Multirotor Unmanned Aircraft Systems (UAS) Advanced Design

for High Tension Power Line Inspection+

Kathy Swift1, 

Christie Bielmeier1, 

Engineering Design Department, 

Pinnacl X Corporation, 

Moodus, CT

Alfred Gates2, 

Fu-Shang Wei2,

Engineering Department, 

Central Connecticut State U.

New Britain, CT

Carl Tyburski3

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 tension 

power lines and support structures. The multirotor is capable of maneuvering into positions that is not possible 

when using helicopters or bucket trucks. The 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 having the ability to fly in higher wind conditions, hover longer 

times on station, navigate 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 

performed 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 (in2)

Aafx: Frontal area of airframe (in2)

Acax : Frontal area of camera arm (in2)

Acx : Frontal area of camera (in2)

Aafy : Top area of airframe (in2)

Cd: Drag coefficient

d: Distance (in)

E: Modulus of Elasticity (lb/in2)

F: Force (lb)

f: Frequency Hz

I: Moment of inertia (in4)

L: Length (in)

Lbat: 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.

1Design Engineer, PinnaclX Corporation, Moodus, CT.

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

3Project Manager, Northeast Utilities, Hadley, MA.

Lc: Camera distance (in)

Lcg: Center of gravity distance (in)

M: Mass (slug)

Ρ: Mass density (slug/ft3)

T: Thrust (lb)

Tx: Thrust in X direction (lb)

Ty: Thrust in Y direction (lb)

V: Wind velocity (ft/sec)

Wc: Camera weight (lb)

Wt: Weight (lb)

: Air density (slug/ft3)

Θ: Airframe angle (deg)

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 applications1-4. CCSU Department of 

Engineering has teamed up with Northeast Utilities 

and Pinnacl X LLC to conduct multirotor unmanned 

aircraft system design, flight testing in close 
proximity of high tension power lines and support 

structures. Recently, CCSU Department of 

Engineering has received several Certification of 

Authorizations (COAs) from the FAA5to 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 Multirotor UAS Testing

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 helicopters, 

bucket trucks and line crews that climb the towers. 

Additionally, the test flight identified 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 wiring6. 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 takeoff and landing 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 

extremely 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 

cause positioning problems with the baseline 

aircraft. A new quadrotor called, Chameleon Pro 

manufactured by Pinnacl X, was developed as a 
result 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 improved 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 be 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 design 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. Additionally a parametric study was 

performed to quantify the improvements cause by 

repositioning the camera. The parametric study 

involved examining the maximum wind velocities 

that the quadrotor can hover in for different angles 

relative to the quadrotor airframe. 

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 determined 

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 are 
incorporated in an advanced multirotor UAS design 

which include6;

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 meters

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 that 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 

designed 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 method7to be well away 

from the hovering frequency range, Figure 3.

The frequency modes of aircraft arms were 

calculated using a cantilevered beam with a 

concentrated mass at the end, equation 18.

(1) 

The arm with the motor at the end was calculated to 

be 65 Hz and finite element results were 62 Hz7.

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 Hz
After 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 under 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 

condition. 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 basic shapes9-12. 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 motors to produce 

the maximum thrust, Equations (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 Aafx cos (θ) V2

Fdx {camera arm} = (1⁄2) ρ Cd Acax cos (θ) V2

Fdx {camera} = (1⁄2) ρ Cd Acx cos (θ) V2

4 Ty – Fdy {airframe} – Wt {airframe and battery} 

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

 

Where; 

Fdy {airframe} = (1⁄2) ρ Cd Aafy sin (θ) V2

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 integrated 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. The 

maximum winds are 27 Knots, 32 Knots and 34 Knots 

for the different configurations, 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 quardortor has a hovering 

performance range 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 plots 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 frequency 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, with 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 higher winds. 

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2014, Hadley MA

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