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58 || AWSAR Awarded Popular Science Stories - 2019
Fixed-wing UAVs can be designed in various sizes depending on the mission and payload to be carried. The larger UAVs are generally powered with internal combustion (IC) engines. The smaller UAVs are powered by either electric motors or IC engines. A brief comparison of these two propulsion systems has been made here. The electric motor propulsion system consists of motors and batteries. Over the past few centuries, battery technology has improved significantly. Specific energy is one of the most important parameters of a power source, which is defined as energy stored per unit mass of the power source. The higher the value of specific power, the more would be the payload carrying capability or the range of the UAV. The specific energy of a lead-acid battery developed during the 1960s is typically around 40 Wh/kg. The modern lithium-based batteries offer specific energy around 750 Wh/ kg. The motors have also improved drastically. Modern-day motors are very
compact, efficient and able to
run at very high speeds. On
the contrary, the IC engine
propulsion system consists of
IC engines and chemical fuels.
The fuel can be gasoline, diesel,
methanol, etc. The specific
energy of gasoline is more
than 11,650 Wh/kg, which is
approximately 15 times greater
than that of modern batteries.
IC engines have an efficiency of
around 25%. This is much less
than the efficiency of motors.
However, since weight comes
as a premium in aerospace
applications, IC engines are
preferred for most medium-to-
high endurance applications.
The goal of the present
work was to further enhance
the specific power of the UAV engines by
increasing the power output of the engine without adding additional weight or by increasing the complexities. It is well known that the engines run at full power only during the take-off of the flight. This lasts for typically 3–10 min of the flight depending on the flight size. For the rest of the flight, the engine runs at only part of the full power; hence, the engine is oversized for this regime. In the present work, it was proposed to size the engine for cruise conditions and obtain the additional power required during the take-off with the use of fuel additives such as nitromethane. This kind of operation is somewhat similar to the afterburners in military jet aircraft, where the engine is sized for cursing while the afterburner provides additional thrust when required. Nitromethane, popularly called as nitro, is a liquid chemical. which is denser than gasoline. Although the calorific value of nitromethane is less than that of gasoline, the advantage of
adding it comes from the fact that the stoichiometric air fuel ratio, which is the amount of air required for complete burning of the fuel, is very low (1.7) compared with gasoline (14.5). Hence, more nitromethane can be pumped in for the same amount of air being drawn by the engine, and so the energy input would be higher. Tests were conducted to study the effect of nitromethane addition to the base fuel on the specific power output of the engine.
An engine test rig to characterize performance, combustion and emission parameters of the UAV engine was built and instrumented. The test rig was capable of measuring the thrust, torque, power, engine
speed, fuel consumption rate, temperatures at
Unmanned aerial vehicle (UAV), popularly known as the drone, has recently grabbed the attention of many engineers for both defence and civil applications. A wedding is almost incomplete without a drone taking photographs of the ceremony. Drones have found applications in surveying, cargo delivery and medical supply for remote locations, traffic monitoring, and disaster management and also combat missions.