Ducted Fan
A ducted fan is an atmospheric type of propulsion device. It is like a modern re-invention of the propeller. The propeller (or rotor) is housed inside a ring-like housing to reduce noise. The rotor also contains more blades than a propeller and each blade is wider in order to generate more thrust. Typically, there is a special round edge on the housing front to provide better flow and increase performance further. Like the propeller, a ducted fan can be powered by anything - it can be driven by either an electrical motor or a combustion engine. For the same thrust output, the propeller consumes less power, but a ducted fan is smaller in size. In addition, flaps called "vanes" can be installed in the duct directly behind the fan. These can be used to vector the thrust or reverse it entirely - a beneficial feature that propellers lack.
Electric Ducted Fans (EDF) are ducted fans powered by an electric motor (brushless is the standard). Today the most common use for the EDF are remote-control airplanes and hobby-grade drones. There are several, more serious, eVTOL (electric vertical take-off and landing such a Lilium ) aircraft applications, but most of such designs have not yet made past the flight-testing phase. The common problem is that an EDF is more power hungry than an electrical propeller for the same thrust output. Powered by electricity, an EDF aircraft will require a lot of lithium batteries to get flight-times of 15-minutes and beyond. For this reason, any EDF powered aircraft mostly consists of batteries - an expensive and sometimes dangerous design as lithium batteries will catch on fire if too much power is drawn.
To solve the issue of the battery storage, a ducted fan should instead be powered by a combustion engine. Energy density of liquid fuel such as Jet-A, is 40-times higher than that of a lithium battery. A liquid fueled aircraft can carry more useful stuff than a battery powered aircraft. For the combustion engine, most designs utilize a turbine jet-engine as it has a higher power-to-weight ratio when compared to a piston engine. When we add a ducted fan to a jet-engine we produce a turbofan. In a turbofan, the jet-engine is not used to provide thrust, instead its sole purpose is to power the fan in front. The larger the fan the greater is the increase to static thrust and fuel efficiency. A bigger fan will provide more thrust at take-off but add drag which greatly limits top-speed. For this reason, turbofans used on passenger aircraft have very large fans to maximize lower speed efficiency. On the other hand, turbofans used on fighter jets have smaller fans to prioritize top speed. The most advanced turbofan is called a "geared turbofan" - it connects the jet-engine with the fan through a gearbox which boosts the fuel efficiency across a wider range of air speeds.
领英推荐
Clearly, turbofans are popular aircraft engines but due to their size, their applications are limited to fairly large aircraft. The future is, in my opinion, the miniaturization of the turbofan engine - a micro turbofan. In essence the micro turbofan is a ducted fan attached to a microturbine jet-engine. Over the years, I have personally explored various ways of implementing microturbine jet-engines for vertical take-off and landing (VTOL) applications. One of the drawbacks of such an aircraft is the power consumption at hover. A micro turbofan would easily solve this problem as it would prioritize fuel efficiency at lower speeds. The resulting VTOL aircraft would be liquid-fueled, hover-efficient and compact. The micro turbofan engine is currently in development but once commercially available it will allow for some incredible new applications for compact VTOL-capable personal aircraft.
at
1 年Additionally, the mentioned 40x energy density difference between Jet-A and today's lithium batteries ignores the "waste heat" of combustion. Which is very effectively used to increase duct temp. Put another way, even if batteries had 40x the power, they still could not do what today's turbofans do, as there is no heat of combustion available.
at
1 年It is incorrect to say the core turbine engine does not provide thrust. At high speeds and altitudes, the core engine's heat and thrust is critical to efficient operation. It is also important to note that while we can drive a fan with an electric motor, we cannot achieve the same high speed cruise results without the core engine's heat. That heat raises the speed of sound, thereby raising the stagnation point (the speed at which the airflow chokes in a duct) and results in far higher duct discharge velocity. The reason fighters have afterburners is to increase the temperature, and thereby allow for a higher discharge velocity at the same pressures. A duct with a 2000 degree gas can have a 2200MPH discharge velocity. The mass flow increase during "reheat" is minimal, the velocity increase is impressive.
CEO @ The Hydroflyer
1 年We use a ducted propellor on The HydroFlyer We experimented with dozens of designs and one of the interesting benefits besides more thrust at low speed is that it provides a stabilizing effect on the craft at high speed.