In conventional micro-air vehicles and other ducted fan unmanned aerial vehicles, the vertical duct is unified and indivisible. Within this indivisible duct, the fuel is typically contained in a shallow, horizontal bladder that lies around the duct. The bladder contains a piccolo tube that similarly circles the entire duct. The piccolo tube comprises an elongated section of tubing that includes one or more openings for taking up fuel from the fuel bladder. The piccolo tube is in turn connected to downstream hoses to facilitate fuel delivery to the engine.
As the engine consumes the fuel contained in the fuel bladder, the air/fuel ratio inside the bladder increases. As the air/fuel ratio reaches high levels (e.g., greater than 1:1), the chances of air or fuel vapor ingestion increases. Thus, when the aerial vehicle pitches forward or banks to the side, all the fuel will rush to one side exposing the remainder of the piccolo tube to air in the bladder, which then allows air to be drawn into the fuel line.
When the engine ingests air or fuel vapor, it typically stalls. With conventional piccolo tubes, the engine often stalls due to air and/or fuel vapor ingestion prior to consumption of all of the fuel contained in the fuel bladder. As a result, the run time of the engine is unduly shortened. Additionally, closed (i.e., unvented) fuel systems conventionally rely on the integrity of the vacuum created and maintained within sealed containers or collapsible bladders to prevent the intrusion of air and/or vapor into the system. Such systems generally do not provide countermeasures to remove internally generated fuel vapor and/or air that enters due to improper fueling or leaks. Accordingly, the total volume of air and/or fuel vapor inside the various components (e.g., fuel bladders, tanks, lines, etc.) of a closed system can reach critical levels capable of progressing through the fuel lines into the engine and thereby inducing engine-seizure.
In order to counteract this, wicking filters have been implemented around the piccolo tube. These wicking filters use the capillary transport properties of a wicking material to increase the amount of fuel that can be reliably drawn by a piccolo tube prior to engine seizure or fuel starvation, even in the presence of excessive ratios of air to fuel (e.g., greater than 1:1), and despite variations in temperature, altitude, and orientation.
The wicking material can be associated with the piccolo tube and can have numerous microporous conduits that extend within a fuel container. The wicking material expands the accessible fuel region within the bladder to nearly any location within the bladder that the wicking material contacts. As a result, the proportion of fuel within the bladder that is consumed prior to engine seizure or fuel starvation is increased.
The configuration of a shallow, horizontal fuel bladder, piccolo tube and wicking filter has several drawbacks. For instance, the bladder is difficult to manufacture and install within the duct without leaks. The bladder can also develop kinks and is a closed, unvented fuel container that increases the likelihood of introducing air into the fuel line. In addition, refueling the bladder is difficult and time consuming, which exposes soldiers to possible enemy fire. The refuel timing issue is exacerbated at night when it is very difficult, if not impossible, to observe the fueling process to ensure that no air is introduced into the fuel system. Further, a fueling syringe or auto fueler is required, which adds additional equipment requirements and weight. Moreover, as the fuel is consumed, the aerial vehicle's center of gravity is significantly altered leading to vehicle instability.