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Scripps Composite Water-landing Multicopter

Background:

CORDC(Coastal Observing and Development Center) has been developing multicopter systems for use as oceanographic sensor platforms. While these systems have unprecedented capability in terms of sensor delivery, operation in the maritime environment presents unique mission challenges. Increasing system range and takeoff/landing capability will result in feasibility for taking new oceanographic measurements.

Objectives:

The primary objectives were to decrease weight of airframe through the use of composites and to be capable of water landing. Moreover, the user should be able to recover the multicopter from the surface of the ocean via the research vessel with limited salt water damage.

The current model's (3D Robotics Hexa-B multicopter) frame is made out of aluminum. With the new lightweight airframe, the overall weight shall not exceed the current weight with the additional components installed. The composite airframe improved the overall appearance. On the other hand, a float system would be implemented to minimize salt water damage. Not only would the float system minimize water damage, but it would also enable quick retrieval. To further protect the electronics from water damage, an enclosure for the electronics was designed.

Description of Final Design:

    The final design was composed of three major components: electronics enclosures, composite airframe and the float system. The configuration of the multicopter was identical to the current hexacopter model with six motors. The composite airframe consists of the six carbon fiber tubes as arms and two carbon fiber plates. 

Waterproofing electronics

In terms of water protection, the electronic speed controllers (ESCs) were coated with thermally conductive epoxy as shown in the figure below. With that, it was possible to mount the ESCs externally to keep them cooled and easily accessible. Furthermore, the enclosures protect the autopilot, battery, power distribution board, radio and all other electronics. For the brushless motors, a WD40 treatment was applied.

Thermally potted ESC

1.Enclosure for Electronics

    The enclosure consists of a lower housing, upper housing and two machined carbon fiber plates. The upper and lower housing prototypes were created using a 3D printer as depicted below. Using these rapid protoype molds, a fiberglass mold was made. After the fiberglass molds cured carbon fiber parts were created. In terms of assembly, quick release fasteners were used to hold the upper and lower housings together to the carbon plates. The plates have a 3D printed pass through for wires to connect the electronics between the upper and lower housing.

Upper and lower housing rapid prototypes

Quick Release Fasteners

    The enclosures are capable of protecting the vital electronics from water splashing and limited water submersion. With the use of quick release fasteners, the enclosures were easy to remove. Moreover, a gore vent may be utilized to allow the Ardupilot's barometer to read ambient air pressure for the purpose of pressure equalization and altitude readings.

2.Composite Airframe

  To fulfill the first primary objective of decreasing the weight of the craft the use of composites, namely carbon fiber, was utilized through the entirety of the structure. The original 3DR Hexa-B multicopter comprised of a 2mm thick G10 composite dual center plate system with (6) 18mm square aluminum tube arms. The final design of the structure ustilizes 1.6mm thick carbon fiber plates for the dual center plate system and (6) 8mm unidirectional carbon fiber square tube arms.

Composite plates and arms

    In comparison the compostite arms are considerably smaller than the original aluminum arms. The weight savings using the composite arms was considerable, a set of 6 composite arms weighs in at 52g compared to 6 aluminum arms weighing a total of 258g. In addition, the smaller arms minimized the surface area thus decreasing the potential drag experienced by the craft. Since the arms of the final design are considerably smaller than the original design, the motors had to be mounted utilizing  cross shaped motor mounts which provided the necessary mounting area.

Size comparison of 1 Aluminum arm to 6 Carbon Fiber arms

Cross shaped motor mount

3. Float System

    The purpose of the float system is to allow the multicopter to land on the surface of the water. With water landings, the copter can be easily recovered while experiencing minimal salt water submergence. There were several functional requirements for the float system. First, the floats should not significantly affect the aerodynamics which would minimize the effects of power consumption of the current model. In addition, the floats should not add significant weight to the multicopter.

For the final design, polyethylene closed-cell foam tube was used with a diameter of 7.62 cm (3 inches). A simple buoyancy analysis was performed to find out the volume of float materials needed for the multicopter assuming the float materials will be half submerged, assuming the Factor of Safety to be 2. Assuming the float materials to be closed-cell polyethylene with a density of 16.02kg/m3 and the salt water density to be 1027kg/m3, the final volume came out to be 3600 cm3  based on the buoyancy force balance equation.

    With the total required volume of floats and the given diameter of the polyethylene tube, the total length of the tube was calculated to be 78.94 cm. In terms of the orientation of the floats, two cylinders were attached to the landing gear in parallel shown in following figure, making each cylinder half of the total length of the tube. The advantages of this orientation are to maximize stability and surface area.

Float Orientation

Aerodynamic effect of floats

     A test flight was performed with and without the float system to examine if the aerodynamic effect of the added float system significantly lowered the performance of the current model. In addition, it should be noted that the test without floats would not have equal mass to that with floats. To ensure that the primary change for the multicopter was the geometry of the floats, a ballast was added to the multicopter when the floats were removed. The current draw from the battery is shown in the figure below, which represents an increase in power of 3.3% with floats. The increase was significant but acceptable with the battery it carried. The battery would provide sufficient amount of power for one mission even with the floats.

Float test flight data with and without floats

Prototype Performance

    After calibration and syncing of the ArduPilot with the ground station the preliminary test flight was conducted to verify the craft was flight capable. The craft was flown for 2 minutes without the float system and was successful. Upon landing the float system was attached to test any adverse effects of the floats on the multicopter. Again the craft was flown for 2 minutes and despite the addition of the floats did not exhibit any adverse behavior during flight. Each flight tested the basic function of the craft up to an altitude of approximately 80-90 meters and performed flawlessly. 

A flight/waterlanding test was also done at the Pier. The following video shows the multicopter being flown and landed on the surface of ocean.

Pier Flight Test