Mechanical Design of a drone can be condensed to the following tasks:
Select a powertrain (motor + battery + controller) that is appropriate to carry the required mission payload, for a required mission speed for a required mission time in required mission conditions.
Design a structure to attach the motors to the battery.
Design a structure to attach the payload to above.
With multirotor drones, aerodynamic design is not often the highest priority in the minds of designers. There are several reasons for this.
Many multirotor drones often operate at low flight speeds and in low winds
Mechanical design of aerodynamic bodies is challenging, technically involved and requires investment and expertise.
Manufacturing of aerodynamic bodies is more involved and expensive.
The incumbent paradigm of industrial drone design is the sandwich plate design type. This is a design style that is easy to implement and operate even for non-aerospace engineers, but has a serious aerodynamic drawbacks.
Flat-plate or sandwich-plate drones are shaped in a way that when they fly forward, air pushes them down. This means the motors need to work harder to keep the drone in the air, making the drone less efficient. To learn more about the aerodynamics of this phenomenon, refer to our post on Aerodynamic Down Force on Industrial Drones
Consider the Sensus 6 Hydrogen fuel cell drone pictured above. The designers simply stuck the fuel cell and tank wherever on the frame it fit and called it a day. The drone can fly an exceptional 120 minutes on hydrogen at low speeds and in low-wind conditions. ISS is able to sell this drone for around $60,000 at the time of this writing because, due to the fuel cell, it can accomplish tasks requiring long flight times in spite of its aerodynamic clumsiness.
Switching to Fixed-wing : Fixed-wing designs are hands-down more efficient in high speed forward flight than a multirotor. In some cases, switching to fixed wing may be a solution to those considering how to get more forward efficiency with their UAV flight. However, there are some shortcomings to this path:
Hard to Use Fixed wing aircraft design and operation is more challenging than multirotor, requiring more experienced pilots and often a runway for takeoff and landing.
Not Optimized for Development Fixed wings require a smooth fuselage, therefore it is more mechanically challenging to integrate experimental accessories or systems.
Stall Speed Fixed wings can't hover or fly slower than stall speed. In applications where UAV is sent to collect specific photographs, this presents challenges.
Switching to VTOL: VTOL designs have many of the advantages of fixed-wings, but they are able to use rotor lift to take off and land vertically, or switch to a hovering mode in flight.
Heavy: Because you must have all the components of both fixed-wings and multirotor, the aircraft is heavier than a fixed-wing or multirotor of equivalent capabilities, and will have less payload weight.
Expensive: Similarly, because you must have all the components of both fixed-wings and multirotor, more parts are needed to produce the aircraft.
Inefficient: While the mission capabilities of the VTOL encompass a broader envelope than airplanes or multirotors, a quadplane will always fly less efficiently than a plane, and hover less efficiently than a quadcopter.
Are there ways to improve efficiency of multirotors in forward flight, without moving to a VTOL design? Aerodynamically speaking, yes. There are two main approaches, but both require a designer to consider the orientation of the airframe in the forward flight condition.
Lifting Body Multirotor Simplistically speaking, lift is drag directed upwards. But when you direct it upwards, it helps the motors to work less, improving the flight efficiency of your aircraft. With this design, we are not looking necessarily to maximize the body lift force such that it can carry the weight of the drone, but simply turning a negative force into a positive one.
Drag-Minimized Multirotor For this style of design, we need to decide on a forward pitch angle at cruise, and commit to it fully. With the airframe designed streamlined to the relative wind, we can eliminate down force and lift from the steady state, and drag is minimized.
The completed aircraft tends to look as though the fuselage is at an angular offset from the motors. The Sonin Hybrid Drone [Right Image] is able to reach flight speeds of 140mph due to this design.
Stay tuned for drone airframes of this type from Mothership Aeronautics.
Tiltrotor: Taking a baby step towards VTOL capabilities without crossing the line out of multirotor territory brings you into Tiltrotor Country. Going with one of the above designs, but choosing not to commit to a fixed cruise angle or angle of attack leads you in the direction of tiltrotors.
Tilting the rotors allows the drone to fly forwards without necessarily pitching the body of the drone. This allows the pilot to keep the lowest-drag profile of the airframe, facing the wind. Sam Reynolds, a Mech. E student at UCSB prototyped a tiltrotor of this type during his Summer 2020 Design Internship at Mothership Aeronautics. See the report here.