WO2025243111A1

WO2025243111A1 - Device and method for monitoring an airplane propulsion system

Info

Publication number: WO2025243111A1
Application number: PCT/IB2025/053825
Authority: WO
Inventors: Vito TEDESCHI
Original assignee: Intuos Srl
Current assignee: Intuos Srl
Priority date: 2024-05-22
Filing date: 2025-04-11
Publication date: 2025-11-27
Anticipated expiration: 2026-11-22

Abstract

The invention concerns a method for monitoring an airplane propulsion system, the propulsion system comprising at least one piston engine and a propeller. The method provides for acquiring an audio signal from inside an airplane cockpit, selecting the frequency with maximum amplitude of the audio signal, calculating the engine revolutions r as (I) Where _-r indicates the engine revolutions per minute, p indicates the value of the maximum frequency and vindicates the number of detonations in the engine necessary to perform an engine revolution, k is a parameter dependent on the ratio between engine revolutions and propeller revolutions and is equal to 1 in the case of a propeller keyed onto a shaft of the engine.

Description

DEVICE AND METHOD FOR MONITORING AN AIRPLANE PROPULSION SYSTEM

TECHNICAL FIELD

The present invention refers to a device and a related method for monitoring an airplane propulsion system, in particular a propeller propulsion system.

STATE OF THE ART

Aircraft, in particular with propeller propulsion system, must be checked periodically to carry out operations of preventive maintenance which is necessary to ensure the good operating condition of the aircraft itself. A failure during flight of the propulsion system can, in fact, be fatal.

To monitor the operating status of the propulsion systems, and in particular of the engines, it is known to equip aircraft with engine monitoring systems that provide for installing sensors on the engine or in the engine compartment. The sensors are then wired to the dashboard where the pilot can check some operating parameters.

The engine monitoring systems are therefore designed as an integral part of the aircraft and are difficult to improve if not with difficulties related, for example, to the need to install new sensors and certify the new system.

The data acquired by the sensors can be stored in the flight data recorder (FDR) of the black box and downloaded in the event of an accident. However, in the event of an accident, if the black box is not recovered, the data are also lost.

To overcome this problem and collect real-time data on the aircraft, Inmarsat has proposed a new solution - called SB-S (SwiftBroadband-Safety) - which provides for the real-time transmission of a large multitude of data thanks to a satellite link, downloading the parameters normally stored by the black box to the cloud. The availability of these parameters allows to manage minor aircraft problems before they become major. The SB-S system, however, also requires many sensors and integration with the aircraft's flight system.

Such complex monitoring systems are also very expensive and are not suitable for use on smaller aircraft with propeller propulsion systems. The need is therefore felt for a new system for monitoring an aircraft propulsion system that requires a few sensors and is easily upgradable to monitor new parameters of the engine itself.

US 5 635 646 describes a method for measuring the rotational speed of a turbocharger of an engine. A microphone captures the sound of the engine, and the signal is filtered to eliminate low-frequency components. An electronic bandpass filter selects the component of the signal with the greatest amplitude, whose frequency is converted into the rotational speed of the turbocharger thanks to the knowledge of its geometry.

CN 115 270 058 describes a method for monitoring the propulsion system of an airplane based on sound signals.

US 2011/246000 Al discloses a system wherein the processing unit is configured to determine the number of revolutions of an aircraft engine based on audio data samples of the ambient sounds of the cockpit.

FR 2 577 673 Al discloses a device for measuring the speed of an engine using a microphone.

OBJECTS AND SUMMARY OF THE INVENTION

Object of the present invention is therefore to overcome at least partially the drawbacks highlighted above in relation to the prior art.

In particular, it is an object of the present invention to present a device for monitoring a propulsion system that does not require the wiring of numerous sensors.

It is also an object of the present invention to present a device for monitoring a propulsion system that can be easily updated with new monitoring functions.

These and other objects and tasks of the present invention are achieved by a system and method for monitoring an airplane propulsion system in accordance with the appended claims that form an integral part of the present description.

In accordance with a first aspect, the invention concerns a method for monitoring an airplane propulsion system, the propulsion system comprising at least one piston engine and a propeller. The method provides for acquiring an audio signal from inside an airplane cockpit, selecting the frequency with maximum amplitude of the audio signal, calculating the engine revolutions r as r = 60 * - */< c

Where rindicates the engine revolutions per minute, p indicates the value of the maximum frequency and c indicates the number of detonations in the engine necessary to perform an engine revolution, k is a parameter dependent on the ratio between engine revolutions and propeller revolutions. The parameter k is 1 if there is no gearbox interposed between the engine and the propeller; otherwise the parameter will assume the rpm reduction value proper to the engine-propeller assembly.

This solution is particularly advantageous as it allows monitoring the revolutions of a piston engine in a propeller powertrain without the need to wire sensors and without the need to access the engine compartment. The engine revolutions are calculated from an audio signal acquired using a microphone that can be positioned in a simple and flexible way inside the cockpit.

In one embodiment, in order to select the frequency with maximum amplitude of the audio signal, the audio signal is digitized, the digitized audio signal is filtered by a band-pass filter, and the frequency having the maximum amplitude of the filtered signal is selected.

By digitizing and filtering the audio signal, it is possible to reduce the size of the data that must be processed to calculate the engine revolutions.

In one embodiment of the monitoring method, a first engine speed calculation is made from a first audio signal acquired (instant to) in the cockpit and, subsequent to the first calculation, a second engine speed calculation is made from a second audio signal acquired (instant ti > to) in the cockpit. For the second calculation of engine revolutions, a band-pass filter centred on the frequency with maximum amplitude selected during the first calculation of engine revolutions is used.

This solution allows to track the frequency with maximum amplitude and concentrate the filtering around said frequency to reduce the effects of noise in the calculation of engine revolutions, thus obtaining a more accurate measurement. Preferably, then, the band-pass filter chosen for the second calculation of engine revolutions has a narrower bandwidth compared to the band-pass filter used for the first calculation. This allows a first broadband filter to be used to determine the frequency with maximum amplitude, and then to focus the analysis of the subsequent audio samples around that frequency with maximum amplitude.

Preferably, the band-pass filter chosen for the second calculation has a bandwidth ranging between 10Hz to 30Hz (preferably settable during setup), while the band-pass filter chosen for the first calculation has a lower cutoff frequency of 10Hz and an upper cutoff frequency of 200Hz. These frequency bands were particularly effective in tracking the base frequencies generated by a propeller propulsion system with a piston engine.

Advantageously, the digitized signal is obtained with a sampling frequency greater than or equal to 1000 Hz.

In a preferred embodiment, then, the method provides for transmitting to a ground control centre data relating to the calculated engine revolutions. This allows a monitoring of the operation of the aircraft from the ground, thus offering the possibility to collect data for the maintenance of the aircraft itself.

According to a further aspect, the invention is directed to a device for monitoring a propulsion system comprising at least one piston engine and a propeller. The device comprises a microphone for acquiring an audio signal, and a control unit operatively connected to the microphone to receive an audio signal from the microphone. The control unit is configured to implement a method as described above and as further detailed in the description below.

Further features and purposes of the present invention will become more evident from the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described hereinbelow with reference to some examples, provided by way of non-limiting example, and illustrated in the appended drawings. These drawings illustrate different aspects and embodiments of the present invention and, where appropriate, similar structures, components, materials and/ or elements in different figures are indicated by similar reference numerals. Moreover, for clarity of illustration, some references may not be repeated in all drawings.

Figure 1 illustrates an airplane with propeller propulsion system and view of the inside of the cockpit;

Figure 2 illustrates a block diagram of a device for monitoring an airplane propulsion system used on the airplane of Figure 1; and

Figure 3 illustrates a flowchart of a method for monitoring an airplane propulsion system.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible to various modifications and alternative constructions, some preferred embodiments are shown in the drawings and are described hereinbelow in detail. It must in any case be understood that there is no intention to limit the invention to the specific embodiment illustrated, but, on the contrary, the invention intends to cover all solutions falling within the scope of protection as defined in the claims.

The description deals in detail with the peculiar aspects and the technical characteristics of the invention, while the aspects and the technical characteristics per se known can only be hinted at. In these respects, the foregoing remains valid with reference to the prior art.

The use of "for example", "etc.", "or" indicates non-exclusive alternatives without limitation, unless otherwise indicated. The use of "comprises" means "includes, but not limited to" unless otherwise indicated.

In the following description, the propulsion system of an aircraft is intended to refer to the system that imparts movement to the aircraft itself. For example, a propeller propulsion system comprises the propeller and the engine that rotates it.

The term engine revolution is intended to refer to a rotation of the engine shaft.

With reference to figure 1, an airplane 1 with propeller propulsion system is shown. For clarity of presentation, the aircraft is shown without a part of the fuselage to allow to see the interior of the cockpit and the engine compartment. In a manner known per se, the airplane comprises an engine compartment 2 in which a piston engine 3 is positioned which drives a propeller 4 which, in the example illustrated in Figure 1, comprises two blades 5. The number of blades, as well as the number of engines, may vary with respect to the example illustrated herein.

The airplane 1 then comprises a cockpit 6 with at least one seat 7 positioned in front of the dashboard 8. In one embodiment, the cockpit 6 is separated from the engine compartment 2.

A device 9 for monitoring the propulsion system of the aircraft is positioned inside the cockpit 6. A block diagram of the device 9 is illustrated in Figure 2.

The device 9 comprises at least one microphone 90 operatively connected to a control unit 91. The microphone 90 may be a directional microphone or a microphone array, while the control unit may comprise one or more processors and/ or microcontrollers.

The microphone 90 acquires the sounds of the cockpit and, in a manner known per se, transforms the sound signal (a pressure wave) into an analogue voltage or current audio signal. The analogue voltage or current signal is digitized by a digital analogue converter that samples the analogue signal with a predetermined sampling rate, e.g., 8000 samples per second. The microphone then provides the control unit 91 with a digital signal obtained by sampling the analogue signal at a first sampling frequency.

Alternatively, the microphone 90 may provide in output the analogue voltage or current signal, in which case the control unit 91 will be provided with an analogue input and with a digital analogue converter capable of digitizing the analogue signal.

The digitized audio signal is therefore analysed by the control unit 91 to determine the revolutions (RPM) of the engine 2. In one embodiment the digitized signal is analysed in real time; therefore, the control unit 91 comprises an internal memory area 910, in particular a log, where the digitized signal is temporarily stored for the time necessary for its processing. Alternatively, the digitized signal may be stored in an external storage area 92 so that it can be downloaded at a later time when the aircraft is on the ground. In order to determine the revolutions of the engine 2, the control unit 91 is configured to implement the method described below with reference to Figure 3. More precisely, the control unit 91 is configured to execute portions of code, which are stored in a memory unit accessible to the control unit (for example the memories 910 or 92) and which, when executed, allow the device 9 to execute the method of figure 3.

If the digitized audio signal is sampled with a very high sampling rate, for example higher than 1000 samples per second, then the control unit 91 proceeds (step 301) to decimation of the digital signal by reducing the number of samples to a predefined value, preferably equal to 1000 samples per second. A higher number of samples may be useful if it is necessary to detect a number of engine revolutions higher than 3000 rpm. This allows to reduce the size of the audio portion to be analysed in order to speed up the subsequent process of identification of the frequency peak associated with the operation of the engine.

The decimation of the samples can be done in different ways, for example by selecting, on a regular basis, a fraction of the incoming samples. For example, if the digitized audio signal is sampled at a microphone native sampling rate of 8kHz, and it is wished to reduce the samples to 1000 samples/ second (i.e. a sampling rate of 1kHz), then one sample every eight received will be taken for decimation.

Alternatively, for decimation the control unit 91 can reconstruct an analogue audio signal from the digital one and proceed to a new sampling - with a desired sampling frequency, e.g. 1kHz, of the reconstructed analogue signal.

After any decimation, the method for determining engine revolutions provides for identifying the frequency peak that represents the fundamental frequency emitted by the propeller of the engine of the aircraft. This peak frequency, in the tests conducted, was less than 500 Hz and typically ranging from 80 Hz to 200 Hz, hence the choice of a sampling frequency at step 301 greater than or equal to 1000 Hz.

To proceed with the process of identifying the frequency peak, the method first provides for checking (step 302) whether it is the first frequency peak that is identified.

In case of first identification of the frequency peak, the method provides for filtering (step 303) the decimated digital signal using a first band-pass filter with a cutoff frequency equal to 10 Hz and an upper cutoff frequency higher than or equal to 200 Hz, but preferably lower than 500 Hz. For example, the filter could be a fourth order band-pass filter of type IIR.

After filtering, the frequency spectrum of the decimated and filtered digital signal is calculated (step 304). Such a spectrum can be calculated, in a manner known per se, using an FFT (Fast Fourier Transform).

Subsequently (step 305) the modulus of the frequency spectrum thus calculated is calculated to then identify the frequency value associated with the highest peak (step 306).

Once this frequency peak is identified, the number of rpm (revolutions per minutes) of the engine is calculated (step 307) using the formula

Where r indicates the engine revolutions per minute (rpm), p indicates the value of the peak frequency and c the number of detonations necessary to perform an engine revolution. For example, in a four-stroke, four-cylinder piston engine, the number of detonations necessary to perform one engine revolution is equal to two. This formula was experimentally very accurate for single-engine airplanes with propeller keyed directly on the engine shaft.

In some aircraft, automotive-derived engines (e.g. Rotax or Mercedes engines) are mounted which achieve a high number of revolutions. In this case the propeller is connected to the engine through a gearbox to prevent the end of the propeller blade from reaching too high speeds, e.g. supersonic.

The above-reported formula can be generalized so that it can be applied also to aircraft with a propeller connected to the engine through a gearbox; in this case, the generalized formula is as follows:

Where k is a parameter that takes into account the reduction factor between the revolutions of the engine and those of the propeller; k will be equal to 1 in the case of propeller keyed on the engine.

Subsequently, the control unit checks whether an instruction to interrupt the calculation process has arrived (step 308) and in the positive case it ends it (step 309). Differently, the control unit continues the process of monitoring the propulsion system by calculating again the engine revolutions. In this case, the method (step 309) preferably provides for filtering the decimated digital signal with a second band-pass filter centred on the frequency peak determined at the previous step. This second band-pass filter has a smaller band-pass compared to that of the band-pass filter used in the first calculation of the engine revolutions. In particular, the second band-pass filter is chosen with bandwidth b preferably ranging from 10Hz to 30Hz, so that the cutoff frequencies will be: and i fH = P + ^b 2

Where ft indicates the lower cutoff frequency, fa the upper cutoff frequency, p the peak frequency, b the filter bandwidth.

In a preferred embodiment, periodically, or continuously, the control unit transmits the data relative to the calculated engine revolutions to a ground collection centre where the data can be further processed. For the transmission of these data, the device 9 is then provided with a transmission unit 92 operatively connected to the control unit 91. The transmission unit 92 is a wireless transmission unit, preferably a satellite transmission unit.

In one embodiment, the transmission unit 92 can be replaced by a transceiver unit that allows not only the transmission of data from the device 9 towards the ground control centre, but also allows data to be received from the control centre. In this embodiment, therefore, the control unit may be configured to receive instructions from the ground control centre to modify parameters of the engine revolution calculation process (e.g. change the bandwidth of the filters) and/ or to modify the frequency of transmission of the data relative to the engine revolutions from the device 9 towards the ground control centre. For example, the ground control centre may request the device 9 to transmit the data continuously or every z seconds, with z being any integer.

In light of the description above, it is clear how the device for monitoring a propulsion system described above allows the objects set to be achieved. The monitoring of the propulsion system (in the case of the engine revolutions described above) is done using a microphone (or a microphone array) that does not need to be placed in the engine compartment and does not require the wiring of sensors or integration with the avionics of the aircraft. The device can be easily improved and integrated with new monitoring functions of the propulsion system without the need to certify again the entire aircraft; in fact, it will be enough to load new monitoring functions into the memory of the device that allow new parameters to be derived from the analysis of the acquired audio signal.

It is therefore clear that the person skilled in the art will be able to make changes to the device described above. For example, instead of changing filter for identifying the peak frequency as described above with reference to steps 302, 303 and 309, a single filter suitably chosen after experimental testing may be used.

Claims

1. Method for monitoring an airplane propulsion system, the propulsion system comprising at least one piston engine (3) and a propeller (4), comprising the steps of

- acquiring an audio signal from inside an airplane cockpit,

- selecting (306) the frequency with the maximum amplitude of the audio signal, characterised by

- calculating (307) the engine revolutions r as r = 60 * - * k c

Where r indicates the engine revolutions per minute, p indicates the value of the maximum frequency and c indicates the number of detonations in the engine necessary to perform an engine revolution, k is a parameter dependent on the ratio between engine revolutions and propeller revolutions and is equal to 1 in the case of a propeller keyed onto an engine shaft.

2. Method according to claim 1, wherein to select the frequency with maximum amplitude of the audio signal the following steps are performed:

- digitization of the audio signal,

- filtering the audio signal (303,310) using a band-pass filter,

- selection (306) of the frequency with maximum amplitude.

3. Method according to claim 2, wherein a first calculation of engine revolutions is made from a first audio signal acquired in the cockpit and wherein, subsequent to the first calculation, a second calculation of engine revolutions is made from a second audio signal acquired in the cockpit, and wherein for the second calculation of engine revolutions a band-pass filter centred on the frequency with maximum amplitude selected during the first calculation of engine revolutions is used (310).

4. Method according to claim 3, wherein the band-pass filter chosen for the second calculation of engine revolutions has a narrower bandwidth compared to the band-pass filter used for the first calculation.

5. Method according to any one of claims 3 to 4, wherein the band-pass filter chosen for the second calculation has a passband width ranging from 10Hz to 30Hz.

6. Method according to any one of claims 3 to 5, wherein the band-pass filter chosen for the first calculation has a lower cutoff frequency of 10Hz and an upper cutoff frequency of 200Hz.

7. Method according to any one of claims 2 to 6, wherein the digitized signal is obtained with a sampling frequency greater than or equal to 1000 Hz.

8. Method according to any one of the preceding claims, further comprising the step of transmitting to a ground control centre data relating to the calculated engine revolutions.

9. Device (9) for monitoring a propulsion system comprising at least one piston engine (3) and a propeller (4), comprising

- a microphone (90) for acquiring an audio signal,

- a control unit (91) operatively connected to the microphone (90) to receive the audio signal from the microphone (90), and wherein the control unit is configured to implement a method according to any one of claims 1 to 8.