EDF with afterburner - Page 11 - RC Groups
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Jan 15, 2005, 02:55 AM
Pull out early!
DiveBombDave's Avatar
Maybe this will help someone....
I learned from building potato cannons that getting the right mix of hydrocarbon (butane or propane) to air was very tricky. Here's some numbers:

Lower Flammability Limit (LFL) and Upper Flammability Limit (UFL), outside of which combustion absolutely cannot occur. The perfect balance "stiochiometrically" is the magic number in between these that the hydrocarbons and air perfectly combust.

Propane 2% to 9%, stio. at 4.02% sea level 72deg etc.
Butane 2% to 10%, stio. at 3.125%

This is measured by volume of gas.

Flame rates for both are in the 1.5-1.8m/s range, slow enough that it's safe as far as "running back in the bottle."

Align the injectors for spiral mixing against the spiral of the inlet air. Provide ignition source a couple of inches downstream from the fan. I've used 4" computer power supply fans, brushless, *inside* the combustion chamber of a potato cannon. Withstood hundreds of 1700degF plus firings without a tarnish.

Rig a servo to control fuel rate, and experiment with different ranges until you get proper combustion. You won't get mach diamonds like the real jets, but it will make a good show (and some decent thrust, too). Use some muffler heat wrap to insulate the flame tube.

Dying to see a video of one of these... even if it called Firework.avi

Oh yeah, and don't blow yourself up.
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Jan 15, 2005, 10:04 AM

Interesting information. Are you sure about the 1.5-1.8 m/s burn speed for propane/butane?

If so, it makes these both very poor fuels for the afterburners. A 60 mph eflux is 88 feet/sec, while 1.8 m/s is only 5.9 feet/sec (4 mph). In this case I would assume the fuel would be ejected before most of it burned if using a sufficiently short (6"-12") combustion chamber and reasonably powerful EDF.

Do you have burn rates/mixture ratios for other fuels, such as methanol or gasoline? I believe the original posted experiments used automotive engine starter spray, and from experience with an old '70 Chevelle, would burn fast enough to explode quickly on a backfire if left to build up in the air cleaner.

Hmmm, I'm tempted to try some experiments myself while I wait for gorilla glue to dry in this cold weather we are having here. Time to find some old lighters to cannabalize for parts.

Jan 17, 2005, 10:53 AM
Dieselized User
gkamysz's Avatar
That's why the afterburner of a jet engine is 1-2 times as long as the engine itself and many times longer than the combustors in the engine. It takes time to burn fuel. In the combustors flow velocities are low and fuel can burn efficiently before leaving the combustion chamber.

Jan 18, 2005, 12:51 PM
Blue Solitude
Cerulean Ghost's Avatar
I'm subscribed...
This looks interesting.
Jan 29, 2005, 12:56 PM
Registered User
you need some way of transfering the energy of exhaust gasses from your engine to your main rotor. it must not be counter rotated but it helps and you might as well since it needs to be geared to accelerate your main rotor. this leans towards a more conventional turbo jet design, but also leads to other possibilities like only using the bateries to start your engine, run your rx pack, and in emergency. might as well build your own engine from scratch. it sounds like a lot to do with a plastic rotor. exhuase eflux will not significantly enhance output without a proper compressor. even the Caproni design incorporates a compressor stage.
Jan 29, 2005, 04:10 PM
A-4 nut!!
skyhawk's Avatar

With an afterburner - there is no exhaust gase energy transfering to a turbine. It's just raw fuel poured in the exhaust stream.
Jan 29, 2005, 05:19 PM
Airbuzz's Avatar
Anyone know if there have been any successful afterburner projects for model turbines?
Jan 30, 2005, 12:01 AM
Registered User
I understand what your saying Skyhawk. im just saying that you wont realize any real thrust increase becasue of the way an EDF is made to function. it is an air mover not a compressor. the hot gasses may go out the wrong way and stall your fan. you may see some advantage if you used a fan that could sustain a higher back pressure. of course i Have no real experiance with this so you fellas might be on to something here. someone get one of these things lit already!!!!

AirBuzz im sure some one has tryed it but its no good for fuel economy...
Last edited by lucent; Jan 30, 2005 at 12:04 AM.
Jan 30, 2005, 12:30 AM
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oldpilot's Avatar
LUCENT said:- "im just saying that you wont realize any real thrust increase because of the way an EDF is made to function. it is an air mover not a compressor. the hot gasses may go out the wrong way and stall your fan. you may see some advantage if you used a fan that could sustain a higher back pressure. of course".

And the "Fan" that produces such pressure is the compressor in a jet engine. It follows then that the only way to have an afterburner on a model plane is in conjunction with a model jet engine.

Any talk of a true afterburner on a ducted fan jet is no more than "Pie in the Sky". (Mind you I would like to see someone try, and I would like to be there with my video camera. I love fireworks. )

Cheers. Patrick.
Jan 30, 2005, 01:02 AM
"Walk of shame" Master!!!
NoM'rcrashPlease's Avatar
I'm subscribed...

I'll finish reading tomorrow

Last edited by NoM'rcrashPlease; Jan 30, 2005 at 05:28 PM.
Jan 30, 2005, 09:06 AM
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gkamysz's Avatar
The hot gasses will not go out the wrong way. Afterburners have been around for quite a few years and operational details are avaialble in texts. Afterburner does not increase the pressure of the gas flow.

Afterburner for EDF is a dream.

Afterburner for model turbine does exist. AMA in the US outlaws it if you want to fly under their insurance.

Jan 30, 2005, 09:36 AM
a really nice guy, really
Ralph Brekan's Avatar
EXACTLY! Eric's right. A small rocket engine mounted so it doest eject into your impeller would give you scale afterburner with smoke and flame. I might try this on my Kamdax M2000. With the brushed 400 EDF unit the Mirage2000 could use a kick in the seat at 300ft. It would become a speck in seconds.

I wouldnt go with a D size engine not unless your trying to exhaust a bulging lipoly pack and one of your "expendable jets" with a spectacular mid air explosion (Please take video).

If I did this, it would be with a small A size engine. I think stage1 engines have a blast flame to ignite the next engine (stage2) That wouldnt be good for the fan. Most engines are designed to eject a parachute (and sometimes the motor itself). You'd want a small rocket engine with a relatively small (or none) ejection blast or you're going to hit a massive speed bump at the end of the ride. If you can find a model motor that doesnt have that ejection blast,I guarantee that would be a great inflight afterburner OR another possible way to get a brushed fan "on step" (like bungee only cooler).

There's also the weight of the engine to consider. EDF's don't need additional weight and obstacles in the exhaust path. AirX (Eric) saw it on a Mig15, so it is possible. Chances are it was a BL motor.

Oh Yeah, Eric B. I got a HiTec Flash5 so you dont have to send that JR radio anymore Now I can start flying cool models (like you) instead of complaining that I cant because of Parkzone . LOL!


Originally Posted by AirX
If you want afterburner, try a small solid rocket motor in the exhaust outlet, saw a Mig 15 once on a video with one, climed out really nicely... Just be carefull how it is used.


Eric B.
Jan 30, 2005, 10:47 PM
a really nice guy, really
Ralph Brekan's Avatar
Lets get drunk and burn !!! Got to love Holland, LOL!
Jan 30, 2005, 10:52 PM
a really nice guy, really
Ralph Brekan's Avatar
Originally Posted by skyhawk

With an afterburner - there is no exhaust gase energy transfering to a turbine. It's just raw fuel poured in the exhaust stream.
Thus, the term AFTERburner. LOL! America rocks! Did we invent that or steal it? LOL!
Last edited by kamikazi; Jan 30, 2005 at 10:54 PM.
Mar 26, 2006, 01:06 PM
Registered User
Been a while since any update on this topic.

I recently received an email from a fellow named Axel from Sweden. He has been doing some serious experimenting on the topic. He has developed a hybrid jet engine. Thrust is 550g. The outlet diameter is only 20mm. 60% of the thrust is from the fan, and 40% is from the afterburner. So the afterburner is multiplying the thrust by a factor of 1.66. Fan is driven by a kontronic 480 4200rpm-motor.

Here is a picture at full thrust.

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