Sandwich Plate Airframes
One of the most common airframe types used in industrial drones is a sandwich plate. These are simple and cheap to design and mass manufacture, especially for larger industrial drones. These designs rely the strength of opposing, sandwiched carbon plates to rigidly hold carbon-fiber motor tubes in an X-type configuration.
The flatness of the center body makes it simple enough to mount electronics, components batteries and payloads on to the frame, that aerospace or mechanical engineering professionals are not necessary which lowers the barriers to entry for drone production.
Flat Plate Aerodynamics and Down Force
There are downsides to sandwich plate design, primarily in the realm of high speed aerodynamics. Consider the main airframe a flat plate.
A plate facing downwards into the relative wind deflects the wind upwards. By Newton's 3rd law of motion, the direction change of the wind imparts a force on the plate. This force is called aerodynamic force, which is generally diagonally downwards and back. For easy analysis we usually resolve this force into vertical and horizontal components called Drag & Lift.
Drag: We define drag force as the backward force, or the force opposing the motion of the vehicle (relative wind)
Lift: Lift is defined as the vertical component of the aerodynamic force, opposed to weight. In cases where lift points downwards, effectively increasing the vehicle's effective weight, we call it Down Force.
Noe consider the flat plate effects on an actual drone flying forwards. For a drone to fly forwards, it pitches into the wind. Thrust from all motors is tilted towards the direction of flight and now also provides the forward acceleration.
In the diagram above, where we see a drone flying towards the left of the page, the flat plate of the airframe is producing an aerodynamic force backward and down. The down force acts with the weight and directly opposes the thrust of the motors. This then increases the power requirement of the drone, resulting in inefficient forward flight.
Value Function and Tradeoffs
If flat plate drones always produce this parasitical down force whenever they fly forwards, why is this design still being used?
The design style is cheap, easy to design, easy to adapt and easy to use. Drones built in this style can be modified in a modular manner without redesigning the drone.
Inefficient in forward flight, can still be efficient in hover flight.
It allows for multi-direction flight of the drone. By not optimizing forward flight efficiency, the drones can just as easily fly sideways, backwards or diagonals, in low speed inspection flights and cinematography.
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.