### Multirotor Unmanned Aircraft Systems (UAS) Advanced Design for High Tension Power Line Inspection +



### K athy Swift 1



### , C hristie Bielmeier 1



### , Engineering Design Department,

Pinnacl X Corporation,
Moodus, C T 

### A lfred Gates 2



### , F u
- S hang Wei 2



### , Engineering Departme nt,

Central Connecticut State U.
New Britain, CT 

### C arl T yburski 3

Transmission Lines and

Contracts

, Northeast Utilities,
Hadley, MA



### Abstract

This paper presents an unmanned multirotor aircraft system design for flight in close proximity of high tensio n power lines and support structures. The multirotor is capable of maneuvering into positions that is not possible
when using helicopters or bucket trucks. T he UAS can be positioned in between the high tension lines and
support structures due to the small sizes

. Over 100 test flights have been performed involving complex maneuvering procedures which require operation within 2 meters of power lines for inspection. The goal of this project is to develop an advanced UAS design ha ving the a bility to fly in high er wind conditions

, hover longer
time s on station, navigat e around power lines autonomously , and maintain position in the event of a loss of GPS signals

. During the UAS system design improvement, a parametric study on a new UAS airframe design has been

per formed with a more compact configuration

. In addition, the design feature

, also includes an integrated camera system that is not suspended below the UAS making it less susceptible to undesired movement caused by aerodynamic forces.



### Nomenclature

A: Area (in 2 )

A afx : Frontal area of airframe (in 2 )

A cax : Frontal area of camera arm (in 2 )

A cx : Frontal area of camera (in 2 )

A afy : Top area of airframe (in 2 )
Cd: Drag coefficient
d: Distance (in)

E: Modulus of Elasticity (lb/in 2 )

F: Force (lb)
f: Frequency Hz

I: Moment of inertia (in 4 )

L: Length (in)

L bat : Battery distance (in)

+ Presented at the Sixth AHS International Specialists Meeting On Unmanned Rotorcraft Systems: Platform Design, Autonomy, Operator Workload Reduction and Network Centric Operations. Scottsdale, Arizona 85215, January 20

- 22, 2015. Copyright@2015 by the American Helicopter Society. All rights reserved.

1 Design Engineer, PinnaclX Corporation, Moodus, CT.

2 UAS Pilot & F aculty, CCSU, New Britain, CT. AHS Member

3 Project Manager, Northeast Utilities, Hadley, MA. L c : Camera distance (in)

L cg : Center of gravity distance (in)

M: Mass (slug)

Ρ: Mass density (slug/ft 3 )

T: Thrust (lb)

Tx: Thrust in X direction (lb) Ty: Thrust in Y direction (lb)

V: Wind velocity (ft/sec)

W c : Camera weight (lb)

W t : Weight (lb)

 : Air density (slug/ft 3 ) Θ: Airframe angle (de g) UAS: Unmanned Aircraft Systems



### Introduction

Multirotor UAS platforms have been found to be a useful tool for video, still photos and data sampling in many applications 1

- 4 . CCSU Department of Engineering has teamed up with Northeast Utilities
and Pinn acl X LLC to conduct multirotor unmanned aircraft system design, flight testing in closeproximity of high tension power lines and support

structures. Recently, CCSU Department of Engineering has received several Certification of Authorizations (COAs) from the FAA 5 to perform flight testing of a baseline multirotor at multiple high tension power line sites owned and operated by Northeast Utilities as shown in Figure 1.

.

Figure 1 : Baseline Multiro tor UAS Test ing

CCSU and Northeast Utilities have conducted the flight test to determine if multirotor UAS can be advantageous as an inspection tool compared with the current high tension and structural inspection
methods which include the use of helico pters, bucket trucks and line crews that climb the towers. Additionally, the test flight identif ied the responsibilities of the pilot and observer along with

areas of needed improvement

.

The UAS flight test have verified that multirotors
with a camera system that includes video and still photo capabilities are useful in providing detailed images of structural hardware and wiring 6 . Real time video is transmitted on a 5.2 GHz frequency to a ground control station. Also telemetry is transmitted

over a 900 MHz down link and the multirotor UAS is controlled over 2.4 GHz frequencies.

The baseline multirotor is capable maneuvering into extremely difficult or impossible positions when using helicopters or bucket trucks. As a result of the
multirotor UAS close proximity positing the photos are more detailed than previous methods. Also due to the small size of the UAS, the UAS can be positioned in between the high tension lines, support lines and structures which are not possible with helicopters, Figure 2 . Also for a bucket truck to be used to obtain the pictures the truck would have to be repositioned for many photos and in some cases it would not be possible. The UAS also can be deployed faster than current methods since it only requires a take off and la nding point within visual

range of a structure. This is very usefully when the flight is conducted over steep terrain or wetlands. This would be very costly and time consuming using
bucket trucks.

The baseline aircraft demonstrated that using UAS is

extr emely beneficial for structural inspection. From the over 100 test flights next to energized high tension power lines and structures also identified areas of needed improvement to expand the flight envelope which would increase the numbers of days
that the aircraft can be deployed. One of the areas is the need to improve the flight envelope, the ability to fly in higher winds. During the flight testing it was observed that turbulence, wind shear and updrafts were common during light wind conditions which

ca use positioning problems with the baseline
aircraft . A new quadrotor called, Chameleon Pro manufactured by Pinnacl X, was developed as aresult of the quadrotor testing. The Chameleon Pro

is capable of station keeping in higher winds in all flight modes.

Figure 2 : Multirotor UAS Operations Involving Complex Maneuvering, Left; UAS Next to or Approaching Structures, Right; Photo from UAS

The overall goals of the design process was to reduce the work load imposed on the Auto Pilot resulting in imp roved performance such as flight in higher winds , wind gust and flight with unbalance payloads. The improved performance was achieved by repositioning the payload from a suspended position to integrating it into the airframe. The improved performance can b e measured by the ability to maintained position in higher winds

. Typically the payload for multicopters is a high resolution camera system.

A natural frequency analysis was first performed to ensure that the primary frequency is not in the range
of the d esign hovering frequency. The quadrotor arm deflection was also analyzed to ensure that the deflection is acceptable. A quadrotor arm that has a
visible deflection will increase the work load of an autopilot and in some cases cause uncontrolled flight. Add itionally a parametric study was performed to quantify the improvements cause by repositioning the camera. The param e tr ic study involved examining the maximum wind velocities that the quad rotor can hover in for different angles relative to the quadrotor air frame

.

The test flights involved operational procedures, complex maneuvering, within 2 meters of power lines and structures, to determine the benefits and deficiencies of using multirotor Unmanned Aircraft Systems, UAS, for inspection. It has been determ ined that multirotor UAS are capable of; power line tower hardware inspection, rapid deployment, and have a minimal environmental impact compared with current inspection methods. Five areas of need improvement have been identified and areincorporated in a n advanced multirotor UAS design

which include 6 ;

1. Ability of flight in higher winds

2. Ability to fly for extended periods of time

3. Ability to program an UAS to navigate around power lines autonomously

4. Ability to hold position with a better accuracy than .5 me ters
5. Ability of maintaining position in the event of a loss of GPS signals

A parametric study on a new multirotor UAS airframe design, has been performed, with more compact configuration. An additional design feature includes an integrated camera system t hat is not suspended below the UAS making it less susceptible to undesired movement caused by aerodynamic
forces.



### i) Resonant Natural Frequency Analysis

The multirotor frame was designed with a specific motor propeller combination. The aircraft was design ed for a combination video and still picture camera payload. Once the electronics battery and camera systems were identified the overall aircraft weight was calculated followed by selecting the motor propeller combination. Next the power required to hover the aircraft is estimated followed by identifying the hovering rpm. From the hovering rpm the primary natural frequency range to avoid
was calculated to be 88Hz + or

– 5% making the primary frequency range to avoid 84Hz to 92Hz. The natural frequencies of the propellers were analyzed using the finite element method 7 to be well away from the hovering frequency range, Figure 3.

T he frequency modes of aircraft arms were calculated using a cantilevered beam with a concentrated mass at the end, equation 1 8

.

(1)

The arm with the motor at the end was calculated to
be 65 Hz and finite element results were 62 Hz 7

. 

### Figure 3: Propellor Frequency Modes

Figure 4: Quadrotot Arm Frequency

The closest airframe frequencies were solving to be
76 Hz and 138 Hz , figure 5, which is far enough away from the hovering frequency.

Figure 5: Quadrotor Frequencies Closest to 88 HzAfter the frequencies have been calculated and

there is no frequency matching with the propeller, arm and aircraft body the parametric study can be performed.



### ii): Parametric Study on Hovering in Windy
Conditions

The first parametric study involved comparing three different quadrotor airframe configurations; airframe without a camera, airframe with a camera suspended unde r the airframe and the airframe with and integrated camera as shown in Figure 6.

Figure 6: Airframe Configurations Used For the

Parametric Study

The parametric study involved determining the available thrust to maintain position in a head wind
conditi on. The required thrust in the horizontal and vertical directions were calculated using angles ranging from 10 to 45 degrees subject to a steady state wind velocity as shown in Figure 7. Aerodynamic forces were calculated using the drag
coefficients for ba sic shapes 9

- 12

. Next the maximum wind velocity was calculated based upon the
available thrust in the horizontal and vertical directions. The steady state equations in the horizontal and vertical directions were balanced by solving the velocity to cause the mo tors to produce the maximum thrust, E quations ( 2 ) and ( 3 )

. This method only provides a potential of performance improvements. Wind tunnel and actual flight testing in wind conditions needs to be performed to verify this methodology.



### 4 Tx

– Fdx {airframe}

– Fdx {camera arm}

- Fdx {camera} = 0 (2)

Where; Fdx {airframe} = (1⁄2) ρ Cd A afx cos (θ) V 2
Fdx {camera arm} = (1⁄2) ρ Cd A cax cos (θ) V 2
Fdx {camera} = (1⁄2) ρ Cd A cx cos (θ) V 2

4 Ty

– Fdy {airframe}

– Wt {airframe and battery}

- Wt {camera and arm} = 0 (3)

Where; Fdy {airframe} = (1⁄2) ρ Cd A afy sin (θ) V 2

Figure 7: Airframe with Suspended camera Subjected to External Forces

The results indicated that the aircraft without a camera will be capable of flight in the highest winds
and the aircraft with the i ntegrated camera will be capable of flight in winds close to the aircraft without the camera as shown in Figure 8

.Figure 8: Quadrotor Capability of Hovering in Winds

Figure 8 indicates that the improvement in performance is dependent on the angle of the quadrotor . Also the results indicate that the angle of 25 degrees from the horizontal produces the highest hovering capability for all of the cases analyzed . T he maximum winds are 27 Knots, 32 Knots and 34 Knots for the different configuration s, with the camera suspended, with the integrated camera and without the camera. Also the percent improvements in performance are plotted in Figure 9 which is based on the hovering capability in higher winds.

Figure 9: Hovering Performance Improvements The maximum hovering performance improvement is at the smallest angle and it decreases as the quadrotor angle from the horizontal increases. The quadrotor without a camera has a hovering performance range from 37 to 16 percent and the integrated camera qu ardortor has a hovering performance r ange from 34 to 10 percent when compaired with the quadrotor with a camera suspended from the aircraft.

An additional parametric study was performed where the aircraft weight was reduced by 17 percent and the same plo ts were produced. The hovering performance improvements were the same as compared with the original aircraft at 100 percent weight, Figure 9. The hovering capability in higher winds increased as the aircraft weight decreased. Figure 10 displays the maximum wind velocity for hover with respect to having a camera payload and a reduction in the quadrotor airframe weight.

Figure 10: Hovering Wind Velocity for Different Quadrotor Airframe Weights 100 % and 83%

### Future Work

The parametric study indicates that positioning a payload around a multirotor airframe will impact the performance resulting in capabilities of hovering in higher winds. An additional parametric study will be performed that examines the quadrotors mass
moment of inertia which if reduce will decrease the response time to a wind load that disturbs the quadrotor from a level attitude . Also wind tunnel testing will be performed to verify the performance
data.



### Conclusion

Basic design requirement for a quadrotor was
discussed which include f requency avoidance of the propellers, arms and airframe. A parametric study was performed that measures a quadrotors hovering capability in winds based on the position of a camera system. Three camera configurations using the same airframe were examined, w ith a camera suspended from the airframe, with a camera integrated into the
airframe and the airframe without a camera. The results indicate that repositioning a camera next to the airframe rather than suspending it will improve the ability to hover in hig her winds.



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