1. Introduction
Technological advances in unmanned aerial vehicles (UAV) have made it possible to operate drone deliveries of packaged products. Since 2011, the drone firm Matternet has used drones to deliver medical supplies and specimens to countries around the world, including Switzerland, Haiti, and the Dominican Republic [
1]. In 2017, Matternet and Mercedes-Benz collaborated and used vans as rolling distribution hubs (drone-van combos) for a three-week aerial package delivery to deliver goods for local e-tailer Siroop in a Swiss city [
2]. JD.com in China plans to build 185 drone ports in China’s mountainous southwest region to cut logistical costs [
3]. The logistics affiliate of e-commerce titan Alibaba, Cainiao, recently used drones to deliver tea leaves from mountain slopes directly to processing centers below, shortening the time to market [
4]. The Icelandic company, Aha, which launched a limited trial with Israeli company Flytrex in 2019, will phase in further drones over the next two years [
5]. UPS and DHL also conducted trial runs for residential delivery [
6].
Similar to other transport means, the drone and its payload experience vibration, shock, and atmospheric hazards during the take-off, in-flight, and landing [
7]. The vibration and shock to the drones are usually associated with two sources. The first is the structural vibration and shock resulting from periodic excitation from drone motors and propeller blades [
8]. Throttling up or down can increase or reduce the periodic excitation frequency. The second is the interaction between the drone quadcopter/propellers and the airflow, which is also variable as it depends on air turbulence and weather patterns. The atmospheric hazards are due to the atmospheric environment acting upon the goods without shielding. Understanding these input vibrations as well as the generated shock and atmospheric conditions will help packaging engineers better understand these hazards and provide sufficient protective design for various goods delivered by the drone.
In contrast to the numerous measurements on the truck [
9,
10,
11] and rail [
12,
13] environment garnered within the last 30 years, only a few major research studies have been conducted in the field of aircraft environment to better characterize its vibration, shock and atmospheric profile. The Forest Products Laboratory’s (FPL) General Technical Report 22 is an early report conducted on aircraft vibration during taxi, takeoff/landing, and cruise mode [
14]. The study indicated that the excitation frequencies are highly dependent on the type of aircraft engines such as turbojet and turboprop whereas the amplitudes are more dependent on the flight modes (takeoff, climb, cruise, and landing). For example, the engine of turboprop had an excitation frequency of 68 Hz for C-130 aircraft and 48 Hz for C-133 aircraft. However, the data included in the FPL report did not indicate overall G
rms (overall energy of random vibration) levels for the different aircraft types. In 1988, Trost conducted a research study to measure an aircraft’s acceleration, involving mechanical stresses during air cargo shipment [
15]. The recording device was mounted on the pallet in the air cargo. The measured maximum G-level ranged from 0.05 G to 0.16 G during cruising of the flight and 0.20 G to 0.42 G during the taxiing, takeoff, touchdown, descent and landing. The Amgen Air PSD profile was developed by mounting the data recorder to the floor of a unit load device (ULD) inside of a jet engine aircraft [
16]. The resulting PSD published had an overall G
rms level of 0.017 G
rms level. The collected vibration was not measured directly from air cargo but from the ULD.
In order to develop a vibration profile as direct input, Dunno and Batt [
17] mounted the recorder in the cargo area next to the wing of the aircraft and recorded the in-flight vibrations of a turbo propeller aircraft. The overall in-flight G
rms value measured during 30 individual flights was 0.155, which is significantly higher than that of the Amgen study. The maximum acceleration recorded was 2.11 G, which occurred predominantly during the ascent and descent of the aircraft. The typical steady-state vibration did not exceed an intensity of 0.2 g.
Figure 1 created by Dunno illustrated the comparison in air profiles between Amgen’s study, Dunno’s study, ASTM D 4169 Air (level II), and ISTA 4AB [
18,
19]. Although the time compression, from the actual field time to the laboratory testing time, was already factored in in both ASTM D 4169 level II and ISTA 4AB air profiles, both Dunno’s and Amgen studies exhibited a significantly lower vibration intensity compared to the ASTM D 4169 air assurance Level II and the ISTA 4AB. Currently, no drone vibration, shock, and atmospheric data specifically linked to the delivery of packaged products have been published.
The aim of this research is to capture and characterize the drone’s input vibration and shock, as well as the in-flight atmospheric condition, which could be utilized by packaging engineers to develop a test method for drone delivery. The vibration and shock profiles obtained from this research are intended to be applied in simulations for in-flight vibrations and shock of the drone chassis in a laboratory environment. The in-flight atmospheric profile was intended for preconditioning for the laboratory simulation.
4. Conclusions
The general shape and vibration intensity of the PSD profiles of the two drones showed no similarities to those that are currently used for representing package delivery via air transport. However, the PSD profiles of the drones selected for this study were similar in their overall shape. They share the same range of exciting vibration frequencies caused by the motor and 2-blade propellers due to the similar propeller speeds, which ranged from the minimal frequency 40–70 Hz for keeping drone steady, to high Grms values in the 165–182 Hz range in which the drones were at cruising speed. Both PSD profiles also share similar patterns in lower frequency zones, for which the interaction between the drone and airflow is responsible. The interaction of the drone with airflow, in the form of bump and lurch, is characterized by low frequencies between about 1.56 to 10 Hz.
The overall recorded Grms levels of the drones were significantly higher than those currently published for package testing for air transport. The recorded Grms values range from 0.49 to 1.69 in the lateral direction and from 0.54 to 0.99 in the vertical direction. The larger size drone generally tended to have higher Grms values than the smaller drone, especially in the horizontal plane. Most of the high vibration Grms values took place at the turning point of the drones.
The motor and 2-propellers are also sources of the shocks at the horizontal level. These shock values can reach numbers as high up as 14 G. The throttle jumps of the drone were likely created by the significant horizontal impacts. In contrast, the vertical shock levels, originally thought to have the greatest severity, were significantly lower than horizontal impacts, even during landing.
Fluctuation of atmospheric data during the flight is unique to the drone and attention should be paid when considering what kind of products can be shipped out as all the atmospheric conditions were affected by the change in altitude.
The data collected by the data logger on the platform of the payload and the data logger on the quadcopter frame are the most relevant information that can be applied to simulate in-flight vibration and shock of the drone chassis in a laboratory environment. Data in
Table 2 and
Table 3 can be used to drive vibration test systems and results shown in
Table 5 can be channeled into a shock tester for simulation. Future studies should involve additional trip destinations, utilization of different models of drones, as well as placing the data logger in different locations to develop test methods that could be used to evaluate packaged products traveling through this emerging delivery channel.