Fixed Wing Versus Rotary Wing For UAV Mapping Applications
UAVs (also known as drones) come in many shapes and sizes. Each of these have their own unique pros and cons. It is these characteristics which ultimately leads to the operator's decision in which platform will best fit the application. It is understanding these key attributes and acting on them will ensure that your mapping mission is a success.
Fixed Wing Or Rotary Wing UAV?
UAV aircraft currently boil down to two categories, fixed wing and rotary wing. As you may have guessed each of these categories can be further broken down, for example a fixed wing UAV can be high wing, mid wing, low wing and flying wing, again each having their own unique characteristic advantages and disadvantages. For the purposes of this article we will be focusing on the “top level” differences between the two.
Fixed Wing UAV
Fixed wing UAVs, such as the Q200 and DATAhawk, consists of a rigid wing that has a predetermined airfoil (again another variable) which make flight capable by generating lift caused by the UAV’s forward airspeed. This airspeed is generated by forward thrust usually by the means of a propeller being turned by an internal combustion engine or electric motor.
Control of the UAV comes from control surfaces built into the wing itself, these traditionally consist of ailerons an elevator and a rudder. They allow the UAV to freely rotate around three axes that are perpendicular to each other and intersect at the UAV’s center of gravity. The elevator controlling the Pitch (Lateral axis), ailerons controlling the Roll (Longitudinal axis) and the rudder controlling the Yaw (Vertical axis).
The main advantage of a fixed wing UAV is that it consists of a much simpler structure in comparison to a rotary wing. The simpler structure provides a less complicated maintenance and repair process thus allowing the user more operational time at a lower cost. More importantly the simple structure ensures more efficient aerodynamics that provide the advantage of longer flight durations at higher speeds thus enabling larger survey areas per given flight.
Another advantage of fixed wing UAVs is that the flght characteristics due to their natural gliding capabilities with no power.
Also worth considering is the fact that fixed-wing aircraft are also able to carry greater payloads for longer distances on less power allowing you to carry some of the bigger (more expensive) sensors as well as twin sensor configurations.
The only disadvantages to a fixed wing solution is the need for a runway or launcher for takeoff and landing however VTOL (vertical take off/landing) and STOL (short take off/landing) solutions are very popular to help eradicate this issue. Also fixed wing aircraft require air moving over their wings to generate lift, they must stay in a constant forward motion, which means they can’t stay stationary the same way a rotary wing UAV can. This means fixed wing solutions are not best suited for stationary applications like inspection work.
Rotary Wing UAV
Rotary wing UAVs consist of 2 or 3 rotor blades that revolve around a fixed mast, this is known as a rotor. Rotary wing UAVs also come in wide range of setups consisting of a minimum of one rotor (helicopter), 3 rotors (tricopter), 4 rotors (quadcopter), 6 rotor (hexacopter), 8 rotors (octocopter) as well as more unusual setups like 12 and 16 rotors! Like fixed wing solutions, these setups can be further broken down, for example a Y6 setup consists of a tricopter with twin rotors on each arm, one pointing upwards and one pointing downwards and an X8 consists of a quadcopter with twin motors on each arm. Again each setup has their own unique characteristic advantages and disadvantages.
Rotor blades work exactly the same way as a fixed wing, however constant aircraft forward movement is not needed to produce airflow over the blades, instead the blades themselves are in constant movement which produce the required airflow over their airfoil to generate lift.
Control of rotary UAVs comes from the variation in thrust and torque from it’s rotors. For example a quadcopter’s downward pitch is generated from the rear rotors producing more thrust than the rotors in the front, this enables the rear of the quadcopter to raise higher than the front thus producing a nose down attitude. Yaw movement uses the rotor’s torque force where diagonal rotors either spool more or less than their counter diagonal rotors thus producing an imbalance in the Yaw axis causing the quadcopter to rotate on the vertical axis.
Tricopters are the only exception to this where their rear rotor requires a servo to physically move the rotor to vector it’s thrust rather than using the rotor’s torque to enable vertical axis control.
The biggest advantage of rotary UAVs is the ability for takeoff and land vertically. This allows the user to operate with in a smaller vicinity with no substantial landing/take off area required. Their capacity to hover and perform agile manoeuvring makes rotary wing UAVs well suited to applications like inspections where precision manoeuvring and the ability to maintain a visual on a single target for extended periods of time is required.
On the flip side rotary wing aircraft involve greater mechanical and electronic complexity which translates generally to more complicated maintenance and repair processes thus meaning the user’s operational time can be decreased, which can occur increases in operational costs.
Finally, due to their lower speeds and shorter flight ranges the operator will require many additional flights to survey any significant areas, another increase in time and operational costs.
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