Micro/Mini Aerial Vehicle Propulsion System Studies Kailash Kotwani, Nishikant Prabhu, Yogesh Bhumkar

Center for Aerospace System and Design Engineering (CASDE) Department of Aerospace Engineering

IIT Bombay-76 Abstract

Selecting correct combination of engine and propeller is very crucial step in design of mini/micro class vehicles. Whether the output power produced by selected engine propeller combination will be sufficient enough to provide mission thrust requirements? What will be the amount of input energy required in terms of Ah of batteries or quantity of fuel, these are questions to be answered for a successful design. The technical information regarding small propellers and engines used for MAVs is not well documented in open literature so far. Another major hurdle is unavailability of experimental facilities and infrastructure for this class of vehicles. The objective of present study is to establish a measurement system and obtain performance maps for mini/micro vehicle class propellers and engines. To reduce the efforts involved and widen the range of analysis, the characteristics are studied using non-dimensional parameters (CP, CT, η, J etc). Another important aspect is optimizing performance by selecting propeller geometry for maximum efficiency in the required velocity range. These characteristics are determined experimentally using the wind tunnel system built as a part of MAV development. Different engine-propeller combinations are tested in wind tunnel at different flow velocities and different RPM. Nomenclature A = Area (m2) c = Chord Length (m) CP = Coefficient of Power CT = Coefficient of Thrust CQ = Coefficient of Torque d = Diameter (inch/m) D = Drag (N)/Diameter (inch/m) Eb = Back EMF (V) I = Current Supplied (A) J = Advance Ratio K = DC motor constant L = Lift (N) N = RPM (1/min) n = RPS or Rotational frequency (1/s) P = Power (W) p = pitch (inch) PI = Input Power (W) Ps = Shaft Power (W) Q = Torque (N.m) r = radius (inch/m)

Ra = Armature Resistance (Ω) T = Thrust (N) TA = Thrust Available (N) V = Voltage Across motor (V)/Flow Velocity (m/s) VR = Resultant flow Velocity (m/s) V∞ = Translational Velocity Component (Upstream flow velocity) (m/s) Vr = Rotational Velocity Component (m/s) ηx = Efficiency of x ϕ = DC motor flux β = Pitch angle or angle of airfoil with plane of rotation (radian/degree) ω = Rotational Velocity (rad/s) φ = Angle to direction of motion to which VR acts (radian/degree) α = Angle of attack of airfoil (radian/degree)

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1. Introduction

A Source of thrust, a driving

medium and available source of energy are basic ingredients of propulsion system. Propeller has been most useful as a source of thrust for conventional and contemporary small aircrafts and µ/MAVs. Among currently available technologies small internal combustion engines and electric motors look promising as a driving medium. For IC engines methanol based fuel is best energy source whereas for electric motors batteries and solar cells are good options (Image 1).

Image 1: Ingredients of Propulsion System

Whether micro, mini or macro

usually for all classes of vehicles propulsion system constitutes around 60% of gross weight of vehicle (Image 2). Sensitivity Analysis study of some of micro aerial vehicles show that an additional 1 gram of drag would decrease the endurance by 3 minutes and an additional 1 gram of mass would decrease the endurance by 30 seconds. Hence sizing and weight of propulsion plays critical role in performance of vehicle.

Image 2: Weight Break-up Aircraft Sub-Systems. Image 3 shows some of the propellers available as the commercial off the shelf components. Propeller diameter varies from 3.8 inch (Black Widow Prop) to 11 inch (APC prop) and made out of variety of materials e.g. carbon, glass filled nylon etc.

Source of Thrust

Driving Medium

Sources of Energy

Image 3: Propellers: Micro to mini

Some of the motors and Engines available are shown in Image 4. Coreless subminiature motors are very light weight (0.8-2 gm), their no load max RPM is very high (25000-27000) and used for micro class vehicles. Cobalt geared motors and OS MAX IC engines are used for mini class vehicles.

Propeller

IC Engine

Electrical Motor

+ Fuel + Battery

Solar Cell

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Image 4: Engine/Motor: Micro to mini 2. Theory and Background Similar to wings, propellers are made up of airfoil sections designed to generate an aerodynamic force. The wing force provides lift to sustain the airplane in the air; the propeller force provides thrust to push the airplane through the air. A sketch of a simple three-blade propeller is given in Fig 1, illustrating that a cross section is indeed an airfoil shape. However, unlike a wing, where the chord lines of the airfoil sections are essentially all in the same direction, a propeller is twisted such that the chord line changes angle from root at prop-hub to tip. This is illustrated in Fig 2, which shows a side view of the propeller, as well as two sectional views, one at the tip and the other at the root. The angle between the chord line and the propeller’s plane of rotation is defined as the pitch angle β. The distance from root to given section is r. Note that β = β(r).1

Cobalt Geared motor

Coreless Subminiature

Motors

OS MAX FX

Engine Fig 1: Crossection of a three bladed propeller1

Fig. 2: Varying pitch angle along the span of propeller1

22rR VVV += ∞ (1)

Where V∞ is the translational

velocity component and Vr is the rotational component. This resultant VR acts at a certain angle φ to the plane of rotation, and this angle is defined as

rnV

rV

VV

r πωφ

2tan ∞∞∞ === (2)

The airflow seen by a given propeller is combination of the airplane’s forward motion and the rotation of the propeller itself. This is sketched in Fig. 3a,

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where the airplane’s relative wind is V∞ and the speed of the blade section due to rotation of the propeller is rω. Here, ω denotes the angular velocity of the propeller in radians per second. Hence, the relative wind seen by the propeller section is the vector sum of V∞ and rω, as shown in Fig 3b.

Fig 3: Resultant local velocity seen by a section of propeller1

If the chord line of the airfoil

section is at an angle of attack α with respect to the local relative wind VR, then lift and drag (Perpendicular and parallel to VR, respectively) are generated. In turn, as shown in Fig. 4, the components of L and D in the direction of V∞ produce a net thrust T:

Fig 4: Lift and drag force on an airfoil of a propeller1

T = L cosφ - D sinφ (3) Where φ = β - α (4)

This thrust, when summed over the entire length of the propeller blade, yields

the net thrust available. (TA) which drives the airplane forward.

Fig 5: Motor propeller combination

There is one more important

parameter, propeller efficiency (η), which is defined as

PCTC

JsP

VAT

sPAP

Pη ×=×

== ‡ (5)

Where Ps is power at the shaft of

engine, J is advance ratio, CT is thrust coefficient and CP is power coefficient. The advance ratio J, which is a measure of how far the propeller moves forward through the medium per rotation of the propeller, is defined as

nDVJ ∞= (6)

For a propeller the non dimensional thrust coefficient is defined as

42 DnTCT ρ

= (7)

Similarly, Power coefficient is defined as

53DnPCP ρ

= (8)

A propeller is nomenclatured in

terms of two parameters, pitch and diameter. Pitch is defined as the horizontal

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Multiplying (1) by I distance moved by propeller in one complete rotation at 100 % efficiency and zero slipping. So airfoil at particular section of propeller performs motion in helical direction as shown in Fig. 5. If the helix is unwrapped onto a two dimensional plane, the length p is defined geometrically as

V I = I Eb+ I2 Ra (11) Where, V I = Input Power = PiI Ra = Electrical losses in armature I Eb = Mechanical power developed Hence eqn (13) can be interpreted as Input power = Mechanical power developed + electrical losses in armature βπ tan.2 rp = (9)

Certain percentage of mechanical power developed is required for overcoming iron and friction losses in the motor and the rest is available as output to drive the thrust source at the shaft of motor.

Hence, one can write equation as Pi = V I= Mechanical losses + Electrical losses + Ps (12)

Fig. 6: Measuring Pitch of a propeller4

It should be noted that, in industry standards, pitch is measured at 75% of radius or values of r and β in eqn (9) are as at 75% of radius.

Once motor has been calibrated that is electrical and mechanical losses are determined as function of RPM, one can calculate efficiency of motor analytically as follows,

For a DC motor, voltage equation is given as2 Motor Efficiency = ηe = Ps/Pi (13)

V = Eb + I Ra (10)

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Fig 7: Optimizing Propeller

Developing Experimental

Facilities

Testing set of propellers

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Developing analytical tools

Validating experimental and

analytical results

Optimizing prop design

3. Wind Tunnel Facility

An open wind tunnel was fabricated at CASDE, IIT Bombay (Image 1) during the 2002-summer for the purpose of power plant characterization and load measurement on MAVs. The size of test section is 1m X 1m X 1.25m and one can use it for the range of velocities from 0 m/s to 10 m/s. Air inside the tunnel was circulated using six single-phase motor-driven fans and voltage was controlled by 230 V range Auto-Transformer.

Image 5: Low Speed wind Tunnel

Two Master-Airscrew propellers 11X7 and 10X5 were characterized using this tunnel. Due to low maximum velocities, it was not possible to map characteristics for all ranges of J. Later on this wind-tunnel was redesigned and redeveloped to produce higher flow velocities (20-25 m/s). Tunnel section design was prepared and theoretical analysis (calculating pressure losses at various sections) was conducted to predict the capacity of motor fans required to produce the velocity of 25 m/s. Qualitative analysis and experimental work with redesigned wind tunnel is in progress. Air inside the tunnel is circulated

Calculating efficiency Analytically

Measuring efficiency experimentally

Validation of Motor model

Fig 8: Motor Efficiency

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using two three phase induction motor driven fans and flow velocity in tunnel is controlled by varying voltage across motors using a 440 volt autotransformer. Specifications of motor are as follows

BHP : 9.0 Motor Rating : 10 HP/960 RPM Various Set-ups for measuring

thrust and power has been developed. Table 1 gives details about parameters which can be measured during wind-tunnel testing.

Type : Axial Capacity : 36000 m3/hr Pressure : 25 mm wg

Fig 9: New Wind-tunnel Lay-out

Sr no.

Parameter

Sensor Pic

1. Thrust produced by propeller Load Cell

2. Torque at the shaft of motor Torque Sensor

3. Flow Velocity inside tunnel Pitot-tube and Micro-manometer

4. Voltage and Current consumption of motor

DC Regulated Power Supply

5. RPM of propeller Proximity Sensor

6. Room temperature and Pressure Digital Barometer

Table 1: Sensors and Parameters

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4. DC Motor and Propeller combination At present Master-Airscrew propellers of various sizes are being tested for characterization. A cobalt 615 geared motor (Image 6) is being used as driving medium as this gives precise control over motor RPM. An electric motor is preferred over IC engine as running an IC engine inside the tunnel involves many complexities e.g. starting trouble, oil in exhaust, variable RPM at constant fuel supply, cumbersome throttle control etc.

Image 6: Motor-Propeller Combination DC Motor Operating Limitation 1) Voltage, Current and Power Limitations Normal voltage range specified = 8 to 12 Volt Though 12 volt is not maximum upper limit because the same motor has been used with 12 Nicads battery (Expected performance mapping, Manufacturer’s mannual) ∴Maximum Applied voltage = 1.2*12=14.4 Volt. For performance mapping manufacturer has crossed 12 Volt, is confirmed from the

fact that with 12 Nicads batteries power consumption was 325 Watt at 24 A current supply ∴ Applied voltage at that time = 325/24=13.54 Volt Another way to obtain maximum limiting voltage across the motor is through maximum continuous current and power specified by manufacturer Max Continuous current = 25 A Max Continuous power= 400 W Max applied voltage = 400/25 = 16 V This analysis shows that it is not unsafe to run motor between 12 to 16 Volt range. One thing should be kept in mind that in high voltage ranges motor should not be run for longer time as motor becomes too hot. Secondly armature resistance is sensible to temperature of motor. 2) RPM and Torque Limitations This cobalt motor uses a gear for reducing the final RPM at the shaft. Gear Ratio = 2.38 to 1 Motor Speed/volt = 1488 rpm/volt Geared motor speed/volt = 652 rpm/volt Lets say maximum applied voltage across the motor = 15 V Geared motor speed = 652*15= 9780 rpm Even with gear motor provides decent rpm which is in the range of 10000. Similarly, Motor torque/amp = 0.91 in-oz/amp Geared torque/amp = 2.17 in-oz/amp At maximum current = 25 A UnGeared torque = 0.91*25=22.75 in-oz Geared torque = 2.17*25 = 54.25 in-oz Propeller operating limitations 1) Noise Considerations

The prop tip speed should not exceed 600 to 650 feet per second (180 to

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200 m/s) to keep it within the noise limit (For a master airscrew nylon prop).6

Tip Speed = Vr (ft/s) = rω = 0.00436*RPM*(Diameter in Inches)

DVRPM r

×=

00436.0 (13)

Lets Say we take limiting propeller

tip speed as 600 ft/s For 11X7 master-airscrew

propeller, limiting RPM can be calculated as

Limiting RPM = 600/(0.00436*11) =12510 rpm 2) Mechanical Considerations

One of the differences between wood and glass-filled nylon propellers is that glass-filled nylon props have suggested RPM limits for mechanical considerations. This varies according to Diameter of prop. For a Master Airscrew prop RPM limit recommended by manufacturer is calculated as follows.6

RPM operating limit = 160,000/(Diameter in inches) (14) For a 11X7 prop, RPM operating limit = 160,000/11 = 14,545 rpm

Minimum of the above two

operating limits will be taken into consideration that is 12510 rpm.

5. Experiment to determine NO LOAD characteristics of Motor The objective of this experiment was 1) To measure the armature resistance of the motor 2) To determine the No Load characteristics of the motor that is obtaining ‘Back EMF vs RPM’ and ‘Mechanical losses vs RPM’ characteristics. The above data will be used to predict the shaft power of DC motor. Under no load conditions, torque applied at the shaft is zero hence output of motor is zero. That is the total power supplied is used to overcome mechanical and electrical losses only. Ps = 0 (under no load conditions) (15) Relationship between speed and back emf of motor is given as2

Eb = N*ϕ/K (16) When RPM is zero back emf is also zero (from (16)). So eqn (10) will become Ra = V/I (17) Using this Ra to calculate back emf for all values of current and voltage. This will enable us to determine characteristic of mechanical losses Vs RPM using eqns (10), (11) and (12). Average Ra from two sets of readings = 0.0885 Ω

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Graph 1: Back EMF vs Motor RPM

Graph 2: Mechanical and Electrical Losses Vs RPM Experiment to predict the shaft power of DC motor

In this experiment, motor loaded with 11X7 propeller is tested in wind tunnel at different flow velocities. For a particular flow velocity, Power consumption is measured at all RPM values. Using the back emf measured at NO Load conditions, Mechanical power developed by motor is calculated. Shaft

power is obtained by subtracting mechanical losses from mechanical power. Important thing is one can measure shaft power just by using motor calibration data and without using torque sensor or any other instrument.

0

2

4

6

8

10

12

0 1000 2000 3000 4000 5000 6000 7000 800

Motor Speed (RPM)

Bac

k E

MF

(V)

0

Cp Vs J

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.00 0.20 0.40 0.60 0.80 1.00 1.20

J (advancd ratio)

Cp

(pow

er c

oeff

icie

nt)

V=2.25 m/s

V=3.19 m/s

V = 3.9 m/s

V = 5.04 m/s

v = 6.36 m/s

V = 7.34 m/s

V = 8.11 m/s

V = 9.18 m/s

V = 9.63 m/s

0

10

20

30

40

50

60

0 1000 2000 3000 4000 5000 6000 7000 8000

Speed (RPM)

Loss

es (W

att)

Mechanical Losses Electrical Losses

Graph 3: Cp Vs J using loss- relationship 5. Thrust Measurement In this experiment thrust produced by propeller is measured at different motor RPMs at different flow velocities inside the tunnel. Depending upon the size of propeller, a prior rough estimate of thrust is made and accordingly load cell is selected. Set up shown in image 7, a propeller-motor combination is mounted over a load cell with the help of suitable mount. An infra-red sensor based RPM sensor is mounted few millimeters behind the propeller. Motor RPM is varied by varying the voltage across the motor. First 11X7 propeller was tested and from the measured data Ct Vs J plot was obtained. To verify repeatability of data the whole set of experiments were repeated on 11X7 and another plot was obtained. The same exercise was repeated on 10X5 propeller.

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Results are shown in graph 4, 5, 6 and 7. Horizontal and vertical bars on a particular point shows uncertainty in respective parameter.

Image 7: Thrust Set-up

Graph 4: Ct Vs J for 11X7 first set

Ct Vs J

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Ct

3000 rpm 4000 rpm 5000 rpm 6000 rpm 7500 rpm Graph 5: Ct Vs J for 11X7 second set

Ct Vs J

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Ct

3000 rpm 4000 rpm 5000 rpm 6000 rpm 7500 rpm

Graph 6: Ct Vs J for 10X5 first set

Ct Vs J

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Ct

4000 rpm 5000 rpm 6000 rpm 7500 rpm

Graph 7: Ct Vs J for 10X5 Second Set

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Ct

3000 rpm 4000 rpm 5000 rpm6000 rpm 7500 rpm

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6. Power Measurement In this experiment power produced by propeller is measured at different motor RPMs at different flow velocities inside the tunnel. From graph 3 (power prediction from losses) a prior rough estimate of maximum torque is made and accordingly correct channel is selected for measuring torque on sensor. Set up shown in image 8, torque sensor is mounted over four bearing support two on each side. On one side propeller is mounted on shaft connecting to sensor and on other side motor is connected to sensor through flexible coupling. This whole set-up is mounted over a channel and a RPM sensor is mounted inside the channel just few millimeters behind propeller. Motor RPM is varied by varying the voltage across the motor. First 11X7 propeller was tested and from the measured data Cp Vs J plot was obtained. To verify repeatability of data the whole set of experiments were repeated on 11X7 and another plot was obtained. The same exercise was repeated on 10X5 propeller. Results are shown in graph 8, 9, 10 and 11.

Image 8: Power measurement set-up

Cp Vs J

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Cp

3000 RPM 4000 RPM 5000 RPM 6000 RPM 7000 RPM

Graph 8: Cp Vs J for 11X7 First Set

Cp Vs J

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Cp

3000 rpm 4000 rpm 5000 rpm 6000 rpm 7500 rpm Graph 9: Cp Vs J for 11X7 Second Set

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Graph 10: Cp Vs J for 10X5 first set

Graph 11: Cp Vs J for 10X5 second set

Calculating Efficiency Once Cp, Ct and J have been

determined, efficiency of propeller can be obtained using eqn 5. Efficiency for 11X7 and 10X5 are shown in Graph 12 and 13 for first set of readings.

Cp Vs J

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8J

Cp

3000 RPM 4000 RPM 5000 RPM 6000 RPM 7000 RPM

eta vs J

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

eta

3000 rpm 4000 rpm 5000 rpm 6000 rpm 7500 rpm Cp Vs J

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

Cp

3000 RPM 4000 RPM 5000 RPM 6000 RPM 7000 RPM

Graph 12: efficiency of 11X7 propeller

eta vs J

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

J

eta

3000 rpm 5000 rpm 6000 rpm 7500 rpm

Graph 13: efficiency of 10X5 propeller

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6. Discussion and Further work 1) Upto this point sufficient technical information about the motor and different propellers has been collected and this will serve as an exhaustive reference for further work in future. 2) Motor’s No load characteristics have been determined which helped in predicting power generated at the shaft and obtaining other necessary plots. Cp Vs J plots obtained by predicting losses resemble with that obtained experimentally by measuring shaft power. 3) A typical plot of Cp, Ct, eta Vs J is shown in fig. 7 taken from reference no. 4

Fig7. Blade Performance Coefficients4

This is being mentioned as plots

obtained experimentally exhibit resemblance in terms of nature of characteristics and order of values with that shown in fig 7. Absolute values cannot be compared because plots are of different propeller. 4) Measured efficiency of 11X7 and 10X5 are of the order of 50 to 60 % but literature survey has revealed that propeller of efficiency 80 % can be designed for desired velocity and rpm ranges. 5) Similarly motor efficiency of the order of 70% is achievable.

6) In future same propeller will be analyzed using blade element theory and the results will be validated by comparison with experiment. Then propeller geometry will be optimized to get most efficient design for required mission. References 1. Anderson, J.D.,”Introduction to

Flight”, Mc Graw Hill publishing company, Fourth Edition, 2000, page no. 595-602.

2. Thareja, B.L., “Introduction to electrical engineering”, S. Chand company and publisher limited, 1993

3. B B DALY, “Woods Practical Guide to Fan Engineering”, Woods of Colchester Limited publisher.

4. Von mises, R., “Theory of flight”, Dover Inc., New York,1959

5. http://www.geocities.com/Yosemite/geyser/2126/flyinggadgets.html

6.http://www.masterairscrew.com/techbull.asp(Masterscrew propeller instruction mannual) 7.http://www.astroflight.com (For DC motor instruction manual) 8.Joel M Grasmeyer and Matthew T Keennon, “ Development of the Black Widow Micro Aerial Vehicle”, AIAA-2001-0127.

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