variable tensions inside blades.

### 1.1 Coordinate systems of the vortex cylinder

The properties of a cylindrical vortex surface are considered in detail by Shaidakov [1]. He analytically investigated the properties of a vortex surface that completely covers a beveled vortex surface. Shaidakov studied a vortex surface that starts from a disk plane, has an arbitrary shape in section, and pointed with its one end to infinity. Vortices on the surface are parallel to the base of the cylinder, which lies in the plane of the beginning of the vortex cylinder. When applied to the rotor disk of a helicopter, it is more appropriate to consider not an arbitrary shape of the vortex surface, but a very specific shape, the cross section of which is shown in Figure 1. The disk plane is filled with closed contours formed on two sides by arcs of circles and on the other two sides by radial segments. The number of closed contours depends on the number of calculated points along the blade and the number of points along the azimuth. However, the shape of closed contours remains the same.

To study the velocity field caused by a discrete vortex cylinder, we will select two typical sections of it. One section is located at the beginning plane of the vortex cylinder (lies in the plane of the screw disk); the other is parallel to it and intersects the cylinder at an infinite distance from the first plane. The sections of the cylinder with these planes are conventionally designated 1-1 and 2-2, respectively.

It is convenient to calculate induced velocities in the coordinate systems

The right rectangular coordinate system

The last two axis systems are most convenient for deriving equations of the vortex cylinder surface. It is enough to know the distance

The ratio of induced-velocity’s components in systems

These component interdependences are true for any vector [3].

In the accepted coordinate system

The modulus of the vector

The projection of the vector

### 1.2 Components of the induced-velocity vector at any point in area around the rotor

Let us consider a certain part of a cylindrical vortex surface, with the beginning at the plane of the disk, bounded on two sides by two generatrices and leaving the other side to infinity (Figure 2). The beginning of the generatrices is denoted by the point’s m and n. Select a vortex element

where

We will calculate the induced velocities from this element at the point

where

We express the vectors included in the formula (Eq. (8)) in terms of affine coordinates

The modulus of the vector

Calculate the components of the inductive velocity vectors from the vortex path

After describing the determinants, we will have the following expressions for induced velocities

Let us perform integration of formulas (Eqs. (16)–(18)) along the cylinder creators

After the conversion, we will have

In formulas (Eq. (22)–(24)) we will denote by

Using the following formulas

Integrals (Eq. (26)) are table integrals and can be denoted by the following:

where

Using the already known relationships

where

Integrals were obtained for the first time by Shaidakov [3] and are referred to in this chapter as Shaidakov’s integrals.

Now it is possible to get expressions for differentials of induced velocity’s components on the axis of a coordinate system

We get the formula for the component of the induced velocity directed along the vortex cylinder generatrices. We use the expression (Eq. (7))

After simple transformations we get

To calculate the velocity from a limited width vortex cylindrical surface, we need to take the contour integral from the projection of this surface on the base plane (the integral along the length of the arc

## 2. Discrete vortex cylinders method

The method of discrete vortex cylinders is based on the linear disk theory of Shaidakov described above. It allows you to calculate the induced velocity from the rotor at any point in the space around the rotor. Consider a main rotor with an infinite number of blades [2]. Imagine a vortex system that descends from the rotor in the form of a vortex column, starting at the plane of the disk and going to infinity. The vortex column is supported by a circle with a radius equal to the radius of the rotor. The angle of vortex column inclination to the disk plane depends on the helicopter forward flight speed and the thrust of the rotor. It is calculated using the Shaidakov formula for the angle of inclination of the vortex column.

Each partial volume of the vortex column can be considered as an elementary column of dipoles with a constant density of circulation. Alternatively consider it as an elementary vortex cylinder of arbitrary shape with a linear circulation of closed vortices

In case of the beveled cylinder filling with dipoles, to calculate the inductive velocity from the entire vortex column, it is necessary to make integral sums from the

When filling a vortex column with vortex cylinders, the area of the base is filled with closed contours of a specific shape (Figure 1), and the column is entirely filled with vortex cylinders of linear circulation

We propose to consider a vortex column as a collection of a finite number of vortex cylinders resting on the plane of the rotor disk. The plane of the disk is filled with closed vortex contours on two sides by arcs of circles and on the other two sides by radial segments (Figure 4). When the disk is split in this way the size of the contours will depend on the number of calculated points along the blade radius and along the circumference of the rotor disk. In this case, the induced velocities will be calculated at points located at the vortex cylinder’s axis, and outside the vortex column at any point other than the vortex cylinders surface. This avoids computational difficulties when calculating contour integrals on the surface of cylindrical columns. This point is indicated by a letter

To calculate the components of the inductive velocity at point A by the method of discrete vortex cylinders we will use Shaidakov’s formulas (Eqs. (31)–(33), (35)), derived for a discrete vortex cylinder (Figure 4).

We assume that the circulation along the contour and along the generatrix of each discrete vortex cylinder is constant. Then in formulas (Eq. (36)) it is possible to take the linear circulation

After the obvious transformations, we will have:

where

The derivatives

Finally, we will have

To calculate integrals along the contour, we will use integrating matrices [3]. For convenience, we will calculate the closed loop integral as the sum of integrals

With the proposed natural partition of the disk into discrete vortex cylinders, the problem of calculating inductive velocities in the disk plane is reduced to multiplying the matrix of influence on the column of linear circulations

In hovering mode, the main diagonal (

## 3. Helicopter trimming

### 3.1 The aerodynamic and inertia loads on the blades

The aerodynamic load on the blades is calculated from the known equations of the aerodynamics of a helicopter rotor is the same for main and tail rotors of the helicopter for the spatial movements relative to the longitudinal and transverse axis of the helicopter

We add inertial loads [4] to the distributed aerodynamic forces in the blade cross section

As a result, we obtain the blade flap equations. The integration over the length of the blade gives us the equation

With a trimmed helicopter flight, the flapping of the blades can be represented as a Fourier series. This uniquely determined by the blade flaps angular velocity

### 3.2 The fuselage aerodynamic loads

The fuselage aerodynamic loads depend on the angle of attack or the angle of sliding is evaluated by wind tunnel or calculation by CFD-method.

The angle of attack

The angle of sliding

In this case, the coefficients in equations (Eqs. (50) and (51)) are calculated with CFD-method and corrected by the results of the tests in the wind tunnel.

### 3.3 Trimming model

The helicopter trimming equations derived from the helicopter dynamics equations. Equating to zero the linear and angular accelerations of the helicopter

The state vector

## 4. Results

To illustrate the comparison of result, we distribute the induced velocity in the plane of the rotor disk and at a far distance from it (Figure 5). This confirms the results of the Momentum theory: the inductive velocities in the plane of the rotor disk are two times less than the inductive velocities at an infinite distance from it [5].

The tip vortices structure of the rotor shown in Figure 6. Comparison of the structure under the main rotor with the blade theory results shows an adequate behavior of the vortex surface in modeling.

Introducing the air flow configuration, we can see which areas of the helicopter are influenced by the induced flow and used in the analysis of information corresponding loads.

Figure 7 shows a comparison of the position of blade vortex theory (disorderly line) and disk theory (black line) for horizontal flight. The tip vortex is shown only from one blade, but the influence of all blades is taken into account.

The results of calculating the normal component of induced velocity were compared with experimental data for forward flight from 75 to 180 km/h. Comparison with the experiment gave good results (Figure 8). Experimental and calculated data have got an adequate correlation.

The application of the described method consists in forming a matrix of influence of the main and tail rotor for any group of points around the rotor. For example, a matrix of influence of the rotor on the fuselage, the matrix of influence of the main rotor on the tail rotor, the matrix of influence of the main rotor on the stabilizer, the matrix effect of the tail rotor on the main rotor, etc., the dimensions of the matrices depend on the selected number of design points on the stabilizer, fuselage, and tail rotor. In this case, these matrices can be formed taking into account the mutual influence of the main and tail rotors.

For a single arbitrary point, this will be a matrix-string for the main rotor and a matrix-string for the tail rotor. Multiplying the row by the column of known air circulations on the disk, we calculate the induced velocities at the selected point.

The values of the influence matrix elements depend on only the rotor geometric characteristics and the position of the selected point relative to the rotor and the angle of inclination

At different flight speeds, induced velocities have different effects on the tail rotor and stabilizer. In the figures, you can see that induced velocity have a special influence on the low flight speeds mode of

The horizontal and vertical components of induced velocity at a characteristic point in the tail rotor and stabilizer area significantly depend on the flight speed (Figures 7 and 9).

By drawing the configuration of the air flow, we can see which areas of the helicopter are affected by the induced flow. This allows us to correctly configure the loads calculating program.

Figure 10 shows the position of the tip vortex according to the blade theory (disordered line) and the disk theory (black line) for forward flight.

Comparison with the blade theory for calculating the position of the tip vortex shows that the method of discrete vortex cylinders gives satisfactory results. The angle of vortex cylinder

As the graphs show, calculating the position of the tip vortex by the method of discrete vortex cylinders gives satisfactory results.

Figure 11 shows the results of comparison of the normal component of the average induced velocity calculated in the cross section along the vane theory [3, 6] and the discrete vortex cylinder method with experiment [2]. Error experiment for flight speed

From these results, we can conclude that the greatest convergence with experimental method of discrete vortex cylinder has got in a cross section

## 5. Conclusion

In this chapter, the authors have developed a method of discrete vortex cylinders based on the Shaydakov’s disk vortex theory. The capabilities of the discrete vortex cylinder method are demonstrated using a helicopter balancing program based on a “semi-rigid” model of the main and tail rotors. Data from numerical calculations are proposed with experimental data from actual flights. Simulations have been conducted. We found the following:

The complex procedure for calculating inductive velocities at any point in the space around the rotor was reduced to the procedure of multiplying the row by column. In this case, the row of influence of discrete vortex cylinders coefficients is multiplied by the column of the corresponding vortex cylinders circulations.

The use of pre-calculated influence matrices makes it much easier to calculate circulations.

The results showed, repeating the results of the classical impulse rotor theory on hover mode in the plane of the rotor and in the wake far behind the rotor.

We showed the capabilities of the method in calculating the distribution of inductive speeds in the plane of the rotor and the space outside it, also in calculating the bevel of the flow in the stabilizer and tail rotor regions.

We confirmed the fine capabilities of the developed method in calculating inductive speeds at the low-speed horizontal flight mode.

To study the effectiveness of the method of discrete vortex cylinder embedded in the helicopter trimming computer program. The calculations are performed with the original data of the helicopter “ANSAT.” The program allows calculating the trimming characteristics of helicopter rotor: aerodynamic performance, the loading of the blades.

The method of discrete vortex cylinders is successfully used for calculating inductive velocities in the aeroelasticity blade model for ANSAT helicopter main rotor loads calculation (for example, see [7]).

## 6. Future work

Modeling of the earth effect using the method of vortices reflected from the earth’s surface.

Construct a discrete model of the fuselage using the CFD-method to study the effect of main rotor blowing with the tail rotor-induced velocity.

Refinement of the discrete cylinders method for calculating the vortex ring regime by the limiting of the vortex cylinder height.

Extension of the discrete vortex cylinder method to the nonlinear Shaidakov’s vortex theory.

Application of the discrete vortex cylinder method to solving helicopter take-off and landing problems.

Simulation of vortex formation when landing on a snow-covered and dusty surface.

Simulation of helicopter dynamics with rotor blades dynamics using the discrete vortex cylinders method.

### Notes

This work was partially supported by the grant in accordance with the Decree of the Government of the Russian Federation No. 220 “on attracting leading scientists to Russian educational institutions” (Agreement No. 11.G34.31.0038).

## Notations

induced-velocity calculated point

element of influence matrix

chord at the blade cross section

fuselage coefficients

section lift and drag coefficients

elementary sectional inertial forces

vortex element

elementary sectional aerodynamic forces

distance from disk to point A location plane

Shaidakov’s integrals for a semi-infinite

elementary oblique vortex cylinder

blade rotational inertia, first moment of inertia

helicopter moment of inertia

helicopter mass and weight

vortex elements location point

aerodynamic flap moment

flap hinge spring constant

flap hinge offset

coordinate systems

rotor radius

main, tile rotors, fuselage forces and moments

the cross-sectional radius

fuselage area and length

first moment of inertia

flight velocity

the induced velocity in hover (at the rotor disk, in the far wake)

related speeds,

rotor induced velocity in hovering (at the rotor disk, in the far wake)

rotor-induced velocity in

rotor-induced velocity in

fuselage attack angle

blade flap angle, Fourier series coefficients of

vortex element circulation, circulation intensity

the helicopter angles of pitch and roll

the inclination angle of the vortex cylinder is calculated by the Shaydakov’s transcendental formula (

rotor advance ratio

air density

main rotor controls

rotor rotational speed

discrete cylinder azimuth

blade azimuth

discrete vortex dimensions

direction of the vector

helicopter angular velocity