On balancing the propeller of the aircraft in the field environment. Part 1

1. V.D. Feldman

Chief Technician of OU Vibromera

On balancing the propeller of the aircraft in the field environment

 “Propeller is the aircraft driver,

and to balance it can only a striver”

1. In lieu of a preface

Two and a half years ago, our company began mass production of the Balanset-1, intended for balancing rotor mechanisms in its own bearings.

To date, more than 180 sets have been produced, which are effectively used in various industries, including production and operation of fans, exhausters, electric motors, work spindles, pumps, crushers, separators, centrifuges, drive shafts and crankshafts and other mechanisms.

Recently, our company has received a large number of requests from organizations and individuals related to the possibility of using our equipment for balancing propellers of aircraft and helicopters in the field environment.

Unfortunately, our specialists, having great experience in balancing a wide variety of machines, have never dealt with this problem. Therefore, the pieces of advice and recommendations we could give to our Customers were general in nature and did not always enable them to effectively solve the problem.

This spring the situation began changing for the better due to the active stand of V. D. Chvokov, who organized and took, along with us, the most active part in the works on balancing propellers of YAK-52 and SU-29, of which he is a pilot.

Fig. 1.1. Yak-52 in the airfield

Fig. 1.2. SU-29 in the parking area

During this process, we have learned a certain skill and technology of balancing propellers of aircraft in the field environment using Balanset-1, including:

• determining the places and methods of installation (mounting) of vibration sensors and phase angle on the facility;
• determining resonant frequencies of a number of structural elements of the aircraft (engine suspension, propeller blade);
• identifying rotational speeds (modes of operation) of the engine, providing a minimum residual imbalance in the process of balancing;
• determining tolerances for residual imbalance of the propeller, etc.

In addition, we have obtained interesting data on the vibration levels of the aircraft equipped with M-14P engines.

Reporting materials are suggested below based on the results of these works.

Along with the results of balancing, they contain the data of vibration surveys of YAK-52 and SU-29 obtained during tests on the ground and in flight.

These data may be of interest, both for pilots of aircraft and for maintenance specialists.

2. The results of balancing the propeller and vibration testing of the aerobatic aircraft YAK-52

2.1. Introduction

In May-July 2014, we carried out the vibration testing of YAK-52 equipped with M-14P aircraft engine, as well as balancing its two-bladed propeller.

Balancing was performed in the same plane using the balancing set Balanset-1, plant No. 149.

The measurement scheme used in balancing is shown in Figure 2.1.

During the balancing process, the vibration sensor (accelerometer) 1 was mounted on the front cover of the engine gear using a magnet on a special bracket.

The laser sensor of the phase angle 2 was also mounted on the gear cover and was guided by a reflecting label applied to one of the propeller blades.

The analogue signals from the sensors were transmitted via cables to the measuring unit of the Balanset-1, in which their preliminary digital processing was performed.

Further, these signals in a digital form were transmitted to the computer, which processed them and calculated the mass and installation angle of the corrective weight required to compensate for the imbalance on the propeller.

Fig. 2.1. Measurement scheme for balancing propeller of YAK-52.

Zk – main gear wheel;

Zс − gear satellites;

Zn − fixed gear wheel.

In the course of this work, taking into account the experience in balancing propellers of SU-29 and YAK-52, we performed a number of additional studies, including:

• determining natural frequencies of oscillations of the engine and propeller of YAK-52;
• examining the value and spectral composition of vibration in the cockpit of the co-pilot in flight after balancing the propeller;
• examining the value and spectral composition of vibration in the cockpit of the co-pilot in flight after balancing the propeller and adjusting the tightening force of the engine damper.

2.2. The results of studies of the natural frequencies of the engine and propeller.

The natural frequencies of the engine mounted on the dampers in the body of the aircraft were determined using the spectrum analyzer AD-3527, f. A @ D, (Japan), by shock excitation of engine oscillations.

We determined 4 main frequencies, namely: 20 Hz, 74Hz, 94 Hz, 120 Hz in the spectrum of natural oscillations of the engine suspension of YAK-52, an example of which is shown in Fig. 2.2.

Fig. 2.2. The spectrum of natural frequencies of oscillations of the engine suspension of YAK-52

Frequencies of 74Hz, 94Hz, 120Hz are probably associated with the features of mounting (suspension) of the engine onto the body of the aircraft.

The frequency of 20Hz is most likely associated with the oscillations of the aircraft on the chassis.

The natural frequencies of oscillation of the propeller blades were also determined by the shock excitation method.

In this case, we revealed four main frequencies, namely: 36Hz, 80Hz, 104Hz and 134Hz.

The data on the natural frequencies of oscillations of the propeller and engine of YAK-52 may be primarily important when choosing the propeller rotation frequency used in balancing. The main condition for choosing this frequency is to ensure its maximum possible detuning from the natural frequencies of oscillations of the structural elements of the aircraft.

In addition, knowledge of the natural frequencies of oscillations of individual components and parts of the aircraft can be useful for identifying the reasons for a sharp increase (in the case of resonance) of certain components of the vibration spectrum at various engine speeds.

2.3. Balancing results.

As we noted above, the propeller was balanced in the same plane, as a result of which compensation was provided for the power imbalance of the propeller in dynamics.

Dynamic balancing in two planes was not possible, which enables (in addition to the force one) to compensate for the moment imbalance of the propeller, since the design of the propeller mounted on YAK-52 makes it possible to form only one correction plane.

The propeller was balanced at a frequency of its rotation equal to 1,150 rpm (60%), at which it was possible to obtain the most stable results of vibration measurement in amplitude and phase from start to start.

The propeller was balanced according to the classical “two start” scheme.

During the first start-up, we determined the amplitude and phase of vibration at the frequency of the propeller rotation in the initial state.

During the second start-up, we determined the amplitude and phase of the vibration at the frequency of the propeller rotation after fixing the proof mass equal to 7g.

Taking into account these data, we calculated the mass M = 19.5g in a programmatic manner and the installation angle of the correction weight F = 32.

Taking into account the design features of the propeller, which do not allow to place the correction weight at the required angle, two equivalent weights are fixed on the propeller, including:

• M1 weight = 14g on the angle F1 = 0º;
• M1 weight = 8.3g on the angle F1 = 60º.

After the above corrective weights were placed the propeller, the vibration measured at a rotational speed of 1,150 rpm and associated with the propeller imbalance decreased from 10.2 mm/s in the initial state to 4.2 mm/s after balancing.

At the same time, the actual imbalance of the propeller decreased from 2,340 g*mm to 963 g*mm.

2.4. Testing the effect of balancing on the vibration level of YAK-52 on the ground at different propeller speeds

Table 2.1 suggests the results of the vibration test of YAK-52, performed in other engine operating conditions obtained during tests on the ground.

As the table shows, balancing had a positive effect on the vibration of YAK-52 in all modes of its operation.

Table 2.1

Moreover, during testing on the ground, a tendency was revealed to significantly reduce the vibration of an aircraft with an increased rotation rate of its propeller.

This phenomenon can be explained by a greater degree of detuning of the rotation speed of the propeller from the natural oscillation frequency of the aircraft on the chassis (presumably 20Hz), which occurs with the increased rotation speed of the propeller.

2.5. Examining the vibration of YAK-52 in the air in the main flight modes before and after adjusting the tightening force of dampers

In addition to the vibration tests carried out after balancing the propeller on the ground (see Section 2.3) we made measurements of the vibration of YAK-52 in flight.

Vibration in flight was measured in the cockpit of the co-pilot in the vertical direction using a portable vibration spectrum analyzer AD-3527 f. A@D (Japan) within the frequency range from 5 to 200 (500)Hz.

The measurements were carried out at five main engine speeds equal to 60%, 65%, 70% and 82% of its maximum rotation speed, respectively.

The results of measurements made before adjusting the dampers are shown in Table 2.2.

Table 2.2

As an example, Fig. 2.3 and 2.4 show graphs of the spectra obtained when measuring vibration in the cockpit of YAK-52 in the modes of 60% and 94% and used when filling in table 2.2.

Fig.2.3. The vibration spectrum in the cockpit of YAK-52 at 60%.

Figure 2.4. The vibration spectrum in the cockpit of YAK-52 at 94%.

As Table 2.2 shows, the main components of vibration, measured in the cockpit of the co-pilot, appear at propeller rotation speeds Vv1 (highlighted in yellow), engine crankshaft Vk1 (highlighted in blue) and air compressor drive gear (and/or frequency sensor) Vn ( highlighted in green), as well as at their higher harmonics Vv2, Vv4, Vv5 and Vk2, Vk3.

The maximum total vibration Vå  was detected at speeds of 82% (1,580 rpm of the propeller) and 94% (1,830 rpm).

The main component of this vibration manifests itself at the 2^nd harmonic of the engine crankshaft speed Vk2 and accordingly reaches the values of 12.5 mm/s at a frequency of 4800 cycles/min and 15.8 mm/s at a frequency of 5,520 cycles/min.

It can be assumed that this component is associated with the operation of the engine piston block (shock processes when the pistons are repositioned twice during one revolution of the crankshaft).

The sharp increase in this component in the 82% (first nominal) and 94% (take-off) modes is most likely caused not by the piston group defects, but by resonant oscillations on the engine fixed in the aircraft body on the damper.

This conclusion is confirmed by the above results of experimental verification of the natural frequencies of oscillations of the engine suspension, in the spectrum of which there are 74Hz (4,440 cycles/min), 94Hz (5640 cycles/min) and 120Hz (7,200 cycles/min).

Two of these natural frequencies, equal to 74 and 94 Hz, are close to the frequencies of the 2^nd harmonic of the crankshaft rotation speed, which take place in the first nominal and take-off ratings of engine operation.

Due to the fact that during the vibration tests we revealed significant vibrations at the 2^nd harmonic of the crankshaft in the first nominal and take-off ratings of the engine, an effort was made to check and adjust the tightening force of the engine suspension dampers.

Table 2.3 shows the comparative test results obtained before and after adjusting the dampers for the propeller rotation speed (Vv1) and the 2^nd harmonic of the crankshaft rotation frequency (Vk2).

Table 2.3

As we can see from table 2.3, adjustment of the dampers did not lead to significant changes in the values of the main components of the aircraft vibration.

Taking into account the above, it is possible to consider a noticeable increase in the component of vibration of YAK-52 in the first nominal and take-off modes (in our opinion) as a constructive miscalculation of the aircraft designers, made when choosing an engine mounting system (suspension) in the aircraft body.

In this regard, we should note that the amplitude of the spectral component associated with the imbalance of the propeller Vv1, detected in the 82% modes and 94% (see Tables 1.2 and 1.3), respectively, 3–7 times lower than the amplitudes Vk2 in these modes.

In other flight modes, the Vv1 component is within 2.8-4.4 mm/s.

Moreover, as Tables 2.2 and 2.3 show, during the transition from one mode to another its changes are mainly determined not by the quality of balancing, but by the degree of detuning of the propeller rotation speed from the natural frequencies of oscillations of certain structural elements of the aircraft.

2.6. Conclusions on the results of the work

2.6.1. Balancing the propeller of YAK-52, carried out at a rotation frequency of 1150 rpm (60%), made it possible to reduce the propeller vibration of 10.2 mm/s to 4.2 mm/s.

Taking into account certain experience gained in the process of balancing propellers of YAK-52 and SU-29 using the Balanset-1, we can assume that there is a possibility of further reducing the vibration level of the propeller of YAK-52.

This effect can be achieved, in particular, by selecting a different (higher) rotation frequency of the propeller during its balancing, which allows a greater degree of detachment from the natural the aircraft oscillation frequency of 20 Hz (1,200 cycles/min) detected during the test.

2.6.2. As the results of vibration tests of YAK-52 in flight show, its vibration spectra (besides the component mentioned above in paragraph 2.6.1, which appears at the propeller rotation frequency), have a number of other components related to the operation of the crankshaft, piston group of the engine, and also drive gear of the air compressor (and/or frequency sensor).

The values of the above vibrations in the modes of 60%, 65% and 70% are proportionate to the value of the vibration, which is associated with the propeller imbalance.

Analysis of these vibrations shows that even the complete elimination of vibration from the imbalance of the propeller will reduce the total vibration of the aircraft in these modes by no more than 1.5 times.

2.6.3. The maximum total vibration Vå of YAK-52 was detected in high-speed modes, namely: 82% (1,580 rpm of the propeller) and 94% (1,830 rpm of the propeller).

The main component of this vibration manifests itself at the 2^nd harmonic of the engine crankshaft rotation frequency Vk2 (at frequencies of 4,800 cycles/min or 5,520 cycles/min), at which respectively reaches values of 12.5 mm/s and 15.8 mm/s.

It can be assumed that this component is related to the operation of the engine piston group (shock processes arising when the pistons are repositioned twice during one revolution of the crankshaft).

The sharp increase in this component in the modes of 82% (first nominal), and 94% (take-off) is most likely caused not by defects in the piston group, but by resonant oscillations on the engine, fixed in the aircraft body on dampers.

During the tests adjustment of the dampers did not lead to significant changes in vibration.

This situation can be considered as a constructive miscalculation of the aircraft designers, made when choosing the mounting system (suspension) of the engine in the aircraft body.

2.6.4. The data obtained during balancing and additional vibration tests (see the results of flight tests in section 2.5) allow us to conclude that periodic vibration monitoring may be useful for diagnostic evaluation of the technical condition of an aircraft engine.

Such procedure can be performed, for example, using the Balanset-1, the software of which implements the function of spectral vibration analysis.