Disk Loading

Often times, when discussing various propulsion methods for VTOL aircraft, the term "Disk Loading" pops up. By definition, Disk Loading is a pressure, it is equal to the total thrust output divided by the effective "rotor" area. For conventional propellers, the rotor area is calculated from the diameter of the propeller. For other engines, like turbojets, the "rotor area" is equal to the nozzle exit area. Disk loading determines whether a given VTOL aircraft is efficient at hover or more optimized for high-speed horizontal travel. To understand this relationship, it is best to compare the disk loading for three common propulsion methods assuming all provide the same thrust output: a propeller, a ducted fan, and a turbojet.

1- A propeller is the most common "rotor" found on drones, eVTOL aircraft and helicopters. Due to its large size, the propeller has low Disk Loading and enables very efficient hover, ideal for long duration surveillance applications. However, in forward flight, the vehicle is forced to pitch forward, exposing a large portion of the propeller to the oncoming air. This causes high drag, and as a result, propeller-powered aircraft are greatly limited in their top-speed.

2- A ducted fan can generate the same thrust as a propeller from a smaller diameter form-factor. Hence, it has a slightly higher Disk Loading than the propeller and can provide an increase to the top-speed of the VTOL aircraft at the expense of a decrease in hover efficiency. Most modern ducted fans are electric and are known to be very demanding on the batteries. For this reason, ducted fans are not a popular choice for eVTOL aircraft (with the exception of Lilium ). It is worth to mention, that ducted fans found great popularity in non-VTOL aviation, specifically modern airliners, which rely on large turbofans (turbine driven ducted fans) to perform trans-oceanic flights.

3- Turbojets generate focused beams of high-temperature gas. They have a very high Disk Loading when compared to propellers and ducted fans. Therefore, turbojet-powered VTOL aircraft are not fuel-efficient at hover but can attain high cruise velocities, maxing out at just under the speed of sound. This enables the aircraft to cover large distances even with smaller fuel tanks. A notable example of this technology is the FusionFlight JetQuad drone. The drone is designed to utilize the hover capability only during VTOL operations, so that the majority of fuel is consumed during down-range flight.

But what if we want hover efficiency AND top speed in the same aircraft? Some companies aim to solve the Disk Loading conundrum using a hybrid approach. Bell Flight (under the DARPA SPRINT program) is currently testing a turbine-based engine with folding propellers. The idea is to use propellers during hover and fold them into the engine nacelle at high speeds (to mitigate the drag issue). Also, take for example the F35 aircraft which combines the direct turbojet thrust output with a ducted fan in hopes of improving the fuel efficiency at hover. Time will only tell which technology dominates. Should we build aircraft for specific missions, or can there be one to rule them all using some fancy hybrid approach?