So an EDF is pretty much a mini-jet engine, right? Wrong!
We love Electric Ducted Fans (EDFs). They're great for model jets and all sorts of crazy RC projects. So, how are they different from full-sized turbojets that power full-scale jets? Well, as you might imagine, there's quite a difference.
How an EDF Works
An Electric Ducted Fan is exactly what it says on the tin: it's simply a fan, with multiple blades, that is mounted inside a duct or shrowd. They work by accelerating the air drawn into them creating thrust. The ones we use in RC airplanes are usually small (with 30mm-90mm fans), powered by high KV brushless motors, and spin extremely quickly.
They're clean and use no fuel, but this means there's no expansion of the air due to heat. If you want to see an early Flite Test experiment where we tried to fit a fire-breathing afterburner to an EDF to see if we could increase the thrust, check out this video.
Check out an article about this plane here.
How a Jet Engine Works
A jet engine is a reaction engine. It works by sucking in air, compressing it, injecting fuel into the mix and igniting it, and then having these gasses expand and blast out of the rear of the turbojet to produce thrust.
The engine is a reaction engine because the exiting gasses pass through a turbine which spins and drives the compressor which compresses the air - it's all a big chain reaction as long as you keep burning the fuel. This was an extremely complex bit of engineering to get right at first. It took many years of experimentation since the idea of the jet was concieved in the early 20th century.
Parts of a jet engine:
A normal turbojet can be broken down into five main block components. These are:
- Fan - a large spinning fan sucks in large quantities of air into the engine.
- Compressor - a collection of smaller fans that progressively compress air to increase air pressure.
- Combuster - a chamber where fuel is mixed with the compressed air and ignited.
- Turbine - a fan that is turned by the exiting exhaust gases and powers the frontal intake fan.
- Nozzle - a device on the end of the engine that accelerates the air further and maximizes the thrust.
Side Note: you can see a large nozzle protruding from the rear of this Italian airplane that was featured on 'number 9' of this list of experimental WW2 aircraft. It looks pretty wild!
Types of Jet Engine
Trubojets
The turbojet is the standard 'flavor' of jet engine. If this jet was an ice cream, it would be plain old vanilla.
Turboprops
Many small airliners use turboprops. You can usually tell which they are at the airport as they'll have propellers. 'Wait!' I hear you shout, 'surely those are just conventional internal combustion engines like the airplanes of old!' - nope, these are just jet engines with a big old prop bolted to the front.
Turbofans
Again, turbofans (like turboprops) are usually found on airliners. They can be massive, so big that you can stand inside their intake without ducking your head.
Turboshafts
This one is less obvious, but basically, this type of jet engine is like a turboprop except for it drives a shaft for a helicopter.
Ramjets
A ramjet is a type of jet engine that 'rams' air into the intake thanks to pure speed alone - to work, it relies on forward speed rather than moving parts. Although one of the fastest types of jet engine out there, ramjets are ironically some of the simplest.
To summarise, the difference between an Electric Ducted Fan used in a model aircraft is far simpler than a jet engine and doesn't produce thrust in exactly the same way - although on a smaller scale an EDF can be used to simulate the effects of a jet engine.
An EDF-powered Durafly Vampire readies for takeoff.
If you found this article interesting, informative or helpful, hit that like button!
Illustrations courtesy of NASA
Article by James Whomsley
Editor of FliteTest.com
Contact: james@flitetest.com
YouTube Channel: www.youtube.com/projectairaviation
Model EDFs have much less power to work with. They rely on "mass flow" not pressure. That is, the mass of the air by sheer volume flowing through the fan. As such they need to breath freely. Large open inlets help. Smooth rounded lips on the inlet also help. Cheater holes can also help. On the output, side, a "thrust tube" can increase speed, but only a little. Choking down the output creates back-pressure and reduces flow, so you can't choke it down very far. High velocity air creates lots of drag, so make sure all internal airflow paths are smooth and un-blocked as possible. Higher Blade counts can often sound more jet like and less whiny, but it often depends on lots of things. EDF's typically run 4, 5, 6, or more cells in series, so high voltage. Batteries need to be high C because the entire flight might last 5 minutes or less, so 12C, but battery manufacturers exaggerate their C ratings greatly, so much higher published ratings are really needed.
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