ANALYSIS OF THE PRE-ROTATION ENGINE LOADS IN THE AUTOGYRO

The paper presents the analyzes of the pre-rotation engine loads in the Taurus autogyro manufactured by Aviation Artur Trendak from Poland. Based on the NACA-9 H-12 airfoil characteristics of the drag coefficient, on which the rotor blade was made, forces acting on the rotor during pre-rotation have been calculated. The paper presents the characteristics of the drag coefficient as a function of angle of attack for Re = 1,800,000 and Re = 2,600,000. For the speed range of 0 to 400 rpm torque resulting from the drag forces and the power required to drive the rotor were calculated.


INTRODUCTION
Pre-rotation is the coupling of the engine which drives the pusher propeller and the main rotor.This combination is designed for the drive of the main rotor for particular rotational speeds to have a short start.During takeoff the main rotor of the gyroplane must achieve the required rotational speed at which the lift force generated enables its separation from the wall.The gyroplane main rotor acts thanks to the phenomenon of autorotation, which is self-propelled by an airfoil through it [1].The airflow is generated by the flight speed of the aircraft.If the gyrocopter is equipped with an additional pre-rotation system the distance that the aircraft must travel on the runway until its separation from the wall due to the increase in the lifting force is several times shorter than a takeoff without pre-rotation.Initial time of increasing the rotor and reached rotational speed have a significant impact on the time and length of a take-off road of a gyroplane [2].
The simplest type of the pre-rotation rotor system for gyrocopters is manual pre-rotation of the impeller.This method is used in amateur structures.A safe untwisting rotor achieves low speed, approx.4-10 rpm.Having such a small value of rotational speed, the gyroplane needs a long way to take off.Along this way, it is necessary to set the rotor at a high angle of attack in order to obtain parameters that are appropriate for takeoff conditions.Systems of pre-rotation use different solutions which include: This work is to check the possibility of using the pre-rotation system equipped with an electric motor.In the existing solutions of constructions, the rotor is driven from 0 rpm to approx.150-200 rpm.The optimum rotor speed reaches approx.40-60% of nominal speed of the main drive motor [2].

RESEARCH OBJECT
A gyroplane to which a system of a prerotation rotor is being developed is Taurus.Taurus is a multi-tasking gyroplane equipped with 3 seats (1 + 2) produced by Aviation Artur Trendak&Son [3]. Figure 1 shows the visualization of Taurus gyroplane.
It is a gyroplane with a curb weight of 285 kg and a maximum take-off weight of 600 kg.It is equipped with an internal combustion engine CA 912 ULT (RST) 135 hp.The diameter of its rotor is 8.8 m.The basic performance parameters of the Taurus are shown in Table 1.

DESIGN ASSUMPTIONS OF THE PRE-ROTATION SYSTEM
The system of pre-rotation should allow the gyroplane rotor to spin from 0 rpm to approx.250 rpm.It is also required to transfer the suitable power to the main rotor with respect to the reached revolution per minute.The gyroplane to which this pre-rotation system is dedicated has a two-blade main rotor with a diameter of 8.8 m.The real value of pre-rotation drive power must be sufficient to achieve the required speed in a relatively short period of time.The pre-rotation system should also have the lowest weight and dimensions possible not   The minimum length the landing distance 0 -5 m

Maximum ceiling flight 4000 m
Fuel consumption 25 l/h Endurance 4.5 h The maximum theoretical range 580 km to limit the gyroplane during flight.This pre-rotation system cannot interfere with the operation of the rotor.Figure 2 shows a diagram of the proposed pre-rotation system where the electric motor as a starter motor drives with a gear rotor.The starter is powered by electrical energy stored in the battery.
In this paper, our calculations are based on the NACA air-9 H-12 airfoil with a chord equal to 220 mm which is very popular in gyroplanes.Figure 3 depicts the applied airfoil with a rotor blade (4.4 m long), the construction of which is based on this airfoil.
Figures 4 and 5 show the characteristics of the applied airfoil drag coefficient as a function of angle of attack for Re = 2,600,000 and Re = 1,800,000.

DETERMINING THE FORCES ACTING ON THE ROTOR DURING PRE-ROTATION
The main elements of the gyroplane rotor are its blades.The rotor blades during prerotation are driven by the propulsion system.Due to the approved scope of work, it was decided to reduce these loads to those that have a direct effect on pre-rotation and the drive system.Therefore, our further calculations take into account only the load induced by the airflow, namely the drag force.Drag is the force that exerts air or other gas on the body.It results from the movement of the body relative to the air [7].Figure 6 shows the mass loads (centrifugal) and aerodynamic loads of the rotor blade.
In order to determine the power of pre-rotation needed to spin the rotor, the algorithm in Figure 7  (2) To determine the drag force of the resultant acting on the blade, the speed was determined at distances of 0.2 m along the entire length of the blade, i.e. from 0 to 4.4 m. Figure 8 shows a diagram illustrating the approach to calculations of linear velocities according to different lengths of the rotor blade.Linear velocity v assumes a linear relationship and takes the value of 115.19 m/s at a predefined required rotational speed n = 250 rpm.(3) Due to the lower Reynolds Number, drag coefficient Cx required to calculate the drag force is read from the drag force coefficient characteristic as a function of angle of attack for Re = 1,800,000 for the roughened surface of the leading edge and at an angle of attack equal to 2 .Cx for the above conditions is 0.009.The drag force is described in Equation 4.  in power value P depending on the blade radius R. The maximum deviation value by polynomial approximations of degree 3 is: -5.307854604817E-7, while the minimum deviation is: 5.98531739350993E-9.The total power resulting from the drive of a single blade rotor was calculated as the sum of the value of the power for the considered blade radii.
According to the above algorithm, the calculations for the speed range of 0 to 400 rpm are made.The dependence of the estimated power required to maintain a certain speed of rotation of the rotor in the TAURUS gyroplane depending on the value of the frequency of rotation is shown in Figure 14.

CONCLUSIONS
At a speed of n = 250 rpm, the power required to drive the single blade rotor is 2.75 kW or for the double blade rotor is 5.5 kW.The total torque for the same rotational speed is 210.23 Nm.This is the torque which drives a rotor under given conditions in the input.For the drive rotor powered by an electric motor or the internal combustion engine coupled to the rotor by gearing, the torque is smaller, which is related to a gear ratio.

Fig. 3 .Fig. 4 .Fig. 5 .
Fig. 3. NACA-9 H-12 airfoil and gyroplane rotor blade based on the NACA-9 H-12 airfoil was formulated.Angular velocity ω [rad/s] determined by substituting the value of rotational speed n [rpm] to Equation 1.It is assumed that the drive system should guarantee pre-rotation speed n = 250 rpm then: (1) In the next step, the linear velocity has been calculated at selected distances from the axis of rotation.This makes it possible to determine distribution of the drag force over the entire length of the blade with the same resolution as the designated speed.To calculate linear velocity v [m/s], Equation 2 is used.By substituting in Equation 2 as already calculated angular velocity and the corresponding values of radius R [m] obtained for R = 0.2 m, linear velocity equals to 5.24 [m/s].

Fig. 6 .Fig. 7 .
Fig. 6.Diagram of the mass and aerodynamic loads of the rotor blades

Fig. 10 .
Fig. 10.Reynolds Number for the individual measuring points

Fig. 12 . 13 .
Fig. 12. Results of the calculations of the torque generated by the rotor blade with α = 2 Fig.

Fig. 14 .
Fig. 14.Power to maintain a certain rotational speed of the TAURUS gyroplane rotor vs. rotational speed n [rpm]

Table 1 .
Basic performance of the Taurus[3]