DE3736141A1 - AIRPLANE PROPELLER - Google Patents

Abstract

A bladed rotor for an aircraft or fan comprises pairs 4, 6; 8, 10 of diametrically-opposed blades, the pairs being arranged at a spacing angle SIGMA of between 15 DEG and 50 DEG . The pairs may be axially spaced, with the rear one of two adjacent blades leading in the direction of rotation. The optimal spacing angle may be calculated from a formula involving the speed of revolution and sound frequency so as to minimise sound emission in use. <IMAGE>

Description

The invention relates to aircraft propellers according to the upper Concept of claim 1.

As a drive for aircraft at cruising speeds below The propeller drive dominates at around 300 km / h. In this air vehicles in the military field are transport planes and in the civilian area for sports and business travel aircraft whose noise emissions - not least due to the large number of flight movements - by the population in the Airfield environment is considered a nuisance.  

The noise emission from smaller propeller aircraft general aviation is largely based on the propeller noise. The noise emission is in particular from depending on the Mach tip number.

So far, efforts have been made to reduce the noise emission by ver reduction in the Mach tip number. A ver Reduction of Mach's tip number is inevitable a loss of thrust. So there are additional ones Take measures to compensate for this loss of thrust.

So it is known, the propeller noise in aircraft of all to reduce common aviation by reducing the speed (B. Bredow, "Quiet drive for general aviation", Bonn: BMFT, final report 1976). Conditional to maintain thrust this, however, an increase in the diameter of the propeller and Use of reduction gears. A bigger propeller diameter in many cases requires compliance with the required ground clearance for the propeller a higher driving plant. Such an increase in the chassis leads together with the necessary transmission gear and the larger propeller a significant increase in mass for the aircraft. That way is therefore only feasible if the required freedom for the larger propeller diameter is available and in any case Reduction gears are provided for the propeller (Z. "VDI-Nachrichten", No. 12, p. 42 / March 20, 1987).

With directly driven propellers, the blade tip mach number only reduced by reducing the propeller diameter will. The propeller must then maintain the thrust Blade profile can be modified accordingly or the propeller sheet with buoyancy aids to boost the thrust receive.  

In summary, it can be said that the way, via a decrease in the Mach tip number to a decrease the noise emission to get essential Make changes to the propeller. In particular an order Arming existing aircraft is therefore very expensive.

The aim of the invention is a significant reduction in propeller noise to achieve without changes in the drive and the Pro peller blade geometry are required and without a thrust reduction occurs.

This object is achieved according to the invention by the in drawing part of claim 1 exposed note times.

Appropriate configurations are the subject of the subclaims.

The particular advantage of the invention is that a German possible, albeit limited, reduction in propeller noise is standard with propellers for the aircraft to be converted are. So no new propeller profiles are necessary. The Mach's blade tip number remains unchanged and therefore the same thrust achievable by the propellers. As mentioned, this is achieved limited reduction in propeller noise. However, it is noise reductions of up to 4 dB (A) can be expected. This means a right one significantly reduce the noise pollution of the population a reasonable effort for the aircraft owner.

The invention is illustrated in the drawing, for example and described in detail below with reference to the drawing. In this shows

Fig. 1 shows a noise reducing arrangement of propeller blade couples the example of a four-bladed propeller;

Fig. 2 is a diagram showing the calculated dependency of the optimum Blatteilungswinkels of the helical blade tip Mach number for a four-bladed propeller;

Figure 3 is a graph of the calculated noise reduction blade propeller in dependence on the Blatteilungswinkel in a four for a helical blade tip Mach number of 0.5.

FIG. 4, in comparison, schematic diagrams of the un-weighted and the A-weighted spectrum of known propellers and of propellers according to the invention;

Fig. 5 lying behind each other in elevation and top view of an arrangement of two in the direction of flight two-blade propellers with an inventive pitch angle;

Fig. 6 measured narrowband level spectra of the noise radiation in the plane of rotation of model propellers.

In recent years, it's not just experimental Possibilities of noise measurement and analysis, but in particular also the possibilities of a theoretical treatment of the Propeller noise emissions have been significantly improved. To Time is therefore the noise emission from propellers that with blade tip Mach numbers below approx. 0.8 in undisturbed, ax parallel inflow can be operated with satisfactory Calculate accuracy (Aircraft Noise Prediction Program - Theoretical Manual, Propeller Aerodynamics and Noise; NASA Technical Memorandum 83 199, Part 3, Hampton / Virginia 1986).  

This shows that the helical Mach tip number (the vector sum related to the local speed of sound Inflow and peripheral speed of the blade tip) Sound generation is the dominant parameter overall. Farther the emitted noise level increases with increasing aero dynamic sheet loading and with the sheet thickness. Profile and Sheet outline shape, however, influence the noise emission in the essential only with high helical blade tip Mach numbers, d. H. those over 0.7.

Measures to reduce noise therefore include, if specified Thrust the use of sheet profiles as thin as possible (with round ones Leaf tips) and as the most important criterion a limitation the blade tip mach number.

For noise testing of propeller aircraft according to the Noise protection requirement for aircraft (LSL) is the maximum A-weighted overall sound pressure level when flying over a measurement microphone the specific unit of measurement. Regarding this level shows a tendency to reduce noise when using Propellers with increasing number of blades and larger diameter the same blade tip Mach number and the same thrust, if applicable a Mach-tip number of about 0.5 is not significantly below is below which the propeller broadband noise Propeller rotary sound covered.

The propeller 2 shown in Fig. 1 has four propeller blades 4, 6, 8, 10 , of which two propeller blades 4 and 6 or 8 and 10 are diametrically opposed to each other, that is, include the angle of 180 °. In a known four-bladed propeller, the individual propellers have the same pitch angle ε , namely a pitch angle ε of 90 °. According to the invention, the two pairs of propeller blades have a pitch angle ε which is less than 50 °, preferably less than 45 °. The optimal pitch angle e depends on the helical blade tip Mach number, which is determined by the propeller diameter and the propeller speed and the flight speed and which should remain essentially unchanged according to the invention compared to a normal propeller drive. The value of the optimal angle ε is to be set smaller, the higher the helical blade tip Mach number M. For example, angles of e ≈ 20 ° for M ∼ 0.7 or ε ≈ 40 ° for M ∼ 0.5 are typical. Small deviations from the optimal pitch angle ε in the order of approximately ± 5 ° have no significant influence on the noise reduction that can be achieved. They lie essentially in the range of fluctuation of the effective Mach-tip number under the various operating states. The pitch angle ε can be optimized in each case in accordance with the operating state of the propeller drive, in which the maximum noise reduction is required. This can be, for example, the start or the climb after the start, at which the maximum number of blade tips Mach is normally reached.

The teaching of the invention is based on the fact that sound pressure signal of a multi-blade propeller through - according to the Number of sheets and the azimuthal sheet position out of phase - multiple superimposition of the sound pressure time signal of a single sheet can be generated with maximum sound pressure amplitudes occur in the time interval in which a sheet is moving towards the observer. Through temporal phases displacement of those emitted during one revolution of the propeller Single sheet sound pressure signals against each other are in the sound pressure level spectrum generates interference minima such that the Frequency of the first minimum with the frequency of the spectral Maximum of the A-weighted level spectrum of the rotary harmonic essentially coincides.  

Corresponding phase shifts are achieved, as already mentioned above, by departing from the uniform (azimuthal) blade division which is customary in multi-blade propellers. In order to avoid unbalance problems, or to be able to continue using the identical propeller blades, an uneven blade pitch angle ε should only be used for blade pairs 4, 6; 8, 10 with sheets opposite each other by 180 ° apply, as shown in FIG. 1. Thus, the invention is limited to propellers with at least 4 blades, or a number of blades divisible by 2.

In a 4-blade propeller, the two blade pairs 4, 6; 8, 10 included pitch angle ε can be optimized with regard to minimal propeller noise radiation. The achievable reduction in the A-weighted overall sound pressure level is based on the corresponding noise level of a conventional propeller with a pitch angle ε = 90 ° and essentially depends on the propeller speed and the propeller diameter of the initial propeller. On the one hand, the characteristics of the propeller rotary sound spectrum are determined by the helical blade tip Mach number (i.e. essentially dependent on the speed and diameter) and on the other hand the frequency position of the harmonics - and thus the effect of the A-rating - is determined by the speed and number of blades.

If the propeller speed deviates from the design value, either the set pitch angle ε - corresponding to the changed operating conditions - must be readjusted to a new, optimal value or a reduction in the maximum achievable noise reduction must be accepted.

Fig. 2 shows an example of a 4-blade propeller calculated typical dependence of the optimum pitch angle of the helical blade tip Mach number. The value of the optimal angle ε therefore becomes smaller with increasing Mach number, or decreases with decreasing Mach number. Typical are the angles already mentioned above of ε ≈ 20 ° for M ≈ 0.7 or ε ≈ 40 ° for M ≈ 0.5 with deviations of ∼ ± 5 °.

In Fig. 3, the typical course of the noise reduction (level difference based on the noise emission of a 4-blade comparison propeller with ε = 90 °) is shown depending on the pitch angle ε . The noise increase occurring here at small angles ( ε <20 °) (dashed curve branch) is due exclusively to acoustic interactions. Any aerodynamic interference that may be expected at very small pitch angles e is not taken into account.

That by acoustic interference in cooperation with the A reduction function resulting in A-evaluation function increases small helical tip mach numbers because in this case the levels of the rotary harmonics with increasing atomic number (Frequency) fall very quickly. The A-rating is thereby achieved Total sound pressure level almost exclusively from the level of one individual harmonics determined, its interference-related Reduction can then come into its own.

In the case examples previously calculated and tested experimentally (on a model scale), this interference effect reduced the level by up to approx. 4 dB (A). Such a reduction can e.g. B. in the legislation in force in the Federal Republic of Germany on approval as a so-called low-noise aircraft. As a result of the additional "subharmonics" that occur with irregular azimuthal leaf division, significantly higher level reductions are not to be expected. Thus, in the case of the 4-blade propeller at ε <90 ° compared to the starting propeller with ε = 90 °, there are twice the number of rotary harmonics, corresponding to the spectrum of a 2-blade propeller, as shown in FIG. 4.

Fig. 4 with reference to schematic propeller rotational sound spectra showing the effect of the underlying invention, the interference effect. In Fig. 4 above the unweighted and the A-spectrum of a conventional four-blade propeller with a pitch angle ε = 90 ° reproduced. In Fig. 4 below the corresponding spectra for a four-bladed propeller with an optimized pitch angle ε according to the invention are shown, together with the values shown in Fig. 4 above. The achievable noise reduction can be clearly seen in the two lower diagrams.

Since the achievable noise reduction and the value of the associated angle ε depend largely on the drop in the level of the harmonics over frequency, appropriate predictions for each application must be made on the basis of either a propeller noise calculation or corresponding experimental studies.

For the application of the inventive concept according to the geometric and operational propeller parameters, for. B. for a 4-blade propeller that determines the "noise-minimal" pitch angle ε and the (with those of the conventional 4-blade propeller identical) propeller blades arranged at this pitch angle ε on the hub.

The propeller configuration shown in FIG. 5 is e.g. B. realized by two conventional 2-blade propellers, whose rotation levels due to the specific radiation properties of the propeller noise (the maximum of the A-weighted total sound pressure level occurs - depending on the operating parameters - mostly in the direction of flight behind the rotation level) a slight axial offset may have. This avoids - particularly at small angles ε - constructive problems with regard to the blade foot clamping in variable pitch propellers. In the case of rigid propellers, there is also the simple possibility for the selected example of mounting two conventional 2-blade propellers axially and indirectly in front of each other.

Furthermore, in the case of axially arranged propellers in pairs at variable propeller speed in flight via an adjustment mechanism arranged between the two 2-blade propellers, the pitch angle ε can be readjusted to the acoustically optimal value depending on the respective propeller speed. This is hardly necessary for variable pitch propellers with their power-dependent regulation of the blade pitch angle, since the propeller speed is almost constant over a large operating range.

In order to achieve large starting thrusts, axially in front of each other arranged leaf pairs also the possibility in this Set appropriate azimuthal leaf arrangements.

In the case of axially arranged propeller pairs, it should be noted for acoustic reasons that of the successive blades at an angle ε in the circumferential direction leads the rear one in the direction of rotation in the direction of rotation, as illustrated in FIG rear sheet is avoided with certainty. When determining the optimum angle ε , the cheapest or permissible axial offset of the rotary planes on the basis of the associated radiation directivity characteristic must be taken into account.

The exact determination of the optimal pitch angle can be done either empirically, on the basis of noise measurements, or with the help of a suitable propeller noise calculation method, e.g. B. with the method specified in the above-mentioned "Aircraft Noise Prediction Program" (ANOPP) of NASA. Approximately, however, the noise-optimal pitch angle can be determined very easily if there is an A-weighted narrowband level spectrum of the sound radiation from the output propeller ( ε = 90 °) from flight measurements (measurement results from standing tests are unsuitable) at the propeller speed to be considered. In this case, the optimal pitch angle can be approximated according to the relationship

ε = (180 ° / f _max ) ( N / 60)

with N = speed in l / min and f _max that frequency (in Hz) at which the level maximum occurs in the measured or calculated A-weighted narrowband spectrum.

The idea of the invention can also be applied to propellers which operate in a disrupted inflow. In this case, however, the achievable noise reduction and the associated azimuthal pitch angle ε can only be determined by experimental investigations.

Experimental investigations were carried out with commercially available 2-blade propellers for model aircraft. Propellers of the "Super 25-8" type (Graupner) with a 25 cm diameter and 8 cm pitch (axial "progress") were used per revolution. The propellers were driven by an electric motor to simulate the flight state in a low-noise and low-turbulence air flow of 30 m / s (free jet test bench), namely at a speed N = 18 060 l / min corresponding to a helical blade tip Mach number of 0.695.

The transferability of propeller noise measurements from the model trying on the large scale is of extreme operating conditions apart, given with sufficient accuracy if same leaf tip Mach numbers are observed. For this, the (smaller) model propeller must be used accordingly higher speed are operated, which, according to the Model scale, higher frequency values for the rotary harmonic of the propeller noise. Before calculating an A- overall sound pressure level is therefore a frequency trans formation required.

Fig. 6 shows examples of model measurement results (unweighted narrowband level spectra) for the conventional 4-blade propeller ε = 90 ° and a propeller according to the invention with the pitch angle of ε = 24 °. The spectrum of the propeller according to the invention clearly shows the interference minimum, the frequency position of which can be calculated with the aid of the selected pitch angle and the speed.

To determine the noise reduction that can be achieved with the corresponding interference propeller with a diameter of 2 m with regard to the A-weighted overall sound pressure level (based on the noise emission of a conventional propeller), first the frequency scales of the measured spectra with the model scale (here 0.125 = 0.25 m / 2.0 m) to multiply (frequency transformation). A corresponding scaling is additionally indicated in FIG. 6 for illustration. The spectra can then be subjected to the A rating and the total sound pressure level can be calculated by summing the sound pressure squares (RMS values) of the rotary harmonic. For the example shown, this results in a noise reduction of approx. 3 dB (A) if a propeller diameter of 2 m is assumed for the large version.

Beyond the application of the invention to aircraft propellers are the interference-related noise reductions described achievable with high-speed axial fans.

Claims (6)

1. aircraft propeller with an even number of propellers, but at least four propeller blades, two of which are diametrically opposed to each other, characterized in that the blade pairs ( 4, 6; 8, 10 ) with a pitch angle ( ε ) between about 15 ° and 50 ° offset from each other so that the levels of the rotary harmonics are weakened by interference. 2. Aircraft propeller according to claim 1, characterized in that the pitch angle ( ε ) essentially corresponds to the formula ε = (180 ° / f _max ) ( N / 60),
with N = speed in l / min
and f _max (Hz) of the frequency at which the level maximum occurs in the measured A-weighted narrowband spectrum. 3. Aircraft propeller according to claim 1 or 2, characterized in that two by the pitch angle ( ε ) staggered two-bladed propellers are arranged axially in front of each other on the motor shaft. 4. Aircraft propeller according to claim 3, characterized in that the two propellers are formed with fixed propeller blades and that a gear for adjusting the pitch angle ( ε ) is arranged between the two propellers. 5. Aircraft propeller according to claim 3, characterized in that the propellers are designed as adjustable propellers. 6. Aircraft propeller according to one of claims 3 to 5, characterized characterized in that of the circumferential at the pitch angle towards successive leaves in the direction of flight rear is arranged leading in the direction of rotation.