Designing Electric Propulsion Systems for UAVs

Mohamed Kara Mohamed, Sourav Patra, and Alexander Lanzon

Control Systems Centre, EEE, The University of Manchester, Manchester, UKMohamed.Karamohamed@postgrad.manchester.ac.uk,

{Sourav.Patra,Alexander.Lanzon}@manchester.ac.uk

1 Introduction

Due to the advantages of electric motors and electric batteries, the electricpropulsion systems are widely used in UAVs [1,2]. The UAVs performance andendurance, the payload capacity and the ight time of the vehicle are aectedhighly by the structure and design of the electric propulsion system. In thispaper, a new systematic design methodology for electric propulsion system (pro-peller, electric motor and battery pack) is proposed based on the design speci-cations given in terms of the required thrust, permissible propulsion systemsweight and required ight time.

For design, a mathematical model of the propeller is considered to quantify thegenerated thrust and the corresponding mechanical power. Using the momentumtheory [3] with certain assumptions, the static thrust T developed by a propellerand the mechanical power P needed to generate this thrust are given respectivelyas T =

128N2d2k1

22R2((1 + 64R

3Ndk1)0.5 1)2 and P =

20482N3d3k31

3R2((1 +64R3Ndk1

)0.5 1)3, where , N and d are respectively the air density, the numberof blades and the chord length of the blade of the propeller. k1 is known as thetwo-dimensional lift slope factor and is considered as k1 = 5.7 [4]. R, and arerespectively the radius, the pitch angle, and the rotational speed of the propeller.

2 Design Methodology

Inputs to the design algorithm: (i) the maximum allowable radius of the propellerRmax, (ii) the total mass requirement of the UAV Mtotal and the allowance forthe propulsion system mass Mp, and (iii) the required minimum ight time tf .

Step 1: Set the required thrust Th = gMtotal, where > 1 is a safety factorto be chosen by the designer (e.g. = 1.2).

Step 2: Selecting a set of propellers: 2a) Choose a set of commercially avail-able propellers Y whose radii are Ry Rmax y Y. 2b) For each propellery Y, calculate the minimum rotational speed y and the corresponding mini-mum mechanical power Py (using the expression given in Section 1) necessary togenerate the required given thrust Th. A propeller y is infeasible for the designif y > maxy , where maxy is the maximum allowed rotational speed of thepropeller y (specied by the manufacturer) and hence the propeller y must beexcluded from the design process. Let Y the set of all feasible propellers. 2c) Over

R. Gro et al. (Eds.): TAROS 2011, LNAI 6856, pp. 388389, 2011.c Springer-Verlag Berlin Heidelberg 2011

Designing Electric Propulsion Systems for UAVs 389

all propellers y Y, nd the minimum rotational speed and minimum mechan-ical power necessary to generate the required thrust; i.e., min = minyY(y)and Pmin = minyY(Py).

Step 3: Selecting a set of motors: 3a) Select a set of commercially availableBLDC motors Z such that z Z the following conditions are fullled: Pmaxz Pmin, maxz min and Mz < Mp where Pmaxz , maxz and Mz are respectivelythe power rate, the maximum rotational speed and the mass of the motor z. 3b)Construct motor-propellers groups Gj , j = 1, 2, .., n(Z), where n(Z) denotes thenumber of the motors in the set Z. The jth group Gj contains a motor zj Z anda subset of propellers Ij Y, where Ij := {y Y : y maxzj , Py Pmaxzj }.3c) For the jth group Gj , calculate V jy and I

jy y Ij , where V jy = ykvzj and I

jy

is obtained from the operational chart of the motor zj. V jy and Ijy are respectivelythe required voltage and current for the motor zj to rotate the propeller y atthe minimum speed y necessary to generate the required thrust. In group Gj ,the pair (zj , y), where y Ij , is feasible for the design if Ijy Imaxzj , whereImaxzj is the maximum allowed continuous current of the motor zj (specied bythe manufacturer). Select all feasible pairs in Gj , j = 1, 2, ..., n(Z).

Step 4: Selecting a set of batteries: 4a) For each feasible pair (zj , y) Gj ,select a set of commercially available battery packs Bjy such that b Bjythe following conditions are fullled: Vb V jy , Imaxb Ijy and Mb Mp Mzj , where Vb and Imaxb are respectively the eective voltage and maximumcontinuous discharging current of the battery b Bjy , and Mb is the massof the corresponding battery pack. 4b) b Bjy, calculate the mass of thepropulsion system (zj , y , b) and the full load (i.e. minimum) ight time as:Mzj,y,b = Mzj + Mb and tzj ,y,b =

I0bIjy

, where (zj , y) Gj is a feasible pair andI0b is the current rate (A.h) of the battery b Bjy. If tzj ,y,b < tf , the batterypack b cannot provide the required ight time when used with the pair (zj , y)and must be excluded from Bjy . 4c) Calculate Mzj ,y,b and tzj,y,b for all feasiblepairs (zj , y) Gj , j = 1, 2, ..., n(Z).

Step 5: Based on the values of Mzj ,y,b and tzj ,y,b for all feasible pairs, wecan choose the best design to achieve maximum ight time (zt , yt , bt) or mini-mum propulsion system weight (zw , yw , bw) or a design that satisfy a trade obetween the two factors.

References

1. Freddi, A., Lanzon, A., Longhi, S.: A feedback linearization approach to fault tol-erance in quadrotor vehicles. In: Proceedings of The 2011 IFAC World Congress,Milan, Italy (2011)

2. Moir, I., Seabridge, A.: Aircraft Systems: Mechanical, Electrical and Avionics Sub-systems Integration, 3rd edn. Wiley, Chichester (2008)

3. Phillips, W.F.: Mechanics of Flight. Wiley & Sons, Inc., Chichester (2004)4. Seddon, J., Newman, S.: Basic Helicopter Aerodynamics, 2nd edn. Blackwell Science,

Malden (2001)

Designing Electric Propulsion Systems for UAVsIntroductionDesign MethodologyReferences

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