RMIT School of Aerospace, Mech. & Manufacturing Eng. AERO2365 Thesis / Project 2007
AERO2365 Thesis / Project 2007: Propeller Aerodynamic Analysis and Design M. F. Shariff
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School of Aerospace, Mech. & Manufacturing Eng., RMIT University, VIC, 3001, AUSTRALIA
Abstract Propeller aircrafts are very efficient for low speed flights up to the speeds where regions of supersonic flow exist on the propeller. This report analyses and designs a propeller system with increased propulsive thrust, with a sufficiently high level of aerodynamic efficiency for a selected radio-controlled aircraft model. All these improvements to the design of the propeller will be achieved by means of a thorough analysis of the effects of varying the different elements and their geometries, together with experimentation of different sizes and types of propellers. The same procedure can then be implemented on UAVs and MicroUAVs, Commercial R/C Aircrafts and Full Scale Airliners, if the findings and results prove successful. The main tool being used for the design of the propeller is the ‘GUI Software’, followed by verifications made with analytical as well as experimental methods for both the “Original” as well as new “Best” propeller designs.
The objective of this project is to apply the same principles and methods used in obtaining the best propeller design for the radiocontrolled aircraft model to: 1.
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3. Introduction If changes were to be made to the propeller systems to such an extent whereby its propulsive thrust can be further improved for low speed propeller aircrafts, in addition to its already known advantage of being fuel efficient at these low speeds, large costs can be saved, in the manufacturing of the propulsion system itself, on its maintenance, and also on running the flights of the aircrafts especially in a world of soaring fuel prices and ever increasing material costs, as a result or mere consequence of exploiting the earth’s natural resources near its limited capacity. It should be noted here that turbines are generally more costly to manufacture and require high levels of skills even in maintenance, whereas the building and construction of propellers are a much simpler craft. The focus of this report is mainly to analyse and design a propeller system with increased propulsive thrust, without too much of a significant loss in efficiency, for a selected radiocontrolled aircraft model.
UAVs and Micro-UAVs: present UAVs and MicroUAVs are mostly made for military use, and are costly to run for short missions, despite the low performance level, in the case of the Micro-UAVs. Commercial R/C Aircrafts: if the radio-controlled propeller aircraft can be designed to be very cost effective, following the improvements to propulsive and fuel efficiencies, the radio-controlled aircrafts can be introduced to more commercial uses such as for dispatch services, traffic watch of a small community or country, security watch and control in large countries with insufficient sheriffs and police officers in rural, country and village areas, or even in farms and for agricultural use. Full Scale Airliners: the propeller design for the selected radio-controlled aircraft model can also be implemented to any full scale aircraft to achieve a proportional improvement to its propulsive efficiency.
The main tools being used for the design of the propeller and its dimensions are softwares such as ‘Javafoil’, ‘JavaProp’, ‘X-foil’ and ‘GUI Software’. Hand calculations and analytical methods are then carried out to verify the results obtained from the softwares, before an actual propeller is printed from the new “Best” propeller design. Following that, final experimental flights for both the “Original” as well as new “Best” propeller designs are executed to further verify the analytical results. Aircraft Information The aircraft selected for this thesis project is a radio-controlled aircraft model. The engine type and capacity will remain constant throughout the course of the thesis project and the only variable is the aircraft propeller.
The engine type and capacity will remain constant throughout the course of the thesis project and the only variable is the aircraft propeller. With the known details of the radio-controlled aircraft, a propeller will be designed for maximum performance under optimum conditions. Propeller efficiency, thrust and torque produced will be the primary guidelines along which the propeller design will be achieved. Following that, the fuel efficiency, aircraft range and endurance of the propeller design may be tested for secondary analysis. All these improvements to the design of the propeller will be achieved by means of a thorough analysis of the effects of varying the different elements introduced earlier and their geometries, together with experimentation of different sizes and types of propellers.
Figure 1: Classic ARF
The Classic was designed as a trainer aircraft and most of its parts are cut by a laser machine. The airframe is conventionally built using balsa, plywood and veneer to make it stronger, yet the design allows the aircraft to be light in weight as well. Also, its semi-symmetrical design makes it easy to roll and loop. The dimensions of the aircraft are measured and displayed as shown in the two figures that follow below. With the dimensions,
the aircraft parameters and wing geometry required for analysis and hand calculations are computed: Wing Geometry: Wing Span, b = 1.445m Wing Width, c = 0.240m Wing Area, S = b.c = 0.347m2 Aspect Ratio, AR = b/c = 6.02 Methodology 1. The “original” propeller is first being scanned using a prototyping scanner which produces spline images. These images are further developed using CAD drawing to obtain the geometry of the “original” propeller, at 12 blade element locations. 2. Following that, the “original” propeller’s performance is tested with the use of GUI (Graphical User Interface) Software which gives a measurement of the thrust (T) and torque (Q) produced, as well as the propeller efficiency (η). 3. With results of the “original” propeller’s performance as benchmark, the “best” propeller is designed by manipulating each individual variable that makes up the geometry of each blade element, to obtain a configuration that produces a significantly increased thrust and torque where possible, while keeping losses in efficiency to a minimum (max loss of 10% accepted). 4. The design variables experimented with were: Radius of Propeller, No. of Blades, Blade Angle as well as Chord Length. 5. The performance of both propellers are compared using GUI Software again, and then verified with Excel Spreadsheet calculations as well Thrust Measurement Experiments. Design Process As stated earlier, GUI software is the main tool being used for the entire design process. The input data are modified accordingly for each of the following design variables. Microsoft Excel spreadsheets are also extensively used for some of the variables, wherever changes need to be made to input values at each of the 12 radial stations. In the literature review part of this thesis report, it has been understood that for most cases, the thrust and torque increases are usually accompanied by efficiency losses. This follows that a choice has to be made on the target area for improvement in performance. Thus, throughout the entire design process, the emphasis is made on affecting as much increase as possible on the thrust and torque produced by the propeller, all the while retaining as much of the efficiency, or even simultaneously have the efficiency increased together with the thrust and torque, wherever feasible. The GUI software results of the “original” propeller are being used as benchmark for the design instead of results from the analytical or the experimental methods, since the bulk of the design will be carried out in GUI anyways. Target improvement: 1. Thrust (T) and Torque (Q): To be increased as much as possible. 2. Efficiency (η): To be retained where possible. Maximum allowable loss of 10%. *Note: the maximum allowable loss of 10% (rather high) for the efficiency is chosen only due to the fact that the “original” propeller has a considerably high efficiency and hence it is possible for some losses to be suffered during the design process.
Design Variables Variable 1: Radius of Propeller The radius of the “original” propeller is varied using the GUI software while leaving everything else, including the hub radius, constant. The output data are then compared between each new propeller radius and the “original” one. Observations: It is observed that a larger propeller radius would produce more thrust and torque than a smaller one, but at the price of reduced efficiency. The opposite is true when the propeller radius is decreased. The main reason for the increase in thrust when the propeller radius is increased, is the fact that the actuator disk area is now larger than before, where the pressure acts upon to create thrust. Variable 2: No. of Blades Most aircraft propellers have between 2 to 8 blades. Thus, in this case, the effect of only increasing the number of blades will be studied in GUI while keeping everything else constant, since the “original” propeller already has the minimum of 2 blades, and hence cannot be further reduced. Observations: Increasing the number of blades has a similar effect to increasing the propeller radius, resulting in improvements of the thrust and torque produced, while the efficiency decreases. It is also seen that the increases in thrust are more significant than the losses in efficiency, and hence, adding one more blade would be advantageous to the propeller performance. The loss in efficiency can be recovered with the change of another variable. Variable 3: Blade Angle Two separate approaches are looked into before being implemented, in the process of modifying the blade angles (β = blade pitch angle and θ = local pitch angle). 1. Changing the blade angle by a certain value 2. Scaling the blade angle by a certain factor Observations: For all 3 cases of decreasing the blade angle by subtraction, both the thrust and torque produced undergo a slight reduction. Though these decreases will not affect the propeller performance much, the increases in efficiency are not that significant either. On the other hand, for the case where the angle is increased by 1 degree, the pattern observed is the total opposite. The thrust and torque produced increase, while the efficiency decreases, all of which are nevertheless, by very insignificant amounts once again. For scaling, both the blade pitch and the local pitch angles are multiplied by the same factor for each case, and hence undergo either a linear increase or decrease. For the purpose of this analysis, the “original” propeller blade angles are multiplied by factors of 0.7, 0.8 and 0.9 for the decreasing angle cases, and multiplied by factors of 1.1, 1.2 and 1.3 for the increasing angle cases. It is observed that all the scaling processes result in increased efficiencies. The advantage of scaling with factors 1.1, 1.2 and 1.3 is that there is no decrease in any of the performance parameters. When analysed for the percentage change in each of the output figures, scale factor 1.3 gave the most promising results and hence, is being shortlisted for possible use in the “best” propeller design. Variable 4: Chord Length The chord length of the aircraft propeller can also be varied to affect changes to its aerodynamic performance. Since the chord
length of the elements at each station are already dissimilar from each other for the “original” propeller, any new variations in chord length will have to be considered at each of these individual stations. As is the case with varying the blade angle, there are also 2 main approaches to vary the chord lengths of the 12 elements. 1. Changing the chord length by a certain value 2. Scaling the chord length by a given factor Observations: Keeping in mind the targeted improvement in performance specified earlier at the start of the design process; which is to achieve an increased thrust produced while keeping efficiency losses minimal, it is apparent that all the above changes brought about by decreasing the chord lengths are not favourable since they all result in a drop in thrust and torque produced, though the efficiency increase is not at all significant either. A similar pattern is observed with chord length scaling, where the minimal increases in efficiency is accompanied by substantial drops in thrust and torque for the cases of decreasing the chord lengths (scale factors 0.7, 0.8 and 0.9). On the other hand, multiplying the chord lengths by scale factor of 1.3 affects a very approving and constructive increase in thrust, with more than 30% increase. However though, there is still a significant loss in efficiency.
Results & Discussions Operating Environment Flight Altitude: h = 50 m Density: ρ= 1.2182 kg/m3 Cruising Velocity: V = 18 m/s Rotational Speed: N = 9000 rpm Ω= 942 rad/s n = 150 r.p.s “Original” Propeller Blade Radius: Hub Radius: Blade Pitch Angle: Number of Blades: No. of blade elements:
R = 0.140 m rh = 0.0125 m β = 0.3161 rad B=2 ne = 12
The local element input properties at the 12 stations of the “original” propeller blade are as obtained from phases 2, 3 and 4 which have been mentioned in the earlier parts and summarized in the table below:
Selection of “Best” Propeller Design From the earlier stages of the design process carried out, certain possible changes have been shortlisted for use. Amongst those, some changes which have been deemed imperative and have been selected for definite use for the increasing the thrust are: 1. 2.
Number of Blades, B = 3 Radius of Propeller, R = 0.165m
Design
Efficiency, η/%
Thrust, T/N
Torque, Q/Nm
Original a) b) c) d) e) f) g)
73.65 66.56 66.53 66.46 66.38 66.37 67.82 65.14
8.4490 20.8606 20.9863 21.3207 21.6493 21.8139 19.9012 23.9321
0.2191 0.5986 0.6025 0.6127 0.6229 0.6280 0.5604 0.7017
↓ ↓ ↓ ↓ ↓ ↓ ↓
↑ ↑ ↑ ↑ ↑ ↑ ↑
↑ ↑ ↑ ↑ ↑ ↑ ↑
Table 1: Final Test Designs
The 7 test designs above are compared for the percentage changes in all 3 of the performance parameters and based on the target set at the beginning of the design process itself, design ‘e’ (1st + 3°: Adding of 3 degrees to blade angles, 2nd * 1.3: Multiplying blade angles by scale factor 1.3) is selected as the final design of the “best” propeller. Design (e) implements a rather sizeable increase in thrust and torque while also ensuring that the efficiency decrease falls within the required limit of 10%. The changes are summarized as follows: 1. 158.2% thrust increase 2. 186.6% torque increase 3. 9.8% efficiency loss It can be said that the increases in thrust and torque far outweigh the minimal loss in efficiency. Hence this design is chosen as the “best” propeller design.
Table 2: Table of “Original” Propeller Input Properties
“Best” Propeller Blade Radius: Hub Radius: Blade Pitch Angle: Number of Blades: No. of blade elements:
R = 0.165 m rh = 0.0125 m β = 0.4790 rad B=3 ne = 12
The 12 blade element stations for the “best” propeller are spaced at 12mm apart from each other, as compared to the “original” propeller’s spacing of 10mm. The new local element input properties at each of these 12 stations of the “best” propeller blade, as obtained from the design process are summarized in the table below:
values and would make the percentage difference closer to those of the thrust produced, if not similar. This assumption was not blindly made, but extracted from the side calculation of the percentage difference between software and analytical thrust figures being 7.4% for the “original” propeller and 11.7% for the “best” propeller. With this argument held in place, the results for torque are still deemed to be acceptable, the larger percentage difference being seen as appropriate to its small value anyways. Conclusions To summarise, the methods used in carrying out the analysis are sufficient to obtain rough estimates of the propellers’ performance, in the case of radio-controlled aircrafts like the one used in this thesis. For real aircrafts though, software with greater accuracy and computability should be used instead, and wind tunnel tests should be carried out.
Table 3: Table of “Best” Propeller Input Properties
Results Summary
“Original” Propeller “Best” Propeller Figure 2: “Original” and “Best” Propellers
Thrust, T (N) ∆T wrt. Experiment Torque, Q (Nm) ∆Q wrt. Software Efficiency, η (%) ∆η wrt. Software
“Original” Propeller Software Analytical Experiment 8.4490 9.0779 8.6800 -2.7% 4.6% 0% 0.2191 0.2563 0% 17.0% 73.65 67.89 0% -7.8% -
Table 4: Table of “Original” Propeller Results
Thrust, T (N) ∆T wrt. Experiment Torque, Q (Nm) ∆Q wrt. Software Efficiency, η (%) ∆η wrt. Software
Software 21.8139 -6.1% 0.6280 0% 66.37 0%
“Best” Propeller Analytical Experiment 24.3583 23.2400 4.8% 0% 0.7658 21.9% 60.95 -8.2% -
Table 5: Table of “Best” Propeller Results
The software and analytical thrust values are both within 10% range of the experimental value. However, the software figures for tend to be slightly lower than the experimental, whereas the analytical figures tend to be slightly higher. Nonetheless, these values are very much within the acceptable range of 10% where differences are clearly due to assumptions and errors, both known and unknown, encountered in the process. On the other hand, the torque figures have quite a significant difference between the software and analytical methods, being closer to 20%. This may be attributed to the fact that no comparison is made to an experimental value for torque, which otherwise most likely fall in between the software and analytical
The design process and the variables tested with have led to the conclusion that not much can be done to increase thrust without penalty on efficiency. In most cases it was found that an increase in one would result in a decrease of the other. Only in certain cases are both able to be increased, but only to a minimal extent. As a whole, the thesis project can be said to have achieved its primary aims on objectives of producing a propeller with greater propulsion (higher thrust generated), while at the same time maintaining a sufficiently high level of efficiency. However, more actions need to be taken to minimise errors, where possible, though in some cases it may be impractical due to increasing difficulty and software runtime, in the progression towards more accurate results. Acknowledgments 1. A/Prof. Hadi Winarto (Supervisor) 2. Joseph Law Wai Ching (Coursemate) 3. Zayd Llouli (Coursemate) 4. Mr. Henry Lim (R/C Aircraft Pilot) 5. Mr. Adrian Reivers (Lab Technician) 6. Mr. Andre Clemann (Lab Technician) References [1] Dr. Jon Watmuff, 2006, Propeller Theories, Fundamentals of Aerodynamics class notes, Melbourne, RMIT University. [2] A/Prof. Hadi Winarto, 2004, BEMT Algorithm for the Prediction of the Performance of Arbitrary Propellers, Melbourne, RMIT University. [3] Theodorsen, Theodore, 1948, Theory of Propellers, Melbourne, Mc Graw. [4] Burstall, M., 1993, Design of A Variable Pitch Propeller System, Melbourne, RMIT University. [5] Ray Preston, Aircraft Performance, Aerodynamics Text, September 2006, viewed 7th http://selair.selkirk.bc.ca/aerodynamics1/ [6] FIU-NASA, 2004, Propellers, Aeronautics Learning Lab for Science, Technology & Research, viewed 23rd August 2006, http://www.allstar.fiu.edu/aero/flight63.htm [7] Thai Technics, 2002, Aircraft Propellers, Thai Technics.Com, viewed 23rd August 2006, http://www.thaitechnics.com/propeller/prop_intro.html [8] Harishkumar Nagarajan, 2004, Propeller Performance Prediction Using GUI Software Based On Blade Element Momentum Theory, Melbourne, RMIT University. [9] Martin Hepperle, 1996-2006, JavaFoil, User’s Manual, viewed 19th July 2007, http://www.mhaerotools.de/airfoils/javafoil.htm [10] Martin Hepperle, 1996-2006, JavaProp, Propeller Design, viewed 19th July 2007, http://www.mhaerotools.de/airfoils/javaprop.htm
