Transitioning Vehicles – Looks Can be Deceiving

It seems that everyone is working on some sort of transitioning UAV these days. Combining a fixed wing aircraft and some sort of multirotor seems to be the thing to do. The resulting vehicle combines the multirotor’s ease of takeoff and landing with the endurance of a fixed wing UAV.

There are two main types of these vehicles: quadplanes and tiltrotors. The quadplane uses a separate set of motors and propellers for lifting than it does for forward flight. During hover, the forward flight motor is off and during forward flight, the lifting motors are off. The tiltrotor shares lifting and forward flight motors. Two, or more, of the lifting motors/propellers tilt forward during the transition from hovering to forward flight. They tilt back to vertical during the transition back to hovering flight.

Of course, there is no free lunch and there is a price to pay for the ability of a fixed wing UAV to hover: complexity, cost, and weight increase. Drag also increases and this has a significant effect on endurance. The effect on endurance is great enough that transitioning vehicles make much more sense for gas powered UAVs. It isn’t difficult to design a fixed wing UAV with an endurance of ten, or more, hours and having to sacrifice a couple hours endurance for the ability to hover is not that big a tradeoff. In an all-electric UAV, the endurance is less and the cost of the ability to hover is a much larger percent of the UAV’s overall endurance.

When comparing a quadplane style UAV to a tiltrotor, the designer might, at first blush, consider the reduction in the number of motors an advantage. Again, this advantage comes at a cost related to the propellers’ pitch. The optimum propeller pitch for forward flight is different from the optimum pitch for hovering. Since a quadplane uses different propellers for forward flight than it does for hovering, the designer is free to choose the most efficient propeller for each phase of flight. The designer of a tiltrotor does not have this freedom. She must compromise between a propeller optimized for hovering versus one optimized for forward flight. She either has to accept less payload or less endurance.

Another disadvantage of tiltrotors is you must use electric propulsion. You cannot mix a gas engine for forward flight with electric motors for hovering.

Endurance is not the only challenge when designing a transitioning UAV; wind also presents some challenges, primarily when hovering. When flying a quadplane in a wind, the wind creates airflow over the wings and tail of the quadplane and this generates forces that are a challenge for the quadplane. The most obvious problem comes from the vertical stabilizer. If the vertical stabilizer is not aligned with the wind, it will generate a yawing moment that tries to turn the UAV into the wind. The problem is that while multirotors have excellent pitch and roll control their yaw control is weak. It is very easy for the torque generated by the vertical stabilizer to overwhelm the qaudplane’s ability to hold a heading when hovering in a wind. This can be a problem if the direction you are facing when your UAV transition to forward flight matters. If you are hovering near a structure and the wind turns your quadplane so that it is facing the structure, you cannot transition without hitting the structure. Tiltrotors do not suffer from this drawback as you can control yaw by tilting one of the motors. This provides very strong yaw control.

Another problem is airflow over the wing when hovering in a wind. In order to hover in a wind, the quadplane must tilt into the wind. This puts the wing at a negative angle of attack and the wing will generate lift in the downward direction. Now the lifting motors must lift not just the weight of the UAV but must also overcome the force generated by the wing. This is a significant challenge because quadplanes are usually heavy – near the maximum lifting capacity of its motors, and the wing’s downward force can seriously limit the UAV’s ability to climb when hovering in a wind. The usual solution to this problem is to use the quadplane’s forward flight engine to help hold position. The quadplane can then hold a more level attitude and the airflow over the wing can help the quadplane climb. Note that using the forward flight engine only helps if the quadplane is pointed into the wind. Tiltrotors also suffer from this challenge. A tiltrotor can tilt its motors into the wind to help hold position.

Another complication with both a quadplane and a tiltrotor is the centre of gravity. The UAV designer has essentially combined two UAVs into one and each of the two UAVs needs the centre of gravity in the correct location. The multirotor works best if the centre of gravity is located centrally between the motors. The fixed wing needs the centre of gravity about a third of the distance between the wing’s leading and trailing edges. It is not difficult to arrange the correct centre of gravity, but it is another detail that needs attention.

As with all UAVs, the designer of a transitioning UAV faces many tradeoffs. The correct choice is dictated by the task the UAV must perform. Given the very poor yaw control of a quad plane and the tiltrotor’s inability to combine gas and electric motors; it may be worth considering a hybrid of hybrids. A quadplane with tilting motors to ensure adequate yaw control provides the best of both worlds and is likely worth the cost and weight of the extra tilting mechanisms.

 

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