Multicopter Photography and Racing: A Comparison of Commercially Available Platforms (UNSY 603 Activity 3.4)

Introduction

This paper will identify two commercial-off-the-shelf (COTS) Unmanned Aircraft Systems (UAS) platforms, each to be used in a different application. One will be used in aerial photography and videography below 400 ft. above ground level (AGL), and the other will be used for racing utilizing a first-person view (FPV) camera. These applications require the same basic task: to achieve flight, carry a camera, and transmit the imagery from that camera to its operator in real-time. The difference between the platforms will be the regimes of flight that they are expected to carry out. The photo/video UAS will need to fly stably, carry a large camera with a mechanical stabilizing device (a gimbal), and fly for extended durations. The racing UAS will need to have a high power to weight ratio, a small imaging device, and a power system optimized for high current draw and short flight times.

Aerial Photography and Videography

DJI is a manufacturer of aerial photography and videography UAS of various sizes and configurations. The decision of which platform to use depends on factors such as the user’s experience level, size of the camera, expected duration of flight, and the user’s budget. This paper will evaluate the DJI Inspire 1. While this is not the best or most capable UAS platform for this task, it is an excellent entry-level UAS at a good price point with good capabilities.

The Inspire 1 was released in late 2014 in time for the Christmas holiday and the Consumer Electronics Show in early 2015. This UAS was advertised as a Ready-to-Fly (RTF) platform for aerial photography and videography and marketed towards individuals with little to no experience with UAS. The Inspire 1 makes use of the operator’s existing portable electronics, such as a smartphone or tablet, and allows for single or dual operator control (one operator flies the aircraft, the other manipulates the camera system). The camera for the Inspire 1 uses a Sony 1/2.3 sensor capable of shooting Ultra High Definition video (4096 x 2160 pixels, also known as 4K video). The sensor is a Complementary Metal-Oxide Semiconductor (CMOS) Sensor, which is cheaper and easier to produce than a Charge-Coupled Device (CCD) Sensor, but may cause some undesirable effects in the images. CMOS Sensors scan line by line, in what’s called a “rolling shutter.” CCD sensors capture every pixel at once, in what’s called a “global shutter.” Rolling shutter becomes apparent when shooting fast moving objects, or when the camera is being moved quickly at the moment the image is taken. The result is that fast moving objects appear to be slanted to one side (Fig. 1).

Fig. 1: Rolling Shutter Effect

For aerial photography UAS, the designer usually places the sensor well below the aircraft, where it will have a good view of the ground, unobstructed by the airframe and propellers. Usually, this requires the airframe to have retractable landing gear, or have landing gear that rotate with the camera (Fig.2). The Inspire 1 utilizes a unique retract system that reconfigures the shape of the airframe after takeoff to give the camera a 360^o unobstructed field of view (Fig 3). The aircraft configures itself automatically using an ultrasonic altimeter to detect its proximity to the ground.

Fig 2: Retractable Landing Gear (Top) and Rotating Landing Gear (Bottom)

Fig. 3: DJI Inspire 1 Flying Configuration (Top) and Landing Configuration (Bottom)

With its low cost, simplistic operation, out-of-the-box functionality, and an approximately 18 minute flight time, the DJI Inspire will accomplish most amateur aerial photographers and videographers’ goals. If the objective is to carry larger, industry standard cameras for professional photography and videography, the author suggests the DJI Spreading Wings S-800, S-900, and S-1000 UAS.

First Person View (FPV) Racing

The foremost goal in racing vehicles of any type is to have a high power-to-weight ratio. That is to say, have a great deal of power compared to the total weight of the vehicle. Multicopters, and helicopters in general, were created to take off and land vertically, and to maneuver in all directions. They are not particularly suited for moving in any direction at great speed, compared to their fixed-wing aerial counterparts. For helicopters, their speed is limited by a phenomenon known as retreating blade stall- when the side of the rotor disk rotating to the aft cannot move through the air fast enough to generate lift and the aircraft stalls sharply to one side, invariably causing catastrophic failure. Multicopters are not limited so drastically. Their speed limitation is a simple physics vector equation (Fig. 4). The aircraft requires a certain amount of thrust to sustain level flight- not surprisingly, this thrust required is equal to the aircraft’s weight. To achieve lateral flight, the propulsion system must produce not only the power to support the aircraft’s weight, but also the power to overcome aerodynamic drag. It becomes possible, if the aircraft has sufficient power available, to angle the aircraft significantly and achieve high lateral speeds. However, because multicopters use the variance of speed of their motors to maneuver, utilizing all of their available power means there is no remaining power left to maneuver. The author has experienced this effect while flying a multicopter at its maximum speed, resulting in an uncontrolled descent and subsequent crash at high speed. For this reason, Multicopter autopilots are programmed with a pitch and roll limit which results in a top forward speed.

Fig. 4: Vertical and Horizontal Components of Lift

The sensing system on an FPV racing multicopter needs to be lightweight, have a large field of view, and should be relatively unobstructed. To achieve high speeds, the aircraft must pitch forward sharply to accelerate, and an unstabilized camera subsequently faces toward the ground and the pilot no longer has a view of the forward flight. Therefore, it is prudent that the camera have a simple servo-controlled pitch stabilizer to maintain forward visibility during acceleration. Many FPV racers use the GoPro camera for its wide field of view (170^o), small size, and high resolution recording capabilities. There are certainly smaller cameras, such as the Sony board camera line, that are more compact and lighter, but if the pilot wishes to record the race in high-definition to review later, the GoPro is a good choice. In order to achieve a view mostly unobstructed by propellers, the camera for an FPV racer is typically placed as far forward on the airframe as possible. There is no compelling reason to place the camera underneath the frame, as photo/video UAS do. This results in unnecessary drag and necessitates the use of large landing gear, which also contribute to drag. Many FPV racers utilize a frame shape that places the midsection of the aircraft, and hence the camera, as far forward as possible, which is grotesquely referred to as the “dead cat frame” (Fig. 5). Most FPV racers purchase the components for their multicopter separately (frame, motors, propellers, speed controllers, battery, and electronics) and assemble the airframe themselves. This is a very cost-effective way to purchase a multicopter.

Fig. 5: “Dead Cat” Quadcopter Frame

As the focus of this paper is to evaluate COTS multicopters, the best off-the-shelf offering for a racing multicopter is most likely the Blade 350 QX. The 350 QX has a nearly 4:1 power to weight ratio, an “agility mode” that gives the pilot a great deal of controllability and maneuvering range, and a flight time of about 15 minutes. With the addition of a GoPro camera or similar, the 350 QX is a very competitive FPV racing UAS.

Fig. 6: Blade 350 QX

References

DJI Inspire 1 Specifications. (n.d.) DJI, Inc. Retrieved from: http://www.dji.com/product/inspire-1/spec

Active Pixel Sensor (APS). (n.d.) Wikipedia. Retrieved from: http://en.wikipedia.org/wiki/Active_pixel_sensor

Rolling-Shutter-Effekt. (n.d.) Wikipedia Deutschland. Retrieved from: http://de.wikipedia.org/wiki/Rolling-Shutter-Effekt

Blade 350 QX. (n.d.). Horizon Hobby, Inc. Retrieved from:  http://www.bladehelis.com/350qx/